Sarcoidosis: Disease mechanisms, diagnostic pathway and treatment.

Sarcoidosis is an inflammatory granulomatous disease that affects people worldwide and can involve virtually any organ but most commonly the lungs and thoracic lymph nodes. The cause of sarcoidosis remains unknown, but occupational and environmental exposures, genetic background, and ethnicity are likely contributors to disease development. Recent immunological studies, including single-cell RNA sequencing and spatial transcriptomics, have increased our understanding of disease pathogenesis. Diagnosing sarcoidosis is often challenging due to the lack of a diagnostic gold standard and the remarkable variability in clinical presentation. Accordingly, the diagnosis requires the presence of compatible clinical and radiological features along with histopathological evidence of noncaseating granulomas and exclusion of other granulomatous diseases. The differential diagnosis includes infection, drug-induced granulomatosis, inborn error of immunity, vasculitis and malignancies. Sarcoidosis often resolves spontaneously, but it is not a benign disease. Up to one-third of patients develops chronic or progressive disease, which carries an increased risk of organ failure or death. Treatment is not always required, but is clearly indicated for progressive pulmonary disease, symptomatic cardiac or central nervous system involvement, and significantly impaired quality of life. Treatment aims to decrease symptom burden and preserve organ function. Corticosteroids have been considered first-line treatment for decades, but their long-term use is associated with substantial toxicity. Recently, methotrexate was found to be equally effective as prednisone as first-line treatment in pulmonary sarcoidosis. The identification of novel pathways involved in disease pathogenesis has suggested JAK inhibitors and mTOR inhibitors as potential therapies. More efficacious and better tolerated therapies are urgently needed; however, the rarity of the disease, its heterogeneous clinical course and the lack of prognostic biomarkers make it difficult to design and implement clinical trials of novel therapies.