(Figure 6b). == Figure 6. by -D-glutamyl-meso-diaminopimelic acid (meso-DAP), which is unique to PGN structures in all Gram-negative bacteria and certain Gram-positive bacteria, including the genusListeriaandBacillus1. In contrast, Nod2 is activated by muramyl dipeptide (MDP), a PGN motif present in all Gram-positive and Gram-negative bacteria1. The importance of NLRs in detecting specific microbial products is underscored by the susceptibility of mice with targeted deletion of various NLR genes2. However, the role of NLRs in sensing intracellular parasites is currently unclear. Toxoplasma gondiiis an obligate intracellular protozoan pathogen capable of infecting various animal species. Infection withT. gondiican cause severe disease, such as pneumonia and encephalitis, in immunocompromised hosts. One major determinant of the outcome ofT. gondiiinfection is the ability of the host to elicit robust cellular immunity against the parasite. This protective immunity is mediated primarily by CLG4B interleukin 12 (IL-12)-induced interferon (IFN)- derived from natural killer (NK) cells and T helper type 1 (TH1) polarized T cells3,4. Host recognition of extracellularT. gondiiby the toll-like receptors (TLRs) expressed on dendritic cells (DCs) promotes secretion of the pro-inflammatory cytokine IL-12.T. gondii-induced IL-12 production and optimal resistance to infection is dependent on TLR11, which recognizes a profilin-like protein fromT. gondii5. Others have implicated TLR2 in parasite-induced responses6, suggesting that the parasite expresses more than one type of TLR ligand. In addition to TLRs, DC-derived IL-12 can also be induced by parasite-derived cyclophilin-18 via CCR5, a Giprotein-coupled chemokine receptor7. From these studies, it is evident the host is endowed with extracellular sensing mechanisms that elicit immune response againstT. gondii. However, given thatT. gondiiresides and replicates in a nonfusogenic vacuole in the cytoplasm, we sought to determine if the host employs an intracellular detection system, such as the mammalian NLRs, to sense intracellular parasites. We found that Nod2 provides a T cell-intrinsic signal that is necessary not Thalidomide fluoride only for generating protective TH1 immunity againstT. gondiibut also for driving T cell-mediated colitis. == Results == == Nod2 promotes resistance toT. gondii == To assess whether NLRs participate in host defense againstT. gondii, mice genetically deficient in various NLR family members were inoculated intraperitoneally (i.p.) with the avirulent ME49 strain ofT. gondii, and the survival of the animals was monitored. Among the mutant mice strains tested, mice lacking Nod1, Nlrc4, ASC, Nlrp6 and Nlrp12 exhibited normal resistance toT. gondiiinfection; only Nod2-deficient mice exhibited impaired survival afterT. gondiiinfection (Figure 1a;Supplementary Figure 1). The enhanced susceptibility of Nod2-null animals was clearly distinct from that of mice lacking IL-12p35 or IFN- (Figure 1a). Unexpectedly, mice lacking RICK (also called Rip2), a caspase-recruitment domain (CARD)-containing kinase that has been implicated in Nod1 and Thalidomide fluoride Nod2 signaling8, were resistant to parasite infection (Figure 1a). The resistance of the Nod2-deficient animals to acute infection was confirmed by examining the peritoneal exudate cells (PECs) harvested from the peritoneum cavity on day 7 post-infection. In bothNod2/and wild-type animals, less than 1% of the cells were infected with the parasite; in contrast greater than 20% of the PECs recovered at the same time point fromIfng/mice were infected (Figure 1b). However, examination of PECs harvested from infected Nod2-deficient mice on day 12 revealed high numbers of infiltrating mononuclear cells harboring parasites as well as numerous extracellular parasites Thalidomide fluoride (Figure 1c). Furthermore, in contrast to wild-type mice, which contained few (<1%) infected cells and lower concentrations of serum IFN-, there was a 20-fold increase in the number of cells infected with tachyzoites and high concentrations of circulating IFN- inNod2/mice (Figure 1dand data not shown). Together, these results suggest that the susceptibility ofNod2/animals following parasite challenge appears to be due to a delayed impairment in the ability to control parasite replication at the site of infection. == Figure 1. == Nod2 in host defense againstT. gondiiinfection. Wild-type (WT),Nod2/,RICK/andIfng/mice were challenged i.p. with 20.