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

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Abstract
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, (fractionE) after removal by precipitation at pH 7 of the 37.5, 42.6, 47.5, and 51.4% SAS fractions (fractionsA-D), including the immunoglobulins, and before removal by precipitation at pH 5 of the albumin fraction (fractionF). The factor, by its retention on XM300 during ultrafiltration of fractionE, 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 fractionE 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.