Background Chronic humoral rejection (CHR) is a major complication after kidney transplantation. The cause of CHR is currently unknown. Autoantibodies have often been reported in kidney transplant recipients alongside anti-donor human leukocyte antigen antibodies. Yet, the lack of comprehensive studies has limited our understanding of this autoimmune component in the pathophysiology of CHR. Methods By using a series of ELISA and immunocytochemistry assays, we assessed the development of autoantibodies in 25 kidney transplant recipients with CHR and 25 patients with stable graft function. We also compared the reactivity of five CHR and five non-CHR patient sera with 8027 recombinant human proteins using protein microarrays. Results We observed that a majority of CHR patients, but not non-CHR control patients, had developed antibody responses to one or several autoantigens at the time of rejection. Protein microarray assays revealed a burst of autoimmunity at the time of CHR. Remarkably, microarray analysis showed minimal overlap between profiles, indicating that each CHR patient had developed autoantibodies to a unique set of antigenic targets. Conclusion The breadth of autoantibody responses, together with the absence of consensual targets, suggests that these antibody responses result from systemic B-cell deregulation.

Pre-existing serum antibodies have long been associated with graft loss in transplant candidates. While most studies have focused on HLA-specific antibodies, the contribution of non-HLA-reactive antibodies has been largely overlooked. We have recently characterized monoclonal antibodies secreted by B cell clones derived from kidney allograft recipients with rejection that selectively bind to apoptotic cells. Here, we assessed the presence of such antibodies in pre-transplant serum from 300 kidney transplant recipients and examined their contribution to the graft outcomes. Kaplan-Meier survival analysis revealed that patients with high pre-transplant IgG reactivity to apoptotic cells had a significantly increased rate of late graft loss. The effect was only apparent after approximately 1 year post-transplant. Moreover, the association between pre-transplant IgG reactivity to apoptotic cells and graft loss was still significant after excluding patients with high reactivity to HLA. This reactivity was almost exclusively mediated by IgG1 and IgG3 with complement fixing and activating properties. Overall, our findings support the view that IgG reactivity to apoptotic cells contribute to pre-sensitization. Taking these antibodies into consideration alongside anti-HLA antibodies during candidate evaluation would likely improve the transplant risk assessment.

Antibody Mediated Rejection (AMR) is associated with a variety of graft-reactive antibodies following kidney transplant. To characterize these antibodies, we immortalized 107 B cell clones from a patient with AMR. In a previous study, we showed that six clones were reacting to multiple self-antigens as well as to HLA and MICA for two of them, thus displaying a pattern of polyreactivity. We show here that all 6 polyreactive clones also reacted to apoptotic but not viable cells. More generally we observed a nearly perfect overlap between polyreactivity and reactivity to apoptotic cells. Functionally, polyreactive antibodies can activate complement, resulting in the deposition of C3d and C4d at the surface of target cells. Testing the serum of 88 kidney transplant recipients revealed a significantly higher IgG reactivity to apoptotic cells in AMR patients than in patients with stable graft function. Moreover, total IgG purified from AMR patients had increased complement activating properties compared to IgG from non-AMR patients. Overall, our studies show the development of polyreactive antibodies cross-reactive to apoptotic cells during AMR. Further studies are now warranted to determine their contribution to the detection of C4d in graft biopsies as well as their role in the pathophysiology of AMR.