Background:  Transmission of HIV via breast milk is a significant source of pediatric infection, yet the majority of infants do not acquire infection through this route. This latter finding may be due to the low levels of HIV RNA in breast milk compared with plasma. We therefore hypothesized that the magnitude and quality of T cells in breast milk are different than the peripheral blood compartment as an explanation for the lower viral load in the former compartment.

Methods:  PBMC and breast milk cells from HIV⁺ (5) and HIV^- (9) women were stained with the following reagents:  MHC class I tetramers, anti-HLA-DR, CD57, CD69, CD38, CD45RO/RA, CD103, CCR7, and CD62L. Cells were also evaluated with the interferon-g ELISpot assay after antigenic stimulation with HIV peptides spanning the entire expressed genome and HLA class I restricted epitopes specific for HIV, CMV, and EBV. The breast milk and the peripheral milk compartments were compared, and hypothesized differences were evaluated using the Mann-Whitney U and Fisher’s exact tests.

Results:  The breast milk compartment contained an increased percentage of lymphocytes that were CD8⁺ compared with peripheral blood (51.1% vs 32.5%; p = 0.04) with an even higher percentage observed in HIV⁺ vs HIV^- women (70.3% vs 49%; p = 0.01). A greater percentage of T cells were CD57⁺CD45RO⁺HLA-DR⁺ in breast milk vs peripheral blood (p <0.01 for all comparisons). A greater percentage of breast milk T cells expressed the intestinal homing receptor, CD103 (11% vs 0.6%; p <0.0001) with more breast milk CD8⁺T cells being CD103⁺ compared with breast milk CD4⁺ T cells (22.5% vs 5.7%; p <0.0001). As expected, breast milk T cells expressed less CD62L (4.7% vs 42.4%; p <0.001) and CCR7 (1.5% vs 61.5%; p <0.001). Using identical antigenic stimulation, cells from the breast milk elicited a greater number of positive responses than the peripheral blood (29/41 vs 15/41, respectively; p <0.01). Furthermore, a higher frequency of CD8⁺ T cells was seen in breast milk (average of 855 SFC/10⁶ per epitope response) as compared with PBMC (65 SFC/10⁶ per epitope; p <0.001).

Conclusions:  T cells found in the breast milk compartment express a highly activated/effector phenotype that preferentially migrate to this area. These findings may not be specific for HIV, as other antigen responses also appear increased in breast milk. Nevertheless, our data are consistent with the hypothesis that increased numbers of activated HIV-specific CD8⁺ T cells result in lower viral RNA levels in the breast milk than peripheral blood compartments.   

Keywords: T cells; Breast milk; MTCT