Intracellular trafficking of Mycobacterium avium ss. paratuberculosis in macrophages.

The granulomatous enteric lesions of cattle with Johne's disease are composed of infected macrophages, and grow by accumulation, re-infection, and expansion of macrophage populations in the intestinal wall. We have examined the growth of bacteria in macrophages to define characteristics of intracellular trafficking for exocytosis, replication, and antigen presentation. Using immunocytochemical markers for light, confocal and electron microscopy, we have examined potential pathway tropisms using data for bacterial attachment, phagosomal acidification, phagolysosomal degradation and apoptosis. Our hypotheses are that pathogenic/wild-type strains block phagosomal acidification so that the phagosome fails to obtain markers of the late phagosome and phagolysosome, and this leads to the replication pathway within bacteriophorous vacuoles. Non-pathogenic strains appear to be processed to exocytosis, and avirulent mutant strains may be degraded and have preference of antigen processing pathways that involve transport vesicles bearing MHC II antigens. Pathogenicity in a nude mouse model of intestinal infection reveals lesion development and confirms pathway preferences of virulent strains for bacteriophorous vacuole formation.