Mannheimia haemolytica is the principal bacterium isolated from respiratory disease in feedlot cattle and is a significant component of enzootic pneumonia in all neonatal calves. A commensal of the nasopharynx, M. haemolytica is an opportunist, gaining access to the lungs when host defenses are compromised by stress or infection with respiratory viruses or mycoplasma. Although several serotypes act as commensals, A1 and A6 are the most frequent isolates from pneumonic lungs. Potential virulence factors include adhesin, capsular polysaccharide, fimbriae, iron-regulated outer membrane proteins, leukotoxin (Lkt), lipopolysaccharide (LPS), lipoproteins, neuraminidase, sialoglycoprotease and transferrin-binding proteins. Of these, Lkt is pivotal in induction of pneumonia. Lkt-mediated infiltration and destruction of neutrophils and other leukocytes impairs bacterial clearance and contributes to development of fibrinous pneumonia. LPS may act synergistically with Lkt, enhancing its effects and contributing endotoxic activity. Antibiotics are employed extensively in the feedlot industry, both prophylactically and therapeutically, but their efficacy varies because of inconsistencies in diagnosis and treatment regimes and development of antibiotic resistance. Vaccines have been used for many decades, even though traditional bacterins failed to demonstrate protection and their use often enhanced disease in vaccinated animals. Modern vaccines use culture supernatants containing Lkt and other soluble antigens, or bacterial extracts, alone or combined with bacterins. These vaccines have 50-70% efficacy in prevention of M. haemolytica pneumonia. Effective control of M. haemolytica pneumonia is likely to require a combination of more definitive diagnosis, efficacious vaccines, therapeutic intervention and improved management practices.

A 4.4-kilobase-pair DNA fragment coding for the leukotoxin of PasteureUa haemolytica Al has been isolated, and its nucleotide sequence has been determined. Two open reading frames, designated ikaC and IkUA, coding for proteins of 19.8 and 101.9 kilodaltons, respectively, were identified. Expression of the two genes in minicell-labeling experiments resulted in the production of the predicted proteins LKTC and LKTA. By using an antiserum against the soluble antigens of P. haemolytica Al in Western blot (immunoblot) analysis of total cellular proteins from the Escherichia coli clones, LKTA was identified as an additional antigenic protein. Results from subcloning of the DNA fragment suggested that expression from both lkiC and lktA is required for leukotoxin activity, indicating that the leukotoxin of P. haemolylica Al is encoded by two genes. A comparison of the organization and the DNA sequence of the leukotoxin genes with those of the E. coli alpha-hemolysin genes showed a significant degree of homology between the two loci. This analysis suggested that the leukotoxin genes of P. haemolylica Al and the E. coli alpha-hemolysin genes may have evolved from a common ancestor and that the two toxins may share similar activities or functional domains or both.

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The toxicity of Pasteurella haemolytica culture supernatant for bovine and porcine cells was evaluated by 51Cr release. Sterile bacterial culture supematant was toxic for bovine pulmonary lavage cells, peripheral blood lymphocytes and neutrophils, and cultured peripheral blood mononuclear cells, resulting in marked 51Cr release. Only slight release was induced from cultured Madin-Darby bovine kidney cells, porcine pulmonary lavage cells, peripheral blood mononuclear cells, lymphocytes, or neutrophils. No release was detected with primary bovine spleen cell cultures, bovine erythrocytes, or porcine erythrocytes. The demonstrated specificity of this cytotoxin for bovine leukocytic cells may be important in the pathogenesis of bovine pneumonic pasteurellosis.

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A clone bank of Pasteureila haemolytica Al was constructed by partial digestion of the genomic DNA with Sau3A and ligation of 5-to 10-kilobase-pair fragments into the BamHI, site of the plasmid vector pBR322. After transformation uito Escherichia coli K-12, a total of 4 x 103 recombinant clones was obtained. These were screened for the production of P. haemolytica soluble antigens by a colony enzyme-linked immunosorbent assay blot method with a rabbit antiserum raised against the soluble antigens. The clones producing P. haemolytica soluble antigens were then analyzed for the production of the leukotoxin by a cytotoxicity assay with cells from a bovine leukemia-derived B.-lymphocyte cell line as the target cells. Positive clones were identified, and subsequent restriction analysis of the recombinant plasmids showed that the same 6.3 kilobase pairs of insert DNA was cloned in either of the two orientations into the plasmid vector pBR322..One of the clones was selected for further characterization of the leukotoxin as produced in E. c6li. Tests for heat lability and target cell species specificity with canine, porcine, and human peripheral blood lymphQcytes indicated that the activity of the cloned leukotoxin wps identical to that of the P. haemolytica leukotoxin. Furthermore, the E. coli-produced leukotoxin was also neutralzed by bovine or rabbit antiserumn known to have antitoxic activity. When cellular proteins from the E. coli clones were subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blot analysis, a 100,000-dalton protein was identified which corresponded to one of the soluble antigens found in the leukotoxic culture supernatant of P. haemolytica. These results demonstrated that the gene(s) for the P. haemolytica leukotoxin have been cloned and that the leukotoxin was expressed in E. coli.

A neutral metalloprotease with marked specificity for an 0-sialoglycoprotein has been isolated from culture supernatants of Pasteurella haemolytica Al. The 35-kDa enzyme cleaves human erythrocyte glycophorin A, which is 0 glycosylated, but does not cleave N-glycosylated proteins or nonglycosylated proteins. Glycophorin A was cleaved when it was present in situ in erythrocyte ghost plasma membranes or when it was free in solution. The glycoprotease did not hydrolyze glycophorin A from which sialate residues had been removed by neuraminidase treatment. An immobilized preparation of the enzyme cleaved glycophorin A at several positions, with a major site of cleavage at Arg-31-Asp-32. The glycoprotease is inhibited by EDTA, citrate, and ascorbate, but inhibition appears to be due to the masking of metal ion activators rather than to their removal. The enzyme is not inhibited by phosphoramidon, an inhibitor of other bacterial neutral metalloproteases.

Innate immune defenses are crucial for survival in the first days and weeks of life. At birth, newborns are confronted with a vast array of potentially pathogenic microorganisms that were not encountered in utero. At this age, cellular components of the adaptive immune system are in a naive state and are slow to respond. Antibodies received from the dam are essential for defense, but represent a finite and dwindling resource. Innate components of the immune system detect pathogen-associated molecular patterns (PAMPs) on microorganisms (and their products) by means of pattern-recognition receptors (PRRs). Soluble mediators of the innate system such as complement proteins, pentraxins, collectins, ficolins, defensins, lactoferrin, lysozyme etc. can bind to structures on pathogens, leading to agglutination, interference with receptor binding, opsonization, neutralization, direct membrane damage and recruitment of additional soluble and cellular elements through inflammation. Cell-associated receptors such as the Toll-like receptors (TLRs) can activate cells and coordinate responses (both innate and adaptive). In this paper, accumulated knowledge of the receptors, soluble and cellular elements that contribute to innate defenses of young animals is reviewed. Research interest in this area has been intermittent, and the literature varies in quantity and quality. It is hoped that documentation of the limitations of our knowledge base will lead to more extensive and enlightening studies.