Sarcoidosis may have more than a single causative agent, including infectious and non-infectious agents. Among the potential infectious causes of sarcoidosis, Mycobacterium tuberculosis and Propionibacterium acnes are the most likely microorganisms. Potential latent infection by both microorganisms complicates the findings of molecular and immunologic studies. Immune responses to potential infectious agents of sarcoidosis should be considered together with the microorganisms detected in sarcoid granulomas, because immunologic reactivities to infectious agents reflect current and past infection, including latent infection unrelated to the cause of the granuloma formation. Histopathologic data more readily support P. acnes as a cause of sarcoidosis compared with M. tuberculosis, suggesting that normally symbiotic P. acnes leads to granuloma formation in some predisposed individuals with Th1 hypersensitivity against intracellular proliferation of latent P. acnes, which may be triggered by certain host or drug-induced conditions. Detection of bacterial nucleic acids in granulomas does not necessarily indicate co-localization of the bacterial proteins in the granulomas. In the histopathologic diagnosis of sarcoidosis, M. tuberculosis-associated and P. acnes-associated sarcoidosis will possibly be differentiated in some patients by immunohistochemistry with appropriate antibodies that specifically react with mycobacterial and propionibacterial antigens, respectively, for each etiology-based diagnosis and potential antimicrobial intervention against sarcoidosis.

Background: There is currently no consensus on the morphology of severe fibrotic pulmonary sarcoidosis, and we examined computed tomography (CT) findings and progression. Methods: We analyzed findings in 10 consecutive patients (three men, seven women) with pulmonary sarcoidosis requiring oxygen therapy for chronic respiratory failure, who were extracted from >2500 sarcoidosis patients (three hospitals, 2000–2018). Patients with comorbidities causing chronic respiratory failure were excluded. Results: Predominant findings were consolidations along the bronchovascular bundles comprising ‘central-peripheral band’, traction bronchiectasis, peripheral cysts/bullae, and upper lobe shrinkage. Traction bronchiectasis arose from opacities comprising ‘central-peripheral band’. Clustering of traction bronchiectasis at the distal side formed honeycomb lung-like structures in three patients. Upper lobe shrinkage progressed in seven patients together with progression of consolidations, ‘central-peripheral band’, traction bronchiectasis clusters, and cysts, while patients without shrinkage included two patients with severe multiple cysts without traction bronchiectasis. Restrictive ventilatory impairment developed in most patients. Pulmonary hypertension (PH) was detected radiologically in five patients, and chronic progressive pulmonary aspergillosis (CPPA) in four patients. Conclusions: During progression, consolidations comprising ‘central-peripheral band’ progressed together with traction bronchiectasis clusters and peripheral cysts, resulting in upper lobe shrinkage. This may lead to respiratory failure with possible complications such as PH and CPPA.

Propionibacterium acnes is implicated in the pathogenesis of sarcoidosis. We investigated the usefulness of immunohistochemistry (IHC) with a commercially available P. acnes-specific monoclonal antibody (PAB antibody) for differentiating sarcoidosis from other granulomatous diseases. Formalin-fixed paraffin-embedded tissue samples from 94 sarcoidosis patients and 30 control patients with other granulomatous diseases were examined by the original manual IHC method. We also compared the detection frequency of P. acnes in sarcoid granulomas between manual and automated IHC methods. P. acnes was detected in sarcoid granulomas of samples obtained by transbronchial lung biopsy (64%), video-associated thoracic surgery (67%), endobronchial-ultrasound-guided transbronchial-needle aspiration (32%), lymph node biopsy (80%), and skin biopsy (80%) from sarcoidosis patients, but not in any non-sarcoid granulomas of the samples obtained from control patients. P. acnes outside granulomas, however, was frequently detected in both groups. The detection status of P. acnes in granulomas did not correlate with the clinical characteristics of sarcoidosis patients. The automated Leica system exhibited the best detection sensitivity (72%) and almost an identical localization for P. acnes in sarcoid granulomas compared with the manual method. IHC with a PAB antibody is useful for differentiating sarcoidosis from other granulomatous diseases by detecting P. acnes in granulomas. An automated method by the Leica system can be used in pathology laboratories for differential diagnosis of granulomas by IHC with the PAB antibody.