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In utero hematopoietic cell transplantation (IUHCT) is an experimental treatment for congenital hemoglobinopathies, including Sickle cell disease and thalassemias. One of the principal advantages of IUHCT is the predisposition of the developing fetus toward immunologic tolerance. This allows for engraftment across immune barriers without immunosuppression and, potentially, decreased susceptibility to graft-versus-host disease (GVHD). We demonstrate fetal resistance to GVHD following T cell-replete allogeneic hematopoietic cell transplantation compared with the neonate. We show that this resistance is associated with elevated fetal serum interleukin-10 conducive to regulatory T cells (Tregs) induction. Finally, we demonstrate that the adoptive transfer of Tregs from IUHCT recipients to neonates uniformly prevents GVHD, recapitulating the predisposition to tolerance observed after fetal allotransplantation. These findings demonstrate fetal resistance to GVHD following hematopoietic cell transplantation and elucidate Tregs as important contributors.

Introduction Opiates are often prescribed after pediatric operations despite safety concerns and lack of evidence confirming superiority compared to other pain control modalities. In this study, we use daily parental surveys to prospectively evaluate a strict non-opioid pain control strategy after laparoscopic appendectomy. Methods After IRB approval, children who underwent laparoscopic appendectomy for nonperforated acute appendicitis were recruited to the study. For these patients, our standard practice is to provide instructions to administer alternating acetaminophen and ibuprofen over-the-counter (OTC) postoperatively, and no opiate prescriptions are written. Parents of enrolled children received a daily RedCap survey via text message or e-mail on postoperative days (POD) 1 through 5 to prospectively assess pain control and medication usage. Trends were compared across postoperative days. Results One hundred twenty patients were enrolled in the study, and none received opiate prescriptions. Postoperative pain survey response rates were 54% on POD1, 47% on POD2, 35% on POD3, 34% on POD4, and 29% on POD5. Pain level was 4.7 ± 2.3 (out of 10) on POD1, and down-trended significantly each postoperative day to reach 0.7 ± 1.2 by POD5. On POD1, 85% of parents administered OTC medications, which reduced significantly to 14% by POD5. Parent-reported success rates to manage pain by OTC regimen were 85% on POD1, 94% on POD2, 91% on POD3, and 100% on POD4 and POD5. Conclusion Strict non-opioid pain control after appendectomy exhibits high performance based upon prospective parental surveys. This strategy should be implemented as standard of care and tested for application to other surgical conditions. Level of Evidence Level II.

In utero hematopoietic cell transplantation (IUHCT) has the potential to cure congenital hematologic disorders including sickle cell disease. However, the window of opportunity for IUHCT closes with the acquisition of T-cell immunity, beginning at approximately 14 weeks gestation, posing significant technical challenges and excluding from treatment fetuses evaluated after the first trimester. Here we report that regulatory T cells can promote alloengraftment and preserve allograft tolerance after the acquisition of T-cell immunity in a mouse model of late-gestation IUHCT. We show that allografts enriched with regulatory T cells harvested from either IUHCT-tolerant or naive mice engraft at 20 days post coitum (DPC) with equal frequency to unenriched allografts transplanted at 14 DPC. Long-term, multilineage donor cell chimerism was achieved in the absence of graft-versus-host disease or mortality. Decreased alloreactivity among recipient T cells was observed consistent with donor-specific tolerance. These findings suggest that donor graft enrichment with regulatory T cells could be used to successfully perform IUHCT later in gestation.

In utero hematopoietic cell transplantation (IUHCT) has the potential to treat a number of congenital hematologic disorders. Clinical application is limited by low levels of donor engraftment. Techniques that optimize donor cell delivery to the fetal liver (FL), the hematopoietic organ at the time of IUHCT, have the potential to enhance engraftment and the clinical success of IUHCT. We compared the 3 clinically applicable routes of injection (intravenous [i.v.], intraperitoneal [i.p.], and intrahepatic [i.h.]) and assessed short- and long-term donor cell engraftment and fetal survival in the murine model of IUHCT. We hypothesized that the i.v. route would promote direct donor cell homing to the FL, resulting in increased engraftment and allowing for larger injectate volumes without increased fetal mortality. We demonstrate that the i.v. route results in (1) rapid diffuse donor cell population of the FL compared with delayed diffuse engraftment after the i.p. and i.h. routes; (2) higher FL and spleen engraftment at early prenatal time points; (3) enhanced stable long-term peripheral blood donor cell engraftment; and (4) improved survival at higher injectate volumes, allowing for higher donor cell doses and increased long-term engraftment. These findings support the use of an i.v. route for clinical protocols of IUHCT.

In utero hematopoietic cell transplantation (IUHCT) is a nonmyeloablative nonimmunosuppressive alternative to postnatal hematopoietic stem cell transplantation for the treatment of congenital hemoglobinopathies. Anti-HLA donor-specific Abs (DSA) are associated with a high incidence of graft rejection following postnatal hematopoietic stem cell transplantation. We determine if DSA present in the mother can similarly cause graft rejection in the fetus following IUHCT. Ten million C57BL/6 (B6, H2kb) bone marrow cells were transplanted in utero into gestational day 14 BALB/c (H2kd) fetuses. The pregnant BALB/c dams carrying these fetuses either had been previously sensitized to B6 Ag or were injected on gestational days 13–15 with serum from B6-sensitized BALB/c females. Maternal–fetal Ab transmission, Ab opsonization of donor cells, chimerism, and frequency of macrochimeric engraftment (chimerism >1%) were assessed by flow cytometry. Maternal IgG was transmitted to the fetus and rapidly opsonized donor cells following IUHCT. Donor cell rejection was observed as early as 4 h after IUHCT in B6-sensitized dams and 24 h after IUHCT in dams injected with B6-sensitized serum. Efficient opsonization was strongly correlated with decreased chimerism. No IUHCT recipients born to B6-sensitized dams or dams injected with B6-sensitized serum demonstrated macrochimeric engraftment at birth compared with 100% of IUHCT recipients born to naive dams or dams injected with naive serum (p < 0.001). In summary, maternal donor–specific IgG causes rapid, complete graft rejection in the fetus following IUHCT. When a third-party donor must be used for clinical IUHCT, the maternal serum should be screened for DSA to optimize the chance for successful engraftment.

Minimally invasive surgery for inguinal hernia repair in children has been a controversial topic for pediatric surgeons. Our method for inguinal hernia repair using laparoscopic techniques has comparable outcomes to the standard open technique. We describe our technique and experience with the laparoscopic needle-assisted repair of inguinal hernia (LNAR). We report 502 cases (710 hernias) from 2009 to 2013 by 3 surgeons. We reviewed our prospectively collected outcomes database of all patients receiving LNAR from 1/2009 to 3/2013. 502 cases in 495 patients

Introduction: We investigated the correlation of amniotic fluid (AF) concentrations of glial fibrillary acidic protein (GFAP) with prenatal features of myelomeningocele (MMC) and neurodevelopmental outcome after fetal MMC (fMMC) surgery. Materials and Methods: AF was collected during fMMC surgery between December 2012 and November 2015. AF-GFAP concentration was determined by ELISA. Retrospective chart review identified the characteristics of the defect. Data regarding delivery and 1-year neurodevelopmental outcome was collected from The Children's Hospital of Philadelphia fMMC Registry. Results: Eighty-two AF samples were collected from fMMC surgeries. Perinatal data were obtained from 77 subjects, and 1-year follow-up data from 65 subjects. GFAP concentrations were significantly elevated in MMC compared to myeloschisis (24.1 ± 2.9 and 10.3 ± 1.5 ng/mL; p < 0.0001). A larger percentage of subjects with myeloschisis defects delivered before their scheduled due date (myeloschisis 88.5%; MMC 55.0%; p = 0.003) and delivered at an earlier mean gestational age (34.6 ± 0.4 weeks, n = 26) compared to those with MMC defects (35.2 ± 0.4 weeks, n = 51) (p = 0.04). Discussion: AF-GFAP levels differentiate between MMC and myeloschisis, and raise interesting questions regarding the clinical significance between the 2 types of defects.

Background Cell-free RNA in amniotic fluid supernatant reflects developmental changes in gene expression in the living fetus, which includes genes that are specific to the central nervous system. Although it has been previously shown that central nervous system–specific transcripts are present in amniotic fluid supernatant, it is not known whether changes in the amniotic fluid supernatant transcriptome reflect the specific pathophysiologic condition of fetal central nervous system disorders. In myelomeningocele, there is open communication between the central nervous system and amniotic fluid. Objectives The purpose of this study was to identify molecular pathophysiologic changes and novel disease mechanisms that are specific to myelomeningocele by the analysis of amniotic fluid supernatant cell-free RNA in fetuses with open myelomeningocele. Study Design Amniotic fluid supernatant was collected from 10 pregnant women at the time of the open myelomeningocele repair in the second trimester (24.5±1.0 weeks); 10 archived amniotic fluid supernatant from sex and gestational age–matched euploid fetuses without myelomeningocele were used as controls (20.9±0.9 weeks). Differentially regulated gene expression patterns were analyzed with the use of human genome expression arrays. Results Fetuses with myelomeningocele had 284 differentially regulated genes (176 up- and 108 down-regulated) in amniotic fluid supernatant. Known genes that were associated with myelomeningocele ( PRICKLE2 , GLI3 , RAB2 3, HES1 , FOLR1 ) and novel dysregulated genes were identified in association with neurodevelopment and neuronal regeneration (up-regulated, GAP4 3 and ZEB1 ) or axonal growth and guidance (down-regulated, ACAP1 ). Pathway analysis demonstrated a significant contribution of inflammation to disease and a broad influence of Wnt signaling pathways ( Wnt1 , Wnt5A , ITPR1 ). Conclusion Transcriptomic analyses of living fetuses with myelomeningocele with the use of amniotic fluid supernatant cell-free RNA demonstrated differential regulation of specific genes and molecular pathways relevant to this central nervous system disorder, which resulted in a new understanding of pathophysiologic changes. The data also suggested the importance of pathways that involve secondary disease, such as inflammation, in myelomeningocele. These newly identified pathways may lead to hypotheses that can test novel therapeutic targets as adjuncts to fetal surgical repair.

Advances in prenatal diagnosis have led to the prenatal management and treatment of a variety of congenital diseases. Although surgical treatment has been successfully applied to specific anatomic defects that place the fetus at a risk of death or life-long disability, the indications for fetal surgical intervention have remained relatively limited. By contrast, prenatal stem cell and gene therapy await clinical application, but they have tremendous potential to treat a broad range of genetic disorders. If there are biological advantages unique to fetal development that favor fetal stem cell or gene therapy over postnatal treatment, prenatal therapy may become the preferred approach to the treatment of any disease that can be prenatally diagnosed and cured by stem cell or gene therapy. Here, we review the field including recent progress toward clinical application and imminent clinical trials for cellular and gene therapy.

INTRODUCTION: In utero hematopoietic cell transplantation (IUHCT) is a non-myeloablative, non-immunosuppressive transplant approach that takes advantage of the immunologic immaturity of the fetus to engraft donor cells across immune barriers and induce donor specific immune tolerance (DST). Though it has the potential to treat many congenital hematologic disorders, IUHCT is limited by levels of engraftment below those anticipated to be therapeutic for most target diseases, including sickle cell anemia. Thus, a likely initial clinical application of IUHCT is to induce DST to allow for non-myeloablative, non-immunosuppressive postnatal "booster" transplants to increase engraftment. We have previously validated this approach in murine sickle cell, beta-thalassemia, and non-diseased models using low dose total body irradiation and busulfan prior to postnatal bone marrow transplantation (BMT). Although encouraging, less toxic approaches that specifically target the hematopoietic system would make IUHCT + postnatal BMT more applicable. CD45 is a membrane glycoprotein expressed by all leukocytes and hematopoietic stem cells. Recent studies have demonstrated enhanced engraftment in a congenic postnatal transplant mouse model following pre-transplant conditioning with an anti-CD45-saporin (SAP) immunotoxin (Palchaudhuri et al., Nat Biotech 2016; 34:738-45). We hypothesized that mixed allogeneic chimerism achieved by IUHCT can be enhanced after birth by conditioning with an anti-CD45-SAP immunotoxin followed by a same-donor allogeneic BMT. METHODS: 5x106 BM mononuclear cells from C57Bl/6 mice (H2Kb, CD45.2+) were injected intravenously into gestational day 14 (E14) Balb/c fetuses (H2Kd, CD45.2+). Donor cell peripheral blood (PB) chimerism was confirmed at 4 weeks of age, at which time chimeric mice were injected via tail vein with PBS (control) or CD45.2-SAP. Six days following treatment, chimeric animals were intravenously injected with 30x106 B6Pep3b (H2Kb, CD45.1+) BM cells. Additional control groups consisted of recipients of IUHCT alone without a postnatal BMT and naïve 5 week-old Balb/c mice which did not undergo IUHCT and only received CD45.2-SAP + postnatal BMT. Following postnatal BMT, PB was assessed bi-weekly by flow cytometry for total donor chimerism and the contribution of prenatal and postnatal donor cells to engraftment. Mice were weighed before each bleeding and evaluated for signs of GVHD. Statistical comparison was performed using the Student's t-test for 2 samples at each time point assuming unequal variances. RESULTS: There was no significant difference in initial donor cell chimerism levels at 4 weeks of age between treatment (IUHCT+CD45.2-SAP+BMT, 12.4±2.2%, n=5) and control (IUHCT alone, 9.3±2.7%, n=18; IUHCT+BMT, 10.0±2.6%, n=7) groups prior to therapy. In contrast to naïve non-chimeric Balb/c mice undergoing a postnatal BMT (n=10) which demonstrated no donor cell engraftment, recipients of IUHCT and a postnatal BMT (both PBS and CD45.2-SAP treated groups) demonstrated postnatal donor cell engraftment, supporting DST achieved by IUHCT. While postnatal BMT after IUHCT failed to demonstrate a significant enhancement of engraftment compared to IUHCT alone (4.4±1.3% vs. 7.1±4.2% donor cell engraftment at 22 weeks, p = 0.7), low-level allogeneic chimerism following IUHCT was significantly enhanced to high-level chimerism following conditioning with CD45.2-SAP+BMT (55.7±9.8% vs. 7.1±4.2% donor cell engraftment at 22 weeks, p=0.0001) (Fig. 1). The majority of donor cell chimerism resulted from the postnatal donor population in recipients of CD45.2-SAP, supporting a mechanism of enhanced engraftment via a competitive advantage following recipient CD45.2-SAP treatment (Fig. 2). Chimerism was stable and multilineage at 22 weeks indicating enhanced stem cell/early progenitor engraftment. All mice demonstrated appropriate weight gain without GVHD. CONCLUSIONS: Prenatal tolerance induction achieved by IUHCT followed by postnatal CD45-SAP conditioning and same-donor BMT results in enhanced stable multilineage allogeneic engraftment at therapeutic levels for many target diseases. This work supports a potentially less toxic approach for the strategy of prenatal tolerance induction to facilitate non-myeloablative, non-immunosuppressive postnatal BMT and has the potential to significantly expand the applicability of IUHCT. Disclosures No relevant conflicts of interest to declare.

Background: In utero hematopoietic cell transplantation (IUHCT) results in long-term, multilineage chimerism without myeloablation/immunosuppression. It has the potential to treat a number of congenital hematologic disorders, including as Sickle cell disease. Central tolerance, through which donor-reactive host T cells and host-reactive donor T cells are deleted, is instrumental to the induction of donor specific tolerance (DST) and the prevention of graft-versus-host-disease (GVHD). Central deletion, however, is incomplete, and donor-reactive host T cells remain following allogeneic IUHCT. Furthermore, central deletion alone cannot explain the absence of GVHD either following IUHCT using non-T cell depleted bone marrow (BM) or in chimeric animals that undergo postnatal donor lymphocyte infusion (DLI) with subsequent conversion to >90% allogeneic donor cell engraftment (Hayashi et al., Blood 2002;100:804-12). Studies in both mice and humans have demonstrated the importance of regulatory T cells, including traditional CD25+FoxP3+ Tregs and CD49b+LAG-3+ Type 1 regulatory T (Tr1) cells, in maintaining tolerance following postnatal allogeneic transplantation. We hypothesized that peripheral regulatory T cells are increased in chimeric animals following IUHCT and that they contribute to the establishment of DST and prevention of GVHD. Methods: 10x106 BM cells from 6-8 week old C57Bl/6 (B6) FoxP3GFP mice (H2Kb, CD45.2+) were injected intravenously (IV) into gestational day 14 Balb/c FoxP3GFP fetal recipients (H2Kd, CD45.2+). Mice were sacrificed at 2, 4, 8, and 24 weeks of age. Splenocytes were analyzed for the expression of CD4, H2Kb, H2Kd, CD49b, LAG-3, CD25, and FoxP3 to determine the percentage of traditional Tregs and Tr1 cells within the CD4+ population as well as for the expression of intracellular IL-10 - a key mediator of regulatory T cell function. A GVHD model was developed in which 10x106 B6 BM cells were injected IV into day of life 0 (P0) naive Balb/c mice. The ability of IUHCT-induced regulatory T cells to suppress GVHD was assessed in two groups: 1) 10x106 B6 BM cells injected into P0 mice that had undergone IUHCT and 2) 10x106 B6 BM cells co-injected with 5x106 CD4+ splenocytes harvested from 2 week old chimeric mice into P0 naive Balb/c mice. To evaluate the role of regulatory T cells following DLI in chimeric mice, 30x106 B6CD45.1 (H2Kb, CD45.1+) splenocytes were injected IV into 4 week old chimeric recipients. Mice were sacrificed at 48 hours and 2 weeks post-DLI, and regulatory T cells were measured as a percentage of CD45.2+CD4+ cells in the spleen. Results: IUHCT was associated with a significant increase in Tr1 cells, predominantly of donor origin, compared to naive controls (Figure 1). There was a corresponding increase in intracellular IL-10 expression in donor CD4+ splenocytes in chimeric animals compared to naive B6 FoxP3GFP controls at 2 weeks of age (MFI 7.07 vs. 3.26, p=0.0017). Elevated levels of regulatory T cells following IUHCT also had a functional effect in the GVHD model (Figure 2). Mice that had undergone IUHCT (group 1) demonstrated a trend toward improved survival compared to naive mice when injected with B6 BM at P0. Naive mice injected at P0 with B6 BM plus CD4+ splenocytes from chimeric mice (group 2) demonstrated significant improvement in survival compared to naive recipients of B6 BM alone (p=0.049). Although we detected no increase in traditional Tregs after IUHCT alone, we noted a 3-fold increase in traditional Tregs of host origin in chimeric animals at 48 hours post-DLI (25.5% vs. 8.3%, p Conclusion: The induction of tolerance following IUHCT is important for maintaining successful long-term engraftment and preventing GVHD. We demonstrate an increase in nontraditional regulatory T cells following IUHCT which can minimize GVHD as well as an increase in traditional Tregs following DLI in an established model of complete allogeneic engraftment. These studies suggest an important role for regulatory T cell populations in the induction of tolerance following IUHCT. They are significant in light of a potential obstacle to the clinical translation of IUHCT: the window of central tolerance induction in humans occurs at a gestational age that will pose significant technical challenges. Augmentation of regulatory T cell-dependent peripheral tolerance is one approach by which IUHCT could be delayed to a technically feasible point later in gestation. Disclosures No relevant conflicts of interest to declare.

Purpose B cell tolerance is critical for donor cell engraftment across immune barriers after in utero hematopoietic cell transplantation (IUHCT). Using our fetal transplant model we studied how B cells achieve tolerance to allogeneic MHC antigens. Methods 10×106 C57Bl/6-GFP (H2Kb) bone marrow (BM) cells were injected intravenously into gestational day 14 Balb/c (H2Kd) fetuses. Mice were sacrificed at 3 days (P3), 1 and 4 months of age and donor-specific B cell tolerance including deletion, receptor editing and functional inactivation were assessed. Age matched naïve C57Bl/6 and Balb/c mice served as controls. Results The % of alloantigen specific splenic B cells was significantly decreased in allogenic chimeras as compared to controls (p=0.016) consistent with clonal deletion. Alloreactive B cells that escape deletion can undergo receptor editing. There was a significant increase in the % of B cells undergoing editing (lambda light chain expression) at P3 in recipients of IUHCT compared to controls (10.8% vs. 8.4%; p=0.02). Alloreactive B cells can be functionally inactivated such that they no longer make antibodies or express IgM. In contrast to naïve Balb/c mice immunized with C57Bl/6 splenocytes, immunization of recipients of IUHCT failed to elicit an anti-donor antibody response (MFI fold change: IUHCT-0.89 naïve-2.77; p Conclusion B cell tolerance after IUHCT is achieved by partial deletion in alloreactive splenic B cells, receptor editing in early mature splenocytes, and the remainder of alloreactive B cells demonstrate a functionally inactive phenotype.

Objective To describe the use of the internal mammary vessels (IMVs) in microvascular head and neck reconstruction in a small case series with select donor sites. Design Retrospective medical record review study. Setting Oregon Health and Science University and University of Alabama. Patients Patients for whom IMVs were used for head and neck reconstruction from January 1, 1998, through December 31, 2010. Main Outcome Measures Intraoperative or postoperative complications, flap survival, and morbidity due to the flap. Results Of 2721 free tissue transfers, 55 (2%) (in 48 patients) used IMVs. Use of IMVs was associated with ablative surgery with sternal resection (25 of 55 [45%]), a vessel depleted neck (23 of 55 [42%]), and fistula repair with gross contamination due to prior flap failure or chronic pharyngocutaneous fistula with vessel depleted neck (7 of 55 [13%]). Flaps included radial forearm (33 of 55 [60%]), jejunum (9 of 55 [16]), ulnar (5 of 55 [9%]), and other (8 of 55 [14%]). No vein grafts were used. Pneumothorax developed in 1 patient (2%). Postoperative fistulas were observed in 14 of 48 patients (29%); the fistulas healed conservatively in 7 patients (50%), rotation of flap tissue was required in 2 patients (14%), and the fistulas persisted in 5 patients (36%). The flap survival rate was 98%. Conclusion Internal mammary vessels provide reliable recipient vessels for cervical and sternal microvascular reconstruction.

Background: In utero hematopoietic cell transplantation (IUHCT) is a nonmyeloablative, nonimmunosuppressive transplant approach that results in donor cell engraftment across immune barriers. Although a significant amount of work has investigated the fate of T cells following IUHCT, little attention has been paid to B cell tolerance and the fate of donor derived host reactive or host derived donor reactive B cells following IUHCT. B cell tolerance is broadly believed to occur by a combination of 3 mechanisms: deletion, receptor editing, and functional inactivation (anergy). In the current study we attempt to elucidate the mechanism(s) by which B cell tolerance occurs following IUHCT. Methods: 10x106 donor bone marrow (BM) cells were injected intravenously via the vitelline vein into gestational day 14 murine fetuses. IUHCT was performed in the congenic (C57Bl/6-GFP [H2Kb ] into C57Bl/6 [H2Kb ]) and allogeneic (C57Bl/6-GFP into Balb/c [H2Kd ]) strain combinations. Naive Balb/c and C57Bl/6 mice served as controls. Mice were sacrificed at day of life 3 (P3), 1 month and 4 months of age at which time their BM, spleen, and serum were harvested. To assess B cell deletion, flow cytometry was used to determine the absolute # and % of host and donor immature and pre B cells in the BM. Additionally, apoptosis of host and donor BM derived B cells was determined by annexin staining. Central receptor editing was evaluated using RT qPCR to measure the amount of Vκ-RS rearrangments in BM pre-B cells. Peripheral receptor editing was studied by calculating the % of λ light chains in mature splenocytes identified by flow cytometry. Finally, functional inactivation of donor reactive host B cells was assessed by measuring anti-H2Kb serum antibodies (ab) of allogeneic chimeras, naive, and immunized mice at 1 month of age. Results: The absolute number of BM immature B cells was decreased in allogeneic recipients of IUHCT compared to noninjected Balb/c controls at 1 month of age (fig 1). This effect was lost by 4 months of age. The decrease in B cells resulted primarily from a decrease in immature donor as opposed to host B cells compared to controls (% immature donor B cells in allogeneic recipients vs. controls: 16.2% vs. 39.9%; p Light chain receptor editing involves rearrangements within the κ and λ gene loci and may occur in BM pre-B cells or mature B cells in the spleen. We found no difference in the Vκ-RS rearrangements of pre B cells in allogeneic chimeras and controls at 1 month. In contrast, the quantity of total λ+ mature splenic B cells was increased in allogeneic chimeras at P3 (10.8 vs. 8.4%; p=0.02) and resulted from an increased host λ+ % compared to controls (10.8 vs. 8.4% p=0.03) suggesting peripheral receptor editing of host cells (fig.2). The λ+ % increase in allogeneic chimeras was lost by 1 month. Autoreactive B cells that escape deletion and receptor editing can be functionally inactivated. Neither allogeneic nor naive mice developed ab to H2Kb splenocytes, however, Balb/c mice immunized to H2kb antigen showed high ablevels (MFI fold change: allo-0.89 naive-1.37 imm-2.77; p Conclusion: B cell tolerance after IUHCT is achieved by distinct mechanisms for host and donor cell populations. Donor derived host reactive B cells undergo deletion and apoptosis while receptor editing and functional inactivation are the primary mechanisms of B cell tolerance of host derived donor reactive B cells. We hope use this and future studies of antigen specific B cell tolerance to harness the immunologic potential of IUHCT for many hematopoietic and immunologic congenital diseases. Disclosures No relevant conflicts of interest to declare.

Background: In utero hematopoietic cell transplantation (IUHCT) results in long-term, multilineage chimerism across immune barriers without myeloablation/immunosuppression. It has the potential to induce donor specific tolerance (DST) for postnatal nonmyeloablative cellular transplants to treat target diseases such as Sickle cell disease. Partial deletion of donor reactive host T cells and host reactive donor T cells is responsible for tolerance and the absence of GVHD following IUHCT. The remaining nondeleted donor and host specific T cells are thought to undergo anergy. The mechanisms by which anergy and peripheral tolerance are achieved following IUHCT are not completely understood. Studies in both mice and humans have demonstrated the importance of regulatory T cells in maintaining tolerance following postnatal allogeneic transplantation. Subsets of regulatory T cells include traditional regulatory T cells (Tregs; CD4+ CD25+ FoxP3+), Type 1 regulatory T cells (Tr1 cells; CD4+ CD49b+ CD223+), and LAP+ regulatory T cells (LAP+ cells; CD4+ CD25- LAP+). These T regulatory populations inhibit T cell activation and proliferation through, among other mechanisms, the secretion of immunosuppressive cytokines including IL-10. During pregnancy, there is upregulation of circulating IL-10 levels in maternal and fetal blood, and maternal antigen specific fetal Tregs are induced in peripheral lymphoid tissue. We propose that IUHCT takes advantage of these tolerogenic features of the fetus to promote the peripheral induction of regulatory T cells capable of suppressing reactive T cell clones that escape thymic deletion. Methods: 10x106 bone marrow cells from 6-8 week old C57Bl/6 (B6) FoxP3GFP (H2Kb) mice were injected intravenously into allogeneic gestational day 14 Balb/c FoxP3GFP fetal recipients (H2Kd). Uninjected B6 FoxP3GFP and Balb/c FoxP3GFP mice served as controls. Mice were sacrificed at 2 and 4 weeks of age. Peripheral blood donor chimerism was determined by flow cytometry at the time of sacrifice. Splenocytes were analyzed for expression of CD4, H2Kb, H2Kd, CD49b, CD223, CD25, and LAP to determine the percentage of donor and host Tregs, Tr1 cells, and LAP+ cells within the CD4+ population. CD4+ splenocytes were also assessed for intracellular IL-10 expression. Statistically significant differences between chimeric and uninjected control animals were determined using a two-tailed student's t-test. Results: Chimeric animals demonstrated a 4- and 5-fold increase in donor Tr1 cells compared to uninjected B6 FoxP3GFP controls at 2 (2.4% vs. 0.6%, p=0.03) and 4 weeks of age (2.4% vs. 0.5%, p=0.007). There was no significant difference in host Tr1 cell levels of chimeric mice compared to uninjected Balb/c FoxP3GFP controls at either 2 (0.73% vs. 0.61%, p=0.4) or 4 weeks of age (1.0% vs. 0.53%, p=0.09). Similar to Tr1 cells, LAP+ cells of donor origin were increased compared to B6 FoxP3GFP controls at 2 (2.2% vs. 0.8%, p=0.008) and 4 weeks of age (3.2% vs. 0.6%, p=0.03). Additionally, host LAP+ cells were increased compared to Balb/c FoxP3GFP controls at 2 (2.6% vs. 1.5%, p=0.0004) and 4 weeks of age (3.0% vs. 1.1%, p=0.0005). The increase in Tr1 and LAP+ cells was associated with a significant decrease in traditional Tregs in both the host and donor population at 2 and 4 weeks of age (Figure 1). Elevated levels of Tr1 and LAP+ cells did not correlate with the absolute levels of donor cell engraftment but was dependent on achieving a threshold level of chimerism known to be associated with DST. Furthermore, the MFI of intracellular IL-10 among donor CD4+ splenocytes was significantly increased compared to those from B6 FoxP3 controls at 2 weeks of age (Figure 2), demonstrating immunosuppressive activity consistent with elevated Tr1 and LAP+ levels. Conclusion: The induction of donor and host tolerance following IUHCT is important for maintaining successful long-term engraftment and preventing GVHD. DST also allows for postnatal nonmyeloablative cellular transplants to increase engraftment to therapeutically relevant levels. We demonstrate a potentially important role that nontraditional Tr1 and LAP+ regulatory T cells play in maintaining donor cell engraftment and peripheral tolerance in the setting of IUHCT. Future investigations into the ability of these cells to augment tolerance induction after inefficient IUHCT have the potential to expand the clinical promise of IUHCT. Figure 1. Figure 1. Figure 2. Figure 2. Disclosures No relevant conflicts of interest to declare.