Impact of vitamin D plasma levels on in vitro natural killer cell cytotoxicity against non-small cell lung carcinoma in the presence of nivolumab

Non-small cell lung cancer (NSCLC) continues to be a leading cause of cancer-related death. Although immune checkpoint inhibitors (ICIs) targeting the PD-1/PD-L1 pathway have improved patient outcomes, responses vary widely. Natural killer (NK) cells are crucial to antitumor immunity but are often suppressed within the tumor microenvironment. Emerging evidence indicates that vitamin D may influence immune activity, potentially boosting NK cell cytotoxicity and enhancing ICI responses. This study investigated the relationship between PD-L1 expression, NK cell function, and donor vitamin D levels in NSCLC models. Serum vitamin D levels from 63 volunteers were measured by RIA, and NK cells from donors grouped by vitamin D status. NK cytotoxicity was tested using MTT assays on A549 (low PD-L1) and HCC827 (high PD-L1) NSCLC cell lines, with or without PD-1 blockade via nivolumab. Sex-related differences were also analyzed. A significant positive correlation was found between donor vitamin D sufficiency and NK cell cytotoxicity against HCC827 cells, but no such link was observed in A549 cells. This effect was the strongest in male donors with serum vitamin D levels of at least 20 ng/mL, with no additional benefit at higher levels. These findings suggest that maintaining adequate vitamin D levels could enhance NK cell responses to PD-1 inhibitors in NSCLC, especially in tumors with high PD-L1 expression. Overall, the results support exploring vitamin D as an affordable, immune-modulating adjunct to immunotherapy and highlight the importance of personalized, biomarker-guided treatment strategies.