B cells and antibody in the development of long-term cardiac graft rejection

Abstract

The long-term survival of heart transplants is limited by the development of allograft vasculopathy (AV), a vascular pathology that develops in spite of the use of modern immunosuppressive therapies. Although it is widely accepted that T cells play a major role in the development of AV, the contribution of B cells and antibody has been less well characterized. A fully MHC-mismatched cell transfer model was used to mimic the antigenic stimulus of a cardiac graft, we examined the production of antibody under conditions of clinically relevant immunosuppression in the form of the calcineurin inhibitor cyclosporine A (CyA). Anti-donor antibody with the capacity to mediate complement-dependent cytotoxicity of donor strain cells, but not third-party cells, developed in the presence of two different doses of CyA (30 mg/kg and 50 mg/kg). When this antibody was passively transferred into immunodeficient B6.RAG1-/- abdominal aortic graft recipients, the antibody alone had the capacity to mediate formation of a neointimal lesion and induce the loss of medial smooth muscle cells. These are two hallmark characteristics of AV in this animal model. A wild-type model, where BALB/c grafts were transplanted into B6 recipients and received daily CyA immunosuppression was used to test the de novo antibody response to the transplant itself. Again, anti-donor antibody was produced with the capacity to mediate complement-dependent cytotoxicity of donor cells. In addition, grafts showed evidence of C4d deposition in the medial area, indicating that area as a sit of antibody binding and activation of the classical complement cascade. The presence of anti-donor antibody has been demonstrated to correlate with poorer graft outcome and a higher risk of developing AV in patients. Examination of human epicardial coronary artery tissue from patients with cardiac transplants demonstrated the presence in the adventitia of ectopic lymphoid structures (ELS) containing CD20+ B cells, plasma cells, IgM, and IgG. These findings illustrate active, antibody-producing ELS in close proximity to the vessels developing AV. Of note was the finding of CD20+CD27+ memory B cells in these ectopic lymphoid structures. Memory B cells are rapidly re-activated following exposure to their cognate antigen and easily differentiate into plasma cells. Taken together, these data suggest that memory B cells and antibody may be contributing to long-term allograft rejection and therapeutic options should be considered to target these immune mechanisms.