Immune Cells Flashcards
Give an overview of immune cells?
Specialisations so they can perform their specific immunological functions e.g.
Plasma B cells secrete huge amounts of antibodies and have a vastly expanded ER
Dendritic cells use macropinocytosis to capture antigens for presentation to T cells
Macrophages use phagocytosis to take up microorganisms
Cytotoxic T lymphocytes kill pathogen infected and cancerous cells via the exocytosis of secretory lysosomes
Describe macrophages?
Macrophages and neutrophils are the major phagocytic cells of the immune system
They are present in tissues and are an important defence against microbial infection and remove cellular debris
Tissue macrophages are derived from monocytes that circulate in the blood stream
Monocytes have a horse shoe shaped nucleus
Monocytes are recruited to infected and damaged tissues, where they differentiate into additional macrophages
Describe phagocytosis?
A specialised type of endocytosis that takes up large particulate species ≥ 0.5mm such as microorganisms
It plays a role in removing dead cells, microbes or foreign debris
Microorganisms are surrounded by a membrane derived from the plasma membrane to form a membrane bound vesicle: the phagosome
Subsequent membrane fusion reactions deliver antimicrobial molecules to the phagosome: a process termed maturation
The phagocytosed microorganism is typically killed within a phagolysosome
What triggers phagocytosis?
It is receptor mediated
Phagocytosis of microorganisms is triggered/initiated by the recognition/binding of a microorganism by receptors on the macrophage cell surface
Macrophage receptors can either bind to the microorganism directly or bind to molecules that have opsonised (coated) the microorganism
Pattern recognition receptors bind to microbes directly examples include:
CD36, mannose receptor, dectin-1 etc.
Opsonic receptors bind opsonised microorganisms
Fc receptors bind immunoglobulin coated microorganisms
Complement receptors bind complement coated microorganisms
They also have receptors e.g. Toll-like receptors and some G-protein coupled receptors involved in priming/regulation of phagocytosis
How is phagocytosis initiated?
The IgG receptor FcgRIII, binds IgG opsinised microorganisms
Clustering of FcgRIII caused by binding to multiple sites on an IgG opsinised microorganism promotes phosphorylation of intracellular tyrosine activation motifs (ITAM) motifs on FcgRIII by Src protein kinases
The protein kinase Syk is recruited and acts on cellular targets to promote phagocytosis and other processes associated with macrophage activation
The small GTPases Rho, Ras, Rac and cdc42 are key downstream targets of Syk
Describe the initial formation of the phagosome?
The rearrangement of the actin cytoskeleton plays a central role in phagosome formation
Rac and cdc42 GTPases interact with Wiskott-Aldrich syndrome protein (WASP), which activates Arp2/3 to promote actin filament formation in FcgR receptor triggered phagocytosis
Psuedopods (finger like membrane projections) are pushed out from the plasma membrane by actin polymerisation and reorganisation
These membrane projections envelope the microorganism to form the phagosome
Describe the maturation of phagosomes?
The phagosome membrane is initially derived from the plasma membrane
Phagosomes mature by the sequential fusion of early endosomes, late endosomes and eventually phagolysosomes
As it matures the phagosome contents become more acidic and resemble that of lysosomes
Early phagosomes can’t remove foreign particles pH - 6.1 and deficient hydrolytic activity
A complex machinery regulates phagosome maturation, including the small GTPases Rab5 and Rab7, which associate with the phagosome as it matures in the late phagosome stage
Rab5 and Rab7 act sequentially to recruit other proteins onto the phagosome membrane
During its maturation the phagosome moves along microtubules towards a perinuclear location (towards the nucleus)
What are some antimicrobial activities of macrophage phagolysosomes?
The phagolysosome has multiple antimicrobial activities:
Nutrient depravation: lactoferrin
Membrane permeabilization via defensins
Hydrolases: lysozyme, phospholipases, proteases
Production of reactive oxygen and reactive nitrogen species
Acidification
What subset of microorganisms can escape/survive within macrophages?
Mycobacterium tuberculosis
Listeria monocytogenes
Describe the evasion of macrophage killing by mycobacterium tuberculosis?
The infectious agent that causes tuberculosis, a disease which primarily affects the lungs
M. tuberculosis is adapted to survive within macrophages
It is phagocytosed by macrophages, but arrests phagosome maturation at an early stage preventing phagolysosome formation
Rab5 is present on the arrested phagosomes, but fails to recruit other proteins onto the phagosome membrane
It utilises a number of mechanisms to block phagosome formation, including:
- SapM hydrolyses phosphatidylinositide-3-phosphate inhibiting phagosome maturation
- Phosphatidylinositide mannoside promotes fusion of early endosomes with M. tuberculosis containing phagosomes - inhibiting phagosome acidification
- Reducing intracellular Ca2+ preventing the development of late phagosomes
What is used to promote the macrophage killing of M.tuberculosis?
Interferon-ɣ (IFN-ɣ)
This is released by T-cells and natural killer cells
IFN-ɣ induces macroautophagy of M. tuberculosis arrested phagosomes
Autophagosomes are delivered to lysosomes, thus circumventing the block in phagosome maturation resulting in microbial killing
Describe the evasion of macrophage killing by Listeria monocytogenes?
L. monocytogenes is the causative agent of listeriosis
It is captured by macrophage scavenger receptors and complement receptors it then modifies and escapes from phagosomes
Listeriolysin O (LLO) is a toxin secreted by the bacterium, creates pores in the phagosome membrane through which H+ and Ca2+ escape, which are required for fusion of the phagosome with endosomes
L. monocytogenes also produces phospholipases which, with LLO, cause the breakdown of the phagosome membrane and enable the escape of the bacterium into the cytosol where it can replicate
Describe cytotoxic T lymphocytes (CTLs)?
CTLs kill other cells that are abnormal e.g. infected with viruses and cancerous
The T cell receptor on CTLs recognise peptides presented by MHC class I molecules on aberrant cells
Target cell recognition triggers the exocytosis of secretory lysosomes (also known as granules) by CTLs
CTL secretory lysosomes contain the cytotoxic molecules perforin and granzymes
Perforin facilitates entry of granzymes into the target cell’s cytoplasm where these enzymes induce apoptosis
What happens with recognition of aberrant cells?
This triggers secretory lysosome exocytosis by CTLs
The perforin and granzymes are released
The contents of CTL secretory lysosomes induce apoptosis in target cells
Describe secretory lysosomes?
They are specialised:
Secretory lysosomes are dual function organelles: combine the degradative function of classical lysosomes with the capacity for regulated exocytosis
Like conventional lysosomes, CTL secretory lysosomes contain lysosomal hydrolases, lysosomal transporters and have an acidic pH
In addition, they store perforin and granzymes and when CTLs recognise an abnormal target cell they undergo regulated exocytosis to release their contents
