Production and characterization of monoclonal antibodies to serine proteinase allergens in Penicillium and Aspergillus species

Aspergillus-specific antibodies - Targets and applications

Aspergillus is a fungal genus comprising several hundred species, many of which can damage the health of plants, animals and humans by direct infection and/or due to the production of toxic secondary metabolites known as mycotoxins. Aspergillus-specific antibodies have been generated against polypeptides, polysaccharides and secondary metabolites found in the cell wall or secretions, and these can be used to detect and monitor infections or to quantify mycotoxin contamination in food and feed. However, most Aspergillus-specific antibodies are generated against heterogeneous antigen preparations and the specific target remains unknown. Target identification is important because this can help to characterize fungal morphology, confirm host penetration by opportunistic pathogens, detect specific disease-related biomarkers, identify new candidate targets for antifungal drug design, and qualify antibodies for diagnostic and therapeutic applications. In this review, we discuss how antibodies are raised against heterogeneous Aspergillus antigen preparations and how they can be characterized, focusing on strategies to identify their specific antigens and epitopes. We also discuss the therapeutic, diagnostic and biotechnological applications of Aspergillus-specific antibodies.

Vacuolar Serine Protease Is a Major Allergen of Fusarium proliferatum and an IgE-Cross Reactive Pan-Fungal Allergen

Purpose

Fusarium species are among prevalent airborne fungi and causative agents of human respiratory atopic disorders. We previously identified a 36.5-kDa F. proliferatum component recognized by IgE antibodies in 9 (53%) of the 17 F. proliferatum-sensitized atopic serum samples. The purpose of this study is to characterize the 36.5-kDa allergen of F. proliferatum.

Methods

Characterization of allergens and determination of IgE cross-reactivity were performed by cDNA cloning/expression and immunoblot inhibition studies.

Results

Based on the finding that the 36.5-kDa IgE-binding component reacted with the mouse monoclonal antibody FUM20 against fungal vacuolar serine protease allergens, the cDNA of F. proliferatum vacuolar serine protease (Fus p 9.0101) was subsequently cloned. Nine serum samples from respiratory atopic patients with IgE binding to the vacuolar serine protease allergen of Penicillium chrysogenum (Pen ch 18) also showed IgE-immunoblot reactivity to rFus p 9.0101. The purified rFus p 9.0101 can inhibit IgE and FUM20 binding to the 36.5-kDa component of F. proliferatum. Thus, a novel and important Fus p 9.0101 was identified. The rPen ch 18 can inhibit IgE binding to Fus p 9.0101. It indicates that IgE cross-reactivity between Fus p 9.0101 and Pen ch 18 also exists. Furthermore, neither rFus p 9.0101 K88A nor rPen ch 18 K89A mutants inhibited IgE binding to rFus p 9.0101. Lys88 was considered a critical core amino acid in IgE binding to r Fus p 9.0101 and a residue responsible for IgE cross-reactivity between Fus p 9.0101 and Pen ch 18 allergens.

Conclusions

Results obtained from this study indicate that vacuolar serine protease may be a major allergen of F. proliferatum and an important IgE cross-reactive pan-fungal allergen, and provide important bases for clinical diagnosis of fungal allergy.

Alt a 15 is a new cross-reactive minor allergen of Alternaria alternata

Alternaria alternata is one of the most common saprophytes worldwide that is clinically and epidemiologically associated with severe asthma. Therefore, the identification and characterization of all A. alternata allergens are of major clinical importance. This study describes a new cross-reactive A. alternata allergen that was officially named Alt a 15 by the official Allergen Nomenclature Subcommittee. The complete coding region for Alt a 15 was amplified using 5' and 3' rapid amplification of cDNA ends and PCR. The recombinant protein was produced in Escherichia coli as a 65-kDa fusion protein, and the protein sequence exhibits high homology with several important fungal allergens. Immunoblotting analyses revealed that IgE antibodies from A. alternata-sensitized patients (n=59) bound to rAlt a 15 with a prevalence of 10.2%. All patients who presented sIgE to rAlt a 15 were apparently poly-sensitized to A. alternata and C. lunata. The extensive cross-reactivity between A. alternata and C. lunata serine proteases was confirmed using immunoblotting inhibition assays. Overall, Alt a 15 is an important new cross-reactive allergen of A. alternata that explains some allergies to A. alternata without Alt a 1 sensitization and initial diagnostic errors for allergies to Alternaria. This molecule may improve the accuracy of the diagnosis, the understanding, and the management of IgE-mediated fungal diseases.

Epitope mapping and in silico characterization of interactions between Der p 7 allergen and MoAb WH9

Der p 7 is an important house dust mite allergen. However, antigenic determinants of Der p 7 are largely unknown. The purpose of this study is to analyze the determinants of Der p 7 and determine the structural basis of interactions between Der p 7 and WH9, an IgE-binding inhibition mouse monoclonal antibody (MoAb). IgE and WH9-reactive determinant(s) was identified by immunoblot using allergen mutants. A 3-D binary complex structure of Der p 7 and WH9 was simulated with homology modeling and docking methods. Our results obtained showed that among the five Der p 7 mutants (S156A, I157A, L158A, D159A, P160A), serum no. 1045 with IgE-binding against Der p 7 exhibited a reduced IgE immunoblot reactivity against Der p 7 L158A and D159A mutants. WH9 showed reduced immunoblot reactivity against S156A, L158A, D159A and P160A and the observation was confirmed by immunoblot inhibition. The WH9-binding determinant on Der p 7 containing S156, L158, D159 and P160 assumes a loop-like structure. The structural model of the Der p 7-WH9 complex suggests residues S156, I157, L158, D159 and P160 of Der p 7 contribute to WH9 binding via potential hydrogen bonds, electrostatic and hydrophobic interactions. In conclusion, MoAb WH9 interacts with critical residues L158 and D159 of Der p 7 and inhibits IgE-binding to Der p 7. Results obtained advance our understanding on molecular and structural bases of the antigenicity of Der p 7, its interactions with MoAb WH9 and facilitate the design of safer immunotherapy of human atopic disorders.

Vacuolar serine protease is a major allergen of Cladosporium cladosporioides

BACKGROUND

Cladosporium is an important allergenic fungus worldwide. We report here a major allergen of C. cladosporioides.

METHODS

Major C. cladosporioides allergens were characterized by immunoblotting, N-terminal amino acid sequencing, protein purification and cDNA cloning.

RESULTS

Seventy-four sera (38%) from 197 bronchial asthmatic patients demonstrated IgE binding against C. cladosporioides extracts. Among these 74 sera, 41 (55%) and 38 (51%) showed IgE binding against a 36- and a 20-kDa protein of C. cladosporioides, respectively. Both IgE-reacting components reacted with FUM20, a monoclonal antibody against fungal serine proteases. N-terminal amino acid sequencing results suggest that they are vacuolar serine proteases, and the 20-kDa component is possibly a degraded product of the 36-kDa allergen. A corresponding 5'-truncated 1,425-bp cDNA fragment was isolated. The mature protein after N-terminal processing starts with an N-terminal serine that is the ninth residue encoded by the 5'-truncated cDNA. The protein sequence deduced shares 69-72% sequence identity with Penicillium vacuolar serine proteases and was designated as Cla c 9. The purified 36-kDa Cla c 9 allergen showed proteolytic activity with peptide Z-Ala-Ala-Leu-pNA as substrate. IgE cross-reactivity was detected between the purified Cla c 9 and serine protease allergens from Aspergillusfumigatus and Penicillium chrysogenum.

CONCLUSION

We identified a vacuolar serine protease as a major allergen of C. cladosporioides (Cla c 9) and a major pan-allergen of prevalent airborne fungi. IgE cross-reactivity among these highly conserved serine protease pan-fungal allergens was also detectable.

Aspergillus and Penicillium allergens: focus on proteases

Penicillium and Aspergillus species are prevalent airborne fungi. It is imperative to identify and characterize their major allergens. Alkaline and/or vacuolar serine proteases are major allergens of several prevalent Penicillium and Aspergillus species. They are also major immunoglobulin (Ig) E-reacting components of the most prevalent airborne yeast, Rhodotorula mucilaginosa, and the most prevalent Cladosporium species, C. cladosporioides. IgE cross-reactivity has been detected among these major pan-fungal serine protease allergens. In addition, the alkaline serine protease of P. chrysogenum (Pen ch 13) induces histamine release from basophils of asthmatic patients, degrades the tight junction protein occludin, and stimulates release of proinflammatory mediators from human bronchial epithelial cells. In addition to induction of IgE and inflammatory airway responses, the alkaline serine protease allergen of A. fumigatus (Asp f 13) has synergistic effects on Asp f 2-induced immune response in mice. Studies of these serine protease major allergens elucidate the diverse allergic disease mechanisms and facilitate the development of better therapeutic strategies.

The spectrum of fungal allergy

Fungi can be found throughout the world. They may live as saprophytes, parasites or symbionts of animals and plants in indoor as well as outdoor environment. For decades, fungi belonging to the ascomycota as well as to the basidiomycota have been known to cause a broad panel of human disorders. In contrast to pollen, fungal spores and/or mycelial cells may not only cause type I allergy, the most prevalent disease caused by molds, but also a large number of other illnesses, including allergic bronchopulmonary mycoses, allergic sinusitis, hypersensitivity pneumonitis and atopic dermatitis; and, again in contrast to pollen-derived allergies, fungal allergies are frequently linked with allergic asthma. Sensitization to molds has been reported in up to 80% of asthmatic patients. Although research on fungal allergies dates back to the 19th century, major improvements in the diagnosis and therapy of mold allergy have been hampered by the fact that fungal extracts are highly variable in their protein composition due to strain variabilities, batch-to-batch variations, and by the fact that extracts may be prepared from spores and/or mycelial cells. Nonetheless, about 150 individual fungal allergens from approximately 80 mold genera have been identified in the last 20 years. First clinical studies with recombinant mold allergens have demonstrated their potency in clinical diagnosis. This review aims to give an overview of the biology of molds and diseases caused by molds in humans, as well as a detailed summary of the latest results on recombinant fungal allergens.

Lys, Pro and Trp Are Critical Core Amino Acid Residues Recognized by FUM20, a Monoclonal Antibody against Serine Protease Pan-Fungal Allergens

BACKGROUND

Alkaline/vacuolar serine proteases comprise a major group of pan-fungal allergens from several prevalent airborne fungal species. It is of importance to characterize antigenic determinant(s) recognized by monoclonal antibodies against these major allergens.

METHODS

The antigenic determinant of fungal serine proteases recognized by a monoclonal antibody, FUM20, was analyzed by dot immunoassay of synthetic peptides immobilized on cellulose membrane. Results obtained were confirmed by wild-type recombinant protease and its mutants. The epitopes were mapped to the structure of serine proteases by molecular modeling.

RESULTS

A linear epitope encompassing 9 amino acids from Pen ch 18 ((6)EKNAPWGLA(14)) binds FUM20. The corresponding peptide ((5)AKGAPWGLA(13)) from Rho m 2 also binds FUM20. Substitution of K6, P9 or W10 with alanine in this peptide resulted in drastic loss of FUM20 binding. Rho m 2 mutants with single K6A, P9A, P9G, W10A or W10F substitute showed negative immunoblot reactivity against FUM20. However, the Rho m 2 K6R mutant can bind FUM20. Three-dimensional structural models of the FUM20 antigenic determinants on serine proteases were constructed. The lysine residue critical for FUM20 interaction is on the surface of the proteases and solvent accessible. The critical core residue proline is located at the beginning of an alpha-helix.

CONCLUSIONS

The lysine, proline and tryptophan residues located on the N-terminal region of fungal serine proteases are critical core amino acid residues recognized by FUM20, a monoclonal antibody against serine protease pan-fungal allergens. These findings advance our understanding of the antigenic structures responsible for the antigenicity of serine protease allergens.

Molecular and structural analysis of immunoglobulin E-binding epitopes of Pen ch 13, an alkaline serine protease major allergen from Penicillium chrysogenum

BACKGROUND

Through proteomic and genomic approaches we have previously identified and characterized an alkaline serine protease that is a major allergen (88% frequency of IgE binding) of Penicillium chrysogenum (Pen ch 13).

OBJECTIVE

The aim of the present study is to identify the linear IgE-binding epitopes of Pen ch 13.

METHODS

IgE-binding regions were identified by dot-blot immunoassay using 11 phage-displayed peptide fragments spanning the whole molecule of Pen ch 13. The minimal epitope requirements for IgE binding were further defined with overlapping peptides synthesized on derivatized cellulose membranes using SPOTs technology. The critical residues on the immunodominant epitopes were mapped through site-directed mutagenesis. The locations of the IgE epitopes identified were correlated with a three-dimensional structure of Pen ch 13.

RESULTS

IgE antibodies in 35 serum samples reacted with at least one of the 11 peptide fragments of Pen ch 13. Peptide f-2n (residues 31-61) showed a high-intensity and the highest frequency (77%) of IgE binding. The frequencies of IgE binding to peptide f-4 (residues 93-133), f-1 (residues 1-37) and f-7 (residues 168-206) were 51%, 34% and 31%, respectively. SPOTs assay narrowed down the region of IgE binding of f-2n to residues 48-55 (GHADFGGR). Three, two and one epitope(s) that are four to nine amino acids in length, within f-4, f-1 and f-7, respectively, were found. Site-directed mutagenesis of Pen ch 13 revealed that substitution of His49 and/or Phe52 on Pen ch 13 with methionine resulted in proteins with drastic loss of IgE binding in seven sera tested. Proteins with amino acid replacements at residues 15-18 (RISS), or at residues 112 (I) and 116 (D) have lower IgE-binding reactivity in one of the two patient's sera tested. Substituting residues 117 (W), 119 (V) and 120 (K) also block most of the IgE binding in one of the two patient's sera tested. In addition, replacing residues 203 (V) and 204 (D) along with a deletion at residue 206 (Y) diminished the IgE binding in two serum samples tested. A model was constructed based on the structure of P. cyclopium subtilisin protease that has >90% (256 out of 283 amino acids) sequence identity with Pen ch 13. The major epitope (GHADFGGR) on Pen ch 13 formed a loop-like structure and was located at the surface of the allergen.

CONCLUSIONS

Several linear IgE-reactive epitopes and their critical core amino acid residues were identified for the Pen ch 13 allergen. The major linear IgE-binding epitope, 48GHADFGGR55, formed a loop-like structure at the surface of the allergen. Substitution of His49 and/or Phe52 with methionine significantly reduced IgE-binding to Pen ch 13. Mapping of these results on a 3D model of the allergen provides valuable information about the molecular basis of allergenicity for Pen ch 13 and for designing specific immunotherapeutics.

Clinical use of immunoassays in assessing exposure to fungi and potential health effects related to fungal exposure

OBJECTIVE

To review and summarize current evidence regarding the proper role of immunoassays in clinical assessments of exposure to fungi and health effects related to fungal exposure.

DATA SOURCES

We reviewed relevant scientific investigations and previously published reviews concerning this topic.

STUDY SELECTION

The authors' clinical, laboratory, and public health experiences were used to evaluate relevant data for scientific merit.

RESULTS

Testing to determine the presence of IgE to specific fungi may be a useful component of a complete clinical evaluation in the diagnosis of illnesses that can be caused by immediate hypersensitivity such as allergic rhinitis and asthma. Detection of IgG to specific fungi has been used as a marker of exposure to agents that may cause illnesses such as hypersensitivity pneumonitis. However, the ubiquitous nature of many fungi and the lack of specificity of fungal antigens limit the usefulness of these types of tests in the evaluation of potential building-related illness and fungal exposure. Specific serologic tests (such as tests for cryptococcal antigen, coccidioidal antibody, and Histoplasma antigen) have been shown to be useful in the diagnosis of some fungal infections, but these are the exception not the rule.

CONCLUSIONS

There is currently not enough scientific evidence to support the routine clinical use of immunoassays as a primary means of assessing environmental fungal exposure or health effects related to fungal exposure. Health care providers who care for persons expressing concerns about the relationship of symptoms to potential exposure to fungi are advised to use immunoassay results with care and only as an adjunct to a comprehensive approach to patient care.

Allergic inflammation induced by a Penicillium chrysogenum conidia-associated allergen extract in a murine model

BACKGROUND

Recent evidence has shown that viable conidia from the fungus Penicillium chrysogenum induce allergic effects in mice. The present study was conducted to determine the specific allergic dose response of C57BL/6 mice to the protease extract, Pen ch, isolated from viable P. chrysogenum conidia.

METHODS

Mice were treated with primary intraperitoneal (IP) injections of 10 or 100 microg of Pen ch adsorbed to alum, followed by weekly IP injections of 0.1, 1.0, or 10.0 microg Pen ch with alum for 4 weeks, and with 10.0 microg of Pen ch by intranasal (IN) inoculations the final 2 weeks before killing.

RESULTS

Intraperitoneal injections of 10 and 100 microg of Pen ch for 5 weeks followed by 2 weeks of IN instillation of 10 microg induced significant increases of total serum immunoglobulin (Ig)E and IgG(1). Bronchoalveolar lavage cell counts revealed increased numbers of eosinophils and neutrophils. Histopathological examination of lungs detected perivascular inflammation by eosinophils and neutrophils and increased mucous production.

CONCLUSIONS

The data presented in this study indicate that sensitization to protease allergens released by viable P. chrysogenum conidia in vivo induce a strong allergic inflammatory response in a murine model, which could have implications for people exposed to high levels of conidia of this organism.

Limitations of monoclonal antibodies for monitoring of fungal aerosols using Penicillium brevicompactum as a model fungus

Molds are ubiquitous in every environment and many species have been recently associated with an increase in opportunistic infections in immunocompromised patients or the exacerbation of asthmatic episodes in allergic patients. The degree of environmental contamination with fungi thus needs to be monitored and in this study we report the development of a monoclonal antibody (mAb)-mediated enzyme-linked immunosorbent assay (ELISA) for the detection of spores of Penicillium brevicompactum in experimental model aerosols. In addition, we have investigated the influence of different parameters of air sampling and sample recovery on ELISA performance. MAbs were produced with standard hybridoma techniques and cross-reactivities were determined against spores of 53 fungal species by indirect ELISA. Standardized experimental fungal aerosols were collected with the Button Personal Inhalable Aerosol Sampler onto polycarbonate or polytetrafluoroethylene filters (PTFE) and the effects of different extraction buffers and filter agitation methods during sample processing on spore recovery and ELISA detection were investigated. Five mAbs were produced and all of them cross-reacted with several of 31 related Aspergillus, Penicillium and Eurotium species. However, cross-reactivities with 21 non-related fungi were rare. Spores were recovered in much higher numbers from polycarbonate filters (PFs) than from polytetrafluoroethylene filters. Optical densities (ODs) in ELISA were higher for spores collected into carbonate coating buffer (CCB) than phosphate-buffered saline (PBS). Filter bath sonication following filter vortexing had no positive effects on ELISA sensitivity. The cross-reactivity patterns of mAbs suggest that Aspergillus and Penicillium species share multiple antigens. Quantitative ELISA results for fungal aerosols were found to be influenced by differential sample processing and thus method standardization will be essential to maintain the comparability of immunometric monitoring results.

Molecular and immunological characterization of Pen ch 18, the vacuolar serine protease major allergen of Penicillium chrysogenum

BACKGROUND

We have suggested previously that the 32 and 34 kDa major allergens of Penicillium chrysogenum (also known as P. notatum) are the vacuolar (Pen ch 18) and the alkaline (Pen ch 13) serine proteases, respectively, of P. chrysogenum. The purpose of this study is to characterize the 32 kDa allergen of P. chrysogenum and its immunoglobulin E (IgE)cross-reactivity with Pen ch 13 allergen.

METHODS

The full-length cDNA of Pen ch 18 was isolated by reverse transcriptase-polymerase chain reaction and the 5'-rapid amplification cDNA end reaction. Recombinant Pen ch 18 was expressed as his-tagged proteins in Escherichia coli. Its reactivity with IgE and monoclonal antibodies against fungal serine protease allergens was analyzed by immunoblotting. The IgE cross-reactivity between Pen ch 18 and Pen ch 13 was analyzed by immunoblot inhibition. Overlapping recombinant fragments and synthetic peptides were used to map the B cell epitopes on Pen ch 18.

RESULTS

In this study, we isolated a 1857 bp cDNA fragment containing an open reading frame of 494 amino acids that encodes the preproenzyme of Pen ch 18. Similar to other vacuolar serine proteases, this precursor appears to undergo N- and possibly C-terminal cleavage upon maturation. The his-tagged recombinant Pen ch 18 containing the putative sequence of the mature protein reacted with IgE antibodies in serum samples from asthmatic patients. In addition, IgE-binding to the 32 kDa major allergen of P. chrysogenum was inhibited when a positive serum sample was absorbed with recombinant Pen ch 18 before immunoblotting. Both inhibition and almost no inhibition of IgE-binding to the 32 kDa major allergen of Pen ch 18 were detected when eight positive serum samples were preabsorbed individually with purified Pen ch 13 before immunoblotting. The major IgE binding region was located in a fragment (PN1) encompassing the N-terminal 102 amino acid residues of the recombinant Pen ch 18. A dominant linear IgE epitope was further mapped within residues 73-95 (peptide PN1-e) of the N-terminally processed allergen. Monoclonal antibody FUM20 that reacts with Pen ch 18 but not with Pen ch 13 binds a synthetic peptide with sequence encompassing the N-terminal 23 residues of the recombinant Pen ch 18. Monoclonal antibody PCM39 that reacts with both Pen ch 13 and Pen ch 18 recognizes a peptide containing residues 132-154 of the allergen.

CONCLUSIONS

Our results confirm that the Pen ch 18 allergen is a vacuolar serine protease of P. chrysogenum that matures through N- and possibly C-terminal processing. The finding that there are cross-reactive and allergen-specific IgE epitopes for Pen ch 18 and Pen ch 13 suggests that both major allergens should be included in clinically diagnostic P. chrysogenum extracts.

The prevalence of IgE antibody reactivity against the alkaline serine protease major allergen of Penicillium chrysogenum increases with the age of asthmatic patients

BACKGROUND

Penicillium species are prevalent airborne fungi. However, the prevalence of allergic sensitization to Penicillium antigens and the true impact of these ubiquitous fungi on atopic respiratory disorders remain to be determined.

OBJECTIVE

The purpose of this study was to analyze the prevalence of immunoglobulin (Ig)E and IgG antibodies against Penicillium chrysogenum (Pen ch 13), the alkaline serine protease major allergen of P. chrysogenum, in asthmatic patients of different age groups.

METHODS

Pen ch 13 was purified from a culture medium of P. chrysogenum. The reactivity of IgE and IgG antibodies to Pen ch 13 in the serum samples of 212 asthmatic patients was analyzed by immunoblotting methods.

RESULTS

Sixty-nine (33%) of the 212 sera analyzed showed IgE and/or IgG immunoblot reactivity to Pen ch 13. Significant differences in the prevalence of IgE and/or IgG antibody reactivity to Pen ch 13 were found among eight different age groups of 212 asthmatic patients. The frequency of IgE-binding reactivity to Pen ch 13 increased significantly with the age of the patients. It was 7% for the group less than 10 years old and 42% for the group older than 70 years old. In addition, a significant difference between the prevalence of IgE (7%) and IgG (33%) antibodies against Pen ch 13 in the group aged 10 or less was also found.

CONCLUSIONS

Our study demonstrates that IgE and IgG antibodies specific for Pen ch 13 were detected in approximately one-third of the 212 asthmatic patients analyzed. Our results suggest that allergic sensitization to Pen ch 13, and possibly to other airborne Penicillium species, is more common in older asthmatic patients.

cDNA cloning and immunologic characterization of Pen o 18, the vacuolar serine protease major allergen of Penicillium oxalicum

Penicillium species are prevalent indoor airborne fungi that have been identified as causative agents of human extrinsic bronchial asthma. In the preparation of standardized diagnostic reagents, it is imperative to define the allergens of these ubiquitous fungi. Results from our previous study on P. oxalicum suggest that the 34-kd major immunoglobulin E-reacting component of this prevalent Penicillium species is probably a vacuolar serine protease. The purpose of the present study was to define this major P. oxalicum allergen (Pen o 18) through cDNA cloning and immunologic characterization. The cDNA of Pen o 18 was isolated through a combination of reverse transcriptase-polymerase chain reaction and 5'- and 3'-rapid amplification cDNA ends reactions. The primers used in these reactions were constructed according to the internal amino acid sequences of Pen o 18 and the conserved amino acid sequences of fungal serine proteases. Our results showed that a 1897-bp cDNA with an open reading frame of 503 residues was isolated for the proenzyme of Pen o 18. The encoded protein has a 16-residue signal peptide and a 119-residue prosequence. On maturation, the protein has an N-terminal glutamate that is the 136th residue encoded by the cDNA. Apparently the precursor also undergoes C-terminal processing with the cleavage of about 47 amino acids. The cDNA for Pen c 18 (the vacuolar serine protease allergen from P. citrinum ) was also isolated for comparison. Contrary to a previous report, the C-terminal region of Pen c 18 is similar to that of Pen o 18. Recombinant proteins (rPen o 18 and rPen c 18) with the putative mature N-termini and a his-tag were obtained by expressing the corresponding cDNAs in Escherichia coli. Serum samples from 7 asthmatic patients with immunoglobulin E reactivity to the 34-kd component of P. oxalicum also react to his-tagged recombinant Pen o 18. The presence of immunoglobulin E cross-reactivity between rPen o 18 and rPen c 18 was detected by immunoblot inhibition. Two monoclonal antibodies (PCM39 and FUM20) against fungal serine proteases react with rPen o 18, rPen c 18, and the 35/34-kd components in the corresponding crude fungal extracts. These components also react with immunoglobulin E antibodies in serum samples from asthmatic patients. In conclusion, results obtained confirm that the 34-kd major allergen of P. oxalicum is a vacuolar serine protease. The cDNAs of Pen o 18 and Pen c 18 encode precursor molecules that appear to undergo both N-terminal and C-terminal processing. Constructs beginning with mature N-terminal can be expressed in E. coli to produce recombinant polypeptides that are reactive to monoclonal antibodies or immunoglobulin E antibodies in serum samples from asthmatic patients. Results obtained may provide useful information and materials for preparation of standardized diagnostic reagents in clinical mold allergy.