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In View: Game Changer

Transplantation Tolerance: Expanded and Selective Roles for B Cells

Cavazzoni, Cecilia B.1; Sage, Peter T.1

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doi: 10.1097/TP.0000000000003411
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Tolerance occurs when a host’s immune system recognizes nonself antigens as self in an antigen-specific manner. This process is distinct from immunosuppression, because it is exquisitely targeted and also persists for long periods of time with little intervention. Generating tolerance is thus a fundamental goal in autoimmunity and transplantation.

In solid organ transplantation, the frequency and complexity of major and minor histocompatibility antigens on the graft results in immune-mediated organ rejection. Although new generations of broad immunosuppressive drugs have had some success in treating acute antibody-mediated rejection, chronic rejection mediated by donor-specific antibody (DSA) is still a major concern. Moreover, as current immunosuppressants are nonspecific, they broadly weaken the immune system, thus augmenting the risk for infections and malignancies. Moreover, broad immunosuppression is expensive and requires frequent, as well as prolonged, administration. Therefore, tolerance induction in transplant recipients remains the “holy grail.” Although immunological tolerance has been studied for many decades, mechanisms are still mysterious. In settings of neonatal and oral tolerance, layers of classical immunoregulation combine in a unique manner to alter lymphocyte activity as a mechanism of induction.1,2 Although progress has been made, antibody-mediated immune responses remain a major hurdle in achieving tolerance induction.

Bone marrow chimerism in the context of human solid organ transplantation generates a pseudo-tolerant like state leading to graft acceptance without long-term immunosuppression.3 However, the ability to replicate this type of tolerance without inducing bone marrow chimerism has been challenging. Elucidating pathways controlling tolerance induction during solid organ transplantation have the potential to facilitate new therapeutic strategies without the need for hematopoietic chimerism.

In a recent article in the Journal of Clinical Investigation, Khiew et al from Anita Chong’s Laboratory performed an in-depth analysis of B cells after induction of transplant tolerance.4 The authors used a fully mismatched heart transplant model in which donor splenocytes were adoptively transferred to recipients receiving costimulation blockade (with anti-CD154 blocking antibody or CTLA4-Ig). Tolerance induction resulted in prolonged allograft survival (past 90 d) and an almost complete absence of DSA. Historically, anti-CD154 has shown efficacy for transplant tolerance induction, although early clinical trials were challenged by thromboembolic complications. Nevertheless, the CD40-CD40L interaction between B cells and T cells is still a relevant target for therapeutics because these molecules mediate the strongest costimulation during cognate antigen recognition leading to pathogenic antibodies. Molecules targeting both sides of the interaction (CD40 and CD40L) are currently under investigation.5 Because anti-CD154 (and CTLA4-Ig) are relevant in humans, and donor cells (splenocytes and peripheral blood mononuclear cells [PBMCs]) can be easily obtained from deceased donors, this strategy may be relevant to clinical organ transplantation.

To identify mechanisms controlling transplant tolerance, the authors used MHC tetramers identifying allospecific B cells (for MHC I and MHC II alloantigens) and performed an in-depth analysis of humoral immunity. Alloreactive B cells in this model were not deleted but were incapable of differentiating into germinal center B cells, thus eliminating the possibility that induced tolerance is due to the absence or deletion of alloreactive B cells. An innovative aspect of the study is the functional assessment of B cells isolated from tolerant animals. The authors adoptively transferred “tolerant” B cells into hosts containing B cells carrying irrelevant B-cell receptors and challenged recipients with alloantigens. Tolerant B cells were activated but did not acquire a germinal center B-cell phenotype, although their ability to induce T-follicular helper (Tfh) cell expansion was preserved. These data suggest that tolerant B cells are capable of responding to antigens but are restricted in performing full effector functions. Interestingly, the authors also showed that tolerant B cells can suppress naive B cells in an alloantigen-specific manner, while antibody responses to unrelated third-party alloantigens remained unaffected. The ability to inhibit adaptive immune responses in an antigen-specific manner is highly desired for transplantation tolerance, preventing graft rejection while maintaining an intact immunity.

Regulatory T (Treg) cells have been extensively studied, and their suppressive capacity has been utilized to induce tolerance. Moreover, it has been shown that Treg cells mediate infectious tolerance and linked suppression, rendering other nonalloreactive T cells suppressed.6-8 Although these features are advantageous for tolerance maintenance, they could also bear the risk of T-cell suppression, limiting the control of infections or malignancies. In the current work, tolerance was not “infectious” per se, but B cells prevented new B cells from responding to alloantigen.

Although kinetics of tolerant B-cell formation and their functional relevance is nicely demonstrated in the work by Khiew et al, mechanisms on how tolerant B cells suppress alloreactive naive B cells (but not third-party B cells nor Tfh cells) remain unclear. It is possible that costimulatory blockade facilitates suboptimal B-cell signaling, potentially leading to epigenetic changes that facilitate tolerance. A similar process has been shown to occur during B-cell suppression by T-follicular regulatory (Tfr) cells.9 In vitro studies suggest truncation of Tfh-mediated B-cell activation signals by Tfr cells causing epigenetic changes in B cells that result in long-lasting defects in effector function (such as class switch recombination), while Tfh-helper functions remain intact.9 Therefore, costimulation blockade during transplantation may be recapitulating a humoral immunoregulatory mechanism to induce tolerance. However, the epigenetic state in tolerant B cells was not assessed by Khiew et al.

A phase I clinical trial has been reported recently utilizing donor PBMC to induce tolerance in kidney transplant recipients. Before transplant, donor PBMC were collected, treated with mitomycin C, and transferred to transplant recipients similar to the strategy presented in Khiew et al.4 Patients in this clinical study did not generate DSA, had limited alloimmunity, and generated Breg cells; however, therapeutic benefit can only be determined with sufficiently expanded trials.10

In summary, the findings by Anita Chong’s group show a new avenue for the understanding of B-cell tolerance, alloantigen recognition, and adaptive immunity.

REFERENCES

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