Unravelling the complexity of the interactions among VACCIMEL, BCG and blood monocytes

VACCIMEL is an immunotherapeutic adjuvant treatment for high-risk cutaneous melanoma. It consists of 13 intradermal injections of four irradiated melanoma cell lines co-adjuvated with BCG and GM-CSF. VACCIMEL prolongs distant metastases-free survival and it induces T lymphocytes reactive against melanoma differentiation antigens, cancer testis antigens, and neoantigens. In this paper, we have studied <i>in vitro</i> the interaction among VACCIMEL, BCG, and blood-derived monocytes, a fundamental component of innate immunity. We have demonstrated that monocytes may phagocytose, separately and jointly, BCG and VACCIMEL. We have shown for the first time by transmission electron microscopy, detailed features of irradiated melanoma cells phagocytosis and its timeline. We have equally demonstrated that monocytes may process and cross-present tumor antigens to CD8⁺ T cells and that BCG interferes with that process in a multiplicity of infection (MOI) - dependent manner. BCG induces high production of IL-10 by monocytes which is several-fold reduced by phagocytosis of tumor antigens. Although cross-presentation is still inhibited in the presence of a high BCG MOI (0.4), it rebounds by reducing tenfold the BCG MOI from 0.4 to 0.04. This suggests that an adequate balance between tumor antigens and BCG phagocytosis is needed to retain the stimulatory properties of activated monocytes and trigger immunogenicity of tumor antigens.