Phagocytosis of a Model Human Immunodeficiency Virus Target by Human Breast Milk Leukocytes Is Predominantly Granulocyte-Driven When Elicited by Specific Antibody

Studies demonstrate a protective effect of antibodies (Abs) in breast milk (BM) against mother-to-child transmission (MTCT) of human immunodeficiency virus (HIV). Contribution of the BM cellular component has been overlooked. The only clinical HIV vaccine trial to demonstrate efficacy, RV144, correlated protection with Abs mediating functions through the constant immunoglobulin region - the crystallizable fragment (Fc). These data support induction of vaccine Abs triggering antiviral activities by leukocytes through Fc receptors (FcRs). To measure Ab-dependent cellular phagocytosis (ADCP), an essential Fc-mediated response, by BM phagocytes. Materials and Methods: Cells were isolated from five human BM samples obtained at 7-183 days postpartum and analyzed for ADCP. Fluorescent beads coated with HIV envelope (Env) epitopes were used as targets. Sixty-seven to 100 mL of milk was utilized. Results: Total cell concentrations per milliliter were 16,083-222,857, with 1.6-12.3% being CD45 ⁺ leukocytes. ADCP activity was measurable using the HIV-specific Ab 830A. Use of the actin inhibitor cytochalasin D and FcR blocker indicated that ADCP was actin dependent and required FcR engagement. ADCP scores were variable, but largely consistent, across the samples studied, exhibiting <4-fold difference from lowest to highest activity for CD45 ⁺ cells. Of the CD45 ⁺ ADCP, significantly more activity was granulocyte derived (72-95%), while the remaining activity was monocyte driven. Conclusions: The data indicate that BM phagocytes can manifest antiviral activities in the presence of specific Abs and therefore may contribute to reduction of MTCT of HIV.