Quantitative and functional analysis of CD69+ NKG2D+ T regulatory cells in healthy subjects

T regulatory (Treg) cells have a key role in immune homeostasis and the pathogenesis of chronic inflammatory and autoimmune diseases. CD69 is an early leukocyte activation molecule that under steady state conditions is detected in a small proportion of lymphocytes in peripheral blood and lymphoid tissues. Although it has been reported that a subset of CD69+ T cells behaves as Treg lymphocytes, the possible relationship between CD69+ Treg cells and CD4+NKG2D+ T lymphocytes, which also exert immunosuppressive activity, has not been explored. In this study, we analyzed the expression of CD69 and NKG2D by T lymphocytes from the peripheral blood of twenty-five healthy subjects by multi-parametric flow cytometry analysis, and their suppressive activity by an assay of inhibition of lymphocyte activation (CD40L expression) and proliferation (carboxyfluorescein partition assay). We found a very small percentage of CD4+CD69+NKG2D+ T cells (median 0.002%, Q1–Q3, 0.001–0.004%), which also expressed TGF-β (Latency Associated Peptide or LAP) and IL-10, in all samples analyzed. These cells exerted an important in vitro suppressive effect on both activation and proliferation of T effector cells. Our data suggest that at very small numbers, CD4+CD69+NKG2D+ lymphocytes seem to exert a relevant functional immune-regulatory role in healthy subjects.