Intracellular triggering of inflammation by the extracellular bacterium Pseudomonas aeruginosa

Abstract P.aeruginosa is an extracellular, Gram-negative opportunistic pathogen. One of the most important virulence factors during infection is the type III secretion system (T3SS). This system is found exclusively in Gram-negative bacteria and it forms a conduit between the bacteria and the host cell through which effector molecules can be translocated. These effectors alter the function of the host cell to promote survival of the bacterium. Infections are detected initially by the innate immune system via germ-line encoded receptors, pathogen recognition receptors (PRRs). These receptors recognise conserved microbial patterns, known as pathogen-associated molecular patterns and molecules which signal danger, danger-associated molecular patterns. PRRs are both membrane bound, such as Toll-like receptors (TLRs), and cytosolic, such as Nod-like receptors (NLRs). Some NLRs are involved in the formation of multimeric protein complexes, the Nod-signalosome and inflammasomes. These lead to the activation of NF-κB and the activation of caspase-1 and subsequent proteolytic processing of interleukin-1β (IL-1β) into its mature form. Both processes contribute to the inflammatory response following infection. In this study we sought to elucidate whether P.aeruginosa is able to trigger cytosolic PRRs and the mechanism of this activation. Initially we studied inflammasome activation by P.aeruginosa. We demonstrated that P.aeruginosa is able to activate the NLRC4/ASC-inflammasome complex. This was found to be dependent on a functional T3SS, but independent of any effectors passing through the system. The activation was discovered by detection of processed, and thus active caspase-1 fragments, as well as by secretion of mature IL-1β. The mechanism of the inflammasome activation was then investigated. We found that the NLRC4-dependent inflammasome activation is also dependent on extracellular potassium. An increase of extracellular potassium leads to a complete abrogation of inflammasome activation by P.aeruginosa and Salmonella. To further elucidate this finding, we investigated the leakiness of the pore formed by the T3SS in the host cell membrane. No flux of ions or small molecules could be detected in the host cell membrane following infection. However, host-membrane repair mechanisms were triggered, which could be detected by lysosomal-associated membrane protein (LAMP)-1-specific staining of the plasma membrane. We hypothesize a role for membrane potential in triggering of inflammasome activation by bacteria possessing a secretion system. Potassium-efflux has previously been identified as a activator of the NLRP3 inflammasome, but no changes in intracellular potassium could be found during this study. The activation of the NLRC4 inflammasome by the Pseudomonal strain PA103 was shown, in this study, to be independent of flagellin. Instead, the bacterial molecule responsible for inflammasome activation was shown to be pilin. Pilin is important for attachment to the host cell and the function of the T3SS. We showed that a strain lacking pilin were still able to translocate effectors through its T3SS. However, it was unable to activate the inflammasome complex. Transfection of purified pilin into cells was shown to trigger inflammasome activation. This was found to be dependent on caspase-1 but independent of NLRC4 and ASC, which is not in agreement with the results found for live bacteria. We hypothesised that the reason for this is the delivery method used, since a T3SS and infection delivers proteins and molecules differently compared to a transfection reagent. Finally, the role for Nod1 in infection by P.aeruginosa was explored. We could not identify Nod1-dependent NF-κB-activation using luciferase reporter gene assays. We therefore hypothesise that Nod1 does not have a role in the innate immune response to P.aeruginosa. In conclusion, we have identified NLRC4- and ASC-dependent inflammasome activation by P.aeruginosa. This activation was shown to be dependent on a functional T3SS and the surface protein pilin, as well as extracellular potassium. This describes a novel NLRC4-activation mechanism dependent on potassium and identifies pilin as a PRR-trigger for the first time

Editorial: Exploring Immune Variability in Susceptibility to Tuberculosis Infection in Humans.

Editorial on the Research Topic - Exploring Immune Variability in Susceptibility to Tuberculosis Infection in Humans. No abstract available

Autoimmunity in Parkinson's Disease: The Role of α-Synuclein-Specific T Cells

Evidence from a variety of studies implicates a role for the adaptive immune system in Parkinson's disease (PD). Similar to multiple sclerosis (MS) patients who display a high number of T cells in the brain attacking oligodendrocytes, PD patients show higher numbers of T cells in the ventral midbrain than healthy, age-matched controls. Mouse models of the disease also show the presence of T cells in the brain. The role of these infiltrating T cells in the propagation of disease is controversial; however, recent studies indicate that they may be autoreactive in nature, recognizing disease-altered self-proteins as foreign antigens. T cells of PD patients can generate an autoimmune response to α-synuclein, a protein that is aggregated in PD. α-Synuclein and other proteins are post-translationally modified in an environment in which protein processing is altered, possibly leading to the generation of neo-epitopes, or self-peptides that have not been identified by the host immune system as non-foreign. Infiltrating T cells may also be responding to such modified proteins. Genome-wide association studies (GWAS) have shown associations of PD with haplotypes of major histocompatibility complex (MHC) class II genes, and a polymorphism in a non-coding region that may increase MHC class II in PD patients. We speculate that the inflammation observed in PD may play both pathogenic and protective roles. Future studies on the adaptive immune system in neurodegenerative disorders may elucidate steps in disease pathogenesis and assist with the development of both biomarkers and treatments

Host Transcriptomics as a Tool to Identify Diagnostic and Mechanistic Immune Signatures of Tuberculosis

Tuberculosis (TB) is a major infectious disease worldwide, and is associated with several challenges for control and eradication. First, more accurate diagnostic tools that better represent the spectrum of infection states are required; in particular, identify the latent TB infected individuals with high risk of developing active TB. Second, we need to better understand, from a mechanistic point of view, why the immune system is unsuccessful in some cases for control and elimination of the pathogen. Host transcriptomics is a powerful approach to identify both diagnostic and mechanistic immune signatures of diseases. We have recently reported that optimal study design for these two purposes should be guided by different sets of criteria. Here, based on already published transcriptomics signatures of tuberculosis, we further develop these guidelines and identify additional factors to consider for obtaining diagnostic vs. mechanistic signatures in terms of cohorts, samples, data generation and analysis. Diagnostic studies should aim to identify small disease signatures with high discriminatory power across all affected populations, and against similar pathologies to TB. Specific focus should be made on improving the diagnosis of infected individuals at risk of developing active disease. Conversely, mechanistic studies should focus on tissues biopsies, immune relevant cell subsets, state of the art transcriptomic techniques and bioinformatics tools to understand the biological meaning of identified gene signatures that could facilitate therapeutic interventions. Finally, investigators should ensure their data are made publicly available along with complete annotations to facilitate metadata and cross-study analyses

T Cell Responses to Neural Autoantigens Are Similar in Alzheimer’s Disease Patients and Age-Matched Healthy Controls

Alzheimer’s disease (AD), a chronic multifactorial and complex neurodegenerative disorder is a leading cause of dementia. Recently, neuroinflammation has been hypothesized as a contributing factor to AD pathogenesis. The role of adaptive immune responses against neuronal antigens, which can either confer protection or induce damage in AD, has not been fully characterized. Here, we measured T cell responses to several potential antigens of neural origin including amyloid precursor protein (APP), amyloid beta (Aβ), tau, α-synuclein, and transactive response DNA binding protein (TDP-43) in patients with AD and age-matched healthy controls (HC). Antigen-specific T cell reactivity was detected for all tested antigens, and response to tau-derived epitopes was particularly strong, but no significant differences between individuals with AD and age-matched HC were identified. We also did not observe any correlation between the antigen-specific T cell responses and clinical variables including age, gender, years since diagnosis and cognitive score. Additionally, further characterization did not reveal any differences in the relative frequency of major Peripheral Blood Mononuclear Cells (PBMC) subsets, or in the expression of genes between AD patients and HC. These observations have not identified a key role of neuronal antigen-specific T cell responses in AD

Immunological consequences of intragenus conservation of Mycobacterium tuberculosis T-cell epitopes

A previous unbiased genome-wide analysis of CD4 Mycobacterium tuberculosis (MTB) recognition using peripheral blood mononuclear cells from individuals with latent MTB infection (LTBI) or nonexposed healthy controls (HCs) revealed that certain MTB sequences were unexpectedly recognized by HCs. In the present study, it was found that, based on their pattern of reactivity, epitopes could be divided into LTBI-specific, mixed reactivity, and HC-specific categories. This pattern corresponded to sequence conservation in nontuberculous mycobacteria (NTMs), suggesting environmental exposure as an underlying cause of differential reactivity. LTBI-specific epitopes were found to be hyperconserved, as previously reported, whereas the opposite was true for NTM conserved epitopes, suggesting that intragenus conservation also influences host pathogen adaptation. The biological relevance of this observation was demonstrated further by several observations. First, the T cells elicited by MTB/NTM cross-reactive epitopes in HCs were found mainly in a CCR6+CXCR3+ memory subset, similar to findings in LTBI individuals. Thus, both MTB and NTM appear to elicit a phenotypically similar T-cell response. Second, T cells reactive to MTB/NTM-conserved epitopes responded to naturally processed epitopes from MTB and NTMs, whereas T cells reactive to MTB-specific epitopes responded only to MTB. Third, cross-reactivity could be translated to antigen recognition. Several MTB candidate vaccine antigens were cross-reactive, but others were MTB-specific. Finally, NTM-specific epitopes that elicit T cells that recognize NTMs but not MTB were identified. These epitopes can be used to characterize T-cell responses to NTMs, eliminating the confounding factor of MTB cross-recognition and providing insights into vaccine design and evaluation

Pseudomonas aeruginosa pilin activates the inflammasome

IL-1 beta is produced from inactive pro-IL-1 beta by activation of caspase-1 brought about by a multi-subunit protein platform called the inflammasome. Many bacteria can trigger inflammasome activity through flagellin activation of the host protein NLRC4. However, strains of the common human pathogen Pseudomonas aeruginosa lacking flagellin can still activate the inflammasome. We set out to identify what non-flagellin components could produce this activation. Using mass spectroscopy, we identified an inflammasome-activating factor from P. aeruginosa as pilin, the major component of the type IV bacterial pilus. Purified pilin introduced into mouse macrophages by liposomal delivery activated caspase-1 and led to secretion of mature IL-1 beta, as did recombinant pilin purified from Escherichia coli. This was dependent on caspase-1 but not on the host inflammasome proteins NLRC4, NLRP3 or ASC. Mutants of P. aeruginosa strain PA103 lacking pilin did not activate the inflammasome following infection of macrophages with live bacteria. Type III secretion remained intact in the absence of pili, showing this was not due to a lack of effector delivery. Our observations show pilin is a novel activator of the inflammasome in addition to flagellin and the recently described PrgJ protein family, the basal body rod component of the type III apparatu

α-Synuclein-specific T cell reactivity is associated with preclinical and early Parkinson’s disease

A diagnosis of motor Parkinson’s disease (PD) is preceded by a prolonged premotor phase with accumulating neuronal damage. Here we examined the temporal relation between α-synuclein (α-syn) T cell reactivity and PD. A longitudinal case study revealed that elevated α-syn-specific T cell responses were detected prior to the diagnosis of motor PD, and declined after. The relationship between T cell reactivity and early PD in two independent cohorts showed that α-syn-specific T cell responses were highest shortly after diagnosis of motor PD and then decreased. Additional analysis revealed significant association of α-syn-specific T cell responses with age and lower levodopa equivalent dose. These results confirm the presence of α-syn-reactive T cells in PD and show that they are most abundant immediately after diagnosis of motor PD. These cells may be present years before the diagnosis of motor PD, suggesting avenues of investigation into PD pathogenesis and potential early diagnosis

A quantitative analysis of complexity of human pathogen-specific CD4 T cell responses in healthy M. tuberculosis infected South Africans

Author Summary: Human pathogen-specific immune responses are tremendously complex and the techniques to study them ever expanding. There is an urgent need for a quantitative analysis and better understanding of pathogen-specific immune responses. Mycobacterium tuberculosis (Mtb) is one of the leading causes of mortality due to an infectious agent worldwide. Here, we were able to quantify the Mtb-specific response in healthy individuals with Mtb infection from South Africa. The response is highly diverse and 66 epitopes are required to capture 80% of the total reactivity. Our study also show that the majority of the identified epitopes are restricted by multiple HLA alleles. Thus, technical advances are required to capture and characterize the complete pathogen-specific response. This study demonstrates further that the approach combining identified epitopes into "megapools" allows capturing a large fraction of the total reactivity. This suggests that this technique is generally applicable to the characterization of immunity to other complex pathogens. Together, our data provide for the first time a quantitative analysis of the complex pathogen-specific T cell response and provide a new understanding of human infections in a natural infection setting