An antibody specific for component 8 of myelin basic protein from normal brain reacts strongly with component 8 from multiple sclerosis brain

Malondialdehyde modification of myelin oligodendrocyte glycoprotein leads to increased immunogenicity and encephalitogenicity

Self proteins may become autoantigenic through structural modification. We studied malondialdehydation of recombinant rat (rr) myelin oligodendrocyte glycoprotein (MOG), an autoantigen in multiple sclerosis. Malondialdehyde (MDA) modification changed protein weight and charge, the location of these adducts being mapped by Fourier transform ion cyclotron resonance. Molecular modelling revealed significant differences in the MDA-rrMOG three-dimensional structure. DBA/1 mice immunised with MDA-rrMOG developed greater proliferative responses and more severe experimental autoimmune encephalomyelitis than mice immunised with unmodified rrMOG. MDA-rrMOG was taken up more effectively by antigen-presenting cells (APC), at least partially through scavenger receptors. Exposure to MDA-rrMOG led to increased expression of IL-23, IL-12 and IL-12R, indicating a role not only for increased antigen uptake but also for activation of APC. We thus provide biochemical, structural, immunological and clinical data that suggest that the post-translationally modified form of this myelin autoantigen is a more relevant form of the molecule.

A Tale of Two Citrullines—Structural and Functional Aspects of Myelin Basic Protein Deimination in Health and Disease

Myelin basic protein (MBP) binds to negatively charged lipids on the cytosolic surface of oligodendrocyte membranes and is responsible for adhesion of these surfaces in the multilayered myelin sheath. The pattern of extensive post-translational modifications of MBP is dynamic during normal central nervous system (CNS) development and during myelin degeneration in multiple sclerosis (MS), affecting its interactions with the myelin membranes and with other molecules. In particular, the degree of deimination (or citrullination) of MBP is correlated with the severity of MS, and may represent a primary defect that precedes neurodegeneration due to autoimmune attack. That the degree of MBP deimination is also high in early CNS development indicates that this modification plays major physiological roles in myelin assembly. In this review, we describe the structural and functional consequences of MBP deimination in healthy and diseased myelin.

Expression of citrullinated proteins in murine experimental autoimmune encephalomyelitis

In this study, we demonstrate for the first time the immunohistochemical expression of citrullinated proteins in the central nervous system (CNS) of mice with myelin oligodendrocyte glycoprotein (MOG)-induced experimental autoimmune encephalomyelitis (EAE). By using an established monoclonal antibody (F95) against natural and synthetic citrullinated proteins (Nicholas and Whitaker [2002] Glia 37:328-336), numerous, small, previously unrecognized "patches" of citrullinated proteins were discovered throughout EAE brains, whereas EAE spinal cords showed similar but much larger lesions. On dual color immunofluorescence, these lesions were found to contain citrullinated myelin basic protein (MBP) and were surrounded by astrocytes immunoreactive for both glial fibrillary acidic protein (GFAP) and F95. These lesions became evident about the time when EAE mice became symptomatic and increased in size and number with increasing disease severity. In some sections of spinal cord but not brains of severely debilitated EAE mice, a widespread gliotic response was seen, with astrocytes containing citrullinated GFAP spread throughout the gray and white matter. Western blot analysis of acidic proteins from the brains and spinal cords of EAE mice had higher levels of multiple citrullinated GFAP isoforms compared with controls, with more F95-positive bands in the EAE brains vs. spinal cords. These results raise the possibility that citrullination of both GFAP and MBP may contribute to the pathophysiology of EAE and that the brains of EAE mice may contain more pathology than previously realized.

Deimination of arginine residues in nucleophosmin/B23 and histones in HL-60 granulocytes

Peptidylarginine deiminases (PADs) convert arginine residues in proteins into citrulline residues Ca(2+)-dependently. PAD V was recently found in granulocyte-differentiated HL-60 cells. To find a target of PAD V, we incubated HL-60 granulocytes with the calcium ionophore A23187 and studied deiminated proteins by immunocytochemistry and immunoblotting using a monospecific antibody to modified citrulline residues. Immunocytochemical signals were found in the nucleus upon incubation with A23187. Immunoblotting indicated that 40-, 18-, 17-, and 14-kDa proteins were preferentially deiminated. The 40-kDa protein, which was focused to pI 5.0 on two-dimensional gel electrophoresis, was identified as nucleophosmin/B23 by mass spectrometry. The 18-, 17-, and 14-kDa proteins extracted with 0.4 N H(2)SO(4) comigrated with histones H3, H2A, and H4, respectively, on two-dimensional gel electrophoresis specialized for histones. The citrulline content of histones amounted to about 10% of the histone molecules. We discuss the implications of deimination of these proteins for their nuclear functions.

Thiopalmitoylation of myelin proteolipid protein epitopes enhances immunogenicity and encephalitogenicity

Proteolipid protein (PLP) is the most abundant protein of CNS myelin, and is posttranslationally acylated by covalent attachment of long chain fatty acids to cysteine residues via a thioester linkage. Two of the acylation sites are within epitopes of PLP that are encephalitogenic in SJL/J mice (PLP(104-117) and PLP(139-151)) and against which increased immune responses have been detected in some multiple sclerosis patients. It is known that attachment of certain types of lipid side chains to peptides can result in their enhanced immunogenicity. The aim of this study was to determine whether thioacylated PLP peptides, as occur in the native protein, are more immunogenic than their nonacylated counterparts, and whether thioacylation influences the development of autoreactivity and experimental autoimmune encephalomyelitis. The results show that in comparison with nonacylated peptides, thioacylated PLP lipopeptides can induce greater T cell and Ab responses to both the acylated and nonacylated peptides. They also enhanced the development and chronicity of experimental autoimmune encephalomyelitis. Synthetic peptides in which the fatty acid was attached via an amide linkage at the N terminus were not encephalitogenic, and they induced greater proportions of CD8+ cells in initial in vitro stimulation. Therefore, the lability and the site of the linkage between the peptide and fatty acid may be important for induction of encephalitogenic CD4+ T cells. These results suggest that immune responses induced by endogenous thioacylated lipopeptides may contribute to the immunopathogenesis of chronic experimental demyelinating diseases and multiple sclerosis.

Enhanced T cell responsiveness to citrulline-containing myelin basic protein in multiple sclerosis patients

Myelin basic protein (MBP), a candidate autoantigen in multiple sclerosis (MS), exists in different isoforms and charge isomers generated by differential splicing of exons and by a combination of posttranslational modifications, respectively. These various isoforms and charge isomers of MBP vary in abundance and most likely serve different functions during myelinogenesis and remyelination. The least cationic among the charge isomers of MBP is citrullinated and is referred to as MBP-C8. MBP-C8 is relatively increased in the population of MBP isomers in more developmentally immature myelin and in MS brain tissue. In a previous study, we found that MBP-C8-reactive T cells could be detected in CD4+ T cell lines (TCL) generated with MBP from both MS patients and normal controls. Here, we examined the frequency and peptide specificity of MBP-C8-specific TCL generated with MBP-C8 in MS patients and controls. Ten subjects grouped in five sets, each an MS patient and a control, were studied. In all cases, the MS patient had either a higher overall number of MBP-C8-responding lines, responded with greater sensitivity to the MBP-C8 antigen or both. Few lines responded to the MBP-C8 peptides but, if they did, they appeared to be specific to the carboxyl-half of the MBP-C8 molecule. Given the large amounts of citrullinated MBP in MS brain tissue, a preferential T cell response to MBP-C8 may be involved in the induction and perpetuation of this disease. Multiple Sclerosis (2000) 6 220 - 225

Molecular characterization of peptidylarginine deiminase in HL-60 cells induced by retinoic acid and 1alpha,25-dihydroxyvitamin D(3)

Three types of peptidylarginine deiminase (PAD), which converts a protein arginine residue to a citrulline residue, are widely distributed in animal tissues. Little is known about PAD of hemopoietic cells. We found that PAD activity in human myeloid leukemia HL-60 cells was induced with the granulocyte-inducing agents retinoic acid and dimethyl sulfoxide and with the monocyte-inducing agent 1alpha,25-dihydroxyvitamin D(3). We cloned and characterized a PAD cDNA from retinoic acid-induced cells. The cDNA was 2,238 base pairs long and encoded a 663-amino acid polypeptide. The HL-60 PAD had 50-55% amino acid sequence identities with the three known enzymes and 73% identity with the recently cloned keratinocyte PAD. The recombinant enzyme differs in kinetic properties from the known enzymes. Immunoblotting and Northern blotting with an antiserum against the enzyme and the cDNA, respectively, showed that a protein of approximately 67 kDa increased concomitantly with increase of mRNA of approximately 2.6 kilobases during granulocyte differentiation. During monocyte differentiation the same mRNA and protein increased as in granulocyte differentiation. Neither the enzyme activity nor the protein was found in macrophage-induced cells. These results suggested that expression of the PAD gene is tightly linked to myeloid differentiation.

Three-dimensional structure of myelin basic protein. I. Reconstruction via angular reconstitution of randomly oriented single particles

Myelin basic protein (MBP) plays an integral role in the structure and function of the myelin sheath. In humans and cattle, an 18.5-kDa isoform of MBP predominates and exists as a multitude of charge isomers resulting from extensive and varied post-translational modifications. We have purified the least modified isomer (named C1) of the 18.5-kDa isoform of MBP from fresh bovine brain and imaged this protein as negatively stained single particles adsorbed to a lipid monolayer. Under these conditions, MBP/C1 presented diverse projections whose relative orientations were determined using an iterative quaternion-assisted angular reconstitution scheme. In different buffers, one with a low salt and the other with a high salt concentration, the conformation of the protein was slightly different. In low salt buffer, the three-dimensional reconstruction, solved to a resolution of 4 nm, had an overall "C" shape of outer radius 5.5 nm, inner radius 3 nm, overall circumference 15 nm, and height 4.7 nm. The three-dimensional reconstruction of the protein in high salt buffer, solved to a resolution of 2.8 nm, was essentially the same in terms of overall dimensions but had a somewhat more compact architecture. These results are the first structures achieved directly for this unusual macromolecule, which plays a key role in the development of multiple sclerosis.

Acute multiple sclerosis (Marburg type) is associated with developmentally immature myelin basic protein

We have studied a case of acute, fulminating multiple sclerosis (MS) (Marburg type) at the pathological and biochemical levels. Postmortem examination of the brain revealed extensive areas of gross rarefaction in the hemispheric white matter. Histologically, well-demarcated areas of demyelination with a large influx of macrophages and a subtle perivascular infiltration of lymphocytes were seen with relative preservation of the axis cylinders. Myelin basic protein (MBP) was isolated and purified [correction of purifed] from noninvolved white matter. It was slightly larger in molecular weight than MBP from normal brain or from chronic MS brain. The increase in mass was accounted for, in part, by the deimination of 18 of 19 arginyl residues to citrulline, making the patient's MBP much less cationic than MBP from normal white matter. When expressed as the ratio of least cationic form of MBP to the most cationic (C-8/C-1), the normal ratio was 0.82, chronic MS 2.5, and the patient in this study 6.7. Because the ratio of 6.7 was similar to 7.5 found for a 15-month-old infant, MBP was considered to be of the immature form. The data are consistent with a genetic factor influencing the charge microheterogeneity of MBP. The resulting less cationic MBP cannot carry out its normal function of compacting multilayers.

The effects of citrullination or variable amino-terminus acylation on the encephalitogenicity of human myelin basic protein in the PL/J mouse

The post-translational modifications of myelin basic protein (MBP) in the form of citrullination and varying length of amino-terminus acylation may modify the biological functions and immunological features of MBP. Both modifications influence the reaction of antibodies and specific T cells recognizing MBP. The present study was undertaken to compare the encephalitogenicity of the citrullinated isomer of MBP (MBP-C8) with the unmodified isomer of MBP (MBP-C1) and to determine if the length of amino-terminal acylation of MBP peptide 1-21 altered an encephalitogenic epitope. MBP-C8, whether from patients with or without multiple sclerosis (MS), and MBP-C1 could induce active experimental allergic encephalomyelitis (EAE) in PL/J mice. A trend of reduced severity of EAE was observed in MBP-C8-injected animals. An increase in the length of amino-terminus fatty acid decreased the encephalitogenicity of MBP peptide 1-21 for both active and adoptive EAE in PL/J mice. Only lymph node cells sensitive to MBP peptide acetyl 1-21 and butyl 1-21 could transfer clinical EAE. In adoptive EAE, MBP peptides hexyl and octyl 1-21 induced moderate histopathological but no clinical change, whereas MBP peptide decyl 1-21 caused neither. A broadening in the antibody response could be detected in the sera of mice with active EAE induced by MBP-acylated peptides 1-21. Our findings demonstrate that encephalitogenicity is retained in the presence of citrullination but that the length of amino-terminus acylation diminishes the encephalitogenicity of MBP in the PL/J mouse.(ABSTRACT TRUNCATED AT 250 WORDS)

Reactivity of normal T-cell lines to MBP isolated from normal and multiple sclerosis white matter

T-cell reactivity to human myelin basic protein (MBP) has been extensively studied using T-cell lines and clones generated from both peripheral blood and cerebrospinal fluid, from normal controls and multiple sclerosis (MS) patients. These studies have largely utilized myelin basic protein isolated from control human adult white matter. In our study, we used MBP reactive T-cell lines as a probe to investigate antigenic differences in a series of MBP preparations isolated from either control human white matter or white matter from the central nervous system (CNS) of MS patients. Autologous peripheral blood derived mononuclear cells were used as antigen presenting cells (APC). Although the majority of T-cells were found to react equally well with all preparations of MBP isolated from both control and MS white matter, we were also able to identify T-cell lines which reacted well with all preparations of MBP isolated from controls but failed to react with MBP isolated from MS white matter. These differences were unlikely to reflect differences in degradation products or excess peptides present in the MS brain since SDS-PAGE and HPLC did not show any difference in the MS samples compared to the controls, and the concentration response curves for a human T-cell clone specific for the 84-102 region of MBP were similar for all the MBP preparations. We did not detect differences in amino acid content amongst MBP preparations although single amino acid substitutions cannot be ruled out. These results raise the possibility that MBP isolated from MS brain may differ in charge microheterogeneity which would affect antigenic determinants.(ABSTRACT TRUNCATED AT 250 WORDS)

Myelin in multiple sclerosis is developmentally immature

The etiology of multiple sclerosis (MS) is considered to involve genetic, environmental, infective, and immunological factors which affect the integrity of a normally assembled myelin sheath, either directly or indirectly resulting in demyelination. In a correlative study involving protein chemical, mass spectrometric, and electron microscopic techniques we have determined that myelin obtained from victims of MS is arrested at the level of the first growth spurt (within the first 6 yr of life) and is therefore developmentally immature. The data supporting this conclusion include (a) the pattern of microheterogeneity of myelin basic protein (MBP); (b) the NH2-terminal acylation of the least cationic component of MBP ("C-8"); (c) the phase transition temperature (Tc) of myelin isolated from victims of MS correlated with the increased proportion of the least cationic component of MBP; and (d) immunogold electron microscopy using an antibody specific for "C-8" showed that the distribution of gold particles in a 2-yr-old infant was similar to the distribution found in a victim of MS. We postulate that this developmentally immature myelin is more susceptible to degradation by one or a combination of factors mentioned above, providing the initial antigenic material to the immune system.

Immunological analysis of the amino terminal and the C8 isomer of human myelin basic protein

The citrullination and N-terminus acylation of myelin basic protein (MBP) increases the heterogeneity among the MBP isoforms. The present study was undertaken to further characterize the immune response to the citrullinated form (C8) of MBP as well as to the variably acylated N-terminus of MBP. Six well-characterized murine monoclonal antibodies (mAbs) to human MBP-C8 or MBP peptides (four mAbs to MBP acetyl 1-9, one mAb to MBP 10-19 and one mAb to MBP 80-89), one murine T cell line (PL11) to human MBP peptide acetyl 1-9 and one Lewis rat T cell line (RT-1) to guinea pig (GP) MBP peptide 68-88 were used to assess reactivity with MBP-C1, MBP-C8, and MBP peptides including a series of MBP peptide 1-21 containing 0, 2, 4, 6, 8 or 10 carbon fatty acids. Enzyme-linked immunosorbent assay (ELISA) results revealed that all of the mAbs reacted with human MBP-C1 and MBP-C8 except anti-MBP 10-19 and anti-MBP-C8. The former reacted only with MBP-C1 and the latter only with MBP-C8. The presence and length of acylation of MBP peptide 1-21 modified reactivity. Three mAbs to MBP acetyl 1-9 reacted only with acetyl 1-21, and one mAb anti-MBP actyl 1-9 reacted with all of MBP 1-21 preparations whether acylated or not. mAb anti-MBP-C8 generally reacted better with acylated MBP 1-21 having longer fatty acids. The PL11 T cell line strongly proliferated to human MBP-C1, MBP-C8 and MBP acetyl 1-9, responded, but less well, to MBP 1-21 with longer fatty acids and failed to respond to nonacylated MBP peptide 1-21. The RT-1 cell line responded strongly to GP MBP peptide 68-88, marginally to MBP-C8 and failed to respond to MBP-C1 or any of the other MBP peptides. Specific immune responses to different MBP charge isomers and different N-terminal acylating groups of MBP may play a role in immune-mediated demyelination.