Mycobacteria in Crohn's disease: A persistent hypothesis

Efforts to explore a mycobacterial origin for Crohn's disease typically have involved an epidemiological approach, searching for Mycobacterium avium subsp. paratuberculosis in patient tissue. An alternative approach involves consideration of genetic and experimental data regarding host resistance to mycobacteria. From human and mycobacterial genetics, it is known that mycobacterial diseases depend on both pathogen and host factors and that tuberculosis and leprosy are effectively genetic diseases. The discovery of a number of Crohn's susceptibility genes, including NOD2/CARD15, demonstrates that Crohn's also is a complex genetic disease. Mutations in NOD2/CARD15 do not necessarily lead to Crohn's disease, so other mitigating factors, genetic and/or environmental, probably are required to produce illness. Recent work has shown that NOD2/CARD15 serves a role in bacterial sensing and activation of innate immune responses, providing a link between Crohn's genetics and an environmental factor, potentially a bacterial trigger. In this review, we discuss the current understanding of mycobacterial and Crohn's genetic susceptibility and review the evidence that NOD2/CARD15 may mediate host resistance to mycobacterial infection.