Innate Immune Memory-Mediated Enhancement of Adaptive Immune Activation

Abstract

Innate immune memory, or trained immunity (TI), is a heterologous form of memory acquired by innate immune cells that confers non-specific immune protection. However, it has not yet been linked to enhancement of adaptive immune activation. Here, we investigated the role of Bacillus Calmette Guérin (BCG)-mediated TI acquisition on antigen presenting cell (APC) function and adaptive immune activation, both ex vivo and in murine models of non-muscle invasive bladder cancer (NMIBC) and coronavirus infection with murine hepatitis virus-1 (MHV-1), a murine coronavirus. BCG-trained dendritic cells (DCs) exhibited significantly increased ovalbumin (OVA) uptake and cross-presentation in vitro. Furthermore, naïve cytotoxic T lymphocytes (CTLs) co-cultured with trained DCs exhibit significantly increased proliferation and expansion. To compare local and systemic immune activation on anti-tumour responses in non-muscle invasive bladder cancer (NMIBC), a syngeneic orthotopic bladder tumour murine model was used, wherein female C57Bl/6 mice were instilled with MB49 bladder cancer cells directly in the bladder followed by intravesical BCG (the standard immunotherapy of bladder cancer) or intravenous (IV) BCG administration to directly activate systemic responses. IV BCG treatment led to significantly reduced tumour burden, measured by ultrasound, and anti-tumour tumour draining lymph node and bone marrow microenvironments, analyzed using polychromatic flow cytometry. IV BCG treatment also led to a heightened tumour-specific B and cytotoxic T lymphocyte (CTL) response in the spleen. To determine if these effects are mediated by TI, mice were co-instilled with MB49 cells and BCG-trained bone marrow derived macrophages, and increased tumour-specific CTLs were present in the tumour microenvironment. Finally, to determine the effect of BCG on coronavirus infection, trained alveolar macrophages (AMs) and monocytes were co-cultured with coronavirus-infected fibroblasts and reduced viral titer was observed in the supernatant. In vivo, mice injected intraperitoneally with BCG displayed reduced viral load in the lungs following coronavirus infection with murine hepatitis virus-1. These results provide evidence that TI enhances APC function, leading to improved adaptive immune activation. Elucidating ways to enhance adaptive immune responses through non-specific treatments, such as BCG vaccination, may help reduce future disease burden.