Donor-specific B cells in Transplantation

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Abstract Kidney transplantation elicits powerful B cell responses that generate antibodies specific for the transplant donor. B cell responses to allogeneic HLA (panel reactive antibodies, PRA) and to the transplant donor (donor- specific antibodies, DSA) correlate with risk of rejection and graft loss and thus serve as diagnostic indices of allo-immunity. B cell responses that generate DSA initiate the most vexing types of rejection (antibody- mediated rejection, ABMR) and chronic ABMR and hence much interest exists in development of therapeutics that specifically target B cells. Some B cell responses however may correlate with or even promote graft survival (immune regulation, accommodation, operational tolerance). Although there has been extensive investigation of the specificity and function of the blood DSA hardly anything is known about the properties of donor-specific B cells. The research we propose will identify for the first time key characteristics of donor-specific B cell responses in kidney transplant recipients to determine what properties associate with and potentially contribute to stable graft function, as opposed to rejection. As a first objective, donor-specific B cell responses will be studied by determining Heavy and Light chain Ig sequences of donor- specific B cells by single cell sequencing and their phenotypic signature by RNA-seq . We will determine key properties of Ig, such as clonal diversity, extent of somatic hypermutation or Ig independent properties such as production of cytokines in recipients with stable function or rejection. As a second objective, donor-specific Ig sequences obtained by NGS will be studied prospectively over periods > a year (up to two years). We will determine whether clonotypes expand and diversify in anticipation of rejection as would be expected of T cell dependent responses; and how donor-specific sequences change following rejection treatment. We will also determine if in recipients with stable grafts, donor-specific B cells encode Igs with distinct properties. As a third objective, we will conduct the first investigation of key properties (e.g. maximum binding, avidity, direct activation of donor cells and antigen specificity, where possible, confirmed using graft tissues) of recombinant monoclonal DSA on donor cells to determine whether these properties could explain differences in pathogenicity. The research might reveal novel approaches to assessing risk and new targets for biological intervention. The research might also inspire rational approaches to modifying B cell responses in transplant recipients in a personalized manner with the goal of promoting protective responses and antagonizing pathogenic ones.