Rectal tissue contains more than 3 times as many macrophages vulnerable to HIV than does the colon, according to results of an in situ fluorescence study in healthy volunteers. The study also identified other important differences in HIV target cells between colon and rectum.A large fraction of HIV transmissions occur rectally, especially in men who have sex with men but also in women. More is known about HIV target cells in colon than in rectum, but assuming the two sites have similar cellular environments may be risky. Understanding the cellular milieu of the rectum is critical to vaccine and microbicide development.
To compare rectal and colonic cell populations, researchers at the NIAID HIV Vaccine Trials Network conducted this in situ fluorescence study of HIV target cells 4 cm and 30 cm distant from the anal canal in 29 healthy individuals. The researchers used computerized analysis of digitized combination stains to analyze cell populations at the two sites. They focused on CCR5-expressing T cells, macrophages, and putative dendritic cells.
Compared with the colon, the rectum contained more than 3 times as many CD68+ (activated) macrophages expressing the HIV coreceptor CCR5 (P = 0.0001). The researchers surmised that “rectal macrophages seem biologically closer to the HIV-susceptible CCR5high phenotype in the vagina than the mostly HIV-resistant CCR5low phenotype in the colon.”
In contrast, putative CD209+ dendritic cells, another HIV target, populated the colon more than the rectum (P = 0.0004), although CCR5 expression levels in these cells were similar in colon and rectum. CD3+ T-cell densities and CCR5 expression were comparable in colon and rectum.
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