HAEMOPHILUS-SOMNUS IMMUNOGLOBULIN BINDING-PROTEINS AND SURFACE FIBRILS

The high molecular-weight (HMW) immunoglobulin binding proteins (IgBPs ) of Haemophilus somnus and a 76-kDa surface protein (p76) are found i n serum-resistant virulent strains hut not in several serum-sensitive strains from asymptomatic carriers. For the first time, p76 was shown to be an IgBP also. This was done by competitive inhibition studies wi th affinity-purified antidinitrophenol (anti-DNP) and DNP to ensure th at binding was not antigen specific. The HMW IgBPs, but not the p76 Ig BP, were partially purified from concentrated culture supernatant in d etergent by fluid-phase liquid chromatography with a gel filtration co lumn. Membrane extraction studies showed that p76 predominated in the Sarkosyl-soluble fraction of the bacterial cell pellet. Since integral outer membrane (OM) proteins are Sarkosyl insoluble, this is consiste nt with our previous finding that implicated p76 as a peripheral OM pr otein. The HMW IgBPs were found predominantly in the Sarkosyl-soluble fraction of the culture supernatant. This suggests that they were not integral membrane proteins and that their presence in the supernatant was not due to OM blebbing. We then showed that two IgBP-positive seru m-resistant virulent strains have a surface fibrillar network but that two IgBP-negative serum-sensitive H, somnus strains from asymptomatic preputial carriers do not. Fibrils on the surfaces of IgBP(+) strains bound gold-labelled bovine immunoglobulin G2 (IgG2) anti-DNP, indicat ing that these fibrils have IgG2 binding activity. Therefore, this stu dy shows that H. somnus has two IgBPs, including a peripheral membrane protein and a fibrillar surface network.