[Mendelian susceptibility to mycobacterial disease: a case report of disseminated infection due to Mycobacterium avium].

Mendelian susceptibility to mycobacterial disease (MSMD) is a rare genetic syndrome that predisposes patients to infections caused by weakly virulent mycobacterial species, such as bacillus Calmette-Guérin (BCG) vaccines and nontuberculous environmental mycobacteria in children free of classical immunodeficiencies. This syndrome consists of impaired antimycobacterial immunity (axis IL12/INF-γ) constituting a new immune deficiency and outlining its major role in mycobacterial immunity. We report a new case of MSMD through the observation of a young girl with a disseminated infection due to Mycobacterium avium. The molecular defect was 2 autosomal recessive mutations of the IL12Rβ1 gene (gene encoding for the β1 chain of the IL12 receptor) leading to the absence of the IL12 receptor on the activated T lymphocytes' surface. IL-12RB1 deficiency is the most common genetic etiology of MSMD. Today, there are 6 MSMD-causing genes, leading to 13 distinct genetic disorders. The clinical phenotype differs between patients. The description of the molecular and immunological basis of this syndrome has allowed us to explain the pathophysiology of antimycobacterial immunity and is essential to understanding and managing these diseases.