Structure-Function Studies of the HIV-1 Envelope Glycoproteins

National Institute Of Allergy And Infectious Diseases

Investigators

Linked publications, trials & patents

Abstract

The development of an HIV vaccine has been hampered by unprecedented challenges, primarily due to the unique properties of the HIV-1 envelope (Env) trimer, a cleverly engineered entry machinery that features an extraordinary assortment of immune-evasion tactics, including antigenic variation, heavy glycosylation of exposed surfaces and conformational masking of key neutralization sites. Further insights into the complex structure-function relationships in the HIV-1 Env trimer and its protective shield may be critical to guide the rational design of a protective vaccine and other immunoprophylaxis measures. 1) Enhancement of broadly HIV-neutralizing antibodies. We previously reported the discovery of a second CD4-binding site in the HIV-1 Env trimer, that we defined as CD4-binding site 2 (CD4-BS2), which opened new perspectives for HIV vaccine and therapy. We also identified selected anti-CD4-BS antibodies, such as VRC03 and VRC06, which mimic the quaternary-binding mode of CD4 and establish contact with two adjacent gp120 protomers via an extended loop in their heavy chain framework region 3 (FR3). Thus, we rationally engrafted the extended FR3 loop of VRC03 onto different CD4-supersite bNAbs and tested the resulting chimeric antibodies against a wide panel of global HIV-1 strains (n = 208). FR3-loop chimerization enhanced the neutralizing activity of several potent bNAbs against a majority of global HIV-1 strains. The interactive quaternary surface was delineated by solving the crystal structure of two FR3 loop-chimeric antibodies in complex with a soluble Env trimer. Furthermore, compared to unmodified antibodies, chimeric antibodies displayed a reduced autoreactivity and a prolonged in vivo half-life in both huFcRn transgenic mice and rhesus macaques. Due to their increased neutralizing potency and favorable biological and pharmacokinetic properties, FR3-loop-chimeric bNAbs are being considered for use in HIV prevention and treatment. We have recently extended these studies to other potent bNAbs, including the N49P family from the IHV in Baltimore and 1-18 from the Klein group in Germany. Interestingly, chimerization of the N49P antibodies was successful resulting in the most potent bNAbs against the CD4-BS ever reported so far. By contrast, modification of antibody 1-18 resulted in a loss of function, confirming that successful chimerization requires specific structural features that ar4 not common to all anti-CD4-BS antibodies. Preclinical studies are underway to assess the efficacy of selected chimeric antibodies in humanized mouse models. 2) Pre-clinical evaluation of a VLP-forming mRNA vaccine for HIV-1 in macaques The development of a protective vaccine remains a top priority for the control of the HIV/AIDS pandemic. We have designed a novel vaccine platform for HIV-1 based on messenger RNA (mRNA), the same technology used for the Covid-19 vaccines, and tested in a preclinical model in macaques. The study was conducted in collaboration with Moderna Inc. Our vaccine design includes: a) co-expression of membrane-anchored HIV-1 envelope (Env) with simian immunodeficiency virus (SIV) Gag proteins in order to generate non-infectious virus-like particles (VLPs) in vivo; b) initial priming with a germline bNAb engaging Env; and c) multiple heterologous booster immunizations with different HIV-1 clades. In preliminary experiments conducted in mice, immunization with co-formulated Env and Gag mRNAs was superior to Env mRNA alone in inducing neutralizing antibodies. In macaques, we tested a complex protocol encompassing an initial priming with a transmitted-founder clade-B Env mRNA lacking a critical glycan (N276) that covers the CD4-binding site glycan, followed by multiple booster immunizations with glycan-repaired autologous and subsequently bivalent heterologous Envs from different clades (clades A and C). All Envs were co-formulated with Gag to produce VLPs. This regimen was highly immunogenic and elicited neutralizing antibodies against the most prevalent (tier-2) HIV-1 strains accompanied by robust anti-Env CD4+ T cell responses. Vaccinated animals had a 79% per-exposure risk reduction upon repeated low-dose mucosal challenges with heterologous tier-2 simianhuman immunodeficiency virus (SHIV AD8). Thus, our multiclade envgag VLP mRNA platform represents a promising approach for the development of an HIV-1 vaccine.