What is a cytokine?
A cytokine is a protein released by an immune cell that has an effect on nearby cells expressing the appropriate receptor. A same cytokine may have different effects depending on the context.
What are the steps of the neutrophil response?
- Recruitment to the site of infection via blood vessels.
- Extravasation from the blood vessels.
- Migration through the tissue to the site of infection.
- Activation via recognition of pathogen, products of pathogen or products of infected cells (via pattern recognition receptors).
- Induction of mechanism of action to eliminate pathogen or infected cells.
- Recruitment of and interaction with other immune cells.
- Contribution to resolution of inflammation and tissue repair.
How do neutrophils know where to exit in the blood vessels?
Neutrophils are circulating in blood vessels.
- Near the site of infection, the endothelial cells of the blood vesses express P-selectin and E-selectin. Neutrophils recognize these via P-selectin glycoprotein 1 and E-selectin ligand. Interaction causes the neutrophils to tether to the walls and slow down.
- Interaction between chemokine receptors and chemokines coating the blood vessels cause adhesion of neutrophils to endothelium of blood vessels.
- Integrins cause full arrest and are required for the neutrophils to extravasate from the blood vessels.
- Migration through the tissues via chemotaxis: concentration gradient of a chemical stimulus guides directional movement of cells
What are chemokines?
Chemical messengers inducing directional movement of cells.
Describe the 5 mechanisms of action of neutrophils.
- Production of antimicrobial substances and proteolytic enzymes:
Neutrophils have granules, which are highly specialized lysosomes producing antimicrobial substances that can be released in the phagosome or extracellularly (degranulation) to disrupt the integrity of the microorganism. Can also limit the availability of essential nutrients inside the phagosome to prevent growth. - Phagocytosis:
- Attachment of microbe or other particle to neutrophil
- Engulfment
- Phagosome
- Phagolysosome
- Digestion
- Residual body
- Discharge of waste - Oxidative burst
- Occurs at phagosome membrane or cell membrane
- Transfer of e- of O2 to cytosolic NADPH, producing reactive oxygen species
If neutrophile not able to make ROIs, recurrent and atypical infections - NETs
Upon activation, neutrophil undergoes a special kind of cell death (NETosis):
- Nucleus swells
- Chromatin is dissolved
- Decondensed DNA is expelled out of the cell
- DNA is covered in histones and antimicrobia proteins, making DNA very stickky
- Form neutrophilic extracellular traps (NETs) that trap microbes and cause them to die - Cytokine secretion
How are eosinophils recruited and what is their mechanism of action?
- Recruitment from blood into tissues via ILK-5
- Release of granule contents involved in anti-parasitic response
- Involved in type II immune responses (including airway dysfunction during asthma)
- Produce inflammatory lipid mediators
- Homeostatic role
How are mast cells and basophils recruited and what is their role?
Mast cells are already sitting in tissues near blood vessels and nerves. Basophils are recruited via blood vessels.
Role:
- Histamine release
- Associated with Th2 CD4 T cell responses
- Protective immunity against helminths and ticks
- Responses against allergens
What are lipid mediators/eicosanoids?
Derived from arachidonic acid, itself derived from cell membrane phospholipids.
- Leukotrienes via LOX enzyme: leukocyte recruitment
- Prostaglandins via COX enzyme: inhibition of platelet aggregation, vasodilation, blood vessel leakage; platelet aggregation
Released by eosinophils, mast cells, macrophages, endothelial cells, etc.
Where are monocytes located and what is their role?
Mostly in blood vessels, but colonies in the spleen can be mobilized. Continuously generated in bone marrow.
- Patrolling monocytes (CD16+) maintain blood vessel integrity and detect pathogens
- Inflammatory monocytes (CD14+) are recruited to the site of infection or injury
Monocytes can differentiate into macrophages and/or dendritic cells upon recruitment to the site of infection/injury during inflammation
Where are macrophages located, what is their role and how do they differentiate?
Location: In tissues
Early insights: Monocytes replenish the pool of macrophages in tissues
Now: Macrophage populations are seeded during embryogenesis and replenish themselves via cell division in tissues
Role:
- Many roles in homeostasis (e.g. alveolar macrophages remove excess surfactant in the lungs, osteoclasts resorb bone marrow)
- Role in response to pathogens. Recruited after neutrophils, takes much longer. Important in the decision-making process as to whether neutrophils need to be recruited or not. If the local damage is small enough, neutrophils can cloack it and eliminate the foreign body by themselves and neutrophils never need to be recruited.
Differentiation:
In response to different cytokines released by other immune cells, macrophages can differentiate in classically activated macrophages (M1) or differentially activated macrophages (M2).
What are the roles of dendritic cells?
- Antigen-processing and presenting cells
- Express MHCI and MHCII (for antigen presentation) and PRR for activation by microbial stimuli
- Establish communication between innate immune response and subsequent adaptive response
- cDC: activate B and T lymphocytes
- pCD: secrete type I interferon, essential in response against viruses
- Resident dendritic cells take up antigen and travel through lymphatics to secondary lymphoid organs and present antigen to B and T lymphocytes to initiate, coordinate and regulate the adaptive immune response. DCs also sit in secondary lymphoid organs themselves and take up antigen travelling through lymphatics.
How are diseases of granulopoiesis treated?
- Reduction of exposure to infections via prophylactic methods
- Anti-microbial drugs to prevent and treat infections
- Administration of colony stimulating factor (G-CSF and GM-CSF)
- Ablation of bone marrow of infected patient with cytotoxic drugs and transplantation of healthy donor cells to replenish the immune cell populations: transplantation of bone marrow, HSCs or blood from ombilical chord
Define inflammation.
Local accumulation of fluid, plasma proteins and white blood cells in response to injury, infection or local immune response.
What was the greatest debate regarding the immune system and what is the right answer to it?
Question: Whether the immune system is cellular (function lies within cells) or humoral (function lies within humor = cell-free bodily fluids)
Asnwer: Both (cell = T lymphocytes, humor = B lymphocytes)
What are the steps of an immune response (describe the first one in more detail)?
- Pathogen must breach anatomical barriers to infect the host:
- Anatomical barriers: Skin, gut, lungs, eyes/nose/oral cavity
- Each site has mechanical, chemical and microbiological (normal microbiota) barriers - If breached, innate immune response: Activation of non-specific and broadly specific effectors
- If fails, early induced innate immune response: Recruitment of effector cells and activation in response to PAMPs, inflammation.
- If fails, adaptive immune response: Transport of antigen and APCs to secondary lymphoid organs, activation of naïve B and T cells, clonal expansion and differentiation of B and T cells to effector cells
Compare the innate and the adaptive immune systems in their mediators, goals and kinetics. How do both interact?
Mediators:
Cellular:
- Innate: Granulocytes, monocytes, macrophages, DCs
- Adaptive: B and T lymphocytes
Humoral:
- Innate: Plasma proteins called complement, anti-microbial proteins
- Adaptive: Abs
Goal:
- Innate: Early clearance, activation of adaptive immunity, wound healing and tissue repair
- Adaptive: Clearance of pathogen, immune memory
Kinetics:
- Innate: Rapid, same upon each exposure
- Adaptive: Slower, rapid upon second exposure
Interaction:
Cell-cell contact or chemical mediators:
- Cytokines
- Chemokines
What are primary and secondary lymphoid organs?
Primary: Site of development of immune cells
- Bone marrow:
Site of self-renewal and differentiation of HCSs
Site of development of B cells
- Thymus:
Site of development of T cells
Plasma B cells and T cell memory
Secondary: Site of initiation of adaptive immune response (priming)
- Spleen:
Filters blood, provides immune response to blood-born antigen (systemic infection)
Red pulp: Where macrophages that filter the blood sit
White pulp: Like mini lymph odes, B and T cells sit in specialized B and T cell zones, initiation of germinal center reaction upon antigen presentation by APCs or antigen itself
- Lymph nodes:
Highly organized structure where immune cells meet (DCs, macrophages, B and T cells)
Communication via blood and lymphatic vessels:
Lymphatic vessels:
- Transport lymph (interstitial fluid derived from plasma, returned to circulation via jugular thoracic duct draining into jugular veins)
- Highway for lymphocytes
- Transport antigen from tissues to secondary lymphoid organs
- No valves to prevent back flow nor pump like heart, flow ensured by muscle contractions only
