Host Immune Response to Histophilus somni

Histophilus somni is known to cause several overlapping syndromes or to be found in genital or upper respiratory carrier states in ruminants. Vaccines have been used for decades, yet efficacy is controversial and mechanisms of protective immunity are not well understood. Since H. somni survives phagocytosis, it has sometimes been considered to be a facultative intercellular parasite, implying that cell-mediated immunity would be critical in protection. However, H. somni not only inhibits phagocyte function, but also is cytotoxic for macrophages. Therefore, it does not live for long periods in healthy phagocytes. Protection of calves against H. somni pneumonia by passive immunization is also evidence that H. somni is more like an extracellular pathogen than an intracellular pathogen. Several studies showed that bovine IgG2 antibodies are more protective than IgG1 antibodies. Even the IgG2 allotypes tend to vary in protection. Of course, antigenic specificity also determines protection. So far, there is most evidence for protection by a 40 K outer membrane protein and by Immunoglobulin binding protein A fibrils. Serology and immunohistochemistry have both been used for immunodiagnosis. Many evasive mechanisms by H. somni have been defined, including decreased phagocyte function, antibodies bound by shed antigens, decreased immune stimulation, and antigenic variation. Interaction of H. somni with other bovine respiratory disease organisms is another layer of pathogenesis. Studies of bovine respiratory syncytial virus (BRSV) and H. somni in calfhood pneumonia revealed an increase in IgE antibodies to H. somni, which were associated with more severe disease of longer duration than with either agent alone. Innate immune mechanisms at the epithelial cell level are also affected by dual infection by BRSV and H. somni as compared to either pathogen alone. Although much more work needs to be done, the complex mechanisms of H. somni immunity are becoming clearer.