Limited or no protection by weakly or nonneutralizing antibodies against vaginal SHIV challenge of macaques compared with a strongly neutralizing antibody

Abstract

To guide vaccine design, we assessed whether human monoclonal antibodies (MAbs) b12 and b6 against the CD4 binding site (CD4bs) on HIV-1 gp120 and F240 against an immundominant epitope on gp41 could prevent vaginal transmission of simian HIV (SHIV)-162P4 to macaques. The two anti-gp120 MAbs have similar monomeric gp120-binding properties, measured in vitro, but b12 is strongly neutralizing and b6 is not. F240 is nonneutralizing. Applied vaginally at a high dose, the strongly neutralizing MAb b12 provided sterilizing immunity in seven of seven animals, b6 in zero of five animals, and F240 in two of five animals. Compared with control animals, the protection by b12 achieved statistical significance, whereas that caused by F240 did not. For two of three unprotected F240-treated animals there was a trend toward lowered viremia. The potential protective effect of F240 may relate to the relatively strong ability of this antibody to capture infectious virions. Additional passive transfer experiments also indicated that the ability of the administered anti-gp120 MAbs to neutralize the challenge virus was a critical influence on protection. Furthermore, when data from all of the experiments were combined, there was a significant increase in the number of founder viruses establishing infection in animals receiving MAb b6, compared with other nonprotected macaques. Thus, a gp120-binding, weakly neutralizing MAb to the CD4bs was, at best, completely ineffective at protection. A nonneutralizing antibody to gp41 may have a limited capacity to protect, but the results suggest that the central focus of HIV-1 vaccine research should be on the induction of potently neutralizing antibodies.

Fig. 1.

Sensitivity of the challenge viruses to the test MAbs in a rhesus PBMC neutralization assay. (A) SHIV-162P4 or (B) SHIV-162P3 was incubated with the indicated concentrations of MAbs b12 (■, □), b6 (●, ○), F240 (▲, △), or DEN3 (◆, ◇) before infection of rhesus macaque PBMCs. Virus replication on day 9 was measured using a p27 antigen assay. Inhibition of virus replication was calculated as percentage neutralization (no MAb = 0% neutralization; no replication = 100% neutralization). The data depicted were derived from a single assay that is representative of three of similar design.

Fig. 2.

The gp120- and virus-binding properties of the test MAbs. Detergent lysates of (A) SHIV-162P4 or (B) SHIV-162P3 were used as the antigen source for a gp120-capture assay. MAbs b12 (■, □), b6 (●, ○), F240 (▲, △), or DEN3 (◆, ◇) were added to the test wells at the indicated concentrations and the amount of bound antibody was determined as an A₄₀₅ value. (C) SHIV-162P4 virions were used as the antigen source for a virus-capture ELISA involving the same MAbs (∇, no MAb). Virion capture was quantified using a TZM-bl cell infection assay, with a luciferase (RLU) endpoint.

Fig. 3.

Plasma viremia in macaques infected with SHIV-162P4 in the presence of test MAbs. The test MAbs were administered vaginally in saline (5 mg in 5 mL), 30 min before addition of SHIV-162P4. Plasma viremia (viral RNA) was measured weekly for 10 wk postchallenge. The mean AUC values for the infected animals in each group are recorded on each panel. The assay sensitivity limit was 125 RNA copies/mL (A) b12, zero of seven infected; (B) b6, five of five infected; (C) F240, three of five infected; (D) DEN3, five of five infected. The infection rates in the b6 and b12 groups were significantly different (P = 0.0013).

Fig. 4.

Number of founder viruses infecting macaques challenged with SHIV-162P4 or SHIV-162P3 in the presence and absence of MAbs. The number of founder viruses measured in each infected animal is shown on the y axis. The animals are grouped on the y axis according to the treatment (MAb, or control agent) they received before challenge. SHIV-162P3 and SHIV-162P4 infections are depicted by different symbols (see labels in figure), as is whether the MAbs were administered intravenously or vaginally before challenge. Four animals received a DC-SIGN-Fc fusion protein (5 mg in 4 mL) vaginally before SHIV-162P3 challenge; each became infected. The data for the gel control group were generated by Athe Tsibris and Steven Wolinsky. For the b6 group vs. DEN3 plus HEC control groups, the difference is statistically significant (P = 0.016).