HIV-Protease Inhibitors Reduce Cell Adherence of Candida Albicans Strains by Inhibition of Yeast Secreted Aspartic Proteases

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1 HIV-Protease Inhibitors Reduce Cell Adherence of Candida Albicans Strains by Inhibition of Yeast Secreted Aspartic Proteases Margarete Borg-von Zepelin,* Ingo Meyer,* Reiner Thomssen,* Reinhard Würzner, Dominique Sanglard, Amalio Telenti, and Michel Monod *Department of Bacteriology, University Clinics of Göttingen, Germany; Institut für Hygiene, University of Innsbruck, Innsbruck, Austria; Institut de Microbiologie, Centre Hospitalier Universitaire Vaudois, Lausanne, Switzerland; Departement des Maladies Infectieuses, Service de Dermatologie (DHURDV), Centre Hospitalier Universitaire Vaudois, Lausanne, Switzerland Since the introduction of new anti-retroviral agents and saquinavir inhibited adherence of Candida albicans such as human immunodeficiency virus (HIV) protease under the chosen experimental conditions similarly to inhibitors, oropharyngeal candidiasis is less often the in vitro results, whereas indinavir had no effect. observed in acquired immune deficiency syndrome This inhibition was shown to be concentration depend- patients. Secretory aspartic proteases of Candida ent. The specificity of these effects with respect to albicans, which have similarities to the HIV aspartic the secretory aspartic proteases was demonstrated by proteases, are pathogenicity factors that have been competitive binding experiments using purified intensively investigated in recent years. The inhibitory recombinant secretory aspartic proteases. On the basis effect of four different HIV aspartic protease inhibitors of these studies we conclude that lower rates of oropharyngeal (ritonavir, saquinavir, indinavir, and nelfinavir), on the candidiasis in individuals receiving potent activity of different Candida albicans secretory aspartic anti-retroviral therapy could reflect not only an proteases was demonstrated. These anti-retroviral improvement in the immune system but also direct agents were able to inhibit Candida albicans secretory inhibition of Candida secretory aspartic proteases by aspartic proteases 1, 2, and 3 which are involved in HIV protease inhibitors. Key words: adherence/aspartic Candida adherence. As a consequence of these results protease inhibitors/aspartic proteases/candida albicans/ we used selected HIV protease inhibitors in an adherence human immunodeficiency virus inhibitors/oropharyngeal can- assay of Candida cells to epithelial cells. Ritonavir didiasis. J Invest Dermatol 113: , 1999 O disease and two-thirds of these patients will have OC as their initial ropharyngeal candidiasis (OC) is the most common fungal disease among patients suffering from human immunodeficiency virus (HIV) infection. Some 80 95% of HIV-infected patients will have at least one episode of OC during the course of their the fungus to host cells and tissues is the initial step leading to the establishment of infection and is thus considered to be essential to virulence (Cutler, 1991). The binding mechanisms are mediated by complementary molecules at both the surface of the fungus and the host. During recent years, C. albicans secreted aspartic proteases (Sap) have attracted a lot of attention as virulence factors, and have been demonstrated on the surface of fungal elements colonizing mucosa and penetrating tissues (Borg and Rüchel, 1988). Evidence for the role of Sap in the adherence process has been derived from experiments with the specific inhibitor pepstatin A, which is able symptomatic illness. Candida albicans is responsible for the majority of these infections (for review see Penzak and Gubbins, 1998). Since the introduction of highly active anti-retroviral therapy using HIV aspartic protease inhibitors and nucleoside analogs, however, HIV-associated opportunistic infections such as OC due to to block adherence of C. albicans cells (Ollert et al, 1993). C. albicans are less often observed even in the presence of viral HIV aspartic proteases and Candida Sap are enzymes which resistance to treatment (Egger et al, 1997; Palella et al, 1998; belong to the same class of proteases (Rao et al, 1990). Six closely Ledergerber et al, 1999) related secreted C. albicans aspartic proteases Sap1 Sap6 have been Candida albicans is a dimorphic fungus that is both a commensal successfully produced as recombinant proteins in the methylotrophic and opportunistic pathogen in humans (Odds, 1988). Adhesion of yeast Pichia pastoris and well characterized (Borg-von Zepelin et al, 1998). The availability of purified recombinant Sap1 Sap6 allowed us to test the effect of four different HIV aspartic protease inhibitors, Manuscript received April 28, 1999; revised July 1, 1999; accepted for publication July 22, Reprint requests to: Dr. Michel Monod, Service de Dermatologie, Laboratoire de Mycologie, BT422, Center Hospitalier Universitaire Vaudois, 1011 Lausanne, Switzerland. Michel.Monod@chuv. hospvd.ch Abbreviations: OC, oropharyngeal candidiasis; Sap, secretory aspartic protease. ritonavir, saquinavir, indinavir and nelfinavir on the C. albicans Sap activity. Additionally, we tested the influence of these HIV protease inhibitors on the adherence of various Candida strains to epithelial cells using a previously described assay (Borg-von Zepelin and Wagner, 1995). The results of the adherence assay were correlated to the direct effect of the HIV protease inhibitors on the different Sap leading to the hypothesis that the decrease in the frequency of X/99/$14.00 Copyright 1999 by The Society for Investigative Dermatology, Inc. 747

2 748 BORG-VON ZEPELIN ET AL THE JOURNAL OF INVESTIGATIVE DERMATOLOGY Table I. Sap enzyme activities in the presence of HIV Cell culture Vero cells were cultivated in medium 1640 containing 5% aspartic protease inhibitors a fetal bovine serum in cell culture flasks. The preparation of the microtest Inhibitor Sap1 Sap2 Sap3 Sap4 Sap5 Sap6 plates for the standard adherence assay was performed as previously described (Borg-von Zepelin and Wagner, 1995). Briefly, the detached Vero cells Ritonavir were suspended in new cell culture medium at a concentration of 10 5 cells per ml. Each position of a flat-bottomed microtest plate was filled with 3.2 µm µl of this Vero cell suspension. Prior to testing adherence, the Vero 16 µm cells were incubated in the microtest plates for 24 h at 37 C (5% CO 2, 32 µm % humidity). Saquinavir 32 µm Standard adherence assay This assay was essentially performed as 160 µm previously described (Borg-von Zepelin and Wagner, 1995). Candida cells 320 µm at the end of the logarithmic growth phase were washed twice with cold Nelfinavir PBS. The number of cells was determined photometrically at 630 nm on 32 µm the basis of standard curves or by hemocytometry. The yeasts were 160 µm cultivated in 0.1 M phosphate buffer 50 mm glucose ph 6.0 or medium 320 µm supplemented with 5% fetal bovine serum (adherence media) at a Indinavir concentration of cells per ml. The wells of the microtest plate 32 µm containing Vero cells were infected with 200 µl of the Candida cell 160 µm suspension. Yeasts and target cells were incubated for at least 1 h at 37 C. 320 µm For the detection of adherent Candida cells, the dye Calcofluor white (stock solution 0.2 mg per ml) was directly applied to the adherence Percent values of enzyme activity in the presence of inhibitors. A value of 100% medium at each position (final concentration 20 µg per ml). Subsequently, corresponds to activity in the absence of inhibitor. the plates were incubated for 30 min at 37 C. All wells of the microtest plate were rinsed three times with PBS to remove nonadherent Candida cells. Finally, the amount of adherent fluorescent Candida cells was oropharyngeal candidiasis might be assisted by inhibition of Sap determined by an automatic fluorescence reader (CytoFluor II, Biosearch, involved in C. albicans adherence. Somerville, NJ) at extinction 360 nm and emission 460 nm. As a standard procedure, each test was performed at least four times in MATERIALS AND METHODS parallel on the same microtest plate in order to determine mean values Strains The C. albicans wild-type strains SC5314, ATCC10231, 10261, and SD , CBS2730, DSM70014, and clinical isolates from the bacteriological department of the university clinics of Göttingen were used in this study. Adherence assay in the presence of different HIV-protease Forty-eight hours before use, the strains were plated on Sabouraud agar inhibitors Prior to the infection of Vero cells, differing concentrations and incubated at room temperature. For the adherence assays, the yeasts of the HIV protease inhibitors were added to Candida yeast cells. Control were grown for 14 h in glucose broth at 35 C. At that time point the tests without inhibitor, either containing 1% methanol as solvent for yeasts were at the end of the logarithmic growth phase. ritonavir or water as solvent for saquinavir and indinavir were performed in parallel on the same microtest plate. These controls without inhibitors Proteases and protease inhibitors The different C. albicans Sap tested were set to 100%. The results of the adherence assay with inhibitors were expressed as recombinant proteins using the P. pastoris expression were expressed as percentages of the corresponding control tests without system and purified as previously described (Borg-von Zepelin et al, 1998). inhibitors. The differences between the control tests without protease Protein concentrations were measured by the method of Bradford (1976) inhibitors and the Candida adherence in the presence of HIV protease using a commercial kit (BioRad, Richmond, CA). inhibitors was analyzed statistically using the Student s t test. The inhibitors ritonavir (ABT 538), saquinavir (Ro ), nelfinavir In order to assay the concentration-dependence of selected inhibitors, (AG 1343), and indinavir (L 735,524), obtained from commercial sources, saquinavir and ritonavir were first added to the adherence medium (0.1 M were prepared as follows: Indinavir and saquinavir were dissolved in Aqua phosphate buffer 50 mm glucose) beginning with a concentration of bidest. at concentrations of 20 mm and 2 mm, respectively. Nelfinavir and 200 µm. Inhibitors concentration was then 2-fold diluted in the same ritonavir were dissolved in methanol at concentrations of 20 mm and adherence medium in five steps. The adherence assay was performed as 40 mm, respectively. Pepstatin A was purchased from Sigma (Taufkirchen, described above. Germany), and a stock solution was prepared in methanol at a concentration In order to test the reversal of adherence inhibition caused by HIV of 10 2 M. inhibitors, ritonavir and the different Sap were mixed and preincubated in 50 mm citrate buffer, ph 4 for 15 min at 37 C. This mixture was then Effect of protease inhibitors on Sap proteolytic activity Each added to the Candida cells prior to the infection of the target cells. compound was added at different concentrations (compare Table I) to 0.5 µg of enzyme in 0.45 ml of 50 mm sodium citrate buffer (ph 3.5 Immunofluorescence microscopy Vero cells were cultivated for 48 h 5.0 depending on the optimum ph of activity of the enzyme). The mixture in 24-well test plates (Nunc no ) containing round glass coverslips was incubated for 10 min at 37 C, and assays were performed after addition (diameter 12 mm) in cell culture medium containing 5% fetal bovine of 50 µl of a 0.2% solution of resorufin-labeled casein. An appropriate serum. The target cells were then infected with Candida cells ( cells control without inhibitor was assayed simultaneously. After a 60 min per ml) in adherence medium in the presence or in the absence of aspartic incubation at 37 C, the undigested substrate was precipitated with trichloroacetic protease inhibitors. Candida and Vero cells were incubated for at least 4 h acid (4% final concentration) and separated from the supernatant by at 37 C. Following coincubation, the wells were rinsed once with PBS. centrifugation. The absorbance of the supernatant as a function of the Sap Freshly prepared paraformaldehyde (2% in PBS) was then applied and activity was measured in the alkaline range at 574 nm following addition incubated for at least 1hatroom temperature. of 30 µl 4 M NaOH. Immunofluorescence microscopy was performed using an anti-serum Candida viability assays in the presence of HIV protease (α-sap2) raised in New Zealand White rabbits reacting with Sap1, Sap2, and Sap3 (Borg-von Zepelin et al, 1998). The fixed specimens were rinsed inhibitors Candida cells at the end of the logarithmic growth phase were with PBS and subsequently incubated with PBS 1% bovine serum washed twice with cold phosphate-buffered saline (PBS). The number of albumin and 0.1% milk powder to reduce nonspecific binding for 1 h at Candida cells was determined photometrically by hemocytometry. The room temperature. The anti-serum was applied (100-fold diluted in yeasts were then transferred to medium RPMI 1640 supplemented with PBS 1% bovine serum albumin 0.1% milk powder) and incubated at 5% fetal bovine serum at a concentration of cells per ml. The 8 C for at least 12 h. After another series of rinses with PBS a second HIV protease inhibitors were added at a final concentration of 200 µm. antibody fluorescein isothiocyanate-conjugate from goat was added (diluted For comparison, control tests without inhibitors containing the solvents 200-fold in the above-mentioned buffer). After 3 h incubation and five only, were incubated in parallel. The cell concentrations in the presence subsequent rinses in PBS, the tests were embedded in p-phenylenediamine and in the absence of HIV protease inhibitors were measured by hemocyto- according to Johnson et al (1982). The binding of the conjugate was metry after 2, 4, 6, 8, and 24 h of cultivation at 37 C (5% CO 2, evaluated with a Zeiss fluorescence microscope using the filter combination, 95% humidity). BP /FT 510/LP 520.

3 VOL. 113, NO. 5 NOVEMBER 1999 EFFECT OF HIV PROTEASE INHIBITORS ON C. ALBICANS ADHERENCE 749 Figure 3 demonstrates that in the presence of ritonavir, only few Candida cells are adherent. Compared with control tests without the HIV protease inhibitor it is additionally obvious that these few adherent Candida cells produce shorter germination tubes (Fig 3a, b). The presence of protease antigen on the fungal surfaces was demonstrated by the binding of the antibody α-sap2, which recognizes the protease antigens of Sap1, Sap2, and Sap3 (Borg- von Zepelin et al, 1998). There were no obvious differences between the Candida cells grown with or without the HIV protease inhibitor (Fig 3c, d). The HIV protease inhibitors did not affect Candida viability. Compared with Candida cells cultured without HIV protease inhibitors, neither differences in cell concentrations nor differences in growth kinetics were observed in the presence of the HIV protease inhibitors ritonavir and saquinavir at various time points of the cultivation (data not shown). The specificity of the adherence inhibition due to Candida Sap was concluded from competitive binding experiments of HIV protease inhibitors with recombinant Sap as shown in Fig 4. Ritonavir and Sap1, Sap2, and Sap3 were mixed and preincubated in 50 mm citrate buffer ph 4 for 15 min at 37 C. This mixture was then added to the Candida cells prior to the infection of the target cells. As controls, ritonavir was mixed with serum proteins as well as bovine serum albumin (5 and 10 µg per ml) under the same experimental conditions. The addition of the different Sap to ritonavir completely abolished the inhibition of Candida adherence, whereas unspecific serum proteins had no effect. The effects of the HIV-protease inhibitors saquinavir and ritonavir at a concentration of 200 µm were demonstrated with eight further C. albicans strains from different origins. As the control, the wildtype strain C. albicans SC5314 was tested in parallel. The comparison of these Candida strains in the presence and in the absence of the HIV protease inhibitors is shown in Fig 5. The addition of ritonavir reduced adherence in the same strains by 40 60% compared with the controls without inhibitor. For all strains, the addition of saquinavir was less effective with only a 20 40% reduction in adherence. RESULTS Effect of HIV protease inhibitors on Sap proteolytic activity in vitro Depending on the Sap isoenzyme produced, a yield of µg of protein per ml of P. pastoris culture supernatant was obtained. Saquinavir, ritonavir, nelfinavir, and indinavir were tested at concentrations in the range of µm(table I). Sap1, Sap2, and Sap3 were inhibited by all four inhibitors in a concentration- dependent manner. Ritonavir was by far the most effective inhibitor. Under our experimental conditions, a 50% inhibition was obtained with µm of ritonavir, which are concentrations comparable with those achievable in patients (Hsu et al, 1997). Ten-fold higher concentrations of saquinavir, nelfinavir, and indinavir were necessary to obtain similar inhibition of Sap1, Sap2, and Sap3. In contrast, only a slight inhibition of activity was detected for Sap4, Sap5, and Sap6 with a concentration of up to 320 µm saquinavir. Effect of HIV protease inhibitors on Candida adherence C. albicans SC5314 was used in an adherence assay to test the effect of ritonavir, which most effectively inhibited Sap activities in vitro, as well as saquinavir and indinavir. C. albicans adherence without inhibitor was arbitrarily set at 100%. Under the chosen experimental conditions, the addition of pepstatin A at a concentration of 100 µm was used as a positive control and revealed an adherence inhibition of 30%, which is in agreement with previously published results (Ollert et al, 1993). Using phosphate buffer ph 6.0 as the adherence medium, the influence of the HIV protease inhibitors on the adherence of C. albicans SC 5314 was shown to differ (Fig 1). Saquinavir and ritonavir used at concentrations of 200 µm revealed a marked inhibition of the adherence. The addition of ritonavir and saquinavir reduced the adherence of C. albicans SC5314 to 30% and 50% of the controls, respectively, at the end of the adherence period. The differences in adherence between C. albicans SC5314 without protease inhibitors and in the presence of each inhibitor at a concentration of 200 µm was found to be highly significant (p 0.001). In contrast, indinavir did not influence the adherence of C. albicans cells under the chosen experimental conditions (Fig 1). The inhibition measured in the presence of ritonavir and saquinavir was found to be concentration dependent (Fig 2). Whereas ritonavir still distinctly inhibited the adherence of C. albicans SC5314 up to a concentration of 25 µm, the end-point of adherence-inhibition caused by saquinavir was only clearly seen at a 4-fold higher concentration only (100 µm, Fig 2). The inhibition of adherence in the presence of HIV protease inhibitors was also visualized by immunofluorescence microscopy. Figure 1. Adherence of C. albicans SC5314 to Vero cells using HIV protease inhibitors. Adherence assay performed using in phosphate- buffered (0.1 M, ph 6.0) glucose (50 mm) adherence medium. In the assay the HIV protease inhibitors were used at final concentrations of 200 µm. Adherence values are a percentage of the control tests without inhibitor (100%). The mean maximal SD is depicted. The SD varied between 9% and 13%. Figure 2. Concentration-dependent adherence of C. albicans SC5314 by the HIV-protease inhibitors ritonavir and saquinavir in 0.1 M phosphate buffer, ph 6.0 plus 50 mm glucose. Adherence values (A%) are a percentage of the results of the control adherence assays without HIV protease inhibitors, which are set to 100%. The mean maximal SD is depicted. The SD varied between 6% and 15%. DISCUSSION Secretory proteases from C. albicans have attracted great interest as defined virulence factors. Nine genes encoding Sap have been cloned from C. albicans to date (Monod et al, 1994, 1998). Sap1, Sap2, and Sap3 as well as Sap4, Sap5, and Sap6 form 2 subgroups each containing closely related enzymes, whereas Sap7, Sap8, and Sap9 do not form a specific group. The fact that C. albicans inhabit diverse host niches leads to the question of whether different Sap are expressed by C. albicans as a reaction to specific adaptation

4 750 BORG-VON ZEPELIN ET AL THE JOURNAL OF INVESTIGATIVE DERMATOLOGY Figure 3. Fluorescence micrograph of C. albicans SC5314 after adherence on Vero cells in the absence (a, c) and in the presence (b, d) of 200 µm ritonavir. The cells were stained with Calcofluor white under the same conditions as used in the adherence assay (a, b). Scale bar: 10µm. Immunoreaction with antibody α-sap2, recognizing Sap1, Sap2, and Sap3 on the fungal surface (c, d). No obvious differences are observed. Scale bar: 20µm Figure 5. Adherence of nine different C. albicans strains supplemented by ritonavir and saquinavir (200 µm). Adherence values (A%) Figure 4. Competitive binding experiments using recombinant Sap are a percentage of the results of the control adherence assays of the same with the inhibitory effect on the Candida adherence caused by Candida strains without inhibitor (100%). The SD varied between 4% ritonavir. Prior to the addition to the Candida cells, ritonavir was mixed and 15%. with Sap1, Sap2, and Sap3 at a concentration of 2 µg per ml each and incubated for 15 min at 37 C. Control experiments were performed in the presence of unspecific proteins (10 µg per ml). The SD varied between and Sap3 appear to be the Sap produced during mucosal adherence. 10% and 17%. In contrast, Sap4, Sap5, and Sap6 have been shown to be secreted in macrophages after phagocytosis of yeast cells (Borg-von Zepelin requirements under different environmental conditions. The first et al, 1998). In these studies we have demonstrated that the Sap studies concerned with such problems were carried out on the involved in adherence, Sap1, Sap2, and Sap3, are those inhibited basis of gene expression. With the help of reverse transcription by HIV protease inhibitors. polymerase chain reaction the expression of genes SAP1, SAP2, We, therefore, tried selected anti-retroviral substances in a SAP3, and SAP6 was demonstrated in samples of experimental and previously described adherence assay (Borg-von Zepelin and clinical candidosis (Schaller et al, 1998). The expression of SAP1 Wagner, 1995) to address possible inhibition of the adherence of and SAP2 was also shown by northern blot analysis using an C. albicans cells to the Vero target cells by HIV protease inhibitors. experimental rat vaginitis model (De Bernardis et al, 1995). In Epithelial cells were used as a model for mucosal infections instead particular, Sap2 seemed to have a certain potential importance for of keratinocytic cells, e.g., HaCat line or keratinocytes obtained the vaginal infection as was deduced from infection assays using from human foreskins which could be used as a model for protease-deficient mutant strains in the rat model (De Bernardis skin infections. The inhibition of adherence was not caused by et al, 1999). From these data it must be concluded that Sap1, Sap2, diminishing the viability of the Candida cells (Diz et al, 1999).

5 VOL. 113, NO. 5 NOVEMBER 1999 EFFECT OF HIV PROTEASE INHIBITORS ON C. ALBICANS ADHERENCE 751 Ritonavir, which is the most active inhibitor of Sap activity in vitro, diminished the adherence of C. albicans SC5314 to epithelial cells We thank Professor Dr. H. Eiffert, Department of Bacteriology, University Clinics by up to 70% of the controls. A distinct reduction in Candida of Göttingen for providing some clinical isolates of Candida albicans, Barbara adherence was found in the range of 25 µm (corresponding to Léchenne for technical assistance and Mrs Cyrilla Maelicke for critical review of the 15 µg per ml), which is a HIV protease inhibitor concentration manuscript and linguistic assistance. This work comprises parts of the medical thesis regularly found as the peak serum level under the proposed of I. Meyer. therapeutic regimen for AIDS patients (Hsu et al, 1997). In our model unspecific serum proteins or bovine serum albumin had no effect on the activity of HIV inhibitors on Candida adherence. So REFERENCES far no data are available on substance concentrations in saliva or in Borg M, Rüchel R: Expression of extracellular acid proteinase by proteolytic Candida mucosal cells so that both decreased bioavailability and accumulation spp during experimental infection of oral mucosa. Infect Immun 56:626 of the drugs in the mucosal cells is possible. In accordance with 631, 1988 Borg-von Zepelin M, Wagner T: Fluorescence assay for the detection of adherent the observed concentration-dependence of Candida adherence Candida yeasts to target cells in microtest plates. 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