SAP modulates B cell functions in a genetic background-dependent manner

•Anti-CD40 in part re-activates humoral responses that are impacted by SAP deficiency.•SAP deficiency affects B cell responsiveness in a genetic background-dependent manner.•Environmental factors affect the severity of humoral deficiencies of SAP−/− mice.

Mutations affecting the SLAM-associated protein (SAP) are responsible for the X-linked lympho-proliferative syndrome (XLP), a severe primary immunodeficiency syndrome with disease manifestations that include fatal mononucleosis, B cell lymphoma and dysgammaglobulinemia. It is well accepted that insufficient help by SAP−/− CD4+ T cells, in particular during the germinal center reaction, is a component of dysgammaglobulinemia in XLP patients and SAP−/− animals. It is however not well understood whether in XLP patients and SAP−/− mice B cell functions are affected, even though B cells themselves do not express SAP. Here we report that B cell intrinsic responses to haptenated protein antigens are impaired in SAP−/− mice and in Rag−/− mice into which B cells derived from SAP−/− mice together with wt CD4+ T cells had been transferred. This impaired B cells functions are in part depending on the genetic background of the SAP−/− mouse, which affects B cell homeostasis. Surprisingly, stimulation with an agonistic anti-CD40 causes strong in vivo and in vitro B cell responses in SAP−/− mice. Taken together, the data demonstrate that genetic factors play an important role in the SAP-related B cell functions. The finding that anti-CD40 can in part restore impaired B cell responses in SAP−/− mice, suggests potentially novel therapeutic interventions in subsets of XLP patients.