Histophilus somni Survives in Bovine Macrophages by Interfering with Phagosome-Lysosome Fusion, but Requires IbpA for Optimal Serum Resistance

Histophilus somni survives intracellularly in professional phagocytic cells, but the mechanism of intracellular survival is not understood. The Fic motif within the DR1/DR2 IbpA fibrillar network protein of H. somni is cytotoxic to epithelial and phagocytic cells, which may interfere with the bactericidal activity of these cells. To determine the contribution of IbpA and Fic on resistance to host defenses, strains and mutants that lack all of or a small region of ibpA or DR1/DR2 were tested for survival in bovine monocytic cells and for serum susceptibility. A mutant lacking IbpA, but not DR1/DR2, was more susceptible to killing by antiserum than the parent. H. somni strains expressing IbpA replicated in bovine monocytes for at least 72 hours, and were toxic for these cells. Virulent strain 2336 with transposon insertions or deletions within IbpA remained toxic for bovine monocytes. However, strain 2336 mutants lacking all of ibpA or both DR1/DR2 were not toxic to the monocytes, but survived within the monocytes for at least 72 hours. Examination of intracellular trafficking of H. somni with monoclonal antibodies to early and late phagosomal markers indicated that early phagosomal marker EEA-1 colocalized with both disease isolate strain 2336 and serum-sensitve mucosal isolate strain 129Pt, but only strain 2336 did not co-localize with late lysosomal marker LAMP-2 and prevented acidification of phagosomes. These results indicate that virulent isolates of H. somni are capable of surviving within phagocytic cells through interference of phagosome-lysosome maturation. Therefore, H. somni may be considered a permissive intracellular pathogen.