A new role of the complement system: C3 provides protection in a mouse model of lung infection with intracellular Chlamydia psittaci

Prognostic analysis of concurrent Pneumocystis jirovecii pneumonia in patients with systemic lupus erythematosus: a retrospective study

BACKGROUND

Systemic lupus erythematosus (SLE) has been less deadly since the advent of corticosteroid-sparing medications. SLE patients still have a higher mortality rate than the general population. Infectious disease is reported as one of the major causes of death in patients with SLE. Although bacteria are the most often isolated pathogens from patients with SLE, Pneumocystis jirovecii pneumonia (PJP) is more deadly than bacterial infection.

METHODS

We retrospectively enrolled consecutive patients with SLE concurrent with PJP (SLE-PJP) in our center between January 2014 and December 2022. The participants were classified into two groups: survivors and non-survivors. Cox regression models and Kaplan‒Meier survival analyses were conducted to explore prognostic factors for survival.

RESULTS

There were 57 patients with SLE (42.0 ± 15.8 years old, 78.9% female) complicated with PJP, 22 (38.6%) of whom died. Compared with the survival group, the non-survival group had more patients with hyperglycemia or diabetes mellitus, invasive ventilation (p < 0.01), respiratory failure, intensive care unit admission, non-invasive ventilation, and hospital-acquired pneumonia (p < 0.05). The non-survival group showed a higher neutrophil percentage, lactate dehydrogenase, D-dimer (p < 0.001), urea, high-sensitivity C-reactive protein (hsCRP), erythrocyte sedimentation rate (ESR), and ferritin (p < 0.05). It also had lower minimal albumin, hemoglobin (p < 0.001), immunoglobulin G, complement 3, peripheral lymphocyte count, platelet, NK cell count, and CD4+ T-cell count (p < 0.05). Multivariate analysis indicated that hyperglycemia or diabetes mellitus (HR = 4.25, p < 0.01, 95% CI: 1.51-11.97), thrombocytopenia (HR = 4.22, p < 0.01, 95% CI: 1.63-10.91) and lower complement 3 (C3) (HR = 4.06, p < 0.01, 95% CI: 1.60-10.33) were independent risk factors for the survival of SLE-PJP patients.

CONCLUSIONS

The mortality rate of patients with SLE-PJP is still high. Hyperglycemia, decreased C3, and thrombocytopenia are independent survival risk factors.

Local complement activation and modulation in mucosal immunity

The complement system is an evolutionarily conserved arm of innate immunity, which forms one of the first lines of host response to pathogens and assists in the clearance of debris. A deficiency in key activators/amplifiers of the cascade results in recurrent infection, whereas a deficiency in regulating the cascade predisposes to accelerated organ failure, as observed in colitis and transplant rejection. Given that there are over 60 proteins in this system, it has become an attractive target for immunotherapeutics, many of which are United States Food and Drug Administration-approved or in multiple phase 2/3 clinical trials. Moreover, there have been key advances in the last few years in the understanding of how the complement system operates locally in tissues, independent of its activities in circulation. In this review, we will put into perspective the abovementioned discoveries to optimally modulate the spatiotemporal nature of complement activation and regulation at mucosal surfaces.

Silent neonatal influenza A virus infection primes systemic antimicrobial immunity

Infections with influenza A viruses (IAV) cause seasonal epidemics and global pandemics. The majority of these infections remain asymptomatic, especially among children below five years of age. Importantly, this is a time, when immunological imprinting takes place. Whether early-life infections with IAV affect the development of antimicrobial immunity is unknown. Using a preclinical mouse model, we demonstrate here that silent neonatal influenza infections have a remote beneficial impact on the later control of systemic juvenile-onset and adult-onset infections with an unrelated pathogen, Staphylococcus aureus, due to improved pathogen clearance and clinical resolution. Strategic vaccination with a live attenuated IAV vaccine elicited a similar protection phenotype. Mechanistically, the IAV priming effect primarily targets antimicrobial functions of the developing innate immune system including increased antimicrobial plasma activity and enhanced phagocyte functions and antigen-presenting properties at mucosal sites. Our results suggest a long-term benefit from an exposure to IAV during the neonatal phase, which might be exploited by strategic vaccination against influenza early in life to enforce the host's resistance to later bacterial infections.

The role of properdin and Factor H in disease

The complement system consists of three pathways (alternative, classical, and lectin) that play a fundamental role in immunity and homeostasis. The multifunctional role of the complement system includes direct lysis of pathogens, tagging pathogens for phagocytosis, promotion of inflammatory responses to control infection, regulation of adaptive cellular immune responses, and removal of apoptotic/dead cells and immune complexes from circulation. A tight regulation of the complement system is essential to avoid unwanted complement-mediated damage to the host. This regulation is ensured by a set of proteins called complement regulatory proteins. Deficiencies or malfunction of these regulatory proteins may lead to pro-thrombotic hematological diseases, renal and ocular diseases, and autoimmune diseases, among others. This review focuses on the importance of two complement regulatory proteins of the alternative pathway, Factor H and properdin, and their role in human diseases with an emphasis on: (a) characterizing the main mechanism of action of Factor H and properdin in regulating the complement system and protecting the host from complement-mediated attack, (b) describing the dysregulation of the alternative pathway as a result of deficiencies, or mutations, in Factor H and properdin, (c) outlining the clinical findings, management and treatment of diseases associated with mutations and deficiencies in Factor H, and (d) defining the unwanted and inadequate functioning of properdin in disease, through a discussion of various experimental research findings utilizing in vitro, mouse and human models.

Prophylactic Multi-Subunit Vaccine against Chlamydia trachomatis: In Vivo Evaluation in Mice

Chlamydia trachomatis is the most frequent sexually-transmitted disease-causing bacterium. Urogenital serovars of this intracellular pathogen lead to urethritis and cervicitis. Ascending infections result in pelvic inflammatory disease, salpingitis, and oophoritis. One of 200 urogenital infections leads to tubal infertility. Serovars A–C cause trachoma with visual impairment. There is an urgent need for a vaccine. We characterized a new five-component subunit vaccine in a mouse vaccination-lung challenge infection model. Four recombinant Pmp family-members and Ctad1 from C. trachomatis serovar E, all of which participate in adhesion and binding of chlamydial elementary bodies to host cells, were combined with the mucosal adjuvant cyclic-di-adenosine monophosphate. Intranasal application led to a high degree of cross-serovar protection against urogenital and ocular strains of C. trachomatis, which lasted at least five months. Critical evaluated parameters were body weight, clinical score, chlamydial load, a granulocyte marker and the cytokines IFN-γ/TNF-α in lung homogenate. Vaccine antigen-specific antibodies and a mixed Th1/Th2/Th17 T cell response with multi-functional CD4⁺ and CD8⁺ T cells correlate with protection. However, serum-transfer did not protect the recipients suggesting that circulating antibodies play only a minor role. In the long run, our new vaccine might help to prevent the feared consequences of human C. trachomatis infections.

Complement and Chlamydia psittaci: Early Complement-Dependent Events Are Important for DC Migration and Protection During Mouse Lung Infection

The zoonotic intracellular bacterium Chlamydia psittaci causes life-threatening pneumonia in humans. During mouse lung infection, complement factor C3 and the anaphylatoxin C3a augment protection against C. psittaci by a so far unknown mechanism. To clarify how complement contributes to the early, innate and the late, specific immune response and resulting protection, this study addresses the amount of C3, the timing when its presence is required as well as the anaphylatoxin receptor(s) mediating its effects and the complement-dependent migration of dendritic cells. Challenge experiments with C. psittaci on various complement KO mice were combined with transient decomplementation by pharmacological treatment, as well as the analysis of in vivo dendritic cells migration. Our findings reveal that a plasma concentration of C3 close to wildtype levels was required to achieve full protection. The diminished levels of C3 of heterozygote C3+/- mice permitted already relative effective protection and improved survival as compared to C3-/- mice, but overall recovery of these animals was delayed. Complement was in particular required during the first days of infection. However, additionally, it seems to support protection at later stages. Migration of CD103+ dendritic cells from the infected lung to the draining lymph node-as prerequisite of antigen presentation-depended on C3 and C3aR and/or C5aR. Our results provide unique mechanistic insight in various aspects of complement-dependent immune responses under almost identical, rather physiological experimental conditions. Our study contributes to an improved understanding of the role of complement, and C3a in particular, in infections by intracellular bacteria.

Complement and Chlamydia psittaci: Non-Myeloid-Derived C3 Predominantly Induces Protective Adaptive Immune Responses in Mouse Lung Infection

Recent advances in complement research have revolutionized our understanding of its role in immune responses. The immunomodulatory features of complement in infections by intracellular pathogens, e.g., viruses, are attracting increasing attention. Thereby, local production and activation of complement by myeloid-derived cells seem to be crucial. We could recently show that C3, a key player of the complement cascade, is required for effective defense against the intracellular bacterium Chlamydia psittaci. Avian zoonotic strains of this pathogen cause life-threatening pneumonia with systemic spread in humans; closely related non-avian strains are responsible for less severe diseases of domestic animals with economic loss. To clarify how far myeloid- and non-myeloid cell-derived complement contributes to immune response and resulting protection against C. psittaci, adoptive bone marrow transfer experiments focusing on C3 were combined with challenge experiments using a non-avian (BSL 2) strain of this intracellular bacterium. Surprisingly, our data prove that for C. psittaci-induced pneumonia in mice, non-myeloid-derived, circulating/systemic C3 has a leading role in protection, in particular on the development of pathogen-specific T- and B- cell responses. In contrast, myeloid-derived and most likely locally produced C3 plays only a minor, mainly fine-tuning role. The work we present here describes authentic, although less pronounced, antigen directed immune responses.

Pathogen Burden, Blood Biomarkers and Functional Ageing in Community-Dwelling Older Adults

BACKGROUND

Lifelong accumulation of latent or persistent or repeated infections may be a contributing factor to the deterioration of physical and cognitive function associated with functional ageing, but the evidence is limited and the biological underpinnings are unclear.

METHODS

We profiled the seropositivity for common viral, bacterial and plasmodial pathogens of local importance in community-living older adults in two studies involving 745 older adults (mean age 67.0, SD: 7.7 years), and 142 older adults (mean age 72.7, SD: 8.3 years). Pathogen load was related to different sets of age-related physical and cognitive measures of functional ageing and the frailty index, and plasma levels of biomarkers of inflammation, innate and adaptive immunity, and other physiological functions.

RESULTS

High pathogen load was associated with impaired gait speed (GS), (p<0.015), functional mobility (POMA) (p<0.029), cognitive function (MMSE) (p<<0.05), and increased frailty index (FI) (p<<0.05). High pathogen load was significantly associated with C3a complement activity (p<0.001), matrix metalloproteinase-7, macrophage inflammatory protein-1α (p<0.05), and monocyte chemoattractant protein 2 (p=0.028). Blood biomarkers did not fully explain the observed association between pathogen load and functional ageing measures.

CONCLUSIONS

The present study provides novel evidence linking lifelong cumulated numbers of latent, persistent or repeated infection to functional ageing, plausibly via inflammatory and immune and other biological factors.

Analysis of complement deposition and processing on Chlamydia trachomatis

Chlamydia trachomatis (C. trachomatis) is the leading cause of sexually transmitted bacterial infections worldwide, with over 120 million annual cases. C. trachomatis infections are associated with severe reproductive complications in women such as extrauterine pregnancy and tubal infertility. The infections are often long lasting, associated with immunopathology, and fail to elicit protective immunity which makes recurrent infections common. The immunological mechanisms involved in C. trachomatis infections are only partially understood. Murine infection models suggest that the complement system plays a significant role in both protective immunity and immunopathology during primary Chlamydia infections. However, only limited structural and mechanistic evidence exists on complement-mediated immunity against C. trachomatis. To expand our current knowledge on this topic, we analyzed global complement deposition on C. trachomatis using comprehensive in-depth mass spectrometry-based proteomics. We show that factor B, properdin, and C4b bind to C. trachomatis demonstrating that C. trachomatis-induced complement activation proceeds through at least two activation pathways. Complement activation leads to cleavage and deposition of C3 and C5 activation products, causing initiation of the terminal complement pathway and deposition of C5b, C6, C7, C8, C9 on C. trachomatis. Interestingly, using immunoelectron microscopy, we show that C5b-9 deposition occurred sporadically and only in rare cases formed complete lytic terminal complexes, possibly caused by the presence of the negative regulators vitronectin and clusterin. Finally, cleavage analysis of C3 demonstrated that deposited C3b is degraded to the opsonins iC3b and C3dg and that this complement opsonization facilitates C. trachomatis binding to human B-cells.

An Ancient Molecular Arms Race: Chlamydia vs. Membrane Attack Complex/Perforin (MACPF) Domain Proteins

Dynamic interactions that govern the balance between host and pathogen determine the outcome of infection and are shaped by evolutionary pressures. Eukaryotic hosts have evolved elaborate and formidable defense mechanisms that provide the basis for innate and adaptive immunity. Proteins containing a membrane attack complex/Perforin (MACPF) domain represent an important class of immune effectors. These pore-forming proteins induce cell killing by targeting microbial or host membranes. Intracellular bacteria can be shielded from MACPF-mediated killing, and Chlamydia spp. represent a successful paradigm of obligate intracellular parasitism. Ancestors of present-day Chlamydia likely originated at evolutionary times that correlated with or preceded many host defense pathways. We discuss the current knowledge regarding how chlamydiae interact with the MACPF proteins Complement C9, Perforin-1, and Perforin-2. Current evidence indicates a degree of resistance by Chlamydia to MACPF effector mechanisms. In fact, chlamydiae have acquired and adapted their own MACPF-domain protein to facilitate infection.

Opsonophagocytosis of Chlamydia pneumoniae by Human Monocytes and Neutrophils

The human respiratory tract pathogen Chlamydia pneumoniae, which causes mild to severe infections, has been associated with the development of chronic inflammatory diseases. To understand the biology of C. pneumoniae infections, several studies have investigated the interaction between C. pneumoniae and professional phagocytes. However, these studies have been conducted under nonopsonizing conditions, making the role of opsonization in C. pneumoniae infections elusive. Thus, we analyzed complement and antibody opsonization of C. pneumoniae and evaluated how opsonization affects chlamydial infectivity and phagocytosis in human monocytes and neutrophils. We demonstrated that IgG antibodies and activation products of complement C3 and C4 are deposited on the surface of C. pneumoniae elementary bodies when incubated in human serum. Complement activation limits C. pneumoniae infectivity in vitro and has the potential to induce bacterial lysis by the formation of the membrane attack complex. Coculture of C. pneumoniae and freshly isolated human leukocytes showed that complement opsonization is superior to IgG opsonization for efficient opsonophagocytosis of C. pneumoniae in monocytes and neutrophils. Neutrophil-mediated phagocytosis of C. pneumoniae was crucially dependent on opsonization, while monocytes retained minor phagocytic potential under nonopsonizing conditions. Complement opsonization significantly enhanced the intracellular neutralization of C. pneumoniae in peripheral blood mononuclear cells and neutrophils and almost abrogated the infectious potential of C. pneumoniae In conclusion, we demonstrated that complements limit C. pneumoniae infection in vitro by interfering with C. pneumoniae entry into permissive cells by direct complement-induced lysis and by tagging bacteria for efficient phagocytosis in both monocytes and neutrophils.

Comparative immunopathogenesis in a murine model of inhalative infection with the mucormycetes Lichtheimia corymbifera and Rhizopus arrhizus

Pathogenic mucormycetes induce diseases with considerable morbidity and mortality in immunocompromised patients. Virulence data comparing different Mucorales species and various underlying risk factors are limited. We therefore compared the pathogenesis of inhalative infection by Rhizopus (R.) arrhizus and Lichtheimia (L.) corymbifera in murine models for predominant risk factors for onset of infection. Mice with diabetes or treated with cyclophosphamide or cortisone acetate were challenged via the intranasal route with an isolate of R. arrhizus or L. corymbifera, respectively. Clinical, immunological and inflammation parameters as well as efficacy of posaconazole prophylaxis were monitored over 14 days. Whereas immunocompetent mice showed no clinical symptoms after mucormycete infection, mice treated with either cyclophosphamide (CP) or cortisone acetate (CA) were highly susceptible. Animals infected with the isolate of R. arrhizus showed prolonged survival and lower mortality, compared to those exposed to the L. corymbifera isolate. This lower virulence of R. arrhizus was risk factor-dependent, since diabetic mice died only after infection with Rhizopus, whereas all Lichtheimia-infected diabetic animals survived. Under posaconazole prophylaxis, both mucormycetes were able to establish breakthrough infections in CA- and CP-treated mice, but the course of infection was significantly delayed. Detailed analysis revealed that susceptibility of CA- and CP-treated mice could not be mimicked by exclusive lack or downmodulation of neutrophils, platelets or complement, but can be supposed to be the consequence of a broad immunosuppressive effect induced by the drugs. Both Lichtheimia corymbifera and Rhizopus arrhizus induce invasive mycoses in immunocompromised hosts after inhalative infection. Key parameters such as virulence and immunopathogenesis vary strongly according to fungal species and underlying risk group. Selected neutropenia is no sufficient risk factor for onset of inhalative mucormycosis.

Complement component 3 prevents imiquimod-induced psoriatic skin inflammation by inhibiting apoptosis in mice

Complement component 3 (C3), a pivotal molecule in the complement system, is an essential immune mediator in various diseases, including psoriasis. However, the mechanistic role of C3 in psoriasis pathology and development remains elusive. Here, we showed that C3 deficiency dramatically augmented imiquimod-induced psoriasis-like skin inflammation, characterized by greater epidermal hyperplasia, inflammatory cell infiltration, and inflammatory gene expression than those in wild-type counterparts. In addition, C3 deficiency promoted imiquimod-induced skin cell apoptosis and supported greater proportions of IFN-γ⁺ T cells in the inflamed tissues. Accordingly, C3 supplement in the C3 deficient mice reduced skin inflammation and cells apoptosis. Moreover, blocking apoptosis with Z-VAD-FMK, a broad caspase inhibitor, markedly attenuated imiquimod-induced psoriasis-like skin inflammation and IFN-γ⁺ T cell responses in C3-deficient mice. Collectively, our results suggest that C3 prevents imiquimod-induced psoriasis-like skin inflammation by inhibiting apoptosis.

Isolation and Characterization of Avian Chlamydia psittaci from Symptomatic Pet Birds in Southern Hunan, China

Chlamydia psittaci is a zoonotic pathogen with multiple hosts, especially avian, and can be transmitted to humans, causing psittacosis or ornithosis. No effective vaccines have been developed. We therefore isolate and genotype avian C. psittaci strains and investigate the pathogenicity of isolates in the southern Hunan area of China. Among 200 suspicious avian specimens, eight were positive for the C. psittaci outer membrane protein A (ompA) gene (4%), and seven were successfully cultured in human epithelial type 2 and Vero cells (87.5%). Genotyping of the ompA gene of the eight PCR-positive samples revealed that all of the cultured strains, except for the E9 strain, belonged to genotype A. Pathologic changes in the mice infected with C. psittaci via intranasal inoculation showed severe pneumonia and intense infiltration of inflammatory cells in the lung in a dose-dependent manner, and immunohistochemical staining displayed different levels of infiltration of C. psittaci inclusions in the heart, liver, spleen, kidney, and, especially, lung. Our findings demonstrate that genotype A dominates all C. psittaci genotypes in the southern Hunan area and that the C. psittaci avian isolates in this region possess dose-dependent pathogenicity.

Simultaneous Intramuscular And Intranasal Administration Of Chitosan Nanoparticles-Adjuvanted Chlamydia Vaccine Elicits Elevated Protective Responses In The Lung

Background

Chlamydia psittaci is a zoonotic bacteria closely associated with psittacosis/ornithosis. Vaccination has been recognized as the best way to inhibit the spread of C. psittaci due to the majority ignored of infections. The optimal Chlamydia vaccine was obstructed by the defect of single immunization route and the lack of availability of nontoxic and valid adjuvants.

Methods

In this study, we developed a novel immunization strategy, simultaneous (SIM) intramuscular (IM) and intranasal (IN) administration of a C. psittaci antigens (Ags) adjuvanted with chitosan nanoparticles (CNPs). And SIM-CNPs-Ags were used to determine the different types of immune response and the protective role in vivo.

Results

CNPs-Ags with zeta-potential values of 13.12 mV and of 276.1 nm showed excellent stability and optimal size for crossing the mucosal barrier with high 71.7% encapsulation efficiency. SIM-CPN-Ags mediated stronger humoral and mucosal responses by producing meaningfully high levels of IgG and secretory IgA (sIgA) antibodies. The SIM route also led to Ags-specific T-cell responses and increased IFN-γ, IL-2, TNF-α and IL-17A in the splenocyte supernatants. Following respiratory infection with C. psittaci, we found that SIM immunization remarkably reduced bacterial load and the degree of inflammation in the infected lungs and made for a lower level of IFN-γ, TNF-α and IL-6. Furthermore, SIM vaccination with CNPs-Ags had obviously inhibited C. psittaci disseminating to various organs in vivo.

Conclusion

SIM immunization with CNPs-adjuvanted C. psittaci Ags may present a novel strategy for the development of a vaccine against the C. psittaci infection.

Complement factor and T-cell interactions during alloimmune inflammation in transplantation

Complement factor and T-cell signaling during an effective alloimmune response plays a key role in transplant-associated injury, which leads to the progression of chronic rejection (CR). During an alloimmune response, activated complement factors (C3a and C5a) bind to their corresponding receptors (C3aR and C5aR) on a number of lymphocytes, including T-regulatory cells (Tregs), and these cell-molecular interactions have been vital to modulate an effective immune response to/from Th1-effector cell and Treg activities, which result in massive inflammation, microvascular impairments, and fibrotic remodeling. Involvement of the complement-mediated cell signaling during transplantation signifies a crucial role of complement components as a key therapeutic switch to regulate ongoing inflammatory state, and further to avoid the progression of CR of the transplanted organ. This review highlights the role of complement-T cell interactions, and how these interactions shunt the effector immune response during alloimmune inflammation in transplantation, which could be a novel therapeutic tool to protect a transplanted organ and avoid progression of CR.

Immunity against the Obligate Intracellular Bacterial Pathogen Rickettsia australis Requires a Functional Complement System

The complement system has a well-defined role in deterring blood-borne infections. However, complement is not entirely efficacious, as several bacterial pathogens, including some obligate intracellular pathogens, have evolved mechanisms for resistance. It is presumed that obligate intracellular bacteria evade complement attack by residing within a host cell; however, recent studies have challenged this presumption. Here, we demonstrate that the complement system is activated during infection with the obligate intracellular bacterium Rickettsia australis and that genetic ablation of complement increases susceptibility to infection. Interaction of Rickettsia australis with serum-borne complement leads to activation of the complement cascade, producing three effector mechanisms that could negatively influence R. australis. The C9-dependent membrane attack complex can lead to deposition of a bacteriolytic membrane pore on the bacteria, but this system does not contribute to control of rickettsial infection. Similarly, complement receptor (CR1/2)-dependent opsonophagocytosis may lead to engulfment and killing of the bacteria, but this system is also dispensable for immunity. Nevertheless, intact complement is essential for naturally acquired and antibody-mediated immunity to Rickettsia infection. Comparison of infection in mice lacking the central complement protein C3 with infection in their wild-type counterparts demonstrated decreases in gamma interferon (IFN-γ) production, IgG secretion, and spleen hyperplasia in animals lacking complement. The correlation between loss of secondary immune functions and loss of complement indicates that the proinflammatory signaling components of the complement system, and not membrane attack complex or opsonophagocytosis, contribute to the immune response to this pathogen.

Complement C3 opsonization of Chlamydia trachomatis facilitates uptake in human monocytes

Chlamydia trachomatis is an obligate intracellular bacterium that causes severe infections, which can lead to infertility and ectopic pregnancy. Although both innate and adaptive immune responses are elicited during chlamydial infection the bacterium succeeds to evade host defense mechanisms establishing chronic infections. Thus, studying the host-pathogen interaction during chlamydial infection is of importance to understand how C. trachomatis can cause chronic infections. Both the complement system and monocytes play essential roles in anti-bacterial defense, and, therefore, we investigated the interaction between the complement system and the human pathogens C. trachomatis D and L2. Complement competent serum facilitated rapid uptake of both chlamydial serovars into monocytes. Using immunoelectron microscopy, we showed that products of complement C3 were loosely deposited on the bacterial surface in complement competent serum and further characterization demonstrated that the deposited C3 product was the opsonin iC3b. Using C3-depleted serum we confirmed that complement C3 facilitates rapid uptake of chlamydiae into monocytes in complement competent serum. Complement facilitated uptake did not influence intracellular survival of C. trachomatis or C. trachomatis-induced cytokine secretion. Hence, C. trachomatis D and L2 activate the complement system leading to chlamydial opsonization by iC3b and subsequent phagocytosis, activation and bacterial elimination by human monocytes.

Circulating and broncho-alveolar interleukin-6 in relation to body temperature in an experimental model of bovine Chlamydia psittaci infection

In rodent models of experimentally induced fever, the important role of interleukin-6 (IL-6) as a circulating endogenous pyrogen is well established. Studies employing larger animal species and real infections are scarce. Therefore, we assessed bioactive IL-6 in peripheral blood and in broncho-alveolar lavage fluid (BALF) of calves after intra-bronchial inoculation with vital Chlamydia psittaci (Cp), with inactivated Cp, or with BGM cells. Only calves inoculated with vital Cp developed fever (peak at 2-3 days after challenge) and significantly increased IL-6 activity. Controls inoculated with either inactivated Cp or BGM cells also expressed increased bioactive IL-6, but no fever developed. Activity of IL-6 in BALF was significantly higher compared to blood serum. This experimental model of Cp infection revealed no apparent relation between IL-6 in blood and body temperature, but did reveal a relation between IL-6 and other markers of inflammation in BALF. We conclude that a local inflammatory response in the lungs of infected calves caused fever, which developed by mechanisms including other mediators besides IL-6.

A novel whole-bacterial enzyme linked-immunosorbant assay to quantify Chlamydia trachomatis specific antibodies reveals distinct differences between systemic and genital compartments

Chlamydia trachomatis (CT) is the leading sexually transmitted bacterial infection. The continued global burden of CT infection strongly predicates the need for a vaccine to supplement current chlamydial control programs. The correlates of protection against CT are currently unknown, but they must be carefully defined to guide vaccine design. The localized nature of chlamydial infection in columnar epithelial cells of the genital tract necessitates investigation of immunity at the site of infection. The purpose of this study was to develop a sensitive whole bacterial enzyme-linked immunosorbent assay (ELISA) to quantify and compare CT-specific IgG and IgA in sera and genital secretions from CT-infected women. To achieve this, elementary bodies (EBs) from two of the most common genital serovars (D and E) were attached to poly-L-lysine-coated microtiter plates with glutaraldehyde. EB attachment and integrity were verified by the presence of outer membrane antigens and the absence of bacterial cytoplasmic antigens. EB-specific IgG and IgA standards were developed by pooling sera with high titers of CT-specific antibodies from infected women. Serum, endocervical and vaginal secretions, and endocervical cytobrush specimens from CT-infected women were used to quantify CT-specific IgG and IgA which were then normalized to total IgG and IgA, respectively. Analyses of paired serum and genital samples revealed significantly higher proportions of EB-specific antibodies in genital secretions compared to sera. Cervical and vaginal secretions and cytobrush specimens had similar proportions of EB-specific antibodies, suggesting any one of these genital sampling techniques could be used to quantify CT-specific antibodies when appropriate normalization methodologies are implemented. Overall, these results illustrate the need to investigate genital tract CT antibody responses, and our assay provides a useful quantitative tool to assess natural immunity in defined clinical groups and CT vaccine trials.

Depletion of the Third Complement Component Ameliorates Age-Dependent Oxidative Stress and Positively Modulates Autophagic Activity in Aged Retinas in a Mouse Model

The aim of the study was to investigate the influence of complement component C3 global depletion on the biological structure and function of the aged retina. In vivo morphology (OCT), electrophysiological function (ERG), and the expression of selected oxidative stress-, apoptosis-, and autophagy-related proteins were assessed in retinas of 12-month-old C3-deficient and WT mice. Moreover, global gene expression in retinas was analyzed by RNA arrays. We found that the absence of active C3 was associated with (1) alleviation of the age-dependent decrease in retinal thickness and gradual deterioration of retinal bioelectrical function, (2) significantly higher levels of antioxidant enzymes (catalase and glutathione reductase) and the antiapoptotic survivin and Mcl-1/Bak dimer, (3) lower expression of the cellular oxidative stress marker-4HNE-and decreased activity of proapoptotic caspase-3, (4) ameliorated retinal autophagic activity with localization of ubiquitinated protein conjugates commonly along the retinal pigment epithelium (RPE) layer, and (5) significantly increased expression of several gene sets associated with maintenance of the physiological functions of the neural retina. Our findings shed light on mechanisms of age-related retinal alterations by identifying C3 as a potential therapeutic target for retinal aging.

Recombinant protein CPSIT_0846 induces protective immunity against Chlamydia psittaci infection in BALB/c mice

Chlamydia psittaci is an obligate intracellular bacteria that causes respiratory disease in poultry and humans. Currently, there are no licensed vaccines against chlamydial infection in humans. The transmembrane head protein CPSIT_0846 of C. psittaci is a putative member of the larger Inc protein family. In this study, we investigated immunogenicity and protective efficacy of the recombinant CPSIT_0846 protein in BALB/c mice. Mice immunized with CPSIT_0846 developed strong T-lymphocyte responses that were recalled by the immunogen CPSIT_0846 in an in vitro restimulation assay. These T cells displayed a strong Th1-biased cytokine profile with high levels of IFN-γ. At the same time, a strong humoral immune response was also detected in the immunized mice with high titers of Chlamydia psittaci-specific serum IgG antibodies. More importantly, the robust immune responses correlated well with significantly reduced chlamydial burden and inflammatory pathology in the mouse lungs upon an airway challenge infection. The above results together suggest that the CPSIT_0846 protein may be a potential vaccine candidate antigen for inducing protection against C. psittaci infection and disease in the airway.

Neutralizing antichlamydial activity of complement by chlamydia-secreted protease CPAF

Ascending infection by sexually transmitted Chlamydia trachomatis is required for chlamydial induction of tubal pathology. To achieve ascension, the C. trachomatis organisms may have to spread from cell to cell, which inevitably exposes the organisms to extracellular mucosal effectors such as complement factors that are known to possess strong antichlamydial activities. Here, we report that the chlamydia-secreted protease CPAF efficiently neutralized complement factor C3-dependent antichlamydial activity. The neutralization was dependent on the proteolytic activity of CPAF and correlated with the CPAF-mediated degradation of complement factor C3 and factor B. As a result, CPAF preferentially inhibited the alternative complement activation pathway. The significance and limitation of these observations were discussed.

Protective immunity induced by recombinant protein CPSIT_p8 of Chlamydia psittaci

Chlamydia psittaci is a zoonotic pathogen with a broad host range that can lead to severe respiratory and systemic disease in humans. Currently, an effective commercial vaccine against C. psittaci infection is not available. The chlamydial plasmid is an important virulence factor and encodes plasmid proteins that play important roles in chlamydial infection and the corresponding immune response. In this study, we assessed the efficacy of vaccination with plasmid proteins at preventing C. psittaci lung infection in a murine model. BALB/c mice were immunized intraperitoneally, three times at 2-week intervals, with purified recombinant CPSIT_p8 protein and then infected with C. psittaci. Immunization significantly decreased chlamydial load in the lungs of infected mice, resulted in a lower level of IFN-γ, and reduced the extent of inflammation. In vivo or in vitro neutralization of C. psittaci with sera collected from immunized mice did not reduce the amount of viable C. psittaci in the lungs of mice, indicating that CPSIT_p8-specific antibodies do not have neutralizing capacity. Furthermore, confocal fluorescence microscopy using a mouse anti-CPSIT_p8 antibody revealed that CPSIT_p8 was localized inside the inclusion of C. psittaci 6BC-infected cells. Our results demonstrate that CPSIT_p8 protein induces significant protective immunity against challenge with C. psittaci in mice and represents a promising new vaccine candidate for the prevention of C. psittaci infection.

CD11b(+)Ly6C(++)Ly6G(-) Cells with Suppressive Activity Towards T Cells Accumulate in Lungs of Influenza a Virus-Infected Mice

Influenza A virus (IAV) infection causes an acute respiratory disease characterized by a strong inflammatory immune response and severe immunopathology. Proinflammatory mechanisms are well described in the murine IAV infection model, but less is known about the mechanisms leading to the resolution of inflammation. Here, we analyzed the contribution of CD11b⁺Ly6C⁺⁺Ly6G^– cells to this process. An accumulation of CD11b⁺Ly6C⁺⁺Ly6G^– cells within the lungs was observed during the course of IAV infection. Phenotypic characterization of these CD11b⁺Ly6C⁺⁺Ly6G^– cells by flow cytometry and RNA-Seq revealed an activated phenotype showing both pro- and anti-inflammatory features, including the expression of inducible nitric oxide synthase (iNOS) by a fraction of cells in an IFN-γ-dependent manner. Moreover, CD11b⁺Ly6C⁺⁺Ly6G^– cells isolated from lungs of IAV-infected animals displayed suppressive activity when tested in vitro, and iNOS inhibitors could abrogate this suppressive activity. Collectively, our data suggest that during IAV infection, CD11b⁺Ly6C⁺⁺Ly6G^– cells acquire immunoregulatory function, which might contribute to the prevention of pathology during this life-threatening disease.

Temperature and host cell-dependent changes in virulence of Chlamydia pneumoniae CWL029 in an optimized mouse infection model

The obligate intracellular bacterium Chlamydia (C.) pneumoniae causes respiratory infections and is associated with vascular diseases. To elucidate how temperature and host cells used for propagation alter chlamydial virulence, C. pneumoniae CWL0129 (Cpn) was cultured at 35 or 37°C in two different cell lines and then applied to mice. These mice infected with differentially propagated chlamydiae showed differences in clinical score, body weight and inflammatory cytokines in the lung. Our study demonstrates that Cpn cultured at 37°C in hamster fibroblast BHK-21 are able to colonize the mouse lung faster and better, and induce stronger symptoms and cytokine induction than bacteria cultured at 35°C. The temperature-triggered virulence alteration could not be observed for Cpn propagated in HeLa cells and was independent of host cell protein synthesis. Transcriptome analysis did not reveal temperature-induced effects on chlamydial gene expression, suggesting that the observed virulence changes are regulated on a different, so far unknown level. Preculture close to the central body temperature of its warm-blooded human or murine host might 'prepare' Cpn for subsequent in vivo infection. Our identification of culture-dependent virulence alteration helps to establish an optimized mouse lung infection model for Cpn and provides the basis to further unravel the molecular mechanisms underlying chlamydial pathogenicity.

A re-evaluation of the role of B cells in protective immunity to Chlamydia infection

Chlamydia trachomatis is the etiological agent of the most commonly reported bacterial sexual transmitted infection (STI) in North America and Europe. The control of Chlamydia infection is hindered by the asymptomatic nature of initial infection but the consequence of untreated infection seriously threatens the reproductive health of young women. Unfortunately, there is no licensed vaccine for Chlamydia vaccine, in part due to our incomplete understanding of the immune response to Chlamydia urogenital infection. It has been well established that T cell-mediated immunity plays a dominant role in protective immunity against Chlamydia and thus the importance of B cells is somewhat underappreciated. Here, we summarize recent progress on understanding the role of B cells during Chlamydia genital tract infections and discuss how B cells and humoral immunity make an effective contribution to host defense against important intracellular pathogens, including Chlamydia.

Chlamydia psittaci: update on an underestimated zoonotic agent

Chlamydia (C.) psittaci is an economically relevant pathogen in poultry and pet birds, where it causes psittacosis/ornithosis, and also a human pathogen causing atypical pneumonia after zoonotic transmission. Despite its well-documented prevalence, the agent has received less attention by researchers than other Chlamydia spp. in the last decades. In the present paper, we review recently published data on C. psittaci infection and attempt to single out characteristic features distinguishing it from related chlamydial agents. It is remarkable that C. psittaci is particularly efficient in disseminating in the host organism causing systemic disease, which occasionally can take a fulminant course. At the cellular level, the pathogen's broad host cell spectrum (from epithelial cells to macrophages), its rapid entry and fast replication, proficient use of intracellular transport routes to mitochondria and the Golgi apparatus, the pronounced physical association of chlamydial inclusions with energy-providing cell compartments, as well as the subversive regulation of host cell survival during productive and persistent states facilitate the characteristic efficient growth and successful host-to-host spread of C. psittaci. At the molecular level, the pathogen was shown to upregulate essential chlamydial genes when facing the host immune response. We hypothesize that this capacity, in concert with expression of specific effectors of the type III secretion system and efficient suppression of selected host defense signals, contributes to successful establishment of the infection in the host. Concerning the immunology of host-pathogen interactions, C. psittaci has been shown to distinguish itself by coping more efficiently than other chlamydiae with pro-inflammatory mediators during early host response, which can, to some extent, explain the effective evasion and adaptation strategies of this bacterium. We conclude that thorough analysis of the large number of whole-genome sequences already available will be essential to identify genetic markers of the species-specific features and trigger more in-depth studies in cellular and animal models to address such vital topics as treatment and vaccination.

Complement modulation of T cell immune responses during homeostasis and disease

The complement system is an ancient and critical effector mechanism of the innate immune system as it senses, kills, and clears infectious and/or dangerous particles and alerts the immune system to the presence of the infection and/or danger. Interestingly, an increasing number of reports have demonstrated a clear role for complement in the adaptive immune system as well. Of note, a number of recent studies have identified previously unknown roles for complement proteins, receptors, and regulators in T cell function. Here, we will review recent data demonstrating the influence of complement proteins C1q, C3b/iC3b, C3a (and C3aR), and C5a (and C5aR) and complement regulators DAF (CD55) and CD46 (MCP) on T cell function during homeostasis and disease. Although new concepts are beginning to emerge in the field of complement regulation of T cell function, future experiments should focus on whether complement is interacting directly with the T cell or is having an indirect effect on T cell function via APCs, the cytokine milieu, or downstream complement activation products. Importantly, the identification of the pivotal molecular pathways in the human systems will be beneficial in the translation of concepts derived from model systems to therapeutic targeting for treatment of human disorders.

Intracellular sensing of complement C3 activates cell autonomous immunity

Pathogens traverse multiple barriers during infection, including cell membranes. We found that during this transition, pathogens carried covalently attached complement C3 into the cell, triggering immediate signaling and effector responses. Sensing of C3 in the cytosol activated mitochondrial antiviral signaling (MAVS)-dependent signaling cascades and induced proinflammatory cytokine secretion. C3 also flagged viruses for rapid proteasomal degradation, preventing their replication. This system could detect both viral and bacterial pathogens but was antagonized by enteroviruses, such as rhinovirus and poliovirus, which cleave C3 using their 3C protease. The antiviral rupintrivir inhibited 3C protease and prevented C3 cleavage, rendering enteroviruses susceptible to intracellular complement sensing. Thus, complement C3 allows cells to detect and disable pathogens that have invaded the cytosol.

Risk factors for drug-resistant bloodstream infections in patients with systemic lupus erythematosus

OBJECTIVE

To identify risk factors for developing drug-resistant bacterial infections in patients with systemic lupus erythematosus (SLE).

METHODS

A retrospective, case-control study was performed. Patients fulfilled American College of Rheumatology criteria for SLE and had an episode of bloodstream infection between 2001 and 2012. Cases were defined as those with bloodstream infection caused by drug-resistant bacteria (Pseudomonas aeruginosa, methicillin-resistant Staphylococcus aureus, or extended-spectrum-β-lactalamase-producing Escherichia coli); while controls had susceptible strains of S. aureus or E. coli. Differences between groups were analyzed by Student t test or Mann-Whitney U test. Association between variables was assessed by OR (CI 95%). Multivariate analysis was performed by binary logistic regression model.

RESULTS

Forty-four patients were included in each group. Variables associated with drug-resistant bloodstream infection were history of central nervous system activity; hematological activity, immunosuppressive treatment and prednisone dose at the time of the infection; and low C3 levels, antibiotic use, or hospitalization in the previous 3 months. In multivariate analysis, variables that remained significant were low C3 previous to infection (OR 3.12, CI 95% 1.91-8.22), previous hospitalization (OR 2.22, CI 95% 1.42-4.10), and prednisone dose at the time of infection (OR 1.10, CI 95% 1.04-1.22).

CONCLUSION

Low C3 levels, recent hospitalization, and prednisone dose at time of infection are independent risk factors for acquiring drug-resistant bacteria in patients with SLE. Although the present data do not fully support a change in initial treatment-decision strategies, this information could lead to prospective studies designed to address this issue, which could determine the best approach in clinical practice.

Complement factor C5 but not C3 contributes significantly to hydrosalpinx development in mice infected with Chlamydia muridarum

Hydrosalpinx is a pathological hallmark of tubal infertility associated with chlamydial infection. However, the mechanisms of hydrosalpinx remain unknown. Here, we report that complement factor 5 (C5) contributes significantly to chlamydial induction of hydrosalpinx. Mice lacking C5 (C5(-/-)) failed to develop any hydrosalpinx, while ∼42% of the corresponding wild-type mice (C5(+/+)) did so following intravaginal infection with Chlamydia muridarum. Surprisingly, deficiency in C3 (C3(-/-)), an upstream component of the complement system, did not affect mouse susceptibility to chlamydial induction of hydrosalpinx. Interestingly, C5 activation was induced by chlamydial infection in oviducts of C3(-/-) mice, explaining why the C3(-/-) mice remained susceptible to chlamydial induction of hydrosalpinx. Similar levels of live chlamydial organisms were recovered from oviduct tissues of both C5(-/-) and C5(+/+) mice, suggesting that C5 deficiency did not affect C. muridarum ascending infection. Furthermore, C5(-/-) mice were still more resistant to hydrosalpinx induction than C5(+/+) mice, even when live C. muridarum organisms were directly delivered into the upper genital tract, both confirming the role of C5 in promoting hydrosalpinx and indicating that the C5-facilitated hydrosalpinx was not due to enhancement of ascending infection. The C5(-/-) mice displayed significantly reduced lumenal inflammatory infiltration and cytokine production in oviduct tissue, suggesting that C5 may contribute to chlamydial induction of hydrosalpinx by enhancing inflammatory responses.

Lack of long-lasting hydrosalpinx in A/J mice correlates with rapid but transient chlamydial ascension and neutrophil recruitment in the oviduct following intravaginal inoculation with Chlamydia muridarum

Lower genital tract infection with Chlamydia trachomatis and C. muridarum can induce long-lasting hydrosalpinx in the upper genital tract of women and female mice, respectively. However, A/J mice were highly resistant to induction of long-lasting hydrosalpinx by C. muridarum. We further compared host inflammatory responses and chlamydial infection courses between the hydrosalpinx-resistant A/J mice and CBA/J mice known to be susceptible to hydrosalpinx induction. Both mouse strains developed robust pyosalpinx during the acute phase followed by hydrosalpinx during the chronic phase. However, the hydrosalpinges disappeared in A/J mice by day 60 after infection, suggesting that some early hydrosalpinges are reversible. Although the overall inflammatory responses were indistinguishable between CBA/J and A/J mice, we found significantly more neutrophils in oviduct lumen of A/J mice on days 7 and 10, which correlated with a rapid but transient oviduct invasion by C. muridarum with a peak infection on day 7. In contrast, CBA/J mice developed a delayed and extensive oviduct infection. These comparisons have revealed an important role of the interactions of oviduct infection with inflammatory responses in chlamydial induction of long-lasting hydrosalpinx, suggesting that a rapid but transient invasion of oviduct by chlamydial organisms can prevent the development of the long-lasting hydrosalpinges.

The complement C3a receptor is critical in defense against Chlamydia psittaci in mouse lung infection and required for antibody and optimal T cell response

BACKGROUND

The complement system protects against extracellular pathogens and links innate and adaptive immunity. In this study, we investigated the anaphylatoxin C3a receptor (C3aR) in Chlamydia psittaci lung infection and elucidated C3a-dependent adaptive immune mechanisms.

METHODS

Survival, body weight, and clinical score were monitored in primary mouse infection and after serum transfer. Bacterial load, histology, cellular distribution, cytokines, antibodies, and lymphocytes were analyzed.

RESULTS

C3aR(-/-) mice showed prolonged pneumonia with decreased survival, lower weight, and higher clinical score. Compared to wild-type mice bacterial clearance was impaired, and inflammatory parameters were increased. In lung-draining lymph nodes of C3aR(-/-) mice the total number of B cells, CD4(+) T cells, and Chlamydia-specific IFN-γ(+) (CD4(+) or CD8(+)) cells was reduced upon infection, and the mice were incapable of Chlamydia-specific immunoglobulin M or immunoglobulin G production. Performed before infection, transfer of hyperimmune serum prolonged survival of C3aR(-/-) mice.

CONCLUSIONS

C3a and its receptor are critical for defense against C. psittaci in mouse lung infection. In this model, C3a acts via its receptor as immune modulator. Enhancement of specific B and T cell responses upon infection with an intracellular bacterium were identified as hitherto unknown features of C3a/C3aR. These new functions might be of general immunological importance.

Genetic variation in Chlamydia trachomatis and their hosts: impact on disease severity and tissue tropism

Chlamydia trachomatis infections are a global health problem. This obligate intracellular bacterial pathogen comprises lymphogranuloma venereum (L1-L3), ocular (A-C) and genital (D-K) serovars. Although genetically similar, each serovar group differs in disease severity and tissue tropism through mechanisms that are not well understood. It is clear that host genetic differences also play a role in chlamydial disease outcome and key host polymorphisms are beginning to emerge from both human and experimental animal studies. In this review, we will highlight pathogen and host genes that link genetic diversity, disease severity and tissue tropism. We will also use this information to provide new insights that may be helpful in developing improved management strategies for these important pathogens.

Severity of allergic airway disease due to house dust mite allergen is not increased after clinical recovery of lung infection with Chlamydia pneumoniae in mice

Chlamydia pneumoniae is associated with chronic inflammatory lung diseases like bronchial asthma and chronic obstructive pulmonary disease. The existence of a causal link between allergic airway disease and C. pneumoniae is controversial. A mouse model was used to address the question of whether preceding C. pneumoniae lung infection and recovery modifies the outcome of experimental allergic asthma after subsequent sensitization with house dust mite (HDM) allergen. After intranasal infection, BALB/c mice suffered from pneumonia characterized by an increased clinical score, reduction of body weight, histopathology, and a bacterial load in the lungs. After 4 weeks, when infection had almost resolved clinically, HDM allergen sensitization was performed for another 4 weeks. Subsequently, mice were subjected to a methacholine hyperresponsiveness test and sacrificed for further analyses. As expected, after 8 weeks, C. pneumoniae-specific antibodies were detectable only in infected mice and the titer was significantly higher in the C. pneumoniae/HDM allergen-treated group than in the C. pneumoniae/NaCl group. Intriguingly, airway hyperresponsiveness and eosinophilia in bronchoalveolar lavage fluid were significantly lower in the C. pneumoniae/HDM allergen-treated group than in the mock/HDM allergen-treated group. We did observe a relationship between experimental asthma and chlamydial infection. Our results demonstrate an influence of sensitization to HDM allergen on the development of a humoral antibacterial response. However, our model demonstrates no increase in the severity of experimental asthma to HDM allergen as a physiological allergen after clinically resolved severe chlamydial lung infection. Our results rather suggest that allergic airway disease and concomitant cellular changes in mice are decreased following C. pneumoniae lung infection in this setting.

Perforin is detrimental to controlling [corrected] C. muridarum replication in vitro, but not in vivo

CD4 T cells are critical for clearing experimental Chlamydia muridarum genital tract infections. Two independent in vitro CD4 T cell mechanisms have been identified for terminating Chlamydia replication in epithelial cells. One mechanism, requiring IFN-γ and T cell-epithelial cell contact, terminates infection by triggering epithelial production of nitric oxide to chlamydiacidal levels; the second is dependent on T cell degranulation. We recently demonstrated that there are two independent in vivo clearance mechanisms singly sufficient for clearing genital tract infections within six weeks; one dependent on iNOS, the other on Plac8. Redundant genital tract clearance mechanisms bring into question negative results in single-gene knockout mice. Two groups have shown that perforin-knockout mice were not compromised in their ability to clear C. muridarum genital tract infections. Because cell lysis would be detrimental to epithelial nitric oxide production we hypothesized that perforin was not critical for iNOS-dependent clearance, but posited that perforin could play a role in Plac8-dependent clearance. We tested whether the Plac8-dependent clearance was perforin-dependent by pharmacologically inhibiting iNOS in perforin-knockout mice. In vitro we found that perforin was detrimental to iNOS-dependent CD4 T cell termination of Chlamydia replication in epithelial cells. In vivo, unexpectedly, clearance in perforin knockout mice was delayed to the end of week 7 regardless of iNOS status. The discordant in vitro/in vivo results suggest that the perforin's contribution to bacterial clearance in vivo is not though enhancing CD4 T cell termination of Chlamydia replication in epithelial cells, but likely via a mechanism independent of T cell-epithelial cell interactions.