The Human Immunodeficiency Virus Type 1 Envelope Spike of Primary Viruses Can Suppress Antibody Access to Variable Regions

Abstract

The human immunodeficiency virus type 1 (HIV-1) envelope spike is a heavily glycosylated trimeric structure in which protein surfaces conserved between different HIV-1 isolates are particularly well hidden from antibody recognition. However, even variable regions on the spike tend to be less antigenic and immunogenic than one might have anticipated for external structures. Here we show that the envelope spike of primary viruses has an ability to restrict antibody recognition of variable regions. We show that access to an artificial epitope, introduced at multiple positions across the spike, is frequently limited, even though the epitope has been inserted at surface-exposed regions on the spike. Based on the data, we posit that restricted antibody access may be the result, at least in part, of a rigidification of the epitope sequence in the context of the spike and/or a highly effective flexible arrangement of the glycan shield on primary viruses. Evolution of the HIV envelope structure to incorporate extra polypeptide sequences into nominally accessible regions with limited antibody recognition may contribute to reducing the magnitude of antibody responses during infection and allow the virus to replicate unhindered by antibody pressure for longer periods.

Introduction of Exogenous Epitopes in the Variable Regions of the Human Immunodeficiency Virus Type 1 Envelope Glycoprotein: Effect on Viral Infectivity and the Neutralization Phenotype

“…In agreement with the findings of previous studies (14,19), we observed that the precise location of the tags within the variable loops (i.e., at the amino terminus, in the central region, or at the carboxy terminus [position "a," "b," or "c," respectively]) differentially affected the fusogenic potentials of Envs in an Env background-dependent manner. Specifically, for SF162, Env FLAG tagged in the amino-terminal or central region of V1 (V1FLAG-a or -b, respectively) mediated entry at 70 to 74% of the WT level, while Env FLAG tagged at the C terminus (V1FLAG-c) mediated entry at only ϳ30% of the WT level.…”

“…Observations from previous studies using tagged Envs (14,19), as well as the results described above, suggest that the introduction of exogenous tags into the variable regions of either the SF162 or the SF33 Env could result in changes in the overall conformation of the individual Env protomers participating in the formation of the trimer or in the overall association of the three Env protomers within the trimer. Such changes could affect the relative exposures of various neutralization epitopes.…”

“…In order to keep the length of the Env constant, amino acid substitutions instead of insertions (which would have increased the size of Env) were used. Thus, our approach differed from that used by others who inserted the tags into the HIV envelope (19,33), but it was similar to the substitution approach used by Laird and Desrosiers (14). The tags were introduced at three different positions (sometimes overlapping) within each of the V1, V2, and V4 loops: at the amino terminus (position "a"), in the central region (position "b"), and at the carboxy terminus (position "c").…”

“…A second implication is that since the FLAG epitope was exposed in the V4 loops of all three isolates, the V4 loop could theoretically be a good target for vaccine-elicited antibodies. In contrast, Pantophlet et al (19) introduced the HA tag into various regions of the JRCSF (neutralization-resistant) and HxB2 (neutralization-sensitive) isolates and reported that JRCSF was intrinsically more resistant than HxB2 to anti-HA antibodies. This observation implies, therefore, that some HIV-1 strains (primary, neutralization-resistant strains) have developed mechanisms that limit the accessibility of multiple Env regions, including variable regions, to antibodies developed during infection.…”

“…Foreign epitopes have been introduced into the variable regions of HIV and simian immunodeficiency virus (SIV) Envs, and their effects on viral neutralization potential have been examined (14,19,22,33). Yang and colleagues (33) introduced the FLAG epitope into the V4 regions of three HIV-1 isolates (YU2, JRFL, and HxB2) displaying distinct neutralization phenotypes in response to anti-HIV NAbs; they found that all three pseudotyped viruses were equivalently neutralized by an anti-FLAG MAb.…”