Survival versus neglect: redefining thymocyte subsets based on expression of NKG2D ligand(s) and MHC class?I

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Abstract
BALB/c thymocytes can be divided into three distinct subsets according to the expression of a ligand for the NK activation receptor NKG2D (NKG2D‐L) and the expression of MHC class I (MHC‐I). The first subset (MHC‐Imid/NKG2D‐Lhigh or “N+”) is predominately CD4+CD8+ double positive (DP), comprises ∼35% of thymocytes in a 6–8‐week‐old adult and contains uncommitted cells that have neither undergone selection nor are committed to death by neglect. The second subset (MHC‐Ilow/NKG2D‐Llow or “M–”), also mostly DP cells, comprises ∼50% of thymocytes and consists of cells committed to death by apoptosis, likely due to neglect. By contrast, the third subset (MHC‐Ihigh/NKG2D‐Llow or “M+”) is largely single positive (SP), represents ∼15% of thymocytes and mostly contains more mature cells that have undergone successful positive selection. The major advantage of the analysis is that it splits DP cells into two subpopulations, one committed to death by apoptosis and the other subjected to selection. The analysis also suggests that NKG2D‐L may play a role in thymocyte development.