How did we get here? Insights into mechanisms of immunity-related GTPase targeting to intracellular pathogens

Selective autophagy receptor hinders antitumor immunity

Autophagy has a dual role in tumor progression and therapy, influenced by specific receptors and cargo selection. Recent research published in Cell by Herhaus et al. identifies immunity-related GTPase Q (IRGQ) as a novel autophagy receptor that facilitates immune evasion in hepatocellular carcinoma (HCC) by degrading histocompatibility complex class I (MHC-I) molecules, highlighting a potential target to enhance immunotherapy.

UID-Dual Transcriptome Sequencing Analysis of the Molecular Interactions between Streptococcus agalactiae ATCC 27956 and Mammary Epithelial Cells

Streptococcus agalactiae ATCC 27956 is a highly contagious Gram-positive bacterium that causes mastitis, has a high infectivity for mammary epithelial cells, and becomes challenging to treat. However, the molecular interactions between it and mammary epithelial cells remain poorly understood. This study analyzed differential gene expression in mammary epithelial cells with varying levels of S. agalactiae infection using UID-Dual transcriptome sequencing and bioinformatics tools. This study identified 211 differentially expressed mRNAs (DEmRNAs) and 452 differentially expressed lncRNAs (DElncRNAs) in host cells, primarily enriched in anti-inflammatory responses, immune responses, and cancer-related processes. Additionally, 854 pathogen differentially expressed mRNAs (pDEmRNAs) were identified, mainly enriched in protein metabolism, gene expression, and biosynthesis processes. Mammary epithelial cells activate pathways, such as the ERK1/2 pathway, to produce reactive oxygen species (ROS) to eliminate bacteria. The bacteria disrupt the host's innate immune mechanisms by interfering with the alternative splicing processes of mammary epithelial cells. Specifically, the bacterial genes of tsf, prfB, and infC can interfere with lncRNAs targeting RUNX1 and BCL2L11 in mammary epithelial cells, affecting the alternative splicing of target genes and altering normal molecular regulation.

Irgm proteins attenuate inflammatory disease in mouse models of genital Chlamydia infection

Chlamydiae are obligate intracellular bacterial pathogens that may cause genital pathology via induction of destructive host immune responses. Human-adapted Chlamydia trachomatis causes inflammatory disease in human hosts but is easily cleared in mice, and mouse-adapted Chlamydia muridarum establishes a productive and pathogenic infection in murine hosts. While numerous anti-chlamydial host resistance factors have been discovered in mice and humans alike, little is known about host factors promoting host fitness independent of host resistance. Here, we show that interferon-inducible immunity-related GTPase M (Irgm) proteins function as such host factors ameliorating infection-associated sequalae in the murine female genital tract, thus characterizing Irgm proteins as mediators of disease tolerance. Specifically, we demonstrate that mice deficient for all three murine Irgm paralogs (pan-Irgm-/-) are defective for cell-autonomous immunity to C. trachomatis, which correlates with an early and transient increase in bacterial burden and sustained hyperinflammation in vivo. In contrast, upon infection of pan-Irgm-/- mice with C. muridarum, bacterial burden is unaffected, yet genital inflammation and scarring pathology are nonetheless increased, demonstrating that Irgm proteins can promote host fitness without altering bacterial burden. Additionally, pan-Irgm-/- mice display increased granulomatous inflammation in genital Chlamydia infection, implicating Irgm proteins in the regulation of granuloma formation and maintenance. These findings demonstrate that Irgm proteins regulate pathogenic immune responses to Chlamydia infection in vivo, establishing an effective infection model to examine the immunoregulatory functions and mechanisms of Irgm proteins.

IMPORTANCE

In response to genital Chlamydia infection, the immune system mounts a proinflammatory response to resist the pathogen, yet inflammation must be tightly controlled to avoid collateral damage and scarring to host genital tissue. Variation in the human IRGM gene is associated with susceptibility to autoinflammatory diseases but its role in ameliorating inflammatory diseases caused by infections is poorly defined. Here, we use mice deficient for all three murine Irgm paralogs to demonstrate that Irgm proteins not only provide host resistance to Chlamydia infections but also limit associated inflammation in the female genital tract. In particular, we find that murine Irgm expression prevents granulomatous inflammation, which parallels inflammatory diseases associated with variants in human IRGM. Our findings therefore establish genital Chlamydia infection as a useful model to study the roles for Irgm proteins in both promoting protective immunity and limiting pathogenic inflammation.

iNOS is necessary for GBP-mediated T. gondii clearance in murine macrophages via vacuole nitration and intravacuolar network collapse

Toxoplasma gondii is an obligate intracellular parasite of rodents and humans. Interferon-inducible guanylate binding proteins (GBPs) are mediators of T. gondii clearance, however, this mechanism is incomplete. Here, using automated spatially targeted optical micro proteomics we demonstrate that inducible nitric oxide synthetase (iNOS) is highly enriched at GBP2+ parasitophorous vacuoles (PV) in murine macrophages. iNOS expression in macrophages is necessary to limit T. gondii load in vivo and in vitro. Although iNOS activity is dispensable for GBP2 recruitment and PV membrane ruffling; parasites can replicate, egress and shed GBP2 when iNOS is inhibited. T. gondii clearance by iNOS requires nitric oxide, leading to nitration of the PV and collapse of the intravacuolar network of membranes in a chromosome 3 GBP-dependent manner. We conclude that reactive nitrogen species generated by iNOS cooperate with GBPs to target distinct structures in the PV that are necessary for optimal parasite clearance in macrophages.

Restriction and evasion: a review of IFNγ-mediated cell-autonomous defense pathways during genital Chlamydia infection

Chlamydia trachomatis is the most common cause of bacterial sexually transmitted infection (STI) in the USA. As an STI, C. trachomatis infections can cause inflammatory damage to the female reproductive tract and downstream sequelae including infertility. No vaccine currently exists to C. trachomatis, which evades sterilizing immune responses in its human host. A better understanding of this evasion will greatly benefit the production of anti-Chlamydia therapeutics and vaccination strategies. This minireview will discuss a single branch of the immune system, which activates in response to genital Chlamydia infection: so-called "cell-autonomous immunity" activated by the cytokine interferon-gamma. We will also discuss the mechanisms by which human and mouse-adapted Chlamydia species evade cell-autonomous immune responses in their native hosts. This minireview will examine five pathways of host defense and their evasion: (i) depletion of tryptophan and other nutrients, (ii) immunity-related GTPase-mediated defense, (iii) production of nitric oxide, (iv) IFNγ-induced cell death, and (v) RNF213-mediated destruction of inclusions.

Human guanylate-binding proteins in intracellular pathogen detection, destruction, and host cell death induction

Cell-intrinsic defense is an essential part of the immune response against intracellular pathogens regulated by cytokine-induced proteins and pathways. One of the most upregulated families of proteins in this defense system are the guanylate-binding proteins (GBPs), large GTPases of the dynamin family, induced in response to interferon gamma. Human GBPs (hGBPs) exert their antimicrobial activity through detection of pathogen-associated molecular patterns and/or damage-associated molecular patterns to execute control mechanisms directed at the pathogen itself as well as the vacuolar compartments in which it resides. Consequently, hGBPs are also inducers of canonical and noncanonical inflammasome responses leading to host cell death. The mechanisms are both cell-type and pathogen-dependent with hGBP1 acting as a pioneer sensor for intracellular invaders. This review focuses on the most recent functional roles of hGBPs in pathways of pathogen detection, destruction, and host cell death induction.

Inducible nitric oxide synthase (iNOS) is necessary for GBP-mediated T. gondii restriction in murine macrophages via vacuole nitration and intravacuolar network collapse

Toxoplasma gondii is an obligate intracellular, protozoan pathogen of rodents and humans. T. gondii's ability to grow within cells and evade cell-autonomous immunity depends on the integrity of the parasitophorous vacuole (PV). Interferon-inducible guanylate binding proteins (GBPs) are central mediators of T. gondii clearance, however, the precise mechanism linking GBP recruitment to the PV and T. gondii restriction is not clear. This knowledge gap is linked to heterogenous GBP-targeting across a population of vacuoles and the lack of tools to selectively purify the intact PV. To identify mediators of parasite clearance associated with GBP2-positive vacuoles, we employed a novel protein discovery tool automated spatially targeted optical micro proteomics (autoSTOMP). This approach identified inducible nitric oxide synthetase (iNOS) enriched at levels similar to the GBPs in infected bone marrow-derived myeloid cells. iNOS expression on myeloid cells was necessary for mice to control T. gondii growth in vivo and survive acute infection. T. gondii infection of IFNγ-primed macrophage was sufficient to robustly induce iNOS expression. iNOS restricted T. gondii infection through nitric oxide synthesis rather than arginine depletion, leading to robust and selective nitration of the PV. Optimal parasite restriction by iNOS and vacuole nitration depended on the chromosome 3 GBPs. Notably, GBP2 recruitment and ruffling of the PV membrane occurred in iNOS knockouts, however, these vacuoles contained dividing parasites. iNOS activity was necessary for the collapse of the intravacuolar network of nanotubular membranes which connects parasites to each other and the host cytosol. Based on these data we conclude reactive nitrogen species generated by iNOS cooperate with the chromosome 3 GBPs to target distinct biology of the PV that are necessary for optimal parasite clearance in murine myeloid cells.

Interferon-Inducible E3 Ligase RNF213 Facilitates Host-Protective Linear and K63-Linked Ubiquitylation of Toxoplasma gondii Parasitophorous Vacuoles

The obligate intracellular protozoan pathogen Toxoplasma gondii infects a wide range of vertebrate hosts and frequently causes zoonotic infections in humans. Whereas infected immunocompetent individuals typically remain asymptomatic, toxoplasmosis in immunocompromised individuals can manifest as a severe, potentially lethal disease, and congenital Toxoplasma infections are associated with adverse pregnancy outcomes. The protective immune response of healthy individuals involves the production of lymphocyte-derived cytokines such as interferon gamma (IFN-γ), which elicits cell-autonomous immunity in host cells. IFN-γ-inducible antiparasitic defense programs comprise nutritional immunity, the production of noxious gases, and the ubiquitylation of the Toxoplasma-containing parasitophorous vacuole (PV). PV ubiquitylation prompts the recruitment of host defense proteins to the PV and the consequential execution of antimicrobial effector programs, which reduce parasitic burden. However, the ubiquitin E3 ligase orchestrating these events has remained unknown. Here, we demonstrate that the IFN-γ-inducible E3 ligase RNF213 translocates to Toxoplasma PVs and facilitates PV ubiquitylation in human cells. Toxoplasma PVs become decorated with linear and K63-linked ubiquitin and recruit ubiquitin adaptor proteins in a process that is RNF213 dependent but independent of the linear ubiquitin chain assembly complex (LUBAC). IFN-γ priming fails to restrict Toxoplasma growth in cells lacking RNF213 expression, thus identifying RNF213 as a potent executioner of ubiquitylation-driven antiparasitic host defense. IMPORTANCE Globally, approximately one out of three people become infected with the obligate intracellular parasite Toxoplasma. These infections are typically asymptomatic but can cause severe disease and mortality in immunocompromised individuals. Infections can also be passed on from mother to fetus during pregnancy, potentially causing miscarriage or stillbirth. Therefore, toxoplasmosis constitutes a substantial public health burden. A better understanding of mechanisms by which healthy individuals control Toxoplasma infections could provide roadmaps toward novel therapies for vulnerable groups. Our work reveals a fundamental mechanism controlling intracellular Toxoplasma infections. Cytokines produced during Toxoplasma infections instruct human cells to produce the enzyme RNF213. We find that RNF213 labels intracellular vacuoles containing Toxoplasma with the small protein ubiquitin, which functions as an "eat-me" signal, attracting antimicrobial defense programs to fight off infection. Our work therefore identified a novel antiparasitic protein orchestrating a central aspect of the human immune response to Toxoplasma.