To improve natural killer group 2 member D (NKG2D)-dependent cytotoxicity, the inhibition of cleavage and release of major histocompatibility complex class 1-related chain (MIC) molecules from the tumor surface are required. Valproic acid (VPA), a histone deacetylase (HDAC) inhibitor, is able to induce cell-surface MICA/B on tumor cells. In the present study, the ability of VPA and gemcitabine (GEM) to upregulate MICA/B in pancreatic cancer cells was investigated, resulting in the inhibition of cleavage and release of MIC molecules from the tumor surface. Flow cytometry was used to quantify MICA/B expression in six human pancreatic cancer lines. Functional cytotoxic activity of γδT cells against pancreatic cancer cells treated with VPA and GEM was determined using cytotoxicity assays. At low doses of VPA (0.7 mM) and GEM (0.001 µM), which did not induce tumor growth alterations, the agents individually increased cell-surface MICA/B expression in MICA/B-positive cell lines, but not in the MICA/B-negative cell line. Furthermore, the combination of VPA and GEM synergistically induced cell-surface MICA/B expression. In MICA/B-positive cell lines, the increase in MICA/B expression was dependent on VPA concentration. The combination of low-dose VPA and GEM enhanced the susceptibility of the PANC-1 cell line to γδT cell-mediated tumor cell lysis. It was observed that soluble MIC was released from PANC-1 in the culture supernatant following treatment with GEM. However, the combination of low-dose VPA with low-dose GEM increased MICA/B expression without inducing soluble MIC, resulting in enhanced tumor cell lysis. The results of the present study suggest that the combined administration of low-dose VPA with low-dose GEM has the potential to enhance the therapeutic effects of immunotherapy in pancreatic cancer. Furthermore, it is proposed that the combination acts, in part, by upregulating MICA/B and prevents soluble MIC from being released.
Keywords: immune evasion; low-dose gemcitabine; low-dose valproic acid; major histocompatibility complex class I-related chain A/B; pancreatic cancer; γδT cell cytotoxicity.