Macrophages Flashcards
Why is phagocytosis important?
- Protects against invading organisms
- Processes foreign antigens -> want to make some adaptive immunity
- Presents parts of foreign antigens activating adaptive immune system (links innate and adaptive immunity)
- Purges debris and damaged/dying host cells > getting rid of cells we don’t want -> apoptosis
Which cells are phagocytic and where can they act?
Macrophages, Dendritic Cells, Neutrophils, Eosinophils and Basophils
They can act at:
- Sites of infection (attracted by inflammatory response) -> active response
- Mucosal lymphoid tissue (patrolling of Respiratory & GI tracts, Liver) -> important to make sure that bacteria doesn’t get here and also cells that have high turnover
- Lymph nodes (filtering from Lymph fluid)
- Spleen (filtering from Blood) -> macrophages want to be here
What are the steps of phagocytosis?
- Activation of resting phagocytes
- Chemotaxis
- Attachment
- Ingestion
- Destruction
Describe activation of phagocytes.
- Inflammatory mediators such as bacterial products, complement, cytokines and prostaglandins activate resting phagocytes
- They up regulate PRRs for increased ability to recognise and adhere to microbes
- They also up regulate metabolic rate, including production of ATP to increase lysosomal enzymes and lethal oxidant production. This increases metabolic and microbicidal activity.
Describe chemotaxis of phagocytes
Chemotaxis of phagocytes is how the phagocyte can move towards the microbe using chemokines using an increasing concentration gradient. Molecules can also be complement, cell wall, capsule or IL-8 (CXCL8)
Describe attachment of phagocytes.
Attachment of the phagocyte to the pathogen can be enhanced or unenhanced
Unenhanced = using PAMPS and PRRs such as LPS and TLRs. They can produce cytokines leading to inflammation e.g. IL-12/-6 for bacteria and IFN-1 for intracellular viruses
Enhanced attachment = opsonin receptors such as complement and Fc receptors attached to the surface of the phagocyte.
Macrophages use both enhanced and un-enhanced attachment whereas neutrophils use mainly enhanced attachment
Describe the process of ingestion by phagocytes
During ingestion, we want to get the microbe into the phagocyte so it has to create a phagosome. A phagosome is made up of Plasma membrane, Endocytic organelles, Endoplasmic reticulum & specific biogenesis and insertion from cytoplasm • Each addition allows inner phagosome membrane to be coated with processing proteins, complexes and receptors (eg MHC class II for Ag loading)
The microbe is engulfed when the actin cytoskeleton starts to rearrange and remodel to start to form the phagosome
What happens during destruction of a microbe by a phagocyte?
There are 2 methods of killing:
Oxygen Independent Killing:
- this involves using lysosomes which are created by the golgi apparatus and contain : digestive enzymes such as:
- Lysozyme – breaks down proteoglycans in bacterial cell walls
– Cathepsins – proteolytic enzymes that degrade bacteria
- Defensins - small peptides that form channels in lipid bilayers
The lysosomes bind with the phagosome to create the phagolysosme inside a vacuole. Here the pathogen is degraded
There is also Oxygen dependent killing
- here you also have a lysosome but it contains microbicidal chemicals and toxic oxygen radicals. There are vacuoles with oxygen radicals inside them. Oxiyfen is oxidised to produce a super oxide which in turn binds with water to create hydrogen peroxide and hydroxyl (OH) radicals. There are 50-60 types of oxidases. Oxidation of proteins, carbs and DN destroys the function of microbes and oxidation of lipids can destabilise their membranes.
However it is important that phagocytes are only active when they need be otherwise if they burst and are active, they could release toxic radicals and enzymes to other cells in the body
How can microbes evade phagocytosis?
- Block phagocyte tasting and feeling:
a. Streptococcus pneumoniae – has a capsule allowing it to escape
b. Resists unenhanced attachment by preventing binding of pattern recognition receptors
c. Also resists C3b opsonisation
- Block swallowing
a. E.g. Yersinia (plague) – stops making actin -> depolymerise actin preventing engulfment
i. Sits in lungs and then you cough it out and pass it on - Block phagolymsosome killing:
a. E.g. Salmonella – more resistant to toxic ROS and defensins
b. Mycobacterium TB – blocks phagosome fusing with lysosome. It sits in phagosomes (latency), so they think they have got rid of the organism so doesn’t induce cascade - Killing phagocytes
a. E.g. Staphylococcus aureus – produces leucocidin (own toxin) which damages membranes
Apoptosis is important for removing active cells. However they need to phagocytosed. Why?
- Important for regulation of immune responses
- Prevents leakage of cytotoxic or antigenic contents
- Rapid, efficient and ‘silent’ (no inflammation) e.g. retinal epithelial cells in the eye remove fragments of photoreceptors that die by apoptosis in response to light – essential for maintaining vision
What are the signals apoptotic cells give off in order to activate phagocytosis?
- “Find Me” signals
- chemoattractants leak when cells are destroyed by apoptosis
- Lysophosphatidylcholine (LPC) , ATP and Chemokines e.g. CX3CL1 -> inflammatory responses
They tell phagocytes where the apoptosis has happened - “Eat me” signals
- These signals activate the phagocytes for engulfment
- Phosphatidylserine (PtdSer), the signal normally expressed on the inner leaflet of the plasma membrane. In apoptosis, when the cell isn’t itself anymore, PtdSer is translocated to the outer part of the lipid bilayer. Phagocytes therefore recognise this
- “Don’t eat me” signals
- Repulse away form the system with good cells
- CD47 : ubiquitously expressed in human cells and overexpressed in many different tumor cells Interferes with actin engulfment pathway
- CD31: cell adhesion endothelial molecule expressed on different cells e.g. platelets, macrophages, lymphocytes that repulses phagocytes. Its disabled during apoptosis so phagocytes can’t access them.
What are the consequences of apoptotic cell removal?
- Secretion of ‘pro-healing’ cytokines (eg IL-10, TGF-β) to reduce inflammation promote wound healing > only healing in certain places
- Presentation of antigens and the maintenance of self tolerance
What problems can occur with apoptotic cell clearance?
- Leakage of content from non-cleared apoptotic cells act as inflammatory factors
- Lack of suppression of inflammatory and immune responses
• no release of anti-inflammatory cytokines e.g. TGF-b, IL-1 - Onset of autoimmune disorders linked to inefficient removal of apoptotic cells e.g. systemic lupus erythematosis
What diseases are linked with defects in phagocytosis?
- Chronic granulomatous disease – boil-like structures seen on the skin. This is due to missing NADPH oxidase components -> generate radicals for destruction.
- Chediak-Higashi Syndrome Hyperpigmentation/albinism delayed phagosome-lysosome fusion low chemotaxis
Both lead to recurrent infections
What are the 2 subsets of macrophages?
M1-phenotype:
- Monocytes recruited from circulation/bone-marrow and differentiate into macrophages (recognise PAMPs )which can help activate DC’s to stimulate Th1/NK cells which then release IFN-γ
- IFN-γ promotes Macrophages to become M1-phenotype
- M1 inflammatory phenotype produce NO, ROS, IL-1, TNF & MMP’s
- Inflammatory microbial response
We need to regulate this by “producing when needed” for M1 as they are toxic to use -> destroy cells (pro-inflammatory response)
M2-phenotype
Making M2s at the same time as M1s
- Alarmins (IL-25, IL-33 TSLP) stimulate Th2 and other cell subsets to release IL-4 & IL13
- Macrophages develop M2 and later regulatory phenotypes which begin to accumulate
- M2s antagonise M1s and promote wound healing
- Regulatory Macrophages stop wound healing when complete
