In this study, which compared four antiretroviral regimens used to decrease MTCT of HIV-1, the use of sdNVP, ZDV/sdNVP, or HAART led to suppression of cell-free HIV-1 RNA in breast milk (Fig. 1). However, concurrently assessed levels and patterns of cell-associated HIV-1 DNA were not significantly different between treatment arms (Fig. 2). Our observation that cell-free HIV-1 RNA in breast milk was suppressed by antiretrovirals is consistent with several other studies [8,14,23,24] (M.H. Chung, J.N. Kiarie, B.A. Richardson, D.A. Lehman, J. Overbaugh, J. Kinuthia, et al., in preparation), whereas there are only limited data describing the effect of antiretrovirals on cell-associated HIV-1 in breast milk [14,24,25]. The findings of this detailed study, in which levels of HIV-1 DNA were quantified at approximately 10 timepoints during a 4–6-week follow-up, reinforce findings from a study that examined breast milk at two timepoints or less and observed no effect of HAART on HIV-1 DNA [14,25]. Studies in blood also report similar findings: levels of HIV-1 DNA are not significantly reduced in the first month of treatment, and decline by only 0.5–1 log by 1 year of HAART treatment, with an estimated half-life of approximately 20 weeks [26–30]. This similar response in breast milk and blood suggests that, systemically, there are large reservoirs of HIV-1-infected cells that persist despite treatment. Importantly, a persistent reservoir of virus in breast milk, which is consumed in large quantities by breastfeeding infants, may have an impact on breast milk transmission.
The study had several strengths and limitations. The strengths of the study included evaluation of three different markers of HIV-1 in breast milk, as well as assessment at up to 10 times during serial evaluation. Although this study involved women from two independent trials with different CD4 exclusion criteria, similar study procedures were utilized and women were derived from the same clinic population, resulting in similar baseline characteristics (Table 1). The similarities between the two trials allowed evaluation of four different regimens commonly used to prevent MTCT. Limitations of the study include the lack of a control arm of untreated women as a reference for normal changes in breast milk virus levels. At the time the study was conducted, it was unethical to include a no-treatment arm because evidence had shown that short-course treatments significantly reduce transmission rates [5,6]. However, the ZDV arm effectively acts as a no-treatment arm after the first 2 days postpartum because treatment ended at delivery and ZDV has a half-life of only 1–2 h. Moreover, the overall pattern of infected cells we observed in all four treated arms is consistent with the pattern observed in a previous study of untreated women, although the sampling in that study was not as intensive as in the current study : the levels of infected cells normalized to total breast milk cells increase during the first few weeks postpartum, presumably due to a change in the proportion of susceptible to total cells . In contrast, when infected cell levels were normalized per milliliter, concentrations per milliliter were highest in colostrum and decreased over the first few weeks postpartum, due to a higher concentration of total cells in early breast milk compared with later milk . Here, we chose to normalize on a per cell basis rather than per volume because we did not have precise volume measurements from which the cells were separated. An additional limitation to this study is the small sample sizes (10–18 women per arm, see Table 1), which could result in a lack of power to detect a significant difference in breast milk HIV-1 DNA between treatment arms. However, these sample sizes were sufficient to detect a significant difference in cell-free HIV-1 RNA, suggesting that there would have been adequate power to detect a difference in HIV-1 DNA of the same magnitude. With this small study, we cannot rule out a modest effect on the infected cell numbers, including a possible reduction in a subset of infected cells such as activated T cells, as discussed below.
The data presented here suggest there is a large reservoir of latently infected cells in breast milk that persist despite treatment, some of which express cell-associated HIV-1 RNA even while cell-free virus is reduced below the level of detection. Because reverse transcriptase inhibitors prevent new cells from being infected, the persistence of an infected cell reservoir indicates that most infected cells in breast milk do not turn over rapidly, suggesting they are either macrophages or resting T cells, which have an estimated half-life of weeks to months , rather than activated T cells, which turn over within days of infection [32,33]. One possible explanation for the suppression of cell-free HIV-1 RNA without suppression of HIV-1 DNA is that infected breast milk cells produce little virus, and that most cell-free virus originates outside of the breast milk compartment  at a site where drugs effectively suppress virus production. However, we propose an alternative model in which the infected and activated T-cell population in breast milk produces most of the cell-free virus, and this infected/activated T-cell population quickly declines in the presence of reverse transcriptase inhibitors due to cytopathic effects of the virus and normal cell turnover. This leads to a large decline in cell-free HIV-1 RNA because these cells typically express high levels of virus . HIV-1 DNA is not significantly reduced because it is estimated that T cells are a minor cell population in breast milk (<5%) [36–38], whereas macrophages are the major cell population [36,39,40]. The large pool of long-lived infected macrophages and resting T cells persists in the presence of reverse transcriptase inhibitors, but does not contribute appreciably to cell-free HIV-1 RNA because these cells express low levels of virus. Thus, as shown in Fig. 4, our model suggests that cell-free HIV-1 RNA can be suppressed by reverse transcriptase inhibitors, even when there is no significant change in the reservoir of infected cells in breast milk. However, a somewhat surprising finding is that cell-associated HIV-1 RNA levels decline less dramatically than cell-free HIV-1 RNA. One explanation for this apparent discrepancy is that virus expressed in macrophages may be sequestered in intracellular compartments rather than contributing to the cell-free virus pool, as suggested by in-vitro studies [41–43]. This supports the idea that even when cell-free virus is undetectable, there may be a reservoir of cell-associated virus capable of facilitating cell-to-cell transmission.
The findings reported here are particularly important in light of studies suggesting that infected cells are a source of transmitted virus in breast milk transmission [15,17,18]. Although infected cell levels are not significantly affected by short-course HAART, cell-free, and cell-associated RNA levels may be reduced, and this may contribute to reduced breast milk transmission rates. Alternatively, HAART may decrease transmission by providing prophylaxis to breastfeeding infants in addition to reducing the levels of infectious virus in breast milk. This idea is supported by data from multiple studies in which infant-only treatment or extended infant prophylaxis significantly reduced transmission rates [44–49]. The ability of short-course antiretroviral regimens to reduce breast milk transmission may depend upon infant prophylaxis (both directly and through passive transfer of antiretrovirals through breast milk) in addition to effects on breast milk HIV-1. These findings have implications for strategies to reduce breastfeeding transmission, which may require continued examination of the role of infant prophylaxis versus maternal treatment.
The authors thank the research personnel, laboratory staff, and data management teams in Nairobi, Kenya and Seattle, Washington; the Mathare North City Council Clinic for their participation and cooperation; the Divisions of Obstetrics and Gynaecology and Paediatrics at Kenyatta National Hospital for providing facilities for laboratory and data analysis; Francis Njiri for data management; Sandy Emery for help with laboratory assays; Daniel Matemo for sample processing; and Anne Piantadosi for helpful discussions and critical reading of the manuscript. Most of all we thank the mothers and children who participated in the trials.
The present work was supported by grants from the National Institutes of Health (HD 23412), the Elizabeth Glaser Pediatric AIDS Foundation, and Fogarty. Dara A. Lehman was supported by a Hearst Fellowship. Michael H. Chung was a scholar in the International AIDS Research and Training Program and is supported by the Fogarty International Center, National Institutes of Health (D43-TW00007) and by an NIH K23 award. Grace John-Stewart is an Elizabeth Glaser Pediatric AIDS Foundation (EGPAF) Scientist.
Dara A. Lehman conducted the laboratory assays, performed statistical analysis, interpreted the data and wrote the article. Michael H. Chung implemented and helped design the study, supervised the on-site data management, and helped write the paper. Grace C. John-Stewart designed the study, obtained funding, helped to implement the study, interpret the data, and write the paper. Barbra A. Richardson advised the statistical analysis, helped to design the study and write the paper. James Kiarie implemented the study, monitored adverse events, and contributed to the study's design. John Kinuthia enrolled and examined the subjects, filled the questionnaires, and implemented the study. Julie Overbaugh supervised the laboratory testing, interpretation of data, design of the study, and writing of the paper.
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