An anti-HIV-1 V3 loop antibody fully protects cross-clade and elicits T-cell immunity in macaques mucosally challenged with an R5 clade C SHIV

Abstract

Neutralizing antibodies have been shown to protect macaques against SHIV challenge. However, genetically diverse HIV-1 clades have evolved, and a key question left unanswered is whether neutralizing antibodies can confer cross-clade protection in vivo. The novel human monoclonal antibody HGN194 was isolated from an individual infected with an HIV-1 clade AG recombinant circulating recombinant form (CRF). HGN194 targets an epitope in the third hypervariable loop (V3) of HIV-1 gp120 and neutralizes a range of relatively neutralization-sensitive and resistant viruses. We evaluated the potential of HGN194 to protect infant rhesus monkeys against a SHIV encoding a primary CCR5-tropic HIV-1 clade C envelope. After high-dose mucosal challenge, all untreated controls became highly viremic while all HGN194-treated animals (50 mg/kg) were completely protected. When HGN194 was given at 1 mg/kg, one out of two monkeys remained aviremic, whereas the other had delayed, lower peak viremia. Interestingly, all protected monkeys given high-dose HGN194 developed Gag-specific proliferative responses of both CD4+ and CD8+ T cells. To test whether generation of the latter involved cryptic infection, we ablated CD8+ cells after HGN194 clearance. No viremia was detected in any protected monkeys, thus ruling out virus reservoirs. Thus, induction of CD8 T-cell immunity may have resulted from transient "Hit and Run" infection or cross priming via Ag-Ab-mediated cross-presentation. Together, our data identified the HGN194 epitope as protective and provide proof-of-concept that this anti-V3 loop mAb can prevent infection with sterilizing immunity after challenge with virus of a different clade, implying that V3 is a potential vaccine target.

Figure 1. HGN194 targets a unique conformational V3-loop epitope.

Plasma from monkey RMf-9 (white bars with black dots), chronically infected with SHIV-1157ip, was used as positive control as well as 2G12 (grey bars), a mAb targeting a conformational mannose-dependent epitope on gp120. Herceptin (white bars) and plasma from naïve monkey RMj-2 (striped bars) were used as negative controls. Results of conformational ELISA are shown. Additional control studies have been published earlier .

Figure 2. Passive immunization with HGN194 against heterologous clade C SHIV challenge in infant rhesus macaques.

(A) Experimental design. Group 1A infant RM (n = 4) were infused twice with 50 mg/kg of HGN194 on days −1 and 7. Group 1B (n = 2) received twice 1 mg/kg of HGN194. Four RM served as untreated controls. On day 0, all 10 animals were challenged intrarectally with 18 AID₅₀ of SHIV-1157ipEL-p. (B) Plasma viral RNA loads after high-dose rectal challenge with SHIV-1157ipEL-p using a quantitative RT assay (detection limit: 50 copies/ml). (C) Average plasma mAb concentrations during the course of the study for Group 1A. (D) Plasma HGN194 concentration for Group 1B. Arrows in C and D indicate mAb treatments. Experiments in C and D were repeated twice or trice.

Figure 3. Induction of specific antiviral T-cell responses after passive immunization.

(A) and (B) Proliferation of T cells. PBMC were stimulated with SIVmac251 Gag protein and proliferation of CD4⁺ and CD8⁺ cells was measured using the CFSE dilution method. (A) Animals given 50 mg/kg of HGN194 (Group 1A) and control group RM (Group 2). PBMC collected 17 weeks post-challenge were tested. Horizontal dashed line, cut-off value based on response shown by a naïve animal. (B) Animals given 1 mg/kg of HGN194 (Group 1B). PBMC collected at weeks 12 (RNc-13) and 17 (ROa-13) post-challenge were tested. (C) Absolute numbers of CD8+ T cells for Groups 1A (black) and 1B (grey) after treatment with a cytotoxic anti-CD8 mAb (Methods). (D) Plasma viral RNA load after CD8+ cell depletion.