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Creativity requires the courage to let get go of certainties.
Erich Fromm
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(PAMPs).
An association between morphogenesis and virulence has long been presumed for dimorphic fungi that are pathogenic to humans, as one morphotype exists in the environment or during commensalism, and another within the host during the disease process. For C. albicans, putative virulence factors include the ability to switch between harmless yeast and pathogenic, filamentous forms of the fungus. DCs sense either form in a specific way, resulting in distinct, T helper cell-dependent protective and non-protective immunities. Recent evidence suggests that the use of distinct recognition receptors contributes to the disparate patterns of reactivity observed locally in response to challenge with C. albicans. The interaction between S. cerevisiae and human societies is so generally recognized to hypothesize domestication of strains of this species.
Successful resolution of pathogenic fungal disease depends on proper coordination of multiple components of the host immune response. The balance between pro- and anti-inflammatory signalling is a prerequisite for safe host-fungal interactions. Although inflammation is an essential component of the protective response to fungi, its deregulation may significantly worsen fungal diseases and limit protective antifungal immune responses.
Host/fungal interactions have so far been studied only for pathogenic fungi such as C.
albicans, but it is not known what determines the commensalism of harmless fungi. Understanding the mechanisms of cohabitation between humans and non-pathogenic fungi is a prerequisite for controlling fungal infections. We investigated the rules of host-fungal cohabitation and dissected the mechanisms responsible for the differential recognition demonstrating that the skewing in the use of pathogen recognition receptors by differences in cell wall composition is a requirement for pathogenicity.
The immune response is a complex entity with many possible inputs, influences and outcomes, and systems biology holds the promise of allowing us to both better understand its nature, and generate predictions and hypotheses about its behaviour under particular conditions.
Immunity is not simply the product of a series of discrete linear signalling pathways; rather it is comprised of a complex set of integrated responses arising from a dynamic network of thousands of molecules subject to multiple influences. Its behavior often cannot be explained or predicted solely by examining its components.
The comparison of the immune response to pure agonists and entire cells clearly illustrated how without addressing the complexity of the response to entire fungi it is impossible to model the immune response. In particular the exposure to entire living cells highlighted the importance of signalling from different cellular compartments dissecting the interplay between outer membrane and phagosomal receptors. Our result indicate the importance of the temporal window between the
initial recognition of the fungus at the cell surface and the stimulation of the other receptors that intensify, complement and sustains the DC activation process.
The integration of multiple stimuli over a defined temporal window is a requirement for an effective response that can be understood only when studying the challenge with pathogenic and non-pathogenic microorganisms in addition to soluble microbial products. This mechanism is of paramount importance for the generation of an adaptive immune response of the appropriate class.
Our transcriptional analyses indicated that one of the major differences between stimulation of DCs with S. cerevisiae yeast cells and Candida hyphae activate a common defined pathway of immune responses, that differs importantly for the differential regulation of sustained transcription of C-type lectins in Candida hyphae, contrary to repression of sensing proteins genes in DCs treated with S. cerevisiae. This evidence supports the observation that the activation of the phagosomal pathways is partially or totally missing from the stimulation with Candida hyphae (similarly to S.
cerevisiae spores) and could be associated with a lack of switch repressing signalling from cell membrane an activating signalling from the phagosome, visible in yeast but not in hyphae. This temporally restricted switch could be crucial in moDCs, balancing tolerogenic and inflammatory responses, in order to guarantee microbial recognition and avoid potentially harmful sustained inflammation processes.
The transcriptional analysis indicated that, far from being inactive microbes, S. cerevisiae yeast cells activate a defined pathway of immune responses in moDCs, balancing each other in order to avoid potentially harmful inflammation processes.
The type of immune response mounted is primarily determined by the initial interaction between the pathogen and the cells of innate immune system. The ability to eradicate fungal infection has been associated to Th1 response. Conversely, a Th17 polarization has been described in several pathological conditions and is activated in response to pathogenic fungi.
It was previously shown that the important differences in the structure of Saccharomyces-derived mannan and Candida-mannan determine differential responses. In contrast to Saccharomyces mannan, Candida-mannan induces potent Th17 response, which suggests that MR is differently used by C. albicans and S. cerevisiae to induce an immune response. We used human monocyte-derived dendritic cells exposed to cells and spores of the yeast S. cerevisiae as well as C. albicans hyphae, as a toolbox to dissect the role of surface mannans which are central for pattern recognition.
Combining transcriptional analysis with receptor-specific blocking and cytokine production assays, we determined that DCs respond differently to C. albicans and S. cerevisiae and in the latter case, the interplay between spores and yeasts is crucial for the commensalism of S.
cerevisiae. In contrast with the yeast cell wall, mannan are present only in the innermost layer of
the spore wall preventing mannan recognition. We identified that, while yeast cells are recognized either by MR and DC-SIGN, spores are recognized by DC-SIGN only, suggesting that not only MR acts differently in response to pathogenic and non-pathogenic fungi but also in response to different form of the same microorganism.
Our results indicate that S. cerevisiae mannan favor a specific individual recognition pathway through mannan-recognizing receptors, leading to different cytokine release and the polarization of a Th1 response. In contrast, the absence of exposed mannan on the spore surface contributes to the Th17 induction by the spores. We demonstrated that the differential recognition of specific mannan structures is the master regulator of the discrimination between harmful and harmless fungi. The higher capacity of Saccharomyces-mannan to induce IL-12/Th1 protective response is likely to play a major role for the non-pathogenicity of Saccharomyces and its commensalism in humans, in contrast with Candida mannan-Th17 induced response.
We demonstrate that the spores are able to elicit a Th17 response, hypothesizing this as a mechanism of escaping host clearance that ultimately may favour yeast dissemination. Thus, we investigated the potential of yeast spores to survive passage through the human host and be disseminated in new environments.
Our results bear importantly on the ecological significance of sporulation and on the central role of fungal cell wall recognition in discrimination between friends and foes.
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Science may set limits to knowledge, but should not set limits to imagination Bertrand Russel