In utero transmission of HIV-1 occurs on average in only 3%–15% of HIV-1-exposed neonates born to mothers not on antiretroviral drug therapy. Thus, despite potential exposure, the majority of infants remain uninfected. Weak HIV-1-specific T-cell responses have been detected in children exposed to HIV-1, and potentially contribute to protection against infection. We, and others, have recently shown that the removal of CD4⁺CD25⁺ T-regulatory (Treg) cells can reveal strong HIV-1 specific T-cell responses in some HIV-1 infected adults. Here, we hypothesized that Treg cells could suppress HIV-1-specific immune responses in young children.

We studied two cohorts of children. The first group included HIV-1-exposed-uninfected (EU) as well as unexposed (UNEX) neonates. The second group comprised HIV-1-infected and HIV-1-EU children. We quantified the frequency of Treg cells, T-cell activation, and cell-mediated immune responses. We detected high levels of CD4⁺CD25⁺CD127⁻ Treg cells and low levels of CD4⁺ and CD8⁺ T cell activation in the cord blood of the EU neonates. We observed HIV-1-specific T cell immune responses in all of the children exposed to the virus. These T-cell responses were not seen in the cord blood of control HIV-1 unexposed neonates. Moreover, the depletion of CD4⁺CD25⁺ Treg cells from the cord blood of EU newborns strikingly augmented both CD4⁺ and CD8⁺ HIV-1-specific immune responses.

This study provides new evidence that EU infants can mount strong HIV-1-specific T cell responses, and that in utero CD4⁺CD25⁺ T-regulatory cells may be contributing to the lack of vertical transmission by reducing T cell activation.