Indoleamine 2,3-dioxygenase 1 is a lung-specific innate immune defense mechanism that inhibits growth of Francisella tularensis tryptophan auxotrophs.

Organs at different anatomic sites of the body have different physiological functions and are exposed to vastly different microbial and environmental challenges on a daily basis (32). As a result, depending on its relative sterility, each organ senses impending danger of infection differently and employs unique innate immune responses in an organ-specific homeostatic manner to effectively fight off any potential infection without compromising organ physiology and function. For instance, a sterile organ like the spleen is likely to induce a strong proinflammatory and bactericidal defense to maintain organ sterility, while such a defense would not be induced in a nonsterile organ like the colon. Instead, innate immune responses in the colon support a peaceful coexistence with the gut microflora rather than achieving sterility. To gain a better understanding of tissue-specific innate immune defense mechanisms, we have used Francisella tularensis as a model bacterial pathogen that is capable of entering and surviving in many different host tissues and organs. Francisella tularensis is a facultative, intracellular Gram-negative bacterium that causes the highly debilitating zoonotic disease tularemia (28). There are currently 4 known subspecies of F. tularensis that cause disease of different severities. The most virulent subspecies of Francisella is F. tularensis subsp. tularensis, which is found predominantly in North America and has an infectious dose of less than 10 CFU in humans (33, 34, 37). It is also associated with lethal pulmonary infections. Francisella novicida is also found primarily in North America but rarely causes disease in immunocompetent individuals (16). However, F. novicida shares the same families of virulence genes as F. tularensis, causes a similar disease in mice (27), and is more genetically amenable than F. tularensis, thus making F. novicida infection of mice a good experimental model for the study of Francisella pathogenesis. Francisella infects mammalian hosts via extremely diverse routes of entry, colonizing and replicating in different organs. Tularemia occurs in several forms, depending on the initial route of infection. The most common form of tularemia is ulceroglandular tularemia, which occurs when the bacterium enters the skin subcutaneously (9). Other infection routes include inoculation via the conjunctiva, which leads to oculoglandular tularemia (9, 38), and the ingestion of contaminated food and water, which leads to oropharyngeal or gastrointestinal tularemia (9). The most acute and fatal form of tularemia is pneumonic tularemia, which is caused by inhalation of the bacterium into the lungs. Pneumonic tularemia can also occur as a result of complications from the above forms of tularemia (9, 12), when the bacteria spread systemically from the initial peripheral site of infection to the lungs. From the perspective of the fitness of a microbe, it is advantageous for a microbe to infect its host via multiple entry routes and infect multiple organs. Thus, it is plausible that F. tularensis, a microbe that infects a range of different organs, has to deal with a diverse repertoire of different innate immune responses launched in these various organs during infection. We utilized a genome-wide genetic screen in F. novicida as a tool to identify tissue-specific interactions between the host and pathogen. Genetic screens are very effective tools that have been successfully used for the large-scale identification of virulence factors of many bacterial pathogens in vivo (6, 19, 44). Here, we utilize a microarray-based, negative-selection technique called transposon site hybridization (TraSH) (44) in an intranasal (i.n.) model of Francisella infection to identify Francisella factors important for bacterial growth and/or survival in the lungs. The screen identified almost all of the known Francisella virulence genes, including the Francisella pathogenicity island (FPI) genes. We also identified novel genes not previously known to be important for bacterial growth and/or survival in the lungs. By comparing the genes that are important for Francisella growth and/or survival in the lungs with the genes identified previously for spleen colonization (44), we identified the Francisella tryptophan biosynthetic pathway as being important for bacterial survival and/or growth specifically in the lungs. We demonstrated that the host enzyme that is responsible for catalyzing the first rate-limiting step of tryptophan degradation, indoleamine 2,3-dioxygenase 1 (IDO1), is induced specifically in the lungs during Francisella infection, suggesting that the host innate immune response to F. novicida restricts the availability of this essential amino acid. Indeed, the F. novicida tryptophan mutant bacteria survived better in the lungs of mice that lack IDO1 compared to wild-type mice. These findings suggest that IDO1 acts as an organ-specific, host innate immune mechanism that defends against a highly virulent bacterial pathogen.