Extracellular vesicles in innate immunity : Proteomic investigations

Extracellular vesicles (EVs) are small, membranous entities secreted from most eukaryotic cells at both homeostatic and stress conditions. Carrying active biological molecules – nucleic acids, lipids and proteins – EVs serve as important means of intercellular communication. In the immune system, EVs circulting in body fluids play important modulatory roles in coordination of responses. EVs are integral part of secretome – all proteins secreted by cells. EVs provide means for secretion of proteins that are not trafficked through conventional, ER/Golgi-mediated mechanisms. Analysis of EV proteome provides basis for fundamental discoveries in understanding the biogenesis, secretion and delivery of these nanosized messengers to target cells. Macrophages are principal tissue-resident effector cells of innate immune system, performing surveillance of their neighborhood in search for danger. Macrophages express pattern recognition receptors (PRRs) which recognize conserved pathogen-associated molecular patterns (PAMPs) – conserved range of molecules expressed by pathogens. PRRs also recognize endogeneous ligands that appear in the human body in other dangerous conditions and diseases. These are collectively called danger-associated molecular patterns (DAMPs). Recognition of PAMPs and DAMPs by PRRs triggers intracellular signaling, leading to activation of macrophage defense mechanisms. These begin with inflammation and secretion of pro-inflammatory cytokines, but many other proteins are also actively secreted by activated macrophages. Although inflammatory cytokine secretion is a well-studied process, very few studies investigating overall and EV-mediated protein secretion from human macrophages were performed. This thesis aims at broadening our understanding of EV-mediated protein secretion from human macrophages activated in response to selected innate immune activators, namely extracellular adenosine triphosphate (ATP), a potent DAMP released during cell damage; (1,3)-β-glucan, a polysaccharide component of fungal cell wall, activating dectin-1-mediated signaling and influenza A virus (IAV) common seasonal pathogenic virus targeting lung epithelia and macrophages. Total secretome and purified EVs were analyzed using different high-throughput mass spectrometry-based methods and further explored using bioinformatic and biochemical studies. The presented results provide evidence that EV-mediated protein secretion is an integral part of responses of human macrophages to studied stimuli. Its activation is demonstrated with quantitative and comparative proteomic approaches. The secreted vesicles are characterized by a broad size range consistent with both exosomes and microvesicles, demonstrating that both types of vesicular structures are involved in protein secretion. The EVs carry distinct set of immunologically important proteins, and bioinformatic analysis suggests that the secreted EVs may exert immunomodulatory effects on recipient cells. It is shown that during IAV infection, EVs are mediators of proinflammatory and antiviral cytokine release, thus they may serve as protective capsules of targeted cytokine delivery. Proteomic analysis identified also a broad set of DAMPs unconventionally secreted in association with EVs, further extrapolating their function towards danger signaling in cellular immune responses. The study on ATP-mediated responses further investigates the intracellular signaling involving calpains, abundant cytosolic proteases, identifying their crucial roles downstream P2X7 receptor: in EV release, as well as inflammasome activation and IL-1β secretion. The data presented here indicate that EVs serve as unconventional means for secretion of a broad range of proteins secreted in PRR-mediated responses in human macrophages. Bioinformatic and functional analysis identifies potential processes involved in their generation as well as their roles in intercellular communication. Together, the presented thesis contributes to our understanding of unconventional, EV-mediated protein secretion in macrophage responses towards common, physiologically releveant threats. The studies presented here will serve as basis for further detailed functional analysis of the roles of EVs in communication between macrophages and other immune system cells. It will also lay the grounds for future studies involving EV-mediated macrophage responses in patients with fungal and viral infections.