The immunmodulatory properties of chitin-derived polymers are dictated by their deacetylation patterns

Abstract

The introduction of vaccines is regarded as one of the most successful medical interventions to date, due to their effectiveness at combating diseases that require the induction of a robust immune response. However there is a clear need for the development of new vaccines for diseases including HIV, TB and malaria and for cancer which require the induction of a potent cellular immune response. Advancements in the field of vaccine research have resulted in a move away from the use of whole organisms and towards the use of subunit vaccines which consist of highly purified antigens and thus offer a much more attractive safety profile. Adjuvants are immunostimulatory components that are included in subunit vaccine formulations to help to direct and amplify an appropriate adaptive immune response. The most commonly used adjuvant to date, alum, is incorporated into vaccine formulations that are aimed at inducing humoral immune responses however alum is a poor inducer of cellular immune responses. Chitosan is a cationic polysaccharide that has been examined in an adjuvant setting due to its biocompatible and biodegradable nature. The polysaccharide has been shown to have the capacity to induce Th1 cell responses following vaccination by injection or mucosal routes, supporting its application as an alternative to alum for vaccines that promote cell-mediated immunity. The objective of this research was to identify the relationship between the physico-chemical properties of chitin-derived polymers and the type of response induced. Treatment of DCs with a range of chitin-derived polymers with varying degrees of deacetylation revealed that polymers with a higher degree of deacetylation promote greater DC maturation and interferon production, dependent on signalling through the type I IFN receptor and the adaptor protein STING. Furthermore, highly deacetylated polymers were also shown to enhance antigen-specific Th1 responses in a STING-type I IFN-dependent manner. Overall these results indicate that through engagement with the STING-IFNAR pathway, the degree of chitosan deacetylation is a key determinant of the immune enhancing effects, with a higher degree of deacetylation leading to greater immunostimulatory effects. To date, no specific receptors have been identified as mediators of the interaction between chitosan and DCs. However several receptors have been associated with the recognition of chitin, chitosan?s naturally occurring precursor, including the C-type lectin receptor (CLR), Dectin-1. Thus a potential role for Dectin-1 in the recognition of chitosan by DCs was investigated. Previous studies have shown that while chitosan uptake by DCs is not dependent on dectin-1, the induction of type 1 interferons is compromised in the absence of the CLR. Furthermore the upregulation of DC activation markers by chitosan are abrogated in the absence of dectin-1, providing further evidence of a role for dectin-1 signalling in chitosan adjuvanticity. The activation of the cGAS-STING pathway by chitosan is induced by mitochondrial DNA release. Highly-deacetylated polymers were found to induce both mitochondrial depolarisation and mitochondrial fission, potentially contributing to the subsequent release of DNA. Overall these results indicate that highly deacetylated chitin-derived polymers induce cGAS-STING dependent DC activation through engagement with the CLR Dectin-1.