W11 Phagocytosis Flashcards
Phagocytosis: Roles
protection from pathogens
disposal of damaged/dying (apoptotic) cells
processing and presentation of antigens (Ag)
activation of adaptive immune system
links innate and adaptive immunity
Phagocytes`examples + origin
neutrophils, macrophages (M), dendritic cells
origin: myeloid lineage; generated in bone marrow
Other cells (apart from phagocytes)
mast cells, eosinophils, basophils (myeloid lineage)
natural killer (NK) cells (lymphoid lineage; bone marrow)
Phagocytes def.
identify, ingest, destroy pathogens
neutrophils, M, dendritic cells
belong to the innate immune system
Phagocytes have receptors for opsonins
Phagocytes: Neutrophils (PMN)
polymorphonuclear (PMN) leukocytes
most abundant WBCs (circulating in blood)
early response (inflammation)
phagocytosis and killing of microbes
enzymes: lysozyme, collagenase, elastase
Phagocytes: Neutrophils (PMN) - life span
life span = 8-10h/blood; 4-5 days in tissues
Phagocytes: Macrophages (M)
monocytes (blood; 20-40hrs)
efficient phagocytosis
killing of microbes
secrete inflammatory factors (cytokines) => inflammation
Dendritic cells
skin, mucosa, tissues
capture microbes
phagocytosis
not just to eliminate
present Ag to T cells
link innate and adaptive immune response
Dendritic cells and signals for T cell activation
Signal 1 = antigen recognition by MHC:peptide (Major histocompatibility complex) onto TCR
Signal 2 = co-stimulation by CD80/CD86 (cluster of differentiation - protein on Dcells activated by B cells) onto CD28 (proteins on T cells that provide co-stimulatory signals for T cell activation/survival)
Signal 3 = cytokines released by macrophages
Phagocytosis: Steps
Chemotaxis (mobilisation to site of infection/injury)
Recognition and attachment to microbe/dead cells
Engulfment
Killing/digestion of ingested microbe/dead cells
Phagocyte mobilization: Chemotaxis
movement of cells towards site of infection
guided by chemoattractants
chemoattractants released by
bacteria
- N-formyl-methionine-leucine-phenylalanine peptides (fMLP)
- inflammatory cells
chemokines (IL-8) - damaged tissues
Phagocytosis: Recognition of pathogens
Requirements:
react to invading pathogens (foreign)
no reaction to body’s own tissues (self)
PAMPs = structures shared by groups of related microbes
Pathogen-Associated Molecular Patterns (PAMPs)
present on pathogens and not on host cells
invariant structures: shared by an entire class of pathogens
essential for survival of pathogens
prevents pathogen evasion of immune responses
e.g. ds viral RNA=> replication
e.g. lipopolysaccharide (LPS) => bacterial membrane
Pattern recognition receptors (PRRs)
present on phagocytes (and other cells, e.g. epithelia)
recognize PAMPs
detect foreign invaders or aged/damaged host cells
Toll-like receptors (TLRs)
PRR
plasma membrane, endosomal membrane
C-type lectin receptors (CTLRs)
PRR
e.g. mannose receptor
NOD-like receptors (NLRs)
PRR
reside as free proteins in cytoplasm
RIG-like helicase receptors (RLRs)
PRR
cytosolic receptors for viral dsRNA
Scavenger receptors
PRR
various bacterial wall components (CD14 scavenges LPS-LBP)
PRRs: Toll-like receptors (TLRs)
essential roles in innate immunity
conserved during evolution
stimulate production of inflammatory cytokines
human TLRs recognize PAMPs:
lipolysaccharide (gram negative) lipoteichoic acid (gram positive) bacterial DNA sequences (unmethylated CpG) single/double-stranded viral RNA glucans (fungi)
Phagocytosis: Opsonization
facilitates phagocytosis (recognition of microbes)
- Opsonized microbes can be phagocytosed easier
(via receptors for opsonins on phagocytes)
Clinical note!
=> Encapsulated microorganisms require opsonization with antibodies to be effectively phagocytosed!
- coating of microbes with opsonins:
a) proteins of complement system (C3b, C4b) (CR1 recognizes breakdown products of C3)
b) antibodies (immunoglobulin, Ig)
Phagosome formation and maturation
Microbe present w/actin cytoskeleton rearrangement
Membrane remodelling occurs w/formation of pseudopods (cytoplasmic projections of CM) which wrap CM around microbe = forming phagosome
Lysosomes fuse forming phagolysosome
Pathogen destruction
Killing of pathogens is dependent/independent of oxygen
Killing of pathogens: oxygen-independent
Lysosomes:
proteolytic enzymes (cathepsins): degrade microbes
lysozyme: breaks bacterial walls
lactoferrin: binds iron => not enough left for bacteria
defensins: destroy bacterial walls
Killing of pathogens: oxygen-dependent
resting phagocyte = NADPH + oxidase are not bound
activated phagocyte = assembly of NADPH oxidase
generation of superoxide anion
Killing of pathogens: oxygen-dependent - equations
O2 → O2- (superoxide)
Through oxidase
H2O + O2- → H2O2;OH (hydrogen peroxide; hydroxyl radicals)
Through dismutase
arginine + O2 → NO + citruline Through iNOS (inducible NO synthase)
NO + H2O2 → peroxynitrite radicals
oxidising radicals (ROS and NOS) kill phagocytosed microbes
blocking phagocyte attachment
Streptococcus pneumoniae – encapsulated bacteria
blocking engulfment
Yersinia
blocking destruction
Salmonella – resistant to ROS
Mycobacterium – blocks phagosome-lysosome fusion
killing of phagocytes
Staphylococcus aureus – toxin => damages membranes
Phagocytosis: Other types of prey
Micro-organisms
Damaged or dying cells
normal turnover of cells = 100-200 billion cells/day
apoptosis (programmed cell death)
fast, efficient removal by phagocytes
‘silent removal’: no inflammation
phagocytes: discriminate apoptotic vs. viable cells
Phagocytosis of apoptotic cells
eat-me signals
apoptotic cell
attraction as Ptdser on apoptotic cell binds to phagocytic receptor
Triggers Rac w/GDP → Rac w/GTP
which then generates myosin-II and force generation for wrapping by pseudopods
Repulsion on viable cell
Discrimination apoptotic / viable cells:
Apoptotic cells
‘Eat-me’ signals
Recognized by phagocytes => promote engulfment
Viable cells
‘Don’t eat-me’ signals
Recognized by phagocytes => no engulfment
Phagocytes that have taken up apoptotic cells:
- secrete‘pro-healing’ cytokines
reduce inflammation (e.g. IL-10)
promote wound healing (e.g. TGB-β) - presentation of self antigens
role in maintenance of self tolerance
