Salmonella escapes antigen presentation through K63 ubiquitination mediated endosomal proteolysis of MHC II via modulation of endosomal acidification in dendritic cells

SIRT1 and SIRT3 Impact Host Mitochondrial Function and Host Salmonella pH Balance during Infection

Mitochondria are important organelles that regulate energy homeostasis. Mitochondrial health and dynamics are crucial determinants of the outcome of several bacterial infections. SIRT3, a major mitochondrial sirtuin, along with SIRT1 regulates key mitochondrial functions. This led to considerable interest in understanding the role of SIRT1 and SIRT3 in governing mitochondrial functions during Salmonella infection. Here, we show that loss of SIRT1 and SIRT3 function either by shRNA-mediated knockdown or by inhibitor treatment led to increased mitochondrial dysfunction with alteration in mitochondrial bioenergetics alongside increased mitochondrial superoxide generation in Salmonella-infected macrophages. Consistent with dysfunctional mitochondria, mitophagy was induced along with altered mitochondrial fusion-fission dynamics in S. typhimurium-infected macrophages. Additionally, the mitochondrial bioenergetic alteration promotes acidification of the infected macrophage cytosolic pH. This host cytosolic pH imbalance skewed the intraphagosomal and intrabacterial pH in the absence of SIRT1 and SIRT3, resulting in decreased SPI-2 gene expression. Our results suggest a novel role for SIRT1 and SIRT3 in maintaining the intracellular Salmonella niche by modulating the mitochondrial bioenergetics and dynamics in the infected macrophages.

The E3 ubiquitin ligase MARCH1 mediates downregulation of plasma membrane GABAB receptors under ischemic conditions by inhibiting fast receptor recycling

GABAB receptors mediate prolonged inhibition in the brain and are important for keeping neuronal excitation and inhibition in a healthy balance. However, under excitotoxic/ischemic conditions, GABAB receptors are downregulated by dysregulated endocytic trafficking and can no longer counteract the severely enhanced excitation, eventually triggering neuronal death. Recently, we developed interfering peptides targeting protein-protein interactions involved in downregulating the receptors. Treatment with these peptides restored GABAB receptor expression after an ischemic insult and thereby inhibited neuronal overexcitation and progressive neuronal death. In this study, we searched for GABAB receptor interactions that specifically occur under ischemic conditions. We found that the E3 ubiquitin ligase MARCH1 is specifically upregulated under ischemic/excitotoxic conditions. Upregulated MARCH1 interacts with GABAB receptors and triggered downregulation of plasma membrane GABAB receptors by inhibiting fast recycling of the receptors. We developed an interfering peptide that inhibits the MARCH1/GABAB receptor interaction. Treatment of cultured neurons subjected to ischemic stress with this peptide restored receptor expression and as a consequence stopped progressive neuronal death. Thus, inhibiting the interaction of GABAB receptors with MARCH1 to restore cell surface receptor expression might be a promising strategy to prevent progressive neuronal death induced by ischemic conditions.

Rationally designed catalytic nanoplatform for enhanced chemoimmunotherapy via deploying endogenous plus exogenous copper and remodeling tumor microenvironment

Chemodynamic therapy represents a novel tumor therapeutic modality via triggering catalytic reactions in tumors to yield highly toxic reactive oxygen species (ROS). Nevertheless, low efficiency catalytic ability, potential systemic toxicity and inefficient tumor targeting, have hindered the efficacy of chemodynamic therapy. Herein, a rationally designed catalytic nanoplatform, composed of folate acid conjugated liposomes loaded with copper peroxide (CP) and chloroquine (CQ; a clinical drug) (denoted as CC@LPF), could power maximal tumor cytotoxicity, mechanistically via maneuvering endogenous and exogenous copper for a highly efficient catalytic reaction. Despite a massive autophagosome accumulation elicited by CP-powered autophagic initiation and CQ-induced autolysosomal blockage, the robust ROS, but not aberrant autophagy, underlies the synergistic tumor inhibition. Otherwise, this combined mode also elicits an early onset, above all, long-term high-level existence of immunogenic cell death markers, associated with ROS and aberrant autophagy -triggered endoplasmic reticulum stress. Besides, CC@LPF, with tumor targeting capability and selective tumor cytotoxicity, could elicit intratumor dendritic cells (mainly attributed to CQ) and tumor infiltrating CD8+ T cells, upon combining with PD-L1 therapeutic antibody, further induce significant anti-tumor effect. Collectively, the rationally designed nanoplatform, CC@LPF, could enhance tumor chemoimmunotherapy via deploying endogenous plus exogenous copper and remodeling tumor microenvironment.

From Eberthella typhi to Salmonella Typhi: The Fascinating Journey of the Virulence and Pathogenicity of Salmonella Typhi

Salmonella Typhi (S. Typhi), the invasive typhoidal serovar of Salmonella enterica that causes typhoid fever in humans, is a severe threat to global health. It is one of the major causes of high morbidity and mortality in developing countries. According to recent WHO estimates, approximately 11-21 million typhoid fever illnesses occur annually worldwide, accounting for 0.12-0.16 million deaths. Salmonella infection can spread to healthy individuals by the consumption of contaminated food and water. Typhoid fever in humans sometimes is accompanied by several other critical extraintestinal complications related to the central nervous system, cardiovascular system, pulmonary system, and hepatobiliary system. Salmonella Pathogenicity Island-1 and Salmonella Pathogenicity Island-2 are the two genomic segments containing genes encoding virulent factors that regulate its invasion and systemic pathogenesis. This Review aims to shed light on a comparative analysis of the virulence and pathogenesis of the typhoidal and nontyphoidal serovars of S. enterica.

The Immunological Synapse: An Emerging Target for Immune Evasion by Bacterial Pathogens

Similar to other pathogens, bacteria have developed during their evolution a variety of mechanisms to overcome both innate and acquired immunity, accounting for their ability to cause disease or chronic infections. The mechanisms exploited for this critical function act by targeting conserved structures or pathways that regulate the host immune response. A strategic potential target is the immunological synapse (IS), a highly specialized structure that forms at the interface between antigen presenting cells (APC) and T lymphocytes and is required for the establishment of an effective T cell response to the infectious agent and for the development of long-lasting T cell memory. While a variety of bacterial pathogens are known to impair or subvert cellular processes essential for antigen processing and presentation, on which IS assembly depends, it is only recently that the possibility that IS may be a direct target of bacterial virulence factors has been considered. Emerging evidence strongly supports this notion, highlighting IS targeting as a powerful, novel means of immune evasion by bacterial pathogens. In this review we will present a brief overview of the mechanisms used by bacteria to affect IS assembly by targeting APCs. We will then summarize what has emerged from the current handful of studies that have addressed the direct impact of bacterial virulence factors on IS assembly in T cells and, based on the strategic cellular processes targeted by these factors in other cell types, highlight potential IS-related vulnerabilities that could be exploited by these pathogens to evade T cell mediated immunity.

GH18 family glycoside hydrolase Chitinase A of Salmonella enhances virulence by facilitating invasion and modulating host immune responses

Salmonella is a facultative intracellular pathogen that has co-evolved with its host and has also developed various strategies to evade the host immune responses. Salmonella recruits an array of virulence factors to escape from host defense mechanisms. Previously chitinase A (chiA) was found to be upregulated in intracellular Salmonella. Although studies show that several structurally similar chitinases and chitin-binding proteins (CBP) of many human pathogens have a profound role in various aspects of pathogenesis, like adhesion, virulence, and immune evasion, the role of chitinase in the intravacuolar pathogen Salmonella has not yet been elucidated. Therefore, we made chromosomal deletions of the chitinase encoding gene (chiA) to study the role of chitinase of Salmonella enterica in the pathogenesis of the serovars, Typhimurium, and Typhi using in vitro cell culture model and two different in vivo hosts. Our data indicate that ChiA removes the terminal sialic acid moiety from the host cell surface, and facilitates the invasion of the pathogen into the epithelial cells. Interestingly we found that the mutant bacteria also quit the Salmonella-containing vacuole and hyper-proliferate in the cytoplasm of the epithelial cells. Further, we found that ChiA aids in reactive nitrogen species (RNS) and reactive oxygen species (ROS) production in the phagocytes, leading to MHCII downregulation followed by suppression of antigen presentation and antibacterial responses. Notably, in the murine host, the mutant shows compromised virulence, leading to immune activation and pathogen clearance. In continuation of the study in C. elegans, Salmonella Typhi ChiA was found to facilitate bacterial attachment to the intestinal epithelium, intestinal colonization, and persistence by downregulating antimicrobial peptides. This study provides new insights on chitinase as an important and novel virulence determinant that helps in immune evasion and increased pathogenesis of Salmonella.

Evolutionary relationship and population structure of domestic Bovidae animals based on MHC-linked and neutral autosomal microsatellite markers

Major histocompatibility complex (MHC) genes are critical for disease resistance or susceptibility responsible for host-pathogen interactions determined mainly by extensive polymorphisms in the MHC genes. Here, we examined the diversity and phylogenetic pattern of MHC haplotypes reconstructed using three MHC-linked microsatellite markers in 55 populations of five Bovidae species and compared them with those based on neutral autosomal microsatellite markers (NAMs). Three-hundred-and-forty MHC haplotypes were identified in 1453 Bovidae individuals, suggesting significantly higher polymorphism and heterozygosity compared with those based on NAMs. The ambitious boundaries in population differentiation (phylogenetic network, pairwise F_ST and STRUCTURE analyses) within and between species assessed using the MHC haplotypes were different from those revealed by NAMs associated closely with speciation, geographical distribution, domestication and management histories. In addition, the mean F_ST was significantly correlated negatively with the number of observed alleles (N_A), observed (H_O) and expected (H_E) heterozygosity and polymorphism information content (PIC) (P < 0.05) in the MHC haplotype dataset while there was no correction of the mean F_ST estimates (P> 0.05) between the MHC haplotype and NAMs datasets. Analysis of molecular variance (AMOVA) revealed a lower percentage of total variance (PTV) between species/groups based on the MHC-linked microsatellites than NAMs. Therefore, it was inferred that individuals within populations accumulated as many MHC variants as possible to increase their heterozygosity and thus the survival rate of their affiliated populations and species, which eventually reduced population differentiation and thereby complicated their classification and phylogenetic relationship inference. In summary, host-pathogen coevolution and heterozygote advantage, rather than demographic history, act as key driving forces shaping the MHC diversity within the populations and determining the interspecific MHC diversity.

Hoodwinking the Big-Eater to Prosper: The Salmonella-Macrophage Paradigm

Salmonella is a major cause of morbidity and mortality in the developing and underdeveloped nations. Being a foodborne disease, Salmonella infection is primarily contracted through the ingestion of contaminated food or water, or due to close contact with infected/carrier individuals. It is an intracellular pathogen, which can survive and replicate in various cells including macrophages, dendritic cells, epithelial cells, and other white blood cells. Once Salmonella crosses the intestinal barrier, it disseminates to various systemic sites by circulation via immune cells. One of the major cell types which are involved in Salmonella infection are host macrophages. They are the niche for intracellular survival and proliferation of Salmonella and a mode of dissemination to distal systemic sites. These cells are very crucial as they mediate the mounting of an appropriate innate and adaptive anti-Salmonella immune response. In this review, we have tried to concise the current knowledge of complex interactions that occur between Salmonella and macrophages.