Itaconate is an effector of a Rab GTPase cell-autonomous host defense pathway against Salmonella

Abstract

The guanosine triphosphatase (GTPase) Rab32 coordinates a cell-intrinsic host defense mechanism that restricts the replication of intravacuolar pathogens such as Salmonella. Here, we show that this mechanism requires aconitate decarboxylase 1 (IRG1), which synthesizes itaconate, a metabolite with antimicrobial activity. We find that Rab32 interacts with IRG1 on Salmonella infection and facilitates the delivery of itaconate to the Salmonella-containing vacuole. Mice defective in IRG1 rescued the virulence defect of a S. enterica serovar Typhimurium mutant specifically defective in its ability to counter the Rab32 defense mechanism. These studies provide a link between a metabolite produced in the mitochondria after stimulation of innate immune receptors and a cell-autonomous defense mechanism that restricts the replication of an intracellular bacterial pathogen.

Effect of pH on the antimicrobial activity of the macrophage metabolite itaconate

“…Intracellular Salmonella are typically located in acidic vacuoles (pH of ca. 5.0) [46], where the concentration of itaconate is 5–6 mM, according to a recent report [27]. Consistent with an antimicrobial role for itaconate in mammals, the MIC of itaconate is much lower at acidic pH and within concentrations that are found in specific macrophage organelles [27].…”

“…coli when the pH of the growth medium is lowered from 7.2 to 6.4 ( and S3), and >100-fold in S. Typhimurium when the pH is dropped from 7.2 to 6.0 ( and S4). Interestingly, at pH 6.0 the MIC 90 of itaconate in S. Typhimurium was found to be 3.7 mM, which is within the concentration range of itaconate found within Salmonella -containing vacuoles (5–6 mM) [27], implying a potential antimicrobial effect of itaconate in this organelle.…”

“…5.0) [46], where the concentration of itaconate is 5–6 mM, according to a recent report [27]. Consistent with an antimicrobial role for itaconate in mammals, the MIC of itaconate is much lower at acidic pH and within concentrations that are found in specific macrophage organelles [27]. Other examples of acidic organelles include phagosomes 10 min after phagocytosis (pH of 4.5–5.0) [47], early phagosomes (pH of 6.1–6.5), and late phagosomes (pH of 5.5–6.0) [48].…”

“…From these numbers, whether the itaconate concentration in macrophages is high enough to effectively inhibit microbial growth has been questioned [26]. It has more recently been demonstrated that itaconate is transported to vacuoles containing Salmonella and reaching 5–6 mM [27], consistent with itaconate playing an antimicrobial role [2, 15]. Furthermore, the amount of itaconate needed to inhibit bacterial growth may also depend on the environment.…”

The Emerging Application of Itaconate: Promising Molecular Targets and Therapeutic Opportunities

“…Meanwhile, the inhibitory effect of itaconate was also found in bacteria that did not express ICL enzyme, which may be due to the inhibition of acetic acid assimilation by inhibiting propionyl-CoA carboxylase (Yang et al, 2020 ). Some bacteria such as Yersinia pestis and Pseudomonas aeruginosa which carried genes encoding enzymes of itaconate degradation can promote pathogenicity and survival (Rao and McFadden, 1965 ; Rittenhouse and McFadden, 1974 ; Chen et al, 2020 ; Riquelme et al, 2020 ). Activated macrophages have been shown to produce itaconate, suggesting that these immune cells may employ this metabolite as a weapon against invading bacteria.…”