Despite improvement in insulin delivery, maintaining tight control of glucose homeostasis continues to be a challenge that results in bouts of severe hypoglycemia and hyperglycemia and serious long term complications in many patients. Therefore, developing an immunotherapy for type 1 diabetes (T1D) remains a major goal. Reaching this goal requires detailed knowledge of the diabetogenic process. Particular knowledge of the molecules that play pivotal roles in driving the pathogenic mechanisms but are not required for mounting normal immune responses could lead to uncovering of safe therapeutic targets. Fas ligand (FasL) is a rare example of such molecules. It is an apoptosis-inducing member of the TNF family that is not required for immune responses. Yet it plays a critical role in driving the diabetogenic process as non-obese-diabetic (NOD) mice bearing spontaneous loss-of-function mutation of FasL (gld) of Fas (lpr) are completely protected from the disease. We hypothesize that aberrant FasL-mediated apoptosis of IL-10-producing regulatory B cells is critical for initiating the diabetogenic process. This hypothesis is supported by strong published and unpublished data in the NOD mouse model. By working with JDRF/nPOD, we are investigating whether a dysregulated Fas pathway is compromising the role of IL-10-producing regulatory B cells in humans. We have collected strong evidence, so far, indicating that, as in NOD mice, aberrant FasL-mediated apoptosis may a pathogenic mechanism in T1D. Such results will not only provide new insights into the disease process, but also a platform for novel therapeutic modalities that do not impair the patient host defense.