MBP-SF, a prominent serum factor in suckling lewis rats that additively inhibits the primary binding of Myelin Basic Protein (MBP) to syngeneic anti-MBP antibodies

Abstract

MBP-SF† is a serum factor in suckling Lewis rats that additively inhibits the binding of 125I-labeled and unlabeled myelin basic protein (MBP) to syngeneic Lewis rat anti-MBP antibodies. It can be detected and measured by 4 different methods of immunological inhibition analysis involving 2 different approaches to radioimmunoassay (salting-out with sodium sulfate, precipitation with rabbit anti-rat immunoglobulin light chain). Thus. MBP-SF contains all the determinants of Lewis rat MBP that are reactive with syngeneic anti-MBP antibodies. Analyses of 47 pools of suckling and control serums, drawn from many different litters at different times, indicate a general rise in activity to a mean peak value of 14ng/μl within 2 weeks after birth. The serum factor is still present in weanling rats but decreases to insignificant levels by the 6th postnatal week. In the pooled serum of one litter 12 ng MBP-SF/μl were present at birth before suckling began. There are at least three hypotheses which can account for the presence of MBP-SF: the factor may (A) represent MBP that is synthesized in excess over that required for the expanding myelin “sink” during the postnatal period; (B) comprise only those peptide regions of MBP whose determinants are involved in humoral immunity: or (C) consist of proteins or peptides of non-neural origin that share the MBP determinants involved in humoral immunity. Regardless of which hypothesis is correct, the cross-reaction of MBP-SF with anti-MBP. coupled with its prominence in early postnatal development, would seem to suggest that it may act as a circulating immunoregulatory molecule or endogenous autoneurotolerogen for humoral immunity to MBP determinants.