β-glucan dependent shuttling of conidia from neutrophils to macrophages occurs during fungal infection establishment

Abstract

41The initial host response to fungal pathogen invasion is critical to infection 42 establishment and outcome. However, the diversity of leukocyte-pathogen 43 interactions is only recently being appreciated. We describe a new form of inter-44 leukocyte conidial exchange called "shuttling". In Talaromyces marneffei and 45 Aspergillus fumigatus zebrafish in vivo infections, live imaging demonstrated 46 conidia initially phagocytosed by neutrophils were transferred to macrophages.47 Shuttling is unidirectional, not a chance event, involves alterations of phagocyte 48 mobility, inter-cellular tethering, and phagosome transfer. Shuttling kinetics 49 were fungal species-specific, implicating a fungal determinant. β-glucan serves as 50 a fungal-derived signal sufficient for shuttling. Murine phagocytes also shuttled 51 in vitro. The impact of shuttling for microbiological outcomes of in vivo infections 52 is difficult to specifically assess experimentally, but for these two pathogens, 53 shuttling augments initial conidial redistribution away from fungicidal 54 neutrophils into the favourable macrophage intracellular niche. Shuttling is a 55 frequent host/pathogen interaction contributing to fungal infection 56 establishment patterns. 57 58 59 150 words / 150 words allowed 60 61 Pazhakh et al: Fungal spore shuttling between phagocytes Page 3/58 62 Introduction63 In vertebrates, two phagocytic cell types have long been recognized as key 64 players in the initial host defense response to infection: neutrophil granulocytes 65 and macrophages [1]. Neutrophils and macrophages share many features: they 66 are both migratory cells, they phagocytose microorganisms on encountering 67 them, and they have intracellular mechanisms for killing microorganisms.68 However, although both phagocyte types engulf microorganisms, individual 69 microorganisms interact with neutrophils and macrophages with different 70 species-specific preferences, in different ways, and using different molecular 71 mechanisms [2]. Conversely, the host has evolved diverse cellular strategies for 72 these two different phagocytes to protect against the panoply of potentially 73 pathogenic microorganisms. 74 75 The exchange of cytoplasmic material through contact-dependent mechanisms 76 between adjacent cells is currently a topical field in cell biology. An example is 77 the contact-dependent exchange of cytoplasm from macrophage to tumor cells as 78 a metastasis-promoting mechanism [3], distinct from the cytoplasmic exchange 79 between macrophages and tumor cells that occur via extracellular vesicles and 80 nanotubes [4-6].81 82 During infections, neutrophils and macrophages also engage in intercellular 83 exchanges. Some microorganisms have evolved mechanisms that exploit these to 84 enhance their pathogenicity and promote their spread between phagocytes. For 85 example, Yersinia pestis and Leishmania promastigotes induce apoptosis in host 86 neutrophils to then exploit efferocytosis, whereby clearance of dead neutrophils Pazhakh et al: Fungal spore shuttling...

Pulses of Class I PI3kinase activity identify the release and recapture of prey from neutrophil phagosomes

“…Cycles of PHAkt-eGFP re-recruitment often occurred multiple times (Fig. 1H), typically x4 on each phagosome, but with up to 21 pulses observed on a phagosome containing a single bacteria which had been 'shuttled' 13 from another neutrophil. Some neutrophils phagocytosed more bacteria than others and we wondered whether these 'hungry phagocytes' had more pulsing phagosomes, perhaps reflecting exhaustion of phagocytic capacity 14 .…”