PD-1, CTLA-4, LAG-3, and TIGIT: The roles of immune checkpoint receptors on the regulation of human NK cell phenotype and functions

The roles of immune checkpoint receptors were defined in many cancers and autoimmune diseases, while there is limited information on their functional roles in the NK cells of healthy individuals. Immune checkpoint receptor expression of NK cell subsets and their association with NK cell functions (cytotoxic capacity and cytokine production) in healthy population were investigated. PD-1, CTLA-4, LAG-3 and TIGIT expression of peripheral blood NK cells, cytokine levels (TNF-α, IFN-γ, IL-10) and cytotoxic functions (granzyme A, perforin, CD107a; with/without K562 target cell stimulation) were evaluated by flow cytometry. CD56dimCD16dim NK cells had the highest expression of TIGIT, while CD56dimCD16- NK cells had highest expression of PD-1, CTLA-4 and LAG-3. PD-1+ NK cells, CTLA-4+ NK cells and LAG-3+ NK cells had increased amount of IL-10 however, reduced IFN-γ and TNF-α levels. Cytotoxic granule expressions (perforin and granzyme A) were reduced in PD-1+ NK cells, CTLA-4+ NK cells and LAG-3+ NK cells. However, TIGIT expression did not alter perforin and granzyme A expressions. Degranulation capacity was reduced in three groups of NK cells (PD-1+ or LAG-3+ or TIGIT+). TIGIT+ NK cells responded strongly to target cell stimulation, while NK cells in the other groups (PD-1+ or CTLA-4+ or LAG-3+) were resistant. PD-1+ NK cells, CTLA-4+ NK cells and LAG-3+ NK cells had a regulatory phenotype, impaired cytotoxic functions, and response to target cell stimulation. In contrast, TIGIT+ NK cells had strong baseline cytotoxic activity that further increased in response to target cell stimulation.