Chitin-derived STING Activators as Adjuvants and Therapeutics

Abstract

A significant challenge in the advancement of vaccine research is a lack of adjuvants that can safely drive potent cellular immunity against intracellular pathogens and cancer, a problem only exacerbated by our limited knowledge of how adjuvants work. As such, there has been an increasing focus on purified pathogen-derived or synthetic agonists for well-defined pathogen recognition receptors as adjuvants. Since the discovery that the cGAS-STING pathway occupies a central role in the activation of tumour-targeting immune responses, there has been a surge in research aimed at identifying natural and synthetic CDNs and non-nucleotidyl STING agonists for use in cancer settings. Despite pre-clinical evidence of efficacy, no widely applicable, clinically effective and safe agonist has been identified or completed phase III trials. The primary barriers to clinical translation are low cellular uptake and intracellular accessibility, poor pharmacokinetics, and STING variability, necessitating personalised STING agonists.

This work has identified C100, a chitosan polymer with no acetyl groups, as an attractive alternative to conventional STING agonists. C100 polymers were demonstrated to promote potent STING and IFNAR-dependent cellular immunity against the TB antigen H56 and tumour growth suppression upon intratumoral injection in B16 melanoma models. Unlike most licensed adjuvants, the study delineated the mechanism and physiochemical properties required for C100-induced immune activation. In a cellular uptake-independent manner, C100 polymers triggered mitochondrial stress that was pivotal for both cGAS-STING and NLRP3 inflammasome activation and subsequent Th1 immunity. Mechanistically, extensive mitochondrial stress damaged nuclear DNA, triggering its accumulation in the cytosol of DCs and activation of STING-dependent type I IFNs. Complete deacetylation of the chitosan backbone was critical for optimal adjuvanticity, as addition of acetyl groups reduced the degree of mitochondrial stress, nuclear damage, IFNAR-dependent DC maturation, NLRP3 activation, Th1 responses and protective anti-tumour immunity. The ability of acetylation to diminish the adjuvanticity of C100 addresses a longstanding mystery of how chitosan polymers can achieve both inert and inflammatory properties. Altogether, these results reveal an effective anti-tumour STING-dependent adjuvant with unique properties that sidestep common limitations of existing STING therapeutics.