Revealing the Biophysics of the Germinal Center Microenvironment

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 the GCME to manipulate B cell
immunodominance in the GCME to favor competitive selection of VRC01-class B cells.

Grant Number: 5DP2AI154410-05
NIH Institute/Center: NIH
Principal Investigator: Robert Abbott