No abstract

The envelope glycoproteins (Env) represent a critical component of a successful antibody-mediated human immunodeficiency virus type 1 (HIV-1) vaccine. However, immunization with soluble Env was reported to induce short-lived antibody responses, suggesting that Env has unusual immunogenic properties. Here, we directly compared the magnitude and durability of B-cell responses induced by HIV-1 Env and an unrelated soluble viral protein, influenza virus hemagglutinin (HA), in simultaneously inoculated macaques. We demonstrate robust peak responses followed by rapid contraction of circulating antibody and memory B cells for both antigens, suggesting that short-lived responses are not unique to HIV-1 Env but may be a common feature of soluble protein vaccines.

Broadly neutralizing antibodies (bNAbs) isolated from chronically HIV-1 infected individuals reveal important information regarding how antibodies target conserved determinants of the envelope glycoprotein (Env) spike such as the primary receptor CD4 binding site (CD4bs). Many CD4bs-directed bNAbs use the same heavy (H) chain variable (V) gene segment, VH1-2*02, suggesting that activation of B cells expressing this allele is linked to the generation of this type of Ab. Here, we identify the rhesus macaque VH1.23 gene segment to be the closest macaque orthologue to the human VH1-2 gene segment, with 92% homology to VH1-2*02. Of the three amino acids in the VH1-2*02 gene segment that define a motif for VRC01-like antibodies (W50, N58, flanking the HCDR2 region, and R71), the two identified macaque VH1.23 alleles described here encode two. We demonstrate that immunization with soluble Env trimers induced CD4bs-specific VH1.23-using Abs with restricted neutralization breadth. Through alanine scanning and structural studies of one such monoclonal Ab (MAb), GE356, we demonstrate that all three HCDRs are involved in neutralization. This contrasts to the highly potent CD4bs-directed VRC01 class of bNAb, which bind Env predominantly through the HCDR2. Also unlike VRC01, GE356 was minimally modified by somatic hypermutation, its light (L) chain CDRs were of average lengths and it displayed a binding footprint proximal to the trimer axis. These results illustrate that the Env trimer immunogen used here activates B cells encoding a VH1-2 gene segment orthologue, but that the resulting Abs interact distinctly differently with the HIV-1 Env spike compared to VRC01.

Neutralizing Abs provide the protective effect of the majority of existing human vaccines. For a prophylactic vaccine against HIV-1, broadly neutralizing Abs (bNAbs) targeting conserved epitopes of the viral envelope glycoproteins (Env) are likely required, as the pool of circulating HIV-1 variants is extremely diverse. The failure to efficiently induce bNAbs by vaccination may be due to the use of sub-optimal immunogens or immunization regimens, or it may indicate that B cells specific for broadly neutralizing Env determinants are selected against during peripheral checkpoints, either before or after antigen encounter. To investigate if perturbation of B cell subsets prior to immunization with recombinant Env protein affects the vaccine-induced Ab response in mice, we used B Lymphocyte Stimulator (BLyS), a cytokine that regulates survival and selection of peripheral B cells. We show that the transient BLyS treatment used here substantially affected naïve B cell populations; in particular, it resulted in an increased number of B cells surviving counter-selection at the transitional stages. We also observed an increased number of mature naïve B cells, especially marginal zone B cells, in BLyS-treated mice. Intriguingly, provision of excess BLyS prior to immunization led to a consistent improvement in the frequency and potency of HIV-1 Env vaccine-induced neutralizing Ab responses, without increasing the number of Env-specific Ab-secreting cells or the Ab binding titers measured after boosting. The results presented here suggest that an increased understanding of BLyS-regulated processes may help the design of vaccine regimens aimed at eliciting improved neutralizing Ab responses against HIV-1.