Antigenic interspecies cross-reaction of subcomponent C1q of the first component of complement: evidence for its cross-reaction in both collagen-like and non-collagen-like regions

Reconstitution of murine resident peritoneal macrophages for antibody-dependent cellular cytotoxicity by homologous serum Clq

Mouse resident peritoneal macrophages (PM) were reconstituted in their response to activation for antibody-dependent cellular cytotoxicity (ADCC) for sheep erythrocyte targets (SRBC) by subhemolytic dilutions of homologous or autologous sera. ADCC-responsive inflammatory PM were largely unaffected in their activation by exogenous serum. Augmentation of resident PM for ADCC by homologous serum was correlated with the complement-activating potential of the mouse monoclonal anti-SRBC IgG isotype in that serum augmented IgG gamma 2a greater than IgG gamma 2b much greater than IgG gamma 1. The active component of mouse serum was heat-labile at 56 degrees C for 30 min and was present in both C5-deficient AKR and C5-sufficient homologous C3H mouse sera. Western blot analysis of the cell lysates for Clq confirmed that oil-elicited and thioglycollate-elicited inflammatory PM had greater levels of endogenous Clq than did resident PM which correlated with their innate responsiveness for ADCC activation. Depletion of Clq from serum by immunoprecipitation with IgG antibody to Clq or by ion exchange chromatography removed the active reconstituting activity for ADCC. Purified mouse Clq (0.4 microgram) partially replenished the ADCC augmenting activity of Clq-depleted AKR mouse serum. SRBC targets preopsonized with IgG gamma 2a and purified mouse Clq (0.075-5.0 microgram/ml) fully reconstituted the ADCC response of resident PM similar to homologous serum indicating that the major active component of serum was Clq. Thus resident PM with low endogenous levels of Clq were reconstituted for ADCC by the addition of exogenous Clq, whereas inflammatory PM with sufficiently high endogenous levels of Clq were not further enhanced by exogenous Clq. Our findings indicate that Clq may provide an essential second signal in concert with Fc receptor binding of IgG to initiate ADCC activation of macrophages.

Isolation of pig Clq and its structural and functional characterization

Pig Clq, a subcomponent of the first component of complement, was isolated in a fully hemolytically active form using precipitation in the presence of chelating agents at low ionic strength. Three times precipitated Clq was highly purified as shown by immunoelectrophoresis, polyacrylamide gel electrophoresis and ultracentrifugation. The yield of Clq using this method ranged from 40 to 55%. Pig Clq was also purified using precipitation with EDTA followed by affinity chromatography on a pig IgG-Sepharose 4B column. However, overall recovery of Clq was only about 10%. The purified Clq was heat-labile (56 degrees C, 30 min) both structurally and functionally, had an apparent molecular weight of 400 600 daltons as determined by ultracentrifugation, and restored the hemolytic complement activity of Clq-depleted sera of various species thus confirming interchangeability of this subcomponent. Cross-reactivity of pig, human, guinea-pig, mouse, rat and frog Clq-containing sera with monospecific anti-pig Clq antiserum showed a high degree of antigenic similarity.