Background:
Infants born to women with HIV (WWH), but not infected (HIV-exposed uninfected, HEU) have two to three times the mortality rate of infants born to women without HIV (WWoH). Tryptophan is an essential amino acid critical for immune development, neurocognitive development, and growth. We investigated the milk metabolome from WWH and WWoH in the pre-ART era to determine if metabolic perturbations may contribute to the impaired immune development of infants born to WWH.
Methods:
Untargeted metabolomics was performed on 1599 breast milk samples collected longitudinally up to 24 months post-partum from 38 WWoH and 288 WWH with known infant outcomes from a randomized clinical trial conducted in Lusaka, Zambia 2001-2008. The milks of WWH were further separated by infant outcome into 4 groups: HEU infants who survived (n=76) or died (n=78) and infants who became infected either via early mucosal transmission (n=53) or through breast milk (n=81). Linear mixed effects models were used to identify differentially abundant compounds in breast milk between the infant outgroups among WWH and WWoH. All p-values were adjusted for multiple comparisons using the Benjamini-Hochberg false discovery rate method.
Results:
1597/1599 samples were successfully analyzed with identification of 765 known biochemicals and 74 unknown biochemicals. Tryptophan levels were significantly lower in the milk samples of WWH compared to WWoH at all timepoints. Log-transformed kynurenine:tryptophan (KT) ratios were elevated in the milk of WWH at all timepoints (p < 0.001). The KT-ratio at 1 month was correlated with baseline maternal plasma and 1 month breast milk HIV RNA (r2 of 0.28 and 0.20, respectively), and inversely with baseline CD4 count (r2 = -0.32); all p<0.001. Dimethylarginine was elevated in the milk of WWH during the first 9 months of lactation. A novel metabolite, X-12127, was significantly elevated in the milk of WWH at all timepoints following the first week of life (p<0.001 through 12 mos, then p<0.1 at 15 and 18 mos).
Conclusions:
Tryptophan is significantly lower in the milk of WWH throughout early infancy. As breast milk serves as the only source of this essential amino acid, this depletion likely contributes to the impaired immune development of children born to WWH. Elevations of dimethylarginines may alter nitric oxide levels. Identifying key alterations and novel therapeutics is of critical importance for the one million children born to WWH each year.
