Background: Breastfeeding is a major mode of mother-to-child HIV-1 transmission in the developing world. The mechanisms of oral HIV-1 transmission are poorly understood. To determine what types of viruses are selectively transmitted and to identify the tissue targets of infection in the alimentary mucosa, we inoculated neonatal macaques orally with simian immunodeficiency virus (SIV) strains that are limited to a single cycle of infection.

Methods: Neonatal macaques were inoculated orally with a mixture of sequence-tagged strains of single-cycle SIV that differ in infectivity, co-receptor utilization, and cellular tropism. These strains included single-cycle SIV_mac239, which uses CCR5 as a co-receptor and preferentially infects memory CD4⁺ T cells, scSIV_mac316, which also uses CCR5, but has enhanced infectivity for macrophages, and scSIV_mac155T3, which uses CXCR4 for infection of naive and memory CD4⁺ T cells. Animals were sacrificed on days 2, 3, and 4 post-inoculation and viral RNA loads in plasma were measured independently for each strain using a multiplex quantitative, real-time polymerase chain reaction (RT-PCR) assay. Nucleic acids were also extracted from tissues and screened for the presence of viral RNA using an RT-PCR assay for the detection of multiply-spliced tat/rev RNA transcripts that are only present in productively infected cells.

Results: Of the 6 animals, 4 had quantifiable plasma viral loads for single-cycle SIV_mac239 and 1 animal had a quantifiable plasma viral load for single-cycle SIV_mac155T3; 2 additional animals had detectable plasma viral loads for single-cycle SIV_mac155T3 that were below the threshold for quantitative analysis. None of the 6 animals had detectable viremia for single-cycle SIV_mac316. RT-PCR amplification of multiply-spliced tat/rev transcripts from tissue RNA revealed that 8 of 52 oropharyngeal samples, 11 of 78 gastrointestinal samples, 6 of 57 lymphoid samples, and 3 of 32 systemic samples were positive for infection (>3 of 6 replicate RT-PCR reactions).

Conclusions: Higher plasma viral loads for single-cycle SIV_mac239 suggest that this strain was better able to establish infection across mucosal barriers of the alimentary canal, perhaps reflecting a greater abundance of susceptible CCR5⁺ target cells. However, the detection of plasma viral loads for single-cycle SIV_mac155T3 indicates that there is no inherent barrier to the transmission of X4-tropic viruses. The presence of SIV tat/rev RNA transcripts in oropharyngeal and gastrointestinal tissues suggests that virus transmission may occur at multiple sites along the alimentary canal.