FAP on human NK cells: insights from NK cell activation and crosstalk with cancer-associated fibroblasts

Abstract

Fibroblast activation protein alpha (FAP) is a post-prolyl proteolytic enzyme highly expressed in the tumor microenvironment (TME), particularly in cancer-associated fibroblasts (CAFs). Although previously thought to be restricted to CAFs, malignant cells and pathological fibroblasts, recent studies have identified FAP expression in natural killer (NK) cells. However, its expression and activity in NK cells remain poorly characterized. Here, we investigated FAP expression and activity in resting and cytokine-stimulated (IL-2 and IL-15) primary human NK cells and NK92 cells. NK cell activation resulted in a significant decrease in FAP protein expression and enzymatic activity. Treatment with the FAP inhibitor UAMC-1110 altered the expression of activating and inhibitory NK cell receptors and reduced perforin expression, though it did not impact degranulation or cytotoxic function. Culturing NK cells in CAF-conditioned medium or direct co-culture with CAFs increased FAP expression and activity in NK92 cells, with donor-dependent effects observed in primary NK cells. These conditions also led to a reduction in NK activating and inhibitory receptor expression. Furthermore, hypoxia upregulated FAP in both NK92 and primary NK cells. Overall, our findings demonstrate that FAP is downregulated in/on NK cells upon activation with IL-2 and IL-15, whereas it is upregulated under TME-mimicking conditions. This may suggest that FAP contributes to the phenotype formation of NK cells, particularly within the TME, where we hypothesize that NK cells might prioritize invasive capacity over cytotoxic capacity, thus upregulating FAP.