MAp19, the alternative splice product of the MASP2 gene

J Immunol Methods. 2011 Oct 28;373(1-2):89-101. doi: 10.1016/j.jim.2011.08.006. Epub 2011 Aug 17.

Abstract

The lectin pathway of complement is a central part of innate immunity, but as a powerful inducer of inflammation it needs to be tightly controlled. The MASP2 gene encodes two proteins, MASP-2 and MAp19. MASP-2 is the serine protease responsible for lectin pathway activation. The smaller alternative splice product, MAp19, lacks a catalytic domain but retains two of three domains involved in association with the pattern-recognition molecules (PRMs): mannan-binding lectin (MBL), H-ficolin, L-ficolin and M-ficolin. MAp19 reportedly acts as a competitive inhibitor of MASP-2-mediated complement activation. In light of a ten times lower affinity of MAp19, versus MASP-2, for association with the PRMs, much higher serum concentrations of MAp19 than MASP-2 would be required for MAp19 to exert such an inhibitory activity. Just four amino acid residues distinguish MAp19 from MASP-2, and these are conserved between man, mouse and rat. Nonetheless we generated monoclonal rat anti-MAp19 antibodies and established a quantitative assay. We found the concentration of MAp19 in serum to be 217 ng/ml, i.e., 11nM, comparable to the 7 nM of MASP-2. In serum all MASP-2, but only a minor fraction of MAp19, was associated with PRMs. In contrast to previous reports we found that MAp19 could not compete with MASP-2 for binding to MBL, nor could it inhibit MASP-2-mediated complement activation. Immunohistochemical analyses combined with qRT-PCR revealed that both MAp19 and MASP-2 were mainly expressed in hepatocytes. High levels of MAp19 were found in urine, where MASP-2 was absent.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Alternative Splicing*
  • Animals
  • Antibodies, Monoclonal / immunology*
  • Antibody Affinity / immunology
  • Blotting, Western
  • Cytoplasm / metabolism
  • Gene Expression Profiling
  • HEK293 Cells
  • Hepatocytes / metabolism
  • Humans
  • Hybridomas
  • Immunohistochemistry
  • Kidney / metabolism
  • Lectins / immunology
  • Lectins / metabolism
  • Macrophages, Alveolar / metabolism
  • Mannose-Binding Lectin / immunology
  • Mannose-Binding Lectin / metabolism
  • Mannose-Binding Protein-Associated Serine Proteases / analysis
  • Mannose-Binding Protein-Associated Serine Proteases / genetics*
  • Mannose-Binding Protein-Associated Serine Proteases / immunology*
  • Mice
  • Protein Binding
  • Protein Isoforms / blood
  • Protein Isoforms / metabolism
  • Protein Isoforms / urine
  • Rats
  • Reverse Transcriptase Polymerase Chain Reaction

Substances

  • Antibodies, Monoclonal
  • Lectins
  • Mannose-Binding Lectin
  • Protein Isoforms
  • ficolin
  • MASP2 protein, human
  • Mannose-Binding Protein-Associated Serine Proteases