Gene expression of the A- and B-chain of mouse C1q in different tissues and the characterization of the recombinant A-chain.

Immunoscreening of a mouse macrophage cDNA library with an anti-mouse C1q-antibody resulted in the isolation of cDNA clones. The deduced amino acid sequence was homologous to the A-chain of human C1q. Homology on the DNA level was found to be 76% and on the protein level 72% thus it appeared the clones coded for the mouse C1q A-chain. An immunoblot of murine serum C1q separated by SDS-PAGE was detected with an A-chain specific antibody that had been affinity purified on recombinant mouse C1q A-chain expressed in Escherichia coli. The antibody preparation reacted exclusively with the mouse C-chain (as defined by SDS-PAGE). Northern blot analysis with strand-specific cDNA probes coding for the A- and B-chain of murine C1q showed that mouse peritoneal macrophages produced the highest concentration of C1q gene transcripts. RNA from mouse spleen, thymus, heart, and brain gave substantial hybridization signals, whereas RNA preparations from liver, kidney, lung, and small intestine appeared to contain only trace amounts of C1q mRNA. In a Northern blot analysis of different guinea pig cells and tissues, only RNA preparations from peritoneal macrophages hybridized with the mouse C1q probes. These results indicate that macrophages are a major site of C1q biosynthesis.