Immune complex-induced anti-inflammatory neutrophil functions
Item statusRestricted Access
Embargo end date20/06/2023
Neutrophils are short lived innate immune cells that perform a range of functions, including chemotaxis, phagocytosis, degranulation, generation of reactive oxygen species (ROS) and production of cytokines which equip them well to defending the host from bacterial or fungal infections. At the end of their short lives neutrophils undergo apoptosis, and are subsequently cleared by professional phagocytes in a specialised process referred to as efferocytosis which helps to promote the resolution of inflammation. Inappropriate recruitment and activation of neutrophils can result in important host damage. Antibodies, in the shape of opsonins of particles or as immune complexes (ICs) bind to, and cross-link Fc receptors on the neutrophil surface, acting as powerful stimuli. In the context of autoimmunity, e.g. rheumatoid arthritis (RA) and other chronic inflammatory events, (auto) antibodies can trigger neutrophilic inflammation. However, observations by our group and others have suggested that neutrophils can promote repair as well as driving inflammation. I addressed the hypothesis that insoluble immune complexes (iICs) trigger antiinflammatory neutrophil functions. I investigated (i) the mechanism of iICinternalisation, (ii) whether iIC internalisation and induction of neutrophil apoptosis are linked and (iii) whether these functions are dysregulated in neutrophils derived from RA patients. By performing a range of functional experiments I showed that insoluble ICs (iICs), which had previously already been shown to induce neutrophil apoptosis, are internalised by receptor-mediated macropinocytosis and subsequently digested by neutrophils. My experiments identified that this internalisation mechanism coincided with, but was separately controlled to the induction of neutrophil apoptosis, with neither event dependent upon the other, suggestive of a mechanism that is distinct from phagocytosis-induced cell death (PICD). However, as with PICD, iIC-induced neutrophil apoptosis was dependent upon ROS production. While both iIC-induced macropinocytosis and apoptosis were both chiefly mediated by FcγRII, simultaneous iICs induced ROS production was dependent upon additional receptors, FcγRI and αMβ2. My results suggest therefore that iICs promote both pro-inflammatory (ROS) and anti-inflammatory neutrophil functions (their own clearance; neutrophil apoptosis). I moreover demonstrated that induction of neutrophil apoptosis via iICs promotes improved neutrophil efferocytosis by macrophages, suggestive of a third antiinflammatory function induced by iICs. I finally compared the induction of some proand anti-inflammatory neutrophil functions induced in healthy donor- and RA patientderived neutrophils, observing increased ROS production, but no differences in terms of iIC internalisation and iIC-mediated induction of apoptosis of RA neutrophils. Taken together, my results suggest that iICs mediate previously underappreciated antiinflammatory neutrophil functions in all neutrophils, even those obtained from RA patients.