Mucosal and systemic immune responses in HIV-1 exposed uninfected individuals

Abstract

Human immunodeficiency virus (HIV) is an infectious agent that, for 26 years, has evaded vigorous attempts to invent either a cure or preventive agent. Even though the immunological understanding of HIV s pathogenesis has greatly improved, most knowledge has been gained from studies of systemic compartments but less from the mucosal environment, where the majority of all infections occur. Strangely, some individuals, despite their exposure to HIV, seem to be less susceptible to the infection, thus categorized as a HIV Exposed Uninfected (EU) group. These fortunate escapees can be considered nature s own experiment and, possibly, have lessons to teach us about superior ways to encounter HIV. That knowledge has obvious potential for application to research directed toward formulating a preventive vaccine.

In the present thesis, the author has evaluated a cohort of Swedish male homosexual EU subjects with regard to their immune responses to HIV, or lack thereof, in systemic as well as mucosal compartments. Plasma, cells and mucosal samples were collected during a period of two years, while each of these individuals had continuous sexual relations with their HIV-positive partners. Questionnaires described sexual behaviors, and plasma samples as well as clinical records were available for the HIV-positive partners. Low-risk HIV-negative individuals were recruited as controls. The questionnaires revealed that the vast majority of these EU subjects were exposed to HIV through the oral (not anal) route. This is considered a low-risk behavior and is rarely seen as an exclusive behavior if compared to other EU cohorts.

We purified the plasma and saliva samples to obtain IgA, an antibody important mostly at mucosal sites. Further, we tested IgA as well as IgA-depleted samples in a PBMC-based neutralization assay, to evaluate whether the EU subjects had any anti-HIV capacity. This was indeed present in salivary IgA, samples of which from 13 of 25 EU subjects could neutralize HIV (in vitro) vs 0 of 22 controls. The neutralizing response was sustained in almost all cases during the study period. Moreover, HIV-neutralizing capacity also detectable in plasma (7 of 25 EU, 0 of 22 controls) was associated with the partner s HIV-RNA viral load. This suggests that the natural anti-HIV response in these EU men is acquired, not predestined. Also in plasma, the vast majority of neutralization was mediated by IgA; however, the neutralizing antibodies (as in saliva) lacked classical HIV-specificity as measured by ELISA or Western Blot.

We then evaluated PBMC from the EU group and found HIV-specific responses in 3 of 25 subjects; this response was detected in both CD4+ and CD8+ subsets of cells as measured by intracellular staining (ICS). Lastly we tested their saliva for innate molecules of known anti-HIV capacity and found significantly higher levels of CC-chemokines than in controls. This outcome was also associated with behavioral aspects of the study subjects, not genetic polymorphisms, which further strengthen the theory of environmental causes for these immune responses.

The conclusions of this work are summarized in the following four points: 1) The HIV exposure was low-grade, since it occurred via the oral route, but could still induce and/or sustain an anti-HIV response mediated by IgA. Therefore, the oral route might be applicable in a vaccine setting. 2) While detectable in >50% of the EU mucosal samples, the HIV-neutralizing capacity also existed to a lesser extent in plasma and, rarely, in systemic T-cells. 3) Amounts of innate soluble molecules were increased in the saliva of EU individuals and were associated with neutralizing capacity. 4) All immune responses against HIV detected in these EU subjects were associated with environmental factors, implying that such responses are acquired. In turn, that supposition favors the possibility that similar or stronger responses can be induced via vaccination.