Impairment of natural killer cell activity in Chlamydia trachomatis infected individuals

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.

NK Cell-Mediated Processing Of Chlamydia psittaci Drives Potent Anti-Bacterial Th1 Immunity

Natural killer (NK) cells are innate immune cells critically involved in the early immune response against various pathogens including chlamydia. Here, we demonstrate that chlamydia-infected NK cells prevent the intracellular establishment and growth of the bacteria. Upon infection, they display functional maturation characterized by enhanced IFN-γ secretion, CD146 induction, PKCϴ activation, and granule secretion. Eventually, chlamydia are released in a non-infectious, highly immunogenic form driving a potent Th1 immune response. Further, anti-chlamydial antibodies generated during immunization neutralize the infection of epithelial cells. The release of chlamydia from NK cells requires PKCϴ function and active degranulation, while granule-associated granzyme B drives the loss of chlamydial infectivity. Cellular infection and bacterial release can be undergone repeatedly and do not affect NK cell function. Strikingly, NK cells passing through such an infection cycle significantly improve their cytotoxicity. Thus, NK cells not only protect themselves against productive chlamydial infections but also actively trigger potent anti-bacterial responses.

Stimulated peripheral blood mononuclear cells from chlamydia-infected women release predominantly Th1-polarizing cytokines

Chlamydia trachomatis infection (chlamydia) is the most prevalent sexually transmitted bacterial infection and causes significant reproductive morbidity in women. Little is known about how immunity to chlamydia develops in women, though animal models of chlamydia indicate that T-helper type 1 (Th1) responses are important for chlamydia clearance and protective immunity, whereas T-helper type 2 (Th2) responses are associated with persisting infection. In chlamydia-infected women, whether the predominant immune response is Th1- or Th2-polarizing remains controversial. To determine the cytokine profiles elicited by peripheral blood mononuclear cells (PBMCs) from chlamydia-infected women, we stimulated PBMCs with C. trachomatis elementary bodies and recombinant C. trachomatis Pgp3 and measured supernatant levels of select cytokines spanning Th1- and Th2-polarizing responses. We found that stimulated PBMCs from chlamydia-infected women secreted cytokines that indicate strong Th1-polarizing responses, especially interferon-gamma, whereas Th2-polarizing cytokines were expressed at significantly lower levels. In chlamydia-infected women, the predominant cytokine responses elicited on stimulation of PBMCs with C. trachomatis antigens were Th1-polarizing, with interferon-gamma as the predominant cytokine.

Modulation of MICA on the surface of Chlamydia trachomatis-infected endocervical epithelial cells promotes NK cell-mediated killing

Chlamydia trachomatis serovars D-K are obligate intracellular bacteria that have tropism for the columnar epithelial cells of the genital tract. Chlamydia trachomatis infection has been reported to induce modifications in immune cell ligand expression on epithelial host cells. In this study, we used an in vitro infection model that resulted in a partial infection of C. trachomatis-exposed primary-like immortalized endocervical epithelial cells (A2EN). Using this model, we demonstrated that expression of the natural killer (NK) cell activating ligand, MHC class I-related protein A (MICA), was upregulated on C. trachomatis-infected, but not on noninfected bystander cells. MICA upregulation was concomitant with MHC class I downregulation and impacted the susceptibility of C. trachomatis-infected cells to NK cell activity. The specificity of MICA upregulation was reflected by a higher cytolytic activity of an NK cell line (NK92MI) against C. trachomatis-infected cells compared with uninfected control cells. Significantly, data also indicated that NK cells exerted a partial, but incomplete sterilizing effect on C. trachomatis as shown by the reduction in recoverable inclusion forming units (IFU) when cocultured with C. trachomatis-infected cells. Taken together, our data suggest that NK cells may play a significant role in the ability of the host to counter C. trachomatis infection.

Prospects and Pitfalls of Pregnancy-Associated Malaria Vaccination Based on the Natural Immune Response to Plasmodium falciparum VAR2CSA-Expressing Parasites

Pregnancy-associated malaria, a manifestation of severe malaria, is the cause of up to 200,000 infant deaths a year, through the effects of placental insufficiency leading to growth restriction and preterm delivery. Development of a vaccine is one strategy for control. Plasmodium falciparum-infected red blood cells accumulate in the placenta through specific binding of pregnancy-associated parasite variants that express the VAR2CSA antigen to chondroitin sulphate A on the surface of syncytiotrophoblast cells. Parasite accumulation, accompanied by an inflammatory infiltrate, disrupts the cytokine balance of pregnancy with the potential to cause placental damage and compromise foetal growth. Multigravid women develop immunity towards VAR2CSA-expressing parasites in a gravidity-dependent manner which prevents unfavourable pregnancy outcomes. Although current vaccine design, targeting VAR2CSA antigens, has succeeded in inducing antibodies artificially, this candidate may not provide protection during the first trimester and may only protect those women living in areas endemic for malaria. It is concluded that while insufficient information about placental-parasite interactions is presently available to produce an effective vaccine, incremental progress is being made towards achieving this goal.

Differential modulating effect of natural killer (NK) T cells on interferon-γ production and cytotoxic function of NK cells and its relationship with NK subsets in Chlamydia muridarum infection

Natural killer T (NKT) cells are a newly identified T-cell population with potential immunomodulatory functions. Several studies have shown modulating effects of NKT cells activated by α-galactosylceramide, a model antigen, on NK cell function. We here report a differential modulating effect of NKT cells on the interferon-γ (IFN-γ) production and cytolytic function of NK cells in a chlamydial infection model, using NKT-cell-deficient mice and antibody blocking (anti-CD1d monoclonal antibody) approaches. Our results showed that both NKT and NK cells became activated and produced IFN-γ following Chlamydia muridarum infection in vitro and in vivo. The NK cells in NKT-cell-deficient mice and CD1d-blocked mice showed decreased CD69 expression, cellular expansion and IFN-γ production but surprisingly showed increased cytolytic activity (degranulation) of immature and more mature NK cell subsets, suggesting an inhibitory role of NKT cells on NK cell killing activity. The results suggest that NKT cells preferentially promote IFN-γ production but are inhibitory for the cytotoxic function of NK cells in this infection model. Furthermore, the differential modulating effect of NKT cells on the IFN-γ production and cytotoxicity of NK cells was observed in immature and mature NK cell subsets, although it was more dramatic in the relatively mature CD11b(high)  CD27(high) NK cell subset. This finding demonstrates the complexity of innate cell interactions in infection and the possible differential impact of NKT cells on the variable functional aspects of other cell(s) even in one infection setting.

Natural killer cell IFN-gamma-activity is associated with Plasmodium falciparum infection during pregnancy

The ability of natural killer (NK) cells to produce gamma interferon (IFN-gamma) after ex vivo stimulation with crude schizont lysate of Plasmodium falciparum was studied in uninfected and P. falciparum-infected pregnant Gabonese women segregated according to the gravidity at the time of delivery. This activity was measured in purified NK cells as well as in whole blood from the periphery and cord. Crude schizont lysate-stimulated NK cells from primiparous women produced significantly more IFN-gamma than those from multiparous women (P<0.001). Women with malaria infection produced more IFN-gamma than negative women in peripheral blood (P<0.001) indicating that immunological determinants regulating the susceptibility to malaria in pregnant women are parasite-specific. These findings reveal that NK cells are major source of IFN-gamma when exposed to P. falciparum antigens in vitro in absence of any other co-stimulant.

NK cell compartment in patients with coronary heart disease

Background

Viral and bacterial infections have been considered as a risk factor for Coronary Heart Disease (CHD). NK cells, as a first line of defense against those infections, may play a role in CHD development. Thus, the main aim of our study was to determine NK cell compartment in patients with CHD undergoing coronary artery by-pass grafting.

Results

Ninety three patients with CHD were included into the study; the control group consisted of 49 healthy volunteers. As compared to controls, CHD patients had lower NK cytotoxic activity. CHD group had also a decreased absolute number and percentage of total NK cells and CD3-CD56dim cytotoxic NK subset. In addition, we observed tendency toward lower percentage of the CD3-CD56bright regulatory NK subset and CD3-CD56+IFN-γ+ cells in CHD patients.

Conclusion

These data indicate that CHD is associated with an impairment of NK cells compartment.

Depressed Natural Killer Cell Cytotoxicity againstPlasmodium falciparum–Infected Erythrocytes during First Pregnancies

We measured natural killer (NK) cell cytotoxicity and cortisol and prolactin concentrations in peripheral venous blood samples obtained from pregnant Gabonese women at the time of delivery. The NK cell-mediated cytotoxicity against Plasmodium falciparum-infected erythrocytes in vitro was lower in samples obtained from primiparous women than in samples obtained from multiparous women; cortisol concentrations were significantly higher in primiparous women than in multiparous women, and prolactin concentrations were significantly lower. The highest cortisol concentrations were found in the plasma of P. falciparum-infected primiparous women. A positive correlation was found between cortisol concentration and parasite load; an inverse correlation was found between the magnitude of the NK cell cytolytic effect and cortisol production. A positive correlation was found between this effect and prolactin production. Thus, depressed NK cell cytotoxicity against P. falciparum-infected erythrocytes is correlated with high cortisol concentrations and may contribute to increased susceptibility to malaria during pregnancy.

Enhancement of natural killer cell activation and antibody-dependent cellular cytotoxicity by interferon-alpha and interleukin-12 in vaginal mucosae Sivmac251-infected Macaca fascicularis

We studied the innate immune system of Cynomolgus monkeys (Macaca fascicularis) experimentally infected via the vaginal mucosae with a virulent simian immunodeficiency virus isolate SIVmac251. Animals were evaluated for their natural killer (NK) cell activity, and for their antibody-dependent cellular cytotoxicity. NK cells from SIVmac251-infected macaques show impaired NK cell activity compared to cells from uninfected animals. Subsequent treatment of NK cells with interferon-a (IFN-alpha) or interleukin-12 (IL-12) alone partially restored the NK activity. However, either treatment of NK cells with both IFN-alpha and IL-12 completely reversed the impairment of cytotoxicity induced by simian immunodeficiency virus (SIV) infection. Incubation of NK cells from infected but not from uninfected monkeys with IFN-alpha and IL-12 for 8 days increased the percentage of CD16+/CD56+ cells twofold to five-fold and enhanced antibody-dependent cellular cytotoxicity (ADCC) activity. Thus IFN-alpha and IL-12 greatly enhance both the NK cell and ADCC activities of peripheral blood cells from SIVmac251-infected animals and increase the number of NK cells in longer term culture. The combined effect of IFN-alpha and IL-12 in enhancing NK cell activity may provide a novel therapeutic approach for the restoration of depressed NK cell activity observed in human immunodeficiency virus (HIV)-infected patients.

A population-based prospective study of Chlamydia trachomatis infection and cervical carcinoma

Persistent human papillomavirus (HPV) infection is an established cause of cervical cancer, but the role of other sexually transmitted agents, most notably Chlamydia trachomatis, has not been well defined. The women participating in the population-based cervical cancer screening program in Västerbotten county of Northern Sweden were followed up for up to 26 years to identify 118 women who developed cervical cancer after having had a normal Pap smear (on average 5.6 years later; range 0.5 months-26 years). As controls, we selected another 118 women who were matched by birth cohort, time-point of sampling of the baseline normal smear and who had a normal smear at the time when the corresponding case was diagnosed with cancer. The Pap smears and cervical cancer biopsies were analyzed by PCR for C. trachomatis DNA and for HPV DNA. At baseline, C. trachomatis DNA was present in 8% of cases but not among any one of the controls. The relative risk for cervical cancer associated with past C. trachomatis infection, adjusted for concomitant HPV DNA positivity, was 17.1 (95% CI 2.6-infinity). The presence of C. trachomatis and of HPV were not interrelated. Whereas C. trachomatis was primarily found in specimens taken many years before cancer diagnosis, HPV DNA was associated with a short lag time before cancer diagnosis. Whereas most women who were HPV DNA-positive in the prediagnostic smear were also positive for the same virus in the cervical cancer biopsy, none of the women were positive for C. trachomatis in both the prediagnostic smear and in the subsequent cervical cancer. In conclusion, a prior cervical C. trachomatis infection was associated with an increased risk for development of invasive cervical cancer.