Immune evasion by acquisition of complement inhibitors: the mould Aspergillus binds both factor H and C4b binding protein.

Pathogenic fungi represent a major threat particularly to immunocompromised hosts, leading to severe, and often lethal, systemic opportunistic infections. Although the impaired immune status of the host is clearly the most important factor leading to disease, virulence factors of the fungus also play a role. Factor H (FH) and its splice product FHL-1 represent the major fluid phase inhibitors of the alternative pathway of complement, whereas C4b-binding protein (C4bp) is the main fluid phase inhibitor of the classical and lectin pathways. Both proteins can bind to the surface of various human pathogens conveying resistance to complement destruction and thus contribute to their pathogenic potential. We have recently shown that Candida albicans evades complement by binding both Factor H and C4bp. Here we show that moulds such as Aspergillus spp. bind Factor H, the splicing variant FHL-1 and also C4bp. Immunofluorescence and flow cytometry studies show that the binding of Factor H and C4bp to Aspergillus spp. appears to be even stronger than to Candida spp. and that different, albeit possibly nearby, binding moieties mediate this surface attachment.

Influence of amphotericin B and amphotericin B colloidal dispersion on the functions of human phagocytes in defence against Aspergillus species.

This study investigated the in vitro effects of amphotericin B and amphotericin B colloidal dispersion (ABCD) on phagocytosis and inhibition of germination of clinical isolates of Aspergillus spp. by monocyte-derived macrophages (MDMs). Both amphotericin B and ABCD caused significant reductions in uptake of conidia of Aspergillus spp. by MDMs (p < 0.01). The inhibition of germination was superior with conidia of Aspergillus fumigatus, Aspergillus niger and Aspergillus terreus isolates. Aspergillus flavus growth was significantly less inhibited of either antimycotic as compared to A. fumigatus and A. terreus (p < 0.01).We demonstrate that amphotericin B or ABCD acts as a potent inhibitor of Aspergillus germination. By contrast, treatment of MDMs with these antimycotics diminished phagocytosis of conidia in vitro.

Effect of increasing inoculum sizes of Aspergillus hyphae on MICs and MFCs of antifungal agents by broth microdilution method.

In order to investigate the influence of different hyphal inoculum sizes on minimal inhibition concentrations (MICs) and minimum fungicidal concentrations (MFCs) of amphotericin B (AMB), voriconazole and itraconazole, five isolates each of Aspergillus fumigatus, Aspergillus flavus, Aspergillus niger and Aspergillus terreus were studied using a broth microdilution method. Three inoculum sizes were used: 1 x 10(3)-5 x 10(3), 1 x 10(4)-5 x 10(4) and 1 x 10(5)-5 x 10(5) cfu/ml. MICs and MFCs were read at 24 and 48 h at 35 degrees C. For all species tested, AMB MICs and MFCs were minimally affected by inoculum size on. However inoculum size significantly affected MICs and MFCs of voriconazole and itraconazole; there was an increase of up to 6-fold in MICs and MFCs for the various aspergilli when the inoculum increased from 10(3) to 10(5) cfu/ml (P<0.05). Thus azoles showed significant inoculum effects, while AMB showed comparatively minimum inoculum effects against Aspergillus spp.

In vitro studies on the activity of amphotericin B and lipid-based amphotericin B formulations against Aspergillus conidia and hyphae.

The minimum inhibitory concentrations (MICs) of amphotericin B and lipid-based amphotericin B formulations against isolates of Aspergillus spp. were tested using a broth microdilution method. Twelve isolates of Aspergillus fumigatus, eight of Aspergillus flavus, six of Aspergillus niger and seven of Aspergillus terreus were examined. In addition, an assay for hyphae of Aspergillus spp. was performed since the invasive form is manifested by the appearance of hyphal structures. MICs of hyphae against lipid-based amphotericin B formulations were within three dilutions higher than those against conidia for almost all isolates of Aspergillus spp. (P < 0.01). In contrast, the differences in the in vitro efficacies of amphotericin B were the lowest for Aspergillus spp. This study demonstrates the importance of the type of inoculum used to test antifungal susceptibilities of Aspergillus spp. The significance of these results for in vivo outcome needs to be determined.

Complement-dependent control of viral dynamics in pathogenesis of human immunodeficiency virus and simian immunodeficiency virus infection.

Since the first contact with the host, human immunodeficiency virus (HIV) exploits the complement system to reach maximal spread of infection. HIV has adapted many strategies to avoid complement-mediated lysis and uses the opsonization with complement fragments for attachment to complement receptors (CR). From the pathogen's perspective, binding to CR-expressing cells is remarkably beneficial, bringing together virus and activated target cells that are highly susceptible to infection. Moreover, complement-mediated trapping on CR+ cells permits HIV to infect surrounding cells even in the presence of an excess of neutralizing antibodies. Thus, complement activation initiates the assumption of power over the host's immune system by HIV and thus augments viral spread and replication throughout the body. On the other hand, natural hosts of primate lentiviruses, such as sooty mangabeys, African green monkeys and chimpanzees, are generally considered to be resistant to the development of AIDS, despite persistent viral replication. This review focuses on the possible link between the resistance to disease and species-specific diversity in function of human and monkey complement system.

HIV protease inhibitors attenuate adherence of Candida albicans to epithelial cells in vitro.

Oropharyngeal candidiasis is one of the first and most commonly reported opportunistic infections of untreated AIDS patients. With the introduction of the new antiviral HAART therapy, including HIV protease inhibitors, this mucocutaneous infection is nowadays only rarely observed in treated patients. It was recently shown that HIV protease inhibitors have a direct attenuating effect on Candida albicans secreted aspartic proteinases (Saps), an investigation prompted by the fact that both Sap and HIV protease belong to the superfamily of aspartic proteinases and by the observation that mucocutaneous infections sometimes resolve even in the absence of an immunological improvement of the host. As these Saps are important fungal virulence factors and play a key role in adhesion to human epithelial cells we tried to assess the effect of the HIV protease inhibitors Ritonavir, Indinavir and Saquinavir on fungal adhesion to these cells. The effect on phagocytosis by polymorphonuclear leukocytes was also assessed. Ritonavir was found to be the most potent inhibitor of fungal adhesion. A dose-dependent inhibition of adhesion to epithelial cells was found already at 0.8 microM and was significant at 4 microM or higher, at 500 microM the inhibition was about 55%. Indinavir and Saquinavir inhibited significantly at 4 microM or 20 microM, respectively; at 500 microM the inhibition was 30% or 50%. In contrast, no protease inhibitor was able to modulate phagocytosis of Candida by polymorphonuclear leukocytes. In conclusion, inhibition of Saps by HIV protease inhibitors may directly help to ease the resolution of mucosal candidiasis. In future, derivatives of HIV protease inhibitors, being more specific for the fungal Saps, may form an alternative in the treatment of mucosal candidiasis insensitive to currently available antimycotics.

The supportive role of complement in HIV pathogenesis.

This review focuses on interactions of HIV with the first-line defence of native immunity, the complement system. In all body compartments tested so far, HIV meets complement. Activation of the complement system results in deposition of C3 fragments on the viral surface, but in contrast to other pathogens, most of HIV is not or is only poorly lysed by membrane attack complexes. To survive complement-mediated lysis, HIV has not only developed resistance mechanisms, but uses opsonisation with complement fragments for its own advantage. Opsonised virions interact with complement receptor-expressing cells, which are either subsequently infected with high efficiency or retain viral particles on their surface, which promotes transmission of virus to other permissive cells. Our knowledge of these mechanisms has increased enormously over the past few years. A complete understanding of these complex interactions of HIV with the complement system opens new perspectives for development of alternative therapeutic strategies.

Human immunodeficiency virus type 1 induces expression of complement factors in human astrocytes.

Since the brain is separated from the blood immune system by a tight barrier, the brain-resident complement system may represent a central player in the immune defense of this compartment against human immunodeficiency virus (HIV). Chronic complement activation, however, may participate in HIV-associated neurodegeneration. Since the level of complement factors in the cerebrospinal fluid is known to be elevated in AIDS-associated neurological disorders, we evaluated the effect of HIV type 1 (HIV-1) on the complement synthesis of brain astrocytes. Incubation of different astrocytic cell lines and primary astrocytes with HIV-1 induced a marked upregulation of the expression of the complement factors C2 and C3. The synthesis of other secreted or membrane-bound complement proteins was not found to be altered. The enhancement of C3 production was measured both on the mRNA level and as secreted protein in the culture supernatants. HIV-1 laboratory strains as well as primary isolates were capable of inducing C3 production with varied effectiveness. The usage of viral coreceptors by HIV-1 was proved to be a prerequisite for the upregulation of C3 synthesis, which was modulated by the simultaneous addition of cytokines. The C3 protein which is secreted after incubation of the cells with HIV was shown to be biologically active as it can participate in the complement cascade.

Studies of in vitro activities of voriconazole and itraconazole against Aspergillus hyphae using viability staining.

The minimal fungicidal concentrations (MFCs) of voriconazole and itraconazole for five clinical isolates each of Aspergillus terreus, Aspergillus fumigatus, Aspergillus flavus, and Aspergillus niger were determined by a broth macrodilution method. Conidial suspensions as inocula were compared to hyphae as inocula since the invasive form of aspergillosis is manifested by the appearance of hyphal structures. In addition, cell viability staining with the dye FUN-1 was performed to assess time-dependent damage of hyphae exposed to various concentrations of the antifungal agents. With conidial inocula the MFC ranges of voriconazole were 0.5 to 4 microg/ml and those of itraconazole were 0.25 to 2 microg/ml, whereas the MFCs (2 to >16 microg/ml) with hyphal inocula were substantially higher (P < 0.01) for both itraconazole and voriconazole. Only minor differences between the tested antifungals were observed since 16 of 20 and 17 of 20 of the isolates of Aspergillus spp. tested appeared to be killed by voriconazole and itraconazole, respectively. The results of FUN-1 viability staining correlated closely to colony counts, but various time- and dose-dependent levels of viability of hyphae were also observed. In conclusion, our study demonstrates the importance of the type of inoculum used to test antifungals and the applicability of FUN-1 staining as a rapid and sensitive method for assaying the viability of hyphae.

Human immunodeficiency virus type 1 Tat binds to Candida albicans, inducing hyphae but augmenting phagocytosis in vitro.

Tat, the human immunodeficiency virus type 1 (HIV-1) transactivating protein, binds through its RGD-motif to human integrin receptors. Candida albicans, the commonest cause of mucosal candidiasis in subjects infected with HIV-1, also possesses RGD-binding capacity. The present study reveals that Tat binds to C. albicans but not to C. tropicalis. Tat binding was markedly reduced by laminin and to a lesser extent by a complement C3 peptide containing the RGD motif, but not by a control peptide. The outgrowth of C. albicans was accelerated following binding of Tat, but phagocytosis of opsonized C. albicans was also increased after Tat binding. Thus, Tat binding promotes fungal virulence by inducing hyphae but may also reduce it by augmenting phagocytosis. The net effect of Tat in vivo is difficult to judge but in view of the many disease-promoting effects of Tat we propose that accelerating the formation of hyphae dominates over the augmentation of phagocytosis.

Culture supernatants of patient-derived Aspergillus isolates have toxic and lytic activity towards neurons and glial cells.

Infection of the central nervous system by the ubiquitous fungi Aspergillus spp. is a life-threatening disease. Therefore we investigated the mechanism of brain damage by fungal infection. To examine whether secretory factors of Aspergillus isolates derived from patients can induce death of different brain cells, culture supernatants of Aspergillus fumigatus, Aspergillus flavus, Aspergillus terreus and Aspergillus niger were added to different astrocytes as well as to the neuroblastoma cell line SK-N-SH, and to the microglial cell line CHME. All four fungal species were shown to secrete toxic factors with neurons being most sensitive against these factors. Very low amounts and short incubation times are sufficient to induce irreversible cell damage, indicating that secreted factors might also affect distant brain regions. Further characterization of the toxic factors revealed that A. fumigatus and A. terreus produced small, heat-stable components whereas the toxic activity of A. niger filtrates was triggered by a high molecular mass factor which could be inactivated by heat. The active component of A. flavus had a molecular mass similar to that of A. niger but was heat-stable and had a significantly lower activity. Taken together these results indicate that secretion of different necrotizing factors might contribute to brain lesions in patients with cerebral aspergillosis.

HIV-1 envelope protein gp41 modulates expression of interleukin-10 and chemokine receptors on monocytes, astrocytes and neurones.

OBJECTIVE: To analyse the effect of HIV-1 transmembrane protein gp41 on cytokine production and chemokine receptor expression in blood and brain. DESIGN: Because previous results had demonstrated that recombinant gp41 contributes to HIV-induced dysfunction of blood immune cells we investigated its effect on interleukin (IL)-10 synthesis and expression of the HIV coreceptors CCR5 and CXCR4 in different human brain cells. METHODS: Astrocytic, microglial and neuronal cell lines were incubated with the extracellular domain of gp41 (aa565-647). Secretion of IL-10 into the medium was measured by ELISA. Chemokine receptor expression was analysed by fluorescence activated cell sorting and by RT-PCR. RESULTS: Incubation of the astrocytic cell line U87 with gp41 induced more than a 10 fold up-regulation of IL-10 secretion. This modulation was shown to be time- and dose-dependent. Use of inhibitors for different signal transduction pathways indicated a similar transduction cascade for the alteration of IL-10 production in astrocytes as in monocytes with participation of cAMP/adenylate cyclase and activation of p70S6 kinase. To a lesser extent IL-10 synthesis was also up-regulated by gp41 in the neuronal cell line SK-N-SH. In all cell types up-regulation of IL-10 paralleled by an enhanced expression of the chemokine receptor and HIV-1 coreceptor CCR5. This up-regulation was driven by IL-10 as shown by use of an IL-10 antibody. Expression of the chemokine receptor CXCR4 was only slightly altered. CONCLUSIONS: These findings suggest a role for gp41 in the modulation of brain-specific host defence, cell migration and cell infectivity by HIV.

Inflammation marker 7,8-dihydroneopterin induces apoptosis of neurons and glial cells: a potential contribution to neurodegenerative processes.

Since neopterin is elevated in the cerebrospinal fluid of patients with inflammatory neurological disorders we investigated the source of neopterin in the brain and a possible contribution of biological active pteridines to the development of brain lesions. Astrocytic, neuronal and microglial cell lines were shown to be negative for neopterin production even after stimulation with interferon-gamma (IFN-gamma) indicating that infiltrating monocytes/macrophages might be responsible for neopterin level in CNS. Whereas neopterin did not affect viability of brain cells, its derivative 7,8-dihydroneopterin (7,8-DHN) induced dose-dependently cellular apoptosis in astrocytes and neurons probably via enhancement of nitric oxide synthase (iNOS) expression. This mechanism might represent a possible link between inflammation in the brain and neurodegeneration.

Tyrosine phosphorylation of a low molecular weight protein induced by RANTES in T-lymphocytes.

The beta-chemokine RANTES, a T-lymphocyte activator, chemoattractant, and inducer of homotypic aggregation, is considered to exert extensive effects on T lymphocytes through either G protein-coupled or protein tyrosine kinase (PTK) signaling pathway. In the present study, we analyzed RANTES-induced signal transduction through PTK as an early event in T-lymphocyte activation. Tyrosine phosphorylation is detected by immunoblots in the human T-cell line H9 after incubation with human recombinant RANTES. The tyrosine phosphorylation of a protein with a molecular mass of about 25 kD is measurable as early as 30 s and maximal at 1-5 min; and is a dose-dependent effect. The phosphorylation response can be abrogated by the tyrosine-kinase inhibitor herbimycin A (HA) but is insensitive to heterotrimeric Galphai protein inhibitor pertussis toxin (Ptx). This phenomenon is also observed in a visible homotypic aggregation response after incubation serum-starved H9 cells with RANTES. The phosphorylation response can not be down-regulated by preincubation with either anti-CC chemokine receptor 5 (CCR5) antibody or HIV-1Bal supernatants. Our results suggest that tyrosine phosphorylation of a protein with molecular mass of about 25 kD via Src-family PTK(s) is an early event in T-lymphocyte activation associated with the homotypic aggregation in response to RANTES.

Dissimilar attenuation of Candida albicans virulence properties by human immunodeficiency virus type 1 protease inhibitors.

The secreted aspartyl proteinase (Sap) of Candida albicans, which is believed to represent an important virulence factor of this opportunistic yeast, and the human immunodeficiency virus type 1 (HIV-1) protease, which is obligatory for the production of infectious virions, both belong to the same family of aspartyl proteinases. We have previously shown that the HIV-1 protease inhibitor Indinavir directly inhibits secretion and proteinase activity of Sap in a dose-dependent manner. Furthermore, at very high concentrations, viability of C. albicans is markedly reduced by Indinavir, indicating that HIV-1 protease inhibitors may possess antifungal activity. We thus proposed that these drugs may add to the resolution of mucosal candidiasis in HIV-1 infected subjects. We have now compared three different HIV-1 protease inhibitors. The rank order of Sap inhibition, already significant at 0.1 mg/ml for all protease inhibitors, was Ritonavir > Indinavir > Saquinavir. However, the cross-reactivity of Ritonavir to pepsin was also more pronounced compared with the other two. Indinavir did not affect Candida viability at concentrations up to 1 mg/ml, in line with our previous study. In contrast, at this concentration Saquinavir was even fungicidal as assessed by three different viability assays (colony formation assay, MTT assay, propidium iodide staining) whereas Ritonavir significantly affected the mitochondrial activity only (MTT assay). No influence on Candida viability was observed for any of the three at concentrations of 0.1 mg/ml or lower. It remains to be examined whether HIV-1 protease inhibitors or derivatives thereof may be suitable for in vivo therapy of subjects suffering from mucosal candidiasis resistant to current antimycotics.

A 60 kD heat-shock protein-like molecule interacts with the HIV transmembrane glycoprotein gp41.

The heat-shock protein hsp60 is typically found in mitochondria, but, in smaller amounts, also in the cell cytoplasm and associated with the cell membrane. Since heat-shock proteins are known to interact with a variety of molecules and since purified HIV-1 particles were described to contain hsp60 molecules, we tested the possibility that a previously described putative receptor for HIV transmembrane protein gp41 is identical to hsp60. The gp41-binding human protein P62 was purified from H9 and Raji cell lysates by a gp41-coupled affinity column. We could show crossreactivity of both polyclonal and monoclonal anti-hsp60 antibodies with the purified P62. In addition we analyzed binding of P18, a soluble gp41 fragment harboring the extracellular domain (Env aa539-684), to recombinant hsp60. Hsp60 bound well to P18-coated ELISA plates whereas HIV-1 surface protein gp120 induced no binding of hsp60. Preincubation of hsp60 with gp41 abolished the binding. The possible role of this molecule as a cofactor in the pathogenesis of HIV disease is discussed.

Epstein-Barr virus infection of human astrocyte cell lines.

Epstein-Barr virus (EBV) is implicated in different central nervous system syndromes. The major cellular receptor for EBV, complement receptor type 2 (CR2) (CD21), is expressed by different astrocyte cell lines and human fetal astrocytes, suggesting their susceptibility to EBV infection. We demonstrated the infection of two astrocyte cell lines, T98 and CB193, at low levels. As infection was mediated by CR2, we used two stable CR2 transfectant astrocyte cell lines (T98CR2 and CB193CR2) to achieve a more efficient infection. We have monitored EBV gene expression for 2 months and observed the transient infection of T98 and T98CR2 cells and persistent infection of CB193 and CB193CR2 cells. The detection of BZLF1, BALF2, and BcLF1 mRNA expression suggests that the lytic cycle is initiated at early time points postinfection. At later time points the pattern of mRNA expressed (EBER1, EBNA1, EBNA2, and LMP1) differs from latency type III in the absence of LMP2A transcription and in the expression of BALF2 and BcLF1 but not BZLF1. A reactivation of the lytic cycle was achieved in CB193CR2 cells by the addition of phorbol esters. These studies identify astrocyte cell lines as targets for EBV infection and suggest that this infection might play a role in the pathology of EBV in the brain.

Human immunodeficiency virus type 1 protease inhibitor attenuates Candida albicans virulence properties in vitro.

The putative virulence factor secreted aspartyl proteinase (SAP) of Candida albicans and the human immunodeficiency virus type 1 (HIV-1) protease both belong to the aspartyl proteinase family. The present study demonstrates that the HIV-1 protease inhibitor Indinavir is a weak but specific inhibitor of SAP. In addition, Indinavir reduces the amount of cell bound as well as released SAP antigen from C. albicans. Furthermore, viability and growth of C. albicans are markedly reduced by Indinavir. These findings indicate that HIV-1 protease inhibitors may possess antifungal activity and we speculate that in vivo SAP inhibition may add to the resolution of mucosal candidiasis in HIV-1 infected subjects.

The complement system: pathophysiology and clinical relevance.

In the present article we review selected aspects of the complement system as a major determinant of innate immunity. Besides a detailed description of the components of the complement system, its mode of action, cellular receptors and regulatory control mechanisms, we have focused on the role of the complement system in mutual defence strategies of invading viral pathogens and the host. Since a detrimental activation of the complement cascade is a critical element of several pathological conditions in diverse organs, we summarise the role of a deteriorated complement system in dermatology, nephrology, neurology and xenotransplantation.