CD8 T cells expressing killer Ig-like receptors and NKG2A are present in cord blood and express a more naïve phenotype than their counterparts in adult blood

We report that natural killer receptors (NKR) for major histocompatibility complex (MHC) class I molecules (MHC-NKR), the inhibitory killer immunoglobulin-like receptors (KIR), and the CD94/NKG2A receptor are present on a small proportion of CD8 T cells in cord blood. On average, 1.67% of CD8 T cells in cord blood express KIR, and 0.74% expresses NKG2A, approximately five-fold less than in adult blood. CD8 T cells expressing MHC-NKR were present at similar levels in cord blood from preterm and term infants, and it is important that their presence was independent of placental pathology or infection. Cord blood CD8 T cells expressing MHC-NKR were relatively homogeneous and entirely CD27⁺, mostly CC chemokine receptor 7^- and granzyme B^-, with a majority being CD45RA⁺ and with no evidence for a skewed distribution of T cell receptor-Vβ when tested in KIR⁺ cells. This contrasted with adult blood, which was more heterogeneous, and where a majority of CD8 T cells expressing MHC-NKR was CD27^- and granzyme B⁺. Functional studies revealed that cord blood KIR⁺ CD8 T cells were as capable as KIR^- CD8 T cells in their ability to proliferate in response to CD3 ligation, yet it is interesting that they were more capable than KIR^- CD8 T cells in their ability to secrete interferon-γ. These data suggest that cord blood CD8 T cells expressing MHC-NKR are a unique subset of cells, distinct from those in adult blood, and may represent a less-differentiated population.