Natural killer cells from malignant pleural effusion show a decidual-like phenotype and an alternative activatory state.

Natural killer (NK) cells are lymphoid cells of the innate immune system crucial in tumor and virus-infected cell recognition and eradication, through cytotoxicity and proinflammatory cytokine release. They are characterized by cytoplasmic granules and exert their cytotoxicity by releasing perforin and granzymes. NK cells also play a crucial role in the production of immunoregulatory cytokines and chemokines, which could have an impact on function of other leucocytes, placing NK cells as orchestrators in the crosstalk between innate and adaptive immunity. However, NK cell activity is often impaired in cancer patients and in NSCLC and colorectal cancer patients, we recently demonstrated that they acquire proangiogenic phenotype and functions. We characterized NK cells from peripheral blood (PB) and pleural effusions (PE) of patients with either inflammatory diseases (iPE, n = 18), primary tumor (ptPE, n = 18), and metastatic tumor (mtPE, n = 27) by multicolor flow cytometry, for surface antigens expression and cytokine release, along with functional assay for angiogenesis. We also investigated whether NK cells from different PEs can acquire decidual-like NK phenotype and whether exposure to IL-2 can restore their killing ability in the presence of pleural fluids. We found that CD56brightCD16- were predominant in mtPE (60%), while representing the 40% and 35% of total NKs from ptPE and iPE, respectively. NK cells from both ptPE and mtPE showed increased expression for the CD49a and CD69 decidual-like NK markers and decreased levels of the CD57 maturation marker. NK from all the PE analyzed showed impaired degranulation capability against K562 tumor cell target and reduced perforin release. PE-NK cells efficiently responded to IL-2 stimulation in vitro, but addition of TGFβ or cell-free pleural fluid blocked IL-2-mediated degranulation and IFNγ release. We observed the same effects when using healthy donor-derived NK cells (n = 6), exposed to PEs. We found that mtPE-NK cells produce VEGF and support the formation of capillary-like structures in human umbilical vein endothelial cells. Our results suggest that the PE tumor microenvironment can shape NK cell polarization towards a low cytotoxic, decidual-like, highly proangiogenic phenotype and that IL-2 treatment is not sufficient to limit this process.