The overall mission of this CHAVI-ID application is to define immunogens and immunization regimens that induce sustained HIV cross-protective B cell and CD4+ T cell responses in preclinical models and, thereby, guide product development strategies for a preventive human AIDS vaccine. For Focus #2, our hypothesis is that long-term humoral immunity is critically dependent on CD4+ T cells, and particularly T follicular helper (Tfh) cells, and that the efficient generation of these cells is an essential and obligatory component of an effective HIV vaccine. Nearly all licensed anti-viral vaccines induce neutralizing antibodies and the development of such antibodies is typically CD4+ T cell dependent. Therefore understanding and controlling CD4+ T cells, and Tfh cells in particular, is important for rational vaccine strategies. Focus #2 is designed to improve our knowledge of: 1) the specificity, phenotype and function of Tfh cells in HIV infection and in humans who have received licensed successful vaccines; 2) the pathways involved that lead to the induction of Tfh cells; 3) the precise Tfh signals that lead to the induction of affinity maturation and broadly neutralizing HIV antibodies; and 4) the role of HIV-specific effector CD4+ T cells in the early focal control of mucosal HIV infection. Tfh cells are also potentially extremely useful biomarkers in human vaccine clinical trials as predictors of long-term humoral immunity and antibody quality. The information gained in Focus #2 will be translated into the design of immunogens and immunization strategies in Focus #1 for evaluation in knock-in mice and NHPs. As we gather together information on optimal immunogens and immunization strategies, we will move forward with small-scale human trials with the advice and close involvement of our Vaccine Discovery Scientific Research Support Component.