The use of 7-amino-actinomycin D in the analysis of Candida albicans phagocytosis and opsonization

Antimicrobial Peptides: a New Frontier in Antifungal Therapy

Invasive fungal infections in humans are generally associated with high mortality, making the choice of antifungal drug crucial for the outcome of the patient. The limited spectrum of antifungals available and the development of drug resistance represent the main concerns for the current antifungal treatments, requiring alternative strategies. Antimicrobial peptides (AMPs), expressed in several organisms and used as first-line defenses against microbial infections, have emerged as potential candidates for developing new antifungal therapies, characterized by negligible host toxicity and low resistance rates. Most of the current literature focuses on peptides with antibacterial activity, but there are fewer studies of their antifungal properties. This review focuses on AMPs with antifungal effects, including their in vitro and in vivo activities, with the biological repercussions on the fungal cells, when known. The classification of the peptides is based on their mode of action: although the majority of AMPs exert their activity through the interaction with membranes, other mechanisms have been identified, including cell wall inhibition and nucleic acid binding. In addition, antifungal compounds with unknown modes of action are also described. The elucidation of such mechanisms can be useful to identify novel drug targets and, possibly, to serve as the templates for the synthesis of new antimicrobial compounds with increased activity and reduced host toxicity.

Innate immune cell response upon Candida albicans infection

Candida albicans is a polymorphic fungus which is the predominant cause of superficial and deep tissue fungal infections. This microorganism has developed efficient strategies to invade the host and evade host defense systems. However, the host immune system will be prepared for defense against the microbe by recognition of receptors, activation of signal transduction pathways and cooperation of immune cells. As a consequence, C. albicans could either be eliminated by immune cells rapidly or disseminate hematogenously, leading to life-threatening systemic infections. The interplay between Candida albicans and the host is complex, requiring recognition of the invaded pathogens, activation of intricate pathways and collaboration of various immune cells. In this review, we will focus on the effects of innate immunity that emphasize the first line protection of host defense against invaded C. albicans including the basis of receptor-mediated recognition and the mechanisms of cell-mediated immunity.

Differential role of NK cells against Candida albicans infection in immunocompetent or immunocompromised mice

Little is known regarding the role of NK cells during primary and secondary disseminated Candida albicans infection. We assessed the role of NK cells for host defense against candidiasis in immunocompetent, as well as immunodeficient, hosts. Surprisingly, depletion of NK cells in immunocompetent WT mice did not increase susceptibility to systemic candidiasis, suggesting that NK cells are redundant for antifungal defense in otherwise immunocompetent hosts. NK-cell-depleted mice were found to be protected as a consequence of attenuation of systemic inflammation. In contrast, the absence of NK cells in T/B/NK-cell-deficient NSG (NOD SCID gamma) mice led to an increased susceptibility to both primary and secondary systemic C. albicans infections compared with T/B-cell-deficient SCID mice. In conclusion, this study demonstrates that NK cells are an essential and nonredundant component of anti-C. albicans host defense in immunosuppressed hosts with defective T/B-lymphocyte immunity, while contributing to hyperinflammation in immunocompetent hosts. The discovery of the importance of NK cells in hosts with severe defects of adaptive immunity might have important consequences for the design of adjunctive immunotherapeutic approaches in systemic C. albicans infections targeting NK-cell function.

Macrophage activation by polysaccharide biological response modifier isolated from Aloe vera L. var. chinensis (Haw.) Berg

A mannose-rich polysaccharide biological response modifier (BRM), derived from Aloe vera L. var. chinensis (Haw.) Berg., was demonstrated to be a potent murine B- and T-cell stimulator in our previous study. We here report the stimulatory activity of PAC-I on murine peritoneal macrophage. The polysaccharide when injected into mice enhanced the migration of macrophages to the peritoneal cavity. Peritoneal macrophage when treated by PAC-I in vitro had increased expression of MHC-II and FcgammaR, and enhanced endocytosis, phagocytosis, nitric oxide production, TNF-alpha secretion and tumor cell cytotoxicity. The administration of PAC-I into allogeneic ICR mice stimulated systemic TNF-alpha production in a dose-dependent manner and prolonged the survival of tumor-bearing mice. PAC-I is thus a potent stimulator of murine macrophage and the in vitro observed tumoricidal properties of activated macrophage might account for the in vivo antitumor properties of PAC-I. Our research findings may have therapeutic implications in tumor immunotherapy.

A single-step, sensitive flow cytofluorometric assay for the simultaneous assessment of membrane-bound and ingested Candida albicans in phagocytosing neutrophils

BACKGROUND

Distinguishing ingested particles from those attached to the cell surface is an essential requirement when performing quantitative studies of phagocytosis. In the present report, we describe a simple, sensitive and reliable flow cytofluorometric method that achieves this goal in a Candida albicans-human neutrophils (PMN) system.

METHODS

The assay is based on the observation that the vital dye trypan blue (TB), while quenching the green fluorescence of fluorescein-labeled C. albicans, causes them to fluoresce red. PMN were incubated with fluorescein-labeled yeast particles for the required time. Aliquots of the incubation mixtures were then promptly diluted with an equal volume of a TB solution at pH 4.0, and subsequently analyzed by flow cytometry for green and red fluorescence.

RESULTS

Since TB does not penetrate into the cells, ingested yeasts retain their green fluorescence, while membrane-bound particles display a red fluorescence.

CONCLUSIONS

Our fluorescence flow cytometric method enables to simultaneously distinguish, within the leukocyte population, cell subsets with attached and ingested yeast particles. Its major features are: (1) accuracy, sensitivity and reproducibility; (2) no further sample manipulations after completion of phagocytosis; (3) possibility of counting free, attached and internalized yeast particles; and (4) use of a nontoxic reagent (TB).

Candida albicansCan Stimulate Stromal Cells Resulting in Enhanced Granulopoiesis

Previously, we have reported that although unperturbed granulocyte colony-stimulating factor (GCSF)-deficient (G-CSF-/-) mice are neutropenic, when challenged with Candida albicans, they develop a profound neutrophilia. In an attempt to understand the basis of Candida-induced neutrophilia in G-CSF-deficient mice, we have modified the Dexter bone marrow culture system to produce an in vitro model that mimics emergency granulopoiesis in vivo. In this model, stromal cultures are overlaid with bone marrow cells in the presence or absence of heat-inactivated (HI) Candida. Irrespective of the genotype of mice used as a source of bone marrow-derived stromal cells, stimulation of these cultures with HI Candida led to a significantly greater recovery of cells compared to unstimulated stromal cultures. In addition, there was a marked increase in the number of colony-forming units granulocyte-macrophage (CFU-GM), as well as in the percentage of granulocytes in the population of nonadherent cells recovered from HI Candida-stimulated cultures. The conditioned medium generated from stromal cultures derived from either wild-type or G-CSF-/- mice exposed to HI Candida, when applied to bone marrow cells in a soft agar clonogenic assay stimulated M-, GM-, and G- type colonies. Interleukin-3 (IL-3) and GM-CSF could not be detected in the conditioned medium from either HI Candida stimulated or unstimulated stromal cultures. However, IL-6 was detected in the conditioned media from both wild-type and G-CSF-/- stromal cultures. Addition of anti-IL-6 antibody significantly impaired granulopoiesis in unstimulated and HI Candida-stimulated, wild type, and G-CSF-/- stromal cultures. Conditioned medium generated from G-CSF/IL-6-deficient stromal cells had the capacity to stimulate bone marrow cells to form colonies comprised of granulocytes and macrophages in soft agar clonogenic assay. This study demonstrates that stromal cells can be stimulated with HI Candida and gives an insight into Candida mediated granulopoiesis.

NK cells mediate increase of phagocytic activity but not of proinflammatory cytokine (interleukin-6 [IL-6], tumor necrosis factor alpha, and IL-12) production elicited in splenic macrophages by tilorone treatment of mice during acute systemic candidiasis

The participation of NK cells in the activation of splenic macrophages or in resistance to systemic candidiasis is still a matter of debate. We had previously reported that there is a correlation between natural killer cell activation and resistance to systemic candidiasis. In those experiments we had used tilorone to boost NK cell activity in mice. Here we show a mechanism elicited by tilorone in splenic macrophages which could explain their effect on mouse survival during acute disseminated Candida albicans infection. The results demonstrate that tilorone treatment elicits, by a direct effect, the production of proinflammatory cytokines (interleukin-6 [IL-6], tumor necrosis factor alpha [TNF-alpha], and IL-12) by splenic macrophages. In addition, it increases the capacity of splenic macrophages to phagocytize C. albicans through activation of NK cells. We also demonstrate that the presence of NK cells is essential for maintaining a basal level of phagocytic activity, which characterizes splenic macrophages of naïve control mice. The results demonstrate that it is possible to identify two phenotypically and functionally peculiar cell populations among splenic macrophages: (i). cells of the "stimulator/secretor phenotype," which show high levels of major histocompatibility complex (MHC) class II surface expression, are poorly phagocytic, and synthesize the proinflammatory cytokines IL-6, TNF-alpha, and IL-12, and (ii). cells of the "phagocytic phenotype," which express low levels of MHC class II molecules, are highly phagocytic, and do not secrete proinflammatory cytokines.

Phagocytosis of apoptotic cells assessed by flow cytometry using 7-Aminoactinomycin D

BACKGROUND

Apoptotic cells are recognized specifically by macrophages and are cleared rapidly by phagocytosis. However, the recognition mechanisms involved in the clearance of apoptotic cells by macrophages are still not fully understood. Therefore, new methods must be designed to better our understanding of the mechanisms of interaction between macrophages and apoptotic cells. 7-Aminoactinomycin D (7-AAD) is a fluorescent DNA-binding stain usually used as a single agent to detect apoptotic cells by flow cytometry. We propose the use of 7-AAD-stained apoptotic cells as targets for a new flow cytometry phagocytosis assay.

METHODS

Murine T-cell lymphoma YAC-1 cells were treated with etoposide to induce apoptosis. Etoposide-treated YAC-1 target cells were stained subsequently with 7-AAD and then coincubated with resident peritoneal macrophages to allow phagocytosis. The samples were analyzed by flow cytometry. Macrophages that had phagocytosed 7-AAD-stained apoptotic cells were identified by their bright red fluorescence and the resulting values were expressed as the percentage of cells.

RESULTS

The phagocytic cells appeared as a distinct population characterized by bright fluorescence, which could not be detected in the negative controls. The effects of a phagocytic enhancer (interferon-gamma [IFN-gamma]) or inhibitor (incubation at 4 degrees C) were assessed accurately with this flow cytometric method.

CONCLUSIONS

We describe the use of 7-AAD in an assay that is easy and quick to perform. This flow cytometric-based assay allows the quantification of phagocytosis of apoptotic cells by macrophages.

Suppression of splenic macrophage Candida albicans phagocytosis following in vivo depletion of natural killer cells in immunocompetent BALB/c mice and T-cell-deficient nude mice

The resistance of mice to systemic infections caused by Candida albicans is associated with activated splenic macrophages. In addition, there is a correlation between natural killer (NK) cell activation and the resistance to systemic candidiasis. The present study was designed to clarify the role of NK cells in the control of splenic macrophage C. albicans phagocytosis by either depleting NK cells (anti-asialo GM(1) treatment) or maintaining them in an activated state (tilorone treatment) in both immunocompetent BALB/c mice and T-cell-deficient nude mice. The results of the in vitro phagocytosis assays were analyzed by flow cytometry and demonstrate the pivotal role of NK cells in controlling the capacity of splenic macrophages to phagocytose C. albicans. In summary, these data provide evidence that the NK cells are the main inducers of phagocytic activity of splenic macrophages and that they mediate the protection against C. albicans systemic infection.

Use of SYTOX green dye in the flow cytometric analysis of bacterial phagocytosis

BACKGROUND

Fluorescein isothiocyanate (FITC) is used widely to label the targets used in flow cytometric phagocytosis assays. Unfortunately, the fluorescence intensity of phagocytosed FITC-labeled targets is influenced by changes in intracellular pH level, making quantitative measurements with this fluorophore problematic. We describe the use of SYTOX green nucleic acid stain to measure phagocytosis by flow cytometry.

METHODS

Suspensions of isopropyl alcohol-permeabilized Escherichia coli DH5alpha were stained with the SYTOX green dye and then incubated with resident peritoneal macrophages. The samples were analyzed by flow cytometry and phagocytosis was determined by gating the cells.

RESULTS

Results are expressed as percentage of phagocyte-associated green fluorescent cells. The validity of the method was shown by the effects of a phagocytosis inhibitor (incubation at 4 degrees C) or enhancer (gamma interferon [IFN- gamma] treatment) being accurately assessed with this assay.

CONCLUSIONS

The method described was reproducible and provides an advantageous alternative to the use of FITC to label bacteria for the flow cytometric measurement of target uptake by phagocytic cells.