Submitted to: Veterinary Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 28, 2007
Publication Date: November 14, 2007
Citation: Palmer, M.V., Waters, W.R., Thacker, T.C. 2007. Lesion Development and Immunohistochemical Changes in Granulomas from Cattle Experimentally Infected with Mycobacterium bovis. Veterinary Pathology. 44(6):863-874. Interpretive Summary: Mycobacterium bovis, causes bovine tuberculosis and is known to persist within sites known as granulomas. To examine changes in granulomas over time, cattle were inoculated with M. bovis. Tissues were examined at various time-points after inoculation. Granulomas were staged based on cell composition. Staining was done for inflammatory mediators such as inducible nitric oxide synthase (iNOS), and for inflammatory cells. Granulomas progressed orderly from early mild lesions to large severe lesions with extensive tissue destruction. Mycobacterium bovis organisms were present in moderate numbers in stage I granulomas 15 to 60 days after inoculation and in late stage IV granulomas. iNOS as abundant through day 60, but was minimal after day 90. Granulomas are dynamic lesions that follow an orderly progression through disease stages. Diminished expression of iNOS late in disease progression may represent a failure of the host response to control infection. This information will be of use to scientists in understanding the progression of tuberculosis and means of preventing disease through vaccination.
Technical Abstract: Mycobacterium bovis, the causative agent of bovine tuberculosis is known to persist within granulomas. To examine temporal changes in granulomas, 32 cattle were inoculated with M. bovis. Tissues from four calves each were examined at various time-points after inoculation. Granulomas in the retropharyngeal lymph node were staged based on cell composition and presence of necrosis and peripheral fibrosis. Immunohistochemistry for inducible nitric oxide synthase (iNOS), CD68, CD4, CD8 and gamma/delta T-cells was performed. Fifteen days after inoculation only stage I granulomas were present; however, by day 60 granulomas of all 4 stages were seen. In later time-points stage I granulomas were seen in proximity to advanced granulomas (stage III-IV), characteristic of “satellite granulomas.” Acid-fast bacilli were present in moderate numbers in stage I granulomas 15 to 60 days after inoculation. In contrast, satellite granulomas contained few acid-fast bacilli. Stage IV granulomas contained large numbers of acid-fast bacteria. iNOS immunoreactivity was present within lesions 15 days after inoculation. Abundant iNOS immunoreactivity was present through day 60, but was minimal after day 90. Lesions at all time-points contained larger numbers of CD4+ T-cells with lesser numbers of CD8+ T-cells and gamma/delta T-cells. The relative number of CD4+ cells remained constant throughout the study while numbers of CD8+ and gamma/delta T-cells diminished. Tuberculous granulomas are dynamic lesions that follow an orderly progression through disease stages. Diminished expression of iNOS late in the progression of tuberculous granulomas may represent a failure of the host response to control infection.