Tracing donor-MHC class II reactive B cells in mouse cardiac transplantation: Delayed CTLA4-Ig treatment prevents memory alloreactive B-cell generation

Background. The dual role of B cells as drivers and suppressors of the immune responses have underscored the need to trace the fate of B cells recognizing donormajor histocompatibility complex class I and class II after allograft transplantation.Methods. In this study, we used donor class II tetramers to trace the fate of I-Ed-specific B cells after immunization with BALB/c spleen cells or cardiac transplantation, in naive or sensitized C57BL/6 recipients. We combined this approach with genetic lineage tracing of memory B cells in activation-induced cytidine deaminase regulated Cre transgenic mice crossed to the ROSA26-enhanced yellow fluorescent protein reporter mice to track endogenous I-Ed-specific memory B cell generation. Results. Immunization with BALB/c splenocytes or heart transplantation induced an expansion and differentiation of I-Ed-specific B cells into germinal center B cells, whereas BALB/c heart transplantation into sensitized recipients induced the preferential differentiation into antibody-secreting cells. A 10.8-fold increase in the frequency of I-Ed-specific memory B cells was observed by day 42 postimmunization. Treatment with CTLA4-Ig starting on day 0 or day 7 postimmunization abrogated I-Ed-specificmemory B cell generation and sensitized humoral responses, but not if treatment commenced on day 14. Conclusions. The majority of donorspecificmemory B cells are generated between days 7 and 14 postimmunization, thus revealing a flexible timeframewhereby delayed CTLA4-Ig administration can inhibit sensitization and the generation of memory graft-reactive B cells.