IFNs Modify the Proteome of Legionella-Containing Vacuoles and Restrict Infection Via IRG1-Derived Itaconic Acid

Abstract

Macrophages can be niches for bacterial pathogens or antibacterial effector cells depending on the pathogen and signals from the immune system. Here we show that type I and II IFNs are master regulators of gene expression during Legionella pneumophila infection, and activators of an alveolar macrophage-intrinsic immune response that restricts bacterial growth during pneumonia. Quantitative mass spectrometry revealed that both IFNs substantially modify Legionella-containing vacuoles, and comparative analyses reveal distinct subsets of transcriptionally and spatially IFN-regulated proteins. Immune-responsive gene (IRG)1 is induced by IFNs in mitochondria that closely associate with Legionella-containing vacuoles, and mediates production of itaconic acid. This metabolite is bactericidal against intravacuolar L. pneumophila as well as extracellular multidrug-resistant Gram-positive and -negative bacteria. Our study explores the overall role IFNs play in inducing substantial remodeling of bacterial vacuoles and in stimulating production of IRG1-derived itaconic acid which targets intravacuolar pathogens. IRG1 or its product itaconic acid might be therapeutically targetable to fight intracellular and drug-resistant bacteria.

The Roles of Type I Interferon in Bacterial Infection

“…While in a mouse model of pulmonary Legionella infection, IFNAR2 deficient mice did not reveal a role for type I IFNs (Ang et al, 2010), proliferation of Legionella in macrophages is inhibited by type I IFNs (Lippmann et al, 2011; Plumlee et al, 2009). Moreover, type I IFN, along with type II IFN, promote host defense likely through induction of cell intrinsic ISGs such as immune-responsive gene 1 (IRG1) (Naujoks et al, 2016). …”

“…Prostaglandin E2 (PGE2), an arachidonic acid derived lipid mediator that promotes host defense against Mycobacterium , is negatively regulated by type I IFN suppression of IL-1β, but, at the same time, negatively regulates type I IFN expression (Mayer-Barber et al, 2014). Conversely, type I IFNs induce IRG1, an ISG that generates, itaconic acid, an antimicrobial metabolite that effectively restricts the growth of intracellular bacteria (Naujoks et al, 2016) (Figure 3). …”

Food Fight: Role of Itaconate and Other Metabolites in Antimicrobial Defense

“…Indeed, growth of MTB is significantly inhibited by addition of itaconate to growth medium (Michelucci et al, 2013). In vitro bacterial growth inhibition by itaconate has also been described for S. enterica , Legionella pneumophilia , a clinical isolate of methicillin-resistant Staphylococcus aureus (MRSA), and a clinical isolate of multidrug-resistant (MDR) Acinetobacter baumannii (Michelucci et al, 2013; Naujoks et al, 2016). Interestingly, ICL has also been shown to be critical for bacterial persistence in addition to acute infection.…”

Type II Secretion Is Necessary for Optimal Association of the Legionella-Containing Vacuole with Macrophage Rab1B but Enhances Intracellular Replication Mainly by Rab1B-Independent Mechanisms

“…These host vesicles are thought to deliver membrane material, foodstuffs, and other "cargo" that facilitate intravacuolar growth of L. pneumophila (86). Among the host GTPases that modulate ER-to-Golgi vesicle traffic, Rab1B is well known for being recruited rapidly and substantially to LCVs in A/J BMD macrophages and U937 cells as well as murine RAW264.7 macrophages, human A549 lung epithelial cells, and human HEK293 embryonic kidney cells (61,66,67,(87)(88)(89)(90)(91)(92)(93)(94)(95)(96)(97)(98). Expression of a dominant negative form of Rab1 restricts L. pneumophila growth in Chinese hamster ovary (CHO) cells and COS1 cells, suggesting that Rab1B recruitment enhances bacterial growth (61,66).…”