Chronic allograft vasculopathy (CAV) in murine heart allografts can be elicited by adoptive transfer of donor specific antibody (DSA) to class I MHC antigens and is independent of complement. Here we address the mechanism by which DSA causes CAV. B6.RAG1−/− or B6.RAG1−/−C3−/− (H-2b) mice received B10.BR (H-2k) heart allografts and repeated doses of IgG2a, IgG1 or F(ab’)2 fragments of IgG2a DSA (anti-H-2k). Intact DSA regularly elicited markedly stenotic CAV in recipients over 28 days. In contrast, depletion of NK cells with anti-NK1.1 reduced significantly DSA-induced CAV, as judged morphometrically. Recipients genetically deficient in mature NK cells (γ-chain knock out) also showed decreased severity of DSA-induced CAV. Direct NK reactivity to the graft was not necessary. F(ab’)2 DSA fragments, even at doses twofold higher than intact DSA, were inactive. Graft microvascular endothelial cells responded to DSA in vivo by increased expression of phospho-extracellular signal-regulated kinase (pERK), a response not elicited by F(ab’)2 DSA. We conclude that antibody mediates CAV through NK cells, by an Fc dependent manner. This new pathway adds to the possible mechanisms of chronic rejection and may relate to the recently described C4d-negative chronic antibody-mediated rejection in humans.

Complement fixation, as evidenced by C4d in the microvasculature, is a widely accepted criterion of antibody-mediated rejection. Complement fixation has been shown to be essential in acute antibody-mediated rejection, but its role in chronic rejection has not been addressed. Previous studies showed that passive transfer of complement fixing monoclonal IgG2a anti-H-2Kk into B6.RAG1−/− KO recipients of B10.BR hearts led to progressive chronic transplant arteriopathy (CTA) over 14–28 days, accompanied by C4d deposition. The present studies were designed to test whether complement was required for these lesions. We report that a noncomplement fixing donor-specific alloantibody (DSA, monoclonal IgG1 anti-H-2Kk) injected into B6.RAG1−/− KO recipients of B10.BR hearts also promotes CTA, without C4d deposition. Furthermore, a passive transfer of DSA (monoclonal IgG2a anti-H-2Kk) initiated endarteritis followed by CTA in B6.RAG1−/− mice genetically deficient in the third component of complement (RAG1−/−C3−/−). These studies indicate that antibody to class I MHC antigens can trigger chronic arterial lesions in vivo without complement participation, in contrast to acute antibody-mediated rejection. This pathway may be relevant to C4d-negative chronic rejection sometimes observed in patients with DSA, and argues that lack of C4d deposition does not exclude antibody-mediated chronic rejection.

Despite extensive research on T cells and potent immunosuppressive regimens that target cellular mediated rejection, few regimens have been proved to be effective on antibody-mediated rejection (AMR), particularly in the chronic setting. C4d deposition in the graft has been proved to be a useful marker for AMR; however, there is an imperfect association between C4d and AMR. While complement has been considered as the main player in acute AMR, the effector mechanisms in chronic AMR are still debated. Recent studies support the role of NK cells and direct effects of antibody on endothelium cells in a mechanism suggesting the presence of a complement-independent pathway. Here, we review the history, currently available systems and progress in experimental animal research. Although there are consistent findings from human and animal research, transposing the experimental results from rodent to human has been hampered by the differences in endothelial functions between species. We briefly describe the findings from patients and compare them with results from animals, to propose a combined perspective.

Monoclonal antibody against the CD45RB protein induces stable transplantation tolerance to multiple types of allograft. We have previously established that this tolerance protocol relies on the regulatory function of B lymphocytes for its effect. B lymphocytes have also been reported to participate in immune regulation in several other settings. In most of these systems, the regulatory function of B lymphocytes depends on the production of IL-10. Therefore, we investigated the role of IL-10 in the anti-CD45RB model of B-cell-mediated transplantation tolerance. Surprisingly, using antibody-mediated neutralization of IL-10, IL-10-deficient recipients and adoptive transfer of IL-10-deficient B lymphocytes, we found that IL-10 actually counter-regulates tolerance induced by anti-CD45RB. Furthermore, neutralization of IL-10 reduced the development of chronic allograft vasculopathy compared to anti-CD45RB alone and reduced the production of graft reactive alloantibodies. These data suggest that the participation of regulatory B lymphocytes in transplantation tolerance may be distinct from how they operate in other systems. Identifying the specific B lymphocytes that mediate transplantation tolerance and defining their mechanism of action may yield new insights into the complex cellular network through which antigen-specific tolerance is established and maintained.

†Authors contributed equally to these studies. These data show that a viral infection in its native host, and not previously implicated in the production of CAV, can contribute to the development of advanced coronary vascular lesions in cardiac allotransplants in mice. These data also suggest that virus-induced CAV can develop via an NK-cell-dependent pathway in the absence of T-and B-lymphocytes.

B cells have many possible mechanisms by which they can affect allograft survival, including antigen presentation, cytokine production, immune regulation, and differentiation into alloantibody-producing plasma cells. This report reviews the last mechanism, which the authors regard as most critical for the long-term survival of allografts, namely, the promotion of chronic rejection by alloantibodies. Chronic humoral rejection characteristically arises late after transplantation and causes transplant glomerulopathy, multilamination of peritubular capillary basement membranes, and C4d deposition in PTCs and glomeruli. Circulating antidonor human leukocyte antigen class II antibodies are commonly detected and may precede the development of graft injury. Prognosis is poor, especially when recognized after graft dysfunction has developed. Improved detection and treatment are critically needed for this common cause of late graft loss.

Background Most studies of vascular disease in transplanted organs have used combinations involving disparities determined by genes of the major histocompatibility complex (MHC). This report describes examples of coronary vascular disease occurring in transplanted mouse hearts involving isolated, non-H2-determined incompatibilities. Methods Mice, incompatible in respect of HY, H4, or H60, were selected. For H60, the incompatibility depended on breeding congenic pairs or the introduction of H60 by transgenic methods because the latter method results in more widespread expression. Transplant survival was determined, and the appearance and prevalence of coronary artery vasculopathy (CAV) was established by appropriate histologic methods. Results Advanced changes of CAV were found at 56 days in transplants involving incompatibilities confined to HY or H4. In both combinations, skin grafts were also rejected. H60 incompatibility does not result in skin graft rejection and only a minority of heart transplants shows evidence of CAV. If heart transplants are preceded by skin grafts bearing both H60 and HY incompatibilities to promote “help” in generating immunity, H60 incompatible hearts develop advanced CAV. Heart transplants in all non-MHC categories ostensibly survive in excellent condition throughout this period despite their CAV. Conclusions CAV can develop as a consequence of non-MHC incompatibilities alone and even when antigens are sparsely expressed on cardiac tissue. Presensitization leads to much more severe vascular disease. Human leukocyte antigen compatible kidney transplants may also develop vascular disease and patients manifesting reactivity to MHC antigens should also be more prone to develop vascular disease because of undetectable non-MHC incompatibilities.

Background A role for NK cells in cardiac allograft vasculopathy (CAV) was suggested by our earlier observation that CAV arises even in the absence of detectable anti-donor T or B cell reactivity in parental to F1 mouse heart grafts. However, prevention of CAV in this setting required the depletion of both NK and CD4+ T cells. Methods To clarify the interrelationship between NK and CD4+ cells we analyzed early events and selective depletion of T regulatory cells (Tregs). Hearts from C57BL/6 (B6) donors were transplanted heterotopically into BALB/c × C57BL/6 (CB6F1) recipients and NK cells, CD4+ T cells, and/or Tregs were depleted with anti-NK1.1 (PK136), anti-CD4 (GK1.5) or anti-CD25 (PC61), respectively. Results In contrast to prior studies in which the prevention of CAV at 8 weeks required the co-depletion of NK and CD4+ T cells, NK cells depletion alone eliminated CAV at 3 weeks. Furthermore, depletion of CD25+ cells accelerated the onset and maturation of CAV at both 2 and 3 weeks (p<0.02 and p<0.001, respectively). However, anti-NK1.1 treatment prevented lesions in CD25-depleted recipients. Finally, CD4+ T cell depletion alone did not prevent or accelerate development of CAV but inhibited the effect of CD25+ T cell depletion. Conclusion These data suggest that NK cells can play and important role in the early pathogenesis of CAV but that their ability to mediate early CAV can be modulated by Tregs.