Functional Dichotomy between NKG2D and CD28-Mediated Co-Stimulation in Human CD8+ T Cells

Both CD28 and NKG2D can function as co-stimulatory receptors in human CD8(+) T cells. However, their independent functional contributions in distinct CD8(+) T cell subsets are not well understood. In this study, CD8(+) T cells in human peripheral blood-and lung-derived lymphocytes were analyzed for CD28 and NKG2D expression and function. We found a higher level of CD28 expression in PBMC-derived naive (CD45RA(+)CD27(+)) and memory (CD45RA(-)CD27(+)) CD8(+) T cells (CD28(Hi)), while its expression was significantly lower in effector (CD45RA(+)CD27(-)) CD8(+) T cells (CD28(Lo)). Irrespective of the differences in the CD28 levels, NKG2D expression was comparable in all three CD8(+) T cell subsets. CD28 and NKG2D expressions followed similar patterns in human lung-resident GILGFVFTL/HLA-A2-pentamer positive CD8(+) T cells. Co-stimulation of CD28(Lo) effector T cells via NKG2D significantly increased IFN-gamma and TNF-alpha levels. On the contrary, irrespective of its comparable levels, NKG2D-mediated co-stimulation failed to augment IFN-gamma and TNF-alpha production in CD28(Hi) naive/memory T cells. Additionally, CD28-mediated co-stimulation was obligatory for IL-2 generation and thereby its production was limited only to the CD28(Hi) naive/memory subsets. MICA, a ligand for NKG2D was abundantly expressed in the tracheal epithelial cells, validating the use of NKG2D as the major co-stimulatory receptor by tissue-resident CD8(+) effector T cells. Based on these findings, we conclude that NKG2D may provide an expanded level of co-stimulation to tissue-residing effector CD8(+) T cells. Thus, incorporation of co-stimulation via NKG2D in addition to CD28 is essential to activate tumor or tissue-infiltrating effector CD8(+) T cells. However, boosting a recall immune response via memory CD8(+) T cells or vaccination to stimulate naive CD8(+) T cells would require CD28-mediated co-stimulation.