Functional properties of antibodies in resistance against HIV-1 infection

Abstract

During the last years a few populations who remain uninfected despite repeated HIV-1 viral exposures have been described. Several mechanisms were suggested to be associated with this natural resistance including HIV-1-specific cell- mediated and humoral immune responses, as well as mutations affecting receptors for virus entry. Exposure to, or infection with, defective viral strains has also been postulated as a possible phenomenon of resistance in some of these risk groups.

Two groups of highly exposed persistently seronegative (HEPS) individuals were examined in the present study: 1) female sex workers from Nairobi, Kenya, where HIV-1 clades A, D and C are prevalent, 2) uninfected heterosexual partners to HIV-1, clade B infected individuals from Italy. The study focused on the humoral immune response in mucosal samples of these two groups, and specifically on the presence of functionally active HIV-1 specific IgA. We have also evaluated if HIV specific IgM monoclonal antibodies may be capable of preventing HIV-1 infection in vitro.

The main aim of the first paper was to develop a HIV-1 p24 antigen capture ELISA with polyclonal IgG from rabbits immunized with different gag antigens. This HIV-antigen detection assay was to be used to evaluate antibody neutralizing capacity, for instance of IgA purified from HEPS individuals. The p24 antigen capture ELISA could be used for HIV-1 p24 antigen detection of at least seven different HIV-1 clades. In the second paper, lgA was purified from plasm saliva and cervicovaginal fluid samples from female sex workers as well as from control individuals. lgA antibodies from some of the HEPS individuals were shown to neutralize an HIV-1 clade B primary isolate in an assay using peripheral blood mononuclear cells as targets. In the third paper, IgA antibodies representing both the HEPS groups were shown to inhibit HIV-1 transcytosis across an epithelial cell layer in vitro. This model system mimics HIV-1 entry into the host and is believed to be one of the mechanisms for sexual transmission of the virus. The fourth paper describes that lgA antibodies, purified from HEPS individuals, could neutralize HIV-1 primary isolates representing different HIV-1 clades in vitro. In this preliminary study, lgA from the female sex workers seemed to have a broader neutralization capacity compared with the Italian HIV-1 discordant couples.

A small group of the Kenyan sex-workers that remained seronegative for many years have seroconverted after having reduced the number of clients or stopped sex-work for two months or longer. This indicates that constant antigen exposure may be necessary to maintain the HIV-1 specific immune responses. In order to evaluate the possibility of inducing long-term B cell immune responses and neutralizing antibodies the study described in the fifth paper was performed. Mice were intranasally immunized with HIV-1gp16O/DNA and boosted with gp41 and CCR5 peptides recognized by HEPS. Long term B-cell mucosal and systemic responses were obtained in addition to HIV-specific IgA responses in the mucosa. Neutralizing antibodies were detected one year after the last immunization.

In order to evaluate the possible role of natural HIV envelope binding IgM in protection against HIV-1 infection in vitro, in the sixth paper we analyzed the capacity of two human monoclonal antibodies to inhibit HIV-1 transcytosis across a human intestinal epithelial cell line. Both gp120 V3 binding Mabs exhibited the capacity to limit the transcytosis of a T-cell laboratory adapted strain and a primary HIV-1 isolate.

Mucosal and plasma IgA from HEPS individuals can thus neutralize HIV-1 primary isolates of different clades as well as inhibit HIV-1 transcytosis across epithelial cell linings. Natural and specific IgM antibodies may also be present in HEPS individuals representing a mechanism supporting resistance to HIV-1 infection. Because preliminary findings have shown that lgA from HEPS individuals have a unique epitope specificity on the HIV1 gp41 envelope proteins compared with HIV-1 infected individuals, and that long term immunity can be obtained in mice suggests a novel design of an HIV-1 vaccine aimed at mucosal protection against multiple HIV-1 clades.