In adults, HIV-1 acquisition across oral mucosal surfaces is infrequent, compared with other modes of transmission, and accounts for <10% of primary HIV-1 infections [10
]. In infants, HIV-1 infection after oral exposure is more common; HIV-1 transmission through breast milk is responsible for ~50% of HIV-1 infections among breast-fed infants [12
]. Salivary antiviral factors and, specifically, the SLPI protein may contribute to protecting oropharyngeal mucosa against invasion by HIV-1 [4
]. Establishing a role for SLPI in preventing oral HIV-1 infection could lead to the development of novel interventions, such as using recombinant SLPI for HIV-1 prevention.
In the present study, the first to evaluate the protective effect of salivary SLPI, we determined salivary SLPI concentrations in a cohort of 188 breast-fed, HIV-1–exposed infants. Salivary SLPI levels among these infants were inversely correlated with infant age and not significantly associated with any maternal or infant characteristics. SLPI concentrations at birth were not associated with the overall risk of HIV-1 infection. However, SLPI did play a role in protection from late HIV-1 transmission through breast milk. Higher salivary SLPI levels at month 1 were associated with a decreased risk of HIV-1 infection among infants who were HIV-1 exposed via breast-feeding after month 1. These findings are consistent with results from a study of SLPI in cervicovaginal secretions from HIV-1–infected pregnant women [7
] but contrast with the results of a study on colostrum and breast-milk SLPI concentrations in a cohort of HIV-1–infected, breast-feeding mothers [6
]. Elevated cervicovaginal SLPI was found to protect against infant HIV-1 infection in the former study [7
], but, in the latter study, neither colostrum nor breast milk SLPI was associated with protection against HIV-1 transmission [6
Differences in results are unlikely to be explained by variations in the mechanism of action of SLPI, because this would be the same in saliva, breast milk, and cervicovaginal secretions. SLPI interferes with HIV-1 entry into monocytes and lymphocytes by binding to a host cell transmembrane protein rather than by interfering directly with viral replication or binding to HIV-1 [13
]. In studies in which virus was pretreated with SLPI, there was no inhibition of viral entry into cells; pretreatment of host cells with SLPI in the absence of virus also did not inhibit viral entry into cells [14
]. Thus, both SLPI and virus need to be present simultaneously at the site of viral invasion for SLPI to exert an inhibitory effect. Cervicovaginal, breast milk, and salivary SLPI, therefore, would be expected to act in a similar manner to prevent HIV-1 infection of infant mucosal cells.
Discrepant results are more likely to be due to differences in SLPI concentrations in infant saliva, cervicovaginal secretions, and breast milk. However, comparing SLPI levels in different body fluids is problematic, because some sample types were diluted before SLPI testing (i.e., infant saliva and cervicovaginal secretions) [7
], whereas others were not diluted (i.e., colostrum and breast milk) [6
]. To compare salivary SLPI collected in this study with colostrum and breast milk, a crude adjustment using the dilution factor must be made. This results in an ~4-fold increase in mean salivary SLPI concentration, from 255 to 1020 ng/mL. This higher value is comparable to adult levels (103
] and to SLPI concentrations that have been reported to maximally inhibit HIV-1 in vitro (103
]. Mean SLPI concentrations in breast milk (~250 ng/mL) were lower than this inhibitory threshold, providing one explanation for the absence of protection associated with breast milk SLPI concentrations at 1 and 6 months.
Low SLPI concentrations do not explain why colostrum, with a mean SLPI concentration ~10-fold higher than that in breast milk, did not influence HIV-1 infection. The transient nature of colostrum may contribute to an inability to alter HIV-1 transmission rates. Alternatively, the small sample size (43 mother-infant pairs) in the study of breast milk SLPI may have limited the ability to define a protective effect on HIV-1 transmission through breast milk, making additional studies in larger populations important to confirm these findings.
In conclusion, this study is the first to report SLPI levels in infants and to evaluate the role of this important innate immune mechanism in mother-to-child HIV-1 transmission. The observation that higher salivary SLPI levels protect against HIV-1 infection after exposure to HIV-1 through breast milk supports further research into the development of pharmacological agents that would mimic the actions of SLPI in vivo. In populations where HIV-1–seropositive women often breast-feed their infants, such agents may contribute to the prevention of mother-to-child HIV-1 transmission. Further investigation of SLPI and other endogenous proteins will be important to develop new HIV-1 prevention strategies, strengthen our understanding of mucosal immunity, and clarify the role of SLPI in reducing mother-to-child HIV-1 transmission via breast milk.