In this maternal cohort from Nairobi, Kenya, women infected with subtype C had 3.0- and 8.0-fold increased odds of vaginal shedding of HIV-1–infected cells than did women with subtypes A and D, respectively. It is possible that disease duration differed between women infected with different subtypes in this cross-sectional evaluation and that this resulted in differences in immune status and plasma viral load between subtypes, which, in turn, affected genital shedding of HIV-1. However, in multivariate logistic regression models that adjusted for age, CD4 cell count, and plasma HIV-1 RNA load, infection with subtype C independently predicted vaginal shedding of HIV-1–infected cells; this suggests that subtype influences genital shedding of HIV-1. Thus, to the extent that genital shedding of HIV-1 influences transmission, we would predict that transmission to infants or partners from women infected with subtype C would be more frequent than that from women infected with subtype A, and we would predict that subtype D would be the least transmissible. Consistent with this hypothesis, the risk of mother-to-child transmission in the present cohort was highest in women infected with subtype C, was next highest in women infected with subtype A, and was lowest in women infected with subtype D, although the differences did not reach statistical significance.
In light of observations of the relatively rapid epidemic spread of HIV-1 in countries such as South Africa, where subtype C is predominant, our observation of increased vaginal shedding of HIV-1 in women infected with subtype C is particularly intriguing. Interestingly, however, during limited serial cross-sectional surveys, we have not observed an increase in the prevalence of subtype C over the last decade in Kenya (J. Overbaugh, unpublished data). It may be that other cofactors, in combination with increased genital shedding, accelerate regional expansion of subtype C epidemics. Cofactors such as partner networks, host factors, and concomitant sexually transmitted diseases in regions with subtype C epidemics may act synergistically with increased genital shedding to fuel rapid sexual spread of HIV-1.
We found that colostrum HIV-1 RNA loads were also highest in women infected with subtype C (for subtype C vs. D, ~1 log higher) and that colostrum and breast milk HIV-1 RNA loads were significantly lower in women infected with subtype D than in women infected with non-D subtypes. However, we did not observe significant differences in risks of mother-to-child transmission. The present study had >80% power to detect at least a 2.8-fold difference in transmission risk between subtypes A and D, but it was underpowered to evaluate a comparable difference in transmission risk for subtype C. Our results are consistent with those of a Ugandan study of vertical transmission, in which there was no difference in transmission risk between subtypes A and D [4
]. The lack of a difference in transmission risk between subtypes A and D is notable, given our observation of significantly increased HIV-1 shedding in genital secretions and breast milk in women infected with subtype A. This observation suggests that the effect of subtype on mucosal shedding is more pronounced than the effect of subtype on transmission risk. It may be optimal to separately analyze the effect of subtype on in utero, intrapartum, and late postnatal transmission. However, such stratified analyses further diminish power to observe associations.
Our study had several limitations. First, because we evaluated shedding in a cross-sectional cohort, confounding effects due to differences in disease duration could not be completely excluded. We adjusted for age, CD4 cell count, and plasma HIV-1 RNA load—all of which reflect disease duration—but a prospective study in which women had defined timing for the beginning of infection would enable better adjustment for disease duration. Second, we limited our evaluation of subtype to the env
region because this viral region is crucial for initial cell binding and infection and, thus, is most likely to influence levels of viral replication and transmission. Because our study was limited to the env
region, we did not comprehensively exclude subtype recombinants that may have unique properties. This is of particular relevance, because our studies suggest that ~20%–50% of the viral genomes circulating in Kenya are unique intersubtype recombinants [13
]. Third, the relative roles of cervical versus vaginal shedding and of HIV-1 DNA versus RNA shedding in transmission have not been well defined, making it difficult to extrapolate the effect of subtype on shedding to its effect on sexual transmission. Finally, perhaps our biggest limitation was a lack of statistical power to assess the effect of subtype C on the risk of mother-to-child transmission. This limitation derives from the low population prevalence of subtype C in Nairobi and reflects the most common obstacle to comparisons of subtypes in population studies.
In conclusion, we found significant differences in mucosal shedding of HIV-1 between subtypes. To our knowledge, this is the first report comparing mucosal shedding between HIV-1 subtypes, although differences in genital shedding have been noted between the more distantly related HIV-1 and HIV-2, with shedding being lower for the latter [14
]. Of note, HIV-2 also causes a more indolent disease course than does HIV-1 and is less efficiently transmitted [15
]. The present study suggests that the magnitude of the effects of the different HIV-1 subtypes on shedding and transmission are more modest than the effects of different HIV types, mirroring their relative genetic divergence. Thus, although the different subtypes may influence viral shedding, transmission, and pathogenesis, it seems unlikely that this alone accounts for the rapid spread of particular subtypes, such as subtype C, in certain parts of the world.