SPHK1 and head and neck squamous cell carcinoma: Next, activated CD8+ T cells were co-cultured with HNSCC in the presence of SPHK1/MMP1 knockdown or overexpression for 48 h, and SPHK1/MMP1 was found to be negatively correlated with the levels of IFN-γ and TNF-α released by CD8+ T cells (Figure 3F).