Human gut microbiota-reactive DP8a regulatory T cells prevent acute graft-versus-host disease in a CD73-dependent manner.

Graft-versus-host disease (GvHD) is a life-threatening complication frequently occurring following allogeneic hematopoietic stem cell transplantation (allo-HSCT). Since gut microbiota and regulatory T cells (Tregs) are believed to play roles in GvHD prevention, we investigated whether DP8a Tregs, which we have previously described to harbor a TCR-specificity for the gut commensal Faecalibacterium prausnitzii, could protect against GvHD, thereby linking microbiota and its effect on GvHD. We observed a decrease in CD73+ DP8α Treg frequency in allo-HSCT patients at 1-month post-transplantation, which was associated with aGvHD development at 1-month post-transplantation, as compared to aGvHD-free patients, without being correlated to hematological disease's relapse. Importantly, CD73 activity was shown to be critical for DP8αTreg suppressive function. Moreover, the frequency of host-reactive DP8α Tregs was also lower in aGvHD patients, as compared to aGvHD-free patients, which could embody a protective mechanism responsible for the maintenance of these cell subset in GvHD-free patients. We also showed that human DP8α Tregs protected mice against xeno-GvHD through limiting deleterious inflammation and preserving gut integrity. Altogether, these results demonstrated that human DP8α Tregs mediate aGvHD prevention in a CD73-dependent manner, likely through host-reactivity, advocating for the use of these cells for the development of innovative therapeutic strategies to preclude aGvHD-related inflammation.

Correct stimulation of CD28H arms NK cells against tumor cells.

Tumor evasion has recently been associated with a novel member of the B7 family, HERV-H LTR-associating 2 (HHLA2), which is mostly overexpressed in PDL-1neg tumors. HHLA2 can either induce a costimulation signal when bound to CD28H or inhibit it by binding to KIR3DL3 on T- and NK cells. Given the broad distribution of CD28H expression on NK cells and its role, we compared two monoclonal antibodies targeting this novel NK-cell engager in this study. We show that targeting CD28H at a specific epitope not only strongly activates Ca2+ flux but also results in NK-cell activation. CD28H-activated NK cells further display increased cytotoxic activity against hematopoietic cell lines and bypass HHLA2 and HLA-E inhibitory signals. Additionally, scRNA-seq analysis of clear cell renal cancer cells revealed that HHLA2+ clear cell renal cancer cell tumors were infiltrated with CD28H+ NK cells, which could be targeted by finely chosen anti-CD28H Abs.

SIRPγ-CD47 Interaction Positively Regulates the Activation of Human T Cells in Situation of Chronic Stimulation.

A numerous number of positive and negative signals via various molecules modulate T-cell activation. Within the various transmembrane proteins, SIRPγ is of interest since it is not expressed in rodents. SIRPγ interaction with CD47 is reevaluated in this study. Indeed, we show that the anti-SIRPγ mAb clone LSB2.20 previously used by others has not been appropriately characterized. We reveal that the anti-SIRPα clone KWAR23 is a Pan anti-SIRP mAb which efficiently blocks SIRPα and SIRPγ interactions with CD47. We show that SIRPγ expression on T cells varies with their differentiation and while being expressed on Tregs, is not implicated in their suppressive functions. SIRPγ spatial reorganization at the immune synapse is independent of its interaction with CD47. In vitro SIRPα-γ/CD47 blockade with KWAR23 impairs IFN-γ secretion by chronically activated T cells. In vivo in a xeno-GvHD model in NSG mice, the SIRPγ/CD47 blockade with the KWAR23 significantly delays the onset of the xeno-GvHD and deeply impairs human chimerism. In conclusion, we have shown that T-cell interaction with CD47 is of importance notably in chronic stimulation.

Toward a better definition of hematopoietic progenitors suitable for B cell differentiation.

The success of inducing human pluripotent stem cells (hIPSC) offers new opportunities for cell-based therapy. Since B cells exert roles as effector and as regulator of immune responses in different clinical settings, we were interested in generating B cells from hIPSC. We differentiated human embryonic stem cells (hESC) and hIPSC into B cells onto OP9 and MS-5 stromal cells successively. We overcame issues in generating CD34+CD43+ hematopoietic progenitors with appropriate cytokine conditions and emphasized the difficulties to generate proper hematopoietic progenitors. We highlight CD31intCD45int phenotype as a possible marker of hematopoietic progenitors suitable for B cell differentiation. Defining precisely proper lymphoid progenitors will improve the study of their lineage commitment and the signals needed during the in vitro process.

Selective SIRPα blockade reverses tumor T cell exclusion and overcomes cancer immunotherapy resistance.

T cell exclusion causes resistance to cancer immunotherapies via immune checkpoint blockade (ICB). Myeloid cells contribute to resistance by expressing signal regulatory protein-α (SIRPα), an inhibitory membrane receptor that interacts with ubiquitous receptor CD47 to control macrophage phagocytosis in the tumor microenvironment. Although CD47/SIRPα-targeting drugs have been assessed in preclinical models, the therapeutic benefit of selectively blocking SIRPα, and not SIRPγ/CD47, in humans remains unknown. We report a potent synergy between selective SIRPα blockade and ICB in increasing memory T cell responses and reverting exclusion in syngeneic and orthotopic tumor models. Selective SIRPα blockade stimulated tumor nest T cell recruitment by restoring murine and human macrophage chemokine secretion and increased anti-tumor T cell responses by promoting tumor-antigen crosspresentation by dendritic cells. However, nonselective SIRPα/SIRPγ blockade targeting CD47 impaired human T cell activation, proliferation, and endothelial transmigration. Selective SIRPα inhibition opens an attractive avenue to overcoming ICB resistance in patients with elevated myeloid cell infiltration in solid tumors.

Multiplex CRISPR/Cas9 system impairs HCMV replication by excising an essential viral gene.

Anti-HCMV treatments used in immunosuppressed patients reduce viral replication, but resistant viral strains can emerge. Moreover, these drugs do not target latently infected cells. We designed two anti-viral CRISPR/Cas9 strategies to target the UL122/123 gene, a key regulator of lytic replication and reactivation from latency. The singleplex strategy contains one gRNA to target the start codon. The multiplex strategy contains three gRNAs to excise the complete UL122/123 gene. Primary fibroblasts and U-251 MG cells were transduced with lentiviral vectors encoding Cas9 and one or three gRNAs. Both strategies induced mutations in the target gene and a concomitant reduction of immediate early (IE) protein expression in primary fibroblasts. Further detailed analysis in U-251 MG cells showed that the singleplex strategy induced 50% of indels in the viral genome, leading to a reduction in IE protein expression. The multiplex strategy excised the IE gene in 90% of all viral genomes and thus led to the inhibition of IE protein expression. Consequently, viral genome replication and late protein expression were reduced by 90%. Finally, the production of new viral particles was nearly abrogated. In conclusion, the multiplex anti-UL122/123 CRISPR/Cas9 system can target the viral genome efficiently enough to significantly prevent viral replication.

Functional Langerinhigh-Expressing Langerhans-like Cells Can Arise from CD14highCD16- Human Blood Monocytes in Serum-Free Condition.

Langerhans cells (LCs) are epithelial APCs that sense danger signals and in turn trigger specific immune responses. In steady-state, they participate in the maintenance of peripheral tolerance to self-antigens whereas under inflammation LCs efficiently trigger immune responses in secondary lymphoid organs. It has been demonstrated in mice that LC-deprived epithelia are rapidly replenished by short half-life langerin-expressing monocyte-derived LCs (MDLCs). These surrogate LCs are thought to be progressively replaced by langerin(high) LCs arising from self-renewing epithelial precursors of hematopoietic origin. How LCs arise from blood monocytes is not fully understood. Hence, we sought to characterize key factors that induce differentiation of langerin(high)-expressing monocyte-derived Langerhans-like cells. We identified GM-CSF and TGF-ß1 as key cytokines to generate langerin(high)-expressing cells but only in serum-free conditions. These cells were shown to express the LC-specific TROP-2 and Axl surface markers and contained Birbeck granules. Surprisingly, E-cadherin was not spontaneously expressed by these cells but required a direct contact with keratinocytes to be stably induced. MDLCs induced stronger allogeneic T cell proliferations but released low amounts of inflammatory cytokines upon TLR stimulation compared with donor-paired monocyte-derived dendritic cells. Immature langerin(high) MDLCs were responsive to MIP-3ß/CCL20 and CTAC/CCL27 chemokine stimulations. Finally, we demonstrated that those cells behaved as bona fide LCs when inserted in a three-dimensional rebuilt epithelium by becoming activated upon TLR or UV light stimulations. Collectively, these results prompt us to propose these langerin(high) MDLCs as a relevant model to address LC biology-related questions.

Infectious Diseases in Transplantation--Report of the 20th Nantes Actualités Transplantation Meeting.

The 20th Nantes Actualités Transplantation (NAT) meeting was held on June 11, 2015, and June 12, 2015. This year, the local organizing committee selected an update on infectious diseases in solid organ and hematopoietic stem cell transplantation. With an attendance of close to 170 clinicians, researchers, students, engineers, technicians, invited speakers, and guests from North and South America, Germany, Switzerland, Netherlands, and France, the meeting was well attended. Invited speakers' expertise covered basic as well as translational microbiology, immunology, transplantation, and intensive care medicine. This report identifies a number of advances presented during the meeting in the care and management of infectious diseases in transplantation and immunocompromised patients. New antiviral immune responses and their modulation by pathogens in addition to novel antimicrobial therapeutic strategies, cell therapies, and genomic analysis were discussed.

IL-7 receptor blockade following T cell depletion promotes long-term allograft survival.

T cell depletion is commonly used in organ transplantation for immunosuppression; however, a restoration of T cell homeostasis following depletion leads to increased memory T cells, which may promote transplant rejection. The cytokine IL-7 is important for controlling lymphopoiesis under both normal and lymphopenic conditions. Here, we investigated whether blocking IL-7 signaling with a mAb that targets IL-7 receptor α (IL-7Rα) alone or following T cell depletion confers an advantage for allograft survival in murine transplant models. We found that IL-7R blockade alone induced indefinite pancreatic islet allograft survival if anti-IL-7R treatment was started 3 weeks before graft. IL-7R blockade following anti-CD4- and anti-CD8-mediated T cell depletion markedly prolonged skin allograft survival. Furthermore, IL-7 inhibition in combination with T cell depletion synergized with either CTLA-4Ig administration or suboptimal doses of tacrolimus to induce long-term skin graft acceptance in this stringent transplant model. Together, these therapies inhibited T cell reconstitution, decreased memory T cell numbers, increased the relative frequency of Tregs, and abrogated both cellular and humoral alloimmune responses. Our data suggest that IL-7R blockade following T cell depletion has potential as a robust, immunosuppressive therapy in transplantation.

Impaired spatial memory in mice lacking CD3ζ is associated with altered NMDA and AMPA receptors signaling independent of T-cell deficiency.

The immunoreceptor-associated protein CD3ζ is known for its role in immunity and has also been implicated in neuronal development and synaptic plasticity. However, the mechanism by which CD3ζ regulates synaptic transmission remains unclear. In this study, we showed that mice lacking CD3ζ exhibited defects in spatial learning and memory as examined by the Barnes maze and object location memory tasks. Given that peripheral T cells have been shown to support cognitive functions and neural plasticity, we generated CD3ζ(-/-) mice in which the peripheral T cells were repopulated to a normal level by syngeneic bone marrow transplantation. Using this approach, we showed that T-cell replenishment in CD3ζ(-/-) mice did not restore spatial memory defects, suggesting that the cognitive deficits in CD3ζ(-/-) mice were most likely mediated through a T-cell-independent mechanism. In support of this idea, we showed that CD3ζ proteins were localized to glutamatergic postsynaptic sites, where they interacted with the NMDAR subunit GluN2A. Loss of CD3ζ in brain decreased GluN2A-PSD95 association and GluN2A synaptic localization. This effect was accompanied by a reduced interaction of GluN2A with the key NMDAR downstream signaling protein calcium/calmodulin-dependent protein kinase II (CaMKII). Using the glycine-induced, NMDA-dependent form of chemical long-term potentiation (LTP) in cultured cortical neurons, we showed that CD3ζ was required for activity-dependent CaMKII autophosphorylation and for the synaptic recruitment of the AMPAR subunit GluA1. Together, these results support the model that the procognitive function of CD3ζ may be mediated through its involvement in the NMDAR downstream signaling pathway leading to CaMKII-dependent LTP induction.

Human cytomegalovirus entry into dendritic cells occurs via a macropinocytosis-like pathway in a pH-independent and cholesterol-dependent manner.

Human cytomegalovirus (HCMV) is a ubiquitous herpesvirus that is able to infect fibroblastic, epithelial, endothelial and hematopoietic cells. Over the past ten years, several groups have provided direct evidence that dendritic cells (DCs) fully support the HCMV lytic cycle. We previously demonstrated that the C-type lectin dendritic cell-specific intercellular adhesion molecule-3-grabbing non-integrin (DC-SIGN) has a prominent role in the docking of HCMV on monocyte-derived DCs (MDDCs). The DC-SIGN/HCMV interaction was demonstrated to be a crucial and early event that substantially enhanced infection in trans, i.e., from one CMV-bearing cell to another non-infected cell (or trans-infection), and rendered susceptible cells fully permissive to HCMV infection. Nevertheless, nothing is yet known about how HCMV enters MDDCs. In this study, we demonstrated that VHL/E HCMV virions (an endothelio/dendrotropic strain) are first internalized into MDDCs by a macropinocytosis-like process in an actin- and cholesterol-dependent, but pH-independent, manner. We observed the accumulation of virions in large uncoated vesicles with endosomal features, and the virions remained as intact particles that retained infectious potential for several hours. This trans-infection property was specific to MDDCs because monocyte-derived macrophages or monocytes from the same donor were unable to allow the accumulation of and the subsequent transmission of the virus. Together, these data allowed us to delineate the early mechanisms of the internalization and entry of an endothelio/dendrotropic HCMV strain into human MDDCs and to propose that DCs can serve as a "Trojan horse" to convey CMV from entry sites to other locations that may favor the occurrence of either latency or acute infection.

Rapid deletional peripheral CD8 T cell tolerance induced by allogeneic bone marrow: role of donor class II MHC and B cells.

Mixed chimerism and donor-specific tolerance are achieved in mice receiving 3 Gy of total body irradiation and anti-CD154 mAb followed by allogeneic bone marrow (BM) transplantation. In this model, recipient CD4 cells are critically important for CD8 tolerance. To evaluate the role of CD4 cells recognizing donor MHC class II directly, we used class II-deficient donor marrow and were not able to achieve chimerism unless recipient CD8 cells were depleted, indicating that directly alloreactive CD4 cells were necessary for CD8 tolerance. To identify the MHC class II(+) donor cells promoting this tolerance, we used donor BM lacking certain cell populations or used positively selected cell populations. Neither donor CD11c(+) dendritic cells, B cells, T cells, nor donor-derived IL-10 were critical for chimerism induction. Purified donor B cells induced early chimerism and donor-specific cell-mediated lympholysis tolerance in both strain combinations tested. In contrast, positively selected CD11b(+) monocytes/myeloid cells did not induce early chimerism in either strain combination. Donor cell preparations containing B cells were able to induce early deletion of donor-reactive TCR-transgenic 2C CD8 T cells, whereas those devoid of B cells had reduced activity. Thus, induction of stable mixed chimerism depends on the expression of MHC class II on the donor marrow, but no requisite donor cell lineage was identified. Donor BM-derived B cells induced early chimerism, donor-specific cell-mediated lympholysis tolerance, and deletion of donor-reactive CD8 T cells, whereas CD11b(+) cells did not. Thus, BM-derived B cells are potent tolerogenic APCs for alloreactive CD8 cells.

Myeloid-derived suppressor cells accumulate in kidney allograft tolerance and specifically suppress effector T cell expansion.

The immune tolerance to rat kidney allografts induced by a perioperative treatment with anti-CD28 Abs is associated with a severe unresponsiveness of peripheral blood cells to donor Ags. In this model, we identified an accumulation in the blood of CD3(-)class II(-)CD11b(+)CD80/86(+) plastic-adherent cells that additionally expressed CD172a as well as other myeloid markers. These cells were able to inhibit proliferation, but not activation, of effector T cells and to induce apoptosis in a contact-dependent manner. Their suppressive action was found to be under the control of inducible NO synthase, an enzyme also up-regulated in tolerated allografts. Based on these features, these cells can be defined as myeloid-derived suppressor cells (MDSC). Interestingly, CD4(+)CD25(high)FoxP3(+) regulatory T cells were insensitive in vitro to MDSC-mediated suppression. Although the adoptive transfer of MDSC failed to induce kidney allograft tolerance in recently transplanted recipients, the maintenance of tolerance after administration of anti-CD28 Abs was found to be dependent on the action of inducible NO synthase. These results suggest that increased numbers of MDSC can inhibit alloreactive T cell proliferation in vivo and that these cells may participate in the NO-dependent maintenance phase of tolerance.

Alloreactive CD8 T cell tolerance requires recipient B cells, dendritic cells, and MHC class II.

Allogeneic bone marrow chimerism induces robust systemic tolerance to donor alloantigens. Achievement of chimerism requires avoidance of marrow rejection by pre-existing CD4 and CD8 T cells, either of which can reject fully MHC-mismatched marrow. Both barriers are overcome with a minimal regimen involving anti-CD154 and low dose (3 Gy) total body irradiation, allowing achievement of mixed chimerism and tolerance in mice. CD4 cells are required to prevent marrow rejection by CD8 cells via a novel pathway, wherein recipient CD4 cells interacting with recipient class II MHC tolerize directly alloreactive CD8 cells. We demonstrate a critical role for recipient MHC class II, B cells, and dendritic cells in a pathway culminating in deletional tolerance of peripheral alloreactive CD8 cells.

Peripheral deletional tolerance of alloreactive CD8 but not CD4 T cells is dependent on the PD-1/PD-L1 pathway.

Although interaction between programmed death-1 (PD-1) and the ligand PD-L1 has been shown to mediate CD8 cell exhaustion in the setting of chronic infection or the absence of CD4 help, a role for this pathway in attenuating early alloreactive CD8 cell responses has not been identified. We demonstrate that the PD-1/PD-L1 pathway is needed to rapidly tolerize alloreactive CD8 cells in a model that requires CD4 cells and culminates in CD8 cell deletion. This protocol involves allogeneic bone marrow transplantation (BMT) following conditioning with low-dose total body irradiation and anti-CD154 antibody. Tolerized donor-reactive T-cell receptor transgenic CD8 cells are shown to be in an abortive activation state prior to their deletion, showing early and prolonged expression of activation markers (compared with rejecting CD8 cells) while being functionally silenced by day 4 after transplantation. Although both tolerized and rejecting alloreactive CD8 cells up-regulate PD-1, CD8 cell tolerance is dependent on the PD-1/PD-L1 pathway. In contrast, CD4 cells are tolerized independently of this pathway following BMT with anti-CD154. These studies demonstrate a dichotomy between the requirements for CD4 and CD8 tolerance and identify a role for PD-1 in the rapid tolerization of an alloreactive T-cell population via a deletional mechanism.

High antigen levels do not preclude B-cell tolerance induction to alpha1,3-Gal via mixed chimerism.

BACKGROUND: Studies of bone marrow transplantation (BMT) from wild-type mice or rats to alpha1,3-galactosyltransferase (GalT) knockout mice have demonstrated that induction of mixed chimerism tolerizes not only T cells, but also natural antibody-producing B cells, even across xenogeneic barriers. Given that rodent cells express lower levels of the alphaGal epitope than the more clinically relevant porcine species, the consequences of exposure to cells expressing high levels of alphaGal on the ability to induce B-cell tolerance are unknown. METHODS: The effects on chimerism and anti-alphaGal B-cell tolerance of an i.p. injection of 10(9) porcine RBC were evaluated in GalT knockout mice receiving wild-type allogeneic BMT after non-myeloablative conditioning with T-cell depleting monoclonal antibodies, thymic irradiation, and low-dose total body irradiation. RESULTS: Achievement of mixed chimerism and tolerance of anti-alphaGal-producing B cells was not affected by exposure to high-density alphaGal at the time of BMT. The absence of induced anti-alphaGal or anti-pig antibody responses in conditioned control mice suggested that the B-cell xeno-response to pig is T-cell-dependent. CONCLUSION: High alphaGal density on pig cells might not preclude the ability to achieve tolerance of pre-existing alphaGal-reactive human B cells via induction of mixed chimerism. This strategy has the potential to induce B-cell tolerance to non-alphaGal epitopes, against which natural antibodies have been found in the sera of healthy humans.

Anti-CD28 antibodies modify regulatory mechanisms and reinforce tolerance in CD40Ig-treated heart allograft recipients.

Blockade of CD40-CD40 ligand (CD40L) costimulation has been shown to synergize with that of CTLA4/CD28-B7 to promote transplant tolerance. To date, however, CD28-B7 interactions have been prevented using B7-blocking reagents like CTLA4-Ig that inhibit CD28-B7 together with CTLA4-B7 interactions. In this study, we have tested anti-CD28 Abs to prevent selectively CD28-B7 interactions while preserving CTLA4-B7 in addition to CD40-CD40L blockade. In the LEW.1W to LEW.1A rat combination, interfering with CD40-CD40L interactions by CD40Ig administration through gene transfer resulted in indefinite heart allograft survival due to the appearance of clonotypic CD8+CD45RClow regulatory T cells that were capable of transferring the tolerant state to naive animals. However, cardiac transplants in these recipients systematically developed chronic rejection lesions. Whereas anti-CD28 Ab monotherapy only delayed acute rejection and failed to induce tolerance, coadministration of anti-CD28 Abs and CD40Ig resulted in the long-term acceptation of allografts without chronic rejection lesions in 60% of the recipients, reduced the level of intragraft mRNA transcripts for cytokines and immune factors, and fully abrogated alloantibody production. In addition, the nature of regulatory cells was modified: the CD8+CD45RClow clonotypic T cells described in the CD40Ig-treated animals could not be found in cotreated animals, and the other CD8+CD45RClow cells had no regulatory activity and a different cytokine expression profile. Instead, in cotreated recipients we found IDO-dependent non-T cells with regulatory activity in vitro. Thus, the addition of a short-term anti-CD28 treatment with CD40Ig resulted in decreased heart allograft chronic rejection lesions, complete inhibition of Ab production, and modified regulatory mechanisms.

Induction of regulatory cells and control of cellular but not vascular rejection by costimulation blockade in hamster-to-rat heart xenotransplantation.

BACKGROUND: In heart allograft in the rat, a sustained costimulation blockade with CTLA4Ig prevents alloreactive T-cell activation and promotes a long-term graft survival through the action of tolerogeneic dendritic cells. It is unclear whether similar mechanisms might occur after xenotransplantation. To test that hypothesis, we have analyzed the action of CTLA4Ig in a model of CD4(+)T cell-mediated xenograft rejection. METHODS: Hamster hearts were transplanted into LEW.1A rats receiving an accommodation-inducing treatment consisting of a short course administration of LF15-0195 and a daily administration of cyclosporine A (CSA). To achieve long-term delivery of CTLA4Ig, an intravenous administration of an adenovirus vector coding for mouse CTLA4Ig (Ad-CTLA4Ig) was added to the accommodation induction protocol. On day 40 post-transplantation, rejection was induced by CSA withdrawal. In other xenograft recipients, CD28/B7 costimulation was inhibited at that time only by injections of CTLA4Ig or anti-CD28 antibodies. Graft survival, immunohistology, as well as development of antibodies and regulatory cells were examined. RESULTS: Xenografts survived 6 days after CSA withdrawal in controls and were rejected, as previously described, through the action of CD4(+) xenoreactive T cells. Interfering with CD28/B7 costimulation inhibited this xenoreactive T cell response and delayed rejection to day 10. In recipients that had received Ad-CTLA4Ig, survival was prolonged to day 19 and this was accompanied by the appearance of regulatory cells exhibiting non-donor-specific suppressive activity dependent on IL-2, NO, and IDO. These regulatory cells were different from those previously identified after Ad-CTLA4Ig administration in heart allograft in the rat. In these recipients, rejection occurred as a consequence of an evoked anti-donor IgM response and complement activation and not of a cellular rejection as complement inhibition with cobra venom factor further prolonged xenograft survival. CONCLUSION: CD28/B7 blockade delays CD4(+) T cell-mediated rejection after CSA withdrawal in accommodated recipients of hamster heart xenografts. In addition, a sustained expression of CTLA4Ig has the potential of inducing cellular regulatory mechanisms. However, such treatment does not prevent the development of xenoreactive IgM antibodies that participate in vascular rejection processes in a complement-dependent manner.

B-cell extrinsic CR1/CR2 promotes natural antibody production and tolerance induction of anti-alphaGAL-producing B-1 cells.

B-1b cells produce IgM natural antibodies against alpha1-3Galbeta1-4GlcNAc (alphaGal). These can be tolerized by nonmyeloablative induction of mixed chimerism using alphaGal-positive (alphaGal+) donor marrow. We assessed the role of CR1/2 in this model for induction of tolerance of B-1b cells. Mixed hematopoietic chimerism was induced in alpha1-3galactosyltransferase (GalT-/-) and GalT-/-Cr2-/- mice with alphaGal+ BALB/c marrow donors. Anti-alphaGal Ab and anti-alphaGal Ab-producing B cells became undetectable in GalT-/- chimeras, whereas they persisted in chimeric GalT-/-Cr2-/- mice. To determine whether CR1/2 expression on stromal cells and/or hematopoietic cells was critical for B-1-cell tolerance, we generated GalT-/- radiation chimeras in which CR1/CR2 was expressed on either stromal cells, hematopoietic cells, neither, or both. After induction of mixed chimerism from alphaGal+ allogeneic bone marrow (BM) donors, anti-alphaGal-producing B cells were rendered tolerant in reconstituted recipients expressing only stromal CR1/CR2. Our results suggest a possible role for follicular dendritic cells that pick up immune complexes via CR1/CR2 receptors in the tolerization of B-1b cells.

Development of CD25- regulatory T cells following heart transplantation: evidence for transfer of long-term survival.

Donor-specific heart allograft acceptance can be induced in the MHC-mismatched LEW.1 W to LEW.1A rat by donor-specific transfusions. Whereas the induction phase of tolerance has been studied in detail, its maintenance remained poorly understood. Here, we performed a side-by-side comparison of CD25+ and CD25- splenic T cells of 100-day tolerant rats. Administration of CD25- T cells from tolerant rats to sublethally irradiated recipients transferred long-term graft survival. These CD25- T cells displayed a decreased donor-specific response in the mixed lymphocyte reaction and presented suppressive activity. These CD25- T cells accumulated IFN-gamma, IL-10 and Foxp3 transcripts. The in vitro suppressive activity of CD25- T cells required both cell contact and soluble factors (IL-10 and IFN-gamma). The CD25+ T cells from tolerant rats did not show any modification of their regulatory properties. We show that splenic CD25- T cells of tolerant rats contribute to the maintenance of tolerance following the transplantation. Our data show that regulatory T cells are not restricted to the CD4+ CD25+ T cell subset and provide new insights on the mechanisms of tolerance to allograft following donor cell priming.