Laminin-binding sites on cell walls of the entomopathogenNomuraea rileyiassociated with growth and adherence to host tissues

Mechanism of Metarhizium rileyi evading cellular immune responses in Helicoverpa armigera

Upon entry into the hemocoel of host insects, entomopathogenic fungi switch to yeast-like hyphal bodies that are not recognized by host hemocytes and replicate extensively in the hemolymph. The mechanism by which hyphal bodies evade host cellular immunity is not well understood. This study compares Metarhizium rileyi conidia and hyphal bodies with respect to elicitation of the immune response of Helicoverpa armigera and recognition by host pattern recognition receptors (PRRs). We found that the ability of host hemocytes to phagocytize and nodulate hyphal bodies was weaker than those responses against conidia, suggesting that hyphal bodies are more able to evade host cellular immunity. Additionally, we found that the binding affinity of H. armigera β-1,3-glucan recognition proteins was much lower for hyphal bodies than for conidia. We observed no agglutination response of H. armigera C-type lectin 3 (HaCTL3) against hyphal bodies, and HaCTL3 bound significantly less to hyphal bodies than to conidia, indicating that host PRRs have a lower affinity for hyphal bodies than for conidia. This study provides direct evidence that the mechanism whereby entomopathogenic fungi escape host cellular immunity involves the inability of host PRRs to sufficiently recognize hyphal bodies to elicit the cellular immune response.

Antifungal immune responses in mosquitoes (Diptera: Culicidae): A review

Mosquitoes transmit many parasites and pathogens to humans that cause significant morbidity and mortality. As such, we are constantly looking for new methods to reduce mosquito populations, including the use of effective biological controls. Entomopathogenic fungi are excellent candidate biocontrol agents to control mosquitoes. Understanding the complex ecological, environmental, and molecular interactions between hosts and pathogens are essential to create novel, effective and safe biocontrol agents. Understanding how mosquitoes recognize and eliminate pathogens such as entomopathogenic fungi may allow us to create insect-order specific biocontrol agents to reduce pest populations. Here we summarize the current knowledge of fungal infection, colonization, development, and replication within mosquitoes and the innate immune responses of the mosquitoes towards the fungal pathogens, emphasizing those features required for an effective mosquito biocontrol agent.

Quorum Sensing Activity and Hyphal Growth by External Stimuli in the Entomopathogenic Fungus Ophiocordyceps sinensis

The entomopathogenic fungus Ophiocordyceps sinensis is one of the best known and most precious medicines and health food in China. The blastospores-hyphae (dimorphism) transition of this fungus in host hemolymph is critical for the virulence and the mummification of host larvae. To regulate this transition, the effects of inoculum density and fifteen chemicals including fungal nutrients, fungal metabolites, quorum-sensing molecules (QSMs) and insect hormones on the dimorphism in O. sinensis were investigated in vitro. The blastospores tended to exhibit budding growth when inoculated at 10⁷ blastospores per mL, and hyphal growth at concentrations lower than 10⁶ blastospores per mL. At 10⁵ blastospores per mL, the percentage of hyphal formation decreased with the addition of filtered spent medium containing 10⁷ blastospores per mL, indicating the quorum-sensing effect. Blastospores-hyphae transition in this fungus by fifteen chemicals was varied from no response to dimorphic reversion. The addition of N-acetylglucosamine at three concentrations significantly stimulated hyphal formation while inhibiting budding growth. For the first time, insect hormone 20-hydroxyecdysone was found to be involved in the hyphal formation in fungi. These results open new possibilities to regulate the dimorphism, which would be beneficial for the cultivation of the Chinese cordyceps.

Artificial Cultivation of the Chinese Cordyceps From Injected Ghost Moth Larvae

The Chinese cordyceps, regarded as the 'Himalayan Viagra', is highly valued for its medicinal benefits. The decline of its yield due to over-exploitation and increased market demand have stimulated efforts to artificially cultivate Chinese cordyceps for over half a century. However, successful cultivation of Chinese cordyceps through caterpillar infection by the fungus Ophiocordyceps sinensis (Berk.) and the induction of the fruiting body from each mummified cadaver remains difficult for its complex life cycle. Herein, we report the developmental dynamics of hyphal bodies in hemolymph of injected Thitarodes xiaojinensis (Tu, Ma & Zhang) larvae and the success in artificial cultivation of sexual fruiting bodies from the mummified cadavers in the low-altitude area. We find that not only the numbers of hyphal bodies but also the conversion of hyphal bodies into hyphae played important roles in the mummification of the injected larvae. This cultivation will be beneficial for sustainable utilization of natural resources and provides the possibility for further research on the mechanism of the interaction between pathogenic fungus and host insect.

Characterization of monoclonal antibodies against cell wall epitopes of the insect pathogenic fungus, Nomuraea rileyi: differential binding to fungal surfaces and cross-reactivity with host hemocytes and basement membrane components

Monoclonal antibodies (MAbs) were generated against epitopes on yeast-like hyphal bodies and hyphae of the entomopathogenic hyphomycete, Nomuraea rileyi. Two MAbs (4C10, 2H4) bind to epitopes common to both hyphal bodies and hyphae, whereas MAb 4E9 binds only to hyphal surfaces. 4C10 and 2H4 appear to be directed towards carbohydrate portions of cell surface mannoproteins, as evidenced by similarities in staining patterns between these MAbs and Concanavalin A on Western blots of N. rileyi cell wall extracts. These MAbs cross-react with antigens on blastospore and hyphal surfaces of two other entomopathogenic fungi, Beauveria bassiana and Paecilomyces farinosus in fluorescence microscopy assays, but do not cross-react with a non-entomopathogenic strain of Candida albicans or with Saccharomyces cerevisiae yeasts. MAb 4C10 also cross-reacts with immunocompetent granular hemocytes from Spodoptera exigua (beet armyworm) and Trichoplusia ni (cabbage looper) larvae and with S. exigua plasmatocytes. Electron microscopy revealed that this MAb binds to a component in cytoplasmic granules in the hemocytes, and that surface labeling may be due to the release of this MAb-positive component upon degranulation. MAb 2H4 does not cross-react with granular hemocytes, but does bind to plasmatocytes and hemocytes that tightly adhere to the substrate in monolayer assays. Additionally, MAb 4C10 specifically labels a basement membrane epitope on S. exigua fat body, suggesting that this antibody binds to mannose residues on extracellular matrix glycoproteins. Cross-reactivity of these N. rileyi MAbs with insect hemocyte and tissue components indicates that fungal surface epitopes can mimic host surface molecules, which could explain why N. rileyi hyphal bodies are not recognized by granulocytes and are able to circulate freely in the hemolymph without binding to basement membranes lining the hemocoel.

Comparative analysis of the in vivo and in vitro metabolites produced by the entomopathogen Beauveria bassiana

Beauveria bassiana, like other insect mycopathogens, has evolved mechanisms to penetrate the insect exoskeleton via germ tubes and to replicate in the host hemocoel. Our initial studies have shown that biologically active metabolites released in the hemolymph during the vegetative growth phase of B. bassiana disrupt the host immune response and metamorphosis. These components cause an immediate reduction in filopodial-producing hemocytes and an increase in the level of serum phenoloxidase. Radiolabeling of tissues explanted from healthy versus infected larvae has demonstrated both the induction and repression of polypeptides in B. bassiana infected hemolymph. None of the polypeptides detected with 35S pulse labeling were responsible for the cytotoxic and insecticidal activities detected in infected hemolymph. Western blots of SDS gels containing chromatographic fractions from healthy and infected sera probed with both antibodies against B. bassiana cell homogenates and culture filtrates contained a complex of antigens. The results of lectin labeling and sodium periodate treatments suggested that carbohydrates were the major epitopes being recognized by both monoclonal and polyclonal probes. Key words: insect mycopathogen, Beauveria bassiana, fungal metabolites, entomopathogen, Spodoptera exigua.