ABSTRACT One biomarker that has a strong association with poor long-term kidney allograft outcomes is the presence of DSA. Recent advances in transplantation medicine have further established that DSA may convey a wide range of injuries to the allograft, from acute ABMR to subclinical ABMR or to the absence of injury. The early immunologic mechanisms that mediate these heterogeneous events are not well understood. If such mechanisms were to be characterized, then IS therapy could be targeted and tailored to provide the most benefit to high risk patients, without wide spread application of IS which can hurt patients who do well. Our goal is to characterize the immunologic mechanisms (cellular and molecular) that proceed and govern the development of DSA leading or not to ABMR, and to long-term poor allograft outcomes. T follicular helper (TFH) cells are CD4+ T cells specialized in the cognate control of antigen (Ag)-specific B cell immunity. They participate to the germinal centers (GC) formation in the secondary lymphatic organs, where they provide critical help to naïve B lymphocytes to differentiate into memory B cells, as well as to generate short-lived plasmablasts (PB) and long-lived plasma cells (PC) that secrete high-affinity, isotype switched IgG responses. TFH cells also help memory B cells during recall responses to differentiate into Ab-producing cells. While TFH cell subsets can distinctly shape the functional quality of antibodies, they themselves can be (re)-programed by inflammatory or anti- inflammatory signals into efficient or suboptimal helpers, underscoring their plasticity. Analysis of TFH cells has proved valuable in humans, as they correlate well with Ab responses post-vaccination and with disease activity in autoimmunity. Very little is known about the role TFH in organ transplantation and their contribution to DSA generation. We have recently identified an association between elevation of Th1-TFH cells and development of asymptomatic DSA in a cohort of Thymoglobulin-induced KTx recipients, whereas activated Th1/Th17-TFH cells were detected in circulation of patients at ABMR diagnosis. Our central hypothesis is that donor-reactive cTFH from patients before KTx are heterogeneous, and depending on how lymphopenia induced proliferation (LIP) (re)programs them into efficient or suboptimal helpers, they can distinctly shape DSA quality with clinical consequences for allograft survival. Therefore in this grant we propose to investigate in Aim 1 the characteristics of allo-reactive memory TFH cells phenotype, TCR clonality and function. In Aim 2 we will ascertain how post-Tx inflammation at the time of T cell repopulation after T cell induction influences cTFH cells (re)-polarization. Aim 3 will assess whether cTFH cells shape DSA quality, correlate with ABMR and early (1 year) and late (3-yr) allograft outcomes.