Acquired immunity

A small but significant number of women in Africa, including a highly-studied cohort of Kenyan sex workers, have remained HIV-negative despite repeated, frequent exposures to HIV. Research has shown that these women may have specific immune defences that prevent HIV infection from becoming established. These include high levels of HIV-specific T cells, CD8+ antiviral factors and high levels of a neutralising antibody called immunoglobulin A (IgA) that is active against HIV.

This phenomenon, acquired immunity, has been studied in other populations as well, although it is rarely so dramatic as in the Kenyan group who appear essentially immune to infection. Studies have shown that an HIV-negative partner exposed to their HIV-positive partner’s virus may in the end build up a degree of immunity to it sufficient to make transmission much less likely.

A recent review concludes that some people are equipped with an array of immune responses that, when activated, lower their susceptibility to HIV infection.¹ This response – linked to various genetic factors as well as an inability to generate long-term effective memory cells following exposure to HIV – enables these individuals to effectively manage repeated viral exposure.

In 2004, Dr Barry Peters and colleagues from St Thomas’s Hospital in London studied 29 monogamous heterosexual serodiscordant couples who had had frequent unprotected intercourse for more than six months. They compared the anti-HIV immune responses in the negative partner to those in a control group of 15 HIV-negative women and 10 HIV-negative men who had had no sex, or only protected sex, for at least the previous six months. They took CD4 cells from all the subjects and attempted to infect them with various different strains of HIV in the test tube.

It was found that CD4 cells in women who had had unprotected sex with HIV-positive men were only 10% as likely to be infected with HIV (of the type that is more often transmitted) as CD4 cells from women who had had protected or no sex.

The HIV-negative men who had had unprotected sex with positive women had half that degree of protection: their CD4 cells were 20% as likely to be infected with HIV as cells from men who had had protected or no sex.

So the mucosal immunity generated by frequent sexual exposure to HIV – meaning the immunity generated by cells lining the genital membranes – can be as high as 90% immunity in women and 80% in men, though the degree of ‘in vivo’ (i.e. real-life) protection will probably be less.

In an accompanying editorial, The Lancet cautioned against jumping to the wrong conclusions from this study:

“Most epidemiological evidence indicates that unprotected intercourse dangerously exposes most individuals to HIV infection...Peters’ observations must never, of course be interpreted as a suggestion for individuals to have unprotected intercourse for the purposes of immunisation.”