Innate and adaptive mechanisms induced by BCG immunotherapy in melanoma murine model

BCG (Bacillus Calmette-Guérin) vaccine has been proposed in clinical trials for the treatment of melanoma since 1970, however its mechanism of action still not fully understood. The purpose of this study was to investigate the immune mechanisms involved in the BCG immunotherapy against melanoma. We first characterized the immunological profile of the mouse melanoma B16–F10 cell line compared to BMDMs (bone marrow-derived macrophages) infected with BCG or stimulated with agonists for different innate immune pathways. B16–F10 were only capable to respond after poly I:C and dsDNA90 stimulus inducing CXCL10 and IFN-β, but no response against BCG infection. Moreover, we evaluated BCG intratumoral treatment in a B16–F10 subcutaneous mouse tumor model in C57BL/6 wild type (WT) and several knockout mice (MyD88 −/−, TLR3 −/−, TLR4 −/−, TLR7 −/−, TLR3/7/9 −/−, cGAS −/−, STING −/−, IFNAR −/−, Caspase 1 −/−, Caspase 11 −/−and IL-1R −/−). BCG dramatically reduces melanoma in all the mice strains, except for MyD88 −/−, suggesting a role played by this adaptor molecule related to the innate immune response. In order to investigate the adaptive immunity, we evaluate BCG treatment in IFNγ −/−and Rag −/−, which demonstrated the importance of lymphocytes for tumor regression. The experiments using B16–F10 infected with BCG and co-cultured with spleen cells or BMDMs from WT or MyD88 −/−mice demonstrated that MyD88 is essential for inflammatory cytokines production. MyD88 is also important to regulate the recruitment of immune cells in the tumor microenvironment. Our data suggests that BCG immunotherapy in melanoma depends on MyD88-related innate immune pathway and adaptive response for an efficient tumor control. This study was funded by Conselho Nacional de Desenvolvimento Científico e Tecnológico - CNPQ (152478/2022-1) and Fundação de Amparo à Pesquisa do Estado de Minas Gerais - FAPEMIG (5.18/2022).