Mycobacterium bovis Bacillus Calmette-Guerin (BCG) reprograms melanoma microenvironment to promote antitumor T cell responses

Intralesional therapy using Mycobacterium bovis Bacillus Calmette-Guérin (BCG) for cutaneous metastatic melanoma induces regression of injected, but also non-injected lesions. Tumor-associated macrophages (also known as M2) infiltrate solid tumors and impair antitumor immunity. Since macrophages play a pivotal role in both tumors and mycobacterial infections, we hypothesized BCG alters M2 to promote antitumor immunity. BCG-treated, in vitro-polarized M2 (M2-BCG) show dramatic transcriptional changes involving inflammation, immune cell recruitment, cross-talk and activation pathways. Mechanistic network analysis indicates potential for M2-BCG to improve IFN-γ responses, frequently used as a marker for successful antitumor immunity. Accordingly, we found an increased frequency of IFN-γ-producing CD4+ T cells responding to M2-BCG vs mock-treated M2 (p<0.05). Moreover, supernatant from M2-BCG increased the frequency of granzyme B-producing CD8+ tumor-infiltrating lymphocytes (TIL) responding to autologous melanoma cell lines (p<0.01). Comparing the transcriptome of injected vs uninjected lesions biopsied from IL-BCG patients showed immune function prevailing in injected lesions, with the most enriched pathways representing T cell activation mechanisms. In vitro BCG-infected tumor cells also stimulated IFN-γ production from HLA-A2-restricted syngeneic TIL from the same melanoma patient (p<0.05). Together, our data indicates BCG can promote an antitumor microenvironment in cutaneous metastatic melanoma.Â