Natural Killer (NK) cells differentiate diseased from healthy cells via a balance between activating and inhibitory receptors; the latter recognizing self-MHC class I. In a process called NK cell education NK cells are granted functional competence. One of the key proteins in this process is SHP-1, a phosphatase which dephosphorylates downstream signalling molecules.

In a previous study, we identified a key role for SHP-1 in NK cell tolerance and established a link between NK cell education and intracellular localization of SHP-1. To gain further insights into the NK cell education process, we studied known cell surface markers and identified Ly49A, Ly49G2, Ly49C, DNAM-1, KLRG1, NKG2D and NKp46 as education markers in H2D^d single mice. Of those, KLRG-1 was not only responsive to education as such, but also to quantitative input from H2D^d during the education process according to the rheostat model.

The rheostat model suggests that NK cells carrying different levels of education receptors or ligands themselves should be differentially responsive. When we tested this hypothesis, we found an expected correlation between responsiveness and KLRG-1 and SHP-1 expression. Surprisingly, cells with high Ly49A expression were less responsive to NK1.1 receptor triggering compared to cells with low Ly49A, opposite from what we expected from the rheostat model. Similar data was obtained also in Ly49C and Ly49G2 NK cell subsets in H2D^d mice, but no link between Ly49 receptor expression and function was seen in subsets carrying the non-educating receptor Ly49I in H2D^d mice, or in NK cells from MHC-deficient mice. Imaging SHP-1 and Ly49A in H2D^d NK cells revealed substantial heterogeneity regarding the cellular localization or both Ly49A and SHP-1 in resting NK cells, corroborating these results. Our results suggest am unexpected functional heterogeneity in a seemingly homogenous NK cell population that warrant investigation.