Neither interleukin-6 nor inducible nitric oxide synthase is required for clearance of Chlamydia trachomatis from the murine genital tract epithelium

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

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

IL-6-mediated signaling pathways limit Chlamydia muridarum infection and exacerbate its pathogenicity in the mouse genital tract

Chlamydia muridarum induction of mouse hydrosalpinx, depending on both tubal infection and inflammation, has been used for investigating Chlamydia trachomatis pathogenesis. We now report that IL-6 both inhibits C. muridarum infection and exacerbates pathogenicity in the mouse genital tract. When intravaginally inoculated with a high dose of C. muridarum, IL-6-deficient mice developed more extensive genital tract infection with severe hydrosalpinx, suggesting that IL-6 is required for controlling the high dose infection but not essential for C. muridarum-induced pathology. However, at a low dose, IL-6-deficient mice still developed more extensive infection in the genital tract but no longer with significant pathology, suggesting that IL-6 is required for both controlling the low dose infection and exacerbating the low dose infection-induced pathology. The lack of hydrosalpinx in IL-6-deficient mice correlated with significantly reduced inflammatory infiltration in the oviduct tissue and decreased spleen CD4+ and CD8+ T cells that produce TNFα. Thus, IL-6-dependent pathways are important for both limiting chlamydial colonization in the genital tract mucosal tissues regardless of the infection doses and exacerbating chlamydial pathogenicity in the upper genital tract when IL-6-independent pathogenic mechanisms are not yet activated with a low infection dose.

The IL-6 response to Chlamydia from primary reproductive epithelial cells is highly variable and may be involved in differential susceptibility to the immunopathological consequences of chlamydial infection

BACKGROUND

Chlamydia trachomatis infection results in reproductive damage in some women. The process and factors involved in this immunopathology are not well understood. This study aimed to investigate the role of primary human cellular responses to chlamydial stress response proteases and chlamydial infection to further identify the immune processes involved in serious disease sequelae.

RESULTS

Laboratory cell cultures and primary human reproductive epithelial cultures produced IL-6 in response to chlamydial stress response proteases (CtHtrA and CtTsp), UV inactivated Chlamydia, and live Chlamydia. The magnitude of the IL-6 response varied considerably (up to 1000 pg ml(-1)) across different primary human reproductive cultures. Thus different levels of IL-6 production by reproductive epithelia may be a determinant in disease outcome. Interestingly, co-culture models with either THP-1 cells or autologous primary human PBMC generally resulted in increased levels of IL-6, except in the case of live Chlamydia where the level of IL-6 was decreased compared to the epithelial cell culture only, suggesting this pathway may be able to be modulated by live Chlamydia. PBMC responses to the stress response proteases (CtTsp and CtHtrA) did not significantly vary for the different participant cohorts. Therefore, these proteases may possess conserved innate PAMPs. MAP kinases appeared to be involved in this IL-6 induction from human cells. Finally, we also demonstrated that IL-6 was induced by these proteins and Chlamydia from mouse primary reproductive cell cultures (BALB/C mice) and mouse laboratory cell models.

CONCLUSIONS

We have demonstrated that IL-6 may be a key factor for the chlamydial disease outcome in humans, given that primary human reproductive epithelial cell culture showed considerable variation in IL-6 response to Chlamydia or chlamydial proteins, and that the presence of live Chlamydia (but not UV killed) during co-culture resulted in a reduced IL-6 response suggesting this response may be moderated by the presence of the organism.

Perforin-2 restricts growth of Chlamydia trachomatis in macrophages

Chlamydia trachomatis is a Gram-negative obligate intracellular bacterium that preferentially infects epithelial cells. Professional phagocytes provide C. trachomatis only a limited ability to survive and are proficient killers of chlamydiae. We present evidence herein that identifies a novel host defense protein, perforin-2, that plays a significant role in the eradication of C. trachomatis during the infection of macrophages. Knockdown of perforin-2 in macrophages did not alter the invasion of host cells but did result in chlamydial growth that closely mirrored that detected in HeLa cells. C trachomatis L2, serovar B, and serovar D and C. muridarum were all equally susceptible to perforin-2-mediated killing. Interestingly, induction of perforin-2 expression in epithelial cells is blocked during productive chlamydial growth, thereby protecting chlamydiae from bactericidal attack. Ectopic expression of perforin-2 in HeLa cells, however, does result in killing. Overall, our data implicate a new innate resistance protein in the control of chlamydial infection and may help explain why the macrophage environment is hostile to chlamydial growth.

Protective role of α-galactosylceramide-stimulated natural killer T cells in genital tract infection with Chlamydia muridarum

Natural killer T (NKT) cells are a unique lymphocyte subpopulation which has an important role in the response to microbial pathogens. In this study, we used α-galactosylceramide (α-GalCer), a specific ligand of NKT cells, to enhance NKT response and examine its effect on host defense against genital tract Chlamydia muridarum infection. The results showed that α-GalCer treatment before infection led to reduced pathological changes and bacterial burden in the genital tract. Moreover, α-GalCer-treated mice showed greater local Th1 cytokine production [interferon γ (IFN-γ) and interleukin 12 (IL-12)] in local lymph node cells and genital tissues following challenge infection compared with untreated mice, as well as an enhanced level of IFN-γ production by NK and T cells. In addition, NKT cells in the mice with genital tract C. muridarum infection, unlike those from naïve mice, showed a polarized IFN-γ production. These results suggest a promoting role of NKT cells on type 1 T cell immune response and host resistance to Chlamydia in genital tract infection.

Nitric oxide synthases and tubal ectopic pregnancies induced by Chlamydia infection: basic and clinical insights

Human ectopic pregnancy (EP) remains a common cause of pregnancy-related first trimester death. Nitric oxide (NO) is synthesized from L-arginine by three NO synthases (NOS) in different tissues, including the Fallopian tube. Studies of knockout mouse models have improved our understanding of the function of NOS isoforms in reproduction, but their roles and specific mechanisms in infection-induced tubal dysfunction have not been fully elucidated. Here, we provide an overview of the expression, regulation and possible function of NOS isoforms in the Fallopian tube, highlighting the effects of infection-induced changes in the tubal cellular microenvironment (imbalance of NO production) on tubal dysfunction and the potential involvement of NOS isoforms in tubal EP after Chlamydia trachomatis genital infection. The non-equivalent regulation of tubal NOS isoforms during the menstrual cycle suggests that endogenous ovarian steroid hormones regulate NOS in an isoform-specific manner. The current literature suggests that infection with C. trachomatis induces an inflammatory response that eventually leads to tubal epithelial destruction and functional impairment, caused by a high NO output mediated by inducible NOS (iNOS). Therefore, tissue-specific therapeutic approaches to suppress iNOS expression may help to prevent ectopic implantation in patients with prior C. trachomatis infection of the Fallopian tube.

Time-dependent disruption of oviduct pacemaker cells by Chlamydia infection in mice

Chlamydia trachomatis is the most commonly reported infectious disease in the United States. In women, this infection can lead to pelvic inflammatory disease and cause ectopic pregnancy and tubal factor infertility. Oviduct interstitial cells of Cajal (ICC-OVI) have been identified as pacemakers, responsible for generating slow waves that underlie myosalpinx contractions that are critical for egg transport. ICC-OVI are damaged in mice by the host inflammatory response to Chlamydia, leading to loss of pacemaker activity and associated contractions. However the inflammatory mediator(s) that causes this damage has not been identified. Mice resolve C. muridarum 3-4 wk postinfection but it remains unexplored whether ICC-OVI and pacemaker activity recovers. We have investigated the time dependence of C. muridarum infection with respect to ICC-OVI loss and examined the inflammatory mediator(s) that may be responsible for this damage. Intracellular recordings from the myosalpinx were made at 1, 2, 4 and 7 wk postinfection with Chlamydia. Immunohistochemistry was performed at similar time points to examine changes in ICC-OVI networks and expression of nitric oxide synthase 2 (NOS2) and prostaglandin synthase 2 (PTGS2). Chlamydia-induced expression of NOS2 occurred in stellate-shaped, macrophage-like cells, and damage to ICC-OVI and pacemaker activity occurred as NOS2 expression increased. Immunohistochemistry revealed that macrophages were in close proximity to ICC-OVI. Changes to ICC-OVI were not correlated with PTGS2 expression. These data suggest that ICC-OVI networks and pacemaker activity may be damaged by nitric oxide produced in NOS2-expressing macrophages in response to C. muridarum infection. As the infection resolves, NOS2 expression decreases, ICC-OVI networks recover, and pacemaker activity resumes.

Antigen-specific CD4+ T cells produce sufficient IFN-gamma to mediate robust protective immunity against genital Chlamydia muridarum infection

Chlamydia has been shown to evade host-specific IFN-gamma-mediated bacterial killing; however, IFN-gamma-deficient mice exhibit suboptimal late phase vaginal Chlamydia muridarum clearance, greater dissemination, and oviduct pathology. These findings introduce constraints in understanding results from murine chlamydial vaccination studies in context of potential implications to humans. In this study, we used mice deficient in either IFN-gamma or the IFN-gamma receptor for intranasal vaccination with a defined secreted chlamydial Ag, chlamydial protease-like activity factor (CPAF), plus CpG and examined the role of IFN-gamma derived from adoptively transferred Ag-specific CD4+ T cells in protective immunity against genital C. muridarum infection. We found that early Ag-specific IFN-gamma induction and CD4+ T cell infiltration correlates with the onset of genital chlamydial clearance. Adoptively transferred IFN-gamma competent CPAF-specific CD4+ T cells failed to enhance the resolution of genital chlamydial infection within recipient IFN-gamma receptor-deficient mice. Conversely, IFN-gamma production from adoptively transferred CPAF-specific CD4+ T cells was sufficient in IFN-gamma-deficient mice to induce early resolution of infection and reduction of subsequent pathology. These results provide the first direct evidence that enhanced anti-C. muridarum protective immunity induced by Ag-specific CD4+ T cells is dependent upon IFN-gamma signaling and that such cells produce sufficient IFN-gamma to mediate the protective effects. Additionally, MHC class II pathway was sufficient for induction of robust protective anti-C. muridarum immunity. Thus, targeting soluble candidate Ags via MHC class II to CD4+ T cells may be a viable vaccine strategy to induce optimal IFN-gamma production for effective protective immunity against human genital chlamydial infection.

Role of secreted conjunctival mucosal cytokine and chemokine proteins in different stages of trachomatous disease

Background

Chlamydia trachomatis is responsible for trachoma, the primary cause of preventable blindness worldwide. Plans to eradicate trachoma using the World Health Organization's SAFE program (Surgery, Antibiotics, Facial Cleanliness and Environment Improvement) have resulted in recurrence of infection and disease following cessation of treatment in many endemic countries, suggesting the need for a vaccine to control infection and trachomatous disease. Vaccine development requires, in part, knowledge of the mucosal host immune responses in both healthy and trachomatous conjuctivae—an area of research that remains insufficiently studied.

Methodology/Principal Findings

We characterized 25 secreted cytokines and chemokines from the conjunctival mucosa of individuals residing in a trachoma endemic region of Nepal using Luminex X100 multiplexing technology. Immunomodulating effects of concurrent C. trachomatis infection were also examined. We found that proinflammatory cytokines IL-1β (r = 0.259, P = 0.001) and TNFα (r = 0.168, P<0.05) were significantly associated with trachomatous disease and concurrent C. trachomatis infection compared with age and sex matched controls from the same region who did not have trachoma. In support of these findings, anti-inflammatory cytokine IL-1 receptor antagonist (IL-1Ra) was negatively associated with chronic scarring trachoma (r = −0.249, P = 0.001). Additional cytokines (Th1, IL-12p40 [r = −0.212, P<0.01], and Th2, IL-4 and IL-13 [r = −0.165 and −0.189, respectively, P<0.05 for both]) were negatively associated with chronic scarring trachoma, suggesting a protective role. Conversely, a pathogenic role for the Th3/Tr1 cytokine IL-10 (r = 0.180, P<0.05) was evident with increased levels for all trachoma grades. New risk factors for chronic scarring trachoma included IL-6 and IL-15 (r = 0.259 and 0.292, respectively, P<0.005 for both) with increased levels for concurrent C. trachomatis infections (r = 0.206, P<0.05, and r = 0.304, P<0.005, respectively). Chemokine protein levels for CCL11 (Eotaxin), CXCL8 (IL-8), CXCL9 (MIG), and CCL2 (MCP-1) were elevated in chronic scarring trachoma compared with age and sex matched controls (P<0.05, for all).

Conclusions/Significance

Our quantitative detection of previously uncharacterized and partially characterized cytokines, a soluble cytokine receptor, and chemokines for each trachoma grade and associations with C. trachomatis infections provide, to date, the most comprehensive immunologic evaluation of trachoma. These findings highlight novel pathologic and protective factors involved in trachomatous disease, which will aid in designing immunomodulating therapeutics and a vaccine.

Trachoma, a disease of antiquity dating back to the 16th century B.C.E., predominates among developing countries, where it remains the primary cause of preventable blindness worldwide. In trachoma, recurrent Chlamydia trachomatis bacterial infections during childhood are thought to result in inflammation and subsequent conjunctival scarring that can progress to trichiasis (TT; chronic trachoma; inversion of ≥1 eyelash that touches the globe of the eye). The trachomatous follicular grade (TF; active disease) is a self-limiting disease, suggesting the coexistence of protective inflammatory proteins. The trachomatous inflammatory grade (TI; active disease) is more likely to progress to trachomatous scarring (TS; chronic trachoma). To date, there are only a handful of studies that have examined the immune response in trachoma, and these were primarily based on gene expression. Characterizing quantified conjunctival mucosal immune differences for secreted proteins among individuals with no, active, and chronic trachoma may identify protein biomarkers associated with protection versus disease, which would greatly aid our understanding of the immunopathogenesis of trachoma. In this study, we characterized 25 cytokine and chemokine proteins for all trachoma grades. We identified eight cytokines and chemokines as risk factors for chronic trachoma and four as protective. Together, these findings further characterize the immunopathologic responses involved during trachoma, which will likely aid in the design of a vaccine and immunomodulating therapeutics for trachoma.

Caspase-1 contributes to Chlamydia trachomatis-induced upper urogenital tract inflammatory pathologies without affecting the course of infection

Chlamydia trachomatis infection induces inflammatory pathologies in the upper genital tract, potentially leading to ectopic pregnancy and infertility in the affected women. Caspase-1 is required for processing and release of the inflammatory cytokines interleukin-1beta (IL-1beta), IL-18, and possibly IL-33. In the present study, we evaluated the role of caspase-1 in chlamydial infection and pathogenesis. Although chlamydial infection induced caspase-1 activation and processing of IL-1beta, mice competent and mice deficient in caspase-1 experienced similar courses of chlamydial infection in their urogenital tracts, suggesting that Chlamydia-activated caspase-1 did not play a significant role in resolution of chlamydial infection. However, when genital tract tissue pathologies were examined, the caspase-1-deficient mice displayed much reduced inflammatory damage. The reduction in inflammation was most obvious in the fallopian tube tissue. These observations demonstrated that although caspase-1 is not required for controlling chlamydial infection, caspase-1-mediated responses can exacerbate the Chlamydia-induced inflammatory pathologies in the upper genital tract, suggesting that the host caspase-1 may be targeted for selectively attenuating chlamydial pathogenicity without affecting the host defense against chlamydial infection.

In pursuit of a human chlamydial vaccine

The best way to control chlamydial diseases is with an effective vaccine. Although previous studies suggest that a vaccine is an achievable goal, progress has been limited even though many vaccination strategies have been tested. A clearer understanding of the immunobiological basis of chlamydial protection and pathology will be critical for future research in chlamydial vaccination.

Comparison of gamma interferon-mediated antichlamydial defense mechanisms in human and mouse cells

Gamma interferon (IFN-gamma)-induced effector mechanisms have potent antichlamydial activities that are critical to host defense. The most prominent and well-studied effectors are indoleamine dioxygenase (IDO) and nitric oxide (NO) synthase. The relative contributions of these mechanisms as inhibitors of chlamydial in vitro growth have been extensively studied using different host cells, induction mechanisms, and chlamydial strains with conflicting results. Here, we have undertaken a comparative analysis of cytokine- and lipopolysaccharide (LPS)-induced IDO and NO using an extensive assortment of human and murine host cells infected with human and murine chlamydial strains. Following cytokine (IFN-gamma or tumor necrosis factor alpha) and/or LPS treatment, the majority of human cell lines induced IDO but failed to produce NO. Conversely, the majority of mouse cell lines studied produced NO, not IDO. Induction of IDO in human cell lines inhibited growth of L2 and mouse pneumonitis agent, now referred to as Chlamydia muridarum MoPn equally in all but two lines, and inhibition was completely reversible by the addition of tryptophan. IFN-gamma treatment of mouse cell lines resulted in substantially greater reduction of L2 than MoPn growth. However, despite elevated NO production by murine cells, blockage of NO synthesis with the l-arginine analogue N-monomethyl-l-arginine only partially rescued chlamydial growth, suggesting the presence of another IFN-gamma-inducible antichlamydial mechanism unique to murine cells. Moreover, NO generated from the chemical nitric oxide donor sodium nitroprusside showed little direct effect on chlamydial infectivity or growth, indicating a natural resistance to NO. Finally, IFN-gamma-inducible IDO expression in human HeLa cells was inhibited following exogenous NO treatment, resulting in a permissive environment for chlamydial growth. In summary, cytokine- and LPS-inducible effectors produced by human and mouse cells differ and, importantly, these host-specific effector responses result in chlamydial strain-specific antimicrobial activities.

Histopathologic Changes Related to Fibrotic Oviduct Occlusion After Genital Tract Infection of Mice With Chlamydia muridarum

OBJECTIVES

We sought to determine if intraluminal occluding fibrosis of the oviduct occurs after urogenital Chlamydia muridarum infection in mice.

STUDY

Oviduct occlusion was assessed by infusing dye into the distal uterus and tracking the diffusion of the dye into the oviduct. We also conducted histologic assessment of the affected tissues using hematoxylin and eosin (H&E) and Masson trichrome stains.

RESULTS

All previously infected susceptible mice had occluded oviducts compared with 17.5% of previously uninfected mice. Oviduct occlusion correlated with hydrosalpinx formation and infertility. Intraluminal oviduct fibrosis was observed in several sections of tissue displaying hydrosalpinx but not in tissues without hydrosalpinx. Fibrosis was localized to the oviduct isthmus and oviduct proper, proximal to the uterus.

CONCLUSION

Intralumenal occluding fibrosis of the oviduct is a sequela of infection with C. muridarum in this model. These observations support the use of the murine model to study pathogenesis of chlamydial upper genital tract infection.

The Radical Site in Chlamydial Ribonucleotide Reductase Defines a New R2 Subclass

Ribonucleotide reductase (RNR) synthesizes the deoxyribonucleotides for DNA synthesis. The R2 protein of normal class I ribonucleotide reductases contains a diiron site that produces a stable tyrosyl free radical, essential for enzymatic activity. Structural and electron paramagnetic resonance studies of R2 from Chlamydia trachomatis reveal a protein lacking a tyrosyl radical site. Instead, the protein yields an iron-coupled radical upon reconstitution. The coordinating structure of the diiron site is similar to that of diiron oxidases/monoxygenases and supports a role for this radical in the RNR mechanism. The specific ligand pattern in the C. trachomatis R2 metal site characterizes a new group of R2 proteins that so far has been found in eight organisms, three of which are human pathogens.

Toll-like receptor-2, but not Toll-like receptor-4, is essential for development of oviduct pathology in chlamydial genital tract infection

The roles of Toll-like receptor (TLR) 2 and TLR4 in the host inflammatory response to infection caused by Chlamydia trachomatis have not been elucidated. We examined production of TNF-alpha and IL-6 in wild-type TLR2 knockout (KO), and TLR4 KO murine peritoneal macrophages infected with the mouse pneumonitis strain of C. trachomatis. Furthermore, we compared the outcomes of genital tract infection in control, TLR2 KO, and TLR4 KO mice. Macrophages lacking TLR2 produced significantly less TNF-alpha and IL6 in response to active infection. In contrast, macrophages from TLR4 KO mice consistently produced higher TNF-alpha and IL-6 responses than those from normal mice on in vitro infection. Infected TLR2-deficient fibroblasts had less mRNA for IL-1, IL-6, and macrophage-inflammatory protein-2, but TLR4-deficient cells had increased mRNA levels for these cytokines compared with controls, suggesting that ligation of TLR4 by whole chlamydiae may down-modulate signaling by other TLRs. In TLR2 KO mice, although the course of genital tract infection was not different from that of controls, significantly lower levels of TNF-alpha and macrophage-inflammatory protein-2 were detected in genital tract secretions during the first week of infection, and there was a significant reduction in oviduct and mesosalpinx pathology at late time points. TLR4 KO mice responded to in vivo infection similarly to wild-type controls and developed similar pathology. TLR2 is an important mediator in the innate immune response to C. trachomatis infection and appears to play a role in both early production of inflammatory mediators and development of chronic inflammatory pathology.

The natural course of asymptomatic Chlamydia trachomatis infections: 45% clearance and no development of clinical PID after one-year follow-up

The natural course of asymptomatic Chlamydia trachomatis infections in women was studied during one year in a cohort based nested case-control study. Healthy women (n = 744, from four company health services in Amsterdam) with a medical check-up prior to job engagement were included. C. trachomatis-positive women (n = 30, cases) and a randomly selected control group of C. trachomatis-negative women (n = 186, controls) were followed for one year. Urine specimens (at one, six and 12 months) were analysed for the presence of C. trachomatis-DNA and the C. trachomatis-serovars, and questionnaires were filled in. The C. trachomatis prevalence and natural course in relation to demographic and sexual characteristics after one, six and 12 months were studied. The main outcome measures were 1) the prevalence of C. trachomatis using urine specimens; 2) self-reported complaints; 3) clinical symptoms reported to the coordinating physicians. The prevalence of asymptomatic C. trachomatis infections was 4% and there was no correlation with demographic and sexual characteristics. The person/year clearance rate was 44.7% per year. None of the C. trachomatis-positive women developed clinical symptoms or used C. trachomatis specific antibiotic treatment. Women with or without an asymptomatic infection had the same number of self-reported urogenital complaints during follow-up. In persisting infections twice as many C. trachomatis-serovar E infections were detected as compared to clearing infections. Our findings showed that almost half of the asymptomatic C. trachomatis infections in women cleared during one year of follow-up and none developed clinical pelvic inflammatory disease (PID), which is a much lower figure than previously suggested. Therefore these data are important for cost effectiveness calculations in screening programmes for asymptomatic C. trachomatis infections.

The role of IFN-gamma in the outcome of chlamydial infection

Chlamydia are intracellular bacteria which infect many vertebrates, including humans. They cause a myriad of severe diseases, ranging from asymptomatic infection to pneumonia, blindness or infertility. IFN-gamma plays an important role in defense against acute infection and in the establishment of persistence. Chlamydia have evolved mechanisms to escape IFN-gamma functions. IFN-gamma-mediated effector mechanisms may involve effects on the metabolism of tryptophan or iron, on the inducible NO synthase (iNOS), on the secretion of chemokines and adhesion molecules or on the regulation of T-cell activities. IFN-gamma is secreted by the innate and the adaptive arms of the immune system. Within the former, Chlamydia-infected macrophages express IFN-gamma that in turn mediates resistance to infection. IFN-alpha/beta are pivotal for both IFN-gamma- and iNOS-mediated resistance to chlamydial infection in macrophages.

The quantity of nitric oxide released by macrophages regulates Chlamydia-induced disease

Intracellular bacteria of the genus Chlamydia cause numerous typically chronic diseases, frequently with debilitating sequelae. Genetic determinants of disease susceptibility after infection with Chlamydia bacteria are unknown. C57BL/6 mice develop severe pneumonia and poor immunity against Chlamydia after moderate respiratory infection whereas BALB/c mice are protected from disease and develop vigorous Th1 immunity. Here we show that infected C57BL/6 macrophages release more NO synthesized by NO synthase 2 (NOS2) than BALB/c macrophages and have lower mRNA concentrations of arginase II, a competitor of NOS2 for the common substrate, l-arginine. Reduction, but not elimination, of NO production by incomplete inhibition of NOS2 abolishes susceptibility of C57BL/6 mice to Chlamydia-induced disease. Thus, the quantity of NO released by infected macrophages is the effector mechanism that regulates between pathogenic and protective responses to chlamydial infection, and genes controlling NO production determine susceptibility to chlamydial disease.

Mouse strain-dependent chemokine regulation of the genital tract T helper cell type 1 immune response

Vaginal infection with the mouse pneumonitis agent of Chlamydia trachomatis (MoPn) produces shorter courses of infection in C57BL/6 and BALB/c mice than in C3H/HeN mice, while C57BL/6 mice are more resistant to oviduct pathology. A robust Th1 response is extremely important in host defense against chlamydia. In this study we examined gamma interferon (IFN-gamma), interleukin 10 (IL-10), and the T-cell-regulatory chemokines macrophage inflammatory protein-1alpha (MIP-1alpha) and monocyte chemoattractant protein-1 (MCP-1) to determine if differences in these responses were associated with the differential courses of infection seen in these three strains of mice. Increased and prolonged IFN-gamma responses and lower IL-10 responses were observed in the C57BL/6 strain compared to BALB/c and C3H. Examination of genital tract chemokines revealed a marked predominance of MIP-1alpha over MCP-1 only in the C57 strain. Thus, a pattern of high MIP-1alpha and low MCP-1 levels during the first week of infection is associated with an increased Th1 response and a shorter, more benign chlamydial infection. Inhibition of the MCP-1 response in C3H mice increased their later T-cell production of IFN-gamma but decreased their early IFN-gamma response and had no effect on the course or outcome of infection. Inhibition of MCP-1 is not beneficial in chlamydial infection because of its pleiotropic effects.

Role for inducible nitric oxide synthase in protection from chronic Chlamydia trachomatis urogenital disease in mice and its regulation by oxygen free radicals

It has been previously reported that although inducible nitric oxide synthase (iNOS) gene knockout (NOS2(-/-)) mice resolve Chlamydia trachomatis genital infection, the production of reactive nitrogen species (RNS) via iNOS protects a significant proportion of mice from hydrosalpinx formation and infertility. We now report that higher in vivo RNS production correlates with mouse strain-related innate resistance to hydrosalpinx formation. We also show that mice with a deletion of a key component of phagocyte NADPH oxidase (p47(phox-/-)) resolve infection, produce greater amounts of RNS in vivo, and sustain lower rates of hydrosalpinx formation than both wild-type (WT) NOS2(+/+) and NOS2(-/-) controls. When we induced an in vivo chemical block in iNOS activity in p47(phox-/-) mice using N(G)-monomethyl-L-arginine (L-NMMA), a large proportion of these mice eventually succumbed to opportunistic infections, but not before they resolved their chlamydial infections. Interestingly, when compared to WT and untreated p47(phox-/-) controls, L-NMMA-treated p47(phox-/-) mice resolved their infections more rapidly. However, L-NMMA-treated p47(phox-/-) mice lost resistance to chronic chlamydial disease, as evidenced by an increased rate of hydrosalpinx formation that was comparable to that for NOS2(-/-) mice. We conclude that phagocyte oxidase-derived reactive oxygen species (ROS) regulate RNS during chlamydial urogenital infection in the mouse. We further conclude that while neither phagocyte oxidase-derived ROS nor iNOS-derived RNS are essential for resolution of infection, RNS protect from chronic chlamydial disease in this model.

Early local cytokine profiles in strains of mice with different outcomes from chlamydial genital tract infection

In this study, we expand on the examination of genetically determined differences in host responses that correlate with clearance of Chlamydia trachomatis from the genital tract. We infected C57BL/6, BALB/c, and C3H/HeN mice with the mouse pneumonitis agent of C. trachomatis (MoPn). C57BL/6 mice had the shortest course of infection (22 days) and the lowest incidence of severe hydrosalpinx. BALB/c mice also had a short course of infection (25 days), but all developed hydrosalpinx. C3H/HeN mice had the longest course of infection (38 days), and all developed severe hydrosalpinx. Determination of local cytokine responses by enzyme-linked immunosorbent assay (ELISA) of genital tract secretions revealed that the levels of the proinflammatory cytokines tumor necrosis factor-alpha (TNF-alpha) and interleukin-1beta (IL-1beta) were significantly increased in the C57BL/6 and BALB/c strains compared to those in the C3H/HeN strain whereas the level of IL-6 was not different. The level of the neutrophil chemokine macrophage inflammatory protein 2 (MIP-2) was increased during the first week of infection in all three strains but was significantly higher in the BALB/c strain, the strain with the most rapid influx of neutrophils into the genital tract. Prolonged detection of MIP-2 in C3H/HeN mice was associated with a protracted presence of neutrophils in the genital tract. Early increases in the levels of the proinflammatory cytokines TNF-alpha and IL-1beta are associated with earlier eradication of infection in the C57BL/6 and BALB/c strains than in the C3H/HeN strain. Increased levels of MIP-2 and neutrophils in BALB/c and C3H/HeN mice relative to C57BL/6 mice suggest that these responses may contribute to pathology.

Immunity to murine Chlamydia trachomatis genital tract reinfection involves B cells and CD4(+) T cells but not CD8(+) T cells

CD4(+) T-helper type 1 (Th1) responses are essential for the resolution of a primary Chlamydia trachomatis genital tract infection; however, elements of the immune response that function in resistance to reinfection are poorly understood. Defining the mechanisms of immune resistance to reinfection is important because the elements of protective adaptive immunity are distinguished by immunological memory and high-affinity antigen recognition, both of which are crucial to the development of efficacious vaccines. Using in vivo antibody depletion of CD4(+) and CD8(+) T cells prior to secondary intravaginal challenge, we identified lymphocyte populations that functioned in resistance to secondary chlamydial infection of the genital tract. Depletion of either CD4(+) or CD8(+) T cells in immune wild-type C57BL/6 mice had a limited effect on resistance to reinfection. However, depletion of CD4(+) T cells, but not CD8(+) T cells, in immune B-cell-deficient mice profoundly altered the course of secondary infection. CD4-depleted B-cell-deficient mice were unable to resolve a secondary infection, shed high levels of infectious chlamydiae, and did not resolve the infection until 3 to 4 weeks following the discontinuation of anti-CD4 treatment. These findings substantiated a predominant role for CD4(+) T cells in host resistance to chlamydial reinfection of the female genital tract and demonstrated that CD8(+) T cells are unnecessary for adaptive immune resistance. More importantly, however, this study establishes a previously unrecognized but very significant role for B cells in resistance to chlamydial reinfection and suggests that B cells and CD4(+) T cells may function synergistically in providing immunity in this model of chlamydial infection. Whether CD4(+) T cells and B cells function independently or dependently is unknown, but definition of those mechanisms is fundamental to understanding optimum protective immunity and to the development of highly efficacious immunotherapies against chlamydial urogenital infections.

Does inhibition of tumor necrosis factor alpha affect chlamydial genital tract infection in mice and guinea pigs?

The role of tumor necrosis factor alpha (TNF-alpha) in host defense against chlamydial infection remains unclear. In order to further evaluate the relevance of TNF-alpha to host resistance in chlamydial genital tract infection, we examined the effect of local inhibition of the TNF-alpha response in normal C57 mice and in interferon gamma gene-deficient C57 mice infected intravaginally with the mouse pneumonitis agent of Chlamydia trachomatis. Since the guinea pig model of female genital tract infection more closely approximates the human in terms of ascending infection and development of pathology, we also examined the effect of local inhibition of the TNF-alpha response in guinea pigs infected intravaginally with the guinea pig strain of Chlamydia psittaci. We successfully blocked the early TNF-alpha response in the respective animal models. This blockade had no effect on the numbers of organisms isolated from the genital tract during the time of TNF-alpha inhibition in mice or guinea pigs. Analysis of interleukin-1beta, macrophage inflammatory protein-2, and granulocyte macrophage-colony stimulating factor in the mouse model revealed that blockade of the TNF-alpha response did not alter the release of these proinflammatory proteins. Yet, in TNF-alpha-depleted mice, increased numbers of neutrophils were detected in the genital tract, and, in TNF-alpha-depleted guinea pigs, increased numbers of neutrophils as well as infiltrating lymphocytes were seen in the endocervix. Blockade of TNF-alpha does not affect the level of infection in mice or guinea pigs, but it may decrease TNF-alpha-induced apoptosis of infiltrating inflammatory cells.

Chlamydia trachomatis infection: incidence, health costs and prospects for vaccine development

Chlamydia trachomatis infection is now the most common sexually transmitted disease worldwide. World Health Organisation figures estimated that 89 million new cases of genital Chlamydia infections occurred in 1995, highlighting the worldwide prevalence of infections and the economic burden on healthcare delivery. A number of methods have been developed for detection of chlamydial infection, which vary in sensitivity and specificity. No single method has yet gained general acceptance and in many countries Chlamydia infections are not reported, suggesting that the above figures may be an underestimate of the problem. As yet there is no consensus as to what constitutes a protective immune response against genital Chlamydia infection. Studies in animal models have shown that cell-mediated immunity, both Th1-driven macrophage activation and cytotoxic T cell responses, as well as antibody can mediate protection at different stages of the chlamydial life cycle. A successful vaccine would probably need to elicit both a systemic cell-mediated immune response to limit/resolve established infections and a mucosal IgA response to reduce bacterial shedding and the resulting spread of infection to partners of infected individuals. The immune response to Chlamydia, either through natural infection or following immunisation, also has the potential to enhance inflammation and to act as a driving force for constant mutation in the variable regions of the major outer membrane protein. As a result a constant prevalence of infection is maintained even in an immune population through the emergence of new allelic variants. Immune responses against antigens such as the 60 kDa heat shock protein can exacerbate inflammation through molecular mimicry and must not be elicited as a result of vaccination. Thus there are many challenges for the development of a successful vaccine which must elicit immunity against multiple serovars while at the same time minimising damaging pro-inflammatory immune responses.

In situ analysis of the evolution of the primary immune response in murine Chlamydia trachomatis genital tract infection

Adaptive immune responses contribute to the resolution of Chlamydia trachomatis genital tract infection and protect against reinfection, but our understanding of the mechanisms of those protective responses is incomplete. In this study, we analyzed by in situ immunohistochemistry the progression of the inflammatory and cytokine responses in the genital tracts of mice vaginally infected with C. trachomatis strain mouse pneumonitis. The cellular inflammatory response was characterized by an initial elevation in myeloid cells in the vagina (day 3) and uterine horns (day 7), followed by a marked rise in the number of T cells, predominantly CD4(+) cells. CD8(+) T cells and CD45R(+) B cells were also detected but were much less numerous. Perivascular clusters of CD4(+) T cells, which resembled clusters of T cells seen in delayed-type hypersensitivity responses, were evident by 2 weeks postinfection. Following the resolution of infection, few CD8(+) T cells and CD45R(+) B cells remained, whereas numerous CD4(+) T cells and perivascular clusters of CD4(+) T cells persisted in genital tract tissues. Interleukin-12 (IL-12)- and tumor necrosis factor alpha (TNF-alpha)-producing cells were observed in vaginal tissue by day 3 of infection and in uterine tissues by day 7. Cells producing IL-4 or IL-10 were absent from vaginal tissues at day 3 of infection but were present in uterine tissues by day 7 and were consistently more numerous than IL-12- and TNF-alpha-producing cells. Thus, the evolution of the local inflammatory response was characterized by the accumulation of CD4(+) T cells into perivascular clusters and the presence of cells secreting both Th1- and Th2-type cytokines. The persistence of CD4(+)-T-cell clusters long after infection had resolved (day 70) may provide for a readily mobilizable T-cell response by which previously infected animals can quickly respond to and control a secondary infectious challenge.

Differential regulation of CD4 lymphocyte recruitment between the upper and lower regions of the genital tract during Chlamydia trachomatis infection

Genital infection with Chlamydia trachomatis results in both the local recruitment of protective immune responses and an inflammatory infiltrate that may also participate in tubal pathology. As a beginning to understanding the etiology of immune system-mediated tubal pathology, we evaluated the regional recruitment of lymphocyte subsets to different areas of the female genital tract (GT) over the course of a murine infection with the mouse pneumonitis agent of Chlamydia trachomatis (MoPn). Using flow cytometric techniques we found that the CD4 lymphocyte subset was preferentially recruited to the upper GT (oviduct and uterine horn) over the lower GT (cervical-vaginal region) throughout the course of MoPn infection. The influx of CD4 cells also correlated with the expression of endothelial cell adhesion molecules (ECAMs) and in vitro lymphocyte adherence in the upper GT. Interestingly, the expression of ECAMs in the lower GT was not maintained longer than 7 days after infection, even in the presence of viable chlamydiae. Taken together, these data suggest that regulatory mechanisms of lymphocyte recruitment differ between the upper and lower regions of the GT and may influence the clearance of chlamydiae and the development of tubal pathology.

Reactive oxygen and reactive nitrogen intermediates in innate and specific immunity

Nitric oxide, nitric oxide derivatives and reactive oxygen intermediates are toxic molecules of the immune system which contribute to the control of microbial pathogens and tumors. There is recent evidence for additional functions of these oxygen metabolites in innate and adaptive immunity; these functions include the modulation of the cytokine response of lymphocytes and the regulation of immune cell apoptosis, as well as immunodeviating effects. Components of several signal transduction pathways have been identified as intracellular targets for reactive nitrogen and oxygen intermediates.

Increased susceptibility to systemic candidiasis in interleukin-6 deficient mice

Interleukin-6 (IL-6) is a multifunctional cytokine that regulates multiple aspects of the innate immune response. It has been recently shown that endogenous IL-6 is crucial for an efficient defence against severe infections with Gram-negative and Gram-positive bacteria. The aim of the present study was to investigate the role of endogenous IL-6 in the defence against infection with the yeast Candida albicans. During experimental candidemia, IL-6 deficient mice (IL-6-/-) had a decreased survival and an increased fungal load in their organs when compared with IL-6+/+ controls, despite increased plasma concentrations of tumour necrosis factor-alpha (TNF), interleukin-1 alpha (IL-1 alpha) and IL-1 beta, IL-6-/- mice were not able to mount an efficient neutrophil response during the infection. When mice were rendered neutropenic by cyclophosphamide, neutropenic IL-6-/- mice were equally susceptible to C. albicans when compared to neutropenic IL-6+/+ mice, implying that neutrophils mediate the beneficial effect of endogenous IL-6. In conclusion, IL-6-/- mice are more susceptible to disseminated candidiasis, and the effect of IL-6 is most likely mediated by neutrophils.

Cytokines and the immunopathology of the spondyloarthropathies

In contrast to rheumatoid arthritis (RA), the triggering antigens are known in reactive arthritis (ReA) and Lyme arthritis. Thus, in these arthritides the antigen-specific T-cell response can be investigated in much detail and lessons possibly learned for other spondyloarthropathies (SpA) such as ankylosing spondylitis (AS) where T cells may well also play an important role in the pathogenesis. This article focusses on the immunopathology of the SpA, ReA, and AS with special reference to T cells and cytokines.

Chlamydia trachomatis: the role of cellular and humoral immune mechanisms in the development of blindness

Ocular infection with Chlamydia trachomatis leading to trachoma is the commonest cause of treatable blindness worldwide. Although effectively controlled by improved sanitation, a chlamydial vaccine may prove a more cost-effective method of protection against this disease in the medium term. The development of vaccines against Chlamydia trachomatis has been hampered by a lack of understanding of the immune responses leading to protection or pathology, particularly in humans. Although a strong cell-mediated immune response (T helper lymphocyte type 1) is almost certainly necessary to resolve intracellular chlamydial infection, a humoral (T helper lymphocyte type 2) response may protect against colonization and limit immunopathological events. Recent studies addressing this potential conflict are reviewed.

Differential Sensitivity of Distinct Chlamydia trachomatis Isolates to IFN-γ-Mediated Inhibition

Resistance to the mouse pneumonitis (MoPn) strain of Chlamydia trachomatis has been mapped to MHC class II-restricted, IL-12-dependent CD4+ T cells that secrete a type 1 profile of proinflammatory cytokines, which includes IFN-γ and TNF-α. The relative contribution of IFN-γ is controversial, however, due to variation in results presented by different laboratories. To determine whether C. trachomatis strain differences contributed to this apparent conflict, the relative resistance of IFN-γ-deficient mice to murine and human strains of C. trachomatis was compared. All human serovars were much more sensitive to the direct inhibitory actions of IFN-γ than the MoPn strain. Furthermore, genital clearance of human serovar D in the C57BL/6 mouse was mediated by class II-independent mechanisms that probably involved local production of IFN-γ by cells of the innate immune system. TNF-α also contributed indirectly to host resistance against all strains tested. The differential susceptibility of distinct C. trachomatis strains to effector cytokines such as IFN-γ could not have been predicted by interstrain biologic variation or by the profile of cytokines stimulated during infection. These findings indicate that strain variation should be considered in situations where related isolates of a given parasite produce conflicting data in models of infection and immunity. They also suggest that stimulation of mucosal IFN-γ activity is a relevant goal for a human chlamydial vaccine.

Clearance of Chlamydia trachomatis from the murine genital mucosa does not require perforin-mediated cytolysis or Fas-mediated apoptosis

The molecular mechanisms of resistance to genital infection with the mouse pneumonitis (MoPn) strain of Chlamydia trachomatis are unknown. A role for major histocompatibility complex class II-restricted, interleukin-12-dependent CD4(+) T cells has been established, but the functional activity of these cells does not depend on secretion of gamma interferon. Here we examined the potential contribution of T-cell-mediated cytotoxicity and apoptosis to mucosal clearance of MoPn by using mice deficient in the molecular mediators of target cell lysis. Animals lacking perforin, Fas, Fas ligand, or both perforin and Fas ligand were infected genitally with C. trachomatis MoPn and monitored for expression of immunity to chlamydial antigens and clearance of MoPn from the genital mucosa. In each case, the profile of spleen cytokine production, the magnitude of the host antibody response, and the kinetics of chlamydial clearance were similar to those of genetically intact controls. Compensatory overproduction of tumor necrosis factor alpha, an alternate mediator of apoptosis in certain cell types, did not appear to account for the ability of mutant mice to resolve Chlamydia infections. These results fail to support CD4(+) T-cell-mediated apoptosis or CD8(+) T-cell-mediated cytotoxicity as being critical to the clearance of C. trachomatis MoPn urogenital infections.

A role for interleukin-6 in host defense against murine Chlamydia trachomatis infection

Interleukin-6-deficient (IL-6(-/-)) knockout mice had significantly increased Chlamydia trachomatis levels in lung tissue and increased mortality compared to B6129F2/J controls early after intranasal infection. Gamma interferon production and chlamydia-specific antibody levels were consistent with a decreased but reversible Th1-like response in IL-6(-/-) mice. IL-6 is needed for an optimal early host response to this infection.

Chlamydial infection in inducible nitric oxide synthase knockout mice

Type 1 CD4+-T-cell-mediated immunity is crucial for the resolution of chlamydial infection of the murine female genital tract. Previous studies demonstrating a correlation between CD4+-T-cell-mediated inhibition of chlamydial growth and gamma interferon (IFN-gamma)-mediated induction of nitric oxide synthase suggested a potential role for the nitric oxide (NO) effector pathway in the clearance of Chlamydia from genital epithelial cells by the immune system. To clarify the role of this pathway, the growth levels of Chlamydia trachomatis organisms in normal (iNOS+/+) mice and in genetically engineered mice lacking the inducible nitric oxide synthase (iNOS) gene (iNOS-/- mice) were compared. There was no significant difference in the course of genital chlamydial infections in iNOS+/+ and iNOS-/- mice as determined by recovery of Chlamydia organisms shed from genital epithelial cells. Dissemination of Chlamydia to the spleen and lungs occurred to a greater extent in iNOS-/- than in iNOS+/+ mice, which correlated with a marginal increase in the susceptibility of macrophages from iNOS-/- mice to chlamydial infection in vitro. However, infections were rapidly cleared from all affected tissues, with no clinical signs of disease. The finding of minimal dissemination in iNOS-/- mice suggested that activation of the iNOS effector pathway was not the primary target of IFN-gamma during CD4+-T-cell-mediated control of chlamydial growth in macrophages because previous reports demonstrated extensive and often fatal dissemination of Chlamydia in mice lacking IFN-gamma. In summary, these results indicate that the iNOS effector pathway is not required for elimination of Chlamydia from epithelial cells lining the female genital tract of mice although it may contribute to the control of dissemination of C. trachomatis by infected macrophages.