Viral and immunological factors associated with breast milk transmission of SIV in rhesus macaques

Abstract

Background: The viral and host factors involved in transmission of HIV through breastfeeding are largely unknown, and intervention strategies are urgently needed to protect at-risk populations. To evaluate the viral and immunological factors directly related to milk transmission of virus, we have evaluated the disease course of Simian Immunodeficiency Virus (SIV) in lactating rhesus macaques (Macaca mulatta) as a model of natural breast milk transmission of HIV.

Results: Fourteen lactating macaques were infected intravenously with SIV/DeltaB670, a pathogenic isolate of SIV and were pair-housed with their suckling infants throughout the disease course. Transmission was observed in 10 mother-infant pairs over a one-year period. Two mothers transmitted virus during the period of initial viremia 14-21 days post inoculation (p.i.) and were classified as early transmitters. Peak viral loads in milk and plasma of early transmitters were similar to other animals, however the early transmitters subsequently displayed a rapid progressor phenotype and failed to control virus expression as well as other animals at 56 days p.i. Eight mothers were classified as late transmitters, with infant infection detected at time points in the chronic stage of the maternal SIV disease course (81 to 360 days). Plasma viral loads, CD4+ T cell counts and SIV-specific antibody titers were similar in late transmitters and non-transmitters. Late breast milk transmission, however, was correlated with higher average milk viral loads and more persistent viral expression in milk 12 to 46 weeks p.i. as compared to non-transmitters. Four mothers failed to transmit virus, despite disease progression and continuous lactation.

Conclusion: These studies validate the SIV-infected rhesus macaque as a model for breast milk transmission of HIV. As observed in studies of HIV-infected women, transmission occurred at time points throughout the period of lactation. Transmission during the chronic stage of SIV-infection correlated with a threshold level of virus expression as well as more persistent shedding in milk. This model will be a valuable resource for deciphering viral and host factors responsible for transmission of HIV through breastfeeding.

Figure 1

CD4+ T Cell Counts in Lactating Macaques. CD3+, CD4+ cell counts in peripheral blood of lactating macaques at various time points post-inoculation with SIV. The day of inoculation is represented by the 0 time point. Cell counts for individual monkeys are shown and the means of each transmission group are represented in the line graph. Early transmitters are shown in yellow, late transmitters in red and non-transmitters in blue.

Figure 2

Viral Load in Lactating Macaques. Viral RNA copies were measured by real-time RT-PCR from (A) peripheral blood plasma and (B) the cell-free fraction of milk samples obtained at indicated time points in each of fourteen lactating macaques. Milk samples that could be amplified by PCR, but had values calculated as ≤50 copies RNA/ml (amplifiable but not quantifiable) were indicated as having 50 copies. Samples from which viral RNA could not be amplified were indicated as having 1 copy. Early transmitters are shown in yellow, late transmitters in red and non-transmitters in blue.

Figure 3

Mean Plasma and Milk Viral Loads in Early, Late and Non-Transmitting Macaques. Viral RNA copies measured by real-time RT-PCR in cell-free fraction of milk samples obtained from lactating macaques. Samples that could be amplified by PCR, but had values calculated as having ≤50 copies RNA/ml (amplifiable but not quantifiable) were indicated as having 50 copies. Samples from which viral RNA could not be amplified were indicated as having 1 copy. Early transmitters are shown in yellow, late transmitters in red and non-transmitters in blue.