What lineage do NK cells come from? Are they a part of the innate or adaptive immune response?
NK cells come from a common lyphoid progenitor (same as B and T cells) but are actually a part of the innate immune response.
What are the 3 primary functions of the innate immune response?
Innate immunity has three primary functions.
(1). It provides a physical-chemical barrier to pathogens. The barrier components include the skin, along with the epithelial lining of mucosal surfaces; cilia in the airway; anti-bacterial molecules such as lysozyme and defensins in mucosal secretions; low pH in the stomach; and commensal bacteria in the epithelial cell surfaces.
(2). It provides an initial and immediate host defense response to a pathogen. The response is activated by recognition of pathogens and is mediated primarily by macrophages, neutrophils, natural killer (NK) cells and the complement system. Recognition of pathogens by the innate immune cells, however, is much less precise than that by cells of the adaptive immune response.
(3). It initiates and regulates the development of the adaptive immune response. This function is also dependent on pathogen recognition and is mediated by dendritic cells and macrophages.
How do skin and mucosal tissue provide a physical and chemical barrier to pathogens?
Skin: One of the largest tissues associated with the immune system is the epithelial layer. The epithelial layer of the skin comprises of a stratified layer of keratin above a layer of epithelial cells, which form a barrier to prevent pathogens from entering the body. The barrier results from the epithelial cells forming “tight” interactions with one another.
Mucosal tissue: Mucosal tissue is made up of a layer of epithelial cells which can be stratified epithelial cells or columnar endothelial cells. This tissue also has the ability to create fluids, which form a “chemical screen” against microorganisms. These include, but are not limited to, sweat, saliva, mucous (viscous polysaccharide fluid), tears. These fluids are able to move microorganisms away from or prevent them from binding to the epithelial layer. Sometimes these fluids may be of low pH or contain enzymes that break down components of pathogens (e.g., lysozymes). Epithelial cells also secrete anti-microbial peptides (e.g. defensin), which further act against microorganisms but leave host cells intact.
What do macrophages develop from? Where are macrophages found?
What are the functions of macrophages?
Breaking the barrier: Pathogens encounter macrophages
When the invading pathogen breaks through the physical/chemical barrier the pathogen enters the connective tissue/mucosal layer below where it will encounter another group of innate immune defenses called macrophages. Macrophages, originating from the bone marrow, are large mononuclear cells, which only reside in almost all tissues. Macrophages are derived from monocytes, which are present in the blood.
Macrophage function: Macrophages function in early defense against microorganisms. They also act as antigen presenting cells to the adaptive immune response (more later in this and subsequent lectures in this block). Macrophages also act as phagocytic scavenger cells to remove dead or dying cells (i.e., “garbage trucks” of the body).
- Macrophages possess receptors to many components unique to pathogens. They recognize specific components on microorganisms, engulf and internalize microorganisms (phagocytosis) and provide substances that ultimately destroy pathogens.
- They also provide soluble factors known as cytokines to recruit and activate a variety of immune-system cells to an ongoing response to a pathogen. Cytokines are molecules secreted by immune cells when the cells are actively responding to a trigger (pathogen component).
What do innate immune cells recognize on pathogens? Give examples. How is this different from how cells of the adaptive immunity recognize pathogens?
Innate immune cells cannot discriminate among the vast heterogeneity of the different antigenic epitopes (like cells of adaptive immunity do). Instead, innate immune cells recognize structures termed pathogen-associated molecular patterns (PAMPs).
- PAMPs are motifs present in structures/molecules that are essential to microbe survival and, thus, are evolutionarily conserved.
- These structures are unique to microbes and are constant among entire classes of pathogens.
- Examples of PAMPs include lipopolysaccharide (outer membrane of gram-negative bacteria), peptidoglycan (cell walls of gram-positive bacteria), and Beta-glucans (fungi cell wall).
What receptors recognize PAMPs? What are the different types of these receptors?
PAMPs are recognized by pattern-recognition receptors (PRRs). PRRs may only be involved in phagocytosis or may only have signaling functions. Those that have signaling functions include Toll-like receptors (TLRs) and Nucleotide-binding oligomerization domain like receptors (NLRs).
All TLRs are members of what receptor family?
How many peptide chains do TLRs have? How many times do they pass through membranes?
Do they function alone or along with other TLRs?
To date, 11 TLRs have been identified and cloned in humans. Human TLR have the capacity to recognize all classes of pathogens that produce infections in humans. Because these structures are unique to pathogens, the recognition is a modified form of non-self recognition.
- All TLRs are members of the Interkeukin (IL-1) receptor family (see lecture notes on Cytokines).
- They are single polypeptide chains that make one pass through the plasma membrane. TLR and IL-1R have similar cytoplasmic tails, but differ in their extracellular regions.
- Each TLR is comprised of a single polypeptide, but all TLRs function as a dimer.
What cells contain TLRs? (just list)
- TLRs are found on all leukocytes (e.g., monocytes/macrophages, neutrophils), dendritic cells, and stromal cells (e.g., epithelial cells). Whereas monocytes/macrophages and dendritic cells express many TLRs, other cells may express only a few types of TLRs.
- TLRs are present on mast cells and eosinophils.
What type of microorganism are mast cells and eosinophils critical in the defense of? What do PAMPs binding to TLRs on these cells trigger?
TLRs are present on mast cells and eosinophils. Both cell types are critical in large parasite response. PAMPs on parasites bind to TLRs on mast cells and induce the secretion of histamine, which increases smooth muscle contraction to expel the parasite from the body. PAMPs on parasite also trigger TLRs on eosinophils to release the content of specific granules to lyse the parasite.
see slides 35-38 of PP
Where are TLRs located within cells?
The localization of TLRs within an individual cell varies.
- TLRs that recognize extracellular pathogens are expressed on the surface of the plasma membrane.
- TLRs that recognize RNA or DNA are present in intracellular organelles (endosomes or phagolysosomes).
In what cell types are NLRs found? What part of the cell do they reside in?
List 3 NLRs.
This is a family of at least 20 proteins present in the cytoplasm of various cells including monocytes, macrophages, and epithelial cells.
RIG-1 (retinoic acid-inducible gene-I)
State what each of the following NLRs respond to:
- Members of family of approximately 22 proteins
- Recognize bacterial structural products, toxins, and live intracellular bacteria (e.g., Listeria)
• NALP proteins recognize DAMPS
The NALP proteins also ‘sense’ stress molecules such as ATP released by damaged cells and uric acid crystals, low K+. These molecules are thus termed DAMPS for danger-associated molecular patterns.
- Viral double-stranded RNA
Production of what type of molecule is a dominant response of TLR mediated cell activation?
What are the two major classes of this molecule produced through TLR signaling?
Cytokine production is a dominant response of TLR-mediated cell activation. Two major classes of cytokines (pro-inflammatory cytokines and Type I interferons) are produced, each via a different intracellular signaling pathway.
Pro-inflammtory cytokines are produced primarily in response to activation of TLR that recognize (intracellar/extracellular) pathogens.
How does TLR signaling lead to cytokine synthesis?
What cells are particularly prominent sources of pro-inflammatory cytokines?
Pro-inflammatory cytokines are produced primarily in response to activation of TLR that recognize
extracellular pathogens (bacteria/fungi/protozoa).
The TLR activate a signaling pathway that leads to activation of NF-κB, a transcription factor that stimulates synthesis of several cytokines (see cytokine lecture).
Monocytes/macrophages, dendritic cells, and stromal cells are particularly prominent sources of pro-inflammatory cytokines. The cytokines, in turn, exert several actions that together initiate the effector response.
What are the major pro-inflammatory cytokines? What are their functions?
Interleukin (IL)-8 (CXCL8) is a chemokine that attracts neutrophils.
Tumor necrosis factor (TNF)-α and IL-1β induce ICAM-1 and selectin expression on endothelial cells, which facilitate movement of neutrophils from the blood into the tissue. They also activate neutrophils.
IL-6 stimulates synthesis of acute phase proteins (e.g., C-reactive protein) by the liver
IL-12 augments NK cell activation (including IFN-γ synthesis).
What cells produce type I interferons (IFNα IFNβ)?
What are the effects of type I interferons?
The type I IFNs are produced by all virus-
infected cells and by plasmacytoid dendritic cells. These two cytokines are produced in response to activation of virus-selective TLR (TLR3, 7, 8, and 9) via a signaling pathway that includes interferon-responsive factor (IRF).
The IFN-α/β have several activities important to host defense against viruses.
(1) induce resistance to viral replication within infected cells by activation of key enzymes, protein-kinase R which diminishes mRNA translation and RNAse L which leads to mRNA degradation;
(2) increase expression of major histocompatibility class I molecules which enhances cytotoxic T-cell responses;
(3) enhance NK cell cytotoxic activity.
What type of primary mediators are produced in response to NOD 1/2 activation?
What does activation of NALP trigger the formation of? What mediatorys are produced and what are their functions?
NLRs: The pro-inflammatory cytokines are the primary mediators produced in response to NOD1/2-mediated activation.
- The NALP triggers formation of a complex of proteins termed the inflammasome that ultimately leads to the production of IL-1β and IL-18.
- The IL-18 drives production of cytokines by TH1 CD4 lymphocytes (to be covered extensively in the cellular immunology lecture).
- IL-1β is important to activation of T-cells by macrophages and dendritic cells (also for increasing permeability of vessels to neutrophils through induction of ICAM-I and selectin expression on endothelial cells).
What is produced through RIG-I signaling?
RIG-I proteins: Pro-inflammatory cytokines as well as Type I interferons are produced in response to RIG-I protein mediated activation.