Investigating the Role of Trained Immunity in Bacillus Calmette-Guérin Immunotherapy of Bladder Cancer Using a Mouse Model

Abstract

The gold-standard treatment for non-muscle invasive bladder cancer (NMIBC) is adjuvant intravesical Bacillus Calmette-Guérin (BCG) immunotherapy following transurethral resection of bladder tumour (TURBT). BCG is a live-attenuated form of Mycobacterium bovis and is utilized in an adjuvant setting to potentiate antitumour immunity. However, the mechanism of action by which BCG induces its therapeutic benefits is not fully elucidated. Recent studies suggest that trained immunity (TI) may be a key mediator of BCG immunotherapy. TI is characterized by an enhanced response to a secondary, inflammatory stimulus, mediated by the innate immune system. A study from our laboratory has shown that monocytes from patients that have undergone BCG immunotherapy can acquire TI, which has shown to be associated with long-term recurrence-free survival. Therefore, we hypothesized that the antitumour effects of BCG therapy are mediated by acquisition of TI. Furthermore, we proposed that systemic modes of training, i.e. intraperitoneal (IP) and intravenous (IV), result in greater levels of TI compared to local intravesical inoculation. The objectives of this study were to establish an orthotopic, syngeneic MB49 mouse model of bladder cancer and BCG immunotherapy, followed by a comparison of the levels of TI acquired by bone marrow derived monocytes of non-tumour-bearing mice inoculated with BCG through the routes of IP, IV or intravesical. Here, we found that intravesical BCG immunotherapy did not elicit a prolonged antitumour effect nor it induced TI in bone marrow monocytes. Similarly, BCG administered IP, IV, or intravesically to non-tumour bearing mice did not result in TI acquisition. However, intravesical administration of BCG, compared to IP and IV administrations, induced BMDMs to secrete heightened levels of proinflammatory cytokines and chemokines (IL-1β, IL-6, and MCP-1) in response to LPS challenge. These results provide some evidence that systemic versus local administration of BCG leads to differential immune responses. Overall, the findings could lead to better approaches to optimize BCG immunotherapy of bladder cancer.