Day 46: The high prevalence of HIV among black MSM

There was an interesting brief report in the journal AIDS recently looking at HIV rates, risk factors, and ethnicity. Using a survey of men who have sex with men (MSM) at a San Francisco clinic, Berry and colleagues found that black men were more likely to have partners of the same race and partners who were at least ten years older than themselves compared with white, Latino, or Asian/pacific islander respondents. The authors undertook the survey to help address why HIV prevalence is higher among the black MSM compared with white MSM, despite lower risk behavior patterns among black MSM. Based on the findings, the authors suggest that older black MSM are passing HIV to younger black partners, potentially explaining the noted prevalence patterns.

This seems the simplest explanation and certainly seems reasonable, but I can’t help but wonder if some other factors are at play. This is just a though, but there have been studies showing that MHC divergence between serodiscordant partners seems to be protective for transmission. One possible explanation for this finding is that alloimmunity may provide a protective response against incoming virus—which is loaded with MHC molecules. Alloimmunity is very strong—a normal reactive antigen may stimulate 1% of T cells, while an allo-antigen can stimulate 5% of T cells. So, if black MSM are having sex with other black men, the level of MHC discordance may be lower than white MSM. This all depends on the amount of MHC divergence within the black community. The other factor that may be involved is the age difference. The authors don’t report the ages within the cohort. Now, a ten year discrepancy between partner ages could be a surrogate for younger age. Younger age coupled with fewer risk behaviors (fewer partners) might mean that many black MSM have not developed alloreactivity to divergent MHCs. Other ethnicities, which have a smaller age discrepancy, may be a surrogate for an older population—one which has a more developed alloimmune response as a result of repeat exposures. It is a bunch of hand waving and there are a bunch of “ifs” involved, but I couldn’t help but think this might be involved.

It reminds me of a friend I had who had a really unique ethnic background—he was pretty much part everything. He had several partners who were positive, yet he was never infected. I always wondered whether he had some really strange MHC combination and had strong alloresponses against any virus he came in contact with. Anyway, I just thought I would throw that out there.