Salmonella, a ubiquitous intracellular Gram-negative bacterium, is a food-borne pathogen with the ability to infect a broad range of hosts. Infection with Salmonella results in two main diseases: typhoid fever, a systemic disease caused by the human restricted Salmonella Typhi and Paratyphi; and salmonellosis, a self-limiting gastroenteritis caused predominantly by the zoonotic Salmonella Typhimurium and Enteritidis. Salmonella Typhimurium infection in mice is a recognized experimental model resembling systemic disease and sepsis of enteric typhoid fever in humans. In order to identify novel susceptibility loci, we have used N-ethyl-N-nitrosourea (ENU) mutagenesis and a three-generation breeding scheme to identify recessively inherited mutations. We have screened over 6000 G3 animals and identified eight deviant pedigrees including Ity15 (Immunity to Typhimurium locus 15) that is responsible for increased susceptibility to Salmonella infection.

The increased susceptibility of the Ity15 pedigree has been mapped to a 4.5 Mb interval on chromosome 15. Analysis of the exomes of two mice with early susceptibility revealed two potential candidate genes within the mapped interval: Gasdermin C4 (Gsdmc4) and Family with sequence homology 49 member b (Fam49b). Quantification of Gsdmc4 and Fam49b mRNA levels in spleen and liver at day 0 and day 5 post-infection presented a significant reduction of Fam49b transcript in mutant mice compared to wild-type or heterozygous littermates. In addition, FAM49B protein expression is completely abrogated in mutant mice. Using FAM49B knockout mice, we have confirmed that the mutation in Fam49b underlies the susceptibility in the Ity15 pedigree. At day 5 post-infection, we observed a small but significant increase in bacterial load in spleen and liver in mutant Ity15 mice compared to wild-type littermates. We also detected an increase in serum levels of IL-6 and IL-10 during infection in Ity15 mutant mice compared to wild-type littermates. By siRNA knockdown of FAM49B, we have also shown that lack of this protein results in increased invasion of Salmonella into HeLa cells. The identification of the exact mechanism of action of FAM49B during Salmonella infection is under study.