Human immunodeficiency virus type 1 coreceptor switching: V1/V2 gain-of-fitness mutations compensate for V3 loss-of-fitness mutations

Abstract

Human immunodeficiency virus type 1 (HIV-1) entry into target cells is mediated by the virus envelope binding to CD4 and the conformationally altered envelope subsequently binding to one of two chemokine receptors. HIV-1 envelope glycoprotein (gp120) has five variable loops, of which three (V1/V2 and V3) influence the binding of either CCR5 or CXCR4, the two primary coreceptors for virus entry. Minimal sequence changes in V3 are sufficient for changing coreceptor use from CCR5 to CXCR4 in some HIV-1 isolates, but more commonly additional mutations in V1/V2 are observed during coreceptor switching. We have modeled coreceptor switching by introducing most possible combinations of mutations in the variable loops that distinguish a previously identified group of CCR5- and CXCR4-using viruses. We found that V3 mutations entail high risk, ranging from major loss of entry fitness to lethality. Mutations in or near V1/V2 were able to compensate for the deleterious V3 mutations and may need to precede V3 mutations to permit virus survival. V1/V2 mutations in the absence of V3 mutations often increased the capacity of virus to utilize CCR5 but were unable to confer CXCR4 use. V3 mutations were thus necessary but not sufficient for coreceptor switching, and V1/V2 mutations were necessary for virus survival. HIV-1 envelope sequence evolution from CCR5 to CXCR4 use is constrained by relatively frequent lethal mutations, deep fitness valleys, and requirements to make the right amino acid substitution in the right place at the right time.

FIG. 1.

Entry efficiency of ADA-3 mutant envelopes in a single-cycle infection assay employing either U87-CD4-CCR5 or U87-CD4-CXCR4 target cells. Mutations 1 to 5 correspond to V2 mutations N157K (loss of PNGS), V178I, and D182N and V3 mutations G312R and E320K, respectively (numbering based on HxB2), and are further described in Table 1. Data are normalized to percentage of ADA wild type (wt) for CCR5-expressing target cells or ADA-3 for CXCR4-expressing target cells. Envelopes that mediate entry only via CCR5 are shown in blue, envelopes that mediate entry via both CCR5 and CXCR4 but more efficiently via CCR5 are shown in green, envelopes that mediate entry via both CCR5 and CXCR4 but more efficiently via CXCR4 are shown in purple, and envelopes that mediate entry only via CXCR4 are shown in brown.

FIG. 2.

Entry efficiency of ADA-1 mutant envelopes in a single-cycle infection assay as described in the legend to Fig. 1. Mutations 1 to 7 correspond to C2 mutations N197D (base of V2 loop, PNGS), and A221T and V3 mutations N301H, S306R, P311R, G319E, and E320K, respectively (numbering based on HxB2), and are further described in Table 1. Color coding is described in the legend to Fig. 1. wt, wild type.

FIG. 3.

Entry efficiency of BaL-derived mutant envelopes in a single-cycle infection assay as described in the legend to Fig. 1. Mutations 1 to 4 in BaL-2A and 1 to 5 in BaL-1B are described in Table 1. Mutations 1 to 4 in BaL-2A correspond to N130D (loss of PNGS), E178K, E320K, and K490T (numbering based on HxB2). Mutations 1 to 5 in BaL-1B correspond to A136T, E178K, A314K, L315I, and E320K. Color coding is described in the legend to Fig. 1. wt, wild type.