Marginal Zone B Cells Are Necessary for the Formation of Anti-donor IgG After Allogeneic Sensitization

Abstract

Background. The formation of anti–major histocompatibility complex (MHC) antibodies is a significant barrier for many patients awaiting organ transplantation. Patients with preformed anti-MHC antibodies have limited options for suitable donors, and the formation of donor-specific anti-MHC antibodies after transplantation is a harbinger of graft rejection. Despite the recognized importance of anti-MHC antibodies, the mechanisms responsible for the differentiation of B cells after exposure to allogeneic antigens are poorly understood. Methods. To evaluate the differentiation of B cells in response to allogeneic antigen, we used a model of H-2b C57Bl/6 sensitization with H-2d antigen. We used a class I MHC tetramer-based approach to identify allogeneic B cells and flow cytometric crossmatch to identify allogeneic IgM and IgG. Results. We found that although the formation of anti-H-2d IgG was robust, few class-switched B cells and germinal center B cells were formed. Antigen-specific B cells did not express classical memory B-cell markers after sensitization but had an IgM+CD21+ marginal zone B-cell phenotype. The frequency of marginal zone B cells increased after sensitization. Depletion of marginal zone B cells before sensitization or skin grafting resulted in a significant diminution of anti-H-2d IgG and fewer germinal center B cells. Adoptive transfer experiments revealed that marginal zone B cells more efficiently differentiated into germinal center B cells and anti-donor IgG-producing cells than follicular B cells. Conclusions. These results demonstrate an important role for marginal zone B cells as a reservoir of alloreactive B cells that are activated by allogeneic antigens.