The Expression and Function of Leukocyte Immunoglobulin-Like Receptor A3: Potential Implication in Multiple Sclerosis

Abstract

LILRA3 is a soluble protein belonging to a family of highly homologous activating and inhibitory receptors mainly expressed on leukocytes and increasingly recognised as key immunoregulatory molecules in the immune system. LILRA3 gene deletion has been associated with multiple sclerosis (MS) susceptibility, but results are not consistent among different study populations. In our study, we found no link between LILRA3 gene deletion and MS susceptibility in a North American cohort. Instead, serum LILRA3 level was significantly increased in patients with MS and strongly associated with disease severity, suggesting that LILRA3 protein may play a role in disease progression. Indeed, we showed that elevated serum LILRA3 had a positive correlation with better clinical outcomes and anti-inflammatory cytokine IL-10, suggesting an anti-inflammatory role of LILRA3 in MS. Importantly, this study also suggested a potential use for measuring serum LILRA3 levels as a biomarker for disease severity in MS. The functions of LILRA3 in MS remain unknown due to the limited knowledge of its ligands. This thesis showed that Nogo 66, a potent neurite outgrowth inhibitor, is a new functional ligand for LILRA3. We showed that the high affinity binding of LILRA3 with Nogo 66 blocked Nogo 66-mediated inhibition of neurite outgrowth and promoted synapse formation in primary cortical neurons. In addition, LILRA3 is able to block Nogo 66-mediated suppression of MAPK signalling in primary cortical neurons. All these results suggested that LILRA3 acts as a broad antagonist to block the interactions between Nogo 66 and its neuronal receptors and their subsequent inhibitory effects and thus promote neuroregeneration. Despite the novel anti-inflammatory and neuroregenerative roles of LILRA3 in MS, little is known about the quaternary structure of LILRA3, which may be crucial for its ligand binding and functions. Preliminary results in this thesis showed that intracellular LILRA3 exists in multiple quaternary forms but is primarily secreted as monomeric protein and surprisingly is abundantly present in the nucleus of primary monocytes. Various quaternary structures of LILRA3 may contribute to its diverse biological functions. Importantly, this thesis unitised a new approach to study the quaternary structure of LILRA3 spatiotemporally at a single cell level.