Regional clustering of shared neutralization determinants on primary isolates of clade C human immunodeficiency virus type 1 from South Africa

Abstract

Clade C is one of the most prevalent genetic subtypes of human immunodeficiency virus type 1 (HIV-1) in the world today and one of the least studied with respect to neutralizing antibodies. Most information on HIV-1 serology as it relates to neutralization is derived from clade B. Clade C primary isolates of HIV-1 from South Africa and Malawi were shown here to resemble clade B isolates in their resistance to inhibition by soluble CD4 and their sensitivity to neutralization by human monoclonal antibody immunoglobulin G1b12 and, to a lesser extent, 2F5. Unlike clade B isolates, however, all 16 clade C isolates examined resisted neutralization by 2G12. Infection with clade C HIV-1 in a cohort of female sex workers in South Africa generated antibodies that neutralized the autologous clade C isolate and T-cell-line-adapted (TCLA) strains of clade B. Neutralization of clade B TCLA strains was much more sensitive to the presence of autologous gp120 V3 loop peptides compared to the neutralization of clade C isolates in most cases. Thus, the native structure of gp120 on primary isolates of clade C will likely pose a challenge for neutralizing antibody induction by candidate HIV-1 vaccines much the same as it has for clade B. The autologous neutralizing antibody response following primary infection with clade C HIV-1 in South Africa matured slowly, requiring at least 4 to 5 months to become detectable. Once detectable, extensive cross-neutralization of heterologous clade C isolates from South Africa was observed, suggesting an unusual degree of shared neutralization determinants at a regional level. This high frequency of cross-neutralization differed significantly from the ability of South African clade C serum samples to neutralize clade B isolates but did not differ significantly from results of other combinations of clade B and C reagents tested in checkerboard assays. Notably, two clade C serum samples obtained after less than 2 years of infection neutralized a broad spectrum of clade B and C isolates. Other individual serum samples showed a significant clade preference in their neutralizing activity. Our results suggest that clades B and C are each comprised of multiple neutralization serotypes, some of which are more clade specific than others. The clustering of shared neutralization determinants on clade C primary HIV-1 isolates from South Africa suggests that neutralizing antibodies induced by vaccines will have less epitope diversity to overcome at a regional level.

FIG. 1.

Neutralizing antibody response over time in infected individuals in South Africa. Neutralizing antibodies were measured in PBMC with the early autologous isolate and in either MT-2 or CEMx174 cells with the clade B TCLA strains IIIB (MT-2), MN (MT-2), and SF2 (CEMx174). Results that were negative (neutralization titers of <4 for autologous isolates and <20 for IIIB, MN, and SF2) were assigned a value of 1 for representation.

FIG. 2.

ELISA reactivity to V3 peptides. Serum samples (1:50 dilution) were evaluated by ELISA for the presence of antibodies that could bind individual peptides derived from the V3 loop of the indicated virus strains. The amino acid sequence of each peptide is shown in Table 2. The dotted line represents the cutoff value for positive reactivity (twice the optical density [OD] of a control serum sample from a healthy, HIV-1-negative individual). The following serum samples were obtained after the estimated number of months of infection: Du123 (13.5 months), Du151 (11.3 months), Du172 (8 months), Du179 (25 months), Du422 (17 months). CC, clade C consensus sequence.

FIG. 3.

Ability of V3 peptides to out-compete the neutralizing activity of clade C serum samples from South Africa. Serum samples shown in Fig. 1 were evaluated for neutralizing activity in the presence and absence of gp120 V3 peptides (50 μg/ml) as described in Materials and Methods. Autologous (Autol.) V3 peptide refers to a peptide bearing the amino acid sequence of the virus from the subject who was the source of serum listed on the x axis. The top panel shows autologous virus-serum combinations, the middle panel shows clade C serum samples assayed with the HIV-1_MN virus, and the bottom panel shows clade C serum samples assayed with the HIV-1_IIIB virus. ID₈₀, 80% inhibitory dose.