Activation of polyclonal CD4+ T cells and B cells is a hallmark of systemic lupus erythematosus (SLE), but the mechanism(s) of activation of T and B cells responsive to self-peptides in lupus is poorly defined. T follicular helper (Tfh) cells are critical for providing help to B cells allowing the formation of germinal center and the subsequent long-lived plasma cells differentiation. There is a growing body of evidence by far pointing toward the crucial roles of Tfh cells in the overproduction of pathogenic autoantibodies and tissue damages in SLE. However, how Tfh cells are deregulated in lupus in mice and humans is currently unknown. Our preliminary results showed that introducing a point mutation within RING finger domain of E3 ubiquitin ligase Cbl-b into lpr mice (carrying a mutation in Fas gene) on a C57BL/6 background (B6-lpr.CblbC373A), exacerbates the disease. This exacerbation is associated with expanded Tfh cells in the spleens and heightened anti-dsDNA titers in the sera. Based upon these data, we hypothesize that Cbl-b controls Tfh development in B6-lpr, which provides strong help for GC B cells to produce autoantibodies, leading to the development of lupus. To test this hypothesis, we will determine 1. Whether and how Cbl-b regulates Tfh cell development in a mouse lupus model. In this aim, we will first confirm the lupus phenotype in B6-lpr.CblbC373A mice, and verify whether Cbl-b indeed regulates Tfh differentiation in vitro and in vivo. We will then determine the potential molecular targets of Cbl-b in Tfh cells. We will further characterize the cellular compositions of B6-lpr.CblbC373A mice, with the particular focus on Tfh cells. Dr. Jian Zhang’s group in the Department of Microbial Infection and Immunity will be responsible for the experiments proposed in this aim. 2. Whether Cbl-b regulates in human Tfh cells in SLE patients. In this aim, we will determine the Cbl-b expression in circulating Tfh cell subsets by multicolor flow cytometry as Tfh17 (CXCR3-CCR6+), Tfh1 (CXCR3+CCR6-) or Tfh2 (CXCR3-CCR6-) cells among CXCR5+CD45RA-CD4+ T cells in the peripheral blood from healthy controls and SLE patients with different severity (SLEDAI). We will then verify whether knocking down Cbl-b by Cbl-b siRNA affects the ability of human Tfh subsets identified in SLE patients on GC B cell function. Finally, we will verify whether Cbl-b regulates human Tfh development by using the same molecular mechanism as proposed in Aim-1. Dr. Wael Jarjour’s group in the Division of Rheumatology will be responsible for the experiments proposed in this aim. Taken together, this proposal should, with its highly focused approach, lead to target identification that will facilitate drug discovery in lupus.