Involvement of CD166 in the Activation of Human γδT Cells by Tumor Cells Sensitized with Nonpeptide Antigens

Abstract We previously reported that human Vγ2Vδ2-γδT cells were activated by many human tumor cell lines treated with pamidronate (PAM) in a γδTCR-dependent manner. In the present study, we indicated that a synthetic pyrophosphomonoester Ag, 2-methy-3-butenyl-1-pyrophosphate, could directly “sensitize” the tumor cells to activate γδT cells independently of the host metabolism, while the sensitizing effect of PAM was reported to be dependent on the pharmacological activity. Some exceptional tumor cells that failed to be sensitized by PAM were incapable of activating γδT cells by the treatment with 2-methy-3-butenyl-1-pyrophosphate either, suggesting a requirement of host factor(s) for the effective γδT cell activation in addition to the nonpeptide Ags. By screening mAbs against a large panel of tumor cell lines, we found that the expression of CD166 closely paralleled the capacity of activating γδT cells upon PAM treatment. The transfection of a CD166-negative tumor cell line with CD166 cDNA caused a marked enhancement of the capacity to activate γδ T cells following PAM treatment. On the contrary, down-regulation of the CD166 expression in a CD166-bearing tumor cell line by short hairpin RNA resulted in a significant reduction of PAM-induced γδΤ cell-stimulatory activity. γδT cells expressed CD6, a receptor of CD166, and CD6 and CD166 were recruited together to the center of synapse between γδ T cells and PAM-treated tumor cells, colocalizing with γδTCR/CD3. The results suggested that the engagement of CD6 with CD166 on tumor cells played an important role in the γδT cell activation by the tumor cells loaded with nonpeptide Ags either endogenously or exogenously.