Chlamydial Lipoproteins Stimulate Toll-Like Receptors 1/2 Mediated Inflammatory Responses through MyD88-Dependent Pathway

Recognition of Chlamydia trachomatis by Toll-like receptor 9 is altered during persistence

Toll-like receptor 9 (TLR9) is an innate immune receptor that localizes to endosomes in antigen presenting cells and recognizes single stranded unmethylated CpG sites on bacterial genomic DNA (gDNA). Previous bioinformatic studies have demonstrated that the genome of the human pathogen Chlamydia trachomatis contains TLR9 stimulatory motifs, and correlative studies have implied a link between human TLR9 (hTLR9) genotype variants and susceptibility to infection. Here, we present our evaluation of the stimulatory potential of C. trachomatis gDNA and its recognition by hTLR9- and murine TLR9 (mTLR9)-expressing cells. Utilizing reporter cell lines, we demonstrate that purified gDNA from C. trachomatis can stimulate hTLR9 signaling, albeit at lower levels than gDNA prepared from other Gram-negative bacteria. Interestingly, we found that while C. trachomatis is capable of signaling through hTLR9 and mTLR9 during live infections in HEK293 reporter cell lines, signaling only occurs at later developmental time points. Chlamydia-specific induction of hTLR9 is blocked when protein synthesis is inhibited prior to the RB-to-EB conversion, exacerbated by the inhibition of lipooligosaccharide biosynthesis, and is significantly altered during the induction of aberrance/persistence. Our observations support the hypothesis that chlamydial gDNA is released during the conversion between the pathogen's replicative and infectious forms and during treatment with antibiotics targeting peptidoglycan assembly. Given that C. trachomatis inclusions do not co-localize with TLR9-containing vacuoles in the pro-monocytic cell line U937, our findings also hint that chlamydial gDNA is capable of egress from the inclusion, and traffics to TLR9-containing vacuoles via an as yet unknown pathway.

Recognition of Chlamydia trachomatis by Toll-Like Receptor 9 is altered during persistence

Toll-like receptor 9 (TLR9) is an innate immune receptor that localizes to endosomes in antigen presenting cells and recognizes single stranded unmethylated CpG sites on bacterial genomic DNA. Previous bioinformatic studies have indicated that the genome of the human pathogen Chlamydia trachomatis contains TLR9 stimulatory motifs, and correlative studies have implied a link between human TLR9 (hTLR9) genotype variants and susceptibility to infection. Here we present our evaluation of the stimulatory potential of C. trachomatis gDNA and its recognition by hTLR9- and murine TLR9 (mTLR9)-expressing cells. We confirm that hTLR9 colocalizes with chlamydial inclusions in the pro-monocytic cell line, U937. Utilizing HEK293 reporter cell lines, we demonstrate that purified genomic DNA from C. trachomatis can stimulate hTLR9 signaling, albeit at lower levels than gDNA prepared from other Gram-negative bacteria. Interestingly, we found that while C. trachomatis is capable of signaling through hTLR9 and mTLR9 during live infections in non-phagocytic HEK293 reporter cell lines, signaling only occurs at later developmental time points. Chlamydia-specific induction of hTLR9 is blocked when protein synthesis is inhibited prior to the RB-to-EB conversion and exacerbated by the inhibition of lipooligosaccharide biosynthesis. The induction of aberrance / persistence also significantly alters Chlamydia-specific TLR9 signaling. Our observations support the hypothesis that chlamydial gDNA is released at appreciable levels by the bacterium during the conversion between its replicative and infectious forms and during treatment with antibiotics targeting peptidoglycan assembly.

The macrophage infectivity potentiator of Trypanosoma cruzi induces innate IFN-γ and TNF-α production by human neonatal and adult blood cells through TLR2/1 and TLR4

We previously identified the recombinant (r) macrophage (M) infectivity (I) potentiator (P) of the protozoan parasite Trypanosoma cruzi (Tc) (rTcMIP) as an immuno-stimulatory protein that induces the release of IFN-γ, CCL2 and CCL3 by human cord blood cells. These cytokines and chemokines are important to direct a type 1 adaptive immune response. rTcMIP also increased the Ab response and favored the production of the Th1-related isotype IgG2a in mouse models of neonatal vaccination, indicating that rTcMIP could be used as a vaccine adjuvant to enhance T and B cell responses. In the present study, we used cord and adult blood cells, and isolated NK cells and human monocytes to investigate the pathways and to decipher the mechanism of action of the recombinant rTcMIP. We found that rTcMIP engaged TLR1/2 and TLR4 independently of CD14 and activated the MyD88, but not the TRIF, pathway to induce IFN-γ production by IL-15-primed NK cells, and TNF-α secretion by monocytes and myeloid dendritic cells. Our results also indicated that TNF-α boosted IFN-γ expression. Though cord blood cells displayed lower responses than adult cells, our results allow to consider rTcMIP as a potential pro-type 1 adjuvant that might be associated to vaccines administered in early life or later.

Role of TRAIL-R in Primary and Secondary Genital and Respiratory Chlamydia muridarum Infections in Mice

The tumor necrosis factor (TNF)-related apoptosis-inducing ligand receptor (TRAIL-R) suppresses inflammation and could therefore affect the course of Chlamydia infections and their long-term sequelae. Wild-type (WT) and TRAIL-R-/- C57BL/6 mice were inoculated vaginally with Chlamydia muridarum; the course of the infection was followed with vaginal cultures and the presence of hydrosalpinx determined. To evaluate the role of TRAIL-R following a secondary infection, the mice were vaginally reinfected. WT and TRAIL-R-/- male mice were also infected and reinfected in the respiratory tract, and the course of the diseases and the infections were followed. Following the primary and secondary vaginal infection, no significant differences in vaginal shedding or hydrosalpinx formation were observed between the WT and TRAIL-R-/- mice. The WT and TRAIL-R-/- mice mounted antibody responses in serum and vaginal washes that were not significantly different. After the primary and secondary intranasal infections of the male mice, changes in body weight were determined, and no significant differences were observed between the WT and TRAIL-R-/- mice. Ten days after the primary and the secondary infections, the weight of the lungs and number of C. muridarum inclusion forming units (IFU) were determined. The lungs of the WT mice weighed less compared with the TRAIL-R-/- mice following a primary infection but not after a secondary infection. No differences in the number of C. muridarum IFU in the lungs were observed between the two groups of mice. In conclusion, despite playing a role in inflammation cell-signaling pathways in vitro, TRAIL-R does not appear to play a major role in the susceptibility, clinical outcomes, or long-term sequelae of C. muridarum infections in vivo. IMPORTANCE TNF-related apoptosis-inducing ligand receptor (TRAIL-R) is involved in suppressing inflammatory responses. Bacterial pathogens such as Chlamydia spp. elicit inflammatory responses in humans following genital, ocular, and respiratory infections. The inflammatory responses are important to control the spread of Chlamydia. However, in certain instances, these inflammatory responses can produce long-term sequelae, including fibrosis. Fibrosis, or scarring, in the genital tract, eye, and respiratory system results in functional deficiencies, including infertility, blindness, and chronic obstructive lung disease, respectively. The goal of this study was to determine if mice deficient in TRAIL-R infected in the genital and respiratory tracts with Chlamydia spp. suffer more or less severe infections, infertility, or lung diseases than wild-type mice. Our results show no differences between the immune responses, infection severity, and long-term sequelae between TRAIL-R knockout and wild-type animals following a genital or a respiratory infection with Chlamydia.

IL-21/IL-21R Regulates the Neutrophil-Mediated Pathologic Immune Response during Chlamydial Respiratory Infection

IL-21/IL-21R was documented to participate in the regulation of multiple infection and inflammation. During Chlamydia muridarum (C. muridarum) respiratory infection, our previous study had revealed that the absence of this signal induced enhanced resistance to infection with higher protective Th1/Th17 immune responses. Here, we use the murine model of C. muridarum respiratory infection and IL-21R deficient mice to further identify a novel role of IL-21/IL-21R in neutrophilic inflammation. Resistant IL-21R^−/− mice showed impaired neutrophil recruitment to the site of infection. In the absence of IL-21/IL-21R, pulmonary neutrophils also exhibited reduced activation status, including lower CD64 expression, MPO activity, and neutrophil-produced protein production. These results correlated well with the decrease of neutrophil-related chemokines (KC and MIP-2), inflammatory cytokines (IL-6, IL-1β, and TNF-α), and TLR/MyD88 pathway mediators (TLR2, TLR4, and MyD88) in infected lungs of IL-21R^−/− mice than normal mice. Complementarily, decreased pulmonary neutrophil infiltration, activity, and levels of neutrophilic chemotactic factors and TLR/MyD88 signal in infected lungs can be corrected by rIL-21 administration. These results revealed that IL-21/IL-21R may aggravate the neutrophil inflammation through regulating TLR/MyD88 signal pathway during chlamydial respiratory infection.

Human genetic diversity regulating the TLR10/TLR1/TLR6 locus confers increased cytokines in response to Chlamydia trachomatis

Human genetic diversity can have profound effects on health outcomes upon exposure to infectious agents. For infections with Chlamydia trachomatis (C. trachomatis), the wide range of genital and ocular disease manifestations are likely influenced by human genetic differences that regulate interactions between C. trachomatis and host cells. We leveraged this diversity in cellular responses to demonstrate the importance of variation at the Toll-like receptor 1 (TLR1), TLR6, and TLR10 locus to cytokine production in response to C. trachomatis. We determined that a single-nucleotide polymorphism (SNP) (rs1057807), located in a region that forms a loop with the TLR6 promoter, is associated with increased expression of TLR1, TLR6, and TLR10 and secreted levels of ten C. trachomatis-induced cytokines. Production of these C. trachomatis-induced cytokines is primarily dependent on MyD88 and TLR6 based on experiments using inhibitors, blocking antibodies, RNAi, and protein overexpression. Population genetic analyses further demonstrated that the mean IL-6 response of cells from two European populations were higher than the mean response of cells from three African populations and that this difference was partially attributable to variation in rs1057807 allele frequency. In contrast, a SNP associated with a different pro-inflammatory cytokine (rs2869462 associated with the chemokine CXCL10) exhibited an opposite response, underscoring the complexity of how different genetic variants contribute to an individual's immune response. This multidisciplinary study has identified a long-range chromatin interaction and genetic variation that regulates TLR6 to broaden our understanding of how human genetic variation affects the C. trachomatis-induced immune response.

Periodontal Disease: The Good, The Bad, and The Unknown

Periodontal disease is classically characterized by progressive destruction of the soft and hard tissues of the periodontal complex, mediated by an interplay between dysbiotic microbial communities and aberrant immune responses within gingival and periodontal tissues. Putative periodontal pathogens are enriched as the resident oral microbiota becomes dysbiotic and inflammatory responses evoke tissue destruction, thus inducing an unremitting positive feedback loop of proteolysis, inflammation, and enrichment for periodontal pathogens. Keystone microbial pathogens and sustained gingival inflammation are critical to periodontal disease progression. However, recent studies have revealed the importance of previously unidentified microbes involved in disease progression, including various viruses, phages and bacterial species. Moreover, newly identified immunological and genetic mechanisms, as well as environmental host factors, including diet and lifestyle, have been discerned in recent years as further contributory factors in periodontitis. These factors have collectively expanded the established narrative of periodontal disease progression. In line with this, new ideologies related to maintaining periodontal health and treating existing disease have been explored, such as the application of oral probiotics, to limit and attenuate disease progression. The role of systemic host pathologies, such as autoimmune disorders and diabetes, in periodontal disease pathogenesis has been well noted. Recent studies have additionally identified the reciprocated importance of periodontal disease in potentiating systemic disease states at distal sites, such as in Alzheimer's disease, inflammatory bowel diseases, and oral cancer, further highlighting the importance of the oral cavity in systemic health. Here we review long-standing knowledge of periodontal disease progression while integrating novel research concepts that have broadened our understanding of periodontal health and disease. Further, we delve into innovative hypotheses that may evolve to address significant gaps in the foundational knowledge of periodontal disease.

Treponema denticola dentilisin triggered TLR2/MyD88 activation upregulates a tissue destructive program involving MMPs via Sp1 in human oral cells

Periodontal disease is driven by dysbiosis in the oral microbiome, resulting in over-representation of species that induce the release of pro-inflammatory cytokines, chemokines, and tissue-remodeling matrix metalloproteinases (MMPs) in the periodontium. These chronic tissue-destructive inflammatory responses result in gradual loss of tooth-supporting alveolar bone. The oral spirochete Treponema denticola, is consistently found at significantly elevated levels in periodontal lesions. Host-expressed Toll-Like Receptor 2 (TLR2) senses a variety of bacterial ligands, including acylated lipopolysaccharides and lipoproteins. T. denticola dentilisin, a surface-expressed protease complex comprised of three lipoproteins has been implicated as a virulence factor in periodontal disease, primarily due to its proteolytic activity. While the role of acylated bacterial components in induction of inflammation is well-studied, little attention has been given to the potential role of the acylated nature of dentilisin. The purpose of this study was to test the hypothesis that T. denticola dentilisin activates a TLR2-dependent mechanism, leading to upregulation of tissue-destructive genes in periodontal tissue. RNA-sequencing of periodontal ligament cells challenged with T. denticola bacteria revealed significant upregulation of genes associated with extracellular matrix organization and degradation including potentially tissue-specific inducible MMPs that may play novel roles in modulating host immune responses that have yet to be characterized within the context of oral disease. The Gram-negative oral commensal, Veillonella parvula, failed to upregulate these same MMPs. Dentilisin-induced upregulation of MMPs was mediated via TLR2 and MyD88 activation, since knockdown of expression of either abrogated these effects. Challenge with purified dentilisin upregulated the same MMPs while a dentilisin-deficient T. denticola mutant had no effect. Finally, T. denticola-mediated activation of TLR2/MyD88 lead to the nuclear translocation of the transcription factor Sp1, which was shown to be a critical regulator of all T. denticola-dependent MMP expression. Taken together, these data suggest that T. denticola dentilisin stimulates tissue-destructive cellular processes in a TLR2/MyD88/Sp1-dependent fashion.

Co- but not Sequential Infection of DCs Boosts Their HIV-Specific CTL-Stimulatory Capacity

Pathogenic bacteria and their microbial products activate dendritic cells (DCs) at mucosal surfaces during sexually transmitted infections (STIs) and therefore might also differently shape DC functions during co-infection with HIV-1. We recently illustrated that complement (C) coating of HIV-1 (HIV-C), as primarily found during the acute phase of infection before appearance of HIV-specific antibodies, by-passed SAMHD1-mediated restriction in DCs and therefore mediated an increased DC activation and antiviral capacity. To determine whether the superior antiviral effects of HIV-C-exposed DCs also apply during STIs, we developed a co-infection model in which DCs were infected with Chlamydia spp. simultaneously (HIV-C/Chlam-DCs or HIV/Chlam-DCs) or a sequential infection model, where DCs were exposed to Chlamydia for 3 or 24 h (Chlam-DCs) followed by HIV-1 infection. Co-infection of DCs with HIV-1 and Chlamydia significantly boosted the CTL-stimulatory capacity compared to HIV-1-loaded iDCs and this boost was independent on the opsonization pattern. This effect was lost in the sequential infection model, when opsonized HIV-1 was added delayed to Chlamydia-loaded DCs. The reduction in the CTL-stimulatory capacity of Chlam-DCs was not due to lower HIV-1 binding or infection compared to iDCs or HIV-C/Chlam-DCs, but due to altered fusion and internalization mechanisms within DCs. The CTL-stimulatory capacity of HIV-C in Chlam-DCs correlated with significantly reduced viral fusion compared to iDCs and HIV-C/Chlam-DCs and illustrated considerably increased numbers of HIV-C-containing vacuoles than iDCs. The data indicate that Chlamydia co-infection of DCs mediates a transient boost of their HIV-specific CTL-stimulatory and antiviral capacity, while in the sequential infection model this is reversed and associated with hazard to the host.

Spontaneous remission of giant cell arteritis: possible association with a preceding acute respiratory infection and seropositivity to Chlamydia pneumoniae antibodies

Recent epidemiological or immunopathological studies demonstrate the possible association between giant cell arteritis and infectious agents including Chlamydia pneumoniae. A 62-year-old Japanese man with type 1 diabetes mellitus developed biopsy-proven giant cell arteritis after acute upper respiratory infection. Serological examination indicated concurrent re-infection with C. pneumoniae. Clinical manifestations of the vasculitis subsided within a month without any immunosuppressive therapy, and no relapse was observed for the following 12 months. The natural history of this disease is unclear and spontaneous remission is rarely reported. The self-limiting nature of the infection could contribute to this phenomenon.

T cell phenotypes in women with Chlamydia trachomatis infection and influence of treatment on phenotype distributions

T cell phenotypes involved in the immune response to Chlamydia trachomatis (CT) have not been fully elucidated in humans. We evaluated differences in T cell phenotypes between CT-infected women and CT-seronegative controls and investigated changes in T cell phenotype distributions after CT treatment and their association with reinfection. We found a higher expression of T cell activation markers (CD38+HLA-DR+), T helper type 1 (Th1)- and Th2-associated effector phenotypes (CXCR3+CCR5+ and CCR4+, respectively), and T cell homing marker (CCR7) for both CD4+ and CD8+ T cells in CT-infected women. At follow-up after treatment of infected women, there were a lower proportion of CD4+ and CD8+ T cells expressing these markers. These findings suggest a dynamic interplay of CD4+ and CD8+ T cells in CT infection, and once the infection is treated, these cell markers return to basal expression levels. In women without reinfection, a significantly higher proportion of CD8+ T cells co-expressing CXCR3 with CCR5 or CCR4 at follow-up was detected compared to women with reinfection, suggesting they might play some role in adaptive immunity. Our study elucidated changes in T cell phenotypes during CT infection and after treatment, broadening our understanding of adaptive immune mechanisms in human CT infections.

Caveolin-mediated endocytosis of the Chlamydia M278 outer membrane peptide encapsulated in poly(lactic acid)-Poly(ethylene glycol) nanoparticles by mouse primary dendritic cells enhances specific immune effectors mediated by MHC class II and CD4+ T cells

We previously developed a Chlamydia trachomatis nanovaccine (PPM) by encapsulating a chlamydial M278 peptide within poly(lactic acid)-poly(ethylene glycol) biodegradable nanoparticles that immunopotentiated Chlamydia-specific immune effector responses in mice. Herein, we investigated the mechanistic interactions of PPM with mouse bone marrow-derived dendritic cells (DCs) for its uptake, trafficking, and T cell activation. Our results reveal that PPM triggered enhanced expression of effector cytokines and chemokines, surface activation markers (Cd1d2, Fcgr1), pathogen-sensing receptors (TLR2, Nod1), co-stimulatory (CD40, CD80, CD86) and MHC class I and II molecules. Co-culturing of PPM-primed DCs with T cells from C. muridarum vaccinated mice yielded an increase in Chlamydia-specific immune effector responses including CD3+ lymphoproliferation, CD3+CD4+ IFN-γ-secreting cells along with CD3+CD4+ memory (CD44high and CD62Lhigh) and effector (CD44high and CD62Llow) phenotypes. Intracellular trafficking analyses revealed an intense expression and colocalization of PPM predominantly in endosomes. PPM also upregulated the transcriptional and protein expression of the endocytic mediator, caveolin-1 in DCs. More importantly, the specific inhibition of caveolin-1 led to decreased expression of PPM-induced cytokines and co-stimulatory molecules. Our investigation shows that PPM provided enhancement of uptake, probably by exploiting the caveolin-mediated endocytosis pathway, endosomal processing, and MHC II presentation to immunopotentiate Chlamydia-specific immune effector responses mediated by CD4+ T cells.

Conceptual Perspectives: Bacterial Antimicrobial Peptide Induction as a Novel Strategy for Symbiosis with the Human Host

Human beta defensins (hBDs) are small cationic peptides, expressed in mucosal epithelia and important agents of innate immunity, act as antimicrobial and chemotactic agents at mucosal barriers. In this perspective, we present evidence supporting a novel strategy by which the oral bacterium Fusobacterium nucleatum induces hBDs and other antimicrobial peptides (AMPs) in normal human oral epithelial cells (HOECs) and thereby protects them from other microbial pathogens. The findings stress (1) the physiological importance of hBDs, (2) that this strategy may be a mechanism that contributes to homeostasis and health in body sites constantly challenged with bacteria and (3) that novel properties identified in commensal bacteria could, one day, be harnessed as new probiotic strategies to combat colonization of opportunistic pathogens. With that in mind, we highlight and review the discovery and characterization of a novel lipo-protein, FAD-I (FusobacteriumAssociated Defensin Inducer) associated with the outer membrane of F. nucleatum that may act as a homeostatic agent by activating endogenous AMPs to re-equilibrate a dysregulated microenvironment. FAD-I has the potential to reduce dysbiosis-driven diseases at a time when resistance to antibiotics is increasing. We therefore postulate that FAD-I may offer a new paradigm in immunoregulatory therapeutics to bolster host innate defense of vulnerable mucosae, while maintaining physiologically responsive states of inflammation.

Reticuloendotheliosis Virus Inhibits the Immune Response Acting on Lymphocytes from Peripheral Blood of Chicken

Chicken reticuloendotheliosis virus (REV) causes the atrophy of immune organs and immuno-suppression. The pathogenic mechanisms of REV are poorly understood. The aim of this study was to use RNA sequencing to analyse the effect of REV on immunity and cell proliferation in chicken lymphocytes from peripheral blood in vitro. Overall, 2977 differentially expressed genes (DEGs) were examined from cells between infected with REV or no; 56 DEGs related to cell proliferation and 130 DEGs related to immunity were identified. MTT, Q-PCR, and FCM indicated that REV reduced the number of lymphocytes by inhibiting the transition of S/G1 phase through FOXO and p53 pathways. Similarly, REV infection would destroy the immune defense of lymphocytes through MAPK-AP1 via Toll-like receptor-, NOD-like receptor-, and salmonella infection pathways to reduce the secretion of IL8 and IL18. In addition, the reduction of lymphocytes also might be responsible for the lower levels of IL8 and IL18, and the rescue of lymphocytes would been activated still through FOXO and p53 pathways. Moreover, the immune response for REV in lymphocytes would activate by up-regulating the expression of NOD1, MYD88, and AP1 through Toll-like receptor-/NOD-like receptor/salmonella-MAPK-AP1 pathways. These results indicate that REV could affect lymphocytes from peripheral blood by inhibit the cell proliferation and the immune system. It also was revealed that NOD1, MYD88, and AP1 were the key genes to activate the immune response through Toll-like receptor-/NOD-like receptor/salmonella-MAPK-AP1 pathways. These findings establish the groundwork and provide new clues for deciphering the molecular mechanism underlying REV infection in chickens.

Bacterial lipids: powerful modifiers of the innate immune response

The innate immune system serves as a first line of defense against microbial pathogens. The host innate immune response can be triggered by recognition of conserved non-self-microbial signature molecules by specific host receptor proteins called Toll-like receptors. For bacteria, many of these molecular triggers reside on or are embedded in the bacterial membrane, the interface exposed to the host environment. Lipids are the most abundant component of membranes, and bacteria possess a unique set of lipids that can initiate or modify the host innate immune response. Bacterial lipoproteins, peptidoglycan, and outer membrane molecules lipoteichoic acid and lipopolysaccharide are key modulators of the host immune system. This review article will highlight some of the research emerging at the crossroads of bacterial membranes and innate immunity.