Histological and immunohistochemical features suggesting aetiological differences in lymph node and (muco)cutaneous feline tuberculosis lesions

Abstract

Objectives To identify and describe histological and immunohistochemical criteria that may differentiate between skin and lymph node lesions associated with Mycobacterium (M.) bovis and M. microti in a diagnostic pathology setting. Materials and Methods Archived skin and lymph node biopsies of tuberculous lesions were stained with haematoxylin and eosin, Ziehl‐Neelsen and Masson's Trichrome. Immunohistochemistry was performed to detect the expression of calprotectin, CD3 and Pax5. Samples were scored for histological parameters (i.e. granulomas with central necrosis versus small granulomas without central necrosis, percentage necrosis and/or multinucleated giant cells), number of acid‐fast bacilli (bacterial index) and lesion percentage of fibrosis and positive immunohistochemical staining. Results Twenty‐two samples were examined (M. bovis n=11, M. microti n=11). When controlling for age, gender and tissue, feline M. bovis‐associated lesions more often featured large multi‐layered granulomas with central necrosis. Conversely, this presentation was infrequent in feline M. microti‐associated lesions, where small granulomas without central necrosis predominated. The presence of an outer fibrous capsule was variable in both groups, as was the bacterial index. There were no differences in intralesional expression of immunohistochemical markers. Clinical Significance Differences in the histological appearance of skin and lymph node lesions may help to infer feline infection with either M. bovis or M. microti at an earlier stage when investigating these cases, informing clinicians of the potential zoonotic risk. Importantly, cases of tuberculosis can present with numerous acid‐fast bacilli. This implies that a high bacterial index does not infer infection with non‐zoonotic non‐tuberculous mycobacteria.

Ocular mycobacterial lesions in cats

“…Feline tuberculous granulomas have been recently categorized as “organized” or “atypical,” depending on the features exhibited in these lesions. 55 These granulomas were infrequently identified across all ocular tissues, but many lesions exhibited an “unstructured” inflammatory infiltrate. Compared with other species, “classical” granuloma formation, that is, central necrosis and an extensive fibrous capsule, 88 is not a routine feature of mycobacterial lesions in cats, 35 , 41 which may reflect differences in the feline immune response or the inciting pathogen, with cases of M. microti more likely to present with “atypical” granulomas than “organized” granulomas.…”

“…Compared with other species, “classical” granuloma formation, that is, central necrosis and an extensive fibrous capsule, 88 is not a routine feature of mycobacterial lesions in cats, 35 , 41 which may reflect differences in the feline immune response or the inciting pathogen, with cases of M. microti more likely to present with “atypical” granulomas than “organized” granulomas. 55 Despite this, the identification of (pyo)granulomatous inflammation as the main histopathological presentation of feline ocular mycobacterial lesions suggests a broadly conserved immunological response to these infections in cats, regardless of the location of lesions.…”

“…Control tissues for IHC were a cutaneous lesion from a cat diagnosed with MTBC infection positive for calprotectin by IHC, 55 the parietal lobe from a cat with cognitive dysfunction, positive for ionized calcium-binding adaptor protein-1 (Iba1), and a feline cutaneous nonepitheliotropic B-cell lymphoma with sparse non-neoplastic T cells. Calprotectin is expressed by granulocytes, monocytes, and recently blood-derived macrophages and granulocytes 74 ; Iba1 by macrophages and microglia 39 ; cluster of differentiation 3 (CD3) by T cells 41 ; and Pax5 by B cells other than plasma cells.…”

“…Slides were scanned using a NanoZoomer-XR scanner, using the NanoZoomer Digital Pathology (NDP) sofware, NDP.scan Ver.3.2.12 (Hamamatsu Photonics) and images viewed on NDP.view2 Ver 2.7.52 (Hamamatsu Photonics, Hamamatsu City, Japan). Sections stained with HE were examined for the presence and population of inflammatory cells, the granuloma type (ie, “organized,” having singular or multifocal zones of central caseous necrosis surrounded by macrophages and neutrophils, with peripherally located lymphocytes and plasma cells; or “atypical,” having well-defined clusters of macrophages and epithelioid macrophages divided by thin fibrous septa), 55 and additional features such as necrosis. The presence of collagen, indicating fibrosis, was assessed using the MT-stained slides.…”

“…The degree of inflammation was scored by counting the number of inflammatory cells present within each lesion using the cell detection feature in QuPath Ver 0.1.2 8 and assigned an inflammation score ( Table 1 ). Slides stained with ZN were examined under light microscopy and the bacterial index for tissues with inflammatory lesions was calculated as previously described, 55 and a bacterial index grade assigned ( Table 2 ).…”