Wednesday 12 November 2003
Asthma is a chronic respiratory condition that affects children and adults. Genetic predisposition and environmental factors interact to produce the disease.
mucosal eosinophilic infiltrate
mucosal basement membrane thickening with sunmembranous collagen deposition
basement membrane thickening of bronchus
submucosal infiltrates of eosinophils with chronic inflammatory cells.
distinctive inclusions in airway mucus
- Charcot-Leyden crystals
- creola bodies
- Curschmann spirals
epithelial shedding (desquamation)
goblet cell metaplasia (goblet cell hyperplasia)
airway wall edema
bronchial mucus gland hyperplasia
airway smooth muscle hyperplasia
Among the genes discovered through association studies, the first group is involved in triggering the immune response and directing CD4+ T helper (TH)-cell differentiation. This group includes genes encoding pattern recognition receptors (CD14, TLR2, TLR4, TLR6, TLR10, NOD1 and NOD2), immunoregulatory cytokines (IL10 and TGFB1), the transcription factor STAT3, molecules involved in antigen presentation (HLA-DR, HLA-DQ and HLA-DP alleles) and the prostaglandin receptor PTGER2.
The second group of asthma susceptibility genes includes those that regulate TH2-cell differentiation and TH2-cell effector functions (GATA3, TBX21, IL4, IL13, IL4RA, FCER1B, IL5, IL5RA, STAT6 and IL12B). The third group of genes is expressed in epithelial cells, and includes chemokines (CCL5, CCL11, CCL24 and CCL26), antimicrobial peptides (DEFB1), CC16 and factors involved in maintaining the integrity of the epithelial-cell barrier (SPINK5 and FLG).
The group of asthma susceptibility genes discovered through positional-cloning approaches includes ADAM33, DPP10, PHF11, GPRA, HLA-G, IRAKM and COL29A1 (which encodes collagen XXIX). These genes are expressed in the epithelium and/or smooth muscle.
Two main mechanisms have been identified that underlie airway obstruction in experimental asthma. The first, type I hypersensitivity, is principally an antibody-mediated reaction.
Following activation by antigen, T-helper type 2 (TH2) cells produce interleukin-4 (IL-4), which is required for B-cell maturation and IgE synthesis, IL-5, which is required for eosinophil growth and differentiation, and IL-4 and IL-9, which are required for mast-cell development. IgE is then captured on the cell surface by its receptor (FcRI), which is present on mast cells and eosinophils.
Crosslinking of this receptor during subsequent encounters with antigen stimulates the release of various toxic products, including histamine, proteases and cytokines, which together elicit three airway changes that contribute to obstruction, airway hyper-responsiveness, goblet-cell metaplasia with mucus overproduction and mucosal oedema.
The second mechanism that contributes to airway obstruction, type IV hypersensitivity, also crucially involves TH2 cells. The TH2 cytokines IL-4 and IL-13 signal through the -chain of the IL-4 receptor (IL-4R), which is present on target tissues of the lung, such as smooth muscle and the airway epithelium, to elicit airway obstruction more directly. IgE, immunoglobulin E.
Asthma by Washington Deceit
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Discovering susceptibility genes for asthma and allergy. Donata Vercelli. Nature Reviews Immunology 8, 169-182 (March 2008)
Kay AB. The role of eosinophils in the pathogenesis of asthma. Trends Mol Med. 2005 Apr;11(4):148-52. PMID: 15823751