Alteration of the antigenic structure of human fibronectin caused by complexing with collagen

Fibronectin potentiates actin polymerization in thrombin-activated platelets

The effect of fibronectin on the polymerization state of actin was studied. Triton X-100-insoluble cytoskeleton was prepared from thrombin-activated platelets, and the conversion of G-actin into F-actin was monitored by an assay involving DNase I inhibition by G-actin. It was found that fibronectin bound to membrane receptors decreased the level of platelet G-actin. This observation suggests that in the presence of fibronectin a larger amount of F-actin becomes incorporated into the Triton X-100-insoluble cytoskeleton. At the same molar concentration, fibrinogen only slightly increased actin polymerization, whereas bovine serum albumin at a much higher concentration caused a small inhibition of actin immobilization. Our data show that fibronectin, through interaction with the platelet actomyosin fibrillar system, facilitates actin polymerization into the cytoskeleton.

Comparison of some procedures detecting circulating immune complexes

Tests for circulating immune complexes were performed by means of 1) plain polyethylene glycol (PEG) precipitation (PEGprec), 2) immunoelectrophoresis of PEG precipitates (IEpp), 3) anti-antibody (AA) inhibition test with sera (AA-Is), and 4) AA inhibition test with PEG precipitates (AA-Ipp). The tests were performed with 156 pathological sera from patients with myasthenia gravis, syphilis, adenocarcinomas of the gastrointestinal tract, rheumatoid arthritis and systemic lupus erythematosus, and 51 normal sera from blood donors. PEGprec was positive with 76 sera, IEpp with 84 sera, AA-Is with 64 sera, and AA-Ipp with 74 sera. Comparison of results in all four tests showed high degree of correlation; all p values were below 0.005. The lower sensitivity of AA inhibition tests was due to the fact that these tests detect only complexes formed by IgG but not by IgM, whereas the remaining two tests detect complexes formed by antibodies of both these immunoglobulin classes. When sera of patients with rheumatoid arthritis and SLE were removed from the material studied, the four tests showed about equal sensitivity. PEGprec gave positive results with two normal sera and the remaining tests were negative with all these sera. It appears that the simultaneous application of PEGprec, IEpp, AA-Is and AA-Ipp will give sensitive and reliable procedure for detecting circulating immune complexes.