Effective, low-titer antibody protection against low-dose repeated mucosal SHIV challenge in macaques

Abstract

Neutralizing antibodies are thought to be crucial for HIV vaccine protection, but studies in animal models suggest that high antibody concentrations are required. This is a major potential hurdle for vaccine design. However, these studies typically apply a large virus inoculum to ensure infection in control animals in single-challenge experiments. In contrast, most human infection via sexual encounter probably involves repeated exposures to much lower doses of virus. Therefore, animal studies may have provided an overestimate of the levels of antibodies required for protection in humans. We investigated whether plasma concentrations of antibody corresponding to relatively modest neutralization titers in vitro could protect macaques from repeated intravaginal exposure to low doses of a simian immunodeficiency virus-HIV chimera (SHIV) that uses the CC chemokine receptor 5 (CCR5) co-receptor. An effector function-deficient variant of the neutralizing antibody was also included. The results show that a substantially larger number of challenges is required to infect macaques treated with neutralizing antibody than control antibody-treated macaques, and support the notion that effector function may contribute to antibody protection. Overall, the results imply that lower amounts of antibody than previously considered protective may provide benefit in the context of typical human exposure to HIV-1.

Figure 1. Protection by b12 and variant LALA during vaginal low-dose repeated challenge with SHIV_SF162P3

Female Indian rhesus macaques were treated weekly with 1 mg kg⁻¹ of either b12 or b12 effector function variant LALA or an isotype control antibody (anti-dengue, DEN3) and challenged vaginally twice weekly. The viral challenge dose began at 3 TCID₅₀ and was subsequently increased to 10 TCID₅₀ SHIV_SF162P3. a, All animals in the isotype control groups became virus positive after a maximum of 4 challenges of 10 TCID₅₀. 4 out of 4 animals were infected after a total number of 10 challenges of 10 TCID₅₀. b, 3 b12-treated animals were virus positive after 6, 23, and 38 viral challenges of 10 TCID₅₀, respectively, and 1 animal (BF68) remained virus negative after 40 challenges. 3 out of 4 animals were infected after a total number of 107 challenges of 10 TCID₅₀. BK10 was infected after 6 challenges of 3 TCID ₅₀. (See Suppl. Fig. 2.) c, Plasma virus was observed in the LALA-treated animals following 6, 8, 12, 17, and 23 viral challenges, respectively. 5 out of 5 animals were infected after a total of 66 challenges of 10 TCID₅₀. Viral challenges and i.v. antibody treatments were suspended after positive detection of virus in plasma but the course of infection was monitored for several weeks. The SIV viral RNA (vRNA) assay detection limit is 125 copies ml^–1 (log 2.1).

Figure 2. Kaplan-Meier analysis and magnitude of protection in low-dose (10 TCID₅₀) repeated challenge by b12 and LALA treatment

a. Kaplan-Meier analysis. The percent of animals remaining uninfected is plotted against the number of 10 TCID₅₀ viral challenges (compare Fig. 1). A single animal (BK10; b12-treated) became infected during the initial repeat 3 TCID₅₀ challenge (see Suppl. Fig. 2). To allow inclusion of this animal in the analysis, it is included as if it was infected in the first 10 TCID₅₀ challenge. The Kaplan-Meier survival curves are significantly different from each other (p = 0.0377; Log-rank (Mantel-Cox) test) b. The reduction in infection susceptibility by b12 and LALA treatment is estimated by counting the number of challenges that did or did not result in infection. Again the animal BK10 is included in this analysis as being infected in the first 10 TCID₅₀ challenge. Both b12 (p=0.0016) and LALA (p=0.0145) are significantly different from the control (Fisher’s exact test).