Project Summary This R03 proposal is to follow-up on recent findings from my SERCA K01 award, which focuses on elucidating the role of the type 1 diabetes (T1D) PTPN2 risk allele in loss of B cell anergy. Previously B cells bearing antigen receptors with high affinity for insulin were found only in the anergic B cell compartment of healthy individuals. Importantly, these cells leave this compartment in a proportion of first-degree relatives (FDRs), and in all autoantibody positive pre-diabetics and recent onset T1D individuals. Departure of these autoreactive anergic B cells in FDRs was shown to be associated with the high risk non-HLA allele, PTPN2 (rs1893217). PTPN2 has been previously shown to be a negative regulator of T cell signaling, but has not been studied in B cells. Recently we demonstrated that mice that lack Ptpn2 specifically in their B cells (Mb1Cre.Ptpn2fl/fl.C57BL/6) have a decrease in anergic B cells, similar to our findings in humans, an increase in autoimmune associated B cells (ABCs), increase in production of autoreactive antibodies, and a hyperresponsive phenotype. Despite these findings, these mice did not develop overt autoimmunity, likely driven by the fact these mice are on the autoimmune resistant genetic background, C57BL/6. Hence, in this study we aim to determine the effect of B cell specific deletion of Ptpn2 in the NOD mouse, which develops spontaneous diabetes by about 20 weeks of age. Specifically, we aim to study whether B cell deletion of Ptpn2 increases the rate and penetrance of diabetes incidence, the effect on frequency and activation status of insulin-reactive and non-reactive B cells, as well as its effect on B cell antigen presentation to T cells and the differentiation of pathogenic T cells. The potential impact of these studies will lie in understanding how risk alleles conspire to undermine maintenance of immune tolerance to autoantigens in T1D.