Differential Cytokine Gene Expression in Granulomas from Lungs and Lymph Nodes of Cattle Experimentally Infected with Aerosolized Mycobacterium bovis

Abstract

The hallmark lesion of tuberculosis in humans and animals is the granuloma. The granuloma represents a distinct host cellular immune response composed of epithelioid macrophages, lymphocytes, and multinucleated giant cells, often surrounding a caseous necrotic core. Within the granuloma, host-pathogen interactions determine disease outcome. Factors within the granulomas such as cytokines and chemokines drive cell recruitment, activity, function and ultimately the success or failure of the host’s ability to control infection. Hence, an understanding of the granuloma-level cytokine response is necessary to understand tuberculosis pathogenesis. In-situ cytokine expression patterns were measured using a novel in situ hybridization assay, known as RNAScope® in granulomas of the lungs, tracheobronchial lymph nodes and caudal mediastinal lymph nodes of cattle experimentally infected with Mycobacterium bovis via aerosol exposure. In spite of microscopic morphological similarities, significant differences were seen between late stage granulomas of the lung compared to those of the tracheobronchial lymph nodes for IL-17A, IFN-γ, TGF-β, IL10 and IL-22 but not for TNF-α. Additionally, significant differences were noted between granulomas from two different thoracic lymph nodes that both receive afferent lymphatics from the lungs (i.e., tracheobronchial and caudal mediastinal lymph nodes) for TNF-α, IL-17A, IFN-γ, TGF-β and IL-10 but not for IL-22. These findings show that granuloma morphology alone is not a reliable indicator of granuloma function as granulomas of similar morphologies can have disparate cytokine expression patterns. Moreover, anatomically distinct lymph nodes (tracheobronchial vs caudal mediastinal) differ in cytokine expression patterns even when both receive afferent lymphatics from a lung containing tuberculoid granulomas. These findings show that selection of tissue and anatomic location are critical factors in assessing host immune response to M. bovis and should be considered carefully.

Measuring bovine γδ T cell function at the site of Mycobacterium bovis infection

“…In our RNAscope analysis, γδ T cells comprised a significant portion of the immune cells expressing IFN-γ within lymph node granulomas, although this proportion was much reduced within lung granulomas. This difference in tissue cytokine expression is in agreement with recent results by Palmer et al , who observed significant differences in inflammatory cytokine expression between lung and lymph node lesions [63]. Although the biological significance of these findings is unclear, it suggests that anatomic location has a significant impact on the host immune response to M. bovis.…”

“…This is in agreement with Aranday-Cortes et al , who observed a decrease in IL-22 levels by qPCR in late-stage granulomas from M. bovis -infected cattle [43, 44]. Similarly, Palmer et al observed very low levels of expression of IL-22 by RNAScope in any stage of lesion collected at 150 days post M. bovis infection [63]. In human patients chronically infected with M. tb , cellular immune responses become depressed [64]; therefore, it may not be surprising that decreased levels of IL-22 were found at the later stages of infection.…”

Animal tuberculosis: impact of disease heterogeneity in transmission, diagnosis, and control

“…A recent work from Palmer, Thacker, and Waters (2016) revealed a differential gene expression of cytokines in lung and lymph node granulomas in cattle experimentally infected with M. bovis (Palmer et al, 2016). Moreover, when comparing cytokines (IFN‐γ, TNF‐α, IL‐10, IL‐17A, IL‐22 and TGF‐β) expression between two lymph nodes with a distinct anatomical localization, tracheobronchial and mediastinal, the expression was also differential (Palmer et al, 2016). This work highlighted the importance of considering the anatomical location of lesions when assessing host immune response and that cytokine expression might be related to the nature of specific granuloma microenvironments.…”

Mycobacterium avium Subspecies paratuberculosis Drives an Innate Th17-Like T Cell Response Regardless of the Presence of Antigen-Presenting Cells

“…that Th17 responses are an important feature of immune cell reactions to MAP. IL-17A is a protective cytokine in some inflammatory diseases (42,43) and in mycobacterial infections following an early infection (25,44,45). Chronic inflammation, a distinct feature of MAP infection and Johne's disease, could be induced by either IL-17A (21) or 47).…”