Combinations of anti-LFA-1, everolimus, anti-CD40 ligand, and allogeneic bone marrow induce central transplantation tolerance through hemopoietic chimerism, including protection from chronic heart allograft rejection

Thalidomide with blockade of co-stimulatory molecules prolongs the survival of alloantigen-primed mice with cardiac allografts

Background

Miscellaneous memory cell populations that exist before organ transplantation are crucial barriers to transplantation. In the present study, we used a skin-primed heart transplantation model in mouse to evaluate the abilities of Thalidomide (TD), alone or in combination with co-stimulatory blockade, using monoclonal antibodies (mAbs) against memory T cells and alloantibodies to prolong the second cardiac survival.

Results

In the skin-primed heart transplantation model, TD combined with mAbs significantly prolonged the second cardiac survival, accompanied by inhibition of memory CD8⁺ T cells. This combined treatment enhanced the CD4⁺Foxp3⁺ regulatory T cells ratio in the spleen, restrained the infiltration of lymphocytes into the allograft, and suppressed the allo-response of spleen T cells in the recipient. The levels of allo-antibodies also decreased in the recipient serum. In addition, we detected low levels of the constitutions of the lytic machinery of cytotoxic cells, which cause allograft damage.

Conclusions

Our study indicated a potential synergistic action of TD in combination with with mAbs to suppress the function of memory T cells and increase the survival of second allografts in alloantigen-primed mice.

Blockade of adhesion molecule lymphocyte function-associated antigen-1 improves long-term heart allograft survival in mixed chimeras

BACKGROUND

The mixed chimerism approach for intentional induction of donor-specific tolerance was shown to be successful in various models from mice to humans. For transplant patients, the approach would obviate the need for long-term immunosuppression and associated side effects; moreover, it would preclude the risk of late graft loss due to chronic rejection. Widespread clinical application is hindered by toxicities related to recipient pre-conditioning. Herein we aimed to investigate a clinically relevant protocol for tolerance induction to cardiac allografts, sparing CD40 blockade or T-cell depletion.

METHODS

B6 mice were conditioned with non-myeloablative total body irradiation, fully mismatched BALB/c bone marrow cells, and short-term therapy, based on either anti- lymphocyte function-associated antigen-1 (anti-LFA-1) or anti-CD40L. Multilineage chimerism was followed by flow-cytometric analysis, tolerance was assessed with skin and heart allografts from fully or major histocompatibility complex-mismatched donors. Mechanisms of tolerance were investigated by analysis of donor-specific antibodies (DSAs), mixed lymphocyte reaction (MLR) assays, and deletion of donor-reactive T cells.

RESULTS

We found that the combination of cytotoxic T-lymphocyte antigen 4 immunoglobulin (CTLA4Ig) and rapamycin with LFA-1 blockade enhanced bone marrow engraftment and led to more efficient T-cell engraftment and subsequent tolerization. Although fully mismatched skin grafts were chronically rejected, primarily vascularized heart allografts survived indefinitely and without signs of chronic rejection, independent of minor antigen mismatches.

CONCLUSIONS

We have demonstarted a robust protocol for the induction of tolerance for cardiac allografts in the absence of CD40 blockade. Our findings demonstrate the potential of a clinically relevant minimal conditioning protocol designed to induce lifelong immunologic tolerance toward cardiac allografts.

Anti-OX40L monoclonal antibody prolongs secondary heart allograft survival based on CD40/CD40L and LFA-1/ICAM-1 blockade

BACKGROUND

Memory T cells (Tms) form a barrier against long-term allograft survival; however, CD4(+)Foxp3(+) regulatory T cells (Tregs) can suppress allograft rejection. The OX40/OX40L pathway is critical to the generation of Tms and turns off Treg suppressor function.

METHODS

B6 mice that rejected BALB/c skin grafts after 4 weeks were used as the secondary heart transplant recipients. The skin recipient mice, termed S0, S2 and S3, were treated with the isotype antibodies, anti-CD40L/LFA-1 or anti-OX40L combined with anti-CD40L/LFA-1 mAbs, respectively. The secondary heart recipients, termed H0 and H2, received anti-CD40L/LFA-1 mAbs or not, respectively (Fig. 1).

RESULTS

Four weeks after primary skin transplantation, the Tms in the S3 group that received anti-OX40L with anti-CD40L/LFA-1 mAbs were reduced compared to those in the S2 group (CD4(+) Tm: 32.61 ± 2.20% in S2 vs. 25.36 ± 1.16% in S3; CD8(+) Tm: 27.76 ± 1.96% in S2 vs. 20.95 ± 1.30% in S3; P < 0.01). Meanwhile, the proportions of Tregs in S3 increased compared to those in S2 (P < 0.05). The anti-OX40L with anti-CD40L/LFA-1 mAbs group (S3H2) prolonged the mean survival time (MST) following secondary heart transplantation from 9.5 days to 21 days (P < 0.001). Furthermore, allogeneic proliferation of recipient splenic T cells and graft-infiltrating lymphocytes were significantly inhibited in the S3H2 group. Additionally, a higher level of IL-10 was detected in sera and allografts.

CONCLUSIONS

Anti-OX40L mAb could prolong secondary heart allograft survival based on CD40/CD40L and LFA-1/ICAM-1 blockade. The mechanism of protecting allografts using anti-OX40L mAb involved impairing the generation of Tm and up-regulating IL-10 producing Tregs, inhibiting the function of T cells.

Mixed chimerism and split tolerance: mechanisms and clinical correlations

Establishing hematopoietic mixed chimerism can lead to donor-specific tolerance to transplanted organs and may eliminate the need for long-term immunosuppressive therapy, while also preventing chronic rejection. In this review, we discuss central and peripheral mechanisms of chimerism induced tolerance. However, even in the long-lasting presence of a donor organ or donor hematopoietic cells, some allogeneic tissues from the same donor can be rejected; a phenomenon known as split tolerance. With the current goal of creating mixed chimeras using clinically feasible amounts of donor bone marrow and with minimal conditioning, split tolerance may become more prevalent and its mechanisms need to be explored. Some predisposing factors that may increase the likelihood of split tolerance are immunogenicity of the graft, certain donor-recipient combinations, prior sensitization, location and type of graft and minimal conditioning chimerism induction protocols. Additionally, split tolerance may occur due to a differential susceptibility of various types of tissues to rejection. The mechanisms involved in a tissue's differential susceptibility to rejection include the presence of polymorphic tissue-specific antigens and variable sensitivity to indirect pathway effector mechanisms. Finally, we review the clinical attempts at allograft tolerance through the induction of chimerism; studies that are revealing the complex relationship between chimerism and tolerance. This relationship often displays split tolerance, and further research into its mechanisms is warranted.

Anti-LFA-1 or rapamycin overcome costimulation blockade-resistant rejection in sensitized bone marrow recipients

While costimulation blockade-based mixed chimerism protocols work well for inducing tolerance in rodents, translation to preclinical large animal/nonhuman primate models has been less successful. One recognized cause for these difficulties is the high frequency of alloreactive memory T cells (Tmem) found in the (pre)clinical setting as opposed to laboratory mice. In the present study, we therefore developed a murine bone marrow transplantation (BMT) model employing recipients harboring polyclonal donor-reactive Tmem without concomitant humoral sensitization. This model was then used to identify strategies to overcome this additional immune barrier. We found that B6 recipients that were enriched with 3 × 10(7) T cells isolated from B6 mice that had been previously grafted with Balb/c skin, rejected Balb/c BM despite costimulation blockade with anti-CD40L and CTLA4Ig (while recipients not enriched developed chimerism). Adjunctive short-term treatment of sensitized BMT recipients with rapamycin or anti-LFA-1 mAb was demonstrated to be effective in controlling Tmem in this model, leading to long-term mixed chimerism and donor-specific tolerance. Thus, rapamycin and anti-LFA-1 mAb are effective in overcoming the potent barrier that donor-reactive Tmem pose to the induction of mixed chimerism and tolerance despite costimulation blockade.

Prope tolerance to heart allografts in mice associated with persistence of donor interleukin-10-transduced stem cells

BACKGROUND

We previously reported that transduction of the human interleukin (IL)-10 gene into the total fetal liver stem cells (hIL-10-TFLs) of mice protects against their rejection in an allogeneic host. In this study, we explored the effects of these cells in two different models of organ transplantation.

METHODS

Balb/c mice were sublethally irradiated before receiving skin or vascularized heterotopic heart grafts from C57Bl/6 mice. TFLs from C57Bl/6 mice transduced with hIL-10 or untransduced TFLs were injected on the day of transplantation into recipient mice once or also every 20 days thereafter.

RESULTS

Skin allograft survival was prolonged for up to 17.8±0.6 days, vs. 9.0±0.4 days, in mice that received hIL-10-TFLs or untransduced TFLs, respectively. Allogeneic heart transplants survived for 86.25±13.8, 46.3±4.6, 28.1±6.1, or 11.5±0.6 days in mice that received repeated injections of hIL-10-TFLs, a single injection of hIL-10-TFLs, repeated injections of untransduced TFLs, or controls, respectively. Histological analyses of the grafts showed fewer inflammatory foci and CD8+ infiltrating cells in mice injected with hIL-10-TFLs compared with untreated mice. Expressions of H-2b and hIL-10 were found in several organs, including the thymus, liver, and the transplant, in hIL-10-TFL-injected mice. Finally, in hIL-10-TFL-injected mice, FoxP3 T cells were present inside the transplanted heart as late as 140 days after transplantation.

CONCLUSIONS

In this study, we showed that repeated injections of hIL-10-TFLs are efficient in mitigating transplant rejection. This "prope" tolerance was associated with survival of donor hematopoietic cells in the host.

Hematopoietic chimerism and transplantation tolerance: a role for regulatory T cells

The immunosuppressive regimens currently used in transplantation to prevent allograft destruction by the host's immune system have deleterious side effects and fail to control chronic rejection processes. Induction of donor-specific non-responsiveness (i.e., immunological tolerance) to transplants would solve these problems and would substantially ameliorate patients' quality of life. It has been proposed that bone marrow or hematopoietic stem-cell transplantation, and resulting (mixed) hematopoietic chimerism, lead to immunological tolerance to organs of the same donor. However, a careful analysis of the literature, performed here, clearly establishes that whereas hematopoietic chimerism substantially prolongs allograft survival, it does not systematically prevent chronic rejection. Moreover, the cytotoxic conditioning regimens used to achieve long-term persistence of chimerism are associated with severe side effects that appear incompatible with a routine use in the clinic. Several laboratories recently embarked on different studies to develop alternative strategies to overcome these issues. We discuss here recent advances obtained by combining regulatory T cell infusion with bone-marrow transplantation. In experimental settings, this attractive approach allows development of genuine immunological tolerance to donor tissues using clinically relevant conditioning regimens.

LFA-1 blockade induces effector and regulatory T-cell enrichment in lymph nodes and synergizes with CTLA-4Ig to inhibit effector function

Despite encouraging results using lymphocyte function antigen-1 (LFA-1) blockade to inhibit BM and solid organ transplantation rejection in nonhuman primates and humans, the precise mechanisms underlying its therapeutic potential are still poorly understood. Using a fully allogeneic murine transplantation model, we assessed the relative distribution of total lymphocyte subsets in untreated versus anti-LFA-1-treated animals. Our results demonstrated a striking loss of naive T cells from peripheral lymph nodes, a concomitant gain in blood after LFA-1 blockade, and a shift in phenotype of the cells remaining in the node to a CD62LloCD44hi profile. We determined that this change was due to a specific enrichment of activated, graft-specific effectors in the peripheral lymph nodes of anti-LFA-1-treated mice compared with untreated controls, and not to a direct effect of anti-LFA-1 on CD62L expression. LFA-1 blockade also resulted in a dramatic increase in the frequency of CD4+ FoxP3+ regulatory T cells in graft-draining nodes. Our results suggest that the differential impact of LFA-1 blockade on the distribution of naive versus effector and regulatory T cells may underlie its ability to inhibit alloreactive T-cell responses after transplantation.

Costimulatory pathways in transplantation

Secondary, so-called costimulatory, signals are critically required for the process of T cell activation. Since landmark studies defined that T cells receiving a T cell receptor signal without a costimulatory signal, are tolerized in vitro, the investigation of T cell costimulation has attracted intense interest. Early studies demonstrated that interrupting T cell costimulation allows attenuation of the alloresponse, which is particularly difficult to modulate due to the clone size of alloreactive T cells. The understanding of costimulation has since evolved substantially and now encompasses not only positive signals involved in T cell activation but also negative signals inhibiting T cell activation and promoting T cell tolerance. Costimulation blockade has been used effectively for the induction of tolerance in rodent models of transplantation, but turned out to be less potent in large animals and humans. In this overview we will discuss the evolution of the concept of T cell costimulation, the potential of 'classical' and newly identified costimulation pathways as therapeutic targets for organ transplantation as well as progress towards clinical application of the first costimulation blocking compound.

LFA-1 antagonism inhibits early infiltration of endogenous memory CD8 T cells into cardiac allografts and donor-reactive T cell priming

Alloreactive memory T cells are present in virtually all transplant recipients due to prior sensitization or heterologous immunity and mediate injury undermining graft outcome. In mouse models, endogenous memory CD8 T cells infiltrate MHC-mismatched cardiac allografts and produce IFN-γ in response to donor class I MHC within 24 h posttransplant. The current studies analyzed the efficacy of anti-LFA-1 mAb to inhibit early CD8 T cell cardiac allograft infiltration and activation. Anti-LFA-1 mAb given to C57BL/6 6 (H-2(b)) recipients of A/J (H-2(a)) heart grafts on days -1 and 0 completely inhibited CD8 T cell allograft infiltration, markedly decreased neutrophil infiltration and significantly reduced intragraft expression levels of IFN-γ-induced genes. Donor-specific T cells producing IFN-γ were at low/undetectable numbers in spleens of anti-LFA-1 mAb treated recipients until day 21. These effects combined to promote substantial prolongation (from day 8 to 27) in allograft survival. Delaying anti-LFA-1 mAb treatment until days 3 and 4 posttransplant did not inhibit early memory CD8 T cell infiltration and proliferation within the allograft. These data indicate that peritransplant anti-LFA-1 mAb inhibits early donor-reactive memory CD8 T cell allograft infiltration and inflammation suggesting an effective strategy to attenuate the negative effects of heterologous immunity in transplant recipients.

Co-stimulation blockade as a new strategy in kidney transplantation: benefits and limits

New immunosuppressive drugs have greatly decreased the frequency of graft failure due to acute rejection but have had little impact on long-term graft survival. This is due, at least in part, to the broad non-immune effects of the current immunosuppressive drugs, which are involved in the death of patients and in chronic allograft dysfunction, particularly due to their nephrotoxicity. Recent progress in the development of biologicals, i.e. antibodies and fusion proteins, allows precise targeting of the immune system, preventing the non-immune side effects encountered with current protocols. In particular, targeting of the two most important co-stimulation pathways critical for T-cell activation, i.e. B7/CD28 and CD40/CD40L, has provided excellent results in many experimental models of organ transplantation. This has led to the clinical development of belatacept, a cytotoxic T-lymphocyte-associated antigen 4 immunoglobulin (CTLA4-Ig) fusion protein, which has proved to be efficient in preventing acute rejection in kidney transplant recipients. Its use is associated with improved renal function and a better metabolic profile than calcineurin inhibitors. However, because belatacept does not selectively target alloreactive T lymphocytes and must be combined with classical immunosuppressive drugs, infectious and neoplastic complications may occur, particularly post-transplantation lymphoproliferative disorders. We also address the current development of molecules targeting other co-stimulatory pathways (CD40/CD40L, leukocyte function-associated antigen [LFA]-1/intercellular adhesion molecule [ICAM], CD2/LFA-3). Many unresolved issues regarding the use of co-stimulation blocking agents are also discussed, e.g. their long half-life, which can be problematic in cases of serious adverse events, their long-term safety and efficacy, and the lack of monitoring tools to allow modulation of their use over time.

Costimulation blockade inhibits the indirect pathway of allorecognition in nerve allograft rejection

Nerve allografts provide a temporary scaffold for host nerve regeneration. The need for systemic immunosuppression limits clinical application. Characterization of the immunological mechanisms that induce immune hyporesponsiveness may provide a basis for optimizing immunomodulating regimens. We utilized wild-type and MHC class II-deficient mice, as both recipients and donors. Host treatment consisted of triple costimulatory blockade. Quantitative assessment was made at 3 weeks using nerve histomorphometry, and muscle testing was performed on a subset of animals at 7 weeks. Nerve allograft rejection occurred as long as either the direct or indirect pathways were functional. Indirect antigen presentation appeared to be more important. Nerve allograft rejection occurs in the absence of a normal direct or indirect immune response but may be more dependent on indirect allorecognition. The indirect pathway is required to induce costimulatory blockade immune hyporesponsiveness.

Translating costimulation blockade to the clinic: lessons learned from three pathways

SUMMARY

As the recognition that costimulatory signals are critical for optimal T-cell activation, proliferation, and differentiation, there has been an explosion in the study of costimulatory molecules and their roles in enhancing anti-donor T-cell responses following transplantation. Here, we focus on the bench-to-beside translation of blocking agents designed to target three critical costimulatory pathways: the CD28/CD80/CD86 pathway, the CD154/CD40 pathway, and the lymphocyte function associated antigen-1/intercellular adhesion molecule pathway. While blockade of each of these pathways proved promising in inhibiting donor-reactive T-cell responses and promoting long-term graft survival in murine models of transplantation, the progression of development of therapeutic agents to block these pathways has each taken a slightly different course. Both logistical and biological pitfalls have accompanied the translation of blockers of all three pathways into clinically applicable therapies, and the development of costimulatory blockade as a substitute for current standard-of-care calcineurin inhibitors has by no means reached completion. Collaboration between both the basic and clinical arenas will further propel the development of costimulation blockers currently in the pipeline, as well as of novel methods to target these critical pathways during transplantation.

Differential susceptibility of allogeneic targets to indirect CD4 immunity generates split tolerance

CD4 T cells frequently help to activate CD8 T and B cells that effect transplant rejection. However, CD4 T cells alone can reject transplants, either directly or indirectly. The relative effectiveness of indirect CD4 immunity in rejecting different types of allogeneic grafts is unknown. To address this, we used a TCR transgenic mouse model in which indirect CD4 alloimmunity alone can be studied. We challenged transgenic recipients with hematopoietic cells and shortly thereafter skin transplants that could only be rejected indirectly, and observed Ag-specific indirect donor B cell and skin rejection, but not T cell elimination, reflecting a state of split tolerance. Deficiency of indirect CD4 alloimmunity in donor T cell rejection was also apparent when acute indirect rejection of donor islets occurred despite generation and maintenance of mixed T cell chimerism, due to migration of the few passenger T cells into recipient circulation. Although passenger lymphocytes delayed indirect islet rejection, they enhanced rejection by a full repertoire capable of both direct and indirect reactivity. Interestingly, the persistence of chimerism was associated with the eventual development of tolerance, as demonstrated by acceptance of donor skin grafts given late to hematopoietic cell recipients, and hyporesponsiveness of transgenic T cells from islet recipients in vitro. Mechanistically, tolerance was recessive and associated with progressive down-regulation of CD4. Collectively, our data indicate that indirect CD4 immunity is not equally destructive toward different types of allogeneic grafts, the deficiency of which generates split tolerance. The futility of these responses can convert immunity into tolerance.

Differential promotion of hematopoietic chimerism and inhibition of alloreactive T cell proliferation by combinations of anti-CD40Ligand, anti-LFA-1, everolimus, and deoxyspergualin

Allogeneic bone marrow (BM) engraftment for chimerism and transplantation tolerance may be promoted by combinations of costimulation blocking biologics and small molecular weight inhibitors. We showed previously in a mouse model that anti-CD40Ligand (anti-CD40L, CD154) combined with anti-LFA-1 or everolimus (40-O-(2-hydroxyethyl)-rapamycin) resulted in stable chimerism in almost all BM recipients, whereas anti-LFA-1 plus everolimus conferred approximately 50% chimerism stability. Here, we investigated whether this lower incidence could be increased with deoxyspergualin (DSG) in place of or in addition to everolimus. However, DSG and everolimus were similarly synergistic with costimulation blockade for stable hematopoietic chimerism. This correlated with allospecific T cell depletion and inhibition of acute but not chronic skin allograft rejection. Different treatments were also compared for their inhibition of alloreactive T cell proliferation in vivo. While anti-CD40L did not impair T cell proliferation, anti-LFA-1 reduced both CD4 and CD8 T cell proliferation, and combining anti-LFA-1 with everolimus or DSG had an additive inhibitory effect on CD4 T cell proliferation. Thus, despite their strong inhibition of alloreactive T cell proliferation, combinations of anti-LFA-1 with everolimus or DSG did not reach the unique potency of anti-CD40L-based combinations to support stable hematopoietic chimerism in this system.

Development of either split tolerance or robust tolerance along with humoral tolerance to donor and third-party alloantigens in nonmyeloablative mixed chimeras

Hematopoietic chimerism is considered to generate robust allogeneic tolerance; however, tissue rejection by chimeras can occur. This "split tolerance" can result from immunity toward tissue-specific Ags not expressed by hematopoietic cells. Known to occur in chimeric recipients of skin grafts, it has not often been reported for other donor tissues. Because chimerism is viewed as a potential approach to induce islet transplantation tolerance, we generated mixed bone marrow chimerism in the tolerance-resistant NOD mouse and tested for split tolerance. An unusual multilevel split tolerance developed in NOD chimeras, but not chimeric B6 controls. NOD chimeras demonstrated persistent T cell chimerism but rejected other donor hematopoietic cells, including B cells. NOD chimeras also showed partial donor alloreactivity. Furthermore, NOD chimeras were split tolerant to donor skin transplants and even donor islet transplants, unlike control B6 chimeras. Surprisingly, islet rejection was not a result of autoimmunity, since NOD chimeras did not reject syngeneic islets. Split tolerance was linked to non-MHC genes of the NOD genetic background and was manifested recessively in F(1) studies. Also, NOD chimeras but not B6 chimeras could generate serum alloantibodies, although at greatly reduced levels compared with nonchimeric controls. Surprisingly, the alloantibody response was sufficiently cross-reactive that chimerism-induced humoral tolerance extended to third-party cells. These data identify split tolerance, generated by a tolerance-resistant genetic background, as an important new limitation to the chimerism approach. In contrast, the possibility of humoral tolerance to multiple donors is potentially beneficial.

Modulation of T cell homeostasis and alloreactivity under continuous FTY720 exposure

The immunomodulator FTY720 inhibits lymph node (LN) and thymic egress, thereby constraining T cell circulation and reducing peripheral T cell numbers. Here, we analyzed in mouse models the as yet scarcely characterized impact of long-term (up to 6 months) FTY720 exposure on T cell homeostasis and possible consequences for alloreactivity. In green fluorescent protein (GFP) hemopoietic chimeras, the turnover of (initially GFP(-)) peripheral T cell pools was markedly delayed under FTY720, while normal homeostatic differences between CD4 and CD8 T cell sub-populations were retained or amplified further. Homeostatic proliferation was enhanced, and within shrinking T cell pools, the proportions of effector memory phenotype CD4 T cells (CD4T(PEM)) increased in spleens and LNs and of central memory phenotype CD8 T cells (CD8T(PCM)) in LNs. By contrast, the fractions of CD8T(PEM) and CD4T(PCM) remained stably small under FTY720. The enrichment for CD4T(PEM) and CD8T(PCM) correlated with larger proportions of IFNgamma-producing T cells upon nonspecific but not allospecific stimulation. Splenic CD4 T cells from FTY720-treated mice proliferated more strongly upon transfer to semi-allogeneic hosts. However, heart allograft survival was not compromised in FTY720 pre-treated recipients. It correlated with reduced intra-graft CD8 T cells, and the longest surviving transplants contained the highest numbers of CD4 T cells. Thus, continuous FTY720 exposure reveals differential homeostatic responses by memory phenotype CD4 and CD8 T cell sub-populations, and it may enhance alloreactive CD4 T cell proliferation and tissue infiltration without accelerating allograft rejection.

Inactivation of T-cell receptor-mediated integrin activation prolongs allograft survival in ADAP-deficient mice

BACKGROUND

Signaling through the T cell receptor (TCR) leads to profound changes in the function and properties of T cells, including integrin activation. Adhesion and degranulation promoting adapter protein (ADAP) is an adapter protein linking T cell receptor stimulation to integrin activation. We aim to clarify how disruption of TCR-mediated integrin activation affects alloreactive immune responses.

METHODS

In vitro T cell proliferation and the cytokine production was determined. In vivo cytotoxic T lymphocyte (CTL) activity was measured as well. Allogenic skin and heart transplantation was used to test the in vivo role of ADAP in alloimmune responses. Histology and flow cytometry was applied to analyze the graft infiltrating lymphocytes.

RESULTS

Upon stimulation with allogenic dendritic cells ADAP-deficient T cells displayed impaired proliferative responses compared to wild type (WT) T cells. This was accompanied by significantly decreased production of the cytokine interleukin-2. In contrast, the in vivo CTL activity in ADAP-deficient mice was comparable to that of WT mice. Consistently, we observed a prolongation of fully major histocompatibility complex (MHC)-mismatched heart transplants in ADAP deficient mice. Protection of allogenic heart grafts in ADAP-deficient mice was accompanied by a decrease in the infiltration, proliferation and activation of T cells in the allograft. However, no effect was observed after fully MHC-mismatched skin transplantation.

CONCLUSIONS

We have shown that although ADAP is dispensable for the rejection of allografts, ADAP function plays an important role for the efficacy of graft rejection. ADAP's main function appears to affect the induction phase of the immune response.

Non-myeloablative mixed chimerism approaches and tolerance, a split decision

Stable mixed chimerism has been considered the most robust tolerance strategy. However, rejection of solid donor tissues by chimeras has been observed, a state termed split tolerance. Since new non-myeloablative mixed chimerism approaches are being actively pursued, we sought to determine whether they lead to full tolerance or split tolerance and to define the mechanisms involved. Fully mismatched mixed chimeras generated by induction with various lymphocyte-depleting antibodies along with either low-dose irradiation or busulfan and temporary sirolimus, maintained stable mixed chimerism but nevertheless rejected donor skin grafts. Generation of stable mixed chimerism using antibody targeting CD40L, but not depleting antibodies to CD4 and CD8, could prevent split tolerance when skin grafts were given together with donor bone marrow. Minor antigen matching abrogated the ability of effector T cells to reject donor skin grafts. A CFSE killing assay indicated that chimeras were both directly and indirectly tolerant of donor hematopoietic cell antigens, suggesting that minor mismatches triggered a tissue-specific response. Thus, split tolerance due to tissue-restricted polymorphic antigens prevents full tolerance in a number of non-myeloablative mixed chimerism protocols and a 'tolerizing' agent is required to overcome split tolerance. A model of the requirements for split tolerance is presented.

Anti–LFA-1 Monotherapy Prevents Neointimal Formation in a Murine Model of Transplant Intimal Hyperplasia

BACKGROUND

Cardiac allograft vasculopathy (CAV) is the pre-eminent cause of late cardiac allograft failure. It is characterized by a concentric intimal hyperplasia, which we designate transplant intimal hyperplasia (TIH). To date, blockade of the adhesion molecule lymphocyte function-associated antigen-1 (LFA-1) has been shown to be effective in preventing TIH in experimental models of transplantation, but only when combined with other immunosuppressants. In this study we explored the impact of monotherapy against LFA-1 in a carotid artery allograft model of TIH.

METHODS

B10A(2R) (H-2(h2)) mice were used as donors and C57BL/6 (H-2(b)) mice used as recipients. The recipients were treated with a monoclonal antibody against LFA-1alpha (M17/4) or isotype-matched control immunoglobulin. Grafts were harvested after 35 days and analyzed by histomorphometry and immunohistochemistry. Blood samples were taken and analyzed by differential cell count and alloantibody levels.

RESULTS

We found that treatment with M17/4 resulted in a significant reduction in TIH compared with controls. Immunostaining revealed that LFA-1alpha blockade inhibited CD45+ leukocyte infiltration, prevented intimal smooth muscle cell (SMC) proliferation, and preserved the medial SMC population. Finally, we demonstrated a reduction in the serum alloantibody titer in the group treated with anti-LFA-1alpha when compared with controls.

CONCLUSIONS

We have demonstrated for the first time that LFA-1alpha blockade on its own can prevent development of TIH in an experimental model. The concept of modulating LFA-1alpha-mediated leukocyte migration and T-cell activation may therefore be of relevance to clinical cardiac transplantation and, as such, represents a potential target for therapeutic intervention against clinical CAV.

Hematopoietic stem cell recipients do not develop post-transplantation immune tolerance to antigens present on minimal residual disease

The immune environment present after allogeneic hematopoietic stem cell transplantation (HSCT) contributes to the control of leukemia. Our laboratory has demonstrated in a murine model that vaccination of recipients after transplantation with recipient tumor vaccines does not exacerbate graft-versus-host disease but does induce meaningful graft-versus-tumor effects. We previously demonstrated that part of the reason for the lack of graft-versus-host disease from post-transplantation vaccination is due to gradual acquisition of tolerance or unresponsiveness to recipient immunodominant minor histocompatibility antigens that are ubiquitously expressed in the recipient. However, our prior studies have not critically addressed the question of whether a similar process of acquisition of unresponsiveness to or tolerance of antigens present on minimal residual disease also occurs. The present study tested the hypothesis that unresponsiveness to antigens present on minimal residual disease present at the time of HSCT would also occur. The answer to this question would have a significant effect on the potential efficacy of post-transplantation tumor vaccines. In a murine model of major histocompatibility complex matched, minor histocompatibility antigen mismatched HSCT (C3.SW female donors and C57BL/6 female recipients), we tested whether transplant recipients would acquire unresponsiveness to antigens present on small numbers of residual leukemia/lymphoma cells. We employed a male C57BL/6 lymphoid malignancy with an immunoglobulin/c-myc oncogene in these studies using as a model of tumor-restricted antigen the well-characterized male (HY) antigen system present only on the tumor but not present as ubiquitous minor antigens in the recipient. After HSCT, recipients did not mount immune responses to the ubiquitously distributed immunodominant recipient strain H7 minor histocompatibility antigen, but did retain the capacity to mount significant T cell responses to HY antigens present on small numbers of HY+ tumor cells present at transplantation. Additional studies using small numbers of nonmalignant recipient male B cells or dendritic cells as models of minimal residual disease also demonstrated that the transplant recipients retained their capacity to mount anti-HY T cell responses. After HSCT, recipients may retain the capacity to mount effective T cell responses to antigens present on minimal residual disease and still acquire relative tolerance to ubiquitously distributed immunodominant minor antigens that are related to graft-versus-host disease.

NK cells mediate costimulation blockade-resistant rejection of allogeneic stem cells during nonmyeloablative transplantation

Although T-cell CD28/CD40 costimulation blockade represents a powerful mechanism to promote immune tolerance during murine allotransplantation, it has not yet been successfully translated to clinical transplantation. We determined the impact of natural killer (NK) cells on costimulation blockade-resistant rejection of donor bone marrow. We found that NK cells represent a potent barrier to engraftment: host NK depletion led to increased donor stem cell survival, increased mixed hematopoietic chimerism and to engraftment of low doses of donor marrow (1 x 10(8)/kg) that were otherwise rejected. To understand the mechanisms of NK alloreactivity, we employed an in vivo NK-specific cytotoxicity assay. We found that an increased proportion of target cells were killed between days 2 and 8 after cell transfer, and that NK killing of parental targets was inducible: NK cells preprimed with allotargets were more efficient at their elimination upon reexposure. Finally, both transplant and in vivo NK-killing models were used to determine the contribution of LFA-1 to NK alloreactivity. Blockade of LFA-1 led to decreased NK-mediated killing, and increased alloengraftment. These results identify NK alloreactivity as an integral component to costimulation blockade-resistant rejection, and suggest that its inhibition may represent an important target in the clinical translation of tolerance-induction transplantation.

LFA-1 (CD11a) as a therapeutic target

Leukocyte function associated antigen-1 (LFA-1) was one of the earliest of cell-surface molecules identified by monoclonal antibodies generated against leukocyte immunogens. This integrin heterodimer is perhaps best known as a classic adhesion molecule facilitating the interaction between T cells and antigen-presenting cells. However, varied studies indicate that LFA-1 has multi-faceted roles in the immune response including adhesion, activation and trafficking of leukocyte populations. While there has been long-standing interest in LFA-1 as a therapeutic target for regulating immunity, anti-LFA-1 therapy is still not a first-line indication for any clinical condition. Antagonism of LFA-1 with monoclonal antibodies, either alone or in combination with other agents, can result in regulatory tolerance in vivo. Furthermore, new generation humanized anti-LFA-1 monoclonal antibodies (Efalizumab) show at least modest promise for continued application in clinical trials. Thus, anti-LFA-1 forms a potential, but still largely unexploited, immunotherapy which may find its greatest application as an agent which augments other therapies.

Robust Tolerance to Fully Allogeneic Islet Transplants Achieved by Chimerism with Minimal Conditioning

BACKGROUND

Whether mixed chimeras induced by nonmyeloablative conditioning are tolerant to challenge with donor allogeneic islet grafts is unknown. Here we investigate whether our nonmyeloablative, costimulation blockade-free and sirolimus (SRL)-based protocol could facilitate mixed chimerism via bone marrow transplantation (BMT) and induce islet allograft tolerance.

METHODS

After low dose (1-3 Gy) total body irradiation (TBI, day -1), with or without prior lymphocyte depletion, C57BL/6 mice were transfused with 40 x 10(6) BALB/c bone marrow cells (day 0) and received SRL (3 mg/kg/day) for 4 weeks. Chimerism was monitored by flow cytometry and the recipients were rendered diabetic chemically and challenged with donor islets.

RESULTS

Mixed chimerism was achieved in mice treated with TBI 3 Gy/SRL but it declined over time in 60% (9/15) of them. Long-term stable chimerism was established in 100% of recipients over 50 weeks with either antilymphocyte serum (ALS, 9/9), anti-CD4 (4/4), or anti-CD4 plus anti-CD8 (5/5) prior to BMT. TBI conditioning could be reduced to 1 Gy, with 90% (9/10) maintaining chimerism in the long-term. When TBI was substituted with cyclophosphamide (CTX) or busulfan (BUS), all mice remained chimeric in the long-term. The chimeras showed no proliferative response to donor antigen and accepted both first and second donor-specific islet grafts indefinitely while rejecting third-party grafts.

CONCLUSIONS

This data provides the first evidence that stable fully allogeneic chimeras induced with BMT after nonmyeloablative conditioning with SRL and lymphocyte-depleting antibodies exhibit robust donor-specific tolerance to islet grafts.