Systemic lupus erythematosus afflicts 1 to 2 million Americans and is associated with significant morbidity and mortality. While wise use of steroids and cytotoxics drugs along with effective use of antibiotics overall survival and quality of life have increased significantly over the last 30 years, we still lack specific treatment, and among others, the lack of full understanding of involved pathogenic mechanisms and of proper disease biomarkers are to be blamed for this. Immune cell and cytokine abnormalities in patients with systemic lupus erythematosus (SLE) are diverse and involve among others decreased cytotoxic responses, decreased activation induced cell death, decreased T regulatory function, increased dendritic cell function, increased IFNa, IL-6 and decreased IL-2, IFN? and TNFa production. Distinct molecular and biochemical abnormalities may account for several cell and cytokine abnormalities. Among peripheral T cells an expanded population of T cells missing CD4 and CD8 from the surface (double negative (DN) T cells exists which when placed in coculture with autologous B cells promoted the production of immunoglobulin and anti-dsDNA. We have recently found that this DN T cell population in SLE patients can be expanded in vitro and that it produces IL- 17 and more importantly, IL-17 producing DN T cells infiltrated kidney tissue of patients with lupus nephritis. We propose, accordingly, that an expanded DN T cell population in patients with SLE produces IL-17 and participates in target organ damage. We will test our thesis in experiments outlined in this application and grouped in four specific aims: In the first aim we will establish the presence of CD4+ and expanded DN T cells producing IL-17 in patients with SLE-determine clinical correlations. In the second we will determine the origin of DN T cells is SLE patients, the requirements for expansion, their early and late signaling profile and their susceptibility to cellular control mechanisms. In the third we will determine the requirements for the generation of TH17 in SLE. In the last aim we study the nature of T cells that infiltrate target tissues in SLE and explore mechanisms which are responsible for the inappropriate homing. These studies intend to propose IL-17 as a novel treatment target in SLE patients and will generate information which will propose Th17 cells as a biomarker of disease activity.