B cell modulation in transplantation

Antibody-mediated rejection in xenotransplantation: Can it be prevented or reversed?

Antibody-mediated rejection (AMR) is the commonest cause of failure of a pig graft after transplantation into an immunosuppressed nonhuman primate (NHP). The incidence of AMR compared to acute cellular rejection is much higher in xenotransplantation (46% vs. 7%) than in allotransplantation (3% vs. 63%) in NHPs. Although AMR in an allograft can often be reversed, to our knowledge there is no report of its successful reversal in a pig xenograft. As there is less experience in preventing or reversing AMR in models of xenotransplantation, the results of studies in patients with allografts provide more information. These include (i) depletion or neutralization of serum anti-donor antibodies, (ii) inhibition of complement activation, (iii) therapies targeting B or plasma cells, and (iv) anti-inflammatory therapy. Depletion or neutralization of anti-pig antibody, for example, by plasmapheresis, is effective in depleting antibodies, but they recover within days. IgG-degrading enzymes do not deplete IgM. Despite the expression of human complement-regulatory proteins on the pig graft, inhibition of systemic complement activation may be necessary, particularly if AMR is to be reversed. Potential therapies include (i) inhibition of complement activation (e.g., by IVIg, C1 INH, or an anti-C5 antibody), but some complement inhibitors are not effective in NHPs, for example, eculizumab. Possible B cell-targeted therapies include (i) B cell depletion, (ii) plasma cell depletion, (iii) modulation of B cell activation, and (iv) enhancing the generation of regulatory B and/or T cells. Among anti-inflammatory agents, anti-IL6R mAb and TNF blockers are increasingly being tested in xenotransplantation models, but with no definitive evidence that they reverse AMR. Increasing attention should be directed toward testing combinations of the above therapies. We suggest that treatment with a systemic complement inhibitor is likely to be most effective, possibly combined with anti-inflammatory agents (if these are not already being administered). Ultimately, it may require further genetic engineering of the organ-source pig to resolve the problem entirely, for example, knockout or knockdown of SLA, and/or expression of PD-L1, HLA E, and/or HLA-G.

Impact of low tacrolimus exposure and high tacrolimus intra-patient variability on the development of de novo anti-HLA donor-specific antibodies in kidney transplant recipients

Introduction: De novo donor-specific antibodies (dnDSA) directed against HLA are a major contributing factor to the chronic deterioration of renal allograft function. Several factors, including the degree of HLA matching, younger recipient age, and past sensitization events have been shown to increase the risk for the development of dnDSA. The development of dnDSA is also strongly associated with modifications in the immunosuppressive regimen, non-adherence, and under-immunosuppression.Areas covered: Tacrolimus is widely used after solid organ transplantation (SOT) and in recent years, both a high intra-patient variability in tacrolimus exposure and low tacrolimus exposure have been found to be associated with a higher risk of dnDSA development in kidney transplant recipients. This article provides an overview of current findings published in the recent 5 years regarding the relationship between tacrolimus exposure and variation therein and the development of dnDSA.Expert opinion: In this review, we describe how combining data on tacrolimus intra-patient variability and mean pre-dose concentration may be an effective tool to identify kidney transplant recipients who are at higher risk of developing dnDSA.

Strategies for long-term preservation of kidney graft function

Kidney transplantation has become a routine procedure in the treatment of patients with kidney failure, and requires collaboration of experts from different disciplines, such as nephrology, surgery, immunology, pathology, infectious disease medicine, cardiology, and oncology. Grafts can be obtained from deceased or living donors, with different logistical requirements and implications for long-term graft patency. 1-year graft survival rates are greater than 95% in many centres but improvement of long-term function remains a challenge. New developments in molecular immunology and computational biology have increased precision of donor and recipient matching of HLA and non-HLA compatibility. Individual omics-wide molecular diagnostics, extracorporeal therapies, and drug developments allow for precise individual decision making and treatment. Tolerance induction by mixed chimerism without toxic conditioning and with a low risk of graft versus host disease is a visionary but realistic goal. Some of these innovations are already used in modern transplant centres and will allow advancement in long-term allograft preservation.

mTOR Inhibition Suppresses Posttransplant Alloantibody Production Through Direct Inhibition of Alloprimed B Cells and Sparing of CD8+ Antibody-Suppressing T cells

BACKGROUND

De novo alloantibodies (donor-specific antibody) contribute to antibody-mediated rejection and poor long-term graft survival. Because the development of donor-specific antibody is associated with early graft loss of cell transplants and reduced long-term survival of solid organ transplants, we hypothesized that conventional immunosuppressives, calcineurin inhibitors (CNi), and mammalian target of rapamycin inhibitors (mTORi), may not be as effective for suppression of humoral alloimmunity as for cell-mediated immunity.

METHODS

Wild-type or CD8-depleted mice were transplanted with allogeneic hepatocytes. Recipients were treated with mTORi and/or CNi and serially monitored for alloantibody and graft survival. The direct effect of mTORi and CNi on alloprimed B cell function was investigated in Rag1 mice adoptively transferred with alloprimed IgG1 B cells. The efficacy of mTORi and/or CNi to suppress CD8-mediated cytotoxicity of IgG1 B cells was evaluated in in vitro and in vivo cytotoxicity assays.

RESULTS

Mammalian target of rapamycin inhibitors, but not CNi, reduced alloantibody production in transplant recipients, directly suppressed alloantibody production by alloprimed IgG1 B cells and delayed graft rejection in both low and high alloantibody producers. Combination treatment with mTORi and CNi resulted in loss of the inhibitory effect observed for mTORi monotherapy in part due to CNi suppression of CD8 T cells which downregulate alloantibody production (CD8 TAb-supp cells).

CONCLUSIONS

Our data support that mTORi is a potent inhibitor of humoral immunity through suppression of alloprimed B cells and preservation of CD8 TAb-supp cells. In contrast, alloantibody is readily detected in CNi-treated recipients because CNi does not suppress alloprimed B cells and interferes with downregulatory CD8 TAb-supp cells.