Common features of tuberculosis and sarcoidosis

Tuberculosis (TB) is a disease caused by Mycobacterium tuberculosis. Despite the availability of novel therapeutic approaches, TB is considered as one of the leading causes of death due to infectious diseases worldwide. Alveolar macrophages are the first line of defense against M. tuberculosis; they ingest and sequester the bacilli within granulomatous structures. Control and resolution of the infection requires activated T lymphocytes as well as Th1 cytokines. There are two forms of TB: active TB and latent TB. Latent TB is a state in which M. tuberculosis survives in the body without causing overt signs and symptoms. People with latent TB are noncontagious. However, M. tuberculosis can become active in the body, multiply, and cause overt TB. Sarcoidosis, on the other hand, is an autoimmune disease of unknown etiology which can affect multiple systems of the body. Nonspecific constitutional symptoms, such as fever, fatigue, malaise, and weight loss, are present in approximately one-third of patients. Chest X-ray usually shows hilar and mediastinal lymphadenopathy. Although the lungs are the most common sites of inflammation, sarcoidosis can also involve other organs, such as the eyes (intraocular and adnexal), skin, lymph nodes, salivary glands, heart, spleen, liver, and the nervous system. Recent investigations have provided further insights into the genetic basis of sarcoidosis and the way genotype determines the clinical presentation and phenotype of patients. Histopathologic features are usually insufficient for diagnosis of sarcoidosis. Diagnosis of sarcoidosis in endemic areas for TB can become a great challenge. Both TB and sarcoidosis are granulomatous diseases; TB is characterized by caseating granulomas, whereas sarcoidosis is characterized by noncaseating granulomas. New cases of sarcoidosis are increasingly being diagnosed in areas endemic for TB due to increased orientation of physicians and availability of diagnostic modalities. However, it is often difficult to differentiate sarcoidosis from TB, especially when caseous necrosis is not seen and acid-fast staining is negative in the biopsy specimen of patient with TB. Granulomatous inflammation in sarcoidosis is believed to be caused by the presence of a persistent poorly degradable unknown antigen in combination with a nonresolving host response. M. tuberculosis has been extensively studied as a possible cause of sarcoidosis. Results suggest that granulomas form in the lungs as a result of the immune response to inhaled M. tuberculosis and serve as the central site of host–pathogen interaction during M. tuberculosis infection. M. tuberculosis DNA detection in sarcoidosis samples by traditional polymerase chain reaction (PCR) has been used for the pathological study of sarcoidosis; however, it is likely that real time quantitative PCR analysis of specific mRNAs and microRNAs will be necessary as a sensitive, precise, and rapid diagnostic test for detecting trace of TB in Sarcoidosis. In conclusion, diagnosis of sarcoidosis in areas with a high burden of TB poses a significant challenge. Improved diagnostic tests including genetic tests can improve our knowledge and help in distinguishing these two diseases.