Size regulates particulate adjuvant induced IL-10 and IL-12p70 production

Abstract

Until recently, vaccines were largely comprised of whole cell killed and attenuated pathogens. However, over recent decades, there has been a shift towards subunit vaccine development, using purified antigens which although much safer are less immunogenic, requiring adjuvants to generate protective immunity. There is an urgent need to develop improved vaccines that generate potent Th1 and CD8+ responses for diseases including TB, malaria, HIV and for cancer. However, adjuvants such as alum and oil in water emulsions are strong inducers of humoral immunity but less effective at promoting cellular immunity.

The hypothesis underlying this project was that this limited capacity to drive cellular immunity was related to adjuvant induced production of the anti-inflammatory cytokine IL-10 and inhibition of the Th1 polarising cytokine IL-12p70 by dendritic cells (DCs) and macrophages. One key parameter dictating the effectiveness of particle based adjuvants is size but how size influences the induction of T cell polarising cytokines has not been elucidated. Using biodegradable poly (lactide-co-glycolide) (PLGA) particles between 100 nm and 30 ?m in size, it was shown that 500 nm-2 ?m particles potently enhanced IL-10 production by DCs and these DCs could inhibit T cell proliferation. These particles also potently enhanced IL-10 production in the draining lymph nodes following intramuscular injection. Moreover, deficiency in IL-10 resulted in enhanced Th1 responses after intra-muscular immunisation with OVA and 500 nm PLGA particles.

Particles of this specific size were shown to selectively interact with lipid rafts on the DC membrane and activate the kinase Syk, leading to p38 and CREB phosphorylation and enhanced transcription and secretion of IL-10. Remarkably it was found that this mechanism is more broadly employed by DCs in response to physical stresses. Specifically, 500 nm PLGA particles, osmotic stress, oxidative stress or disruption of actin polymerisation resulted in DCs becoming rounded, decreasing their membrane fluidity and enhancing their capacity for IL-10 production. Conversely, increasing membrane fluidity using local anaesthetics or causing cell swelling inhibited IL-10 production. Overall, a novel mechanism by which particulate adjuvants and mechanical signals enhance IL-10 production by DCs is proposed, highlighting the therapeutic potential of tailored biodegradable particles to block inflammation and promote tolerance.