Screening assays and methods of using same for screening to identify modulator agents or compounds that affect pcdh18 mediated inhibition of T cell effector function are described herein. Pharmaceutical and immunogenic compositions comprising agents or compounds that modulate pcdh18 mediated inhibition of T cell effector function are also encompassed. Methods for modulating pcdh18 mediated inhibition of T cell effector function using agents identified using assays described herein in pharmaceutical and immunogenic compositions are also envisioned. Adenocarcinoma is an exemplary tumor type that expresses pcdh18 and for which such pharmaceutical and immunogenic compositions would confer benefit to patients. Also encompassed are methods for reducing pcdh18 mediated inhibition of T cell effector function so as to achieve more effective T cell responses to pcdh18 expressing tumors.