Genetic determinants of susceptibility to Mycobacterial infections: IRF8, a new kid on the block.

Genetic and population studies suggest that onset, progression and ultimate outcome of infection with Mycobacteria, including the agent of tuberculosis Mycobacterium tuberculosis, are strongly influenced by genetic factors. Family-based and case-control linkage and association studies have suggested a complex genetic component for susceptibility to tuberculosis. On the other hand, patients with inborn errors in the IL12/IFNγ circuit may develop disseminated mycobacterial infections following perinatal BCG vaccination. The study of such MSMD (Mendelian Susceptibility to Mycobacterial Diseases) patients has provided much insight into innate and acquired immune defenses against mycobacteria. Parallel genetic analyses in mouse models of mycobacterial infections have also indicated complex genetic control, and have provided candidate genes for parallel testing in humans. Recently, mutations in human IRF8 were discovered and shown to cause two distinct forms of a novel primary immunodeficiency and associated susceptibility to mycobacteria. Autosomal recessive IRF8 deficiency is caused by mutation K108E and associated with severe disease with complete depletion of monocytes and dendritic cells. Mutation T80A causes autosomal dominant IRF8 deficiency and a milder form of the disease with selective loss of a subset of dendritic cells. These findings have established that IRF8 is required for ontogeny of the myeloid lineage and for host response to mycobacteria. The ongoing study of the IRF8 transcriptome has shown promise for the identification of IRF8 dependent pathways that play a critical role in host defense against mycobacteria in particular, and against intracellular pathogens in general.