All immunogens are not created equal

From the Stamatatos lab, Vaccine and Infectious Disease Division

Vaccines that elicit broadly neutralizing antibodies (bnAbs), antibodies that can neutralize a wide variety of related but distinct viral strains, are a promising strategy to prevent HIV infection. Although bnAbs can develop over time in people infected with HIV-1, they have not yet been elicited by vaccination. One reason for this is that the viral envelope forms that first engage the naïve, unmutated “germline” (gl) precursor forms of bnAbs, which are expressed on naïve B cells, are not well defined and in many cases remain unknown. The Stamatatos lab (Vaccine and Infectious Disease Division) and others have previously designed a series of targeted HIV-1-derived envelope (Env) immunogens that express an epitope—a region targeted by antibodies — that successfully bind the gl form of a class of antibodies called VRC01 that recognize the CD4 binding site on Env.

Dr. Yu-Ru Lin (Neoleukin Therapeutics, formerly of the Stamatatos lab) and Dr. Rachael Parks (Newell and McElrath labs, formerly of the Stamatatos lab) led a study to investigate whether the VRC01 epitope can equally activate glVRC01 B cells when expressed on three distinct, previously designed gl-targeting immunogens. This work, which is a collaboration among the Stamatatos and Pancera groups at the Fred Hutch and the group of Dr. Possu Huang at Stanford was recently published in Immunity.

The B-cell receptor (BCR) and its secreted antibodies consist of paired “heavy” (VH) and “light” (VL) chains, each with their own variable region responsible for recognition of antigens. Mice do not encode the VRC01 VH allele found in humans, so transgenic mice with a knock-in containing the human allele encoding the heavy chain for VRC01 can be used to study VRC01 bnAbs.  These humanized mice were immunized with each of the three gl-targeting immunogens and VRC01-like B cells were isolated at various timepoints.  VH and VL genes of the resulting BCRs were sequenced and the resulting antibodies were characterized.

B cell selection with HIV Env immunogens.
B cell selection with HIV Env immunogens. Figure provided by Dr. Parks.

All three immunogens elicited robust VRC01-like antibodies in vivo, and sequencing of VH and VL genes revealed that all immunogens elicited antibodies containing the same heavy and light chain pairings. However, the antibodies from each immunogen differed by a few amino acids at critical positions, conferring the antibodies from each immunogen with distinct ranges of cross-reactivity, the crucial characteristic of bnAbs. This finding suggests that the subsequent “boost” immunizations needed to guide VRC01 maturation must be tailored based on the original immunogen, as the primary wave of antibodies generated from one immunogen may not recognize a sequential boost designed from a different immunogen.

Following this initial selection of VH and VL pairings after immunization, B cell affinity, or the strength of the binding interaction between BCR and antigen, is thought to govern which B cells continue to mature. The authors reported that the relative frequencies of high-affinity B cells initially selected by the immunogens decreased over a six-week period, while the frequencies of lower affinity B cells increased. These low-affinity B cells also accumulated somatic hypermutations in their BCR, a necessary process for further maturation into bnAbs, suggesting that contrary to current paradigm, “affinity may not actually be the sole determining factor” of B cell selection during the priming phase of immunization, Dr. Parks explained.

“This work has revealed that while immunogens are designed to engage VRC01-class B cell receptors, they will select for different subpopulations of VRC01-class B cell receptors. These results were found in a transgenic mouse model with a relatively homogenous B cell repertoire,” Dr. Parks summarized. “This raises the question as to what will happen in an even more polyclonal system – the human. So, in humans, will these immunogens effectively be able to engage with VRC01-class B cell receptors and what will the unique properties of these BCRs be? I look forward to finding out,” she said.

This work was supported by the National Institutes of Health, the National Institute of General Medical Sciences, the Office of Basic Energy Sciences of the United States Department of Energy, the Office of Science, Office of Advanced Scientific Computing Research, Scientific Discovery through Advanced Computing (SciDAC) program, and Stanford’s Bio-X Interdisciplinary Initiatives Seed Grants Program.

Lin YR, Parks KR, Weidle C, Naidu AS, Khechaduri A, Riker AO, Takushi B, Chun JH, Borst AJ, Veesler D, Stuart A, Agrawal P, Gray M, Pancera M, Huang PS, Stamatatos S. 2020. HIV-1 VRC01 Germline-Targeting Immunogens Select Distinct Epitope-Specific B Cell Receptors. Immunity. 2020 Oct 13;53(4):840-851.e6. doi: 10.1016/j.immuni.2020.09.007.