Role of nitric oxide and melanogenesis in the accomplishment of anticryptococcal activity by the BV-2 microglial cell line

In the present paper, we investigated the involvement of cryptococcal melanogenesis and macrophage nitric oxide (NO) production in the accomplishment of anticryptococcal activity by microglial effector cells, using the murine cell line BV-2. We demonstrate that the constitutive levels of anticryptococcal activity exerted by BV-2 cells is significantly enhanced upon interferon gamma plus lipopolysaccharide treatment. The phenomenon, which occurs with no enhancement of phagocytic activity, is associated with the production of high levels of NO and is abolished by addition of NG-monomethyl-L-arginine. Comparable patterns of results are observed employing either unopsonized or opsonized microbial targets, the latter microorganisms being markedly more susceptible to BV-2 cell antimicrobial activity. Furthermore, melanization of Cryptococcus neoformans significantly reduces its susceptibility to BV-2 antimicrobial activity, regardless of the fact that activated macrophages or opsonized microorganisms have been employed. In conclusion, our results provide evidence that NO-dependent events are involved in the fulfillment of anticryptococcal activity by activated microglial cells and that fungal melanization is a precious escamotage through which C. neoformans overcomes host defenses.

Systemic infection with Herpes bovis virus 2 evokes a biphasic immune response in the mouse

We evaluated the effects of systemic infection by Herpes bovis virus 2 (HBV-2) on a murine experimental system. We provide evidence that such infection is lethal for the immunocompromised but not for the immunocompetent mouse in which a biphasic immune response is elicited. In particular, 1 day post-infection, we observed a rapid transient depression induced by the virus, as documented by a decrease in peripheral leukocyte counts, mitogenic spleen cell response and resistance to a secondary microbial challenge. Later, HBV-2 infection boosted cytokine secretion and enhanced antimicrobial and antitumoral activities by the splenic district. In conclusion, our experimental model discloses some immunological aspects underlying the complex host-virus interaction.

Different events involved in the induction of macrophage tumor necrosis factor by Candida albicans and lipopolysaccharide

Using an in vitro experimental model, we have recently demonstrated that Candida albicans in its hyphal form (H-Candida), similarly to lipopolysaccharide (LPS), enhances tumor necrosis factor (TNF) secretory response in the cloned macrophage (M phi) population ANA-1. Here we show that H-Candida and LPS each differ in their requirements for intact protein kinase functions, susceptibility to 0.4-microns micropore-size membranes, and sensitivity to polymyxin B. These results, together with the synergistic effect occurring between H-Candida and LPS in inducing TNF response, indicate the existence of different receptor(s) and/or signal-transduction pathway(s) through which the two stimuli act.

Comparative studies on functional and secretory properties of macrophage cell lines derived from different anatomical sites

In the present study, we compared four macrophage (M phi) cell lines from different anatomical origins for functional and secretory activities against the two morphogenetic forms of the fungus Candida albicans. We show that all the cell lines actively phagocytize the yeast and exert antimicrobial activity against both forms of Candida, although M phi of microglial origin are the most effective. When assessed for secretory properties, microglial M phi exhibit a peculiar pattern with respect to other M phi populations under either basal or stimulated conditions. In particular, only microglial M phi fail to respond to the hyphal form of the fungus (H-Candida), which instead acts as a potent tumor necrosis factor inducer in the other M phi cell lines. When exposed to H-Candida, microglial M phi are indistinguishable from other M phi in their ability to modulate specific surface adhesion molecules. In addition to strengthening the knowledge on functional heterogeneity among M phi, our data provide evidence on the peculiar behavior of microglial M phi. To what extent M phi heterogeneity may be related to tissue homeostasis is discussed.

Anticryptococcal resistance in the mouse brain: beneficial effects of local administration of heat-inactivated yeast cells

Using a murine model, we have previously shown that brain resistance to local infection with opportunistic fungi is affected by manipulation of the host myelomonocytic compartment. Here, we demonstrate that intracerebral administration of heat-inactivated Cryptococcus neoformans (H-CN) yeast cells results in a consistent enhancement of mouse survival to subsequent local challenge with lethal doses of C. neoformans. The phenomenon, more pronounced upon double H-CN treatment, is associated with (i) massive local inflammatory response, (ii) reduced growth of the fungus within the brain, and (iii) induction of delayed-type hypersensitivity. Moreover, H-CN treatment confers protection against local heterologous challenges. Our data provide initial evidence that intracerebral administration of H-CN results in the establishment of aspecific and specific immune responses; the mechanisms of elicitation and relative contributions to host antimicrobial resistance remains to be elucidated.

Pattern of cytokine gene expression in brains of mice protected by picolinic acid against lethal intracerebral infection with Candida albicans

Recently, we demonstrated that intracerebral (i.c.) administration of picolinic acid (PLA) confers protection against a lethal local challenge with the opportunistic pathogen Candida albicans. By histopathological studies, we show here that mice receiving PLA treatment survive challenge and no evidence of fungal invasion is found within the brain compartment. In contrast, PLA-untreated mice succumb to infection within 7-10 days and show massive brain colonization with extensive granulomatous reaction. By PCR analysis, we show that, unlike naive brains, PLA-treated brains show transient activation of TNF alpha, IL-1 beta and IL-6 genes. C. albicans infection results in high levels of all cytokine transcripts, the phenomenon being long-lasting in PLA-untreated brains, while gradually declining in PLA-treated brains. The only exception is IL-1 beta, whose levels remain high at the latest time-points tested, also in PLA-treated brains. Finally, IL-1 alpha, constitutively detectable in naive brains, is slightly enhanced by C. albicans challenge, regardless of prior treatment. These findings, together with the knowledge that PLA is a potent co-stimulus for macrophages, suggest the involvement of cytokine circuits, likely of macrophage origin, in anti-Candida resistance established by PLA at the cerebral level.

Tumor necrosis factor as an autocrine and paracrine signal controlling the macrophage secretory response to Candida albicans

We have previously demonstrated that the hyphal form of Candida albicans (H-Candida), but not the yeast form (Y-Candida), acts as a macrophage-stimulating agent. The early response (1 to 3 h) of the macrophage cell line ANA-1 to H-Candida results in enhanced tumor necrosis factor (TNF) transcription and production. Here we show that when coincubation times are prolonged (3 to 24 h), Y-Candida also exhibits stimulatory properties. This phenomenon has been ascribed to the occurrence of the dimorphic transition, as demonstrated by microscopic evaluation of the cultures and by experiments in which both killed Y-Candida and the agerminative strain C. albicans PCA-2 failed to induce cytokine production. TNF produced in response to H-Candida acts as an autocrine and paracrine signal controlling the macrophage secretory response to C. albicans. In fact, addition of anti-TNF polyclonal antibodies to the coculture of ANA-1 macrophages and H-Candida results in a marked and time-dependent decrease of TNF transcript levels. Moreover, pretreatment of macrophages with recombinant TNF for 3 h enhances TNF and induces interleukin-1 production in response to both forms of Candida, while pretreatment for 18 h renders macrophages refractory to any stimuli. Interestingly, the kinetics of interleukin-1 transcription and secretion in response to H-Candida are delayed with respect to those of TNF. Overall, these data indicate that TNF, produced by macrophages in response to H-Candida, regulates its own production as well as that of other soluble factors, thus suggesting that this cytokine plays multiple roles in the immune mechanisms involved in Candida infection.

Heterogeneous secretory response of phagocytes from different anatomical districts to the dimorphic fungus Candida albicans

In the present study, we have examined the ability of phagocytes from different anatomical districts to discriminate between the two morphogenetic forms of Candida albicans. We have demonstrated that resident peritoneal macrophages (RP-M phi) and thioglycollate-elicited peritoneal macrophages (TP-M phi) were able to distinguish between the hyphal (H-Candida) and the yeast (Y-Candida) form of the fungus, since TNF production was observed only upon exposure of RP-M phi and TP-M phi to H-Candida. In contrast, splenic macrophages (S-M phi), bone marrow-derived macrophages (BM-M phi) and polymorphonuclear neutrophils (PMN) did not discriminate between the two forms because S-M phi and PMN produced TNF regardless of the morphogenetic status of the fungus, while BM-M phi did not. Under the same experimental conditions, we failed to observe IL-1 production from any of the phagocytic cell populations examined, with the exception of PMN. This implies that the interaction between phagocytes and C. albicans triggers differential secretory responses depending upon the morphogenetic status of the fungus and the anatomical localization of the immune cells.

Tetanus toxin selectively impairs anti-tumoral but not anti-microbial macrophage-mediated effector functions

The present study was designed to establish the susceptibility of macrophage-mediated effector functions to tetanus toxin (TT). Using the murine macrophage cell line, GG2EE, generated in vitro by v-raf/v-myc oncogenes, we have previously provided evidence that TT selectively inhibits interferon gamma (IFN-gamma), but not basal, lysozyme activity. Here we show that while neither phagocytic nor candidacidal activities are affected by TT treatment, antitumoral activity is significantly impaired after exposure to TT. This phenomenon, which is dose-dependent, is fully ascribed to the holotoxin, as heat inactivated TT, C or A-B fragments result ineffective. Furthermore, C but not A-B fragment competes with TT in abrogating its inhibitory effects. Overall, these data indicate that TT is not a broad-spectrum, down-regulating signal on macrophage-mediated functions, thus implying that its toxic action is exerted on specific molecular targets.

Protective effect of picolinic acid on mice intracerebrally infected with lethal doses of Candida albicans

We have studied the effects of picolinic acid (PLA), a product of tryptophan degradation, on mouse susceptibility to intracerebral infection with Candida albicans. We show that intraperitoneal administration of PLA significantly enhances the median survival time of mice inoculated with the lethal challenge. Furthermore, intracerebral administration of this agent induces a protective state against the local lethal infection, the phenomenon depending upon the administration schedule and doses of PLA employed. According to survival data, yeast growth in the brain as well as yeast colonization of the kidneys are drastically reduced in PLA-treated mice compared with those for untreated controls. Northern (RNA) blot analysis of brain tissues demonstrates that mRNA levels specific for tumor necrosis factor and interleukin 1 are augmented and induced, respectively, after inoculation of PLA. These results indicate that PLA has a protective effect likely involving elicitation of a cytokine response in vivo against fungal infections.

Toxic effects of tetanus toxin on GG2EE macrophages: prevention of gamma interferon-mediated upregulation of lysozyme-specific mRNA levels

By using a nonneuronal cell system, evidence has previously been provided that tetanus toxin (TT) intoxication occurs in macrophages, impairing their secretory activity as well as their antitumoral activity. In particular, both secreted and total lysozyme (LZM) activities are reduced by TT treatment, provided that GG2EE macrophages have been preexposed to gamma interferon (IFN-gamma). In an attempt to provide insight into the molecular mechanisms underlying this phenomenon, we focused our attention on the levels of LZM-specific transcripts. GG2EE macrophages preexposed to IFN-gamma exhibited augmented levels of LZM-specific mRNA. Such an effect was detected 1 h after removal of IFN-gamma, peaked at 3 h, and gradually decreased with time in culture. Exposure of IFN-gamma-pretreated GG2EE macrophages to TT resulted in the prevention of the IFN-gamma-mediated upregulation of LZM mRNA levels. The phenomenon was mediated by the holotoxin (> or = 1 micrograms/ml) and abrogated by preexposure of the macrophages to the C fragment of TT. Protein kinase C (PKC) and Ca(2+)-calmodulin-dependent PK were likely involved in the IFN-gamma-mediated upregulation of LZM mRNA levels and biological activity, as assessed by PK inhibitors. Furthermore, PK inhibitors mimicked TT in impairing LZM activity of GG2EE macrophages, thus suggesting that impairment of PKC and/or the Ca(2+)-calmodulin-dependent PK pathway(s) may be one of the events involved in TT intoxication of macrophages.

Differential host susceptibility to intracerebral infections with Candida albicans and Cryptococcus neoformans

To investigate the immune defense mechanisms employed against fungi in the brain, mice were experimentally infected by intracerebral inoculation of Candida albicans or Cryptococcus neoformans. Parameters such as median survival time and numbers of yeast cells in the brains were assessed for naive and immunomodulated mice. We found that no mice survived either C. albicans or C. neoformans challenge at doses of > or = 10(6) yeast cells per mouse. However, when the inoculum size was decreased (< or = 10(5) yeast cells per mouse), C. albicans was no longer lethal (100% survival), whereas 100 and 70% of the mice still succumbed to challenge doses of 10(4) and 10(3) C. neoformans yeast cells, respectively. Pharmacological manipulation and transfer experiments revealed that the myelomonocytic compartment had a minor role against C. neoformans but was deeply involved in the control of intracerebral C. albicans infection. By counting the number of yeast cells in the brains of naive and immunomodulated animals, we established that, unlike C. albicans, C. neoformans remained essentially in the brain, where massive colonization and damage occurred whether naive or immunomodulated defense mechanisms were employed by the host. Overall, these data suggest that the differential role of the myelomonocytic compartment, together with the diverse tropisms of the two fungi, can explain the different development and outcome of intracerebral C. albicans and C. neoformans infections.

Inhibition of proliferation of retrovirus-immortalized macrophages by LPS and IFN-gamma: possible autocrine down-regulation of cell growth by induction of IL1 and TNF

The GG2EE macrophage tumor cell line was previously established by immortalization of C3H/HeJ mouse bone marrow cells with the J2 retrovirus which contains the v-myc and v-raf oncogenes. Studies on the control of GG2EE cell proliferation in vitro have recently been performed. We observed that the combination of 5-25 U/ml recombinant mouse interferon-gamma (rmIFN-gamma) plus 0.03-0.3 micrograms/ml lipopolysaccharide (LPS) markedly inhibited the proliferation of GG2EE cells (by greater than 95%) in vitro, while either agent alone inhibited only by less than 40% and 0-10%, respectively. Subsequent studies established that biologically active IL1-like (2-4 U/ml) and TNF alpha-like (50-100 U/ml) activities were released into the supernatants of LPS-treated GG2EE cells. The combination of IFN-gamma + LPS induced more (6-8 U/ml) IL1 release. These results suggested that the inhibition of proliferation of GG2EE cells by IFN-gamma + LPS could have been mediated in part by cytokines produced by the cells themselves. rhIL1 alpha at a concentration of 10 U/ml inhibited GG2EE proliferation by 25-30%, while rmIFN-gamma (25 U/ml) + rhIL1 alpha (10 U/ml) inhibited proliferation by 98%. Thus, 10 U/ml rhIL1 alpha could completely replace LPS in the LPS + rmIFN-gamma combination. Further, the combination of low doses of rhIL1 alpha (0.1 to 1 U/ml) plus rmTNF alpha (250 U/ml), which together inhibited proliferation by less than 20% synergized with doses of 5 to 25 U/ml rmIFN-gamma to inhibit proliferation of GG2EE cells by 98-99%. These results suggest that cytokines produced by the cells themselves can synergize with rmIFN-gamma to inhibit the oncogene-driven proliferation of GG2EE cells.

Experimental model of intracerebral infection with Cryptococcus neoformans: roles of phagocytes and opsonization

A murine model of intracerebral (i.c.) infection with Cryptococcus neoformans in which naive mice receiving an i.c. fungal inoculation developed a severe disease has been established. The effect was strictly dependent on the number of microorganisms injected and evolved as lethal meningoencephalitis. Murine susceptibility to i.c. infection with C. neoformans was enhanced by treatment with chloroquine and colchicine, agents known to greatly affect the host phagocytic compartment. Furthermore, the life spans of both naive and drug-treated mice were significantly augmented when opsonized fungi were injected. Therefore, phagocyte-mediated mechanisms are likely involved in local resistance to i.c. infection with C. neoformans. Further support for this conclusion was supplied by in vitro data showing that microglial cells were proficient anticryptococcal effectors, provided opsonized microorganisms were used.

Calcium ionophore A-23187 inhibits the secretion of beta-hexosaminidase from the GG2EE mouse macrophage cell line

Secretion of the lysosomal enzyme beta-N-acetylhexosaminidase is inhibited by calcium ionophore A-23187 in the GG2EE macrophage cell line. Such inhibition is time and dose dependent. Calcium ionophore A-23187 treatment causes a change in the pattern of hexosaminidase isoenzymes detectable in the cell extract, as assessed by DEAE-cellulose chromatography. In particular, control cells show two hexosaminidase isoenzymes corresponding to hexosaminidase A and B, whereas cells treated with calcium ionophore A-23187 express a third isoenzyme form with properties similar to hexosaminidase S.

Candida albicans hyphal form enhances tumor necrosis factor mRNA levels and protein secretion in murine ANA-1 macrophages

We have demonstrated that Candida albicans in its hyphal form (H-Candida) acts as a stimulating agent in the cloned macrophage population ANA-1. Both tumor necrosis factor (TNF) mRNA levels and secreted biological activity augment in ANA-1 macrophages exposed to H-Candida. Such effects are observed at an effector-to-target cell ratio of 1:1 and occur after 1 and 3 hr of coincubation, respectively. The phenomenon is independent of the metabolic status of the fungus, since viable as well as heat-killed H-Candida are comparable in inducing TNF mRNA levels. The extent and kinetics of H-Candida-mediated effects are similar to those observed following exposure of ANA-1 macrophages to lipopolysaccharide (LPS). This implies that C. albicans in its hyphal form is a potent macrophage modulator; whether it acts through the same mechanism(s) as LPS remains to be elucidated.

An immortalized cell line expresses properties of activated microglial cells

Murine cultured microglial cells were immortalized after infection with a v-raf/v-myc recombinant retrovirus. This immortalized cell line (BV-2) shares properties with body macrophages with respect to the antigen profile, their phagocytic capacity and antimicrobial activity. BV-2 cells are not constitutively able to kill tumor cells in vitro, but acquire antitumor activity following an increase in [Ca++]i. BV-2 cells, like microglial cells, are however, distinct from peripheral macrophages by their expression of inwardly rectifying K+ channels in concert with a lack in outwardly rectifying K+ channels and the formation of spineous processes. The BV-2 cell line thus represents a suitable model for in vitro studies of activated microglial cells.

Early differential molecular response of a macrophage cell line to yeast and hyphal forms of Candida albicans

The dimorphic transition of Candida albicans from the yeast (Y-Candida) to the hyphal (H-Candida) form is a complex event; the relevance of this transition in fungal pathogenicity is still poorly understood. By using a cloned macrophage cell line (ANA-1), we questioned whether the interaction between macrophages and Y-Candida or H-Candida could affect specific cell functions, i.e., tumor necrosis factor and lysozyme production. We found that ANA-1 macrophages selectively responded to H-Candida with increased tumor necrosis factor and downregulated lysozyme, as assessed by measurement of relative mRNA levels and secreted biological activities. The H-Candida-mediated effects were (i) dependent upon the ratio between ANA-1 macrophages and H-Candida, (ii) detectable after 1 h of coincubation, and (iii) accomplished without fungal ingestion. Conversely, Y-Candida, which was found inside the ANA-1 macrophages, did not affect tumor necrosis factor and lysozyme production, nor did it prevent the macrophage response to other stimuli. Overall, these results indicate that a macrophage can distinguish between Y-Candida and H-Candida and that only the latter is able to modulate specific functions. H-Candida is recognized and probably processed as an extracellular target. The possible implication of macrophages as autocrine and paracrine regulatory cells during Candida infections is discussed.

Fungicidal activity of Candida albicans-induced murine lymphokine-activated killer cells against C. albicans hyphae in vitro

Multiple intraperitoneal injections of inactivated Candida albicans cells resulted in the generation of cytotoxic peritoneal cells with phenotypical and functional properties similar to in vitro-generated lymphokine-activated killer (LAK) cells. Using an in vitro [3H]glucose uptake assay, C. albicans-induced LAK-like (CA-LAK) cells exhibited high levels of anti-hyphal activity, the effects being effector to target cell (E:T) ratio- and time-dependent. Maximal levels of anti-C. albicans activity (approximately 60%) were observed after 4 h and at E:T greater than or equal to 300:1. Similar patterns of anti-C. albicans activity were exerted by in vivo-activated natural killer (NK) cells, in vitro interleukin-2- (IL-2) generated LAK cells and polymorphonuclear cells. The anti-hyphal activity of CA-LAK cells was enriched by separation on a Percoll gradient, F2 and F3 fractions retaining most of the activity. Experiments using immunodepressed animals demonstrated that the in vivo lethality of the C. albicans hyphal form is significantly affected by in vitro pre-exposure to CA-LAK cells. While control mice receiving C. albicans alone had a median survival time of 2 d, mice receiving C. albicans pre-exposed to CA-LAK cells (E:T = 300:1) had a median survival time of 15 d. Overall, the susceptibility of the C. albicans hyphal form to CA-LAK cells suggests that C. albicans-induced effectors might play a significant role as a second-line defence mechanism against the C. albicans hyphal form.

Microglial cell-mediated anti-Candida activity: temperature, ions, protein kinase C as crucial elements

An in vitro established microglial cell line, BV-2, constitutively exhibits high levels of anti-Candida activity. To elucidate the cascade of events leading to the accomplishment of such activity, we studied its dependence on temperature and ion availability. The role of protein kinases has also been studied by the specific inhibitors, 1-(5-isoquinolinesulfonyl)-2-methylpiperazine dihydrochloride (H7) and N-(2-guanidinoethyl)-5-isoquinoline sulfonamide hydrochloride (HA 1004). We found that (a) the BV-2 cell/Candida conjugate formation is a discrete step, temperature-, ion- and protein kinase-independent; (b) the phagocytic event, which is protein kinase-independent, is significantly impaired by temperature decrease and ion deprivation; (c) the fulfillment of anti-Candida effects is strictly dependent upon temperature, ion availability and functional protein kinase. Functional protein kinase C, but not other kinases, is required for the accomplishment of anti-Candida activity, which, in fact, is selectively abrogated by H7 but not HA. Furthermore, protein kinase C activators, such as 12-O-tetradecanoylphorbol 13-acetate (TPA) or 1-oleoyl-2-acetyl glycerol (OAG), consistently potentiate BV-2 cell-mediated anti-Candida activity, the phenomena being dose-dependent. These results indicate that the multistep events leading a microglial cell to express anti-Candida activity can be dissected and differentiated for biochemical and biological demands, the latest along the cascade being the most demanding steps.

Intracerebral transfer of an in vitro established microglial cell line: local induction of a protective state against lethal challenge with Candida albicans

An in vitro generated BV-2 microglial cell line has been transferred intracerebrally into syngeneic immunocompetent mice prior to local challenge with Candida albicans. The transfer resulted in the establishment of local protection against a lethal dose of C. albicans, which was accompanied by an impairment of yeast growth in the brain and kidneys. Upon histological examination of brain sections from BV-2 cell-pretreated mice, it was found that the size and number of granulomas was reduced as compared to untreated controls receiving Candida alone. These observations provide direct evidence that microglia play a crucial role in the local defense against intracerebral infections.

A rapid Candida albicans hyphal-form growth inhibition assay: determination of myelomonocytic-mediated antifungal activity

An in vitro microassay for the measurement of Candida albicans hyphal-form growth inhibition by myelomonocytic cells is described. The assay is rapid, easy-to-perform and objective. A Candida strain capable of in vitro dimorphic transition from yeast to hyphal form has been employed. The assay is based on the incorporation of 3H-glucose by the fungus, the effect being dependent upon the time of pulse, size of the inoculum and concentration of radiolabelled metabolite. In particular, C. albicans hyphal form, obtained by a 3 h incubation in vitro in the presence of 10% fetal calf serum, is co-incubated with the effector cells. A pulse with 3H-glucose in water is then performed and the radioactivity incorporated by the residual Candida is taken as an indication of hyphal growth. We found that polymorphonuclear cells, peritoneal macrophages and the cloned GG2EE macrophage cell line significantly inhibited hyphal growth, the effects being time and effector-to-target cell ratio dependent.

A rapid objective immunofluorescence microassay. Application for detection of surface and intracellular antigens

An indirect immunofluorescence microassay, which permits automated reading, has been employed for simple, rapid and objective detection of surface and intracellular antigens. Initially, the cells, spun in microplates, are fixed with glutaraldehyde (0.25% v/v in PBS). Following fixation, the cells can be stored at 4 degrees C for up to 2 weeks before being used in the immunofluorescence microassay. The fixed cells are then stained according to standard procedures using appropriate first and fluorescein-conjugated second antibodies. An automated and quantitative evaluation of the fluorescence intensity of the cell samples was achieved using the Titertek Fluoroskan II automatic reader. This microassay was shown to be suitable for the detection of the surface MAC1 antigen and intracellular v-myc protein in the GG2EE macrophage cell line.