Dynamic Host-Pathogen Interactions Result in Fungal Epitope Unmasking

Molecular camouflage is used by a diverse set of pathogens to disguise their identity and avoid recognition by protective host receptors. The opportunistic fungal pathogen Candida albicans is a good example, as it masks the inflammatory component β-glucan in its cell wall to evade detection by the immune receptor Dectin-1. Interestingly, it has been seen that β-glucan becomes unmasked during infection in vivo, though the underlying mechanisms remained unclear. Exposure levels of this epitope may be important, as Dectin-1 mediates protection from some strains of C. albicans and alterations in the organization and composition of the Candida cell wall can influence host responses. This research sought to understand C. albicans cell wall dynamics, particularly within the context of host-pathogen interactions. Special attention was paid to elucidating mechanisms of β-glucan unmasking and we have revealed a novel and dynamic interaction in which neutrophils damage the fungal cell wall via a mechanism involving neutrophil extracellular traps. This damage provoked the disruption of fungal cell wall architecture including increased chitin deposition and β-glucan unmasking at sites of immune attack. Surprisingly, these cell wall changes were also dependent on an active fungal response, which required cell wall integrity signaling and involved relocalization of cell wall remodeling components. Importantly, neutrophil mediated β-glucan unmasking could result in enhanced immune responses to fungi from macrophages, suggesting that this epitope unmasking could have functional consequences during infection. Work we participated in helped elucidate mechanisms involved in baseline fungal epitope masking in the form of Cho1 and phosphatidylserine biosynthesis and also demonstrated that changes to cell wall composition and architecture influence the importance of β-glucan exposure to host responses to C. albicans. Overall, this work helps elucidate the importance of host-pathogen interactions in influencing fungal cell wall dynamics during disseminated candidiasis. Given the importance of the cell wall as a drug target, understanding how this fungus maintains integrity and epitope masking during attack may identify therapeutic targets to aid the treatment of candidiasis. This work also highlights an important concept, which is that microbial cell walls can change dynamically during infection with important consequences for host recognition and immunity

Dynamic Fungal Cell Wall Architecture in Stress Adaptation and Immune Evasion

Deadly infections from opportunistic fungi have risen in frequency, largely because of the at-risk immunocompromised population created by advances in modern medicine and the HIV/AIDS pandemic. This review focuses on dynamics of the fungal polysaccharide cell wall, which plays an outsized role in fungal pathogenesis and therapy because it acts as both an environmental barrier and as the major interface with the host immune system. Human fungal pathogens use architectural strategies to mask epitopes from the host and prevent immune surveillance, and recent work elucidates how biotic and abiotic stresses present during infection can either block or enhance masking. The signaling components implicated in regulating fungal immune recognition can teach us how cell wall dynamics are controlled, and represent potential targets for interventions designed to boost or dampen immunity

Differential adaptation of Candida albicans in vivo modulates immune recognition by dectin-1

Author Summary Dectin-1 is a pattern recognition receptor recognising the fungal cell-wall component, β-glucan, and plays an essential role in controlling C. albicans infections in both mouse and man. Candida albicans is part of the normal human microflora, yet is capable of causing superficial mucosal infections as well as life-threatening invasive diseases, particularly in patients whose immune function is compromised. Here we found that the contribution of Dectin-1 is limited to specific strains of C. albicans ; effects which are due to the differential adaptation of these pathogens during infection. Importantly, C. albicans strains showed variations in both the composition and nature of their cell walls, and it was these differences which influenced the role of Dectin-1. Crucially, we found that we could alter the fungal cell wall, and subsequent interactions with the host, using antifungal drugs. These findings have substantial implications for our understanding of the factors contributing to human susceptibility to infections with C. albicans , but also treatment strategies

Correction: Differential Adaptation of Candida albicans In Vivo Modulates Immune Recognition by Dectin-1

The b -glucan receptor Dectin-1 is a member of the C-type lectin family and functions as an innate pattern recognition receptor in antifungal immunity. In both mouse and man, Dectin-1 has been found to play an essential role in controlling infections with Candida albicans , a normally commensal fungus in man which can cause superficial mucocutaneous infections as well as life-threatening invasive diseases. Here, using in vivo models of infection, we show that the requirement for Dectin-1 in the control of systemic Candida albicans infections is fungal strain-specific; a phenotype that only becomes apparent during infection and cannot be recapitulated in vitro . Transcript analysis revealed that this differential requirement for Dectin-1 is due to variable adaptation of C. albicans strains in vivo , and that this results in substantial differences in the composition and nature of their cell walls. In particular, we established that differences in the levels of cell-wall chitin influence the role of Dectin-1, and that these effects can be modulated by antifungal drug treatment. Our results therefore provide substantial new insights into the interaction between C. albicans and the immune system and have significant implications for our understanding of susceptibility and treatment of human infections with this pathogen

Chemotaxis and swarming in differentiated HL-60 neutrophil-like cells

Abstract The human leukemia cell line (HL-60) is an alternative to primary neutrophils in research studies. However, because HL-60 cells proliferate in an incompletely differentiated state, they must undergo differentiation before they acquire the functional properties of neutrophils. Here we provide evidence of swarming and chemotaxis in differentiated HL-60 neutrophil-like cells (dHL-60) using precise microfluidic assays. We found that dimethyl sulfoxide differentiated HL-60 cells (DdHL-60) have a larger size, increased length, and lower ability to squeeze through narrow channels compared to primary neutrophils. They migrate through tapered microfluidic channels slower than primary neutrophils, but faster than HL-60s differentiated by other protocols, e.g., using all-trans retinoic acid. We found that dHL-60 can swarm toward zymosan particle clusters, though they display disorganized migratory patterns and produce swarms of smaller size compared to primary neutrophils

Large-scale patterning of living colloids for dynamic studies of neutrophil–microbe interactions

© 2018 The Royal Society of Chemistry. Neutrophils are the first white blood cells to respond to microbes and to limit their invasion of the body. However, the growth of the microbes into colonies often challenges the neutrophils ability to contain them. To study the interactions between neutrophils and microbial colonies, we designed an assay for arranging microbes in clusters of controlled size (i.e. living colloids). The patterned microbes in the living colloid are mechanically trapped inside the wells and fully accessible to neutrophils. Using the assay, we studied the interactions between human neutrophils and Candida albicans and Staphylococcus aureus, two common human pathogens. We also probed the susceptibility of C. albicans colloids to caspofungin, a common antifungal drug

Neutrophil Attack Triggers Extracellular Trap-Dependent <i>Candida</i> Cell Wall Remodeling and Altered Immune Recognition

<div><p>Pathogens hide immunogenic epitopes from the host to evade immunity, persist and cause infection. The opportunistic human fungal pathogen <i>Candida albicans</i>, which can cause fatal disease in immunocompromised patient populations, offers a good example as it masks the inflammatory epitope β-glucan in its cell wall from host recognition. It has been demonstrated previously that β-glucan becomes exposed during infection <i>in vivo</i> but the mechanism behind this exposure was unknown. Here, we show that this unmasking involves neutrophil extracellular trap (NET) mediated attack, which triggers changes in fungal cell wall architecture that enhance immune recognition by the Dectin-1 β-glucan receptor <i>in vitro</i>. Furthermore, using a mouse model of disseminated candidiasis, we demonstrate the requirement for neutrophils in triggering these fungal cell wall changes <i>in vivo</i>. Importantly, we found that fungal epitope unmasking requires an active fungal response in addition to the stimulus provided by neutrophil attack. NET-mediated damage initiates fungal MAP kinase-driven responses, particularly by Hog1, that dynamically relocalize cell wall remodeling machinery including Chs3, Phr1 and Sur7. Neutrophil-initiated cell wall disruptions augment some macrophage cytokine responses to attacked fungi. This work provides insight into host-pathogen interactions during disseminated candidiasis, including valuable information about how the <i>C</i>. <i>albicans</i> cell wall responds to the biotic stress of immune attack. Our results highlight the important but underappreciated concept that pattern recognition during infection is dynamic and depends on the host-pathogen dialog.</p></div