Combination of STING and TLR 7/8 agonists as vaccine adjuvants for cancer immunotherapy

Abstract

Cancer vaccines have evolved as a new treatment modality to eradicate cancer and has gained in popularity since the approval of Provenge® vaccine. By activating antigen-presenting cells (APCs), which, in turn, activate innate or adaptive immune responses, immunoadjuvants have become promising tools for cancer immunotherapy. Different types of immunoadjuvants such as toll-like receptor (TLR) agonists, exosomes, and metallic immunoadjuvants have been studied for their immunological effects. However, the clinical use of immunoadjuvants is limited by transient responses and various side-effects. Therefore, there is a need for exploring novel adjuvant technologies such as adjuvant combinations to achieve effective immune responses.In this study, we investigated the use of a multi-adjuvant approach by combining two different classes of adjuvants, STING and TLR 7/8 agonists because of their distinct immune cell targets, signaling pathways, and significant roles in the activation and maintenance of immune responses. STING and TLRs are pattern recognition receptors that act as sensors for “danger signals” and aid in identification and elimination of foreign bodies. Activation of these receptors induces an immune cascade, leading to secretion of numerous cytokines, chemokines and phenotypical changes in an immune cell. Owing to their potent immunological effects, clinical and preclinical studies have demonstrated great potential of STING and TLR agonists as cancer vaccine adjuvants. We evaluated the potential of combining 522, a TLR 7/8 agonist, and DMXAA, a STING agonist, to stimulate an immune response in APCs. Using flow cytometry and ELISA, we investigated the costimulatory molecule expression and cytokine secretion in dendritic cells, and M2 to M1 polarization after treatment with the agonists, individually or in combination. Also, PLGA nanoparticles were formulated and characterized in vitro. We observed that the combined treatment showed enhanced DC activation and M2 repolarization compared to the individual agonists. However, we also observed an increased induction of IL-10, an immunosuppressive cytokine. Small molecules display poor pharmacokinetic profiles in vivo, therefore, we fabricated nanoparticles to improve their in vivo disposition. PLGA NPs displayed similar DC activation to free drugs and hence will be used for further in vivo studies.