Lysates of S. pyogenes Serotype M49 Induce Pancreatic Tumor Growth Delay by Specific and Unspecific Antitumor Immune Responses

Treatment of pancreatic cancer by active unspecific bacterial immunotherapy is a promising new strategy. Recently, we showed that a single intratumoral injection of wildtype Streptococcus pyogenes M49 results in complete regression of pancreatic carcinoma in mice mediated both by unspecific cytotoxicity and by specific immune reactions against tumor cells. As for potential clinical use, conditioning and especially inactivation of bacteria would abolish the risk of systemic bacterial infections; we here explored the potential of a streptococcal lysate prepared by bacteriophage lysine to affect pancreatic carcinoma growth in vivo. Application of the lysate into established Panc02 tumors resulted in pronounced growth cessation accompanied by raises in levels of circulating monocytes, granulocytes, and natural killer cells. Detailed analysis of splenocyte subsets revealed lysate-induced transient increases in pre-B cells followed by raised levels of activated T cells. Moreover, blood levels of proinflammatory, T helper-1–type cytokines were significantly elevated. These systemic immunologic effects were accompanied by massive infiltrations of cytotoxic T cells into the tumors. Concomitantly, lymphocytes obtained from treated mice specifically recognized Panc02 tumor cells in IFN-γ–enzyme-linked immunosorbent spot and in cellular cytotoxicity assays. In rechallenge experiments, these immunologic effector cells were found to delay, but not completely prevent growth of secondary tumors. However, when considering the notoriously depressed immune status of individuals suffering from pancreatic carcinoma, the orchestrated antitumoral immune responses we analyzed here in detail significantly strengthen the potential usefulness of microbial compounds as active unspecific immunotherapeutic agent for treatment of pancreatic carcinoma.