This web page was produced as an assignment for an undergraduate course at Davidson College.
This web-page is an informative survey on the immunological mechanisms and effector cells involved in allergic asthma. If you are concerned that you have asthma or suffer from asthmatic symptoms please consult your physician.
The following information provides a general synopsis of the immunological pathology of asthma from effector ligands, cell-cell mediated interactions, and effector cell functions to the classification and differentiation of clinical symptoms. There is also a section that discusses current asthma treatments and new potential targets for future immune-based asthma therapy.
Asthma can be generally considered a hyper-responsive airway disease involving chronic inflammation of varying austerity arising from various (genetic and environmental) etiology ((Hang et al., 2003), (King 1999)). Asthma is a multi-cellular redundant and self-amplifying disease stemming from an individuals hyper-response to innocuous environmental antigens. The pathophysiology of asthma includes mucus hyper-secretion, bronchial hyper-responsiveness, smooth muscle hypertrophy (Schmitz et al., 2003 ), and airway obstruction (King 1999) (Bronchial hyper-responsiveness is measure y a bronchial inhalation challenge test where a patient is stimulated with metacholine or histamine (King 1999)). Sometimes, chronic inflammation of the lungs is persistent even in the absence of innocuous antigens and asthmatics can have hypersensitive airways to other environmental antigens including viral and some bacterial infections. Inflammation causes patients to suffer from some if not all of the following clinical symptoms: chest tightness, wheezing, breathlessness, and coughing. (King 1999)
On a cellular level these asthmatic symptoms arise from the activation of sub-mucosal mast cells by innocuous antigens (allergens) in the lower airways resulting in mucous and fluid accumulation subsequently followed by bronchial constriction. (Berin 2002) The immune response to asthmatic allergens is mediated by CD4+ T helper 2 (Th2) cells, eosinophils, neutrophils, macrophages, and IgE antibodies. Not surprisingly, these effector cells release cytokines that also affect expression of adhesion molecules on epithelial cells (Janeway et. al., 2001). Without effective treatment, proimflammatory cells in a dysregulated asthmatics immune response initiate remodeling of airway tissues, commonly called subbasement membrane fibrosis. For patients with severe cases, there is a higher frequency of structural remodeling of the small airway matrix compared to patients with less severe cases; however, the later are not precluded from structural remodeling of the small airway matrix. This data emphasizes the importance of treatment in both severe and mild asthma cases. (King 1999)
Asthma phenotypes are differentiated based upon the development of symptoms and the severity of asthmatic lung inflammation. Asthma symptoms are typically manifested at certain stages in life and can be classified into three general categories: childhood asthma, late-onset asthma and occupational asthma. Childhood asthma can arise from several different factors. Typically, a covirial infection such as the rhinovirus, a family history of allergy, or atopy can result in the development of childhood asthma. In childhood asthma, atopy usually results from innocuous substances such as dust mites, pet dander, and fungi. Late onset and occupational asthma exhibit different characteristics from childhood asthma and probably have a different etiology. Asthma’s causation in these circumstances may arise from constant exposure to environmental innocuous antigens. The current distinction between late-onset asthma and occupational asthma is merely the fact that the latter happens usually because of specific antigen exposure related to work. It should be noted that 30-50% atopic individuals do not develop asthma supporting the causation hypothesis that asthma is polygenic and environment related.
Asthmatic inflammation is differentiated into three broad categories: acute, subacute and chronic. Acute asthmatic inflammation involves the early recruitment of cells in to the airway, while subacute asthmatic inflammation is characterized by the activation of recruited and residual effector cells resulting in incessant inflammation. Chronic asthma is defined by constant inflammation leading to cellular damage, which in turn activates cellular repair. (King 1999)
Clinical identification of various aberrant asthmatic phenotypes and family histories of asthma indicate a genetic predisposition to asthma development. Moreover, population studies have recently correlated various genetic and phenotypic polymorphisms to the onset and development of asthma.
Individuals with a genetic predisposition to asthma are designated as atopic and have a robust response to allergens due to increased levels of IgE antibodies (Ab) (Berin 2002). Clinical studies in Germany by Illi et al. corrlelated atopic sensitization in atopic children with childhood asthma at age 7 compared to children with out childhood asthma. Children in the study were considered more susceptible to developing asthma by the age of seven if they had a family history of asthma or atopy. This led the authors to concluded that a fundamental factor relating asthma and maternal transmission may lead to different degrees of sensitization and asthma phenotypes (Care must be taken when reviewing these findings since approximately 1/3 of atopic children developed asthma). (Illi et al., 1999)
At the molecular level, linkage analysis revealed that chromosomal regions 2q33, 5p15, 11p15, 17q11.1-q11.2, 19q13, and 21q21 (Hang et al., 2003) are coupled to asthma and several polymorphic markers have been identified at chromosomal region 14q (Kusser et al., 2001). PCR analysis of TAP1 ( Transporters associated with antigen processing (TAP1) is a protein heterodimer responsible from translocation of pepetides to MHC I (major histocompatibility complex) surface glycoprotein. TAP is found in the chromosomal regions DQB1 and DRB1.) gene polymorphism for 43 healthy people and 116 asthmatics revealed that TAP1 Acc1 allele polymorphism correlates to atopic bronchial asthma. Couple these results with the fact that TAP1 may also activate intercellular adhesion molecule-1 (ICAM-1) and IFN-y responsiveness, TAP1 may be a potential genetic marker for asthma. However Hang et al. point out that there results might also be due to direct causation, natural selection, population stratification, statistical artifact, and linkage disequilibrium. (Hang et al., 2003)
Another study by Kusser et al, showed a significant correlation between the inheritance of the exon 1 allele BE1-2G for the B2 bradykinin receptor gene and late-onset asthma among 77 children with asthma and 73 controls. Previous studies showed that increased levels of bradykinin in asthmatics and stimulation with bradykinin could induce bronchial constriction in asthmatics. Out of three common alleles for B2 receptor gene: 2G, 3G, and 3T, the 2G allele had the highest transcription rate while the 3T had the lowest transcription rate suggesting transcriptional regulation of the B2 receptor gene may lead to the development of asthma. (Kusser et al., 2001) However, this hypothesis warrants further investigation due to a limited population sampling and demographics. Moreover, asthma is a complex, polygenic disease influenced by numerous environmental factors and there could be more than one genetic marker or causation.
A discussion of the molecular mechanisms underlying immune system activation for allergen induced asthma includes several critical components of the immune response: immune related receptors, effector ligands, functional roles of effector T cells, and costimulatory molecules. Generally, allergen exposure stimulates a Cd4+ Th2 immune response and the subsequent production of IgE Ab. Re-exposure to allergen results in the recruitment of mast cells (via high affinity IgE Fce receptors), eosinophils and other leukocytes. In particular, mast cells that release the vasoactive amine, histamine, and other ligands from large granules produce a local systemic hypersensitivity reaction (Janeway et al., 2001). The ensuing inflammation amplifies an individual’s hypersensitivity reaction by the recruitment of other cells and perpetuates the clinical symptoms (wheezing, shortness of breath, and chest tightness) (King 1999).
There are three receptors linked to asthmatic pathogenesis: IL-1 receptpr 1 (IL-1R1), the Toll-like Receptor (TLR) and CCR8. The cytokine IL-1 promotes the proliferation of Th2 cells and specific antibody responses. IL-1 also induces the expression of exotaxin, an eosinophil chemoattractant, on pulmonary epithelia cells. Schmitz et al. showed that, post allergen exposure, null IL-1R1 mice had reduced CD4+ T cell proliferation relative to wild type controls. Il-1R-/- mice also had limited pulmonary antibody responses, eosinophillia, and globlet cell mucus production due to impaired CD4+ Th2 function and development. These results, however, are for a mild asthma model and a more severe asthmatic model with IL-R1null mutations did not alter immune responses. The authors hypothesize, with data from concurrent studies, that IL-1 may play a role in T-cell priming through induction of CD40L and OX40 receptor. (Schmitz et al., 2003)
Another potential receptor involved in asthma responses is the Toll-like Receptor (TLR). There are currently 10 known isoforms (TLR 1-10). The reactivity and activation of TLR’s with host proteins suggests a functional role in inflammatory states and in autoimmune responses. For example, the “hygiene hypothesis” proposes that fewer bacterial infections in industrialized and modernized nations is inversely proportional to the incidence of allergic diseases. Since bacterial infections mount T helper cell 1(Th1) responses through various TLRs it is possible that a reduction in TLR ligand on pathogens coupled to allergen exposure means a reduction in shifts from a Th2 to Th1 immune response for hypersensitivity reactions. This also suggests that activating TLR receptors in conjunction with allergen may be necessary to induce immune deviation. (Heine et al., 2003)
Th2 and Th1 Cd4+ cells show differential receptor expression indicative of the different effector and recruitment mechanisms for the respective cells. Knockout mice for the CCR8 Th2 receptor showed deficient development in allergic airway responses. These mice had reduced eosinophilia and cytokine production. However, the bronchial hyperresponsivness was not reduced. Interestingly, the correlation between these results and deficient CCR8 remains to be determined. (Berin 2002)
Cytokines and chemokines play a critical role in the development and recuirtment of effector cells, respecitively. Asthma is typically characterized by IL-4, Il-5 and Il-13 cytokine production for IgE, eosinophils and mucus (Schmitz et al., 2003). Specifically, IL-4 and IL-13 induce isotype switching and IgE production, while IL-5 exherts regulatory activity on eosinophil growth, differentiation and activation (Berin 2002). Other notable cytokine expression in the asthmatic lung includes granulocyte macrophage-colony-stimulating factor mRNA. Dissimilarly, local concentrations of INF-gamma do not increase in the lung. The presence and expression of the aforementioned cytokines suggests a CD4+ Th2 cell immune response to asthma. (Berin 2002)
Additional studies have shown that pro-inflammatory cytokines IL-1B and IL-6 alter asthmatics airways. In status asthmaticus (acute respiratory failure) there are substantial increases in IL-1, IL-6 and TNF-alpha. These cytokines can have both immune and non immune related functions. Most notably, IL-1 alpha or IL-1 beta can induce loss of appetite, acute phase protein production, increases in adhesion molecules, vasodialation, increased hematopoesis, fever, growth factors, and release of matrix metalloproteinases. (Schmitz et al., 2003)
Chemokines are produced by a variety of cells and yet only eotaxin, a CC chemokine that acts on eosinophils, correlates with asthmatic symptoms (Berin 2002). Animal studies suggest, however, that chemokines play a combitorial role in the development of asthma by acting on the same cells. Neutralization of MIP-1alpha, eotakin, MCP-5 or MCP-1 or inhibited RANTES in murine models decreased bronchial hperresonsivness after ovalbumin challenge. Different receptors on Th1 and Th2 cells suggest different recruitment mechanisms for different immune responses. For example, Th2 cells preferentially express CCR8 and CCR4. TARC, a ligand for the foregoing Th2 receptors, is upregulated in bronchial epithelial cells in allergen challegened astmatics. Figure 1 outlines a proposed mechanism for TARC in the chemotaxis of Th2 cells during allergen simulation. Therefore, TARC may also represent a new protein involved in allergen induced asthma and a potential therapeutic target for neutralization or small molecule inhibition.
Figure 1: Production of IL-1 and TNF-alpha leads to the induction of the transcription factor NF-kB and TARC expression. TARC expression leads to the recruitment of Th2 cells (Berin 2002). Premission pending for image use.
These effector ligands also up regulate a distinct set of transcriptions factors. In particular, there are two transcription factors that have a predominant role in asthma hypersensitivity reactions. Altering the concentrations or effectiveness of these transcription factors illustrates the importance of effector molecules in mediating a hypersensitivity reaction. One of these transcription factor is Stat-6 and is involved in the signal transduction pathway affected by IL-4, IL-5 and IL-13. It is believed that IL-4 and IL-13 are necessary to induce Stat-6, and in turn Stat-6 activates genes responsible for bronchial hyper-responsiveness and allergic inflammation. Stat-6 is also vital for the development of CD4+ Th2 cells and Stat-6 null mutants fail to induce asthma due to a lack of IL-5 production. Adoptive transfer of Stat 6 +/+ T cells into Stat 6 null mutants also demonstrated that Stat 6 is essential for eosinophilia mucus production. The second important transcription factor in allergic asthma is GATA 3, which is involved in Th0 to Th2 differentiation and induced in the asthmatics airways. (Berin 2002)
Several costimulatory factors are believed to play a crucial role in the development of tolerance and immunity. One set of costimulatory molecules is OX40 and OX40L, members of the tumor necrosis factor family of receptors. In null mutant mice for OX40, large numbers of recruited eosinophils, IgE production and concentration of Th2 cytokines in the serum and bronchalveolar lavage were dependent upon the presence of OX40. Moreover, globlet cell hyperplasia, mucus production, and airway hyperresponsiveness (AHR) were suppressed in the absence of OX40, illustrating the importance of OX40 in Th2 asthma responses.
Another important costimulatory set of molecules is ICOS and ICOSL (Note: ICOS is expressed on T cells and ICOSL is expressed on dendritic cells). Its importance in the immune response is highlighted by the fact that blocking ICOS-ICOSL interactions inhibits respiratory tolerance and suppresses regulatory T (Tr) cells development. Inhibition of ICOS during initial stimulation and differentiation of naive T cells results in the production of more Th1 cells suggesting smaller concentrations of Th2 developmental cytokines. Murine models deficient in ICOS have decreased IgE production, Th2 cytokines, and (AHR). ICOS null mutations in a murine model suppress IL-4 and IL-13 production but make the mice susceptible to inflammatory lung disease by airway challenge in primed mice. This result and additional data led Akbari et al. to hypothesize that a Th2 immune response to allergens may abate Tr cell counts due to diminished IL-10 production. If this hypothesis is correct it represents a new possibly avenue for therapy: IL-10 induced tolerance. (Akbari et al. 2003)
The exact mechanism, factors and nature of an allergic asthma immune response are still being defined. It is believed that symptoms are manifested because of a Th2 mediated immune response. Figure 2 provides a schematic representation of the ligands involved in the regulation, recruitment, and effector functions of Th2 cells (Larche et al., 2003). However, other T cells may play a role in allergic asthma. Zuany-Amorim et al. observed that pulmonary allergic inflammation in mice with out gamma-delta T cells lead to decreases in specific IgE, IgG1, pulmonary IL-5 concentration, and eosinophil and T cell recruitment relative to wild type mice. The authors concluded that gamma-delta T cells are crucial for IL-4 dependent IgE and IgG1, and Th2 cell-mediated pulmonary inflammation. (Zuany-Amorin et al., 1998) Data for the importance of human gamma-delta T-cells is still being debated (Larche et al., 2003). Futhermore, there is evidence that CD8+ cytotoxic T-cells, in particular, cytotoxic type 2 lymphocytes, have a role in the asthma process. For example, in a murine model virus-specific CD8+ T cells switched to IL-5 production and caused airway eosinophilia, suggesting that pathogens or chemical haptens can modify antigens thereby inducing CD8+ cytotoxic T cell responses. (Larche et al., 2003) Lastly, there has been a report that IL-10 secreting Tr cells prevent AHR development in allergen sensitized mice, indirectly suggesting that wild type responses to allergens involve Tr mediated immune suppression. (Akbari et al., 2003)
Figure 2: Schematic representation of the ligands secreted and effected by Th2 cells during an allergic asthma. The numerous ligands suggest particular cytokine neutralization and injection strategies to minimize allergen induced asthmatic symptoms (Larche et al. 2003). Premission pending for image use.
Immunotherapy / Treatments:
Since allergic asthma constitutes both genetic and environmental components there are numerous possibilities for developing new treatments for asthmatics. To minimize the manifestation of allergic symptoms due to environmental stimulus, patients could simply avoid allergens, however, this is not pragmatic and exposure to allergens is often unavoidable. Neutralizing the genetic component of asthma may be more difficult, simply because of the polygenic nature of the disease. However, either treatment of the symptoms or altering the immune response can often lead to minimizing asthmatic symptoms. Current treatments rely on aersol inhalers, anti-histamine treatments, and regiments of corticosteroids (Larche et al., 2003) to diminish aberrant symptoms. Several studies showed that steroids reduce T-cell activity. (Larche et al., 2003) However, prolonged steroid use exacerbates unwanted side effects and Woisetschlager et al. suggest altering the immune response to asthma by direct or indirect abrogation of IgE Ab. Figure 3 outlines how decreases in IgE affect other immune related cells. New potential therapeutics would ideally minimize the “direct” functionality of IgE by neutralizing IgE, for example with mAb rhuMab-E25 or neturalizing the family of IgE receptors. Indirect abrogation of IgE would involve developing therapeutics to inhibit isotype switching and transcription. Additionally, the potential for therapeutics designed to alter cell-cell interactions could mitigate asthmatic symptoms. Furthermore, Woisetschlager et al. suggest that curtailing isotype switching would target two key mechanisms, first, the activation of IgE germline gene transcription and, second, DNA switch recombination. Since Stat 6 is a critical transcription factor in germline gene activation Woisetschlager et al. propose that a possible inhibitor of Stat 6 would mitigate IgE production and have a direct effect on B lymphocytes. This inhibitor could also reduce Th2 cell differentiation since Stat-6 is necessary for naive T-cell differentation. One potential inhibitor is the (S)-(+) enantiomer 4-(1-phenylethylamino)quinazoline which works at uM concentrations by inhibiting IgE germline transcription. Small molecules inhibitors could also bind to promoter regions to deactivate transcription. One such class of molecules, 2-aminoetoxy-modified poly pyrimidine oligonucleotides formed a triplex DNA structure that inhibited transcription factor binding for the promoter region of IgE. Another target of IgE production was the protein SWAP-70, which is B-cell specific and involved in recombination. SWAP 70 -/- mice had defective IgE immune responses while other isotypes were not as severely affected. Another therapeutic agent in clinical development is a mAb to CD23 or FceRII, a low affinity IgE receptor on B cells. (Woisetschlager et al. 2002) There are many possibilities for new asthma therapies but the foregoing therapies merely represent ideas on paper that will not necessary show positive results at the clinical bedside. Additionally, some of these therapies my elicit unwanted side effects or leave individuals susceptible to other pathogens.
Figure 3: Reducing IgE concentration results in: ihibition of mast cell and eosinophil degranulation and cyokine production, decreased mast cell counts, decreased IgE receptor expression, and reduced Cd23 expression, and/ or new Th2 cells from CD23 presentation of allergen (Woisetschlager et al. 2002). Premission pending for image use.
The dichotomous nature of asthma, simulated by both environmental innocuous antigen and an atypical immune response makes it an interesting and problematic disease to study. Numerous factors could be the causation of the disease and it is up to future research both at the molecular and clinical levels to find the critical mediators of allergen induced hypersensitivity reactions to make new and more effective treatments. Ultimately, the best treatments will probably aim at immune deviation, switching the allergic asthma response from Th2 cells to Th1 cells
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© Copyright 2002 Department of Biology, Davidson College, Davidson, NC 28035
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