NKG2D engagement of colorectal cancer-specific T cells strengthens TCR-mediated antigen stimulation and elicits TCR independent anti-tumor activity

The NKG2D receptor is expressed by human NK, γδ T and α/β T lymphocytes and its engagement results in the stimulation of effector cells. We evaluated the role of NKG2D receptor in anti-colorectal cancer (CRC) immune response. The cell surface expression of stress-inducible NKG2D ligands MICA/B (MHC class I-related chain molecules A/B) and ULBP (UL16 binding protein) by a panel of CRC lines was evaluated by flow cytometry. MICA and ULBP2/3 were widely expressed by the analyzed lines, with a minority of them being also ULBP-1⁺, whereas MICB was undetectable. CD8⁺ and CD4⁺ HLA-restricted anti-tumor T cell clones of a CRC patient were used to evaluate whether NKG2D engagement could mediate tumor recognition. Three out of four CD8⁺ T cell clones recognized the autologous tumor with a marginal NKG2D engagement, a finding that was correlated with the weak expression of NKG2D ligands by the autologous tumor. On the contrary, NKG2D triggering of these CD8⁺ T cell clones induced recognition of allogeneic CRC lines showing high expression of MICA and ULBP. A costimulatory role of NKG2D was observed with one CD4⁺/NKG2D⁺ T cell clone when stimulated by tumors sharing the HLA class II alleles and expressing NKG2D ligands. Taken together these data indicate that the engagement of NKG2D, depending on the expression of its ligands by target cells, can influence the pattern of antitumor reactivity by T lymphocytes.