Anti-HIV IgM protects against mucosal SHIV transmission

Abstract

Objective: Worldwide, most new HIV infections occur through mucosal exposure. Immunoglobulin M (IgM) is the first antibody class generated in response to infectious agents; IgM is present in the systemic circulation and in mucosal fluids as secretory IgM. We sought to investigate for the first time the role of IgM in preventing AIDS virus acquisition in vivo.

Design: Recombinant polymeric monoclonal IgM was generated from the neutralizing monoclonal IgG1 antibody 33C6-IgG1, tested in vitro, and given by passive intrarectal immunization to rhesus macaques 30 min before intrarectal challenge with simian-human immunodeficiency virus (SHIV) that carries an HIV-1 envelope gene.

Results: In vitro, 33C6-IgM captured virions more efficiently and neutralized the challenge SHIV with a 50% inhibitory molar concentration (IC50) that was 1 log lower than that for 33C6-IgG1. The IgM form also exhibited significantly higher affinity and avidity compared with 33C6-IgG1. After intrarectal administration, 33C6-IgM prevented viremia in four out of six rhesus macaques after high-dose intrarectal SHIV challenge. Five out of six rhesus macaques given 33C6-IgG1 were protected at a five times higher molar concentration compared with the IgM form; all untreated controls became highly viremic. Rhesus macaques passively immunized with 33C6-IgM with breakthrough infection had notably early development of autologous neutralizing antibody responses.

Conclusion: Our primate model data provide the first proof-of-concept that mucosal IgM can prevent mucosal HIV transmission and have implications for HIV prevention and vaccine development.

Figure 1

33C6 mAb characteristics. (a) SDS-PAGE and western blot analysis for 33C6-IgM and 33C6-IgG1. (b) Dynamic light scattering assay to determine particle size of 33C6-IgM and 33C6-IgG1. Data are representative of 4 independent experiments. (c) 33C6-IgM binding to consensus HIV clade C peptides representing V3 and to Env proteins. (d) Binding avidity. Data are representative of two independent experiments. (e, f) Binding affinities of captured antibodies for solution phase SHIV-1157ip gp120, with representative concentration series of SPR sensorgrams ranging from 41 pM - 10 nM gp120 are shown. Global fits to a 1:1 binding model are overlaid in black. Average k_a, k_d, and K_D with standard errors from 3 replicates are indicated.

Figure 2

Neutralization and virion capture by 33C6 mAbs. (a) Neutralization of the challenge virus, SHIV-1157ipEL-p, by 33C6-IgM and 33C6-IgG1. (b) Capture of physical virus (pVirus) and infectious virus (iVirus) particles by 33C6-IgM and 33C6-IgG1. Data are representative of two independent experiments. Error bars, mean ± SEM. VRC01-IgG1 was used as positive control, while Fm-6-IgG1 and IgM isotype control were used as negative controls. Because a different secondary anti-IgM capture antibody was required for 33C6-IgM to capture cell-free virus using Protein G micro-beads, virion capture by 33C6-IgM could not be compared directly to that of 33C6-IgG1.

Figure 3

Study design. Rhesus macaques were randomized into three groups (n = 6 per group). Red arrow, Group 1 received 33C6-IgM intrarectally (i.r.); blue arrow, Group 2 was given 33C6-IgG1 i.r.; empty arrow, Group 3 received only phosphate-buffered saline (PBS) i.r.; black arrow, high-dose i.r. SHIV-1157ipEL-p challenge with 31.5 50% animal infectious doses (31.5 AID₅₀). 33C6-IgM, 33C6-IgG1 or PBS was given 30 min before the virus challenge.

Figure 4

Plasma viral loads in macaques challenged with SHIV-1157ipEL-p. Plasma viral RNA (log vRNA copies/ml). (a) Group 1 (33C6-IgM); (b) Group 2 (33C6-IgG1); (c) Group 3 (controls). Black dotted line, limit of viral RNA detection (50 copies/ml). (d) Kaplan-Meier analysis of time until peak viremia. Log-rank test was used to determine significance.

Figure 5

Titers of anti-SHIV plasma antibodies and their neutralization profiles against the challenge virus, SHIV-1157ipEL-p. (a) Titers of gp120-binding antibodies in plasma from rhesus macaques (RMs) with breakthrough infection. (b) In vitro neutralization of SHIV-1157ipEL-p of day 42 and (c) day 84 RM plasma samples. Data are representative of two independent experiments; each sample was analyzed in duplicate.