Table 1.

Different mechanisms of eosinophil-mediated host defense against various types of microbial pathogens

Types of antimicrobial immunity	Pathogen recognition by eosinophils	Eosinophil-mediated innate immune responses	Eosinophil-mediated adaptive immune responses	Ref.
Anti-parasitic	eosinophils are stimulated by sIgA-, IgA- and IgG-coated helminthes in vitro via the immunoglobulin receptor FcaR	i) cytotoxic granule proteins (MBP, EPO, ECP, and EDN) are released onto helminthic parasites via eosinophil degranulation (compound exocytosis), causing parasitekilling effects; ii) eosinophils down-regulate parasite-induced IgE responses and airway goblet cell mucus production during infection with Brugia malayi microfilariae	eosinophils present parasitespecific antigens to T cells, promoting polarization of Th2 response and production of parasite-specific IgM; enhance host immunity upon reinfection; and promote schistosomiasisinduced liver fibrosis by production of profibrotic mediators IL-13 and Th2 polarization	[5, 36, 37, 131, 132]
Anti-fungal	eosinophil PAR-2 receptor is cleaved by aspartate proteases produced by Alternaria alternata, exposing neo-ligands that activate eosinophils	the activated eosinophil adheres to the fungi Alternaria alternata by the binding of its CD11b integrin to the fungal cell wall component, β-glucan, leading to eosinophil degranulation and release of cytotoxic granule proteins (MBP and EDN) and chemokines (MIP-1a, MCP-1, IL-8)	N/A	[133, 134]
Anti-bacterial	eosinophil surface receptor PRRs, e.g. TLRs, NOD-like receptors, recognize specific PAMPs expressed on bacteria	i) ingesting bacteria by phagocytosis, and thereafter killing these bacteria in phagocytic vacuoles with MBP and ECP translocated from cytoplasmic granules; ii) bactericidal activity of MBP and ECP, and (or) oxygen-dependent killing by superoxide and EPO, towards extracellular bacteria; iii) eosinophil extracellular traps, comprised of mitochondrial DNA and granule proteins, bind to and kill the trapped bacteria	N/A	[3, 5, 21, 135]
Anti-viral	eosinophil surface receptor PRRs, e.g. TLR7, recognize specific PAMPs of viruses	i) eosinophil-derived EDN has ribonuclease activity with anti-ssRNA virus effect; ii) in RSV infection, stimulation of TLR7-MyD88 signaling by viral ssRNA binding induces expression of anti-viral mediators including IRF-7, NOS-2, and ribonucleases EAR-1 and EAR-2; iii) in RSV or PVM infection, viral replication in eosinophils results in release of infectious virions and proinflammatory mediator IL-6	eosinophils present viral peptides (e.g. RV16 virus) to antigen-specific T lymphocytes, inducing T-cell proliferation and IFN- γ production	[21, 136, 137, 138, 139, 140]

MIP-1α = Macrophage inflammatory protein 1α; MCP-1 = monocyte chemotactic protein 1; NOD = nucleotide-binding oligomerization domain; TLRs = Tolllike receptors; PRRs = pattern-recognition receptors; PAMPs = pathogen-associated molecular patterns; RSV = respiratory syncytial virus; PVM = pneumonia virus of mice.