AbstractsMedical & Health Science

The leukocyte immunoglobulin-like receptor A3 (LILRA3) is a unique soluble molecule that belongs to a family of highly homologous activating and inhibitory receptors, critical in the regulation of immune responses. Although LILRA3 is predicted to act as broad soluble antagonist to its closely related LILRs via the shared ligands, its biological roles remain unknown. This is due to the poor biochemical property characterisation of a full length mammalian recombinant LILRA3 (rLILRA3) precluding identification of its ligands and target cells. Limited data has suggested MHC class I as LILRA3 ligands, but the use of truncated, non-gylcosylated rLILRA3 produced in bacteria made them non-physiological and incomplete. Especially, LILRA3 is a mammalian protein predicted to be cysteine-rich and heavily glycosylated, where these characteristics are likely to affect folding, ligand binding, and ultimately functions. To address these, this project aimed at producing mature LILRA3 proteins in various hosts including bacterial and mammalian expression systems to compare the biochemical properties and functions. The bacterial produced rLILRA3 was biologically inactive, whilst the mammalian produced rLILRA3 was biologically active with characteristics similar to native protein. Indeed, mammalian rLILRA3 had suppressed LPS-mediated TNF production and promoted neurite outgrowth in vitro, but the mechanisms were unknown. Interestingly, the common 66 amino acid loop of the Nogo proteins (Nogo-66) was identified as candidate LILRA3 ligand. Such mechanism is similar to the finding in the literature search, where Nogo-66 is a ligand to a LILR murine orthologue, paired Immunoglobulin-like receptor B (PIRB). Preliminary work in this project showed PIRB and Nogo proteins were significantly downregulated on the airway epithelium in mouse lungs with chronic and acute exacerbation asthma model. Co-immunoprecipitation of these proteins from lung macrophages suggested PIRB-Nogo-B interaction may regulate macrophage functions. Taken together, this thesis had identified several novel findings where the identification of N-glycosylation as key for ligand binding was a major breakthrough, which may have wider implication in the field of LILRs and PIRs. These significant findings provide important preludes for future studies including modulation of lung inflammation using human LILRA3 protein and/or by targeted in vivo transgenic overexpression.