Subtype-Specific Conformational Differences within the V3 Region of Subtype B and Subtype C Human Immunodeficiency Virus Type 1 Env Proteins

Abstract

The V3 region of the human immunodeficiency virus type 1 gp120 Env protein is a key domain in Env due to its role in interacting with the coreceptors CCR5 and CXCR4. We examined potential subtype-specific V3 region differences by comparing patterns of amino acid variability and probing for subtype-specific structures using 11 anti-V3 monoclonal antibodies (V3 MAbs). Differences between the subtypes in patterns of variability were most evident in the stem and turn regions of V3 (positions 9 to 24), with the two subtypes being very similar in the base region. The characteristics of the binding of V3 MAbs to Env proteins of the subtype B virus JR-FL and the subtype C virus BR025 suggested three patterns, as each group of MAbs recognized a specific conformation-or sequence-based epitope. Viruses pseudotyped with Env from JR-FL and BR025 were resistant to neutralization by the V3 MAbs, although the replacement of the Env V3 region of the SF162 virus with the JR-FL V3 created a pseudotyped virus that was hypersensitive to neutralization. A single mutation in V3 (H13R) made this chimeric Env selectively resistant to one group of V3 MAbs, consistent with the mAb binding properties. We hypothesize that there are intrinsic differences in V3 conformation between subtype B and subtype C that are localized to the stem and turn regions and that these differences have two important biological consequences: first, subtype B and subtype C V3 regions can have subtype-specific epitopes that will inherently limit antibody cross-reactivity, and second, V3 conformational differences may potentiate the frequent evolution of R5-into X4-tropic variants of subtype B but limit subtype C virus from using the same mechanism to evolve X4-tropic variants as efficiently.The binding of the human immunodeficiency virus type 1 (HIV-1) to its target cell is mediated by the viral envelope glycoprotein Env. The glycoprotein is expressed as a gp160 precursor that is proteolytically cleaved into two mature subunits, the gp120 surface protein and the gp41 transmembrane protein. On the surface of the virus, Env is present as a trimer of the gp120 and gp41 heterodimers (2, 62). When the virus binds to its primary receptor, CD4, Env undergoes conformational changes, exposing previously hidden regions and creating new structural elements. Env then binds to a coreceptor, usually the chemokine receptor CCR5 or CXCR4 (23, 26, 80), which triggers further structural changes that promote fusion to the target cell (27, 35).The Env sequence can be viewed as being composed of regions that are relatively constant (C1 to C5) and regions that on a population basis are much more variable (V1 to V5) (70). As the only viral protein expressed on the surface of the virus, Env is the sole target of neutralizing antibodies that supply selective pressure to favor mutated variants capable of evading the immune response (47, 79). The V3 region is associated with coreceptor preference and physically contacts the chemokine receptor as part of the fusion process (23,26,80). Sequen...

An Introduction to the Current State of HIV Vaccine Research

“…They examined HIV-1 clade C-infected individuals, naïve for antiretroviral therapy, at different disease stages, in order to characterize the V1 to V5 variable regions with respect to sequence length, glycosylation pattern and net electric charge. In the chronic stage of the disease, they observed in the V1, V2 and V4 loops, an increase in sequence length, amino acid variability and putative N-glycosylation sites but very little change in the V3 loop as reported previously for clade C [44]. These data suggest that the V1 and V4 loops are likely to be the main drivers of clade C HIV-1 virus evolution which agrees with the finding that the V4 loop is a major target of neutralization activity in clade C infections [45].…”

Human Immunodeficiency Virus Type 2 (HIV-2)/HIV-1 Envelope Chimeras Detect High Titers of Broadly Reactive HIV-1 V3-Specific Antibodies in Human Plasma

“…Previously, we along with others have demonstrated that HIV-2 Env could be used as a scaffold to present HIV-1 MPER and CD4i epitopes in the context of a functional glycoprotein and that such viruses could serve as sensitive and specific probes for HIV-1-elicited epitope-specific NAbs (18,19,34,61,102 In the present study, we focused on HIV-1 V3, which continues to attract substantial attention as a target of NAbs (2,54,72,74,76,99,107). Mamounas and colleagues first demonstrated that a chimeric virus containing an HIV-2 backbone and an HIV-1 MN V3 loop was capable of infecting human T cells and showed susceptibility to HIV-1 V3-specific antiserum (64).…”

“…A third strategy has been to make site-directed mutations in NAb epitopes in Env glycoproteins and to use these proteins as probes in binding, absorption, or infectivity experiments (54,61,62,72). A fourth approach has been to make reciprocal exchanges between homologous regions of different HIV-1 Env glycoproteins to look for gain or loss of NAb sensitivity (54,68,76,86,87,89). The last three approaches share a common limitation in that V3 neutralizing activity is analyzed against a backdrop of other HIV-1 gp120/41 neutralizing reactivities, making a clear dissection of V3-specific NAb activity challenging.…”

Two N-Linked Glycosylation Sites in the V2 and C2 Regions of Human Immunodeficiency Virus Type 1 CRF01_AE Envelope Glycoprotein gp120 Regulate Viral Neutralization Susceptibility to the Human Monoclonal Antibody Specific for the CD4 Binding Domain

“…Such diversity in the amino acid sequence may affect the protein structure; thus, we cannot rule out the possibility that the structure of Env gp120 is somewhat different among diverse HIV-1 subtypes. Indeed, structural differences of the conserved and variable regions of gp120 are reported to exist between subtype B and C viruses (11,33). The b12 antibody recognizes a conformational epitope; thus, b12 susceptibility of HIV-1 Env is necessarily affected by the protein structure of gp120.…”