Yeast killer systems

Immunity to K1 killer toxin: internal TOK1 blockade

K1 killer strains of Saccharomyces cerevisiae harbor RNA viruses that mediate secretion of K1, a protein toxin that kills virus-free cells. Recently, external K1 toxin was shown to directly activate TOK1 channels in the plasma membranes of sensitive yeast cells, leading to excess potassium flux and cell death. Here, a mechanism by which killer cells resist their own toxin is shown: internal toxin inhibits TOK1 channels and suppresses activation by external toxin.

A viral double-stranded RNA up regulates the fungal virulence of Nectria radicicola

Double-stranded RNAs (dsRNAs) are widespread in plant pathogenic fungi, but their functions in fungal hosts remain mostly unclear, with a few exceptions. We analyzed dsRNAs from Nectria radicicola, the causal fungus of ginseng root rot. Four distinct sizes of dsRNAs, 6.0, 5.0, 2.5, and 1.5 kbp, were detected in 24 out of the 81 strains tested. Curing tests of individual dsRNAs suggested that the presence of 6.0-kbp dsRNA was associated with high levels of virulence, sporulation, laccase activity, and pigmentation in this fungus. The 6.0-kbp dsRNA-cured strains completely lost virulence-related phenotypes. This 6.0-kbp dsRNA was reintroduced by hyphal anastomosis to a dsRNA-cured strain marked with hygromycin resistance, which resulted in the restoration of virulence-related phenotypes. These results strongly suggest that 6.0-kbp dsRNA up regulates fungal virulence in N. radicicola. Sequencing of several cDNA clones derived from 6.0-kbp dsRNA revealed the presence of a RNA-dependent RNA polymerase (RDRP) gene. Phylogenetic analysis showed that this gene is closely related to those of plant cryptic viruses. Biochemical analyses suggested that the 6.0-kbp dsRNA may regulate fungal virulence through signal-transduction pathways involving cyclic AMP-dependent protein kinase and protein kinase C.

Therapy of mucosal candidiasis by expression of an anti-idiotype in human commensal bacteria

Two recombinant strains of Streptococcus gordonii, secreting or displaying a microbicidal single-chain antibody (H6), and stably colonizing rat vagina, were used to treat an experimental vaginitis caused by Candida albicans. A post-challenge intravaginal delivery of the H6-secreting strain was as efficacious as fluconazole in rapidly abating the fungal burden. Three weeks after challenge, 75% and 37.5% of the rats treated with the H6-secreting or displaying bacteria, respectively, were cured of the infection, which persisted in 100% of the animals treated with a S. gordonii strain expressing an irrelevant single-chain antibody. Thus, a human commensal bacterium can be suitably engineered to locally release a therapeutic antibody fragment.

The incidence of killer activity of non-Saccharomyces yeasts towards indigenous yeast species of grape must: potential application in wine fermentation

Fourteen killer yeasts were assayed for their ability to kill species of yeast that are commonly associated with fermenting grape must and wine. A total of 147 of a possible 364 killer-sensitive interactions were observed at pH 4.5. Of the killer yeasts studied, Pichia anomala NCYC 434 displayed the broadest killing range. At a pH value comparable with those of wine ferments, pH 3.5, the incidence of killer-sensitive interactions was reduced by 700% across all the yeasts. Williopsis saturnus var. mrakii CBS 1707 exhibited the broadest killing range at the lower pH, killing more than half of the tester strains. Intraspecific variation in sensitivity to killer yeasts was observed in all species where more than one strain was tested. Also, in strains of Pichia anomala, Kluyveromyces lactis and Pichia membranifaciens, the three species in which more than one killer yeast was analysed, intraspecific variation in killer activity was observed.

Prevention of yeast spoilage in feed and food by the yeast mycocin HMK

The yeast Williopsis mrakii produces a mycocin or yeast killer toxin designated HMK; this toxin exhibits high thermal stability, high pH stability, and a broad spectrum of activity against other yeasts. We describe construction of a synthetic gene for mycocin HMK and heterologous expression of this toxin in Aspergillus niger. Mycocin HMK was fused to a glucoamylase protein carrier, which resulted in secretion of biologically active mycocin into the culture media. A partial purification protocol was developed, and a comparison with native W. mrakii mycocin showed that the heterologously expressed mycocin had similar physiological properties and an almost identical spectrum of biological activity against a number of yeasts isolated from silage and yoghurt. Two food and feed production systems prone to yeast spoilage were used as models to assess the ability of mycocin HMK to act as a biocontrol agent. The onset of aerobic spoilage in mature maize silage was delayed by application of A. niger mycocin HMK on opening because the toxin inhibited growth of the indigenous spoilage yeasts. This helped maintain both higher lactic acid levels and a lower pH. In yoghurt spiked with dairy spoilage yeasts, A. niger mycocin HMK was active at all of the storage temperatures tested at which yeast growth occurred, and there was no resurgence of resistant yeasts. The higher the yeast growth rate, the more effective the killing action of the mycocin. Thus, mycocin HMK has potential applications in controlling both silage spoilage and yoghurt spoilage caused by yeasts.

A molecular target for viral killer toxin: TOK1 potassium channels

Killer strains of S. cerevisiae harbor double-stranded RNA viruses and secrete protein toxins that kill virus-free cells. The K1 killer toxin acts on sensitive yeast cells to perturb potassium homeostasis and cause cell death. Here, the toxin is shown to activate the plasma membrane potassium channel of S. cerevisiae, TOK1. Genetic deletion of TOK1 confers toxin resistance; overexpression increases susceptibility. Cells expressing TOK1 exhibit toxin-induced potassium flux; those without the gene do not. K1 toxin acts in the absence of other viral or yeast products: toxin synthesized from a cDNA increases open probability of single TOK1 channels (via reversible destabilization of closed states) whether channels are studied in yeast cells or X. laevis oocytes.

Novel strategies for treating candidiasis

In this review the recent achievements in the field of chemotherapy and immunotherapy of candidiasis are outlined. The current limitations of chemotherapeutic approaches and the lack of well defined preventive tools and strategies make it imperative to exploit the potential of novel immunotherapeutic venues. In this prospect the rather extensive work performed on cytokine patterns in experimental models and the upsurge of antibodies as one mechanism of anti-Candida protection raises great promise for the clinical use of immunotherapy possibly in conjunction with chemotherapy to improve the fight against this increasingly prevalent human opportunistic infection.

Effect of a killer toxin of Pichia anomala to Pneumocystis. Perspectives in the control of pneumocystosis

Despite the development of drugs in the prophylaxis of pneumocystosis, Pneumocystis carinii remains a major opportunistic microorganism in immunosuppressed individuals, especially in human immunodeficiency virus-infected patients. Since side effects were frequently observed after administration of trimethoprim-sulfamethoxazole or pentamidine, the drugs which are mainly used in treating human P. carinii pneumonia (PCP), new therapeutic strategies should be developed. Over the last years, the inhibitory effect of a Pichia anomala killer toxin (PaKT), a molecule with a wide spectrum of antimicrobial activity, was characterized on P. carinii. The susceptibility of mouse and rat-derived Pneumocystis to PaKT has been demonstrated by in vitro attachment tests and in vivo infectivity assays. Nevertheless, PaKT is strongly antigenic, toxic and could not be used directly as a therapeutic agent. Then, a new strategy using killer toxin-like anti-idiotypic antibodies (KT-antiIds) mimicking the fungal toxin activity has been developed. Different KT-antiIds were obtained by idiotypic immunization with a monoclonal antibody (mabKT4). This mabKT4 neutralized the killer properties of the PaKT. KT-antiIds were produced by immunization against the variable domain (idiotype) of mAbKT4 (internal image of the killer toxin receptor), or they were obtained directly from vaginal fluid of patients affected by recurrent vaginal candidiosis. In this last case, such natural KT-antiIds were immunopurified by affinity-chromatography with mAbKT4 and their anti-P. carinii activity was then evaluated. Our results showed that both the in vitro attachment of rat-derived parasites and their infectivity to nude rats were inhibited by the KT-antiIds. With regard to KT-antiIds obtained by immunization, the antimicrobial activity of a monoclonal KT-antiIds (mAbK10) has been evaluated by using a PCP experimental nude rat model treated by mAbK10 administered by aerosol. The pneumocystosis extension was significantly reduced in this model. The monoclonal KT-antiIds were effective against P. carinii in reducing parasite proliferation in lungs of nude rats. Further experiments are in progress to study the in vivo anti-P. carinii activity of KT-antiIds by using recombinant single-chain of the variable fragment of KT-antiIds. Yeast killer toxin-like recombinant molecules could provide the basis for a new therapeutic strategy towards the control of pneumocystosis.

A transphyletic anti-infectious control strategy based on the killer phenomenon

A strategy for the prevention and control of candidiasis, pneumocystosis, and tuberculosis, based on the idiotypic network of the yeast killer effect has been envisaged. Anti-idiotypic antibodies representing the internal image of a candidacidal, pneumocysticidal, and mycobactericidal killer toxin from Pichia anomala and idiotypes of killer toxin-neutralizing monoclonal antibodies mimicking the specific cell wall receptor of sensitive microorganisms might provide a unique approach for engineering innovative antibiotics and vaccines active against taxonomically unrelated pathogenic microorganisms. The rationale of the strategy relies on a phenomenon of microbial competition which has been mutated by the immune system in the response to natural infections.

Inhibitory effect of human natural yeast killer toxin-like candidacidal antibodies on Pneumocystis carinii

BACKGROUND

Human natural antibodies have been found that owe their candidacidal action to the mimicry of a yeast killer toxin produced by the yeast Pichia anomala (PaKT). Candidacidal human natural antibodies (KTAb) are elicited by and bind to a KT receptor (PaKTR) present on the cell surface of infectious PaKT-sensitive microorganisms. Because of the recognized susceptibility of Pneumocystis carinii organisms to PaKT upon the occurrence of specific PaKTR, we examined whether human natural KTAb could also bind to and inhibit P. carinii.

MATERIALS AND METHODS

Immunoaffinity-purified KTAb from the vaginal fluid of patients affected by candidiasis were tested and compared with PaKT for their ability to inhibit rat-derived P. carinii attachment to epithelial lung cells as well as infectivity to nude rats. Immunofluorescence studies were also performed by biotinylated PaKT in competition with human KTAb to establish their specific binding to PaKTR on the surface of rat-derived and human P. carinii organisms.

RESULTS

Human natural candidacidal KTAb exerted a strong, specific inhibitory activity against rat-derived P. carinii organisms that are susceptible to PaKT itself. The antimicrobial activity of human KTAb was abolished by adsorption with a specific PaKT-neutralizing mAb KT4. Immunofluorescence studies of competition with PaKT showed that human KTAb efficiently bind to the specific PaKTR on the surface of rat-derived and human P. carinii organisms.

CONCLUSIONS

The results strongly suggest that human KTAb, elicited by a common transphyletic receptor of different pathogenic microorganisms during infection, may play a role in antibody-mediated cross-immunity and, if properly engineered, as functionally equivalent recombinant antibodies they could exert a therapeutic activity against pneumocystosis in vivo.