Antibodies to macrophage stimulating protein (MSP): Specificity, epitope interactions, and immunoassay of MSP in human serum

Macrophage stimulating protein (MSP) is a member of a family of proteins characterized by a triple disulfide loop structure (kringle). We developed antibodies to human MSP for detection in Western blots, quantification in biological fluids, and neutralization of activity. Immunogens included native MSP, reduced and alkylated α and β chains, and peptides of MSP regions with minimal sequence similarity to other kringle proteins. We found three antibody categories based on interaction with the following types of epitope: primary sequence, discontinuous (dependent on disulfide bonds), and cryptic (not exposed in native MSP). None of the antibodies reacted with related kringle proteins. A specific sandwich ELISA was developed for measuring human MSP. The mean serum concentration was 4 nM. Serum MSP did not increase over a 24-h period in response to intravenous lipopolysaccharide, indicating that MSP is not an acute phase protein. These findings are consistent with the hypothesis that regulation of MSP activity is by conversion of pro-MSP to MSP rather than by rapid changes in rates of synthesis.