Mycobacterium indicus pranii therapy induces tumor regression in MyD88- and TLR2-dependent manner

Abstract

Objectives Mycobacterium indicus pranii (MIP) is an atypical mycobacterium species with potent antitumor efficacy. Macrophages and dendritic cells (DCs) are antigen-presenting cells, playing key roles in the activation of antitumor immunity. We have previously shown the potent activation of macrophages and DCs by MIP, which is mediated by MyD88–TLR2 signaling axis. In the present study, we further examined the role of MyD88 and TLR2 in MIP-mediated tumor regression. Results Wild-type and MyD88−/− mice were implanted with B16F10 tumor cells, treated with MIP or phosphate-buffered saline (PBS) and monitored for tumor growth. As expected, MIP therapy led to significant tumor regression in wild-type mice. However, antitumor efficacy of MIP was lost in MyD88−/− animals. Both PBS-treated (control) and MIP-treated MyD88−/− mice developed tumors with comparable volume. Since MyD88 relays TLR engagement signals, we analyzed the antitumor efficacy of MIP in TLR2−/− and TLR4−/− mice. It was observed that MIP therapy reduced tumor burden in wild-type and TLR4−/− mice but not in TLR2−/− mice. Tumor volume in MIP-treated TLR2−/− mice were comparable with those in PBS-treated wild-type animals. These results implicated the MyD88–TLR2 signaling axis in the antitumor efficacy of MIP. Introduction Mycobacterium indicus pranii (MIP) is an atypical mycobacterium possessing strong immunomodulatory properties [1]. It was selected from a panel of mycobacteria for evoking cell-mediated immunity against M. leprae and was approved for the treatment of leprosy in 1998. Small clinical studies, wherein MIP was evaluated against head and neck cancer, bladder cancer, and lung cancer, also suggested the potent antitumor efficacy of MIP [2, 3]. Mycobacterium species vary widely in their antigenicity and biochemical properties. Recent studies have uniquely placed MIP between slow- and fast-growing mycobacteria [4]. Whole-genome sequencing has revealed the higher levels of putative antigenic molecules in MIP, compared with BCG [1]. We have shown previously that MIP therapy results in significant tumor regression and prolongs the survival of tumor-bearing mice [5]. Immunological studies in these animals revealed that MIP therapy promotes tumor-specific T cell responses and NK cell cytotoxicity [5]. It has been shown that macrophages and dendritic cells (DCs) play key roles in the processing and presentation of tumor antigens and mounting of antitumor immunity. Consistently, MIP was found to activate macrophages and DCs and induce TH1 polarization potential in these cells [6, 7]. It was further observed that MIP led to macrophage and DC activation in MyD88- and TLR2-dependent manner [6, 7]. In this study, we explored the role of MyD88 and TLRs in MIP-induced tumor regression. Mice were implanted with B16F10 melanoma cells, treated with MIP and monitored for tumor growth. It was observed that MIP therapy resulted in significant tumor regression in wild-type, but not in MyD88−/− mice. Experiments with TLR knockout mice demonstrated that MIP induced comparable tumor regression in wild-type and TLR4−/− mice. However, antitumor efficacy of MIP was significantly reduced in TLR2−/− mice. These findings shed light on the innate immune mechanisms involved in the antitumor efficacy of MIP.