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Antibodies to red blood cell (RBC) alloantigens can increase morbidity and mortality in transfusion recipients. However, alloimmunization rates can vary dramatically, with some patients never generating alloantibodies following transfusion, while others not only become alloimmunized, but may be prone to generating additional alloantibodies following subsequent transfusion. Previous studies suggest that CD4 T cell responses that drive alloantibody formation recognize the same alloantigen engaged by B cells. However, as RBCs express numerous antigens, both internally and externally, it is possible that CD4 T cell responses directed against intracellular antigens may facilitate subsequent alloimmunization to a surface RBC antigen. Here we show that B cells can acquire intracellular antigens from RBCs. Using a mouse model of donor RBCs expressing two distinct alloantigens, we demonstrate that immune priming to an intracellular antigen, which would not be detected by any currently used RBC compatibility assays, can directly influence alloantibody formation following exposure to a subsequent distinct surface RBC alloantigen. These findings suggest a previously under-appreciated mechanism whereby transfusion recipient responders may exhibit an increased rate of alloimmunization due to prior immune priming toward intracellular antigens.

BACKGROUND: Alloimmunization can be a significant barrier to red blood cell (RBC) transfusion. While alloantigen matching protocols hold promise in reducing alloantibody formation, transfusion-dependent patients can still experience RBC alloimmunization and associated complications even when matching protocols are employed. As a result, complementary strategies capable of actively preventing alloantibody formation following alloantigen exposure are warranted. STUDY DESIGN AND METHODS: We examined whether pharmacological removal of macrophages using clodronate may provide an additional strategy to actively inhibit RBC alloimmunization using two preclinical models of RBC alloimmunization. To accomplish this, mice were treated with clodronate, followed by transfusion of RBCs expressing the HOD (HEL, OVA and Duffy) or KEL antigens. On days 5 and 14 post transfusion, anti-HOD or anti-KEL IgM and IgG antibodies were evaluated. RESULTS: Low dose clodronate effectively eliminated key marginal zone macrophage populations from the marginal sinus. Prior treatment with clodronate, but not empty liposomes, also significantly inhibited IgM and IgG anti-HOD alloantibody formation following transfusion of HOD RBCs. Similar exposure to clodronate inhibited IgM and IgG antibody formation following KEL RBC transfusion. CONCLUSIONS: Clodronate can inhibit anti-HOD and anti-KEL antibody formation following RBC transfusion in preclinical models. These results suggest that clodronate may provide an alternative approach to actively inhibit or prevent the development of alloantibodies following RBC transfusion, although future studies will certainly be needed to fully explore this possibility.

Red blood cell (RBC) transfusions can result in alloimmunization toward RBC alloantigens that can increase the probability of complications following subsequent transfusion. An improved understanding of the immune mechanisms that underlie RBC alloimmunization is critical if future strategies capable of preventing or even reducing this process are to be realized. Using the HOD (hen egg lysozyme [HEL] and ovalbumin [OVA] fused with the human RBC antigen Duffy) model system, we aimed to identify initiating immune factors that may govern early anti-HOD alloantibody formation. Our findings demonstrate that HOD RBCs continuously localize to the marginal sinus following transfusion, where they colocalize with marginal zone (MZ) B cells. Depletion of MZ B cells inhibited immunoglobulin M (IgM) and IgG anti-HOD antibody formation, whereas CD4 T-cell depletion only prevented IgG anti-HOD antibody development. HOD-specific CD4 T cells displayed similar proliferation and activation following transfusion of HOD RBCs into wild-type or MZ B-cell–deficient recipients, suggesting that IgG formation is not dependent on MZ B-cell–mediated CD4 T-cell activation. Moreover, depletion of follicular B cells failed to substantially impact the anti-HOD antibody response, and no increase in antigen-specific germinal center B cells was detected following HOD RBC transfusion, suggesting that antibody formation is not dependent on the splenic follicle. Despite this, anti-HOD antibodies persisted for several months following HOD RBC transfusion. Overall, these data suggest that MZ B cells can initiate and then contribute to RBC alloantibody formation, highlighting a unique immune pathway that can be engaged following RBC transfusion.

Alloantibodies developing after exposure to red blood cell (RBC) alloantigens can complicate pregnancy and transfusion therapy. The only method currently available to actively inhibit RBC alloantibody formation is administration of antigen-specific antibodies, a phenomenon termed antibody-mediated immune suppression (AMIS). A well-known example of AMIS is RhD immune globulin prophylaxis to prevent anti-D formation in RhD− individuals. However, whether AMIS is specific or impacts alloimmunization to other antigens on the same RBC remains unclear. To evaluate the specificity of AMIS, we passively immunized antigen-negative recipients with anti-KEL or anti-hen egg lysozyme (HEL) antibodies, followed by transfusion of murine RBC expressing both the HEL-ovalbumin-Duffy (HOD) and human KEL antigens (HOD × KEL RBC). Significant immunoglobulin G deposition on transfused HOD × KEL RBC occurred in all passively immunized recipients. Complement deposition and antigen modulation of the KEL antigen occurred on transfused RBC only in anti-KEL–treated recipients, whereas HEL antigen levels decreased only in the presence of anti-HEL antibodies. Western blot analysis confirmed the specificity of antigen loss, which was not attributable to RBC endocytosis and appears distinct for the 2 antigens. Specifically, removal of KEL was attenuated by clodronate treatment, whereas loss of HEL was unaffected by clodronate in vivo but sensitive to protease treatment in vitro. Antigen-specific modulation correlated with antigen-specific AMIS, with anti-KEL treated recipients forming antibodies to the HOD antigen and anti-HEL–treated recipients developing antibodies to the KEL antigen. Together, these results demonstrate that passively administered antibodies can selectively inhibit the immune response to a specific antigen.

Introduction: The impact of red blood cell (RBC) storage on alloimmunization rates after transfusion remains controversial, with clinical experience demonstrating conflicting results. Indeed, some reports suggest increased rates of alloimmunization associated with longer storage of RBC units, while others suggest no association between alloimmunization rates and length of storage. We hypothesized that this discrepancy may reflect the differential impact of storage on alloimmunization related to each unique antigen and the immune response elicited. Here we define the effect of red cell storage on two distinct antigens, including the Hel-Ova-Duffy (HOD) and KEL antigens, using a preclinical murine model of RBC storage and transfusion. Methods: RBCs singly expressing HOD (HOD RBC) or KEL (KEL RBC) antigens, in addition to those dually expressing both HOD and KEL (HOD x KEL RBC) antigens, were collected from respective transgenic donor mice into CPDA. Units of HOD, KEL or HOD x KEL RBC were generated and used immediately (fresh RBC), or stored for 8 (D8) or 14 (D14) days at 4C before transfusion into wildtype recipients. Recipient cytokine profiles were assessed 2 hours post-transfusion using a multiplex inflammation panel. Donor RBC survival at 24 hours post-transfusion was assessed by flow cytometry. Serum was collected from recipients on days 5 and 14 post-transfusion and analyzed for IgM or IgG development by flow-cytometric crossmatch using HOD or KEL RBC targets. Results: Post-transfusion RBC survival was decreased with increased storage, but was not significantly different between donor cell types (HOD, KEL or HOD x KEL). Recipient cytokine profiles demonstrated a distinct inflammatory profile associated with storage. Development of IgM antibodies against HOD in mice transfused fresh versus D8 or D14 HOD or HOD x KEL RBCs were not significantly different; however, significantly increased (p Conclusions: Although the impact of RBC storage on various measures of transfusion efficacy have been determined, the impact of storage on alloimmunization remains incompletely understood. Using a murine model of transfusion, we demonstrate that storage differentially impacts immunization to red cell antigens, increasing the response to certain antigens, like HOD, while decreasing the response to others, like KEL. Although it is unclear whether these findings apply to other RBC antigens and what relevance this holds for human patients, our study provides insight into the impact of storage on red cell alloimmunization and may enlighten future storage protocols. Disclosures Stowell: Argenx: Speakers Bureau; Grifols: Speakers Bureau; Alexion: Consultancy.

Background Anti-RhD administration can prevent de novo anti-RhD formation following RhD+ red blood cell (RBC) exposure, termed antibody-mediated immunosuppression (AMIS). Recent studies suggest that AMIS may occur through target antigen alterations, known as antigen modulation. However, studies suggest that AMIS may occur independent of antigen modulation. In particular, AMIS to RBCs that transgenically express the fusion hen egg lysozyme-ovalbumin-Duffy (HOD) antigen have been shown to occur independent of activating Fcγ receptors (FcγRs) thought to be required for antigen modulation. Therefore, we sought to determine the mechanism behind AMIS following HOD RBC exposure. Study design and methods Following transfer of HOD RBCs into wild-type or FcγR-chain knockout recipients in the presence or absence of monoclonal anti-hen egg lysozyme (HEL) antibody, individually or in combination, HOD antigen levels and anti-HOD antibody formation were examined. Results Our results demonstrate that anti-HEL antibodies individually or in combination suppressed anti-HOD IgM, which correlated with the rate of detectable decrease in HEL on HOD RBCs. Furthermore, exposure to anti-HEL antibodies alone or in combination equally suppressed anti-HOD IgG formation. Unexpectedly, combination or individual anti-HEL antibodies induced AMIS and antigen modulation in an FcγR-independent manner. Pre-exposure of HOD RBCs to anti-HEL antibodies reduced antigen levels and suppressed anti-HOD antibody formation following HOD RBC exposure. Conclusion These results suggest that antibody-mediated antigen modulation may reflect a mechanism of AMIS that can occur independent of activating FcγRs and may provide a surrogate to identify antibodies capable of inducing AMIS against different RBC antigens.

Red blood cell (RBC) alloimmunization represents a significant immunological challenge for some patients. While a variety of immune constituents likely contribute to the initiation and orchestration of alloantibodies to RBC antigens, identification of key immune factors that initiate alloantibody formation may aid in the development of a therapeutic modality to minimize or prevent this process. To define the immune factors that may be important in driving alloimmunization to an RBC antigen, we determined the specific immune compartment and distinct cells that may initially engage transfused RBCs and facilitate subsequent alloimmunization. Our findings demonstrate that the splenic compartment is essential for formation of anti-KEL antibodies following KEL RBC transfusion. Within the spleen, transfused KEL RBCs are found within the marginal sinus, where they appear to specifically co-localize with marginal zone (MZ) B cells. Consistent with this, removal of MZ B cells completely prevented alloantibody formation following KEL RBC transfusion. While MZ B cells can mediate a variety of key downstream immune pathways, depletion of follicular B cells or CD4 T cells failed to similarly impact the anti-KEL antibody response, suggesting that MZ B cells may play a key role in the development of anti-KEL IgM and IgG following KEL RBC transfusion. These findings highlight a key contributor to KEL RBC-induced antibody formation, wherein MZ B cells facilitate antibody formation following RBC transfusion.

BACKGROUND: Factors influencing the development of alloantibodies against blood group antigens on transfused red blood cells are poorly defined. We hypothesised that transfused platelets may act as a danger signal to recipients and affect humoral immune responses to transfused red blood cells. MATERIALS AND METHODS: Platelet-rich plasma prepared from wild-type C57BL/6 or CD40L knock-out donors was transfused into wild-type or CD40L knock-out recipients. Leucoreduced red blood cells from transgenic donors expressing high levels of the human KEL glycoprotein in an erythrocyte-specific manner (KEL(hi) donors) were transfused after the platelets, and anti-KEL responses were measured longitudinally. In some experiments, recipients were treated with poly (I:C), monoclonal CD40L-blocking antibody, or CD4-depleting antibody prior to transfusion. RESULTS: Transfusion of wild-type C57BL/6 platelets or treatment with poly (I:C) prior to KEL(hi) red blood cell transfusion led to an anti-KEL alloimmune response in wild-type recipients. Transfusion of platelets from wild-type but not CD40L knock-out donors prior to KEL(hi) red blood cell transfusion led to an IgG anti-KEL alloimmune response in CD40L knock-out recipients; unexpectedly, transfusion of platelets from CD40L knock-out donors prior to KEL(hi) red blood cell transfusion led to a robust anti-KEL alloimmune response in wild-type recipients. Recipient treatment with MR1 CD40L-blocking antibody or CD4-depleting antibody prevented KEL alloimmunisation altogether. DISCUSSION: Transfused platelets serve as an adjuvant in this T-dependent murine model of anti-KEL red blood cell alloimmunisation, with CD40/CD40L interactions being involved to some degree but with additional mechanisms also playing a role. These findings raise questions about the role that transfused or endogenous platelets may play in other innate/adaptive immune responses.

Red blood cell (RBC) alloimmunization can make it difficult to procure compatible RBCs for future transfusion, directly leading to increased morbidity and mortality in transfusion-dependent patients. However, the factors that regulate RBC alloimmunization remain incompletely understood. As complement has been shown to serve as a key adjuvant in the development of antibody (Ab) responses against microbes, we examined the impact of complement on RBC alloimmunization. In contrast to the impact of complement component 3 (C3) in the development of an immune response following microbial exposure, transfusion of C3 knockout (C3 KO) recipients with RBCs expressing KEL (KEL RBCs) actually resulted in an enhanced anti-KEL Ab response. The impact of C3 appeared to be specific to KEL, as transfusion of RBCs bearing another model antigen, the chimeric HOD antigen (hen egg lysozyme, ovalbumin and Duffy), into C3 KO recipients failed to result in a similar increase in Ab formation. KEL RBCs experienced enhanced C3 deposition and loss of detectable target antigen over time when compared to HOD RBCs, suggesting that C3 may inhibit Ab formation by impacting the accessibility of the target KEL antigen. Loss of detectable KEL on the RBC surface did not reflect antigen masking by C3, but instead appeared to result from actual removal of the KEL antigen, as western blot analysis demonstrated complete loss of detectable KEL protein. Consistent with this, exposure of wild-type B6 or C3 KO recipients to KEL RBCs with reduced levels of detectable KEL antigen resulted in a significantly reduced anti-KEL Ab response. These results suggest that C3 possesses a unique ability to actually suppress Ab formation following transfusion by reducing the availability of the target antigen on the RBC surface.

Calabro et al. show that 33D1+ dendritic cells present in the bridging channel of the spleen are essential for alloantibody response to transfused red blood cells., Red blood cell (RBC) transfusion is a life-saving therapeutic tool. However, a major complication in transfusion recipients is the generation of antibodies against non-ABO alloantigens on donor RBCs, potentially resulting in hemolysis and renal failure. Long-lived antibody responses typically require CD4+ T cell help and, in murine transfusion models, alloimmunization requires a spleen. Yet, it is not known how RBC-derived antigens are presented to naive T cells in the spleen. We sought to answer whether splenic dendritic cells (DCs) were essential for T cell priming to RBC alloantigens. Transient deletion of conventional DCs at the time of transfusion or splenic DC preactivation before RBC transfusion abrogated T and B cell responses to allogeneic RBCs, even though transfused RBCs persisted in the circulation for weeks. Although all splenic DCs phagocytosed RBCs and activated RBC-specific CD4+ T cells in vitro, only bridging channel 33D1+ DCs were required for alloimmunization in vivo. In contrast, deletion of XCR1+CD8+ DCs did not alter the immune response to RBCs. Our work suggests that blocking the function of one DC subset during a narrow window of time during RBC transfusion could potentially prevent the detrimental immune response that occurs in patients who require lifelong RBC transfusion support.

Although factor VIII (FVIII) replacement therapy can be lifesaving for patients with hemophilia A, neutralizing alloantibodies to FVIII, known as inhibitors, develop in a significant number of patients and actively block FVIII activity, making bleeding difficult to control and prevent. Although a variety of downstream immune factors likely regulate inhibitor formation, the identification and subsequent targeting of key initiators in inhibitor development may provide an attractive approach to prevent inhibitor formation before amplification of the FVIII immune response occurs. As the initial steps in FVIII inhibitor development remain incompletely understood, we sought to define early regulators of FVIII inhibitor formation. Our results demonstrate that FVIII localizes in the marginal sinus of the spleen of FVIII-deficient mice shortly after injection, with significant colocalization with marginal zone (MZ) B cells. FVIII not only colocalizes with MZ B cells, but specific removal of MZ B cells also completely prevented inhibitor development following FVIII infusion. Subsequent rechallenge with FVIII following MZ B-cell reconstitution resulted in a primary antibody response, demonstrating that MZ B-cell depletion did not result in FVIII tolerance. Although recipient exposure to the viral-like adjuvant polyinosinic:polycytidylic acid enhanced anti-FVIII antibody formation, MZ B-cell depletion continued to display similar effectiveness in preventing inhibitor formation following FVIII infusion in this inflammatory setting. These data strongly suggest that MZ B cells play a critical role in initiating FVIII inhibitor formation and suggest a potential strategy to prevent anti-FVIII alloantibody formation in patients with hemophilia A.

To the editor: The recently approved anti-CD38 monoclonal antibody daratumumab (DARA) provides a unique therapeutic strategy that more selectively targets plasma cells in patients with plasma cell myeloma.[1][1][⇓][2][⇓][3][⇓][4][⇓][5]-[6][6] While DARA recognizes and removes malignant

Although RBC transfusion can result in the development of anti-RBC alloantibodies that increase the probability of life-threatening hemolytic transfusion reactions, not all patients generate anti-RBC alloantibodies. However, the factors that regulate immune responsiveness to RBC transfusion remain incompletely understood. One variable that may influence alloantibody formation is RBC alloantigen density. RBC alloantigens exist at different densities on the RBC surface and likewise exhibit distinct propensities to induce RBC alloantibody formation. However, although distinct alloantigens reside on the RBC surface at different levels, most alloantigens also represent completely different structures, making it difficult to separate the potential impact of differences in Ag density from other alloantigen features that may also influence RBC alloimmunization. To address this, we generated RBCs that stably express the same Ag at different levels. Although exposure to RBCs with higher Ag levels induces a robust Ab response, RBCs bearing low Ag levels fail to induce RBC alloantibodies. However, exposure to low Ag–density RBCs is not without consequence, because recipients subsequently develop Ag-specific tolerance. Low Ag–density RBC–induced tolerance protects higher Ag–density RBCs from immune-mediated clearance, is Ag specific, and occurs through the induction of B cell unresponsiveness. These results demonstrate that Ag density can potently impact immune outcomes following RBC transfusion and suggest that RBCs with altered Ag levels may provide a unique tool to induce Ag-specific tolerance.

Introduction: Though a vital therapy for patients with hemophilia A, exposure to factor VIII (fVIII) can lead to the development of neutralizing anti-fVIII IgG. Interestingly, not all patients with hemophilia A form inhibitors, with approximately 20-30% of patients with severe hemophilia A and 5% of patients with mild to moderate hemophilia A developing inhibitors. Several factors have been hypothesized to govern the propensity of patients to develop inhibitors, including the type of F8 gene mutation a patient possesses. Approximately, 60-70% of hemophilia A patients with large deletions (greater than 50 base pairs) of the F8 gene form inhibitors, while only 20-30% of patients with smaller deletions develop inhibitors. However, no significant difference in inhibitor formation has been observed between single exon 16 (E16) and complete (TKO) F8 gene deficient hemophilia A mice exposed to human fVIII (hfVIII). As E16 mice are on a mixed S129-B6 background and strain differences can impact the immunological outcome to a given immunogen, it is possible that the mixed background influenced the humoral immune response to hfVIII in E16 mice. However, E16 mice that are backcrossed >10 generations onto the same B6 background as TKO mice (B6-E16) were found to develop a comparable IgG response to hfVIII as B6-TKO and S129/B6-E16 mice. As human and murine B domain deleted (BDD) fVIII are 85% identical, we hypothesize that the diversity between human and murine fVIII may not allow for elucidation of immune differences that may exist between hemophilia A mice with large or small F8 mutations. Accordingly, we herein sought to compare the relative immunogenicity of murine fVIII (mfVIII) in these distinct hemophilia A mice. Methods:S129/B6 E16, B6-E16 and B6-TKO mice were administered four weekly retro-orbital infusions of 1 μg recombinant BDD mfVIII or hfVIII. Two weeks later, mice were challenged with a 2 μg dose of recombinant mfVIII or hfVIII. One week prior to challenge and one-week post challenge, plasma was collected for examination of anti-fVIII IgG titers by an enzyme linked immunosorbent assay. Results:Consistent with previous reports, no statistically significant difference in anti-hfVIII IgG titers were observed between S129/B6-E16, B6-E16 and B6-TKO mice. When exposed to mfVIII the majority of B6-TKO mice developed inhibitors (87.5%), while the single exon B6-E16 or S129/B6-E16 mice had 50% and 56% rates of inhibitor development, respectively. The B6-E16 mice had a lower median anti-mfVIII IgG ELISA titer of 12.3 (range 1 to 190.3) compared to S129/B6-E16 and B6-TKO that had median ELISA titers of 473 and 289, respectively. When assessing the anti-mfVIII IgG titers only in the mice that had detectable titer levels, the IgG titers for S129/B6-E16 mice ranged from 34.2 to 1519.4 and for B6-TKO mice ranged from 31.3 to 1377. Conclusion: Given the difference in rates of immune response to mfVIII in mice carrying a single exon or total F8 gene deletion, mfVIII may be useful to elucidate the correlation between F8 gene mutations and the propensity of an individual to respond to factor replacement therapy, and in particular modulators of the immune response in lower risk F8 gene mutations. Disclosures Batsuli: Genentech: Other: Advisory board participant; Octapharma: Other: Advisory board participant; Bayer: Other: Advisory board participant. Meeks:HEMA Biologics: Membership on an entity's Board of Directors or advisory committees; Takeda-Shire: Membership on an entity's Board of Directors or advisory committees; Bioverativ: Membership on an entity's Board of Directors or advisory committees; Novo Nordisk: Membership on an entity's Board of Directors or advisory committees; Bayer: Membership on an entity's Board of Directors or advisory committees; Genentech: Membership on an entity's Board of Directors or advisory committees.

Introduction: The immune response to factor VIII (fVIII) is a CD4+ T cell dependent process initiated by fVIII recognition and processing by antigen presenting cells. The C1 and C2 domains have been proposed as the primary domains that mediate fVIII internalization by dendritic cells. Our prior studies demonstrate that fVIII pre-bound to anti-C1 and C2 domain monoclonal antibodies (MAbs) reduces fVIII internalization by murine derived dendritic cells. However, anti-A1 and A3 domain antibodies increase fVIII endocytosis by dendritic cells. In this study, we analyzed the antibody titers of hemophilia A mice immunized with binary fVIII and MAb complexes to determine whether changes in fVIII internalization observed in vitro correspond to changes in the immune response to fVIII in vivo. Methods: Exon16 (E16) knockout mice deficient in fVIII were immunized with B-domain deleted fVIII in the presence of either anti-fVIII MAb 2-116 (anti-A1, IgG2a), 4A4 (anti-A2, IgG2a), 2-113 (anti-A3, IgG1), B136 (anti-C1, IgG2a), or 3D12 (anti-C2, IgG2b) versus fVIII alone. Mice were immunized by weekly retro-orbital injections of 0.1 µg fVIII incubated with 1 µg anti-fVIII MAb for 4 weeks followed by a boost dose of 0.2 µg fVIII and 2 µg anti-fVIII MAb one week later at week 5. Anti-fVIII ELISA titers (in arbitrary units, AU) were analyzed from plasma samples collected at week 7 to account for IgG half-life. A separate cohort of mice immunized with anti-fVIII MAbs alone served as controls to normalize ELISA titers in each fVIII/MAb group to account for residual injected MAbs in the plasma samples. Results: Hemophilia A mice immunized with fVIII/2-116 and fVIII/4A4 complexes significantly increased anti-fVIII ELISA titers compared to mice immunized with fVIII alone (figure 1). Mice immunized with fVIII/2-116 and fVIII/4A4 had median ELISA titers of 7,401 AU (interquartile range, IQR, 2,654 - 11,910 AU) and 3,620 AU (IQR 1,062 - 7,969 AU), respectively, compared to mice immunized with fVIII (median titer 1,063 AU, IQR 402 - 2,476 AU). MAb 2-116 is a non-inhibitory antibody with a titer of Conclusions: Immunization of hemophilia A mice with fVIII/MAb complexes, specifically anti-A1 MAb 2-116 and anti-A2 MAb 4A4, enhance the immune response to fVIII. MAb 2-116 significantly increased anti-fVIII antibody titers in vivo, which correlates with increased fVIII internalization by immature dendritic cells observed in vitro. A better understanding of the effect of anti-fVIII antibodies on fVIII conformational changes could provide insight into whether these changes alter fVIII recognition by immune cells and subsequently propagate the immune response to fVIII at the onset of inhibitor formation or during immune tolerance induction. Disclosures Batsuli: Genentech: Other: Advisory board participant; Bayer: Other: Advisory board participant; Octapharma: Other: Advisory board participant. Meeks:HEMA Biologics: Membership on an entity's Board of Directors or advisory committees; Novo Nordisk: Membership on an entity's Board of Directors or advisory committees; Genentech: Membership on an entity's Board of Directors or advisory committees; Bioverativ: Membership on an entity's Board of Directors or advisory committees; Takeda-Shire: Membership on an entity's Board of Directors or advisory committees; Bayer: Membership on an entity's Board of Directors or advisory committees.

Human bone marrow–derived mesenchymal stromal cells (MSCs) inhibit proliferation of activated T cells, and IFN-γ plays an important role in this process. This IFN-γ–licensed veto property is IDO-dependent. To further decipher the mechanistic underpinnings of MSC veto function on T cells, we investigated the effect of MSCs and IFN-γ–licensed MSCs on T cell effector function as assayed by cytokine secretion of T cells. Although MSCs and IFN-γ–licensed MSCs inhibit T cell proliferation, only IFN-γ–licensed MSCs significantly inhibit Th1 cytokine (IFN-γ, TNF-α, and IL-2) production by T cells. Additionally, IFN-γ–licensed MSCs inhibit T cell degranulation as well as single, double, and triple cytokine–producing T cells. Although IFN-γ–licensed MSCs upregulate their IDO activity, we found that MSC IDO catalytic function is dispensable with regard to MSC-driven inhibition of T cell effector function. Novel flow cytometry based functional screening of MSC-expressed, IFN-γ–licensed inhibitory molecules identified B7H1 and B7DC/PD1 pathways as essential effectors in blocking T cell function. Small interfering RNA–mediated blocking of B7H1 and B7DC reverses the inhibitory potential of IFN-γ–licensed MSCs on T cell effector function. Mechanistic analysis revealed that clustering of MHC and coinhibitory molecules are indispensable for the inhibitory effect of IFN-γ MSCs. Although exogenous IL-2 reverses B7H1-Ig–mediated inhibition of T cell proliferation, it does not affect the veto function of IFN-γ MSCs on both T cell proliferation and effector function. Our results reveal a new immunosuppressive property of IFN-γ–licensed MSCs that inhibits T cell effector function independent of IDO but through the ligands for PD1.

Traditionally, alloimmunization to transfused blood products has focused exclusively on recipient antibodies recognizing donor alloantigens present on the cell surface. Accordingly, the immunologic sequelae of alloimmunization have been antibody mediated effects (ie, hemolytic transfusion reactions, platelet refractoriness, anti-HLA and anti-HNA effects, etc). However, in addition to the above sequelae, there is also a correlation between the number of antecedent transfusions in humans and the rate of bone marrow transplant (BMT) rejection-under reduced intensity conditioning with HLA-matched or HLA-identical marrow. Bone marrow transplant of this nature is the only existing cure for a series of nonmalignant hematologic diseases (eg, sickle cell disease, thalassemias, etc); however, rejection remains a clinical problem. It has been hypothesized that transfusion induces subsequent BMT rejection through immunization. Studies in animal models have observed the same effect and have demonstrated that transfusion-induced BMT rejection can occur in response to alloimmunization. However, unlike traditional antibody responses, sensitization in this case results in cellular immune effects, involving populations such as T cell or natural killer cells. In this case, rejection occurs in the absence of alloantibodies and would not be detected by existing immune-hematologic methods. We review human and animal studies in light of the hypothesis that, for distinct clinical populations, enhanced rejection of BMT may be an unappreciated adverse consequence of transfusion, which current blood bank methodologies are unable to detect.

For many nonmalignant hematological disorders, HLA-matched bone marrow transplantation (BMT) is curative. However, due to lack of neoplasia, the toxicity of stringent conditioning regimens is difficult to justify, and reduced intensity conditioning is used. Unfortunately, current reduced intensity regimens have high rates of BMT rejection. We have recently reported in a murine model that mHAs on transfused platelet products induce subsequent BMT rejection. Most nonmalignant hematological disorders require transfusion support prior to BMT and the rate of BMT rejection in humans correlates with the number of transfusions given. Herein, we perform a mechanistic analysis of platelet transfusion-induced BMT rejection and report that unlike exposure to alloantigens during transplantation, platelet transfusion primes alloimmunity but does not stimulate full effector function. Subsequent BMT is itself an additional and distinct immunizing event, which does not induce rejection without antecedent priming from transfusion. Both CD4(+) and CD8(+) T cells are required for priming during platelet transfusion, but only CD8(+) T cells are required for BMT rejection. In neither case are antibodies required for rejection to occur.

Background Platelet (PLT) transfusions can induce humoral and cellular alloimmunity. HLA antibodies can render patients refractory to subsequent transfusion, and both alloantibodies and cellular alloimmunity can contribute to subsequent bone marrow transplant (BMT) rejection. Currently, there are no approved therapeutic interventions to prevent alloimmunization to PLT transfusions other than leukoreduction. Targeted blockade of T-cell costimulation has shown great promise in inhibiting alloimmunity in the setting of transplantation, but has not been explored in the context of PLT transfusion. Study design and methods We tested the hypothesis that the costimulatory blockade reagent CTLA4-Ig would prevent alloreactivity against major and minor alloantigens on transfused PLTs. BALB/c (H-2(d)) mice and C57BL/6 (H-2(b)) mice were used as PLT donors and transfusion recipients, respectively. Alloantibodies were measured by indirect immunofluorescence using BALB/c PLTs and splenocytes as targets. BMTs were carried out under reduced-intensity conditioning using BALB.B (H-2(b) ) donors and C57BL/6 (H-2(b)) recipients to model HLA-identical transplants. Experimental groups were given CTLA4-Ig (before or after PLT transfusion) with control groups receiving isotype-matched antibody. Results CTLA4-Ig abrogated both humoral alloimmunization (H-2(d) antibodies) and transfusion-induced BMT rejection. Whereas a single dose of CTLA4-Ig at time of transfusion prevented alloimmunization to subsequent PLT transfusions, administration of CTLA4-Ig after initial PLT transfusion was ineffective. Delaying treatment until after PLT transfusion failed to prevent BMT rejection. Conclusions These findings demonstrate a novel strategy using an FDA-approved drug that has the potential to prevent the clinical sequelae of alloimmunization to PLT transfusions.

Generation of alloantibodies to transfused RBCs can be a serious medical problem for patients who require chronic RBC transfusion therapy. Patients with sickle cell disease have a substantially increased rate of alloimmunization compared to other chronically transfused populations. A recent study has forwarded the hypothesis that a polymorphism in an immunoregulatory gene in close proximity to beta-globin (TRIM21 rs660) plays a role in the increased rates of RBC alloimmunization in sickle cell patients. In particular, it was hypothesized that rs660C/T decreases expression of TRIM21, resulting in loss of a negative feedback pathway in immune responses and increased RBC alloimmunization. To test the effects of TRIM21 expression on alloimmunization, we analyzed antibody responses to alloantigens on RBCs and platelets transfused into wild-type and TRIM21 KO mice. No significant increases were seen in the frequency or magnitude of humoral immunization to alloantigens on transfused RBCs or platelets in adult or juvenile TRIM 21 KO recipients compared to wild-type controls. Moreover, recipient inflammation with poly (I:C) enhanced RBC alloimmunization to similar degrees in both TRIM 21 KO and wild-type control recipients. Together, these data rule out the hypothesis that decreased TRIM 21 expression enhances transfusion induced humoral alloimmunization, in the context of a reductionist murine model.

Objective Fungal keratitis is a serious ocular infection that is considered to be rare among contact lens wearers. The recent Fusarium keratitis outbreak raised questions regarding the background rate of Fusarium -related keratitis and other fungal keratitis in this population. Design Retrospective, multicenter case series. Participants Six hundred ninety-five cases of fungal keratitis cases who presented to 1 of 10 tertiary medical centers from 2001 to 2007. Methods Ten tertiary care centers in the United States performed a retrospective review of culture-positive fungal keratitis cases at their centers between January 2001 and December 2007. Cases were identified using microbiology, pathology, and/or confocal microscopy records. Information was collected on contact lens status, method of diagnosis, and organism(s) identified. The quarterly number of cases by contact lens status was calculated and Poisson regression was used to evaluate presence of trends. The Johns Hopkins Medicine Institutional Review Board (IRB) and the IRBs at each participating center approved the research. Main Outcome Measures Quarterly number of fungal keratitis cases and fungal species. Results We identified 695 fungal keratitis cases; 283 involved the use of contact lenses. The quarterly number of Fusarium cases increased among contact lens wearers (CLWs) during the period that ReNu with MoistureLoc (Bausch & Lomb, Rochester, NY) was on the market, but returned to prior levels after withdrawal of the product from the market. The quarterly frequency of other filamentous fungi cases showed a statistically significant increase among CLWs comparing October 2004 through June 2006 with July 2006 through December 2007 with January 2001 through September 2004 ( P Conclusions The quarterly number of Fusarium fungal keratitis cases among CLWs returned to pre-Renu with Moistureloc levels after removal of the product from the market. However, the number of other filamentous fungal keratitis cases, although small, seems to have increased among refractive CLWs. Reasons for these apparent increases are unclear. Financial Disclosure(s) Proprietary or commercial disclosure may be found after the references.

The intestinal immune system must elicit robust immunity against harmful pathogens but must also restrain immune responses directed against commensal microbes and dietary antigens. The mechanisms that maintain this dichotomy are poorly understood. Here we describe a population of CD11b+F4/80+CD11c- macrophages in the lamina propria that expressed several anti-inflammatory molecules, including interleukin 10 (IL-10), but little or no proinflammatory cytokines, even after stimulation with Toll-like receptor ligands. These macrophages induced, by a mechanism dependent on IL-10, retinoic acid and exogenous transforming growth factor-beta, the differentiation of Foxp3+ regulatory T cells. In contrast, lamina propria CD11b+ dendritic cells elicited IL-17 production. This IL-17 production was suppressed by lamina propria macrophages, indicating that a dynamic interaction between these subsets may influence the balance between immune activation and tolerance.

Adaptive immunity provides the unique ability to respond to a nearly infinite range of antigenic determinants. Given the inherent plasticity of the adaptive immune system, a series of tolerance mechanisms exist to reduce reactivity toward self. While this reduces the probability of autoimmunity, it also creates an important gap in adaptive immunity: the ability to recognize microbes that look like self. As a variety of microbes decorate themselves in self-like carbohydrate antigens and tolerance reduces the ability of adaptive immunity to react with self-like structures, protection against molecular mimicry likely resides within the innate arm of immunity. In this review, we will explore the potential consequences of microbial molecular mimicry, including factors within innate immunity that appear to specifically target microbes expressing self-like antigens, and therefore provide protection against molecular mimicry.

Platelet transfusion provides an important therapeutic intervention in the treatment and prevention of bleeding. However, some patients rapidly clear transfused platelets, preventing the desired therapeutic outcome. Although platelet clearance can occur through a variety of mechanisms, immune-mediated platelet removal often plays a significant role. Numerous studies demonstrate that anti-platelet alloantibodies can induce significant platelet clearance following transfusion. In fact, for nearly 50 years, anti-platelet alloantibodies were considered to be the sole mediator of immune-mediated platelet clearance in platelet-refractory individuals. Although nonimmune mechanisms of platelet clearance can often explain platelet removal in the absence of anti-platelet alloantibodies, many patients experience platelet clearance following transfusion in the absence of a clear mechanism. These results suggest that other processes of antibody-independent platelet clearance may occur. Our studies demonstrate that CD8(+)T cells possess the unique ability to induce platelet clearance in the complete absence of anti-platelet alloantibodies. These results suggest a previously unrecognized form of immune-mediated platelet clearance with significant implications in the appropriate management of platelet-refractory individuals.

Alloantibodies against red blood cell (RBC) antigens, which may be generated following exposure to foreign antigens on transfused RBCs or on fetal RBCs during pregnancy, can be clinically significant from the standpoint of morbidity and mortality.[1][1] In the transfusion setting, RBC alloantibodies

The size of the thymus can be greatly influenced by changes in the small number of early progenitors in the thymus. However, it is not known whether thymic cellularity feeds back to regulate the recruitment, survival, and expansion of progenitors. The transcription factor early growth response gene 1 (Egr1) has been implicated in controlling proliferation and survival in many cell types. We have previously shown that mice deficient in Egr1 have increased thymic cellularity. We now show that Egr1 regulates a negative feedback signal that controls the entry of cells into the thymus. Egr1-deficient mice have higher percentages of early T lineage progenitors in the thymus, yet Egr1-deficient mice have normal numbers of myelolymphoid progenitors in the bone marrow, and Egr1-deficient thymocytes show normal rates of apoptosis and proliferation at all stages of development. Evidence from mixed bone marrow chimeras shows that the ability of Egr1 to control progenitor recruitment is mediated by bone marrow-derived cells, but is not cell autonomous. Furthermore, Egr1-deficient thymuses have increased P-selectin expression. The data suggest that Egr1 mediates a feedback mechanism whereby the number of resident double negative thymocytes controls the entry of new progenitors into the thymus by regulating P-selectin expression on thymic endothelial cells.

Background: While red blood cell (RBC) alloantibodies can increase the probability of transfusion-related complications, not all patients become alloimmunized following transfusion. Several factors have been hypothesized to govern susceptibility to alloimmunization, including general differences in immune function and the potential impact of recipient inflammation at the time of transfusion. However, as individuals that do generate alloantibodies appear to experience an increased rate of additional alloantibody formation following subsequent transfusion, distinct immunological responses induced following exposure to certain antigens expressed by transfused RBCs may directly facilitate the development of alloantibodies following subsequent transfusion. Thus, while CD4 T cell help classically occurs through direct recognition of a peptide that resides within a target B cell antigen, these clinical observations suggest that CD4 T cells that respond to one RBC antigen may directly facilitate immunity to a completely distinct RBC alloantigen following subsequent transfusion. As RBCs express a variety of possible polymorphic antigens, both inside RBCs and at the RBC surface, exposure to alloantigens within RBCs may possess the capability to induce a CD4 T cell response that would be undetectable by clinical serological analysis, but which may facilitate subsequent alloimmunization. To test this, we determined whether cellular adaptive immunity to an intracellular alloantigen might enhance alloimmunization following subsequent RBC transfusion. Methods: B6 recipients were immunized three times a week apart against an intracellular antigen, green fluorescent protein (GFP). Two weeks following the last immunization, recipients were transfused with RBCs expressing the model antigen HOD (a fusion protein consisting of hen egg lysozyme fused to ovalbumin and human Duffy b) and GFP (HOD x GFP RBCs) or RBCs expressing the human Glycophorin A (hGPA) and GFP antigens (hGPA x GFP RBCs). Serum was collected at days 7, 14, 21 and 28 post-transfusion and the levels of anti-HOD or anti-GPA antibodies were determined by incubation of serum with HOD RBCs, GPA RBCs or B6 RBCs, followed by detection with fluorescently labeled anti-IgG antibodies. The adjusted mean fluorescent intensity (MFI) for anti-HOD or anti-GPA antibodies was calculated by subtracting the MFI observed following incubation with B6 RBCs from the MFI observed following incubation with HOD or GPA RBCs, respectively. Results: Recipients that underwent GFP immunization experienced a statistically significant enhancement of anti-HOD IgG compared to non-GFP immunized recipients following subsequent transfusion of HOD x GFP RBCs (p < 0.05), strongly suggesting that prior exposure to a single intracellular antigenic determinant can enhance antibody formation following subsequent exposure to RBCs expressing both the intracellular antigen and a clinically relevant surface antigen. To determine whether immunological priming toward an intracellular antigen can impact RBC alloimmunization toward other cell surface RBC alloantigens, we likewise transfused recipients that had been previously immunized against GFP with RBCs that express hGPA and GFP. Similar to the impact of prior GFP immunization on HOD alloimmunization, prior GFP immunization rendered recipients responsive to the hGPA antigen following transfusion of hGPA x GFP RBCs. Importantly, non-GFP immunized recipients were non-responsive to the hGPA antigen regardless of being expressed on the same RBCs as GFP (p < 0.05), suggesting that prior immunization toward an intracellular antigen may not only enhance subsequent alloimmunization in previously alloimmunized individuals, but also render non-responding recipients responsive to RBC-induced alloimmunization. Conclusion: These results demonstrate that immunity to an intracellular alloantigen can directly influence the immunological outcome following exposure to a subsequent extracellular RBC alloantigen. Moreover, these findings suggest a mechanism whereby alloantibody responders may exhibit an increased rate of additional alloantibody formation and highlight a previously under appreciated mechanism by which cellular adaptive immunity can impact the ability of an individual to respond to unrelated immunogens. Disclosures Chonat: Agios Pharmaceuticals: Honoraria.

Background: Exposure to red blood cell (RBC) alloantigens during pregnancy or transfusion can lead to the development of alloantibodies and result in transfusion-related complications, like hemolytic transfusion reactions, or hemolytic disease of the fetus and newborn. Currently, antibody-mediated immune suppression (AMIS) is the only known methodology to actively inhibit RBC alloantibody formation. The best-known example of AMIS is Rh immune globulin, which is given to Rh(D) negative women to prevent the development of anti-D alloantibodies. Mouse models to elucidate the mechanism by which AMIS occurs have used mice genetically engineered to specifically express the human KEL antigen (KEL RBCs) or the Hel-Ova-Duffy antigen (HOD RBCs) on murine RBCs, and found that AMIS to these two antigens is dependent on antigen loss (also called antigen modulation). As RBCs express many antigens simultaneously, we sought to determine whether AMIS to one antigen can impact the immune response to a completely different antigen on the same RBC by generating mice that express both the KEL and HOD antigens (HOD x KEL RBCs). Methods: WT recipients received polyclonal anti-KEL antibody, a cocktail of two anti-HEL monoclonal antibodies, or PBS, followed by transfusion with WT and HOD x KEL donor RBCs. HOD x KEL RBCs were labeled with the lipophilic dye DiI prior to transfusion, while WT RBCs were labeled with the lipophilic dye DiO and served as a RBC tracer population. RBC survival and levels of detectable IgG, Kel antigen, Hel antigen, Duffy antigen, and complement (C3) on transfused cells were measured by flow cytometry at 10min, 1hr, 2hr, and 4hr, and on days 1, 2, 3, and 5 post-transfusion. Serum was collected on day 5 and analyzed for IgM antibody development by flow-crossmatch using KEL or HOD RBCs. Results: Anti-KEL and anti-HEL treated WT recipients demonstrated significant IgG deposition on transfused HOD x KEL RBCs at 10min post-transfusion. Levels of IgG on transfused RBCs decreased over time but did not correlate with complete RBC clearance. RBC survival was nearly 100% over 5 days in recipients passively immunized with anti-HEL antibodies, while a fraction of RBCs (~20%) were cleared within 4hr post-transfusion but not thereafter (survival plateaued at 80% thru day 5) in anti-KEL immunized recipients. Conversely, decreased bound IgG on transfused RBCs was found to correlate with a decrease in target antigen levels. KEL antigen levels decreased on transfused RBCs in the anti-KEL immunized recipients beginning at 1hr post transfusion, as compared to anti-HEL and PBS recipients. Similarly, HEL antigen expression decreased on transfused RBCs in the anti-HEL immunized recipients, but not in the anti-KEL or PBS recipients, demonstrating that passive immunization causes antigen modulation for the corresponding antigen only. C3 deposition was found to occur on transfused RBCs at 10min post-transfusion in the anti-KEL recipients that remained significantly increased at other time-points, but was not detected on RBCs in the anti-HEL or PBS recipients until day 3, when recipients develop IgM antibodies. Flow crossmatch of serum samples showed that anti-KEL treated recipients did not make anti-KEL IgM but did make anti-HOD IgM, while anti-HEL treated recipients did not make anti-HOD IgM but did make anti-KEL IgM. Conclusions: Although AMIS serves as the basis of a widely successful clinical intervention, the mechanism by which it occurs, including any impact on additional antigens, remains incompletely understood. Here we demonstrate that AMIS is antigen specific - that is, passively acquired antibody can inhibit the immune response only to the corresponding antigen. Specifically, murine RBCs expressing both the human and model RBC antigen KEL and HOD, respectively, demonstrate similar clearance kinetics, antigen modulation, complement deposition, and antibody development after transfusion into WT mice as is observed when KEL-only or HOD-only RBCs are used. Although it is unclear whether these findings apply to all RBC antigens and what relevance this holds for human patients, our study provides insight into the specificity of AMIS that may enlighten development of additional therapeutics based on this mode of immune suppression. Disclosures No relevant conflicts of interest to declare.

Few clinical scenarios are as diagnostically demanding as the workup of acute meningitis in the emergency room setting. In humans, most cases of meningitis are due to enteroviral infection, which usually resolves with supportive care and does not generally require hospitalization. The difficulty in evaluating meningitis is often related to its nonspecific presentation; rapid identification of the underlying etiologic agent is critical to selection of the most appropriate course of therapy.To develop a rapid, automated polymerase chain reaction-based test for enteroviral meningitis that can be run on demand, and to make the test easy enough to operate that most hospital laboratories can offer it on an immediate basis in order to provide maximum medical impact.A sensitive and specific test on the GeneXpert was developed for detection of enteroviral RNA in cerebrospinal fluid. The automated test was carried out in a disposable GeneXpert cartridge, which conducts the steps of nucleic acid purification, amplification and detection of enteroviral genomic RNA in ∼ 2 h. Preclinical and clinical studies showed sensitivities of 97.1 and 98%, respectively, and specificities of 100 and 97.1%, respectively. In February 2006, the test received FDA clearance as an aid in the diagnosis of viral meningitis.Rapid and accurate molecular diagnostic testing for enterovirus infection provides clinicians with information they need to make the best patient decisions in managing patients with meningitis. The GeneXpert enterovirus assay features on-demand availability and a level of simplicity that allows nearly any hospital laboratory to offer a sophisticated test that was once the exclusive domain of reference laboratories.

Background:Red blood cell (RBC) transfusion can result in the development of alloantibodies that can make it difficult to find compatible RBCs for future transfusions and increase the risk of hemolytic transfusion reactions. Despite the consequences of RBC alloimmunization, the factors that regulate this process remain relatively unknown. Recent studies suggest that complement deposition on an antigen surface can significantly enhance the immune response to foreign antigen. As many anti-RBC alloantibodies fix complement and RBCs otherwise lack known adjuvants, early antibody-mediated complement deposition may serve as a key regulator that enhances antibody production. To test this, we employed the KEL RBC model system, which employs RBCs that transgenically express the human KEL antigen specifically on RBCs (KEL RBCs). Using this system, we examined the immunological consequence of KEL RBC exposure following transfusion into C57BL/6 wild-type (WT) or complement component 3 (C3) knockout (KO) recipients. Methods: KEL RBCs were transfused into WT or C3 KO recipients, followed by serum collection on days 3, 5, 7, 14, and 21 post-transfusion. Antibody development in WT or C3 KO recipients was examined by flow crossmatch, where serum was incubated with KEL RBCs followed by antibody detection with fluorescently-tagged secondary anti-IgM and anti-IgG antibodies using flow cytometry. To determine the impact of complement deposition on the level of detectable antigen on the RBC surface, RBCs were labeled with the lipophilic dye, DiI, prior to transfusion and then sampled 1, 2, 3, 5, 7 and 9 days post-transfusion. The level of detectable KEL antigen, complement deposition, KEL RBC survival and antibody bound to the RBC surface was measured by flow cytometry. To examine the effect of complement deposition on the level of KEL protein in the RBC membrane post-transfusion, RBCs stroma was isolated at various time points post transfusion, followed by western blot analysis for the KEL protein. Results: While KEL RBCs induced robust anti-KEL antibody formation and C3 deposition in WT recipients, similar exposure to KEL RBCs in C3 KO recipients actually resulted in an unexpected increase in IgM and IgG anti-KEL antibodies when compared to WT recipients. To determine the consequence of C3 deposition, we examined the potential impact of antibody engagement and complement fixation on KEL antigen levels. Consistent with a potential role for complement in directly impacting KEL antigen availability to the immune system, KEL RBCs transferred into WT recipients experienced a decrease in the level of detectable KEL antigen over time that paralleled the development of anti-KEL antibodies and C3 deposition. In contrast, C3 KO recipients failed to experience the same degree of KEL antigen reduction despite the development of significant anti-KEL antibodies over this same time period. Western blot analysis of RBCs post-transfusion revealed that loss of detectable KEL antigen on the RBC surface paralleled a complete lack of detectable KEL antigen in RBC membranes, indicating that C3 may actually facilitate the removal of KEL from the RBC surface. Conclusion: These results suggest an unexpected role for C3 in negatively regulating antibody responses following RBC transfusion. The impact of C3 on the developing alloantibody response strongly suggests that C3-mediated loss of antigen over time likely reduces antigen availability to the immune system, thereby facilitating the inhibition of antibody production over time. These results not only provide novel insight into potential impact of antigen modulation on the development of an immune response to a RBC alloantigen, but also suggest a completely unexpected role for complement in negatively regulating alloantibody production. In doing so, these results suggest that unique differences in complement activity and overall activation following RBC alloantigen exposure between individuals may represent a previously unrecognized factor that influences alloantibody formation following RBC transfusion. Disclosures No relevant conflicts of interest to declare.

Background: While red blood cell (RBC) transfusion can be beneficial, exposure to allogeneic RBCs can result in the development of RBC alloantibodies that can make it difficult to obtain compatible RBCs for future transfusions. Aside from phenotype matching protocols, no strategy currently exists that is capable of preventing RBC alloimmunization following therapeutic transfusion. As RBC alloantigens represent diverse determinants capable of driving distinct immune pathways, common immunological nodes must be identified in order to successfully prevent RBC alloimmunization against a variety of different alloantigens. Recent results demonstrate that marginal zone (MZ) B cells mediate anti-KEL antibody formation in the complete absence of CD4 T cells. However, whether MZ B cells similarly regulate RBC alloantibody formation against other RBC alloantigens remains unknown. As a result, we examined the role of MZ B cells and CD4 T cells in the development of RBC alloantibodies following exposure to the HOD (hen egg lysozyme, ovalbumin and duffy) antigen. Methods: Each recipient was transfused with HOD or KEL RBCs following either MZ B cell or CD4 T cell depletion using a cocktail of MZ B cell (anti-CD11a and anti-CD49d) or anti-CD4 depleting antibody, 4 and 2 days prior to transfusion. Control groups received isotype control injections in parallel. MZ B cell deficient (CD19cre/+ X Notch2flx/flx) and CD4 T cell deficient (MHC class II knockout) recipients were also used to examine the role of MZ B cells and CD4 T cells, respectively. Serum collected on days 5 and 14 post-transfusion was evaluated for anti-HOD or anti-KEL antibodies by incubating HOD or KEL RBCs with serum, followed by detection of bound antibodies using anti-IgM and anti-IgG and subsequent flow cytometric analysis. Evaluation of antibody engagement and overall survival of HOD or KEL RBCs was accomplished by labeling RBCs with the lipophilic dye, DiI, prior to transfusion, followed by examination for bound antibody and RBC clearance on days 5 and 14 post-transfusion by flow cytometry. Results: Similar to the ability of MZ B cell depletion to reduce anti-KEL antibody formation following KEL RBC exposure, depletion of MZ B cells significantly reduced anti-HOD IgM and IgG antibodies following HOD RBC transfusion. In contrast, injection of recipients with isotype control antibodies in parallel failed to prevent alloantibody formation following HOD or KEL RBC transfusion. Similar results were obtained following HOD or KEL RBC transfusion into recipients genetically deficient in MZ B cells. In contrast, although MZ B cells were required for HOD and KEL RBC-alloantibody formation, manipulation of CD4 T cells differentially impacted the ability of each antigen to induce alloantibodies. While transfusion of HOD or KEL RBCs resulted in robust IgM alloantibodies in the absence of CD4 T cells, depletion or genetic elimination of CD4 T cells significantly inhibited anti-HOD IgG antibody formation, while failing to impact IgG anti-KEL antibody formation. Consistent with this, while manipulation of CD4 T cells protected HOD RBCs from antibody deposition and subsequent RBC clearance, this same approach failed to similarly protect KEL RBCs following transfusion. In contrast, depletion of MZ B cells not only prevented detectable alloantibody production, but also completely protected HOD or KEL RBCs from antibody deposition and subsequent RBC clearance. Conclusion: These results suggest that while MZ B cells mediate a robust IgM antibody response following either KEL or HOD antigen exposure, MZ B cells appear to possess the capacity to orchestrate unique downstream IgG responses through CD4 T cell dependent and independent pathways contingent on target alloantigen. As a result, while manipulation of CD4 T cells may prevent alloantibody formation against some antigens, targeting this immune population inadequately prevents RBC alloantibody formation against all RBC antigens. As chronic transfusion therapy exposes recipients to a wide variety of alloantigens, these results suggest that MZ B cells may represent a central initiating node that governs RBC alloimmunization against a variety of RBC alloantigens, and may therefore serve as a useful target in preventing alloantibody formation in chronically transfused individuals. Disclosures No relevant conflicts of interest to declare.

Introduction: Anti-ABO antibodies represent the earliest recognized immunological barrier to transfusion and transplantation. However, despite Landsteiner's discovery of ABO blood group antigens over a century ago, the factors that regulate anti-ABO antibody formation remain relatively unknown. Anti-ABO antibodies develop spontaneously within the first few months of life, in the absence of a known antigenic exposure. However, antibody levels vary considerably between individuals suggesting that differences in exposure to environmental triggers may regulate anti-ABO antibody formation. As distinct microbes can stimulate very specific anti-carbohydrate antibodies, we hypothesized that variation in exposure to microbes that decorate themselves in ABO carbohydrates may regulate anti-blood group antibody formation. However, no model currently exists to examine the potential impact of microbial exposure on the development of naturally occurring anti-blood group antibodies. Methods: To examine the regulatory capacity of specific microbial exposure on naturally occurring antibody formation, we generated a model of ABO blood group antigens. As lower mammals do not express ABO antigens, we used recipients knocked out (KO) for the glycosyltransferase responsible for the synthesis of the carbohydrate B disaccharide (Bdis) antigen, an antigen very similar to the human blood group B antigen. Microbial flora was assessed in WT (Blood group B-like) or Bdis KO (Blood group O-like) by sequencing ribosomal DNA isolated from stool samples. Serum was assessed for Bdis reactivity at baseline and following Bdis+ microbial exposure using a glycan microarray. WT or Bdis KO recipients were transfused with Bdis+ RBCs followed by the evaluation of RBC clearance, antibody engagement and complement fixation by flow cytometry. Results: Similar to blood group O individuals, Bdis KO recipients spontaneously develop varied amounts of anti-Bdis antibodies within the first few weeks of life capable of inducing an acute hemolytic transfusion reaction following incompatible Bdis+ RBC transfusion. To determine whether specific microbial exposure is the primary regulating factor in anti-Bdis antibody formation, we separately housed Bdis KO recipients with low titer anti-Bdis antibodies, yielding an entire Bdis KO colony that never developed detectable anti-Bdis antibodies. Exposure of Bdis KO recipients that lacked detectable anti-Bdis antibodies to specific microbes that express the Bdis antigen induced robust anti-Bdis antibodies. However, the timing of microbial exposure was critical in dictating the likelihood of anti-Bdis antibody formation as younger mice produced anti-Bdis antibodies much more readily than adult mice following Bdis+ microbial exposure. Consistent with this, only Bdis KO recipients that experienced early Bdis+ microbial exposure possessed anti-Bdis antibodies with the capacity to induce an acute hemolytic transfusion reaction following Bdis+ RBC transfusion. Conclusions: These results demonstrate that Bdis KO recipients provide an attractive model to study naturally occurring antibody formation and suggest that anti-ABO antibodies are not an inevitable outcome of not expressing ABO blood group antigens. Instead, naturally occurring antibody formation appears to be temporally regulated by specific microbial exposure. As younger Bdis KO recipients developed antibody more readily than older Bdis KO recipients, in addition to Bdis+ microbial exposure, the actual timing of exposure appears to be a key regulator of anti-Bdis antibody formation. Thus variations in an individual's microbiota, particularly during early immunological development, likely dictate the level of anti-ABO antibody formation. As a result, intentional manipulation of an individual's microbial flora may provide a unique and previously unrecognized approach to prevent anti-ABO(H) antibody development in patients requiring transplantation or chronic transfusion. Disclosures No relevant conflicts of interest to declare.

The thymus and parathyroids are pharyngeal endoderm-derived organs that develop from common organ primordia, which undergo a series of morphological events resulting in separate organs in distinct locations in the embryo. Previous gene expression and functional analyses have suggested a role for BMP4 signaling in early thymus organogenesis. We have used conditional deletion of Bmp4 or Alk3 from the pharyngeal endoderm and/or the surrounding mesenchyme using Foxg1-Cre, Wnt1-Cre or Foxn1-Cre. Deleting Bmp4 from both neural crest cells (NCC) and early endoderm-derived epithelial cells in Foxg1-Cre;Bmp4 conditional mutants resulted in defects in thymus–parathyroid morphogenesis. Defects included reduced condensation of mesenchymal cells around the epithelium, partial absence of the thymic capsule, a delay in thymus and parathyroid separation, and failed or dramatically reduced organ migration. Patterning of the primordia and initial organ differentiation were not affected in any of the mutants. Deleting Bmp4 from NCC-derived mesenchyme or differentiating thymic epithelial cells (TECs) had no effects on thymus–parathyroid development, while loss of Alk3 from either neural crest cells or TECs resulted in only a mild thymic hypoplasia. these results show that the processes of cell specification and morphogenesis during thymus–parathyroid development are independently controlled, and suggest a specific temporal and spatial role for BMP4-mediated epithelial–mesenchymal interactions during early thymus and parathyroid morphogenesis.

Stromal cells play a crucial role in the organogenesis of lymphoid tissues. We previously identified VCAM-1+ stromal cells in cryptopatches (CP) and isolated lymphoid follicles (ILF) in the small intestine of C57BL/6 mice. Nonhemopoietic stromal cell networks in CP and ILF of adult mice also expressed FDC-M1, CD157 (BP-3), and TNF-related activation-induced cytokine (TRANCE). Individual stromal cells were heterogeneous in their expression of these markers, with not all stromal cells expressing the entire set of stromal cell markers. Expression of VCAM-1, FDC-M1, and CD157 on CP stromal cells was absent in alymphoplasia mice deficient in NF-κB-inducing kinase (NIK) and NIK knockout mice. Administration of lymphotoxin β receptor (LTβR)-Ig to wild-type mice on day 13 resulted in the absence of CP on day 20; delaying administration of LTβR-Ig until day 18 resulted in an 80% decrease in the number of CP on day 22 and diminished expression of VCAM-1, FDC-M1, and CD157 on the remaining CP. In sharp contrast, TRANCE expression by stromal cells was completely independent of NIK and LTβR. In addition, expression of TRANCE in ILF was concentrated just beneath the follicle-associated epithelium, a pattern of polarization that was also observed in Peyer’s patches. These findings suggest that TRANCE on stromal cells contributes to the differentiation and maintenance of organized lymphoid aggregates in the small intestine.