Functional exhaustion of human follicular cytotoxic T cells is associated with CEACAM1 expression and dysregulation of the SHP1–LCK–ZAP70 signaling axis

Effective antitumor immunity in ovarian cancer remains limited, partly due to profound dysfunction of tumor-infiltrating T cell subsets. Follicular cytotoxic T cells (Tfc), a recently identified CD8 + T cell population sharing characteristic features of follicular helper T cells, serve as key mediators of antitumor immune responses; yet the molecular mechanisms governing Tfc cell activity remain poorly characterized, especially within the tumor microenvironment. Here, using in vitro cell culture models, we show that the immune checkpoint protein carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1) is enriched on a distinct subset of functionally impaired, in vitro-differentiated Tfc cells and is associated with impaired effector functions. Mechanistically, CEACAM1 expression correlates with elevated SHP-1 activation, which disturbs proximal TCR signal transduction, accompanied by reduced total LCK and ZAP70 protein abundance, which are associated with the functionally exhausted phenotype of Tfc cells. Genetic or pharmacological modulation of the SHP-1/LCK/ZAP70 signaling axis in CEACAM1-positive Tfc cells partially restored proximal TCR signaling together with cytotoxic and B cell helper functions. Collectively, our in vitro findings support the involvement of a CEACAM1-associated SHP-1 signaling axis in the functional impairment of human Tfc cells and provide a rationale for further investigating this pathway as a potential target to enhance antitumor immunity.