Equilibrium and nonequilibrium competitive inhibitions of antipeptide antibody binding by parent myelin basic protein and 18 related peptide sequences

A major epitope of synthetic rabbit encephalitogen S24 disclosed through the use of a tritiated peptide probe

The antibody recognition of synthetic rabbit encephalitogen S24 (TTHYGSLPQKG), which had been inactivated by extensive iodination, was fully restored by tritium-iodine exchange during boron hydride reduction. A highly active form of 3H-S24 was obtained by reverse-phase high-performance liquid chromatography (HPLC) purification of the crude product and could be further separated into three continuous subfractions, the latter two of which were fully active from an immunochemical point of view. A major epitope was disclosed through use of the 3H-S24 probe, which required the intact S24 peptide for its expression. Confirmation of two linear epitopes previously encountered, GSLPQK and THYGSL, was also obtained, the latter through the use of the third HPLC subfraction of 3H-S24.

Polyclonal antibodies to the encephalitogenic neighborhoods of myelin basic protein: singular affinity populations neutralized by specific synthetic peptide probes

Specific ligand neutralization was used to probe the extent to which singular antibody affinity populations signified specific determinants in the neighborhood myelin basic protein (MBP) encephalitogens. The probes were individual members of a panel of synthetic peptide analogs subsuming encephalitogenic regions. Comparative Scatchard analyses of neutralized and unneutralized antisera helped to identify the particular peptide determinants involved in the original polyclonal antibody responses to the multiple antigenic determinants of encephalitogenic peptides. The range of affinities for an antibody population against a singular MBP peptide determinant was found to be relatively restricted while the range of affinities overall for all populations within a given antipeptide antiserum was found to be relatively wide and invariably discontinuous. Consequently, the individual discontinuous affinity populations could readily be dissected by application of the Rosenthal method of Scatchard curve analysis. It was found that the singular high affinity antibody population (5.6 x 10(7) M-1) of a Lewis rat antiserum to rat encephalitogenic GSLPQKAQRPQDENG (S49) was against a determinant near the N-terminal non-encephalitogenic end of the peptide. Only the low affinity antibody populations were found that had reactivity for determinants within the encephalitogenic region itself. The singular high affinity antibody population (5.97 x 10(7) M-1) of a rabbit antiserum to rabbit encephalitogenic TTHYGSLPQKAQGHRPQDEG (S82) was against a determinant centered about the tyrosyl residue, within the encephalitogenic region for the rabbit, but was completely cross-reactive with a specific circulating endogenous inhibitor. The results obtained with the rat and rabbit EAE sera were consistent with a previously advanced hypothesis that antibodies to determinants within encephalitogenic neighborhoods would effectively block the onset of EAE if high enough in affinity and not neutralized by an endogenous inhibitor.

Monoclonal and polyclonal antibodies to myelin basic protein determinants

A detailed immunochemical examination of monoclonal and polyclonal antibody responses to myelin basic protein (MBP) and its peptides has revealed the existence of as many as 27 antigenic determinants, many of them conformational. Topological mapping of the potential antigenic determinants onto a model of MBP secondary structure places these determinants within 11 separate regions of the molecule, including those portions that have been found to be encephalitogenic. MBP and its peptides, therefore, fall under the umbrella of the Multideterminant-Regulatory Model of Benjamin et al. (1984). However, in the case of MBP, multideterminant immunogenicity appears to represent mainly an escape from tight regulation through the avenue of conformational change.

A serum factor cross-reactive with antibodies to a determinant of rabbit encephalitogenic sequence 65-74 of myelin basic protein

A serum factor, cross-reactive with antibodies to a defined determinant of myelin basic protein (residues 66-71), has been found in the sera of nine mammalian species where it may function as a specific neuroautotolerogen. In equilibrium competitive inhibition radioimmunoassays the factor appears to be completely competitive with synthetic peptide S24 (TTHYGSLPQKG) at high affinity and is therefore termed MBP-SF-24 (myelin basic protein serum factor of the S24 type). The bulk of the activity can be recovered by ammonium sulfate fractionation at 61.1% saturated ammonium sulfate (SAS), pH 7, (fraction E) after removal by precipitation at pH 7 of the 37.5, 42.6, 47.5, and 51.4% SAS fractions (fractions A-D), including the immunoglobulins, and before removal by precipitation at pH 5 of the albumin fraction (fraction F). The factor, by its retention on XM300 during ultrafiltration of fraction E, can be purified 20-fold from serum proteins without much loss through a combination of SAS fractionation and ultrafiltration. The yield of MBP-SF-S24 in fraction E may range from a low 26 pmol S24 equivalents from 10 ml in sheep serum to a high 1.7 nmoles from 10 ml rat serum. The serum factor is reactive at high affinity with each of two populations of S24-reactive antibodies in one rabbit reagent antiserum and with one of two populations of S24-reactive antibodies in another. It appears to express a determinant involving residues THYGSL (66-71) of myelin basic protein with the same conformation as found in intact S24.

Affinity purification of two populations of antibodies against format determinants of synthetic myelin basic protein peptide S82 from S82-AH- and S82-CH-Sepharose 4B columns

Two different kinds of immunosorbents were prepared that contained the synthetic myelin basic protein didecapeptide S82 (TTHYGSLPQKAQGHRDQDEG)--one coupled with AH-Sepharose 4B through hexanoate spacers to the C-terminal glycyl residue; the other, with CH-Sepharose 4B through hexanoate spacers to the N-terminal threonine residue. An antiserum rich in antibodies to a format determinant of S82 was passed through each column, and, by means of affinity purification, two homogeneous populations of anti-format antibodies were obtained, each with a binding affinity of 1 X 10(8)M-1 for S82. The population recovered from S82-AH-Sepharose 4B cross-reacted to a considerable extent with synthetic peptide S8 (GSLPQKAQGHRPQDENG) but only to a limited extent with S79 (AQGHRPQDEG). The population recovered from S82-CH-Sepharose 4B cross-reacted poorly, if at all, with S8. An equimoler mixture of S8 + S79, however, reacted well with either population of anti-format antibodies, thus showing that the mixture could mimic the format of S82. It was concluded that secondary structural conformation of S82 could be preserved during the coupling procedure and that the resulting immunosorbents could be used for the affinity purification of anti-S82 antibodies to the format determinants.

Format determinants of synthetic myelin basic protein peptide S82 mimicked by a mixture of synthetic peptides S8 and S79

Antibodies to synthetic myelin basic protein peptide S82 (TTHYG-SLPQKAQGHRPQDEG) did not react with synthetic peptide S8 (GSLPQKAQGHRPQDENG) and only partially so with synthetic peptide S79 (AQGHRPQDEG); however, the antibodies did react to a considerable extent with an equimolar mixture of S8 and S79. Since the anti-S82 antibodies had previously been shown to be directed to a non-sequential format determinant dependent on the conformation of secondary structure, it seems probable that the mixture of S8 and S79 assumed a format that neither one individually possessed to any great degree.

Immunochemical cross-reactivity between intact purified myelin basic protein (MBP) and the synthetic encephalitogenic peptide S49

Three antisera to myelin basic protein--a rabbit antiserum pool against rat myelin, a rabbit antiserum pool against rat myelin basic protein (MBP), and a monkey antiserum against bovine MBP--were found to contain detectable levels of antibodies that would bind radiolabeled S49 (GSLPQKAQRPQDENG). Strongly encephalitogenic in Lewis rat, S49 is a synthetic peptide representing residues 69-84 of bovine MBP with a deletion of glycine-76 and histidine-77 to make it analogous to rat and guinea pig MBPs. The rabbit antimyelin antiserum and the monkey anti-MBP antiserum contained antibodies directed against a non-sequential determinant that required asparagine 84, the glycine-histidine deletion, and residues 69-71 for maximal activity. S49-reactive antibodies from the rabbit anti-MBP antiserum were directed solely against a sequential determinant comprising residues 69-71, S49-reactive antibodies from all three antisera reacted in liquid phase with purified intact rat, guinea pig, and bovine MBP showing that the determinant is exposed for B cell recognition even in bovine MBP and can serve both as immunogen and reactant.

Affinity purification of an acylated and radiolabelled synthetic derivative of residues 75-83 of bovine myelin basic protein, [125I]S79. A model for the purification of picomole quantities of specific peptide fragments of myelin basic protein and antibodies against them

Among the antibodies contained in a rabbit antiserum to synthetic peptide sequence TTHYGSLPQKAQGHRPQDEG (S82) of bovine myelin basic protein (residues 65-83 plus glycine), was a population reactive with a C-terminal determinant of S82 and cross-reactive with S79 (AQGHRPQDEG) but not S6 (AQGHRPQDENG). This antibody population was purified 153-fold by affinity chromatography from a minicolumn containing S79 coupled to CH-Sepharose 4B(TM) and eluted with 3 M MgCl2. The purified antibodies were then coupled to CNBr-activated Sepharose 4B(TM) and used to purify 125I-labelled, acylated S79 ([125I]S79), 3 M MgCl2 once again having been used to elute the labelled ligand. Sips distribution studies revealed appreciable heterogeneity of binding affinities of unpurified antibodies in their reaction with affinity-purified [125I]S79 or of purified antibodies in their reaction with unpurified [125I]S79 (heterogeneity constant a = 0.34 and 0.36, respectively). In contrast Sips distribution data indicated considerable restriction of binding of the purified antibodies in their reaction with purified labelled ligand (a = 0.92) with an average affinity constant of K0 = 1.56 X 10(8) M-1. The results indicate that the heterogeneous spectrum of binding affinities originally displayed by the unpurified S79-reactive antibodies in their reaction with unpurified labelled S79 was due both to the presence of some antibodies characterized by high affinity binding (K0 greater than 10(9) M-1) and of some labelled ligand with low binding affinity. The affinity chromatographic method as here described should prove advantageous in purifying and eventually characterizing picomolar amounts of serum factors, previously postulated to be fragments of myelin basic protein, that are reactive with reagent antibodies up to an affinity level of 10(8) M-1.