Use of immunoblotting assay improves the sensitivity of paracoccidioidomycosis diagnosis

ORIGINAL PAPER

Use of immunoblotting assay improves the sensitivity of paracoccidioidomycosis diagnosis

Silva D. F.^I; Assis C. M.^II; Zamboni I. M.^I; Barreto L. C.^I; Kohara V. S.^I; Vicentini-Moreira A. P.^I

^ILaboratory of Immunodiagnosis in Mycosis, Immunology Section, Adolfo Lutz Institute, São Paulo, São Paulo State, Brazil

^IIBasic Service Division, Adolfo Lutz Institute, São Paulo, São Paulo State, Brazil

^{Correspondence to}

ABSTRACT

The purpose of this work was to evaluate two serological assays: double immunodiffusion (DI) and immunoblotting (IB) in immunodiagnosis of paracoccidioidomycosis (PCM). We evaluated by IB assay 23 sera samples from patients with clinical confirmation of PCM, all of them with negative DI results against culture filtrate from Paracoccidioides brasiliensis isolate 113. For IB, as well as for comparative DI assay, we employed soluble components of the cell wall outer surface (SCCWOS) from P. brasiliensis isolate 113 cultivated at 36°C in Fava-Neto's agar medium for 5 and 10 days. Among the 20 sera samples analyzed by DI, 13 (65%) were negative and 7 (35%) were positive against SCCWOS obtained on the 5^th and 10^th days. By IB assay, 95.4% and 100% of sera reacted against gp43 and gp70 present in SCCWOS from the 5^th day and 95.6% recognized these fractions when evaluated against SCCWOS from the 10^th day. Our results demonstrated that the use of an immunoenzymatic assay significantly improves the sensitivity of PCM immunodiagnosis and also suggests that at least two serological tests for antibody detection should be adopted in cases of questionable diagnosis.

Key Words: paracoccidioidomycosis, immunodiffusion, immunoblotting.

INTRODUCTION

Paracoccidioidomycosis (PCM) is a systemic, chronic, granulomatous, progressive disease caused by the dimorphic fungus Paracoccidioides brasiliensis (5). The definitive diagnosis of PCM is usually based on the demonstration of multibudding yeast cells in different biological specimens and/or the isolation of the fungus by culturing; however, the former is insensitive, and the latter is time-consuming (5). Consequently, serological techniques are an important tool not only for disease diagnosis but also for monitoring the patients' response to treatment (6, 8). Circulating antibodies to P. brasiliensis can be detected in the patients' sera by serological assays such as DI, counterimmunoelectrophoresis and immunoenzymatic assays (6, 20). DI test is routinely used by clinical laboratories due to its easy procedure, low cost involved in its execution as well as its high specificity (about 100%) and sensitivity (65%–90%) (6, 8). The use of undefined antigens has imposed important limitations on DI test; cross-reactivity has been a problem, as has the absence of antigen standardization (6, 8). Variation in antigen production can arise from differences in the used strains, in the chosen fungus growth phase, in the incubation time, and in the employed culture media (6, 8). Among the antigens used in serological approaches, gp43 is the most frequently studied P. brasiliensis component (6, 8, 11). Del Negro et al. (9) described that rarely a patient presents negative DI results. However, when further analyzed by an immunoenzymatic assay such as IB, these sera demonstrate anti-gp43 antibodies, probably due to the relatively low sensitivity of precipitation assays. The purpose of this work was to evaluate two serological tests: DI and IB in immunodiagnosis of PCM.

MATERIALS AND METHODS

Paracoccidioides brasiliensis Isolates

Paracoccidioides brasiliensis isolate 113, kindly provided by Professor Dr. Carlos da Silva Lacaz-Culture Collection of the São Paulo Institute of Tropical Medicine [Instituto de Medicina Tropical de São Paulo], Brazil, was used. The isolate was maintained in Potato agar medium at 25ºC and subcultured every three months at the Laboratory of Immunodiagnosis in Mycosis, Adolfo Lutz Institute, São Paulo, Brazil.

Paracoccidioides brasiliensis Antigens

Two types of antigens were used in this study. The first was a 20-day culture filtrate (Ag CF) obtained according to Garcia et al. (10) from the yeast phase of P. brasiliensis. Briefly, the fungus was cultured in NGTA liquid medium for 20 days at 36ºC with shaking. After incubation time, the culture was treated with borate-thimerosal solution (1:5,000), filtered, divided into small volumes and stored at 4ºC until use. The second antigen employed was P. brasiliensis SCCWOS obtained 15 years ago by Assis (1–4). Yeast cells were grown in Fava Netto's agar at 36ºC. The cells were collected on days 5 and 10, suspended in 0.85% NaCl solution and pelleted by centrifugation. Supernatants, treated with borate-thimerosal solution (1:5,000) and containing the soluble components, were divided into small volumes and stored at 4ºC until use.

Sera Samples

Twenty-three sera samples from patients with proven cases of active PCM (13 sera obtained from patients with chronic and 10 from patients with acute form of the disease), but non-reactive, with culture filtrate or SCCWOS antigens by DI assay, were analyzed. The diagnosis was established by histological or direct microscopic examination and/or by isolation of P. brasiliensis.

Double Immunodifusion Assay

Reactions were performed according to the modified method of Ouchterlony (18). Glass slides were covered with 3.0ml of a gel composed of 1% agarose type II medium (Sigma Chemical Co. St. Louis, Mo, USA) in a buffered saline (pH 6.9) containing 0.4% sodium citrate and 7.5% glycine. Antigen (12µl) was placed in the central well, reference and patients' sera (12µl) in the surrounding wells. The slides were incubated in a humid chamber at room temperature for 48h. They were then washed in saline with several changes over a 24h period. The gels were dried and stained with 0.4% Coomassie Brilliant Blue R-250 (Sigma Chemical Co. St. Louis, Mo, USA) in an ethanol-acetic acid-water mixture as the solvent.

SDS-PAGE and Immunoblotting Assay

For sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis (PAGE), P. brasiliensis antigens were diluted in a buffer containing 62mM Tris-HCl (pH 6.8); 2% (wt/vol) SDS, 50mM 2-mercaptoethanol, 10% glycerol, and 0.01% bromophenol blue were boiled for 3min and centrifuged before application to gels. Cell wall antigens were then subjected to electrophoresis (20mA at room temperature) on a 10% discontinuous SDS buffer system (13) in a Mini-protean II, Electrophoresis Cell (Bio Rad Laboratories, Richmond, CA, USA), and the molecular mass was determined by the use of a 6.5–175kDa standard prestained protein marker (New England BioLabs). Immunobloting assays were conducted as previously described (19). Proteins from SDS-PAGE were electrotransferred onto 0.20µm nitrocellulose membranes (Sigma Chemical Co. St. Louis, Mo, USA) in a Mini Trans-Blot Cell (Bio Rad Laboratories, Richmond, CA, USA), with 25mM Tris, 192mM glycine, pH 8.3, and methanol 20% (v:v). The nitrocellulose membranes containing electrophoresed antigens were blocked with 5% non-fat dry milk in PBS for 1h at room temperature. Membranes were incubated for 2h at room temperature with human sera diluted 1:40 in PBS containing 0.05% Tween-20 (PBS-T), then washed six times with PBS-T and developed with peroxidase-conjugated goat anti-human IgG antibody (Sigma Chemical Co. St. Louis, Mo, USA) for 2h at room temperature. The reactions were visualized with 4-chloro-1-naphtol substrate (Sigma Chemical Co. St. Louis, Mo, USA).

RESULTS

Table 1 summarizes the DI results. Among the 23 sera samples analyzed, 100% showed negative DI results against culture filtrate antigen; 13 (65%) were negative and 7 (35%) were positive against SCCWOS antigens obtained on the 5^th and 10^th days of culture. However, when these negative sera were further analyzed by IB assay, we observed that 95.4% and 100% of sera reacted against gp43 and gp70 present in SCCWOS from 5 days, and 95.6% recognized these fractions when evaluated against SCCWOS from 10 days. (Figure 1)

DISCUSSION

For several years, many different antigenic preparations have been used for the immunodiagnosis of PCM by DI assay. However, these various antigenic preparations lacked a standardized preparation from one laboratory to another and included cytoplasmatic extracts from the yeast forms, concentrated filtrates, lyophilized filtrates, and cell wall components to cite a few approaches (7). Moreover, these antigens were prepared from different P. brasiliensis isolates grown in different culture media as well as under different growth conditions (6, 12, 16). Distinct P. brasiliensis isolates, on the other hand, can produce remarkably different qualities and quantities of antigens, a response that is also influenced by the culture medium (6, 12, 16, 17). With all these variations in P. brasiliensis antigen production, it is not surprising that there is considerable disagreement regarding the sensitivity and specificity in DI assay (6, 12, 16, 17). There are reports of DI false negative results in PCM patients with active infection (9, 14, 16, 18, 21). Do Valle et al. (20) reported that the negative DI tests were more frequent in the acute compared with the chronic form. In contrast, Vidal et al. (21) reported that negative DI in acute form is rarely observed, and the few negative results are most exclusively related to the localized chronic form of PCM. In our study, we observed that 100% of DI negative PCM sera samples were positive in IB assay showing strongly reactivity with 43 and 70kDa antigenic fractions. The gp43 is the immunodominant and species-specific component of P. brasiliensis, being recognized by virtually all patients' sera employing different serological tests (6–8). Another important P. brasiliensis antigen is gp70, recognized by 96% PCM sera (6). Neves et al. (14) analyzed 46 sera samples of patients with proven PCM and with DI negative versus DI positive results, showing by immunoblotting that both DI negative and positive sera recognized predominantly the gp43 fraction of P. brasiliensis. Takahachi et al. (18) evaluated by immunoblotting 78 PCM sera samples with DI negative results, showing 100% of positivity. Some hypotheses could explain the negative DI results such as: prozone effect (20), production of low-avidity IgG2 antibodies directed against carbohydrate epitopes (14, 20). In conclusion, our results demonstrated that the use of an immunoenzymatic assay significantly improves the sensitivity of PCM immunodiagnosis and also suggest that at least two serological tests for antibody detection should be adopted in cases of questionable diagnosis.

ACKNOWLEDGEMENTS

The authors thank Antonio Roberto de Souza Ferreira, Edina Maria and Vasco Gouveia of Adolfo Lutz Institute for their technical assistance. This study was supported by Adolfo Lutz Institute – Coordination for Disease Control – São Paulo State Secretariat of Health (CTC-IAL # 107/97, # 13/2002 and # 05/04). Silva D.F. was the recipient of a fellowship of the Graduation Program in Sciences (PG-CCD-SES-SP), supported by the Coordination for the Improvement of Higher Education Personnel (CAPES).

Received: September 3, 2007

Accepted: November 20, 2007

Abstract published online: November 21, 2007

Full paper published online: May 31, 2008

Conflicts of interest: There is no conflict.

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Publication Dates

• Publication in this collection
  16 June 2008
• Date of issue
  2008

History

• Accepted
  20 Nov 2007
• Received
  03 Sept 2007