Our primate model data show: 1) SHIV-1157ipd3N4 is transmissible across all mucosal routes tested; 2) relative mucosal permeability was rectal>vaginal>oral, reflecting the risk order of sexual HIV acquisition among humans; 3) SHIV-1157ipd3N4 was transmissible across inflamed, but not normal buccal mucosa; and 4) SHIV-1157ipd3N4 showed signs of pathogenicity during acute infection and caused gradual progression to AIDS.
Our R5 SHIV-C mucosal transmission data contrast our earlier rhesus macaque study involving SIVDeltaB670, where the oral route was 60x more permissive than the rectal route [14
], an unexpected result we ascribe to expanded SIVDeltaB670 coreceptor usage. In contrast, SHIV-1157ipd3N4 solely uses CCR5. As such, this R5 SHIV-C better reflects HIV-1 strains typically transmitted sexually among humans. Indeed, the permeability of intact macaque mucosae to SHIV-1157ipd3N4 was rectal>vaginal>oral, a pattern that not only followed the rank-order but also fell within the ranges extrapolated from HIV-1-exposed humans [9
] (reviewed in [1
]). These findings attest to the biological relevance of our new R5 SHIV-C/primate model.
Estimating the relative risks of HIV-1 acquisition due to exclusive orogenital contact among humans is difficult, and not surprisingly, a recent survey [1
] was unable to perform a meta-analysis of earlier reports. The complexity of human sexual practices makes it difficult to study sufficiently large numbers of individuals whose only risk of sexual HIV-1 acquisition is orogenital exposure. Assessing the route of HIV-1 acquisition depends on recall, which may be inaccurate and underestimate the influence of sexual practices known to be high-risk, such as lack of condom use for rectal intercourse. Several human cohort studies reported no cases of HIV-1 seroconversion attributable solely to orogenital contact (reviewed in [1
]); the only quantitative risk-per exposure estimate we could locate in the literature was a risk of 0.4 per 1000 exposures [35
]. Consequently, human epidemiological studies would have to enroll very large cohorts to more accurately estimate the relative risks of oral in relation to vaginal and rectal HIV-1 exposure. In contrast, primate model studies allow stringent control of virus dose, strain and tropism, timing, mucosal route and status of mucosal tissues. Our R5 SHIV-C/primate model system can address basic questions of mucosal permeability to a virus encoding HIV-1 env
with the tropism typical of that of sexually transmitted HIV-1. As such, our data confirmed that the oral route carried the lowest risk, but the difference between oral as compared to vaginal exposure was less than 10-fold in the absence of mucosal trauma or inflammation.
Possible sites of virus entry after oral challenge and subsequent viral dissemination have been examined previously in SIV/macaque models [17
]. When concentrated SIV was swabbed directly onto tonsils, rapid infection ensued at this site, followed by spread to local and regional LN [42
]. A subsequent study assessed initial virus target tissues and the rapidity of virus dissemination in infant/juvenile macaques upon repeated high-dose SIV challenge via buccal mucosa, gingiva and tonsils, followed by swallowing of the inoculum [17
]. Sacrifice one day after this method of virus exposure revealed high SIV DNA copy numbers in gingival tissues, esophagus, submandibular and peripheral LN, and Peyer’s patches. Given the number of positive tissues, initial portal(s) of viral entry remain unclear. To test whether local inflammation enhanced transmission, our study limited R5 SHIV exposure to buccal mucosa. The increased transmission via inflamed buccal mucosa we observed is compatible with clinical observations that the presence of oral ulcers was associated with seroconversion after oral HIV exposure [40
]. Given the infiltration of CD4+
cells into inflamed buccal mucosa in our macaques, the increased presence of viral target cells probably accounted for enhanced SHIV-C transmission.
Several MHC class I alleles have been linked to control of chronic SIV/HIV infection. Prior studies have shown that Mamu-A*01+
Indian rhesus better controlled chronic SIVmac infection and survived longer compared to animals without this allele (reviewed in [43
]). In our study, 4 out of 27 Indian macaques were Mamu-A*01+
. We found no correlation between Mamu-A*01 status and susceptibility to de novo
infection or viral set points. In fact, some Mamu-A*01+
animals had high viral RNA levels during acute infection. To our knowledge, no link has been established between MHC class I alleles and primate susceptibility to lentiviral acquisition. A priori, we would not expect to find any correlation, because MHC class I presentation of viral antigenic peptides can only occur after productive infection of the host has taken place.
Acute infection is associated with marked depletion of CD4+
memory T cells, primarily in mucosae in SIV-infected macaques and HIV-infected humans [44
]. SHIV89.6P and X4 SHIVs predominantly target/destroy naïve CD4+
T cells, leading to their rapid loss in peripheral blood [46
]. In contrast, during R5 SHIV-1157ipd3N4 infection, initial declines in peripheral blood memory T cells were followed by gradual loss of absolute numbers of CD4+
T cells, a pattern described for other R5 viruses [46
]. AIDS occurred in two SHIV-1157ipd3N4-infected macaques thus far. Gut CD4+
cell depletion together with the gradual T-cell depletion in blood mimic HIV disease progression in humans. Of note, another R5 SHIV, SHIVSF162P3
, also gradually induced AIDS in some but not all infected macaques [47
In summary, SHIV-1157ipd3N4 exhibits biological characteristics that parallel many aspects of HIV-1 transmission and pathogenesis in humans. This R5 SHIV-C could be a biologically relevant tool to assess mechanisms of mucosal transmission, including the role of local inflammation and coinfection with other pathogens [22
], and it could also be used to assess the protective potential of microbicides or vaccines in macaques of either Indian [48
] or Chinese origin against mucosal challenge.