Fibroblast activation protein on human natural killer cells: insights from natural killer cell activation and crosstalk with cancer-associated fibroblasts

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