The Biology of HIV Infections

National Institute Of Allergy And Infectious Diseases

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Abstract

SEQUENTIAL IMMUNIZATION OF MACAQUES ELICITS HETEROLOGOUS NEUTRALIZING ANTIBODIES TARGETING THE V3-GLYCAN PATCH OF HIV-1 ENV. Broadly neutralizing antibodies (bNAbs) against HIV-1 develop after prolonged virus and antibody coevolution. Previous studies showed that sequential immunization with a V3-glycan patch germline-targeting HIV-1 envelope trimer (Env) followed by variant Envs can reproduce this process in mice carrying V3-glycan bNAb precursor B cells. However, eliciting bNAbs in animals with polyclonal antibody repertoires is more difficult. A V3-glycan immunogen, multimerized in virus-like particles (VLPs), was used to elicit heterologous neutralizing antibodies in nonhuman primates (NHPs). Structures of antibody/Env complexes after prime and boost vaccinations demonstrated target epitope recognition with apparent maturation to accommodate glycans. However, increasing off-target antibodies were observed after boosting. Eight vaccinated NHPs were subsequently challenged with simian-human immunodeficiency virus (SHIV), and seven of eight animals became infected. The single NHP that remained uninfected after viral challenge exhibited one of the lowest neutralization titers against the challenge virus. These results demonstrate that more potent heterologous neutralization resulting from sequential immunization is necessary for protection in this animal model. Improved prime-boosted regimens to increase bNAb potency and stimulate other immune protection mechanisms are essential for developing anti-HIV-1 vaccines. A MULTICLADE ENVGAG VLP MRNA VACCINE ELICITS TIER-2 HIV-1-NEUTRALIZING ANTIBODIES AND REDUCES THE RISK OF HETEROLOGOUS SHIV INFECTION IN MACAQUES. A messenger RNA (mRNA) vaccine, co-expressing membrane-anchored HIV-1 envelope (Env) and simian immunodeficiency virus (SIV) Gag proteins has been used to generate virus-like particles (VLPs) to induce antibodies capable of broad neutralization and reducing the risk of infection in rhesus macaques. In mice, immunization with co-formulated env and gag mRNAs was superior to env mRNA alone in inducing neutralizing antibodies. Macaques were primed with a transmitted-founder clade-B env mRNA lacking the N276 glycan, followed by multiple booster immunizations with glycan-repaired autologous and subsequently bivalent heterologous envs (clades A and C). This regimen was highly immunogenic and elicited neutralizing antibodies against the most prevalent (tier-2) HIV-1 strains accompanied by robust anti-Env CD4+ Tcell responses. Vaccinated animals had a 79% per-exposure risk reduction upon repeated low-dose mucosal challenges with heterologous tier-2 simianhuman immunodeficiency virus (SHIVAD8). This multiclade envgag VLP mRNA platform represents a promising approach for the development of an HIV-1 vaccine. CONCORDANCE OF IMMUNOLOGICAL EVENTS BETWEEN INTRARECTAL AND INTRAVENOUS SHIVAD8-EO INFECTION WHEN ASSESSED BY FIEBIG-EQUIVALENT STAGING. Primary HIV-1 infection can be classified into six Fiebig stages based on virological and serological laboratory testing, whereas simian-HIV (SHIV) infection in nonhuman primates (NHPs) is defined in time post-infection, making it difficult to extrapolate NHP experiments to the clinics. We identified and extensively characterized the Fiebig-equivalent stages in NHPs challenged intrarectally or intravenously with SHIVAD8-EO. During the first month post-challenge, intrarectally challenged monkeys were up to 1 week delayed in progression through the six Fiebig stages. Nonetheless, regardless of the challenge route, stages III predominated before, and stages VVI predominated after, peak viremia. Decrease in lymph node (LN) CD4+ T cell frequency and rise in CD8+ T cells occurred at stage V. LN virus-specific CD8+ T cell responses, dominated by degranulation and TNF, were first detected at stage V and increased at stage VI. A similar late elevation in follicular CXCR5+ CD8+ T cells occurred, consistent with higher plasma CXCL13 levels at these stages. LN SHIVAD8-EO RNA+ cells were present at stage II, but appeared to decline at stage VI when virions accumulated in follicles. Fiebig-equivalent staging of SHIVAD8-EO infection revealed concordance of immunological events between intrarectal and intravenous infection despite different infection progressions and can inform comparisons of NHP studies with clinical data. NEUTRALIZING ANTIBODIES INDUCED IN IMMUNIZED MACAQUES RECOGNIZE THE CD4-BINDING SITE ON AN OCCLUDED-OPEN HIV-1 TRIMER. Two heterologously-neutralizing CD4-binding site (CD4bs) antibodies (Ab1303 and Ab1573), isolated from sequentially immunized macaques, have been characterized. Ab1303/Ab1573 binding is observed only when Env trimers are not constrained in the closed, prefusion conformation. Fab-Env cryo- EM structures show that both antibodies recognize the CD4bs on Env trimer with an occluded- open conformation between closed, as targeted by bNAbs, and fully open, as recognized by CD4. The occluded-open Env trimer conformation includes outwardly rotated gp120 subunits, but unlike CD4-bound Envs, does not exhibit V1V2 displacement, 4-stranded gp120 bridging sheet, or co-receptor binding site exposure. Inter-protomer distances within trimers measured by double electron-electron resonance spectroscopy suggest an equilibrium between occluded-open and closed Env conformations, consistent with Ab1303/Ab1573 binding stabilizing an existing conformation. Studies of Ab1303/Ab1573 demonstrate that CD4bs neutralizing antibodies that bind open Env trimers can be raised by immunization, thereby informing immunogen design and antibody therapeutic efforts.