High Cell-Free Virus Load and Robust Autologous Humoral Immune Responses in Breast Milk of Simian Immunodeficiency Virus-Infected African Green Monkeys

Abstract

The design of immunologic interventions to prevent postnatal transmission of human immunodeficiency virus (HIV) will require identification of protective immune responses in this setting. Simian immunodeficiency virus (SIV)-infected rhesus monkeys (RMs), a species that develops an AIDS-like illness following experimental infection, transmit the virus at a high rate during breastfeeding. In contrast, postnatal transmission of SIV occurs rarely or not at all in natural, asymptomatic primate hosts of SIV. These contrasting transmission patterns provide a unique opportunity to study mechanisms that evolved to protect suckling infants from SIV infection. We compared the virologic and immunologic properties of milk of SIV-infected and uninfected natural hosts of SIV, African green monkeys (AGMs), to that of RMs. Interestingly, despite a low number of milk CD4؉ T lymphocytes in uninfected AGMs, milk virus RNA load in SIV-infected AGMs was comparable to that of SIV-infected RMs and that in AGM plasma. This observation is in contrast to the relatively low virus load in milk compared to that in plasma of SIV-infected RMs and HIV-infected women. Milk of SIV-infectedAGMs also displayed robust virus-specific cellular immune responses. Importantly, an autologous challenge virus-specific neutralization response was detected in milk of five of six SIV-infected AGMs that was comparable in magnitude to that in plasma. In contrast, autologous challenge virus neutralization was not detectable in milk of SIV-infected RMs. The autologous virus-specific adaptive immune responses in breast milk of AGMs may contribute to impedance of virus transmission in the infant oral/gastrointestinal tract and the rarity of postnatal virus transmission in natural hosts of SIV.

Lack of B Cell Dysfunction Is Associated with Functional, gp120-Dominant Antibody Responses in Breast Milk of Simian Immunodeficiency Virus-Infected African Green Monkeys

“…Interestingly, these animals only rarely transmit the virus to their infants (22)(23)(24)(25), despite repeated daily exposure to high milk virus RNA loads (26,27). This phenomenon is in contrast to pathogenically SIVinfected, Asian-origin nonnatural SIV hosts and HIV-infected humans, which exhibit higher rates of postnatal virus transmission through breastfeeding (28).…”

“…We have previously shown that the milk of SIV-infected AGMs harbors RNA virus loads comparable to that of SIV-infected RhMs, nonnatural hosts that readily transmit the virus via breastfeeding, suggesting that the rarity of postnatal SIV transmission in AGMs is not attributable to lowlevel virus exposure of the infant (27). Therefore, it is plausible that the milk of SIV-infected, lactating AGMs contains immunologic factors which could mediate protection of breastfeeding infants from virus acquisition.…”

“…We previously described robust autologous virus-specific neutralizing antibody responses in the milk of chronically SIV-infected AGMs (27). To further characterize the antibody responses in these natural SIV hosts, we evaluated the plasma and milk autologous SIV gp120-specific IgG and IgA binding responses at 4 months postinfection and compared them to that of RhMs at the same time point following infection and chronically HIV-infected lactating women.…”

“…Automated complete cell counts are skewed by the fat droplets present in milk and cannot be used to quantitate lymphocyte numbers in milk. In order to calculate absolute lymphocytes per microliter of milk, the absolute number of lymphocytes determined by flow-cytometric analysis of each sample type was normalized to the ratio of fluorospheres (AccuCheck counting beads; Invitrogen, Carlsbad, CA) added to the original sample and quantitated by flow cytometry at the end of each staining procedure, as described previously (27). The normalized number of lymphocytes was then divided by the volume of the original milk sample volume.…”

“…African green monkeys (AGMs), a natural host species of SIV, are an excellent model in which to study the potentially protective immune responses in milk of natural SIV hosts. Strikingly, we previously identified strong autologous virus neutralizing antibody responses in the milk of chronically SIV-infected AGMs (27), in contrast to the lack of detectable autologous neutralizing response in milk of HIV-infected women (13) or SIV-infected RhMs (14). Thus, we sought to characterize the mucosal and systemic SIV-specific B cell responses in lactating AGMs that arise during acute infection and may contribute to this robust mucosal antibody response.…”