(1) Background: OK-432 is a penicillin-killed, lyophilized formulation of a low-toxicity strain (Su) of Streptococcus pyogenes (Group A). It is a potent immunotherapy agent for several types of cancer, including oral cancer. We previously showed that (i) OK-432 treatment induces a high amount of IFN-? production from peripheral blood mononuclear cells (PBMCs), and (ii) conditioned medium (CM) from oral cancer cells suppresses both the IFN-? production and cytotoxic activity of PBMCs driven by OK-432. The aim of this study was to determine the inhibitory mechanism of OK-432-induced IFN-? production from PBMCs by CM. (2) Methods: We performed cDNA microarray analysis, quantitative RT-PCR, and ELISA to reveal the inhibitory mechanism of CM. (3) Results: We found that CD40 plays a key role in IFN-? production via IL-12 production. Although OK-432 treatment upregulated the expression levels of the IL-12p40, p35, and CD40 genes, CM from oral cancer cells downregulate these genes. The amount of IFN-? production by OK-432 treatment was decreased by an anti-CD40 neutralizing antibody. (4) Conclusions: Our study suggests that uncertain soluble factor(s) produced from oral cancer cells may inhibit IFN-? production from PBMCs via suppressing the CD40/CD40L-IL-12 axis.
Keywords: CD40; OK-432; conditioned medium; interferon-γ; oral cancer.