This study demonstrates that SIV can be transmitted to male rhesus macaques by immersing the glans and shaft of the penis in a suspension of cell-free virions. Although penile transmission of SIV is possible, it is not as efficient as vaginal SIV transmission. For example, when exposed to an SIVmac251 inoculum containing 103
, four males resisted infection despite 7–14 penile exposures, while seven of eight female macaques became infected after 14 vaginal exposures and all eight were infected by 16 challenges.10
Furthermore, 21 of 21 female rhesus macaques became infected after two vaginal challenges (separated by a 4
h interval) in a single day with 105
while only three of five male macaques became infected after penile exposure to the same inoculum and schedule (, ). The relative inefficiency of penile SIV transmission is consistent with the epidemiologic studies in stable HIV-discordant couples that concluded that women are more susceptible to acquiring HIV per heterosexual act than men.27–29,66
The reasons for the difference in gender susceptibility remain to be determined, but as shown here these differences persist even if the doses of virus to which the genders are exposed are the same. It seems likely that differences in the relative surface area of the genital mucosa (the surface area of the vaginal mucosa is much greater than the surface area of the glans and foreskin), the relative density of susceptible target cells in the genital mucosa of men and women, and the time that the inoculum is in contact with the genital mucosa contribute to the differences in the efficiency of HIV transmission between the genders.
In heterosexual men, the presence of an intact foreskin is associated with an increased risk of HIV acquisition30,34–36,67,68
and the efficiency of penile SIV transmission would likely increase if an effort was made to directly inoculate the foreskin mucosa. In addition, HIV transmission to men is dramatically increased by the presence of genital ulcers69
; thus experimentally inducing penile inflammation prior to SIV inoculation may be another strategy to enhance penile SIV transmission efficiency while also modeling populations at greatest risk of acquiring HIV.
Transmission of HIV is relatively inefficient compared to other sexually transmitted infections (STI), which is reflected in the fact that in most cases a single variant or only a few variants from the viral quasispecies in an HIV-infected person successfully establishes systemic infection.70–73
As systemic infection with a single variant occurred in five of five animals after penile SIVmac251 transmission, this route of experimental SIV inoculation accurately reflects the virology of HIV transmission. In fact, when infection is transmitted, single SIV env
variants establish systemic infections more consistently after penile SIVmac251 exposure compared to vaginal SIVmac251 challenge. After vaginal challenge of randomly selected mature cycling multiparous rhesus macaques with a complex SIVmac251 quasispecies11,17
some animals became infected with one variant, but in about half of the animals, systemic infection was established by five or more unique variants.11
The transmission of multiple variants to some female rhesus may be due to the changes in physiology of the female reproductive tract during the menstrual cycle or the presence of vaginal inflammation at the time of inoculation. Although ulcerative STIs have not been described in macaques, female rhesus monkeys commonly have bacterial vaginosis74,75
that causes genital inflammation.76
Alternatively, the same factors (discussed above) that may make women relatively more susceptible to HIV infection may also increase the number of variants that are transmitted by vaginal SIV inoculation. It remains to be seen if experimentally induced inflammation alters the number of variants transmitted by penile SIV inoculation.
The genetic bottleneck that results in a single viral variant establishing an infection after HIV transmission43,52,70,71,77–79
could be due to a the presence of only a limited number of viral variants in secretions of the infected partner, selective transmission of viral variants, or selective amplification of viral variants that cross the mucosal barrier. Although there could be a reduction in overall diversity in mucosal secretions compared to plasma, this penile infection study is not consistent with first explanation since all five animals inoculated with the genetically diverse SIVmac251 quasispecies became infected with a unique, single virus variant. It should be noted that analyzing 20 SGA-derived sequences provides a 95% confidence estimate that variants representing greater than 15% of the population are not be missed.43
Thus while the most likely conclusion is that all five animals were infected with a single variant we cannot exclude the possibility an individual animal has more than one variant in the plasma sample tested.
Despite this caveat, this is now the fourth study to reach the conclusion that rhesus macaques became infected with a single env
variant after mucosal SIVmac251 exposure; the other studies tested animals after vaginal and rectal SIVmac 251 inoculation.11,17,56
Further, because all variants at set-point were derived from the variants we identified at ramp-up, all biologically relevant variants seem to have been found with our approach. Determining the relative importance of selective transmission versus selective amplification of a single variant prior to systemic dissemination will be important for understanding the challenges associated with preventing HIV transmission through vaccination. In some HIV clades, the transmitted HIV env
variants share features that presumably enhance transmission to a seronegative host, but they also make the transmitted variants relatively susceptible to antibody neutralization.80
In this case preexisting antibodies may be able to efficiently block infection by the founder viruses. On the other hand, if the neutralization-sensitive variants appear as the founder population in blood by outcompeting less fit viruses for limited target cells in the first few days after transmission, then a neutralization-resistant variants may emerge as the primary founder viruses in the presence of established vaccine-induced neutralizing antibodies.
Finally, although infection is established relatively inefficiently by penile SIV inoculation, these data are consistent with the epidemiology of HIV transmission in human populations and represent the first use of this route in NHP models. Even after penile inoculations with a high virus dose the pattern of viremia and the single variant founder SIV populations that establish infection accurately reflect the virology of HIV infection. Given the consistency of infection, this route of SIV inoculation can be used to model penile HIV transmission to better understand virus transmission and dissemination. Furthermore, this route of virus challenge will be very useful for accurately modeling HIV transmission in the preclinical testing of HIV vaccines.