At the Sixteenth Conference on Retroviruses and Opportunistic Infections in Montreal on Monday morning, David Butler of the University of California San Diego presented data on four cases of male-to-male sexual transmission, showing that cell-free virus in semen – not proviral DNA in infected cells – was the means of transmission in all four cases.
HIV is present in semen both as cell-free virus, measurable as RNA in the seminal fluid itself, and as proviral DNA contained within lymphocytes (white blood cells) in the semen. Until now, it has not been known which of these is responsible for sexual transmission. Little is also known about the genetic characteristics that distinguish transmitted from non-transmitted virus. The objective of this study was to study cases of male-to-male sexual transmission, to determine whether the infected partner's viral genome was more closely associated with the cell-free RNA (CF-RNA) or the cell-associated DNA (CA-DNA) in the source (infecting) partner, and explore some of the genetic differences between transmitted and non-transmitted virus strains.
The study team selected four male sexual couples, in which HIV transmission was known to have recently occurred, from a cohort study of primary infection. Samples were collected from the source partner's semen a mean of 72 days after infection took place, and from the newly infected partner's blood a mean of 59 days after infection. First of all, the link between the infecting and infected partner was verified by analysis of the viral pol gene. More detailed genomic sequencing was then performed on HIV RNA from the newly infected partner's blood, and on cell-free RNA and proviral DNA from the source partner's seminal fluid.
Analysis of a specific region of the viral genome – the C2V3 region of the viral env gene – showed that in all four cases, the newly infected partner's virus was closely associated with the original partner's CF-RNA and distinct from the CA-DNA. Butler concluded that cell-free viral RNA, and not cell-associated DNA, was the seminal component responsible for male-to-male HIV transmission.
Questioned as to whether this conclusion could be drawn from such a small number of cases, Butler replied that the patterns of phylogenetic clustering were distinct and significant enough to be quite definitive, and that the results would likely extrapolate to larger numbers. The next question, he said, was the source of the cell-free virus itself – whether it stems from seminal lymphocytes or from the blood.
Additionally, the analysis found six specific "signature" amino acids sequences that were strongly associated with transmission, suggesting that one or more of these could potentially be targeted for protective vaccine development. Further research is intended to explore larger numbers, including heterosexual pairs, to compare blood and seminal virus in source partners, and to find whether circulating blood cells or seminal lymphocytes are the source of seminal cell-free RNA.
Butler D et al. Cell-free virus in seminal plasma is the origin of sexually transmitted HIV among men who have sex with men. Sixteenth Conference on Retroviruses and Opportunistic Infections, Montreal. Abstract 49LB, 2009.