Chronic pneumonia in calves after experimental infection with Mycoplasma bovis strain 1067: Characterization of lung pathology, persistence of variable surface protein antigens and local immune response

Abstract

BackgroundMycoplasma bovis is associated with pneumonia in calves characterized by the development of chronic caseonecrotic lesions with the agent persisting within the lesion. The purposes of this study were to characterize the morphology of lung lesions, examine the presence of M. bovis variable surface protein (Vsp) antigens and study the local immune responses in calves after infection with M. bovis strain 1067.MethodsLung tissue samples from eight calves euthanased three weeks after experimental infection with M. bovis were examined by bacteriology and pathology. Lung lesions were evaluated by immunohistochemical (IHC) staining for wide spectrum cytokeratin and for M. bovis Vsp antigens and pMB67 antigen. IHC identification and quantitative evaluation of CD4+ and CD8+ T lymphocytes and immunoglobulin (IgG1, IgG2, IgM, IgA)-containing plasma cells was performed. Additionally, expression of major histocompatibility complex class II (MHC class II) was studied by IHC.ResultsSuppurative pneumonic lesions were found in all calves. In two calves with caseonecrotic pneumonia, necrotic foci were surrounded by epithelial cells resembling bronchial or bronchiolar epithelium. In all calves, M. bovis Vsp antigens were constantly present in the cytoplasm of macrophages and were also present extracellularly at the periphery of necrotic foci. There was a considerable increase in numbers of IgG1- and IgG2-positive plasma cells among which IgG1-containing plasma cells clearly predominated. Statistical evaluation of the numbers of CD4+ and CD8+ T cells, however, did not reveal statistically significant differences between inoculated and control calves. In M. bovis infected calves, hyperplasia of bronchus-associated lymphoid tissue (BALT) was characterized by strong MHC class II expression of lymphoid cells, but only few of the macrophages demarcating the caseonecrotic foci were positive for MHC class II.ConclusionsThe results from this study show that infection of calves with M. bovis results in various lung lesions including caseonecrotic pneumonia originating from bronchioli and bronchi. There is long-term persistence of M. bovis as demonstrated by bacteriology and immunohistochemistry for M. bovis antigens, i.e. Vsp antigens and pMB67. The persistence of the pathogen and its ability to evade the specific immune response may in part result from local downregulation of antigen presenting mechanisms and an ineffective humoral immune response with prevalence of IgG1 antibodies that, compared to IgG2 antibodies, are poor opsonins.

In VitroInfection of Bovine Monocytes with Mycoplasma bovis Delays Apoptosis and Suppresses Production of Gamma Interferon and Tumor Necrosis Factor Alpha but Not Interleukin-10

“…Interestingly, whereas IL-10 is known to impair the antigen-presenting capacity of monocytes and macrophages through decreased expression levels of surface major histocompatibility complex class II (MHC-II) and other costimulatory molecules, in our case, M. bovis-infected monocytes exhibited enhanced expression of MHC-II molecules (data not shown). Some reports have demonstrated that M. bovis preferentially induces a host Th2 immunological response characterized by strong IgG1 responses that do not confer protection in experimental vaccines (6,7). The absence of IFN-␥ and TNF-␣ from M. bovis-infected monocytes agrees with those observations.…”

“…Observations from experimentally infected cattle have revealed stimulation of both innate and adaptive immunity (5,6). The pulmonary bovine immune response to M. bovis is mainly anti-inflammatory, with high levels of interleukin-4 (IL-4) and moderate levels of gamma interferon (IFN-␥) and IgG1 antibodies (6,7). Although M. bovis infects predominantly mucosal and serosal surfaces, live bacteria can be detected in brain of infected cattle (8).…”

Mycoplasma bovis pneumonia in feedlot cattle and dairy calves in Argentina

“…In the lung, M. bovis produces chronic bronchopneumonia characterized by caseonecrotic nodules in the cranioventral lung lobes (2,10,24). Microscopic findings suggest that lung necrosis is originated in small bronchi and bronchioles (15,18). The persistence of the agent in the lung tissue and the chronicity of the pulmonary lesions imply that the immune response is ineffective in eliminating the infection (6,18,21).…”

Mycoplasma bovis in Australian feeder cattle

“…Consequently, M. bovis is not the only bacteria isolated from pneumonic lung lesions, and the presence of M. bovis antibody is not always associated with successful culture of M. bovis. This suggests that M. bovis alone may not always be the cause of BRD, but in some way may predispose animals to LRT colonisation by other bacteria, which are also often commensals of the URT, leading to the development of BRD (Gourlay and Houghton, 1985, Hermeyer et al, 2012.…”

“…Experimental infection trials conducted using calves have shown that M. bovis has both stimulating and suppressing effects on the bovine immune response (Hermeyer et al, 2012). The production of nitric oxide and tumor necrosis factor-alpha by macrophages is stimulated (Jungi et al, 1996); apoptosis of lymphocytes (Vanden Bush and Rosenbusch, 2002) and the production of a lympho-inhibitory peptide is induced (Vanden Bush and Rosenbusch, 2004); while lymphocyte responses to mitogens and the neutrophil oxidative burst is suppressed (Wiggins et al, 2010).…”