(233c) ?-Glucan Induced Innate Immune Memory Alters the Tumor Microenvironment and Enhances Checkpoint Inhibitor Therapy

Honing the anti-tumor immune responses of patients has allowed cancer immunotherapies to be highly effective for some individuals. However, many of these therapies are unable to overcome the immunosuppressive tumor microenvironment which greatly limits their effectiveness in many solid tumors. There is a need for increasing the inflammation in the tumor microenvironment in order to increase the efficacy of FDA approved immunotherapies. Most cancer immunotherapies, including checkpoint inhibitors, help initiate and prolong the adaptive immune system’s ability to kill tumor cells. There is now growing interest in developing immunotherapies that not only activate the adaptive immune system, but also harness the innate immune system’s anti-tumor characteristics. The innate immune system is imperative in facilitating and maintaining a proper adaptive immune response, and innate immune cells are highly responsible for maintaining a proinflammatory or immunosuppressive tumor microenvironment. This study focused on the concept of trained immunity, also known as innate immune memory. Trained immunity is defined as the long-term functional modification of innate immune cells, allowing trained cells to have faster and stronger pro-inflammatory responses to subsequent challenges (i.e. tumor cells). The goal of this study was to determine if β-glucan peptide derived from trametes versicolor could increase the inflammation in the tumor microenvironment in a mouse model of melanoma. Additionally, it was determined if β-glucan peptide could augment the anti-tumor responses of the adaptive immune system and increase response rate to checkpoint inhibitors in a tumor model that is normally resistant.

β-glucan peptide was found to induce both in vitro and in vivo trained immunity. The peptide enhanced proinflammatory cytokine secretion, TNFα and IL-6, in murine macrophages when cells were restimulated with lipopolysaccharide. Additionally, bone marrow cells of mice receiving intravenous injections of β-glucan peptide showed increased levels of TNFα and IL-6 secretion upon lipopolysaccharide stimulation. A time course study was done, and seven days post-training resulted in the strongest trained immune response in the bone marrow. This experiment informed the dosing regimen in tumor models. Treatment of mice inoculated with B16F10 melanoma tumors with β-glucan peptide showed a significant reduction in tumor growth rate and increased survival compared to control mice. Flow analysis of the tumor microenvironment revealed significant reductions in the percentage of macrophages and myeloid derived suppressor cells in mice treated with β-glucan peptide. Additionally, T-cell infiltration and total numbers were analyzed using immunofluorescence staining. Staining revealed larger CD8+/Treg ratios in the tumor microenvironment of mice treated with β-glucan peptide. CD45+ immune cells in the tumor microenvironment of treated mice showed increased gene expression of proinflammatory cytokines and checkpoint molecules, indicating enhanced immune activation. Lastly, mice were co-treated with β-glucan peptide and an anti-PD1 antibody. The B16F10 melanoma model is resistant to anti-PD1 therapy, but it was found that β-glucan peptide significantly increased checkpoint inhibitor effectiveness in this model.

In conclusion, β-glucan peptide increased the pro-inflammatory responses of the innate immune cells, and it may give preference to the proinflammatory responses of myeloid cells as indicated by flow data. Not only do training agents act directly on the innate immune cells, but it appears that innate immune stimulation increases the adaptive immune system’s anti-tumor responses; evidenced by immunofluorescence data and synergy with checkpoint inhibitor therapy.