Anti-HIV-1 B cell responses are dependent on B cell precursor frequency and antigen binding affinity

Abstract

The discovery that humans can produce potent broadly neutralizing antibodies (bNAbs) to several different epitopes on the HIV-1 spike has reinvigorated efforts to develop an antibody based HIV-1 vaccine. Antibody cloning from single cells revealed that nearly all bNAbs show unusual features that could help explain why it has not been possible to elicit them by traditional vaccination, and instead that it would require a sequence of different immunogens. This idea is supported by experiments with genetically modified immunoglobulin knock-in mice. Sequential immunization with a series of specifically designed immunogens was required to shepherd the development of bNAbs. However, knock-in mice contain super-physiologic numbers of bNAb precursor expressing B cells and therefore how these results can be translated to a more physiologic setting remains to be determined. Here we make use of adoptive transfer experiments using knock-in B cells that carry a synthetic intermediate in the pathway to anti-HIV-1 bNAb development to examine how the relationship between B cell receptor affinity and precursor frequency affects germinal center B cell recrutiment and clonal expansion. Immunization with soluble HIV-1 antigens can recruit bNAb precursor B cells to the germinal center when there are as few as 10 such cells per mouse. However, at low precursor frequencies the extent of clonal expansion is directly proportional to the affinity of the antigen for the B cell receptor, and recruitment to germinal centers is variable and dependent on re-circulation. Significance statementAn essential requirement for an HIV-vaccine is to elicit antibodies to conserved regions of the spike protein (Env) becasue these antibodies can protect against infection. Although broadly neutralizing antibodies develop naturally in rare individuals after prolongued HIV infection, eliciting them by vaccination has only been possible in artificial knock-in mouse models wherein the number of B cells expressing the antibody precursor is superphysiologic. To understand the relationship between precursor frequency, antigen affinity and germinal center recruitment we have performed adoptive transfer experiments in which fixed numbers of precursor cells are engrafted in wild type mice. Our results provide a framework for understanding how precursor frequency and antigen affinity shape humoral immunity to HIV.

B cells expressing authentic naive human VRC01-class BCRs can be recruited to germinal centers and affinity mature in multiple independent mouse models

“…Germline targeting requires the ability to target BCRs of precursors of the desired bnAb, specifically BCRs actually found in the human naive B cell repertoire. While knockin mouse models have provided substantial value in understanding the in vivo challenges that exist in maturing B cells toward broad neutralization (12,13,(22)(23)(24)(25)(26)(27), it is unclear if these models are capable of revealing all of the hurdles that authentic VRC01class B cells will face with germline-encoded CDR3s. Second generation germline-targeting immunogens have incorporated broader sets of precursors of representative bnAbs and BCR sequences identified from next-generation sequencing (NGS) of HIV-negative donors predicted to be related to bnAb precursors (28,29).…”

Strategies for inducing effective neutralizing antibody responses against HIV-1

“…Recently, two elegant studies have started to address this question by transferring different amounts of (partly) germline bNAb B cells into WT mice. Subsequently, these mice were immunized with antigens with different affinities for the transferred germline B cells [168,169]. In both studies, the authors found that the activation of the antigen-specific B cells was dependent on B cell frequency and antigenbinding affinity.…”

Outflanking immunodominance to target subdominant broadly neutralizing epitopes

“…This phenomenon, termed immunodominance, is just now being defined and mechanistically dissected at the level of serum Abs and B cell responses (8). Two recent studies suggest that B cell precursor frequency and BCR avidity contribute to the subdominance of conserved HIV GP160 epitopes (9,10). Other studies suggest, however, that after initial B cell seeding, GCs are more permissive than previously thought, allowing B cells with BCRs of even 100-fold differences in avidity to emerge from the same LN (11,12).…”