ABSTRACT The objective of this grant application is to understand how loss-of-function genetic variants of the PTPN2 gene -encoding the T cell-protein tyrosine phosphatase (TC-PTP)- enhance risk of rheumatoid arthritis (RA). PTPN2 is ubiquitous, and very highly expressed in immune cells and is a critical negative regulator of Janus kinases and signal transducers and activators of transcription downstream multiple cytokine receptors. In order to model the mechanism of action of PTPN2 autoimmunity-associated variants in RA, we assessed mice carrying Ptpn2 haploinsufficiency (Ptpn2+/- mice), which causes a loss of expression of PTPN2 comparable to the human PTPN2 RA-risk variants. We found that in the SKG RA model- characterized by CD4 T cell-driven disease- partial loss of function of PTPN2 caused significant enhancement of arthritis severity. By leveraging conditional Ptpn2 haploinsufficiency and fate-mapping mice, we showed that the phenotype of SKG.Ptpn2+/- mice is due to enhanced inflammation-induced FoxP3+ regulatory T cell (Treg) instability, a process known to lead to conversion of peripheral FoxP3+ Treg into pathogenic FoxP3- “exTreg” expressing interleukin-17 (IL-17). We have evidence that the enhanced conversion of Ptpn2+/- Tregs into IL-17-producing “exTreg” is due to increased STAT3 phosphorylation after stimulation with IL-6 and potentially other inflammation-induced factors. Here we apply for funding to further understand the mechanism of action of PTPN2 in Treg instability and the pathogenesis of RA via mouse immunology and cell signaling studies. In Aim 1 and 2 we will elucidate the mechanism and topology of enhanced inflammation-induced instability and pathogenicity of SKG.Ptpn2+/- Treg. In Aim 3 we will assess whether overexpression of PTPN2 in Treg can reverse the Treg and arthritis phenotype induced by Ptpn2+/- in SKG mice. Our long-term goal is to acquire knowledge of PTPN2 functional genetics to enable the discovery of personalized and non-immunosuppressive therapies for RA patients carrying genetic PTPN2 risk variants.