| Sensorineural and Systemic Aspects of Inflammatory
At an industry-sponsored symposium held in conjunction with the 99th International Conference of the American Thoracic Society, four experts in the diagnosis and treatment of airway diseases presented the latest information on allergic airway disease and its potential to propagate systemically or through the nervous system. They discussed how such propagation might be responsible for interactions between the nasal and lower airways. Topics included systemic aspects of allergic airways disease, systemic response and the small airway, sensorineural pathways in airway disease, and sensorineural interplay between the upper and lower airways.
This program was supported by an unrestricted educational grant from Merck & Co., Inc.
Systemic Aspects of Allergic Airway Disease
The presence of an antigen sets off a very complex response in the body with dendritic cells (DC) and T-cells at the core. Although less than 1% of mononuclear cells, they are the key antigen-presenting cells (APC) in the lung.
Immature DCs reside in the epithelium and submucosa. They take up the antigen (Ag), process it and express peptides as MHC molecules on the surface. After capturing the Ag, mediators such as the cytokines, chemokines and others are expressed by these cells.
After Ag exposure, the DC migrates to the lymph node where it stimulates a CD-4 T-helper cell response. The T-helper cell can be differentiated into Th1 type, which secretes interferon-gamma or Th2 that secretes other cytokines such as interleukin (IL)-4 and IL-5. Th1 cells are usually produced in response to viral or bacterial antigens that replicate intracellularly while Th2 cells are dominant in allergen responses. Cytokine signaling determines which type is expressed, as IL-4 promotes Th2 and IL-12 is responsible for Th1 responses.
The T-cells are differentiated into effector and memory cells. The effector cells immediately go to the site of inflammation, secrete cytokines, do their job, and then die out by apoptosis.
“Memory cells are an evolving field,” said Anuradha Ray, PhD, Professor, University of Pittsburgh School of Medicine, Pittsburgh, PA. “Most go to the lymph nodes and await activation if the Ag returns.”
Regulatory T-cells (T-regs) are another facet of the immune response. They are generated during the first few months of life and are very important in the maintenance of immune homeostasis. They are able to suppress Th1 and Th2 function.
As an example, Dr. Ray and colleagues intranasally administered high doses of albumin to mice with and without the adjuvant choleratoxin. The adjuvant promotes Th2 response when present and tolerance when only Ag is delivered. They then looked at the response of mice to Ag exposure alone and noted that T-cells in the lymph node showed an initial increase in GATA-3 secretion, the transcription factor for Th2, expression that diminished over time. When the mice were subjected to the inflammation model, there was a progressive increase in GATA-3 expression.
When the T-cell finds an antigen, it rolls on the endothelial walls and sets up a signaling event so that it can adhere to the wall using additional molecules known as integrins. The integrins are important because they regulate the type of tissue that the cell can adhere to.
For example, the CLA+ subpopulation has been detected in atopic dermatitis (AD) and is probably specialized for the skin. In contrast, those with asthma but without atopic dermatitis had only the CLA- subset. There is currently no known integrin specifically for the lung.
“One of the interesting speculations is that these other kinds of cells may migrate to the lung,” said Dr. Ray. “If they were stimulated at some later point, the expansion of these cells in the lung could also expand the CLA+ cells on the skin.”
In the past, it was thought that dendritic cells did not express IgE receptors. It is now known that high affinity receptors for IgE are expressed on epidermal Langerhans cells and blood DCs.
“Put in this framework, we might have low levels of IgE leaking into the blood, which then could go to the lung and bind to dendritic cells there,” said Dr. Ray. “We need to ask whether patients with severe AD harbor circulating antigen specific T-cells that home in on the lung and conversely do patients with asthma generate cells that might home in on the skin?”
Systemic Response and the Small Airway
The physiological reasons why small airways are so important, and in particular those less than 2 mm in size, has been lost in a kind of time warp. The first suggestions came from Weibel’s landmark morphometric study from the early 1960s showing that a large part of the lung’s volume was in the smaller airways (Weibel E. Morphometry of the Human Lung. New York Academic Press, 1963).
“Malcolm Greene then took Weibel’s data and calculated the resistance found in each generation of the airway,” said Charles Irvin, PhD., Professor and Director of the Vermont Lung Center at the University of Vermont. “ What he found was very interesting; small airways probably contribute little to total lung airflow resistance” (Green M. St. Thomas Hospital Gazette.1964;63:136).
His theory was later proved correct in both animal and human experiments showing that if central resistance is doubled, total resistance is nearly doubled. A doubling of resistance in the peripheral airway showed little change. This means it is possible to have very bad disease in the peripheral lung that is not detectable by standard measures.
More recently, Wagner and others provided direct evidence of peripheral lung involvement in asthma in their seminal paper. By measuring peripheral airflow resistance with a wedged bronchoscope, they found very little pressure differences in controls. In asthmatics, there was a more diverse response to increased flows with the pressures becoming quite high in some subjects (Wagner EM, et al. Am Rev Resp Disease. 1990; 141:584).
“The reason these findings were surprising is that the asthmatics had normal spirometry results,” said Dr. Irvin. “Yet these mild asthmatics had peripheral resistance of their lung that was ten times normal and not detected with standard lung function tests.”
Dr. Irvin and colleagues then studied peripheral resistance in controls and nocturnal asthmatics at 4:00 pm and 4:00 am. Although peripheral resistance was higher in the more severe asthmatics in the afternoon, the resistance measured was even higher in the early morning (Kraft M, et al. Am J Respir Crit Care Med. 2001;163:1551) (Figure 1).
“We concluded that as asthma severity worsened, the changes in the peripheral lung became more pronounced,” said Dr. Irvin. “ The best way to clinically assess small airway dysfunction is a question that has been around for a long time. Residual volume measured in a plethysmograph is probably the most telling since it correlates well to increases in peripheral resistance.”
“The bronchial pressures showed peripheral resistance to be very unstable,” he continued. “When flow was increased, instead of resistance going up like it should do if the airways were simple tubes, peripheral lung resistance often abruptly fell. Only one phenomenon would account for that; the sudden opening of a closed airway and recruitment of a collapsed lung.”
One way to directly assess this possibility involved Magnetic Resonance Imaging (MRI). Helium that had been polarized by a laser was inhaled. In normal subjects, the distribution was uniform. In asthmatics, there were parts with clearly attenuated helium suggesting poor distribution that worsened with exercise-induce bronchospasm (Wagers S. J Allergy & Clin Immunol. 2003;111: 1201).
How airway closure and lung de-recruitment leads to airway hyperactivity may be explained by inflammation-caused leakage of serum proteins.
Inflammation can form fibrin monomers, which, in turn, is the most potent inactivator of surfactant (Wagers SS, et al. Am J Respir Crit Care Med. 2003;167: A882.[poster].
“The fibrin formation we created experimentally was exactly the same we see in patients who die of asthma,” said Dr. Irvin. “When we create fibrin within the airway of an animal, we create closure and hence airway hyperactivity.”
The unanswered question then becomes whether allergic disorders, such as asthma, influences airways via a systemic response.
Dr. Irvin and others measured the frequency dependence of resistance in the lung. When they infused the subjects with histamine, the airways responded with inhomogeneity indicating a peripheral lung response to systemic factors (Bhansali PV, et al. J Appl Physiol 1979; 47:161).
“There is no question that asthma, especially mild asthma, involves the small airways and alveolar spaces,” said Dr. Irvin. “A better understanding of these changes in the depths of the lung is likely to yield new insights into the pathophysiology of asthma and suggest new approaches for treatment.”
Sensorineural Pathways in Airway Disease
There are three major neural networks that control many airway functions. The parasympathetic nerves are dominant in the neural control of human airways and responsible for cough, contraction, and mucous secretion. Sensory nerves respond to changes in the local environment in the lung and trigger reflex responses such as cough. Sympathetic nerves are sparse in human airways compared to other species.
Electrophysiological studies have shown that the A-delta and C-fibers respond to local environmental changes. The A-delta fibers respond primarily to mechanical stimulation. The C-fibers are nociceptors that are triggered by inflammatory mediators and capsaicin.
“The normal function of the airway sensory fibers is protective,” said Maria G. Belvisi, PhD, Professor of Respiratory Pharmacology, Faculty of Medicine, at the Imperial College in London. “It is thought that in airway inflammatory diseases, these protective reflexes go out of control and may become deleterious.”
One mechanism described that could lead to exaggerated sensory nerve function is peripheral sensitization. Fox and others studied the function of airway sensory nerves using single-fiber recording studies in guinea pig airways. Starting with a vagus nerve still attached to the trachea, they isolated a single fiber, identified it as either a C- or A-delta fiber and then found the receptive field. Upon perfusing the field with inflammatory mediators such as bradykinin, the C-fibers were activated so that a low concentration of the agent could sensitize these fibers resulting in an increased firing in response to other agents such as capsaicin. The A-delta fibers were more mechanically sensitive.
Peripheral sensitization of airway sensory nerves, demonstrated in vivo as enhanced cough is found following inhaled bradykinin prior to a tussive challenge with citric acid. This enhanced tussive response could be abolished by a B2 receptor agonist (Fox AJ, et al. J. Physiol.1993;469:21).
Phenotypic changes have also been described under inflammatory conditions and could lead to a state of sensory nerve hyper-responsiveness. In the vagal-sensory system, neuropeptides such as substance P and calcitonin gene-related peptide (CGRP) are synthesized almost exclusively in small-diameter nociceptive type C-fiber neurons that are designed to respond to noxious or tissue-damaging stimuli.
Myers, et al. have shown that allergic inflammation of guinea pig airways leads to induction of Substance P and CGRP production in large-diameter vagal sensory neurons. Electrophysiological and anatomical evidence reveals that peripheral terminals of these neurons are low-threshold A-delta mechanosensors insensitive to nociceptive stimuli such as capsaicin and bradykinin. Thus, inflammation causes a qualitative change in chemical coding of vagal primary afferent neurons (Figure 1).
These results support the hypothesis that inflammatory reactions stimulate sensory neuropeptide release from primary afferent nerve endings in the periphery simply as a result of stimulating the low-threshold mechanosensors. These phenotypic changes with the sensory nerves are thought to lead to exaggerated central and peripheral reflexes (Myers AC, et al. Am J Physiol Lung Cell Mol Physiol. 2002;282:L775).
C-fibers are not only activated for core central reflexes, but also to elicit a variety of other events. These events, such as bronchoconstriction, mucous secretion, and vasodilatation, are grouped together as neurogenic inflammation. However, “the major controversy is whether neurogenic inflammation actually happens in human airways,” said Dr. Belvisi.
Ichinose and others produced encouraging data showing that the dipeptide receptor antagonist FK-224 inhibited bradykinin-induced bronchoconstriction in asthmatics supporting a role for neurogenic inflammation in human airways. Enthusiasm for this was dampened when follow-up studies were not able to reproduce the response and other workers did not show much success with NK1 or NK2 receptor antagonists (Ichinose M, et al. Lancet. 1992;340:1248; Joos GF, et al. Am J Respir Care. 1996;153:1781).
According to Dr. Belvisi, it may be necessary to block more than one receptor, and possibly all three, to stop these effects.
An alternative strategy that could be employed to inhibit sensory nerve function is to inhibit nerve activation directly using cannabinoids or large conductance calcium activated channel openers. This should lead, in theory, to blocking the activation of the nerve inhibiting central reflexes such as cough and peripheral release of neuropeptides leading to neurogenic inflammation (Fox AJ, et al. J Clin Invest. 2002; 239:239; Belvisi MG. Curr Opin Pharmacol. 2003;3:239).
Dr. Belvisi and colleagues used an isolated guinea pig vagal nerve preparation and found that a cannabinoid CB 2 receptor agonist was very effective at inhibiting capsaicin-induced depolarization. This can be mimicked in the human vagus nerve (Patel HJ, et al. Br. J Pharma-col.2003, in press).
“This is very interesting because it is potentially a new therapy and not hampered by the usual CB1 receptor-mediated side effects,” said Dr. Belvisi. “It has been shown that airway sensory nerves are important in airway disease and in generating the neuropeptide release that causes neurogenic inflammation. Several treatment options may become available.”
Sensorineural Interplay Between the Upper and Lower Airways
Many clinical studies indicate an interaction between the nasal and lower airways. The mechanisms of this interaction remain hazy.
A study undertaken by Corren and others looked at subjects with asthma who received a nasal challenge in a way that none of the allergen reached the lower airway. Methacholine provocations were performed at baseline and at various time points after the challenge (Corren J, et al. J Allergy Clin Immunol. 1992;89:611).
“When you look at those given the diluent instead of allergen, there is a diurnal improvement in their methacholine responsiveness over time,” said Alkis Togias, MD, Associate Professor of Medicine at Johns Hopkins University Asthma and Allergy Center. “That improvement was ablated by the allergen challenge. That was one of the first experimental pieces of evidence that allergic reactions in the nasal airways may have consequences in the lower.”
Treating the nose may actually protect the lower airway. For example, a Mayo Clinic study randomized four groups of subjects with rhinitis to receive placebo, cromolyn, or two different steroid nasal sprays during pollen season. Those subjects who also had asthma were completely protected from a seasonal increase in asthma symptoms with nasal steroid treatment (Welsh PW et al. Mayo Clinic Proc.1987;62:125).
“A logical mechanism is that asthma may be taking the beneficial functions of the nose out of the equation,” said Dr. Togias. “This puts significant burdens on the lower respiratory tract. The additional pathway of systemic allergic response also needs to be considered.”
In a seminal study by Braunstahl and colleagues, nasal and bronchial biopsies were performed at baseline and 24-hours after a nasal allergen challenge. At 24-hours, they found not only the expected eosinophilia in the nasal mucosa, but also increased eosinophils in the bronchial mucosa accompanied by up regulation of various adhesion molecules (Braunstahl C-J, et al. J Allergy Clin Immunol.2001;107:469).
The systemic propagation of allergic reactions probably explains more interactions such as between the lungs and the skin. In a study by Brinkman and colleagues, patients with atopic dermatitis with and without asthma were given a dust mite inhalation challenge. There was an increase in skin symptoms 24 hours later which was more evident in those with asthma (Brinkman L, et al. J Clin Exper Allergy. 1997;27:1043).
“The model I’d like to propose, is one that begins with a local allergic reaction but also generates a systemic component,” said Dr. Togias. “That component feeds back into the site of the original reaction, but also is responsible for distant manifestations.”
Several mechanisms can be proposed to explain the distant manifestations of allergic reactions. Humoral factors created during an allergic reaction in one area may directly or indirectly impact on another. Histamine and leukotrienes should be seriously considered, especially in light of the fact that histamine and leukotriene receptors reside on many immune cells.
“There is no doubt that we have to treat the local components of allergic reactions,” said Dr. Togias. “We should not assume that the systemic component is inactive after the local problems are suppressed. It is important to attack both components at the same time to reduce both the local feedback and the distant manifestations that can be quite significant.”
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