Humoral Response to Carbohydrate Antigens in the Context of ABO-Incompatible Transplantation and Xenotransplantation

Antibody-mediated rejection is central to ABO incompatible transplantation as well as to xenotransplantation. The xenoantigen alpha-Gal has a highly analogous carbohydrate structure to the human blood group antigens, and both require memory B cell activation for antibody production. We hypothesize that B cells, reactive to the alpha-Gal xenoantigen and B blood group antigen, require the presence of fully activated T cells in order to survive and proliferate in vitro, contrary to the traditional theory that humoral response to carbohydrate antigens is a T cell-independent process. When we compared the capacity of B cells to proliferate, we observed that activated T cells were necessary for B cell proliferation even in the presence of carbohydrate-derived antigens. A relevant question was also to investigate the role of a specific class of T cells: the CD1d-restricted iNKT cells, in the activation of alpha-Gal and B blood group-reactive B cells. The iNKT cells have the specificity of being reactive to glycolipids and are capable of producing both T helper 1 and T helper 2 cytokine responses. We therefore wanted to determine the role of the iNKT cells as mediators of a T helper 2-type response when B cells were exposed to a glycolipid antigen expressing the alpha-Gal epitope or the human B blood group antigen. We observed that, if the interaction between B cells and iNKT cells is blocked, neither B cell proliferation nor antibody production occurs. These results suggest therefore the importance of the iNKT cell category of T helper cells in the response to alpha-Gal and ABO-blood group glycolipids.