The development of Type 1 Diabetes (T1D) relies on complex interrelationships between cells of the immune system [e.g., DC, CD8+ T cells] and genes imparting susceptibility or resistance to the disease that underlie the autoimmune destruction of insulin producing pancreatic ? cells. While a broad body of evidence certainly exists to support this notion (and we ourselves believe it true), the exact mechanism by which autoimmune ? cell destruction is facilitated remains unclear. In addition, the relative contributions of each facet (i.e., cells, genes) play in the process remain, to a large extent, unknown. Mechanistic studies of T1D-associated susceptibility alleles are complicated by polygenic inheritance such that no two individuals are truly alike. Hence, studies are severely hampered by a lack of power in populations, and the inability to isolate the functional impact of a variant to a specific cell type. Here we present a solution that focuses on individual alleles usin an innovative isogenic mode that takes advantage of cutting edge technologies. We have created an experimental platform to study how specific genetic risk variants precipitate immune dysregulation leading to cytotoxic CD8+ T lymphocyte (CTL) activation and ? cell destruction. We hypothesize that genetically regulated defects in PTPN22 promote; i) immunogenic DC, ii) TH1 responses, iii) pancreatic vascular inflammation and CTL homing, and iv) pathogenic CTL activity towards ? cells coupled with reduced activation induced CTL death: each of these tenants form an aim of this grant. Further, we posit that defects reach full potential when immune cells and endothelial cells are excessively sensitive to activation by endogenous or exogenous factors that stimulate inflammation, thus linking environment and immunogenetics in T1D. Here we will utilize a novel experimental pipeline where PTPN22R (T1D resistant), PTPN22W (T1D susceptible) or PTPN22 deletion (PTPN22-/-) alleles are carried by isogenic human immune and endothelial cells engineered from induced pluripotent stem cells [iPSC]. The iPSC system allows exquisite control of T1D disease alleles, where the susceptible allele can be replaced by the resistant allele (and vice versa) providing a constant genetic background upon which effects of a single risk allotype can be studied without complicating epistatic effects, in a manner analogous to studies in genetically modified mice. This system proposed here will provide an unprecedented capacity to interrogate molecular and cellular interactions under isogenic conditions to provide mechanistic understanding of how PTPN22 alleles regulate individual steps of T1D pathogenesis and how those steps interrelate to bring upon T1D onset. Importantly, this study will also lay the groundwork for future investigations of single or multipl T1D susceptibility genes using this innovative strategy.