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Facilitating cells (FC) are a CD8+TCR− bone marrow subpopulation that enhance engraftment of purified hematopoietic stem cells (HSC) and induce antigen-specific CD4+CD25+FoxP3+ regulatory T cell (Treg) in vivo. The major subpopulation in FC resembles plasmacytoid precursor dendritic cells (p-preDC) both phenotypically and functionally. Here, we report that the number of FC was significantly reduced in Fms-like tyrosine kinase 3-ligand-knockout (Flt3-L-KO) mice. Specifically, there was a selective decrease in the B220+CD11c+CD11b− p-preDC FC subpopulation. The p-preDC FC subpopulation in FC total is restored after Flt3-L administration to Flt3-L-KO mice. FC from Flt3-L-KO donors exhibit impaired facilitation of allogeneic HSC engraftment in ablatively conditioned mice (B6 → NOD) as well as in mice conditioned with reduced intensity conditioning (B6 → BALB/c). In addition, the number of CD4+CD25+Foxp3+ Treg from Flt3-L-KO mice is significantly decreased. This was associated with the expression of chemokine receptor CXCR3+ or CCR5+ on Treg. Treg from the spleen of Flt3-L-KO mice showed impaired facilitation of engraftment of allogeneic HSC compared to wild-type Treg. Flt3-L treatment significantly expanded Treg, and restored their facilitating function. These results suggest that Flt3-L is an important growth factor in the development and homeostasis of p-preDC FC and in the role of FC inducing generation of Treg. Flt3-L provides potent immunoregulatory properties that may be clinically useful to improve tolerance induction and enhance the function of allogeneic cell therapies.

Successful solid organ transplantation currently requires the life-long use of medications to suppress the immune system in order to prevent transplant rejection. Drug-based immunosuppression significantly increases the risk of infection and cancer, as well as being very costly. Development of new therapies to minimize or eliminate entirely the need for anti-rejection drugs is of great interest to the transplant community. Therapeutic cell transfer for the control of the human immune system represents a compelling approach to reduce or eliminate the need for anti-rejection drugs. Establishment of durable hematopoietic chimerism through hematopoietic stem cell transplantation (HSCT) has been shown in preclinical models and patients to lead to donor specific tolerance. However, the application HSCT is limited by the potential toxicity of conditioning regimens, the risk of graft versus host disease (GVHD) and the challenge of HLA mismatching. In this review we describe the clinical outcomes and science behind a CD8+/TCR- facilitating cell-based hematopoietic stem cell transplant approach (termed FCRx) to induce tolerance to mismatched renal allografts while minimizing the risk of graft-versus-host GVHD and achieving avoidance of long-term immunosuppressant drugs in living donor kidney transplant recipients.

Introduction We previously reported the induction of kidney transplant tolerance by the establishment of durable whole blood and T-cell chimerism and withdrawal of immunosuppression (IS) in 26 of 37 highly mismatched recipients of combined stem cell and living donor kidney transplant recipients (KTx) with low risk of graft versus host disease (GvHD), using FCR001, an investigational cell therapy. The focus of the current analysis is to evaluate the impact of degree of bidirectional donor/recipient mismatching using high resolution allele typing at HLA-A, -B, -C, -DRB1, -DQB1 and -DPB1, on the ability to establish durable chimerism, allowing full immunosuppression (IS) withdrawal and the induction of transplant tolerance. Methods In this phase 2 trial, recipients received nonmyeloablative conditioning with fludarabine 30mg/m2 (Days -5, -4, -3), cyclophosphamide 50mg/kg (Days-3 and +3), 200 cGy TBI (Day-1) followed by KTx (Day 0). G-CSF mobilized peripheral blood mononuclear cells were apheresed from the donor >2 weeks prior to KTx, processed to remove GvHD-producing cells yet retain CD34 +cells and tolerogenic CD8+/TCR facilitating cells (FC) and cryopreserved until infusion day+1 after KTx. IS consisted of mycophenolate and tacrolimus. IS was weaned and discontinued in the presence of sustained donor T-cell chimerism (>50%), stable renal function and no evidence of biopsy-proven rejection. Results Samples from 32 of the 37 subject pairs were available for analysis; of these, three subjects did not have sufficient DNA to test for locus DPB1.All 32 recipients, ranging from age 18 - 65 years, have reached at least 4.5 years of follow-up up to 12 years. Two recipients were re-transplants. Of the 29 D/R pairs with data from all 12 alleles, 21 were mismatched between 6 to 12 alleles (6 related, 15 unrelated), 8 were mismatched between 2 and 5 alleles (all related). Of the three D/R pairs missing DP data, one was mismatched 6 of 10, another 10 of 10, and a third 2 of 10 alleles. Despite the high degree of mismatch, durable chimerism allowed for full IS withdrawal in 25 of these 32 subjects (time off IS from 3.5 - 11 years). 12/25 off IS were from unrelated D/R pairs and ≥ 8 HLA mismatches; the majority showed >95% donor whole blood/T cell chimerism. Three have exhibited stable mixed chimerism ranging between 40% - 60%. Of the subjects not off IS, two failed to engraft their cells; four lost chimerism by 4 months, and one developed GVHD. Durably chimeric patients retained chimerism after removal of IS, remain rejection-free without donor-specific antibody (DSA) with up to 12 years following KTx and have not resumed IS. Transiently chimeric subjects resumed endogenous hematopoiesis and are maintained on low dose IS with stable renal function. There have been two cases of GvHD: one grade 2 lower GI acute GvHD that developed during conversion from tacrolimus to sirolimus and responded to steroids; this patient has developed moderate chronic GvHD of the skin. He is off IS with normal renal function. The second presented late following development of severe gastrointestinal symptoms and manifested treatment-resistant lower GI GvHD with associated tissue-invasive CMV colitis that proved fatal at 11 months post-transplant. Conclusions In summary, high levels of durable chimerism and tolerance with a low (5.5%) incidence of GvHD has been achieved in highly mismatched related and unrelated recipients of FCR001 + kidney transplant. Figure 1 Figure 1. Disclosures Krieger: Talaris Therapeutics, Inc.: Current Employment, Current equity holder in publicly-traded company. Tambur: Viela Bio and Sanofi: Consultancy; ThermoFisher: Speakers Bureau. Schneider: Talaris Therapeutics, Inc.: Current Employment. Olsen: Talaris Therapeutics, Inc.: Current Employment. Ildstad: Talaris Therapeutics, Inc.: Current Employment, Current equity holder in publicly-traded company, Membership on an entity's Board of Directors or advisory committees.

Renal transplantation has become the preferred treatment for end stage kidney failure. Although short-term graft survival has significantly improved as advances in immunosuppression have occurred, long-term patient and graft survival have not. Approximately only 50% of renal transplant recipients are alive at 10 years due to the toxicities of immunosuppression and alloimmunity. Emerging research on cell-based therapies is opening a new door for patients to receive the organs they need without sacrificing quality of life and longevity because of drug-based immunosuppression. Research has focused on inducing tolerance, a state in which the body accepts the transplant and graft function is stable. Cell-based therapies to facilitate chimerism and achieve tolerance in major histocompatibility disparate recipients have been developed in mouse, swine, canine, and nonhuman primate models. These findings are now being translated into the clinic in several trials currently underway. Protocols that use a combination of traditional therapeutic agents paired with cell populations including hematopoietic stem cells, regulatory T cells, and facilitating cells are being conducted with the objective to harness the donor immune system to protect the transplanted tissue. The benefits and feasibility of the clinical application of cell-based therapy has been demonstrated, and promising results have been achieved. Here we discuss the preclinical work that has led to the clinical application of the various approaches and a summary of the most current clinical data from groups throughout the world.

BACKGROUND Immunoglobulin-cytokine fusion molecules have been shown to be the new generation of immunomodulating agents in transplantation tolerance induction. In the present study, we tested whether immunoregulatory cytokine fusion proteins of IL-10/Fc, TGF-β/Fc, or IL-2/Fc would enhance allogeneic bone marrow cell (BMC) engraftment and promote tolerance induction. METHODS B6 (H2) mice were conditioned with anti-CD154 (MR1) and rapamycin (Rapa) plus 100 cGy total body irradiation (MR1/Rapa/100 cGy) and transplanted with allogeneic B10.D2 (H2) BMC. Recipients were treated with lytic IL-2/Fc, nonlytic IL-2/Fc, TGF-β/Fc, or IL-10/Fc fusion proteins to promote chimerism to induce tolerance. RESULTS Donor chimerism was achieved in 20% of recipients conditioned with MR1/Rapa/100 cGy. The addition of TGF-β/Fc (5- or 10-day treatment) or nonlytic IL-2/Fc (10-day treatment) fusion proteins to the conditioning resulted in engraftment in nearly 100% of recipients. In contrast, lytic IL-2/Fc or IL-10/Fc had no effect. The combination of nonlytic IL-2/Fc and TGF-β/Fc had a synergistic effect to promote engraftment and resulted in significantly higher donor chimerism compared with recipients conditioned with TGF-β/MR1/Rapa/100 cGy. Engraftment was durable in the majority of chimeras and increased over time. The chimeras accepted donor skin grafts and promptly rejected third-party skin grafts. Moreover, specific T cell receptor-Vβ5.½ and TCR-Vβ11 clonal deletion was detected in host T cells in chimeras, suggesting central tolerance to donor alloantigens. CONCLUSIONS Allogeneic BMC engraftment is enhanced with TGF-β/Fc fusion protein treatment. TGF-β/Fc and nonlytic IL-2/Fc exert a synergistic effect in promotion of alloengraftment and donor-specific transplant tolerance, significantly decreasing the minimum total body irradiation dose required.

Background Mobilization of hematopoietic stem cells (HSCs) has become the preferred approach for HSC transplantation. AMD3100, a competitive inhibitor of C-X-C motif chemokine receptor-4, has been found to be a rapid mobilizing agent. The present study evaluated approaches to optimize the product collected. Methods Mobilized peripheral blood mononuclear cells (mPBMCs) from B6 mice were transplanted to recipient BALB/c mice conditioned with ablative or nonmyeloablative approaches. Results The optimal dose of AMD3100 was found to be 5.0 mg/kg. Optimal HSC mobilization was observed when AMD3100 (day 10) was coadministered with Flt3 ligand (FL) (days 1-10) and granulocyte colony-stimulating factor (GCSF) (days 4-10). There was a 228.8-fold increase of HSC with FL/GCSF/AMD3100 compared with AMD3100 treatment alone. When unmodified mPBMCs were transplanted into ablated allogeneic recipients, all recipients expired by day 40 from severe acute graft versus host disease (GVHD). When T cells were depleted from mPBMC, long-term survival and engraftment were achieved in majority of the recipients. When PBMC mobilized by FL/GCSF/AMD3100 were transplanted into recipients conditioned nonmyeloablatively with anti-CD154/rapamycin plus 100, 200, and 300 cGy of total body irradiation, 42.9%, 85.7%, and 100% of mice engrafted, respectively. Donor chimerism was durable, multilineage, and stable. Lymphocytes from mixed chimeras showed no response to host or donor antigens, suggesting functional bidirection T-cell tolerance in vitro. Most importantly, none of the engrafted mice exhibited clinical features of GVHD. Conclusions FL/GCSF/AMD3100 is an efficient treatment to maximally mobilize HSC. Durable engraftment and donor-specific tolerance can be achieved with mPBMC in nonmyeloablative conditioning without GVHD.

CD8(+)/TCR(-) facilitating cells (FCs) in mouse bone marrow (BM) significantly enhance engraftment of hematopoietic stem/progenitor cells (HSPCs). Human FC phenotype and mechanism of action remain to be defined. We report, for the first time, the phenotypic characterization of human FCs and correlation of phenotype with function. Approximately half of human FCs are CD8(+)/TCR(-)/CD56 negative (CD56(neg)); the remainder are CD8(+)/TCR(-)/CD56 bright (CD56(bright)). The CD56(neg) FC subpopulation significantly promotes homing of HSPCs to BM in nonobese diabetic/severe combined immunodeficiency/IL-2 receptor γ-chain knockout mouse recipients and enhances hematopoietic colony formation in vitro. The CD56(neg) FC subpopulation promotes rapid reconstitution of donor HSPCs without graft-versus-host disease (GVHD); recipients of CD56(bright) FCs plus HSPCs exhibit low donor chimerism early after transplantation, but the level of chimerism significantly increases with time. Recipients of HSPCs plus CD56(neg) or CD56(bright) FCs showed durable donor chimerism at significantly higher levels in BM. The majority of both FC subpopulations express CXCR4. Coculture of CD56(bright) FCs with HSPCs upregulates cathelicidin and β-defensin 2, factors that prime responsiveness of HSPCs to stromal cell-derived factor 1. Both FC subpopulations significantly upregulated mRNA expression of the HSPC growth factors and Flt3 ligand. These results indicate that human FCs exert a direct effect on HSPCs to enhance engraftment. Human FCs offer a potential regulatory cell-based therapy for enhancement of engraftment and prevention of GVHD.

Purpose of review This review discusses the role and mechanisms by which facilitating cells promote stem cell engraftment and induce tolerance in HLA-disparate kidney transplant recipients. Recent finding Facilitating cells in both mice and human are heterogeneous, consisting of several subpopulations. They have been shown to enhance stem cell engraftment in allogeneic recipients. They also increase hematopoietic stem cells (HSC) clonogenicity, enhance migration and homing of stem cells via secretion of cytokines/chemokines/growth factors, prevent apoptosis of stem cells and induce regulatory cells. This review summarizes the findings that led to the development of chimerism-based induction of tolerance using FCRx (a mobilized blood product enriched in stem cells and facilitating cells) in allogenic kidney transplant patients. Summary A phase-2 clinical trial based on FCRx therapy has been successful in inducing tolerance to living donor kidney allografts, leading to withdrawal of immunosuppression in over 70% of patients transplanted. The ultimate goal of establishing tolerance in the absence of immunosuppresive drugs can be achieved using FCRx therapy.

Kidney transplantation is now the preferred approach to treat end-stage renal failure. However, graft survival is dependent upon the lifelong use of immunosuppressive drugs, which unfortunately are associated with harmful side effects ranging from kidney damage to shortened life-span. The desire to achieve better long-term outcomes for solid organ transplants has led to the emergence of chimerism-based tolerance. Hematopoietic stem cells from the organ donor are used to modulate the recipient’s immune system to provide long-term, drug-free tolerance to the transplanted organ. Three centers in the United States have conducted chimerism-based clinical trials in kidney transplant recipients and have proven safety, feasibility, and efficacy of the approach. In this chapter, we discuss the mechanisms underlying the establishment of chimerism-based tolerance and review in detail protocols used in clinical trials. This approach to kidney transplant tolerance will likely have significant impact on other solid organ and cellular transplants.

The development of safe conditioning protocols has reduced the morbidity and mortality of hematopoietic stem cell transplantation (HSCT), allowing broader application for treatment of a variety of non-malignant conditions including autoimmune diseases, hemoglobinopathies, metabolic disorders etc., as well as inducing tolerance to solid organ transplants. One of the most successful clinical trials using a facilitating cell enhanced HSCT paired with a kidney transplant has effectively induced tolerance in the absence of immunosuppressive drugs, maintaining the function of the transplanted kidney, and reconstituting the immunocompetence of the recipient. This novel protocol eliminates the need for immunosuppressive drugs, the key source of kidney and liver toxicity, increased malignancy, and shortened life span. CD8+ TCR facilitating cells (FC) are a population of tolerogenic cell which promote hematopoietic stem cell engraftment across human leukocyte antigen (HLA) barriers. In this review, we discuss the bench to bedside journey of FC, from discovery in mouse models, characterization of the subpopulations of FC, the mechanisms by which FC induce tolerance and clinical application. As a novel personalized medicine, FC may change the approach to overcoming HLA barriers for both HSCT and solid organ transplant recipients.

Facilitating cells (FC) are a CD8

Nineteen subjects have more than 18 months' follow-up in a phase IIb tolerance protocol in HLA-mismatched recipients of living donor kidney plus facilitating cell enriched hematopoietic stem cell allografts (FCRx). Reduced intensity conditioning preceded a kidney allograft, followed the next day by FCRx. Twelve have achieved stable donor chimerism and have been successfully taken off immunosuppression (IS). We prospectively evaluated immune reconstitution and immunocompetence. Return of CD4 and CD8 T central and effector memory cell populations was rapid. T-cell receptor (TCR) Excision Circle analysis showed a significant proportion of chimeric cells produced were being produced de novo. The TCR repertoires posttransplant in chimeric subjects were nearly as diverse as pretransplant donors and recipients, and were comparable to subjects with transient chimerism who underwent autologous reconstitution. Subjects with persistent chimerism developed few serious infections when off IS. The majority of infectious complications occurred while subjects were still on conventional IS. BK viruria and viremia resolved after cessation of IS and no tissue-invasive cytomegalovirus infections occurred. Notably, although 2 of 4 transiently or nonchimeric subjects experienced recurrence of their underlying autoimmune disorders, none of the chimeric subjects have, suggesting that self-tolerance is induced in addition to tolerance to alloantigen. No persistently chimeric subject has developed donor-specific antibody, and renal function has remained within normal limits. Patients were successfully vaccinated per The American Society for Blood and Marrow Transplantation guidelines without loss of chimerism or rejection. Memory for hepatitis vaccination persisted after transplantation. Chimeric subjects generated immune responses to pneumococcal vaccine. These data suggest that immune reconstitution and immunocompetence are maintained in persistently chimeric subjects.

The modern immunosuppression regimen has greatly improved short-term allograft outcomes but not long-term allograft survival. Complications associated with immunosuppression, specifically nephrotoxicity and infection risk, significantly affect graft and patient survival. Inducing and understanding pathways underlying clinical tolerance after transplantation are, therefore, necessary. We previously showed full donor chimerism and immunosuppression withdrawal in highly mismatched allograft recipients using a bioengineered stem cell product (FCRx). Here, we evaluated the gene expression and microRNA expression profiles in renal biopsy samples from tolerance-induced FCRx recipients, paired donor organs before implant, and subjects under standard immunosuppression (SIS) without rejection and with acute rejection. Unlike allograft samples showing acute rejection, samples from FCRx recipients did not show upregulation of T cell- and B cell-mediated rejection pathways. Gene expression pathways differed slightly between FCRx samples and the paired preimplantation donor organ samples, but most of the functional gene networks overlapped. Notably, compared with SIS samples, FCRx samples showed upregulation of genes involved in pathways, like B cell receptor signaling. Additionally, prediction analysis showed inhibition of proinflammatory regulators and activation of anti-inflammatory pathways in FCRx samples. Furthermore, integrative analyses (microRNA and gene expression profiling from the same biopsy sample) identified the induction of regulators with demonstrated roles in the downregulation of inflammatory pathways and maintenance of tissue homeostasis in tolerance-induced FCRx samples compared with SIS samples. This pilot study highlights the utility of molecular intragraft evaluation of pathways related to FCRx-induced tolerance and the use of integrative analyses for identifying upstream regulators of the affected downstream molecular pathways.

Nonmyeloablative conditioning followed by donor bone marrow infusion (BMI) to induce tolerance has not been robustly tested in liver transplantation (LT) and may be unsafe at the time of LT. We hypothesized T cell-depleted BMI is effective in inducing tolerance when delayed after LT, resulting in potentially safer future clinical applications.Nonimmunosuppressed syngeneic (Lewis to Lewis) and allogeneic (ACI to Lewis) rat LT transplants were initially performed as controls. Three experimental allogeneic LT groups were treated with tacrolimus (TAC) for 3 to 4 weeks and then underwent: (1) TAC withdrawal alone; (2) nonmyeloablative conditioning (anti-αβTCR mAb + total body irradiation [300 cGy]) followed by TAC withdrawal; (3) Nonmyeloablative conditioning + donor BMI (100 × 10 T cell-depleted bone marrow cells) followed by TAC withdrawal.All group 1 recipients developed chronic rejection. Group 2 had long-term survival but impaired liver function and high donor-specific antibody (DSA) levels. In contrast, group 3 (conditioning + BMI) had long-term TAC-free survival with preserved liver function and histology, high mixed chimerism and blood/liver/spleen CD4 + CD25 + Foxp3+ regulatory T cells, and low DSA titers, similar to syngeneic grafts. While donor-specific tolerance was observed post-BMI, graft-versus-host disease was not.These results support that donor-specific tolerance can be achieved with BMI even when delayed after LT and this tolerance correlates with increased mixed chimerism, regulatory T cell generation, and diminished DSA.

Regulatory B cells (Breg) are a subpopulation of B cells that play a suppressive role in the immune system. The mechanism of how these immune cells perform their effects has been explored by experiments in mice and in humans. Intracellular staining for interleukin 10 continues to be a consistent and reproducible method of identifying Breg in mouse and human studies. The lack of Breg is associated with a worsening of several autoimmune diseases such as collagen-induced arthritis, systemic lupus erythematosus, and experimental autoimmune encephalomyelitis in murine studies. The purpose of this review is to provide a concise summary of the role of Breg in the immune system, including the most recently studied cell surface markers associated with Breg, and to describe the role of Breg in the etiology of several autoimmune diseases, the current understanding of Breg development, their role in the development of autoimmune diseases, and their role in inducing tolerance after transplantation.

37 patients have been transplanted in a phase 2 protocol to establish chimerism to induce tolerance in up to 0 of 6 matched related and unrelated recipients of living donor renal allografts (KTx). The protocol is based upon tolerogenic CD8+/TCR- facilitating cells (FC) and nonmyeloablative conditioning. Recipients were conditioned with fludarabine (30mg/m2 days -5,-4,-3), cyclophosphamide (50mg/kg day-3 and+3), 200 cGy TBI (day-1) followed by KTx (day0). G-CSF mobilized peripheral blood mononuclear cells were apheresed from the donor >2 weeks before kidney transplantation, processed to remove graft-versus-host disease (GVHD)-producing cells yet retain CD34 +cells and FC, and cryopreserved until transplantation on day+1 after kidney transplantation. Immunosuppression consisted of mycophenolate mofetil and tacrolimus. 36 patients have reached at least 1 year of follow up (range 12-105 months) and are the focus of this analysis. Patients ranged in age from 18-65 yrs. Enrollment was agnostic to degree of HLA match; 2 were 6/6 and 3 5/6 matched related using high resolution allele level typing, with the remainder 4/6 to 0/6 matched related (n=15) or unrelated (n=16) Two of the recipients were renal re-transplants. Tacrolimus/MMF immunosuppression (IS) was weaned and discontinued at 1 year if chimerism (>50% whole blood and T cell), normal renal function and normal kidney transplant biopsy were noted. 34 of 36 subjects exhibited peripheral blood donor chimerism at one month post KTx. Durable chimerism allowing for full IS withdrawal developed in 26 (time off IS from 1- 88 months); the majority (23/26) showed >95% donor whole blood/T cell chimerism. Three have exhibited stable-mixed chimerism ranging between 40% - 60%. Three patients failed to develop chimerism. Transient chimerism occurred in 8 patients. Durably chimeric patients retained chimerism after removal of IS, remain rejection-free without donor-specific antibody (DSA) and show immunocompetence to vaccinations. None have had to resume immunosuppression. Transiently chimeric subjects resumed endogenous hematopoiesis and are maintained on low-dose IS with stable renal function. There have been two cases of GVHD: one grade 2 lower GI acute GVHD that developed during conversion from tacrolimus to sirolimus that responded to steroids; this patient has developed moderate chronic GVHD of the skin. He is off IS. The second presented late following development of severe gastrointestinal symptoms and manifested treatment-resistant lower GI GVHD with associated tissue-invasive CMV colitis that proved fatal at 11 months post-Tx. There have been two additional kidney graft losses, both related to early infections. A second subject death occurred in a heavy (>100 pack yr) smoker who developed advanced stage lung cancer 4.5 years after Tx. A third subject developed pneumococcal sepsis > 4 years after transplant. He had not undergone the recommended pneumococcal vaccination post HSCT. Overall patient survival is 91.8% and death censored graft survival 94.1%. These rates compare favorably to treatment-related mortality rates after standard of care renal transplant. In summary, high levels of durable chimerism and tolerance with a low (5.5%) incidence of GVHD has been achieved in up to 0 of 6 matched related and unrelated recipients of FCR001 + kidney transplant. Figure Disclosures Ildstad: Talaris Therapeutics Inc: Employment, Equity Ownership, Other: Founder and CSO.

Approaches to safely induce tolerance in vascularized composite allotransplantation (VCA) with chimerism through bone marrow transplantation (BMT) are currently being pursued. However, VCA was historically performed sequentially after donor chimerism was established. Delayed VCA is not clinically applicable due to the time constraints associated with procurement from deceased donors. A more clinically relevant approach to perform both BMT and VCA simultaneously was evaluated.Wistar Furth (RT1A) rats were treated with a short course of immunosuppressive therapy (anti-αβ-TCR monoclonal antibody, FK-506, and anti-lymphocyte serum). One day before BMT, rats were treated with varying doses of total body irradiation (TBI) followed by transplantation of heterotopic osteomyocutaneous flaps from hindlimbs of August Copenhagen Irish (RT1A) rats.Eighty percent of rats conditioned with 300 cGy TBI and 40% of rats receiving 400 cGy TBI accepted the VCA. Mixed chimerism was detected in peripheral blood at 1 month after VCA, but chimerism was lost in all transplant recipients by 4 months. Most peripheral donor cells originated from the BMT and not from the VCA. Acceptors of VCA were tolerant of a donor skin graft challenge and no anti-donor antibodies were detectable, suggesting a central deletional mechanism for tolerance. Regulatory T cells (Treg) from spleens of acceptors more potently suppressed lymphocyte proliferation than Treg from rejectors in the presence of donor stimulator cells.These studies suggest that simultaneous BMT and VCA may establish indefinite allograft survival in rats through Treg-mediated suppression and thymic deletion of alloreactive T cells.

In the failing heart, iNOS is expressed by both macrophages and cardiomyocytes. We hypothesized that inflammatory cell-localized iNOS exacerbates left ventricular (LV) remodeling. Wild-type (WT) C57BL/6 mice underwent total body irradiation and reconstitution with bone marrow from iNOS−/− mice (iNOS−/−c) or WT mice (WTc). Chi-meric mice underwent coronary ligation to induce large infarction and ischemic heart failure (HF), or sham surgery. After 28 days, as compared with WTc sham mice, WTc HF mice exhibited significant (p < 0.05) mortality, LV dysfunction, hypertrophy, fibrosis, oxidative/nitrative stress, inflammatory activation, and iNOS upregulation. These mice also exhibited a ∼twofold increase in circulating Ly6Chi pro-inflammatory monocytes, and ∼sevenfold higher cardiac M1 macrophages, which were primarily CCR2– cells. In contrast, as compared with WTc HF mice, iNOS−/− c HF mice exhibited significantly improved survival, LV function, hypertrophy, fibrosis, oxidative/nitrative stress, and inflammatory activation, without differences in overall cardiac iNOS expression. Moreover, iNOS−/−c HF mice exhibited lower circulating Ly6Chi monocytes, and augmented cardiac M2 macrophages, but with greater infiltrating monocyte-derived CCR2+ macrophages vs. WTc HF mice. Lastly, upon cell-to-cell contact with naïve cardiomyocytes, peritoneal macrophages from WT HF mice depressed contraction, and augmented cardiomy-ocyte oxygen free radicals and peroxynitrite. These effects were not observed upon contact with macrophages from iNOS−/− HF mice. We conclude that leukocyte iNOS is obligatory for local and systemic inflammatory activation and cardiac remodeling in ischemic HF. Activated macrophages in HF may directly induce cardiomyocyte contractile dysfunction and oxidant stress upon cell-to-cell contact; this juxtacrine response requires macrophage-localized iNOS.

For over 50 y the association between hematopoietic chimerism and tolerance has been recognized. This originated with the brilliant observation by Dr. Ray Owen that freemartin cattle twins that shared a common placental blood supply were red blood cell chimeras, which led to the discovery that hematopoietic chimerism resulted in actively acquired tolerance. This was first confirmed in neonatal mice by Medawar et al. and subsequently in adult rodents. Fifty years later this concept has been successfully translated to solid organ transplant recipients in the clinic. The field is new, but cell-based therapies are being used with increasing frequency to induce tolerance and immunomodulation. The future is bright. This review focuses on chimerism and tolerance: past, present and prospects for the future.

The immunoregulatory properties of hematopoietic stem cells (HSC) have been recognized for over 60 years, beginning when Owen (1945) reported that genetically disparate freemartin cattle sharing a common placenta were red blood cell chimeras, and subsequently when Billingham, Brent, and Medawar (1953) demonstrated that murine neonatal chimeras prepared by infusion of donor-derived hematopoietic cells exhibited donor-specific tolerance to skin allografts. Various approaches using HSC in organ transplantation have gradually brought the dream of inducing donor-specific tolerance in organ transplant recipients closer to reality. Several hurdles needed to be overcome, especially avoiding graft-versus-host disease, the toxicity of ablative conditioning and eliminating the need for close donor/recipient matching. For wide acceptance, HSC therapy must be safe and reproducible in mismatched donor/recipient combinations. Discoveries in other disciplines have often unexpectedly and synergistically contributed to progress. This review presents an historic perspective of the quest for tolerance in organ transplantation, highlighting current clinical approaches.

Background Mixed chimerism is an effective approach for tolerance induction in transplantation. Strategies to achieve mixed chimerism with relatively low toxicity have significantly expanded the clinical use of chimerism.

BACKGROUND Currently, the donor-recipient matching process for vascularized composite tissue allotransplantation (VCTA) closely follows the standard practices for solid organ transplantation. Sensitization is considered a contraindication to VCTA. However, the role of sensitization in VCTA rejection is largely unstudied. METHODS Major histocompatibility-mismatched ACI (RT1) donors and Wistar Furth (WF) (RT1) recipients were used to determine whether sensitization would lead to hyperacute rejection in VCTA as in other organs, such as kidneys. WF rats were presensitized to ACI antigens by skin transplantation and received heterotopic osteomyocutaneous VCTA flaps. Kidney transplants served as controls. RESULTS Production of anti-donor antibody was detected in WF recipients after rejection of the ACI skin grafts. Sensitized WF rats rejected VCTA grafts from ACI rats significantly faster (P

Hematopoietic stem cell (HSC) transplantation (HSCT) has been utilized for treatment of many hematologic malignancies, genetic and metabolic disorders, and hemoglobinopathies such as sickle cell disease and thalassemia. It also induces donor-specific tolerance to organ and tissue transplants. The widespread success of HSCT is hampered by the toxicities of immunosuppression and development of graft-versus-host disease (GVHD). The mechanism of induction of transplantation tolerance (reciprocal donor/host) is still an elusive challenge in allogeneic HSCT. An understanding of the mechanisms for induction of tolerance and the critical cells involved in this process has resulted in novel cell-based therapies poised to be translated to clinical application. The focus of this review is those cells of interest. Bone marrow-derived plasmacytoid dendritic cells induce naive T cells to differentiate to become antigen-specific regulatory T cells (Treg), creating a milieu for the induction of transplantation tolerance....

CD8-positive/T-cell receptor–negative (CD8+/TCR−) graft facilitating cells (FCs) are a novel cell population in bone marrow that potently enhance engraftment of hemopoietic stem cells (HSCs). Previously, we showed that the CD11c+/B220+/CD11b− plasmacytoid-precursor dendritic cell (p-preDC) FC subpopulation plays a critical but nonredundant role in facilitation. In the present study, we investigated the mechanism of FC function. We report that FCs induce antigen-specific CD4+/CD25+/FoxP3+ regulatory T cells (Tregs) in vivo. The majority of chimeric Tregs were recipient derived. Chimeric Tregs harvested at ≥ 4 weeks after transplantation significantly enhanced engraftment of donor- and recipient-derived HSCs, but not third-party HSCs, in conditioned secondary recipients, demonstrating antigen specificity. Although Tregs were present 2 and 3 weeks after transplantation, they did not enhance engraftment. In contrast, week 5 and greater Tregs potently enhanced engraftment. The function of chimeric Tregs was directly correlated with the development of FoxP3 expression. Chimeric Tregs also induced significantly stronger suppression of T-cell proliferation to donor antigen in vitro. Removal of p-preDC FCs resulted in impaired engraftment of allogeneic HSCs and failure to produce chimeric Tregs, suggesting that the CD8α+ p-preDC subpopulation is critical in the mechanism of facilitation. These data suggest that FCs induce the production of antigen-specific Tregs in vivo, which potently enhance engraftment of allogeneic HSCs. FCs hold clinical potential because of their ability to remain tolerogenic in vivo.

The prospect of eliminating the long term costs and side effects of drug-based immunosuppression (IS) is a compelling reason to pursue the establishment of transplant tolerance. Tolerance in kidney transplant (KTx) recipients has been achieved through the infusion of donor HSC under some form of conditioning. The increased upfront cost and intensive treatment required by tolerance induction protocols demands that the long term outcomes be superior to that achieved with SOC IS regimens. Since 2009 we have conducted a Phase 2 trial of combined stem cell/living donor kidney transplantation in mismatched and unrelated subjects where the stated goal has been the establishment of durable donor macrochimerism. Our protocol is based upon tolerogenic CD8+/TCR-facilitating cells (FCRx) and 200 cGy TBI-based nonmyeloablative conditioning. Recipients were conditioned with fludarabine (30mg/m2/dose, days -5,-4,-3), cyclophosphamide (50mg/kg/dose, day-3 and+3), 200 cGy TBI (day-1) followed by a living donor KTx (day0). A G-CSF mobilized peripheral blood mononuclear cell product was apheresed from the donor >2 weeks pre-KTx, processed to remove graft-versus-host disease (GVHD)-producing cells yet retain CD34 + cells and FC, and cryopreserved until administration day+1 post-KTx. MMF and tacrolimus-based immunosuppression (IS) was weaned and discontinued at 1 year if chimerism, normal renal function and normal KTx biopsy were noted. 37 subjects were transplanted between 2009 and 2016. 17 of 37 subjects were transplanted from 0-2 of 6 HLA matched unrelated donors. 26 of 37 subjects achieved high levels of durable donor chimerism and have been removed from all immunosuppression (7-95 months). Durably chimeric subjects show normal yearly protocol biopsies at 6 months, 12 months, and yearly thereafter, while standard of care patients begin to show abnormal biopsies and deterioration of renal function as early as 24 months post-transplant. Among the 26 subjects who were removed from immunosuppression, none have experienced biopsy proven acute rejection, none have lost chimerism, and none have had to have immunosuppression resumed. Patient and graft survival at 5 years post-transplant are similar to standard of care (SOC) subjects. Two cases of graft-versus-host disease (GVHD) have occurred (6% incidence). One subject experienced grade 2 GVHD that responded to therapy and was successfully removed from all immunosuppression. One subject experienced steroid unresponsive grade 3-4 GVHD resulting in death. Among 7 durably chimeric subjects who had an underlying autoimmune disease that caused their renal failure, none have experienced recurrence of the autoimmune disease (expected rate of recurrence 40-60%). In summary, patient and graft survival at five years was comparable between FCRx and SOC pts. Tolerant FCRx subjects had significantly better renal function than SOC. Medical therapy for hypertension and hyperlipidemia was more common in SOC than tolerant FCRx pts. We conclude there are significant long-term medical benefits to establishing tolerance in KTx recipients using the FCRx approach that become apparent within < 5 years. Disclosures Horwitz: Gamida Cell: Research Funding. Ildstad:Regenerex, LLC: Equity Ownership, Other: CEO.

β-Cell regeneration and β-cell preservation are two promising therapeutic approaches for the management of patients with type 1 diabetes (T1D). Stem cell-based strategies to address the problems of shortage in β cells, autoimmune and alloimmune responses have become an area of intense study.This review focuses on the progress that has been made in obtaining functional, insulin-producing cells from various types of stem/progenitor cells, including the current knowledge on the immunomodulatory roles of hematopoietic stem cell and multipotent stromal cell in the therapies for T1D.A broad overview of recent advancements in this field is provided. The hurdles that remain in the path of using stem cell-based strategies for the treatment of T1D and possible approaches to overcome these challenges are discussed.Stem cell-based strategies hold great promise for the treatment of T1D. In spite of the progress that has been made over the last decade, a number of obstacles and concerns need to be cleared before widespread clinical application is possible. In particular, the mechanism of ESC and iPSC-derived β-cell maturation in vivo is poorly understood.

In this review, we summarized the latest results from the interventional clinical trials on inducing clinical tolerance in the recipients of human leukocyte antigen (HLA)-matched or mismatched living donor kidney transplantation via establishment of either transient or durable donor chimerism by hematopoietic stem cell (HSC)-based cell therapies. The protocols utilized by the three medical centers in the United States are different in the donor cells composition of hematopoietic stem cells transplantation (HSCT) and its timing, and the conditioning regimen prepared for the transplantation. Tolerant recipients from the clinical trials benefited from better long-term outcome, improved quality of life and reductions in lifetime healthcare expenses compared with Standard of Care recipients on conventional immunosuppression. Durable chimerism is indispensable to achieve immunologic tolerance to kidney transplantation and to protect against recurrence of the original renal disease in HLA-mismatched kidney transplant recipients by concomitant HSCT.

Retinal degenerative diseases that target photoreceptors or the adjacent retinal pigment epithelium (RPE) affect millions of people worldwide. Retinal degeneration (RD) is found in many different forms of retinal diseases including retinitis pigmentosa (RP), age-related macular degeneration (AMD), diabetic retinopathy, cataracts, and glaucoma. Effective treatment for retinal degeneration has been widely investigated. Gene-replacement therapy has been shown to improve visual function in inherited retinal disease. However, this treatment was less effective with advanced disease. Stem cell-based therapy is being pursued as a potential alternative approach in the treatment of retinal degenerative diseases. In this review, we will focus on stem cell-based therapies in the pipeline and summarize progress in treatment of retinal degenerative disease.

Sensitization to major histocompatibility complex (MHC) alloantigens is critical in transplantation rejection. The mechanism of sensitization to minor histocompatibility antigens (Mi-HAg) has not been thoroughly explored. We used a mouse model of allosensitization to Mi-HAg to study the Mi-HAg sensitization barrier in bone marrow transplantation (BMT). AKR mice were sensitized with MHC congenic Mi-HAg disparate B10.BR skin grafts. Adaptive humoral (B-cells) and cellular (T cells) responses to Mi-HAg are elicited. In subsequent BMT, only 20% of sensitized mice engrafted, while 100% of unsensitized mice did. In vivo cytotoxicity assays showed that Mi-HAg sensitized AKR mice eliminated CFSE labeled donor splenocytes significantly more rapidly than naïve AKR mice but less rapidly than MHC-sensitized recipients. Sera from Mi-HAg sensitized mice also reacted with cells from other mouse strains, suggesting that Mi-HAg peptides were broadly shared between mouse strains. The production of anti-donor-Mi-HAg antibodies was totally prevented in mice treated with anti-CD154 during skin grafting, suggesting a critical role for the CD154:CD40 pathway in B-cell reactivity to Mi-HAg. Moreover, anti-CD154 treatment promoted BM engraftment to 100% in recipients previously sensitized to donor Mi-HAg. Taken together, Mi-HAg sensitization poses a significant barrier in BMT and can be overcome with CD154:CD40 costimulatory blockade.

Background. The role of bone marrow (BM)-derived cells in pancreatic β-cell regeneration remains unresolved. We examined whether BM-derived cells are recruited to the site of moderate pancreatic injury and contribute to β-cell regeneration. Methods. Low-dose streptozotocin (STZ) treatment was used to induce moderate pancreatic damage and hyperglycemia. Enhanced green fluorescent protein-positive (EGFP + ) BM chimeras were evaluated for β-cell regeneration after STZ treatment. Results. To test the hypothesis that pancreatic tissue injury induces a stromal cell-derived factor (SDF)-1 gradient to chemoattract the stem cells, we evaluated the expression of mRNA for SDF-1 in damaged pancreatic tissue. SDF-1 was significantly increased in the pancreas after damage, peaking at day 10. The majority of BM cells expressing mRNA for pancreatic development markers were detected in the subpopulation of CD45 ― /Sca-1 + /Lin ― very small embryonic-like (VSEL) cells. VSEL cells mobilized from BM to peripheral blood in response to pancreatic damage, peaking in peripheral blood at day 5, and were enriched in the pancreas 10 to 15 days after STZ treatment. To confirm a role for BM-derived cells in pancreatic β-cell regeneration, we prepared EGFP + →B6 chimeras. In the EGFP + chimeras, EGFP + cells were detected around duct and islets and were positive for insulin after STZ treatment. However, STZ-induced hyperglycemia was reduced only transiently (49―77 days) after pancreatic injury. Conclusions. These data suggest that VSEL cells are mobilized into injured pancreatic tissue and contribute to β-cell regeneration. Transplantation of BM-derived cells improves the function of injured pancreas, although the response is not sufficient to restore sustained normoglycemia.

Plasmacytoid dendritic cells (pDCs) are specialized type I IFN producers, which play an important role in pathogenesis of autoimmune disorders. Dysregulated autoreactive B cell activation is a hallmark in most autoimmune diseases. This study was undertaken to investigate interactions between pDCs and autoreactive B cells. After coculture of autoreactive B cells that recognize self-Ag small nuclear ribonucleoprotein particles with activated pDCs, we found that pDCs significantly enhance autoreactive B cell proliferation, autoantibody production, and survival in response to TLR and BCR stimulation. Neutralization of IFN-α/β and IL-6 abrogated partially pDC-mediated enhancement of autoreactive B cell activation. Transwell studies demonstrated that pDCs could provide activation signals to autoreactive B cells via a cell-to-cell contact manner. The involvement of the ICAM-1-LFA-1 pathway was revealed as contributing to this effect. This in vitro enhancement effect was further demonstrated by an in vivo B cell adoptive transfer experiment, which showed that autoreactive B cell proliferation and activation were significantly decreased in MyD88-deficient mice compared with wild-type mice. These data suggest the dynamic interplay between pDCs and B cells is required for full activation of autoreactive B cells upon TLR or BCR stimulation.

We have previously reported that administration of granulocyte colony-stimulating factor (G-CSF) + Flt-3 ligand (FL) or G-CSF + stem cell factor (SCF) improves left ventricular (LV) function and halts LV remodeling at 35 d after myocardial infarction (MI). In the current study, we investigated whether these beneficial effects are sustained in the long term — an issue of fundamental importance for clinical translation. Mice undergoing a 30-min coronary occlusion followed by reperfusion received vehicle (group I), G-CSF + FL (group II), G-CSF + SCF (group III), or G-CSF alone (group IV) starting 4 h after reperfusion and were euthanized 48 wk later. LV structure and function were assessed by serial echocardiography before and at 48 h and 4, 8, 16, 32, and 48 wk after MI. During follow-up, mice in group I exhibited worsening of LV function and progressive LV remodeling. Compared with group I, both groups II and III exhibited improved LV EF at 4 wk after MI; however, only in group II was this improvement sustained at 48 wk. Group II was also the only group in which the decrease in infarct wall thickening fraction, the LV dilatation, and the increase in LV mass were attenuated vs. group I. We conclude that the beneficial effect of G-CSF+FL on postinfarction LV dysfunction and remodeling is sustained for at least 11 months, and thus is likely to be permanent. In contrast, the effect of G-CSF + SCF was not sustained beyond the first few weeks, and G-CSF alone is ineffective. To our knowledge, this is the first long-term study of cytokines in postinfarction LV remodeling. The results reveal heretofore unknown differential actions of cytokines and have important translational implications.

Background Mixed chimerism induces donor-specific tolerance to composite tissue allotransplants (CTAs). In the present studies, we used a nonmyeloablative conditioning approach to establish chimerism and promote CTA acceptance. Methods Wistar Furth (RT1A(u)) rats were conditioned with 600 to 300 cGy total body irradiation (TBI, day-1), and 100 x 10(6) T-cell-depleted ACI (RT1A(abl)) bone marrow cells were transplanted on day 0, followed by a 11-day course of tacrolimus and one dose of antilymphocyte serum (day 10). Heterotopic osteomyocutaneous flap transplantation was performed 4 to 6 weeks after bone marrow transplantation. Results Mixed chimerism was initially achieved in almost all recipients, but long-term acceptance of CTA was only achieved in rats treated with 600 cGy TBI. When anti-alphabeta-T-cell receptor (TCR) monoclonal antibody (mAb) (day-3) was added into the regimens, donor chimerism was similar to recipients preconditioned without anti-alphabeta-TCR mAb. However, the long-term CTA survival was significantly improved in chimeras receiving more than or equal to 300 cGy TBI plus anti-alphabeta-TCR mAb. Higher levels of donor chimerism were associated with CTA acceptance. The majority of flap acceptors lost peripheral blood chimerism within 6 months. However, donor chimerism persisted in the transplanted bone at significantly higher levels compared with other hematopoietic compartments. The compartment donor chimerism may be responsible for the maintenance of tolerance to CTA. Long-term acceptors were tolerant to a donor skin graft challenge even in the absence of peripheral blood chimerism. Conclusions Mixed chimerism established by nonmyeloablative conditioning induces long-term acceptance of CTA, which is associated with persistent chimerism preferentially in the transplanted donor bone.

OBJECTIVE—Plasmacytoid precursor dendritic cell facilitating cells (p-preDC FCs) play a critical role in facilitation of syngeneic and allogeneic hematopoietic stem cell (HSC) engraftment. Here, we evaluated the phenotype and function of CD8+/TCR− FCs from NOD mice.RESEARCH DESIGN AND METHODS—The phenotype of CD8+/TCR− FCs was analyzed by flow cytometry using sorted FCs from NOD, NOR, or B6 mice. The function of NOD FCs was evaluated by colony-forming cell (CFC) assay in vitro and syngeneic or allogeneic HSC transplantation in vivo.RESULTS—We report for the first time that NOD FCs are functionally impaired. They fail to facilitate engraftment of syngeneic and allogeneic HSCs in vivo and do not enhance HSC clonogenicity in vitro. NOD FCs contain subpopulations similar to those previously described in B6 FCs, including p-preDC, CD19+, NK1.1+DX5+, and myeloid cells. However, the CD19+ and NK1.1+DX5+ subpopulations are significantly decreased in number in NOD FCs compared with disease-resistant controls. Removal of the CD19+ or NK1.1+DX5+ subpopulations from FCs did not significantly affect facilitation. Notably, Flt3 ligand (FL) treatment of NOD donors expanded FC total in peripheral blood and restored facilitating function in vivo.CONCLUSIONS—These data demonstrate that NOD FCs exhibit significantly impaired function that is reversible, since FL restored production of functional FCs in NOD mice and suggest that FL plays an important role in the regulation and development of FC function. FCs may therefore be linked to diabetes pathogenesis and prevention.

Composite tissue allotransplantation (CTA) is emerging as a potential treatment for complex tissue defects. It is currently being performed with increasing frequency in the clinic. The feasibility of the procedure has been confirmed through 30 hand transplantation, 3 facial reconstructions, and vascularized knee, esophageal, and tracheal allografts. A major drawback for CTA is the requirement for lifelong immunosuppression. The toxicity of these agents has limited the widespread application of CTA. Methods to reduce or eliminate the requirement for immunosuppression and promote CTA acceptance would represent a significant step forward in the field. Multiple studies suggest that mixed chimerism established by bone marrow transplantation promotes tolerance resulting in allograft acceptance. This overview focuses on the history and the exponentially expanding applications of the new frontier in CTA transplantation: immunology associated with CTA; preclinical animal models of CTA; clinical experience with CTA; and advances in mixed chimerism–induced tolerance in CTA. Additionally, some important hurdles that must be overcome in using bone marrow chimerism to induce tolerance to CTA are also discussed.

Background. Fms-related tyrosine kinase 3 (Flt3)-ligand (FL) promotes the proliferation, differentiation, development, and mobilization ofhematopoietic cells. We previously found that FL-mobilized hematopoietic stem cells (HSC) engraft efficiently, whereas FL-expanded bone marrow HSC do not. The function of FL-mobilized c-Kit + Sca-1 + Lin - (KSL) subpopulations has not been systematically evaluated. A precise definition of the repopulating ability is needed to define which HSC subpopulations are critical for long-term chimerism and tolerance induction. FL significantly mobilized c-Kit hi and c-Kit lo Sca-1 + Lin - cells into peripheral blood (PB). Here, we evaluated the influence of Flt3 expression on long-term repopulating ability of HSC subpopulations. Methods. c-Kit hi or c-Kit lo KSL cells were sorted from PB of FL-treated green fluorescent protein-positive donors. The function of these cells was evaluated using competitive reconstitution assays, colony-forming units spleen, and colony forming cell assays. The function of c-Kit hi CD34 - Flt3 - KSL, c-Kit hi CD34 + Flt3 - KSL, c-Kit hi CD34 + Flt3 + KSL were investigated in an in vivo transplantation model. Results. Only FL-mobilized PB c-Kit hi KSL cells exhibited high spleen colony-forming unit activity, generated high numbers of both lymphoid and myeloid colonies in vitro, and rescued ablated recipients. FL-mobilization expanded both c-Kit hi CD34 + Flt3 - cells (short-term HSC) and c-Kit hi CD34-Flt3- KSL cells (long-term HSC). There was a significant decrease in c-Kit hi CD34 + Flt3 + KSL late multipotent progenitors in PB. A combination of c-Kit hi CD34 + Flt3 - and c-Kit hi CD34-Flt3- KSL cells offered the most effective rescue of ablated recipients. Conclusions. These data suggest that engraftment of purified HSC is influenced by both short- and long-term repopulating populations and that Flt3 expression may be useful for selecting the most critical HSC subpopulations for transplantation.

The use of tolerogenic cells as an approach to induce tolerance to solid organ allografts is being aggressively pursued. A major limitation to the clinical application of cell-based therapies has been the ability to obtain sufficient numbers and also preserve their tolerogenic state. We previously reported that small numbers of bone marrow-derived CD8+/TCR− graft facilitating cells (FC) significantly enhance hemopoietic stem cell (HSC) engraftment in allogeneic and syngeneic recipients. Although the majority of FC resemble precursor plasmacytoid dendritic cells (p-preDC), p-preDC do not replace FC in facilitating function. In the present studies, we investigated the mechanism of FC function. We show for the first time that FC significantly enhance HSC clonogenicity, increase the proportion of multipotent progenitors, and prevent apoptosis of HSC. These effects require direct cell:cell contact between FC and HSC. Separation of FC from HSC by transwell membranes completely abrogates the FC effect on HSC. p-preDC FC do not replace FC total in these effects on HSC function. FC produce TNF-α, and FC from TNF-α-deficient mice exhibit impaired facilitation in vivo and loss of the in vitro effects on HSC. Neutralizing TNF-α in FC similarly blocks the FC effect. The antiapoptotic effect of FC is associated with up-regulation of Bcl-3 transcripts in HSC and blocking of TNF-α is associated with abrogation of up-regulation of Bcl-3 transcripts. These data demonstrate a critical role for TNF-α in mediating FC function. FC may have a significant impact upon the safe use of chimerism to establish tolerance to transplanted organs and tissue.

Background. Bone marrow (BM) chimerism has been shown to have a beneficial effect on allograft survival. We recently found that production of donor T-cells was highly correlated with induction of tolerance in minimally conditioned chimeras. In the present studies, we demonstrate that nonmyeloablative conditioning and BM cell infusion modulate innate and adaptive host immune responses. Methods. Chimeras were generated by bone marrow transplantation (B10.BR to B10). Recipients were preconditioned with T-cell depleting antibodies and total body irradiation with or without cyclophosphamide. Donor-specific tolerance was tested by skin grafting. Results. Transfer of tolerant splenocytes to immunocompetent secondary recipients did not transfer tolerance, nor did infusion of tolerant CD4+/CD25+ T-cells into chimeras without donor T-cell production, demonstrating that linked suppression is an unlikely mechanism in tolerance induction in the context of BM cell infusion. The addition of a single dose of cyclophosphamide to the conditioning enhanced engraftment and tolerance. This was associated with production of donor T-cells and effective clonal deletion, and a significant reduction in activated recipient plasmacytoid dendritic cells (pDC) and natural killer (NK) cells. Chimeras without donor T-cell production that eventually lost their chimerism did not generate an antidonor humoral response, whereas unconditioned controls infused with similar numbers of BM cells did, indicating that infusion of donor BM cells into conditioned recipients induced immune deviation for adaptive B-cell immunity, preventing sensitization to major histocompatibility complex (MHC) alloantigens. Conclusions. These results demonstrate that recipient T-cells, pDC, and NK cells contribute to the host barrier for establishing chimerism, implicate deletional tolerance as the mechanism for total body irradiation-based nonmyeloablative conditioning for BM transplantation, and show a beneficial effect of BM cells in preventing sensitization to MHC alloantigens.

Bone marrow transplantation (BMT) is the treatment of choice for many hematological malignancies and immunopathologies. Unfortunately, success is often impeded by engraftment failure and graft-versus-host disease (GVHD). A rare bone marrow population known as the facilitating cell (FC) has been identified which facilitates stem cell engraftment and circumvents these obstacles in murine experimental models. This review discusses the identification and characterization of this rare population and provides an emerging portrait of FC origin, ontogeny and function. The promotion of durable stem cell engraftment in MHC disparate recipients, GVHD inhibition and tolerance induction by the FC suggests that future therapies in hematopoietic cell transplantation and tolerance induction for solid organ transplants may be significantly improved through the application of FC transplantation.