The Biochemistry of Asthma PHM142 Fall 2015 Coordinator: Dr. Jeffrey Henderson Instructor: Dr. David Hampson The Biochemistry of Asthma Candice Biback, Kristen Conte, Sarah James, Man Yen Ng October 27, 2015

Interactive Activity It’s a beautiful Spring day! You walk out the house and your neighbour is mowing their lawn. You inhale and shortly after you start experiencing problems with your breathing. You begin experiencing shortness of breath and chest tightness followed by continuous cough and intermittent wheezing. You just experienced an asthma attack. Let’s get into the details of the events that led up to your attack.

What is Asthma? Definition: Chronic illness that leads to inflammation and narrowing of the airways Causes: Many contributors not well understood: Genetic, environmental and immune factors Triggers include: exercise, allergens, irritants Asthma is a heterogeneous, chronic inflammatory disease of the airways that is characterized by airway hyperresponsiveness, obstruction, and inflammatory cell infiltrates. The pathogenesis of the disease involves a complex interaction of genetic, environmental, and immune factors. Interactions between airway epithelial cells, antigen presenting cells, allergic cells, and other inflammatory leukocytes are central to disease progression. Cytokines and other inflammatory mediators are the effectors of allergic inflammation and orchestrate the chronic inflammation and structural changes in the respiratory tract. Causes: There is a genetic factor that can increase your risk of having asthma, but other factors especially environmental factors are important contributors to the onset of asthma. For example, one environmental factor that we came across was the chronic exposure to saw dust leading to development of asthma The triggers for asthma include exercise. allergen irritant exposure. Like in the interactivity that we went through there can be a delay in the onset of symptoms where you may experience symptoms within the first few minutes of exposure, but symptoms can persist or re-occur hours after exposure During the remainder of our presentation we will be focusing on the allergen induced asthma which is why we have bolded it.

Symptoms of Asthma Include: Coughing Wheezing Shortness of breath Chest tightness As you can see this image depicts a normal airway vs one during an asthma attack. One glance at the diagram and you can probably see that the diameter of the normal airway is significantly larger than that of someone experiencing an asthma attack. The bronchoconstriction as well as the inflammation and increased mucous secretion will lead to an overall narrowing of the airway. This will then lead to the onset of symptoms of asthma which include coughing, wheezing, shortness of breath and chest tightness. All in all these individuals will experience difficulty in breathing.

Initiation of Allergic Response Allergen binds antigen- presenting cell Antigen is presented to naive T cell Interaction between antigen-presenting cell and naive T cell leads to differentiation into T- helper (Th2) cells Now that we know the symptoms of asthma, let's look at the biochemical pathways which lead to these symptoms Epithelial cells are the primary barrier between the environment and the host, for this reason they play a crucial role in not only innate immunity but also adaptive immunity where they initiate the immune response. The respiratory epithelium contains a subset of dendritic cells known as intraepithelial dendritic cells. These cells are responsible for binding and processing antigens (in this case more specifically allergens) and are sometimes referred to as antigen presenting cells. Antigen- presenting cells are influenced by thymic stromal lymphopoietin (TSLP), a cytokine released from the respiratory epithelium, to begin an allergic response. This occurs due to TSLP recruiting leukocytes and T cells to the airway. Upon the antigen binding the the dendritic cell (antigen presenting cell) it is endocytosed by the dendritic cell. The antigen/dendritic cell complex migrates to the local lymphoid collections (lymph node), here the dendritic cell presents the antigen to naive T cells which contain antigen specific receptors. This interaction leads to the differentiation of naive T cells to T-helper (Th) cells. In this case dendritic cells contain the differentiation marker CD86 which interacts with CD28 on T cells to produce T-helper 2 cells (Th2). These specific helper cells are related to allergen response.

Primary Allergic Response Pathway B cells are stimulated by IL-4 and IL-13 B cell releases IgE antibodies IgE antibodies bind to Mast cells The Th2 cells migrate back to the site of antigen presentation in the airway (because of influence form chemokines). Here the Th2 cell produces many cytokines, which leads to the production of many interleukins, such as IL-4 and IL-13. Th2 cells attract allergen-specific B cells. IL-4 and IL-13 stimulate B cell to produce and release Immunoglobulin E antibodies specific to the allergen. These antibodies are secreted into circulation where they interact with receptors (such as FceR1) on mast cells.

Causes of Asthma - Allergic Response Upon interaction between the allergen and the IgE antibody, mast cell is stimulated to release granules containing histamine, leukotrienes and prostaglandins. Mast cells also alter the transcription of these mediators, allowing for release over a longer period of time. These mediators are known to lead to the asthmatic symptoms previously mentioned. For the sake of time, we focused on allergic asthma, where Mast cells are the central effector, however, it is important to note that basophils and eosinophils also play a role in the biochemistry of asthma. (I.e., Eosinophils are recruited by IL-5 and are similarly believed to release a number of chemokines and cytokines, including IL-4, IL-5, and IL-13.)

Review: PG Synthesis Liberation of AA from membrane phospholipids by phospholipase A2 in response to inflammatory stimuli AA converted to PGH2, a common precursor of several PGs PGD2 is a major product of COX-catalyzed reactions in a variety of cells, including T cells, macrophages, mast cells, and platelets As we recently learned in class, PGs are synthesized by phospholipase A2 releasing arachidonic acid from membrane phospholipids, Arachidonic acid is then converted to PGH2, a common precursor of several PGs, including PGD2 – the main PG released from mast cells

Role of PGD2 in Asthma PGD2 may play an important role in the pathogenesis of allergic asthma, through orchestrating interactions between mast cells, Th2 cells, and eosinophils. When PGD2 is released from mast cells → Binds the chemoattractant receptor expressed on TH2 cells (CRTH2) → Leading to: 1) accelerated inflammation by promoting chemotaxis and, 2) upregulated production of cytokines and interleukins Ultimately leading to furthered symptom presentation

Causes of Symptoms Mediators released by leukocytes ultimately responsible for symptoms Histamine Leukotrienes Chronic Inflammation Important to note that the symptoms of asthma are all caused by mediators released by these leukocyte cells. For example, histamine released by mast cells can trigger goblet cells in the respiratory tract to release mucus, which is intended to “sweep” away the allergen. In patients with asthma however, histamine is overactive and so the goblet cells will release more mucus than normal, which will partially block the airway. Histamine is also responsible for inflammation due to the recruitment of fluid to a site of infection.This is an attempt to deliver cells that can protect the body, but since histamine is overactive, this will cause inflammation. Since this is often an ongoing response and many leukocytes will be recruited to the area, this can ultimately act on the epithelia cells to remodel the airway. Histamine and PGD2 are also responsible for bronchoconstriction, or smooth muscle contraction. Leukotrienes produced by mast cells are released and bind to their G-coupled protein receptors on the cell surface of structural airway cells. This produces smooth muscle contraction and increases vascular permeability of small blood vessels. Cycle of chronic inflammation in patients with asthma. Allergic inflammation develops from an interplay between the respiratory epithelium and leukocytes. Environmental and inflammatory stimuli induce the production of mediators from the airway epithelium, which activates and recruits inflammatory cells. Inflammatory cells infiltrate the lungs and release mediators that augment the inflammatory response in the epithelium,creating a cycle of chronic inflammation.This process causes bronchoconstriction and epithelial damage,and it can result in remodeling of the airway.

Possible Treatments Inhaled corticosteroids provide an anti-inflammatory response (example, Flovent) Advair Combination drug Both corticosteroid and long-acting β2 adrenergic agonist Inhaled corticosteroids are commonly used to treat the inflammation component of asthma. FLOVENT is a standard steroid inhaler. It is a highly potent glucocorticoid anti-inflammatory steroid. It’s mechanism of action is to act on the glucocorticoid responsive elements in the promoter region of genes and will eventually inhibit the release of many inflammation-associated molecules. These include but are not limited to cytokines, chemokines, arachidonic acid metabolites, and adhesion molecules. Without these mediators, no new inflammatory immune cells will be recruited to the area and so the inflammatory response will be suppressed. ADVAIR is a combination steroid inhaler.It is a long acting bronchodilator that prevents breakthrough systems of wheezing and chest tightness.(Two therapeutic agents within this are salmeterol and fluticasone propionate if they ask) It acts as a bronchodilator by relaxing the smooth muscle in the airway that may become inflamed in a reaction.

Summary Slide Definition: Chronic illness that leads to inflammation and narrowing of the airways Symptoms: Coughing, Wheezing, Shortness of breath, Chest tightness Pathway: Allergen binds antigen-presenting cell Antigen presented to naive T cell Interaction between antigen-presenting cell and naive T cell leads to differentiation into T-helper (Th2) cells IL-4 and IL-13 stimulate B cells B cells releases IgE antibodies IgE antibodies bind to Mast cells Mast cells release mediators, i.e., histamine, leukotrienes and prostaglandin D2 (PGD2) Causes of Symptoms: PGD2 → promotes chemotaxis, upregulates cytokine and interleukin production Histamine → increased mucus secretion, increased bronchoconstriction, inflammation Leukotrienes → smooth muscle contraction, increased vascular permeability of small blood vessels

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