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Histophilus somni causes bovine pneumonia, septicemia, myocarditis, thrombotic meningoencephalitis and arthritis, as well as a genital or upper respiratory carrier state in normal animals. However, differences in virulence factors among strains are not well studied. The surface and secreted immunoglobulin binding protein A (IbpA) Fic motif of H. somni causes bovine alveolar type 2 (BAT2) cells to retract, allowing virulent bacteria to cross the alveolar monolayer. Because H. somni IbpA is an important virulence factor, its presence was evaluated in different strains from cattle, sheep and bison to define whether there are syndrome specific markers and whether antigenic/molecular/functional conservation occurs. A few preputial carrier strains lacked IbpA by Western blotting but all other tested disease or carrier strains were IbpA positive. These positive strains had either both IbpA DR1/Fic and IbpA DR2/Fic or only IbpA DR2/Fic by PCR. IbpA Fic mediated cytotoxicity for BAT2 cells and sequence analysis of IbpA DR2/Fic from selected strains revealed conservation of sequence and function in disease and IbpA positive carrier strains. Passive protection of mice against H. somni septicemia with antibody to IbpA DR2/Fic, along with previous data, indicates that the IbpA DR1/Fic and/or DR2/Fic domains are candidate vaccine antigens for protection against many strains of H. somni. Since IbpA DR2/Fic is conserved in most carrier strains, they may be virulent if introduced to susceptible animals at susceptible sites. Conservation of the protective IbpA antigen in all disease isolates tested is encouraging for development of protective vaccines and diagnostic assays., (Copyright © 2010 Elsevier B.V. All rights reserved.)

Haemophilus somnus immunoglobulin binding proteins (IgBPs) are virulence associated but only one (p76) has been genetically defined. We determined the nucleotide sequence of the 5'-flanking region of the p76 gene. This region had been identified as the coding region for a series of high molecular weight (HMW)-IgBPs. Analysis of the nucleotide sequence indicated the gene (immunoglobulin binding protein A, ibpA) encoding the HMW and p76 IgBPs comprised a single open reading frame of 12,285 base pairs (bp). The ibpA gene is flanked by an upstream ORF of 1758bp, designated ibpB. The predicted amino acid sequences of these two genes demonstrate similarity to virulence exoproteins and their transporter proteins that comprise a two-partner secretion pathway in various Gram-negative bacteria. Motifs associated with binding to mammalian cells were also identified within the sequence. Competitive inhibition studies implicated a putative heparin-binding domain in adherence to bovine endothelial cells. Expression plasmids for glutathione S-transferase (GST)-fused recombinant fragments covered amino acid residues 972-3201. IgG2 Fc binding studies identified fragment 972-1515 (GST-IbpA3) as an Fc binding peptide. This peptide and GST-IbpA5 (aa 2071-2730) reacted strongly with convalescent phase serum. In a small preliminary study, calves immunized with the purified GST-IbpA3 peptide were protected against an intrabronchial H. somnus challenge.