Allergen-triggered airway hyperresponsiveness and lung pathology in mice sensitized with the biopesticide Metarhizium anisopliae

Differential allergy responses to Metarhizium anisopliae fungal component extracts in BALB/c mice

Intratracheal aspiration (IA) exposure to Metarhizium anisopliae crude antigen (MACA), which is composed of equal protein amounts of mycelium (MYC), conidia (CON) and inducible proteases/chitinases (IND) extracts/filtrates, has resulted in responses characteristic of human allergic asthma in mice. The study objective was to evaluate the potential of each component extract to induce allergic/asthma-like responses observed in this mouse model. BALB/c mice received 4 IA exposures to MACA, CON, MYC, IND, or bovine serum albumin (BSA; negative control) or appropriate vehicle control or inflammatory control over a 4-wk period. Mice were assessed by whole-body plethysmography for immediate airway responses and airway hyperresponsiveness to methacholine (Mch) challenge (PenH). Serum and bronchoalveolar lavage fluid (BALF) were collected 3 d after the final exposure. Additionally, BALF neurotrophin levels and extract protease and chitinase activity levels were evaluated. Western blot analysis showed that each component contained different IgE-reactive proteins. All fungal extract exposures resulted in elevated BALF total and differential cell counts, IgE and IgA and total serum IgE compared to HBSS and BSA controls. MYC-exposed mice had the highest responses except for neutrophil influx, which was highest in MACA and IND exposures. However, the MYC-exposed mice had significantly lower PenH values compared to other treatments. By comparison IND and MACA induced significantly higher PenH values. Additionally, IND had substantially higher protease activity levels but induced the lowest neurotrophin levels compared to the other fungal exposures. In this allergic asthma model extract chitinase activity was not associated with allergic responses. In summary, multiple exposures to any of the M. anisopliae component extracts induced allergic/asthma-like responses in BALB/c mice but the response magnitude was different for each component and each appears to contain unique IgE-reactive proteins. Therefore, hazard identification and/or risk assessment for molds must test both mycelia and conidia.

Increased neurotrophin production in a Penicillium chrysogenum-induced allergic asthma model in mice

Neurotrophins, including nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophin (NT)-3, have been implicated in the pathogenesis of many features and symptoms of asthma. The role of neurotrophins in fungal allergic asthma, however, is unknown. Repeated pulmonary challenge with Penicillium chrysogenum extract (PCE) induces dose-dependent allergic asthma-like responses in mice. The aim of this study was to investigate whether neurotrophins are involved in the PCE-induced allergic airway response in mice. Mice were exposed to 10, 20, 50, or 70 microg PCE by involuntary aspiration 4 times over 1 mo. Bronchial alveolar lavage fluid (BALF) was collected immediately before and after the final exposure. The levels of NGF, NT-3, and NT-4 were determined by enzyme-linked immunosorbent assay (ELISA). The lungs were fixed and processed for immunohistochemical examination of NGF production. PCE-exposed mice had dose-dependent increases in NGF, NT-3, and NT-4 in both BALF and sera. Exposures to PCE produced elevation in positive immunohistochemical staining for NGF in the airway epithelium and smooth muscle cells, in addition to infiltrated cells such as mononuclear cells, eosinophils, and macrophages. Taken together, this is the first study to link fungal allergic asthma in an experimental model with enhanced production of neurotrophins in the airways, and suggests that neurotrophins may play a role in the etiology of mold-induced asthma in humans.

Animal models for protein respiratory sensitizers

Protein induced respiratory hypersensitivity, particularly atopic disease in general, and allergic asthma in particular, has increased dramatically over the last several decades in the US and other industrialized nations as a result of ill-defined changes in living conditions in modern western society. In addition, work-related asthma has become the most frequently diagnosed occupational respiratory illness. Animal models have demonstrated great utility in developing an understanding of the etiology and mechanisms of many diseases. A few models been developed as predictive models to identify a protein as an allergen or to characterize its potency. Here we describe animal models that have been used to investigate and identify protein respiratory sensitizers. In addition to prototypical experimental design, methods for exposure route, sample collection, and endpoint assessment are described. Some of the most relevant endpoints in assessing the potential for a given protein to induce atopic or allergic asthma respiratory hypersensitivity are the development of cytotropic antibodies (IgE, IgG1), eosinophil influx into the lung, and airway hyperresponsiveness to the sensitizing protein and/or to non-antigenic stimuli (Mch). The utility of technologies such as PCR and multiplexing assay systems is also described. These models and methods have been used to elucidate the potential for protein sources to induce allergy, identify environmental conditions (pollutants) to impact allergy responsiveness, and establish safe exposure limits. As an example, data are presented from an experiment designed to compare the allergenicity of a fungal biopesticide Metarhizium anisopliae (MACA) crude extract with the one of its components, conidia (CON) extract.

Lack of acute pathogenicity and toxicity in mice of an isolate of Metarhizium anisopliae var. anisopliae from spittlebugs

A monospore strain of Metarhizium anisopliae var. anisopliae (EH-479/2), isolated in Mexico from Aeneolamia sp., was tested for oral acute intragastric pathogenicity and toxicity in CD-1 mice, including a thorough histological study. Animals were inoculated by gavage with one dose (10(8) conidia/animal) of viable (72 mice) and nonviable (24 mice) conidia and compared to 18 control mice. Clinical observations were done daily; mycological and histological tests were performed during necropsies at days 3, 10, 17, and 21 after the inoculation. At the end of the study, no mice showed clinical symptoms of illness, and the animals' mean weight corresponded to that of healthy adults. No inflammatory reactions were identified in analyzed organs, suggesting the nonimmunogenic status of this fungal strain. Evidence of fungal germination was noted in two lymph nodes and in liver and lung of one dead mouse, out of 72 viable-conidia treated mice. There was no evidence of toxicity symptoms in mice inoculated with nonviable conidia. The results obtained support the nonpathogenic and nontoxic status of this fungal strain when administered in a sole intragastric dose in mice.

Molecular and immunological characterization of allergens from the entomopathogenic fungus Beauveria bassiana

Background

Entomopathogenic fungi such as Beauveria bassiana are considered promising biological control agents for a variety of arthropod pests. Beauveria species, however, have the potential to elicit allergenic reactions in humans, although no specific allergens have been characterized to date.

Methods

Four putative allergens were identified within B. bassiana expressed sequence tag (EST) datasets. IgE-reactivity studies were performed using sera from patients displaying mold allergies against recombinant B. bassiana proteins expressed in E. coli.

Results

Full length cDNA and genomic nucleotide sequences of four potential B. bassiana allergens were isolated. BLASTX search results led to their putative designation as follows; Bb-Eno1, with similarity to fungal enolases; Bb-f2, similar to the Aspergillus fumigatus major allergen, Asp f2 and to a fibrinogen binding mannoprotein; Bb-Ald, similar to aldehyde dehydrogenases; and Bb-Hex, similar to N-acetyl-hexosaminadases. All four genes were cloned into E. coli expression systems and recombinant proteins were produced. Immunoblots of E. coli extracts probed with pooled as well as individual human sera from patients displaying mould allergies demonstrated IgE reactivity versus recombinant Bb-Eno1 and Bb-Ald.

Conclusion

Four putative Beauveria bassiana allergens were identified. Recombinant proteins corresponding to two of the four, Bb-Eno1 and Bb-Ald were bound by sera IgEs derived from patients with fungal allergies. These data confirm the potential allergenicity of B. bassiana by identification of specific human IgE reactive epitopes.

The fungal biopesticide Metarhizium anisopliae has an adjuvant effect on the allergic response to ovalbumin in mice

The parasitic fungus, Metarhizium anisopliae, is non-pathogenic to humans and licensed for indoor control of cockroach infestation. An important reason for the elimination of this vermin is that sensitisation to cockroaches is associated with asthma. Previously M. anisopliae has been shown to cause allergic- and asthma-like responses in mice and in the present study we have examined the adjuvant activity of M. anisopliae on the allergic response to the model allergen ovalbumin (OVA) in a mouse model. Levels of OVA-specific IgE, IgG1 and IgG2a in serum were measured and the weight and cell number of the excised popliteal lymph node were determined. Mice primed with mycelium+OVA and boosted with OVA had increased anti-OVA IgE and IgG1 levels compared with mice primed with OVA alone or mycelium. Priming with M. anisopliae (as mycelium or MACA) increased weight or cell number of the excised PLNs. These results suggest that M. anisopliae has the ability to increase an allergic response to an allergen and consequently, may worsen allergy in susceptible individuals.

Dose-dependent allergic responses to an extract of Penicillium chrysogenum in BALB/c mice

Indoor mold has been associated with the development of allergic asthma. Penicillium chrysogenum, a common indoor mold, is known to have several allergens and can induce allergic responses in a mouse model of allergic penicilliosis. Our hypothesis is that soluble components of P. chrysogenum (PCE) can dose-dependently induce responses typical of allergic asthma in BALB/c mice. Mice were exposed to 10, 20, 50, or 70 microg of PCE by involuntary aspiration four times over a 4-week period. Serum and bronchoalveolar lavage fluid (BALF) were collected before (day 0), and at days 1 and 3 following the final exposure. PCE-exposed mice demonstrated dose-dependent increases in: BALF total cell numbers including eosinophil, serum and BALF total IgE levels, BALF IL-5 levels, and increased severity of histopathologic lesions. A single exposure to the highest dose of PCE resulted in edema and cellular damage but not immune responses. Four exposures to Metarhizium anisopliae crude antigen (10 microg, positive control) resulted in equivalent or greater allergic asthma-like responses than those demonstrated by multiple exposures to 50 or 70 microg of PCE. Multiple exposures to 70 microg of PCE showed increased allergen-triggered immediate respiratory responses as well as non-specific airway hyperresponsiveness to methacholine as assessed by barometric whole-body plethysmography. Taken together, repeated pulmonary challenge with P. chrysogenum extract induced dose-dependent allergic asthma-like responses in mice.

Biopesticides

The term 'biopesticide' encompasses a broad array of microbial pesticides, biochemicals derived from micro-organisms and other natural sources, and processes involving the genetic incorporation of DNA into agricultural commodities that confer protection against pest damage (plant-incorporated protectants). Some microbial pesticides, such as Bacillus thuringiensis, have a long history of safe and effective use as a biological insecticide. More recent developments in microbial pest control include the utilisation of other bacterial and fungal species that may competitively inhibit the growth of pathogenic and toxigenic micro-organisms on important agricultural commodities. The use of microbes and their gene products introduces additional considerations to the toxicological dose-response relationship, including a need to determine the plausibility of infectious and immunological effects in association with human exposure to these biopesticides in food or the environment. Studies of substantial equivalence suggest that foods currently derived from plant-incorporated protectants are not likely to differ from conventional foods. However, there is general consensus that the scientific methods to assess risks from genetically modified foods and micro-organisms will continue to evolve in the future.

Comparison of respiratory responses to Metarhizium anisopliae extract using two different sensitization protocols

Metarhizium anisopliae, an entomopathogenic fungus, is a prototypic microbial pesticide licensed for indoor control of cockroaches, a major source of allergens. We have previously demonstrated allergy and asthma-like responses in BALB/c mice intraperitoneally (IP) sensitized in the presence of adjuvant and intratracheally (IT) challenged with the soluble factors from M. anisopliae crude antigen (MACA) (Ward et al., 1998, 2000). This protocol has been used frequently to establish animal models of allergenicity. However, the sensitization protocol is artificial and not representative of an environmental exposure. Concern has been raised that this protocol might produce allergic responses that would not occur under normal environmental exposure conditions. The objective of this study was to compare responses in mice to MACA by two exposure protocols: (1) exclusive respiratory exposures without adjuvant (representative of environmental exposures) and (2) intraperitoneal sensitization in the presence of adjuvant followed by IT challenge (the traditional approach). The intratracheal protocol consisted of four IT exposures of 10 microg MACA in 50 microl HBSS each over a 4-week period. A vehicle control group of mice was exposed IT to HBSS. The intraperitoneal protocol consisted of IP sensitization with 25 microg MACA in 0.2 ml of 1.3% alhydrogel (aluminum hydroxide) followed 14 days later with an IT challenge (10 microg MACA/50 microl HBSS). Airway reactivity responsiveness to methacholine was assessed, serum and bronchoalveolar lavage fluid (BALF) samples were obtained, and the lungs were fixed for histopathology at 1, 3, and 8 days following the last MACA IT challenge. Both groups exhibited immune and pulmonary responses typical of allergic asthma. In general, local responses in the lung, including inflammatory responses (eosinophils, lymphocytes, and macrophages), BALF IgE, and functional responses to methacholine were greater in the IT sensitized group compared to the IP sensitized group, whereas the systemic IgE response was greater in the IP sensitized group. The BALF IL-5 cytokine levels were elevated before and throughout the eosinophil influx. IL-4 was detected in the BALF of IP sensitized, but not IT sensitized mice. Histopathologic changes in the two groups were similar in nature but more severe in the IT mice. The results suggest that the IP sensitization protocol does not induce the level of respiratory responsiveness that results from sensitization by a physiologically relevant route of exposure. Thus total serum IgE levels, which were greater following IP sensitization, may not be the best indicator of allergen potency, at least with respect to respiratory responses.