Selective CD28 Blockade Results in Superior Inhibition of Donor-Specific T Follicular Helper Cell and Antibody Responses Relative to CTLA4-Ig

Donor-specific antibodies (DSAs) are a barrier to improved long-term outcomes after kidney transplantation. Costimulation blockade with CTLA4-Ig has shown promise as a potential therapeutic strategy to control DSAs. T follicular helper (Tfh) cells, a subset of CD4⁺ T cells required for optimal antibody production, are reliant on the CD28 costimulatory pathway. We have previously shown that selective CD28 blockade leads to superior allograft survival through improved control of CD8⁺ T cells relative to CTLA4-Ig, but the impact of CD28-specific blockade on CD4⁺ Tfh cells is unknown. Thus, we identified and characterized donor-reactive Tfh cells in a murine skin transplant model and then used this model to evaluate the impact of selective CD28 blockade with an anti-CD28 domain antibody (dAb) on the donor-specific Tfh cell-mediated immune response. We observed that the anti-CD28 dAb led to superior inhibition of donor-reactive CXCR5⁺ PD-1^high Tfh cells, CD95⁺ GL7⁺ germinal center B cells and DSA formation compared with CTLA4-Ig. Interestingly, donor-reactive Tfh cells differentially upregulated CTLA4 expression, suggesting an important role for CTLA4 in mediating the superior inhibition observed with the anti-CD28 dAb. Therefore, selective CD28 blockade as a novel approach to control Tfh cell responses and prevent DSA after kidney transplantation warrants further study.

Pathogen Stimulation History Impacts Donor-Specific CD8(+) T Cell Susceptibility to Costimulation/Integrin Blockade-Based Therapy

Recent studies have shown that the quantity of donor-reactive memory T cells is an important factor in determining the relative heterologous immunity barrier posed during transplantation. Here, we hypothesized that the quality of T cell memory also potently influences the response to costimulation blockade-based immunosuppression. Using a murine skin graft model of CD8(+) memory T cell-mediated costimulation blockade resistance, we elicited donor-reactive memory T cells using three distinct types of pathogen infections. Strikingly, we observed differential efficacy of a costimulation and integrin blockade regimen based on the type of pathogen used to elicit the donor-reactive memory T cell response. Intriguingly, the most immunosuppression-sensitive memory T cell populations were composed primarily of central memory cells that possessed greater recall potential, exhibited a less differentiated phenotype, and contained more multi-cytokine producers. These data, therefore, demonstrate that the memory T cell barrier is dependent on the specific type of pathogen infection via which the donor-reactive memory T cells are elicited, and suggest that the immune stimulation history of a given transplant patient may profoundly influence the relative barrier posed by heterologous immunity during transplantation.

Rapamycin ameliorates the CTLA4-Ig-mediated defect in CD8(+) T cell immunity during gammaherpesvirus infection

Latent viral infections are a major concern among immunosuppressed transplant patients. During clinical trials with belatacept, a CTLA4-Ig fusion protein, patients showed an increased risk of Epstein-Barr virus-associated posttransplant lymphoproliferative disorder, thought to be due to a deficient primary CD8(+) T cell response to the virus. Using a murine model of latent viral infection, we observed that rapamycin treatment alone led to a significant increase in virus-specific CD8(+) T cells, as well as increased functionality of these cells, including the ability to make multiple cytokines, while CTLA4-Ig treatment alone significantly dampened the response and inhibited the generation of polyfunctional antigen-specific CD8(+) T cells. However, the addition of rapamycin to the CTLA4-Ig regimen was able to quantitatively and qualitatively restore the antigen-specific CD8(+) T cell response to the virus. This improvement was physiologically relevant, in that CTLA4-Ig treated animals exhibited a greater viral burden following infection that was reduced to levels observed in untreated immunocompetent animals by the addition of rapamycin. These results reveal that modulation of T cell differentiation though inhibition of mTOR signaling can restore virus-specific immune competence even in the absence of CD28 costimulation, and have implications for improving protective immunity in transplant recipients.

An anti-CD154 domain antibody prolongs graft survival and induces Foxp3(+) iTreg in the absence and presence of CTLA-4 Ig

The use of monoclonal antibodies targeting the CD154 molecule remains one of the most effective means of promoting graft tolerance in animal models, but thromboembolic complications during early clinical trials have precluded their use in humans. Furthermore, the role of Fc-mediated deletion of CD154-expressing cells in the observed efficacy of these reagents remains controversial. Therefore, determining the requirements for anti-CD154-induced tolerance will instruct the development of safer but equally efficacious treatments. To investigate the mechanisms of action of anti-CD154 therapy, two alternative means of targeting the CD40-CD154 pathway were used: a nonagonistic anti-CD40 antibody and an Fc-silent anti-CD154 domain antibody. We compared these therapies to an Fc-intact anti-CD154 antibody in both a fully allogeneic model and a surrogate minor antigen model in which the fate of alloreactive cells could be tracked. Results indicated that anti-CD40 mAbs as well as Fc-silent anti-CD154 domain antibodies were equivalent to Fc-intact anti-CD154 mAbs in their ability to inhibit alloreactive T cell expansion, attenuate cytokine production of antigen-specific T cells and promote the conversion of Foxp3(+) iTreg. Importantly, iTreg conversion observed with Fc-silent anti-CD154 domain antibodies was preserved in the presence of CTLA4-Ig, suggesting that this therapy is a promising candidate for translation to clinical use.

Integrin antagonists prevent costimulatory blockade-resistant transplant rejection by CD8(+) memory T cells

The success of belatacept in late-stage clinical trials inaugurates the arrival of a new class of immunosuppressants based on costimulatory blockade, an immunosuppression strategy that disrupts essential signals required for alloreactive T-cell activation. Despite having improved renal function, kidney transplant recipients treated with belatacept experienced increased rates of acute rejection. This finding has renewed focus on costimulatory blockade-resistant rejection and specifically the role of alloreactive memory T cells in mediating this resistance. To study the mechanisms of costimulatory blockade-resistant rejection and enhance the clinical efficacy of costimulatory blockade, we developed an experimental transplant system that models a donor-specific memory CD8(+) T-cell response. After confirming that graft-specific memory T cells mediate costimulatory blockade-resistant rejection, we characterized the role of integrins in this rejection. The resistance of memory T cells to costimulatory blockade was abrogated when costimulatory blockade was coupled with either anti-VLA-4 or anti-LFA-1. Mechanistic studies revealed that in the presence of costimulatory blockade, anti-VLA-4 impaired T-cell trafficking to the graft but not memory T-cell recall effector function, whereas anti-LFA-1 attenuated both trafficking and memory recall effector function. As antagonists against these integrins are clinically approved, these findings may have significant translational potential for future clinical transplant trials.

Antigenic disparity impacts outcome of agonism but not blockade of costimulatory pathways in experimental transplant models

Treatment regimens consisting of CTLA-4 Ig/anti-CD154 or agonistic anti-CD28/rapamycin have both been shown to prevent GVHD in fully allogeneic murine model systems. Using a transgenic approach to track the fate of host-reactive T cells in a minor antigen disparity model of GVHD, we found that while treatment with CTLA-4 Ig/anti-CD154 retained efficacy, the costimulation agonist anti-CD28 combined with rapamycin failed to prevent GVHD. Analysis of the host-reactive CD4(+) and CD8(+) T-cell responses revealed that in contrast to CTLA-4Ig/anti-CD154-treated recipients, host-reactive T cells in recipients treated with agonistic anti-CD28/rapamycin displayed enhanced and accelerated T-cell proliferation and failed to undergo activation-induced cell death. An increase in systemic levels of inflammatory cytokines was observed in the anti-CD28/rapamycin-treated recipients of the minor but not major antigen disparity bone marrow transplants. Our results demonstrate the intricacies of costimulatory receptor signaling pathways, and reveal how agonism of the CD28 pathway can have opposing outcomes depending on the degree of antigenic disparity.

Failure to induce neonatal tolerance in mice that lack both IL-4 and IL-13 but not in those that lack IL-4 alone

Current evidence suggests that neonatal tolerance to a foreign Ag is the consequence of IL-4-mediated Th2 immunity rather than the thymic deletion of Ag-specific T cells. Here, we addressed the role of IL-4 in neonatal tolerance by testing whether tolerance to a minor histocompatibility Ag can be induced in newborn mice that lack IL-4 (IL-4(-/-)). We found that IL-4 does not play a dominant role in the induction of neonatal tolerance as newborn female IL-4(-/-) mice could be readily tolerized to the H-Y male Ag. In contrast, mice that lack both IL-4 and IL-13 (IL-4(-/-)/IL-13(-/-)) were resistant to the induction of neonatal tolerance, and their splenocytes produced exaggerated amounts of IFN-gamma on rechallenge with the same Ag encountered during the neonatal period. These findings argue against the view that IL-4 alone is critical for the induction of neonatal tolerance and suggest that the combined actions of both IL-4 and IL-13 are essential for this process.

Alloantigen-driven T cell death mediated by Fas ligand and tumor necrosis factor-alpha is not essential for the induction of allograft acceptance

BACKGROUND

Fas ligand (FasL)-Fas and tumor necrosis factor alpha (TNFalpha)-tumor necrosis factor receptor (TNFR) interactions regulate immune responses and contribute to self-tolerance by mediating antigen-driven T cell apoptosis. It is not known whether FasL and TNFalpha, expressed by the recipient's lymphoid or nonlymphoid cells, are essential for the apoptosis of alloreactive T lymphocytes and the induction of allograft acceptance.

METHODS

We compared the survival of fully allogeneic vascularized cardiac allografts between wild-type (wt) and FasL-mutant (gld) recipient mice. In addition, we studied cardiac allograft survival in gld mice injected with TNFalpha-neutralizing antibody. Allograft acceptance (graft survival >100 days) was induced by treating the recipients with CTLA4Ig, a recombinant fusion protein that blocks B7-CD28 T cell costimulation. In vivo alloantigen-driven apoptosis of mature CD4+ and CD8+ T lymphocytes was analyzed in mice repeatedly stimulated with allogeneic splenocytes.

RESULTS

We found that CTLA4Ig induces 100% long-term acceptance of cardiac allografts in wt and gld mice. Similarly, CTLA4Ig induced 100% allograft acceptance in gld recipients injected with TNFalpha-neutralizing antibody. In vivo alloantigen-driven apoptosis of mature CD4+ and CD8+ T cells was significantly reduced in gld mice and in wt mice treated with anti-TNFalpha antibody. However, neutralizing TNFalpha activity in gld mice failed to abrogate alloantigen-driven T cell apoptosis.

CONCLUSIONS

These data indicate that: (1) FasL and TNFalpha expression are not obligatory for the induction of long-term allograft acceptance by CTLA4Ig and (2) FasL- and TNFalpha-independent death pathways contribute to alloantigen-driven T cell apoptosis.

Emery-Dreifuss muscular dystrophy

Emery-Dreifuss muscular dystrophy (EDMD) is the third most common X-linked muscular dystrophy. This disorder is characterized by childhood onset of early contractures, humeroperoneal muscle atrophy, and cardiac conduction abnormalities. Weakness is slowly progressive, but there is a broad spectrum of clinical severity. Patients and carriers are at risk of sudden death. Regular cardiac evaluation is mandatory to assess the risk of cardiac arrhythmias. Unique atrial pathology is seen at autopsy. The mutated gene in EDMD is localized to the long arm of the X chromosome. Mutations in the gene lead to abolished synthesis of the gene product, emerin. Emerin is localized to the nuclear membrane of skeletal, cardiac, and smooth muscle. The term Emery-Dreifuss syndrome describes patients who have the EDMD phenotype without X-linked inheritance. There is no treatment for the underlying disease, but early placement of pacemakers may be lifesaving.