Post-translational modifications of myelin oligodendrocyte glycoprotein in CNS autoimmunity

Abstract

Autoimmunity towards components of the central nervous system (CNS) is a driving factor of nerve demyelination in Multiple Sclerosis (MS). The etiology of autoimmunity in MS pathogenesis remains elusive, but a combination of genetic predisposition and environmental triggers is theorized to initiate this complex disease.

Oxidative stress and lipid peroxidation represent insults to tissue homeostasis and are associated with inflammation and tissue damage. Reactive molecules associated with these processes covalently modify proteins forming post-translational modifications (PTMs). These PTMs alter the topology and biochemical properties of carrier proteins, which potentially impinges on their recognition by the immune system. Immunological self-tolerance to rare determinants derived from PTM antigen is poorly established, which is a theoretical basis for the development of autoimmunity.

Myelin Oligodendrocyte Glycoprotein (MOG) is a minor surface antigen expressed specifically on myelinating oligodendrocytes in the CNS but is a major target in CNS autoimmunity.

Study I addresses the innate recognition and immunological consequences of MOG modified by malondialdehyde (MDA), a reactive product of lipid peroxidation. MOG-MDA is phagocytosed efficiently and selectively by macrophages via Scavenger Receptor A (SRA), most efficiently by homeostatic M2-type macrophages. Higher uptake of MOG-MDA enhances the proliferation of autoreactive T cells in vitro but induces equal severity in experimental autoimmune encephalomyelitis (EAE), the animal model of MS. Taken together the results imply primarily homeostatic clearance of MDA-modified antigen by macrophages, but also a presentation to previously licensed T cells for screening purposes. The phenotype and activation state of the macrophage may influence the response to MDA-modified antigen. Furthermore, the results imply a dissociation of effects mediated by MDA adducts as opposed to the soluble chemical.

Study II introduces a useful in silico molecular modeling plugin tool for various PTMs that can be used to address related research questions and predictions.

Study III combines approaches from Study I+II to address the immunology of MOG nitration, a modification related to oxidative stress and inflammation. A combination of molecular modeling, bioinformatics, in vitro and in vivo data supports the conclusion that nitration denies the presentation of the dominant MGO35-55 epitope by H2-IAb due to nitration of the critical anchoring residue Tyrosine-40 rendering it incompatible with the p1 pocket. Nitration thus represents a potential silencing effect depending on the major histocompatibility complex (MHC) haplotype. Accordingly, nitrated MOG is poorly encephalitogenic in H2b C57BL/6 mice, but fully encephalitogenic in H2q DBA1 mice.

Study IV functionally and phenotypically fully characterizes two fundamentally different sets of bone marrow-derived dendritic cells (BMDCs) generated in vitro by alternative protocols. These BMDCs resemble certain populations of antigen presenting cells in vivo and can be used to study immunological principles related to immunology.

Collectively, this thesis provides both innovative tools to approach and novel insights relating to the impact of post-translational modifications on autoimmunity.