Abstract Is an HIV vaccine possible? Vaccines are one of the most successful public health interventions over the past century. Nearly all vaccines work by induction of protective antibodies. However, our understanding of the cellular dynamics of immune responses to vaccines, particularly the biology surrounding B cell competition within germinal centers (GC) to complex vaccine antigens is limited. This lack of understanding of fundamental B cell biology has contributed to the inability to develop an effective HIV vaccine. Promisingly, a small population of HIV+ individuals have developed broadly neutralizing antibodies (bnAbs), giving renewed hope that an HIV vaccine is possible. Recent work has found that many HIV negative healthy human donors have VRC01-class bnAb precursor B cells. However, work from these studies revealed that these potential bnAb precursor B cells are found at an unusually rare frequency. This suggested that following immunization these B cells may be outcompeted by more frequent non-neutralizing B cells. To answer immunological questions surrounding this problem, I developed a model system utilizing mice containing human genes for the germline-reverted VRC01 bnAb (VRC01gHL). Through this B cell transfer model, we found that antigen affinity, avidity, and precursor frequency all played interdependent roles in competitive success of rare VRC01gHL B cells in GCs. Critically, we found that rare VRC01gHL B cells with physiological affinities could be primed to successfully compete within GCs. However, these responses were limited to specific “GC” islands suggesting B cell competition to seed individual GCs is critical in addition to competition within the GC. Taken together, these observations suggest that B cell immunodominance in the GC microenvironment (GCME) is a major obstacle to overcome in developing a successful HIV vaccine. However, there are significant knowledge gaps pertaining to the physiological conditions in which B cells compete to enter GCs, and compete within the GCME. To start, what do we know about the biophysical and metabolic characteristics of the GCME? We hypothesized and found that GCs form a hypoxic microenvironment. I hypothesize that other biophysical constraints may be acting to control GC selection events as many pathways have been shown to be both active in hypoxic tumor microenvironments (TMEs) and in the hypoxic GCME. I hypothesize that in further correlation with TMEs, the GCME may contain high lactate levels, induce multiple metabolic GPCRs, reduced pH, increased temperature, and cellular pressure. I posit that these biophysical parameters of the GC can and do influence B cell selection events to complex antigens. In this DP2 proposal I will investigate the nature of the extracellular milieu of the GCME through multi- photon targeted direct measurements and define the biophysical constraints that limit the success of VRC01- class B cell responses. We will then apply what we learn from studying...