An intensive study looking at the protective impact on adult male circumcision for reducing the risk of sexual HIV transmission suggested new biological mechanisms for protection.
This study was presented as an oral presentation by Minh Dinh from Northwestern University Feinberg School of Medicine.1
Other groups have suggested two mechanisms for protection with both lower keratinisation and a higher number of target cells closer to the tissue surface in the inner compared to the outer foreskin. The current study is a development from previous work from the group that suggested that a reduced keratin layer in the inner foreskin is unlikely to contribute to protection.2 For the first time, the current study reported evidence for the route of entry to be through the mucous membrane of the glans (penis head) and that circumcision has an impact on target cell distribution in this tissue.
Both labs were unable to indentify differences in kerantinisation for the inner and outer foreskin or for the Frener band (the area between where these different tissues meet).
The group then looked at viral interactions again using fluorescent viral labeling with explants both from U.S. donors undergoing circumcision but also penile tissue post-autopsy, from a U.S. national donation tissue bank, to understand how HIV enters the tissue. While most R5-bound viral particles remained close to the surface, caught in keratinised tissue, this analysis showed a significant difference between viral penetration of the inner foreskin and outer foreskins or shaft tissue (p=0.02). An analysis in 14 samples of glans tissue suggested similar difference to inner foreskin. However a 3-fold higher proportion of virions were likely to enter glans tissue compared to shaft tissue in the uncircumcised compared to circumcised samples. Viral penetration increased in proportion to the concentration of Langerhans cells close to the epithelial surface (and this was higher in uncircumcised samples), but were also found at deeper distances from the surface where CD4 cells were commonly found in both tissues.
No difference was seen in uretheral tissue between circumcised and uncircumcised samples with little evidence of viral penetration for either group, suggesting that this may not be a major site of infection.
This study answers the suggestion from researchers involved in circumcision research that the groups previous findings may have been confounded by using foreskins donated by 16 U.S. donors who were being circumcised relating to underlying medical conditions.3
This research is helpful in trying to understand the route of transmission in detail and if validated is of importance not only to people at risk of infection but for production of resources focused on reducing transmission.
A similar approach would help understand the likely risk and mechanism for sexual transmission of hepatitis C, which is particularly poorly understood.
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