New insights into the development of B cell responses: Implications for solid organ transplantation

Memory T follicular helper cells drive donor-specific antibodies independent of memory B cells and primary germinal center and alloantibody formation

Human leukocyte antigen antibodies are important immunologic mediators of renal allograft loss and are difficult to control. The inability to permanently eliminate donor-specific antibodies (DSA) is partly due to an incomplete understanding of the cellular mechanisms driving alloantibody formation, recurrence, and maintenance. Memory T follicular helper (mTfh) cells rapidly interact with memory B cells upon antigen re-exposure for anamnestic humoral responses, but little is known about Tfh memory in transplantation. We hypothesized that alloreactive mTfh cells form after transplantation and play a critical role in DSA formation following alloantigen re-encounter. To test this hypothesis, we utilized murine skin allograft models to identify and characterize Tfh memory and interrogate its ability to mediate alloantibody responses. We identified alloreactive Tfh memory as a mediator of accelerated humoral alloresponses independent of memory B cells and primary germinal center, or DSA, formation. Furthermore, we demonstrate that mTfh-driven alloantibody formation is susceptible to CD28 costimulation blockade. These findings provide novel insight into a pathologic role for memory Tfh in alloantibody responses and strongly support shifting therapeutic focus from the singular targeting of B cell lineage cells and alloantibodies themselves to multimodal strategies that include inhibition of mTfh cells to treat DSA.

Desensitization in Crossmatch-positive Kidney Transplant Candidates

Access to kidney transplantation is limited by HLA-specific sensitization. Desensitization strategies enable crossmatch-positive kidney transplantation. In this review, we describe clinical experience gained over the last 20 y using desensitization strategies before kidney transplantation and describe the different tools used (both drugs and apheresis options), including IVIg, rituximab, apheresis techniques, interleukin-6 interference, proteasome inhibition, enzymatic degradation of HLA antibodies, complement inhibition, and B cytokine interference. Although access to transplantation for highly sensitized kidney transplantation candidates has been vastly improved by desensitization strategies, it remains, however, limited by the recurrence of HLA antibodies after transplantation and the occurrence of antibody-mediated rejection.

Rationale for the IMAGINE study for chronic active antibody-mediated rejection (caAMR) in kidney transplantation

Chronic active antibody-mediated rejection (caAMR) in kidney transplantation is a major cause of late graft loss and despite all efforts to date, there is no proven effective therapy. Indeed, the Transplant Society (TTS) consensus opinion called for a conservative approach optimizing baseline immunosuppression and supportive care focused on blood pressure, blood glucose, and lipid control. This review provides the rationale and early evidence in kidney transplant recipients with caAMR that supported the design of the IMAGINE study whose goal is to evaluate the potential impact of targeting the IL6/IL6R pathway.

Antibody-suppressor CXCR5+ CD8+ T cellular therapy ameliorates antibody-mediated rejection following kidney transplant in CCR5 KO mice

CCR5 KO kidney transplant (KTx) recipients are extraordinarily high alloantibody producers and develop pathology that mimics human antibody-mediated rejection (AMR). C57BL/6 and CCR5 KO mice (H-2^b ) were transplanted with A/J kidneys (H-2^a ); select cohorts received adoptive cell therapy (ACT) with alloprimed CXCR5⁺ CD8⁺ T cells (or control cells) on day 5 after KTx. ACT efficacy was evaluated by measuring posttransplant alloantibody, pathology, and allograft survival. Recipients were assessed for the quantity of CXCR5⁺ CD8⁺ T cells and CD8-mediated cytotoxicity to alloprimed IgG⁺ B cells. Alloantibody titer in CCR5 KO recipients was four-fold higher than in C57BL/6 recipients. The proportion of alloprimed CXCR5⁺ CD8⁺ T cells 7 days after KTx in peripheral blood, lymph node, and spleen was substantially lower in CCR5 KO compared to C57BL/6 recipients. In vivo cytotoxicity towards alloprimed IgG⁺ B cells was also reduced six-fold in CCR5 KO recipients. ACT with alloprimed CXCR5⁺ CD8⁺ T cells (but not alloprimed CXCR5^- CD8⁺ or third-party primed CXCR5⁺ CD8⁺ T cells) substantially reduced alloantibody titer, ameliorated AMR pathology, and prolonged allograft survival. These results indicate that a deficiency in quantity and function of alloprimed CXCR5⁺ CD8⁺ T cells contributes to high alloantibody and AMR in CCR5 KO recipient mice, which can be rescued with ACT.

Romidepsin (FK228) improves the survival of allogeneic skin grafts through downregulating the production of donor-specific antibody via suppressing the IRE1α-XBP1 pathway

Antibody-mediated rejection (AMR) is one of the major causes of graft loss after transplantation. Recently, the regulation of B cell differentiation and the prevention of donor-specific antibody (DSA) production have gained increased attention in transplant research. Herein, we established a secondary allogeneic in vivo skin transplant model to study the effects of romidepsin (FK228) on DSA. The survival of grafted skins was monitored daily. The serum levels of DSA and the number of relevant immunocytes in the recipient spleens were evaluated by flow cytometry. Then, we isolated and purified B cells from B6 mouse spleens in vitro by magnetic bead sorting. The B cells were cultured with interleukin-4 (IL-4) and anti-clusters of differentiation 40 (CD40) antibody with or without FK228 treatment. The immunoglobulin G1 (IgG1) and IgM levels in the supernatant were evaluated by enzyme-linked immunosorbent assay (ELISA). Quantitative reverse transcription-polymerase chain reaction (RT-qPCR) and western blotting were conducted to determine the corresponding levels of messenger RNA (mRNA) and protein expression in cultured cells and the recipient spleens. The results showed that FK228 significantly improved the survival of allogeneic skin grafts. Moreover, FK228 inhibited DSA production in the serum along with the suppression of histone deacetylase 1 (HADC1) and HDAC2 and the upregulation of the acetylation of histones H2A and H3. It also inhibited the differentiation of B cells to plasma cells, decreased the transcription of positive regulatory domain-containing 1 (Prdm1) and X-box-binding protein 1 (Xbp1), and decreased the expression of phosphorylated inositol-requiring enzyme 1 α (p-IRE1α), XBP1, and B lymphocyte-induced maturation protein-1 (Blimp-1). In conclusion, FK228 could decrease the production of antibodies by B cells via inhibition of the IRE1α-XBP1 signaling pathway. Thus, FK228 is considered as a promising therapeutic agent for the clinical treatment of AMR.

Plasma cell biology: Foundations for targeted therapeutic development in transplantation

Solid organ transplantation is a life-saving procedure for patients with end-stage organ disease. Over the past 70 years, tremendous progress has been made in solid organ transplantation, particularly in T-cell-targeted immunosuppression and organ allocation systems. However, humoral alloimmune responses remain a major challenge to progress. Patients with preexisting antibodies to human leukocyte antigen (HLA) are at significant disadvantages in regard to receiving a well-matched organ, moreover, those who develop anti-HLA antibodies after transplantation face a significant foreshortening of renal allograft survival. Historical therapies to desensitize patients prior to transplantation or to treat posttransplant AMR have had limited effectiveness, likely because they do not significantly reduce antibody levels, as plasma cells, the source of antibody production, remain largely unaffected. Herein, we will discuss the significance of plasma cells in transplantation, aspects of their biology as potential therapeutic targets, clinical challenges in developing strategies to target plasma cells in transplantation, and lastly, novel approaches that have potential to advance the field.

Bone marrow-derived AXL tyrosine kinase promotes mitogenic crosstalk and cardiac allograft vasculopathy

Cardiac Allograft Vasculopathy (CAV) is a leading contributor to late transplant rejection. Although implicated, the mechanisms by which bone marrow-derived cells promote CAV remain unclear. Emerging evidence implicates the cell surface receptor tyrosine kinase AXL to be elevated in rejecting human allografts. AXL protein is found on multiple cell types, including bone marrow-derived myeloid cells. The causal role of AXL from this compartment and during transplant is largely unknown. This is important because AXL is a key regulator of myeloid inflammation. Utilizing experimental chimeras deficient in the bone marrow-derived Axl gene, we report that Axl antagonizes cardiac allograft survival and promotes CAV. Flow cytometric and histologic analyses of Axl-deficient transplant recipients revealed reductions in both allograft immune cell accumulation and vascular intimal thickness. Co-culture experiments designed to identify cell-intrinsic functions of Axl uncovered complementary cell-proliferative pathways by which Axl promotes CAV-associated inflammation. Specifically, Axl-deficient myeloid cells were less efficient at increasing the replication of both antigen-specific T cells and vascular smooth muscle cells (VSMCs), the latter a key hallmark of CAV. For the latter, we discovered that Axl-was required to amass the VSMC mitogen Platelet-Derived Growth Factor. Taken together, our studies reveal a new role for myeloid Axl in the progression of CAV and mitogenic crosstalk. Inhibition of AXL-protein, in combination with current standards of care, is a candidate strategy to prolong cardiac allograft survival.

Graft-derived extracellular vesicles transported across subcapsular sinus macrophages elicit B cell alloimmunity after transplantation

Despite the role of donor-specific antibodies (DSAs) in recognizing major histocompatibility complex (MHC) antigens and mediating transplant rejection, how and where recipient B cells in lymphoid tissues encounter donor MHC antigens remains unclear. Contrary to the dogma, we demonstrated here that migration of donor leukocytes out of skin or heart allografts is not necessary for B or T cell allosensitization in mice. We found that mouse skin and cardiac allografts and human skin grafts release cell-free donor MHC antigens via extracellular vesicles (EVs) that are captured by subcapsular sinus (SCS) macrophages in lymph nodes or analog macrophages in the spleen. Donor EVs were transported across the SCS macrophages, and donor MHC molecules on the EVs were recognized by alloreactive B cells. This triggered B cell activation and DSA production, which were both prevented by SCS macrophage depletion. These results reveal an unexpected role for graft-derived EVs and open venues to interfere with EV biogenesis, trafficking, or function to restrain priming or reactivation of alloreactive B cells.

What have we learned about how to prevent and treat antibody-mediated rejection in kidney transplantation?

Antibody-mediated rejection (ABMR) in kidney transplantation is a major cause of late graft loss, and despite all efforts to date the "standard of care" remains plasmapheresis, IVIg, and steroids, which itself is based on low quality evidence. This review focuses on the risk factors leading to memory and de novo donor-specific antibody (DSA)-associated ABMR, the optimal prevention strategies for ABMR, and advances in  adjunctive and emerging therapies for ABMR. Because new agents require regulatory approval via a Phase 3 randomized control trial (RCT), an overview of progress in innovative trial design for ABMR is provided. Finally, based on the insights gained in the biology of ABMR, current knowledge gaps are identified for future research that could significantly affect our understanding of how to optimally treat ABMR.