MMP1 and head and neck squamous cell carcinoma: The results showed that compared to the NC group, knocking down SPHK1 and MMP1 made HNSCC cells more sensitive to CD8+ T-cell-mediated killing, whereas overexpression of SPHK1 and MMP1 increased the survival rate of HNSCC cells co-cultured with T cells, indicating that SPHK1 positively regulates PD-L1 via MMP1 to reduce the ability of CD8+ T cells to kill HNSCC cells (Figure 3A-B).