Interaction between human interleukin-2-activated natural killer cells and heat-killed germ tube forms of Candida albicans

Unexpected kidney-restricted role for IL-17 receptor signaling in defense against systemic Candida albicans infection

Kidney injury is a frequent outcome in patients with disseminated Candida albicans fungal infections. IL-17 receptor (IL-17R) signaling is critical for renal protection against disseminated candidiasis, but the identity and function of IL-17-responsive cells in mediating renal defense remains an active area of debate. Using BM chimeras, we found that IL-17R signaling is required only in nonhematopoietic cells for immunity to systemic C. albicans infection. Since renal tubular epithelial cells (RTEC) are highly responsive to IL-17 in vitro, we hypothesized that RTEC might be the dominant target of IL-17 activity in the infected kidney. We generated mice with a conditional deletion of IL-17 receptor A (Il17ra) in RTEC (Il17raΔRTEC). Strikingly, Il17raΔRTEC mice showed enhanced kidney damage and early mortality following systemic infection, very similar to Il17ra-/- animals. Increased susceptibility to candidiasis in Il17raΔRTEC mice was associated with diminished activation of the renal protective Kallikrein-kinin system (KKS), resulting in reduced apoptosis of kidney-resident cells during hyphal invasion. Moreover, protection was restored by treatment with bradykinin, the major end-product of KKS activation, which was mediated dominantly via bradykinin receptor b1. These data show that IL-17R signaling in RTEC is necessary and likely sufficient for IL-17-mediated renal defense against fatal systemic C. albicans infection.

Human natural killer cells acting as phagocytes against Candida albicans and mounting an inflammatory response that modulates neutrophil antifungal activity

BACKGROUND

Natural killer (NK) cells are innate lymphocytes with potent cytotoxic activity. Whereas activity of NK cells has been demonstrated against the fungal pathogens Aspergillus fumigatus and Cryptococcus neoformans, little was known about their interaction with Candida albicans.

METHODS

Primary human NK cells were isolated from buffy coats, primed with a cytokine cocktail and used for confrontation assays with C. albicans. Interaction was monitored and quantified using live cell imaging, confocal microscopy, flow cytometry, and enzyme-linked immunosorbent assay.

RESULTS

Human NK cells actively recognized C. albicans, resulting in degranulation and secretion of granulocyte-macrophage colony-stimulating factor, interferon γ, and tumor necrosis factor α . Uniquely, activation of NK cells was triggered by actin-dependent phagocytosis. Antifungal activity of NK cells against C. albicans could be detected and mainly attributed to secreted perforin. However, NK cells were unable to inhibit filamentation of C. albicans. Human polymorphonuclear neutrophils (PMNs) counteracted the proinflammatory reaction of NK cells by preventing direct contact between NK cells and the fungal pathogen. Activation of PMNs was enhanced in the presence of NK cells, resulting in increased fungicidal activity.

CONCLUSIONS

Our results show a unique pattern of NK cell interaction with C. albicans, which involves direct proinflammatory activation and modulation of PMN activity. For the first time, phagocytosis of a pathogen is shown to contribute to NK cell activation.

Structural and functional alterations of cellular components as revealed by electron microscopy

Scanning (SEM) and transmission electron microscopy (TEM) are two fundamental microscopic techniques widely applied in biological research for the study of ultrastructural cell components. With these methods, especially TEM, it is possible to detect and quantify the morphological and ultrastructural parameters of intracellular organelles (mitochondria, Golgi apparatus, lysosomes, peroxisomes, endosomes, endoplasmic reticulum, cytoskeleton, nucleus, etc.) in normal and pathological conditions. The study of intracellular vesicle compartmentalization is raising even more interest in the light of the importance of intracellular localization of mediators of the signaling in eliciting different biological responses. The study of the morphology of some intracellular organelles can supply information on the bio-energetic status of the cells. TEM has also a pivotal role in the determination of different types of programmed cell death. In fact, the visualization of autophagosomes and autophagolysosomes is essential to determine the occurrence of autophagy (and also to discriminate micro-autophagy from macro-autophagy), while the presence of fragmented nuclei and surface blebbing is characteristic of apoptosis. SEM is particularly useful for the study of the morphological features of the cells and, therefore, can shed light, for instance, on cell-cell interactions. After a brief introduction on the basic principles of the main electron microscopy methods, the article describes some cell components with the aim to demonstrate the huge role of the ultrastructural analysis played in the knowledge of the relationship between function and structure of the biological objects.

pH-dependent disruption of Escherichia coli ATCC 25922 and model membranes by the human antimicrobial peptides hepcidin 20 and 25

The human hepcidin 25 (hep-25) and its isoform hepcidin 20 (hep-20) are histidine-containing, cystein rich, β-sheet structured peptides endowed with antimicrobial activity. We previously reported that, similar to other histidine-containing peptides, the microbicidal effects of hep-25 and hep-20 are highly enhanced at acidic pH. In the present study, we investigated whether pH influences the mode of action of hep-25 and hep-20 on Escherichia coli American Type Culture Collection 25922 and model membranes. A striking release of β-galactosidase by hepcidin-treated E. coli was observed at pH 5.0, whereas no inner membrane permeabilization capacity was seen at pH 7.4, even at bactericidal concentrations. Similar results were obtained by flow cytometry when assessing the internalization of propidium iodide by hepcidin-treated E. coli. Scanning electron microscope imaging revealed that both peptides induced the formation of numerous blebs on the surface of bacterial cells at acidic pH but not at neutral pH. Moreover, a phospholipid/polydiacetylene colourimetric vesicle assay revealed a more evident membrane damaging effect at pH 5.0 than at pH 7.4. The leakage of entrapped dextrans of increasing molecular size from liposomes was also assessed at pH 7.4. Consistent with the lack of β-galactosidase release from whole E. coli observed at such a pH value, evident leakage of only the smallest 4-kDa dextran (and not of dextrans of 20 or 70 kDa) was observed, indicating a poor ability of hepcidin peptides to permeabilize liposome vesicles at pH 7.4. Altogether, the data obtained in the present study using different approaches strongly suggest that the ability of hepcidins to perturb bacterial membranes is markedly pH-dependent.

Natural killer cells and antifungal host response

As a result of improved experimental methodologies and a better understanding of the immune system, there is increasing insight into the antifungal activity of natural killer (NK) cells. Murine and human NK cells are able to damage fungi of different genera and species in vitro, and they exert both direct and indirect antifungal activity through cytotoxic molecules such as perforin and through cytokines and interferons, respectively. On the other hand, recent data suggest that fungi exhibit immunosuppressive effects on NK cells. Whereas clear in vivo data are lacking in humans, the importance of NK cells in the host response against fungi has been demonstrated in animal models. Further knowledge of the interaction of NK cells with fungi might help to better understand the pathogenesis of invasive fungal infections and to improve treatment strategies.

Host defence against disseminated Candida albicans infection and implications for antifungal immunotherapy

The different manifestations of Candida albicans infection are dictated by an underlying defect in the immune response of the host. Protective immunity to disseminated candidiasis, the manifestation of C. albicans infection discussed in this review, has traditionally been ascribed to innate immunity with emphasis on the role of granulocytes. Lately, however, immunological studies have learned that host defence against disseminated candidiasis is based on a complex interplay between innate and cell-mediated immunity. Despite the availability of new antifungal agents, mortality associated with disseminated C. albicans infection remains high. Immunotherapy that augments host defence is an important strategic option in the battle against disseminated candidiasis. Here, the authors review the chronological events in the pathogenesis of disseminated candidiasis that aid in predicting the impact of existing immunotherapy and the development of future immunomodulating strategies.

Killed Candida albicans yeasts and hyphae inhibit gamma interferon release by murine natural killer cells

Killed yeasts and hyphae of Candida albicans inhibit gamma interferon secretion by highly purified murine NK cells in response to the Toll-like receptor ligands lipopolysaccharide and zymosan. This effect, which is also observed in the presence of NK-activating cytokines (interleukin-2 [IL-2], IL-12, and IL-15), may represent a novel mechanism of immune evasion that contributes to the virulence of C. albicans.

Inflammation induced by inoculation of the joint with Candida albicans

In humans Candida albicans is the most frequently isolated opportunistic fungal pathogen. In immunocompromized host the balance with the commensal fungus easily turns to life-threatening disseminated infection. The asymptomatic Candida persistence in organs and the recurrent infections suggest continuous circulation of yeast cells and their degradation products. Under certain conditions, joints might become one of the infectious sites. More easily a reactivation and destructive process can be provoked in individuals with established arthritis. We have investigated the joint inflammation caused by inoculation of the paw with live C. albicans, in intact mice and mice with collagen-induced arthritis (CIA). The results demonstrate that C. albicans infection when localized into the joints caused rapidly progressing septic arthritis. The effect was associated with a strong swelling, a rapid influx of polymorphonuclear (PMN) cells, and an elevated secretion of TNF-alpha and IFN-gamma by lymph node cells. Joint infection exacerbated the established CIA which correlated with an increased level of anti-collagen antibodies.

Host's innate immune response to fungal and bacterial agents in vitro: up-regulation of interleukin-15 gene expression resulting in enhanced natural killer cell activity

Natural killer (NK) cells play an important role in the first line of defence against viral infections. We have shown earlier that exposure of human peripheral blood mononuclear cells (PBMC) to viruses results in rapid up-regulation of NK cell activity via interleukin-15 (IL-15) induction, and that this mechanism curtails viral infection in vitro. By using Candida albicans, Escherichia coli and Staphylococcus aureus, we now show here that exposure of PBMC to fungi and bacteria also results in an immediate increase of NK cytotoxicity. Reverse transcriptase-polymerase chain reaction and Western blot analyses as well as the use of antibodies against different cytokines revealed that IL-15 induction played a predominant role in this NK activation. These results indicate that IL-15 is also involved in the innate immune response against fungal and bacterial agents.

Reduced cytokine secretions by LAK cells of pulmonary tuberculosis patients in response to tumor targets in vitro

Activation of macrophages and other immune components to release a series of proinflammatory cytokines is one of the first events in innate resistance to intracellular infections. Severe manifestations of tuberculosis (TB) could be caused by alterations in the balance of these cytokines. In this study, lymphokine-activated killer (LAK) cells of TB patients and normal individuals were generated by stimulation with cytokines in vitro. The LAK cells of both groups were further triggered with allogeneic tumor targets. Cytokines interferon-gamma (IFN-gamma), tumor necrosis factor-alpha (TNF-alpha), and granulocyte-macrophage colony-stimulating factor (GM-CSF) were estimated in the supernatants generated in the two groups. The aim was to see if infection with TB influenced the secretory capacity of the immune cells in vitro. Reduced cytokine profiles were observed in TB patients, indicating defective interactions between patient effector cells with allogeneic transformed cells compared with normal individuals. Partial restoration of IFN-gamma production was seen with a combination of cytokines interleukin-2 (IL-2) and IL-12 in TB patients. Based on the in vitro observations, we hypothesize that in vivo also there is diminished immune cell activation of effector cells in response to the presence of infected macrophages. This probably leads to a diminished secretory function that can be corrected by the use of such cytokines as IL-2 and IL-12. The effector populations of TB patients are probably in a state of target-induced anergy, allowing the bacteria to thrive, and immunomodulatory cytokines that improve the host immune response toward countering mycobacterial infection.

The differential effect of stress on natural killer T (NKT) and NK cell function

When C57Bl/6 mice were exposed to restraint stress for 12 h or 24 h, lymphocytopenia was induced in the liver, spleen, and thymus. We examined which types of lymphocytes were sensitive or resistant to such stress by a immunofluorescence test. T cells of thymic origin were sensitive while NKT and NK cells were resistant. In contrast to the increase in the proportion of NK cells, NK activity of liver lymphocytes against YAC-1 targets decreased at 24 h after stress. On the other hand, their NKT cytotoxicity against syngeneic thymocytes increased in parallel with an increase in their proportion. In perforin -/- B6 mice and B6-gld/gld (Fas ligand-) mice, NK cells were found to mediate cytotoxicity through perforin while NKT cells mediated self-reactive cytotoxicity through Fas ligand. These results suggest that stress increases the proportion of both NK and NKT cells, but that NK cytotoxicity is suppressed while self-reactive NKT cytotoxicity is not, due to a diversity of their functional mechanisms.