Involvement of Toll-like receptor 4 signaling in interferon-gamma production and antitumor effect by streptococcal agent OK-432.

BACKGROUND The streptococcal agent OK-432 has been used for immunotherapy of head and neck cancer, among other malignancies, but its mechanism of action is unknown. Because the Toll-like receptor 4 (TLR4)/MD-2 complex is important in enabling the mammalian immune system to recognize bacterial components, we investigated whether expression of the TLR4 and MD-2 genes is associated with OK-432-induced anticancer immunity. METHODS Peripheral blood mononuclear cells (PBMCs) from 28 patients with head and neck cancer were analyzed for TLR4 and MD-2 mRNA expression by reverse transcription-polymerase chain reaction (RT-PCR) analysis. PBMCs were treated in vitro with OK-432 or with OK-PSA (a lipoteichoic-acid-related molecule that is an active component of OK-432), and interferon-gamma (IFN-gamma) mRNA expression, an immune response measure, was analyzed by RT-PCR. Patient sera collected 24 hours after OK-432 administration were examined for IFN-gamma protein using an enzyme-linked immunosorbent assay. Lewis lung carcinoma-bearing wild-type C57BL/6 and TLR4-deficient mice (four mice per group) received intraperitoneal injections of OK-432, and tumor volumes and sera IFN-gamma levels were measured over time. All statistical tests were two-sided. RESULTS Twenty patients expressed both TLR4 and MD-2. Expression of TLR4 and MD-2 genes was associated with the in vivo IFN-gamma induction in 19 patients administered OK-432 (Fisher's exact test P<.001). Although both OK-432 and OK-PSA induced IFN-gamma expression from PBMCs in vitro, expression of TLR4 and MD-2 was associated only with IFN-gamma expression induced by OK-PSA (P<.001). In vivo intraperitoneal administration of OK-432 resulted in an increase of IFN-gamma in sera from wild-type mice but not in sera from TLR4-deficient mice. Tumors in wild-type mice treated with OK-432 were statistically significantly smaller than those in mice treated with saline (P =.007). By contrast, in TLR4-deficient mice, there was no difference in tumor volume between the two treatment groups. CONCLUSIONS TLR4 and MD-2 may mediate OK-432-induced anticancer immunity.