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INTRODUCTION |
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INTRODUCTION
WHAT IS KNOWN
WHAT IS CONFLICTING
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Passive sensitization provides a useful model for the examination of the effect of the allergic process on smooth muscle reactivity and growth. This technique involves the incubation of bronchial tissue segments or bronchial smooth muscle cells in culture with serum from patients with allergic asthma or, alternatively, serum from nonallergic nonasthmatic patients. The allergic serum contains a high concentration of total IgE, usually greater than 1,000 IU/ml, and is taken from patients who exhibit a positive skin test response to one or more common allergens, whereas control nonallergic serum has a total IgE concentration of less than 20 IU/ml and is derived from patients who are skin test negative (1). One of the major advantages of this model is that comparisons can then be made between tissues or cells from the one patient, thus minimizing the inevitable variability in between-patient responses so familiar to those who work with human tissue.
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INTRODUCTION
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Passive Sensitization Results in Changes in Human Airway Smooth Muscle Tone, Growth, and Cytokine Production
Initially, the observation was made that passive sensitization
results in an increased contractile response to histamine and K+ and that this was likely to be associated with increased calcium influx into the airway smooth muscle cell (1). Since then, the findings have been confirmed (2) and extended by numerous other groups who have reported that passive sensitization
results in increased contractile responses to tachykinins and
cholinergic agonists and decreased relaxation responses to calcium channel antagonists, potassium channel openers, vasoactive intestinal polypeptide, and, inconsistently, 
-adrenoceptor agonists (3).
However, there are several different types of functional consequences of passive sensitization (Figure 1). First, bronchial ring segments exposed to allergic serum exhibit a contractile response to exogenous allergen, whereas segments from the same patient exposed to nonallergic serum do not. That this response is mediated by IgE and that the site of this reaction is mast cells located within the tissues is now well established. This evidence arises from studies in which chimeric IgE has been substituted for allergic serum (8), serum has been depleted of IgE (7), and anti-IgE has been substituted for allergen (13). Moreover, immunohistochemical analysis of passively sensitized tissue has revealed that there is an increase in IgE-bearing cells in these tissues and that these cells are principally mast cells (14).
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Second, there are the changes in contraction and relaxation responses outlined above. Evidence for the mechanisms underlying these changes is conflicting, but the increase in contractile responses is consistent except for a single study in which differences in protocol may be relevant (15). In addition, the alteration in smooth muscle response is observed whether the changes are recorded as shortening velocity and capacity (4), isometric force (1, 2, 3, 5, 7, 11), or induction of myogenic response to quick stretch (6).
Third, when human airway smooth muscle cells in culture
are exposed to allergic serum and challenged with allergen,
there is increased protein expression of the oncogenes c-fos
and c-jun and cycling of the cells as measured by incorporation
of tritiated thymidine (16). Hakonarson and colleagues have
reported that passive sensitization of human bronchus markedly increases the expression of interleukin 1 (IL-1) (10) as
well as the mRNA for the low-affinity IgE receptor Fc
RII
(9). Moreover, in one study, in response to passive sensitization, human airway smooth muscle cells have been reported to
express increased mRNA for both helper T cell type 1 (Th1)
and Th2 cytokines, as well as protein for the respective receptors (17). Hirata and colleagues (18) found that allergen challenge of passively sensitized human bronchi produced upregulation of intercellular adhesion molecule (ICAM), vascular
cell adhesion molecule (VCAM), and E-selectin on bronchial
vessels and this was associated with increased release of tumor
necrosis factor 
(TNF-), but not IL-1 into the supernatant.
Fourth, passive sensitization produces changes in the extracellular matrix proteins secreted by human airway smooth muscle cells in culture. Exposure to allergic serum produces increased amounts of fibronectin, perlecan, and chondroitin sulfate compared with cells exposed to nonallergic serum (19).
Thus, passively sensitized human airway smooth muscle exhibits properties that are of relevance to the asthmatic airway: increased contraction, decreased relaxation, increased growth, which could contribute to hyperplasia, and participation in the inflammatory/allergic reaction by a response to, as well as elaboration of, important cytokines.
There Are Important Differences in the Properties of Actively Sensitized and Nonsensitized Bronchial Smooth Muscle
The relationship between the sensitization status of the lung
tissue and the clinical definition of atopy cannot always be studied directly. In our laboratory tissues from approximately one-third of the patients contract in response to application of
allergen. Thus we describe these patients as "actively sensitized" but since we do not have available corresponding data
on skin test reactivity, we cannot strictly call them atopic. Nevertheless, we have noticed marked differences in the in vitro
characteristics of tissue taken from this subset of patients,
compared with those exhibiting no such response to allergen.
Supernatants from stimulated neutrophils, when applied to
human bronchial segments in vitro, caused a direct contractile
response in nonsensitized tissue that was not observed in sensitized tissues (20). In contrast, when these supernatants were
applied to tissues subjected to electrical field stimulation (EFS),
a potentiation of the contractile response was observed in sensitized but not nonsensitized tissues: an effect mediated prejunctionally on parasympathetic nerve fibers. Exogenous TNF-,
however, potentiated the contractile response to EFS only in
nonsensitized tissues (21). Johnson and coworkers (22) reported that the mast cell mediator tryptase causes potentiation of the contractile response to histamine in sensitized but not nonsensitized tissues. Moreover, the myosin light chain kinase content of actively sensitized tissues was increased compared with that of nonsensitized patients (23). In addition, in an immunohistochemical study (24) enumerating inflammatory cells in sections of human bronchus taken from sensitized and nonsensitized patients, whose sensitization status was established
in vitro, the number of mast cells located in the smooth muscle
of sensitized patients was increased compared with that of
controls. Thus, there are significant differences in the in vitro
properties of human airway tissue that can be directly related
to the allergic status of the patient. It is interesting to contemplate the relationship between these findings and those in a
study by Robinson and coworkers (25), who tested the hypothesis that immature progenitors, which may have the potential to develop into eosinophils, are present in the bronchial mucosa in asthma. They examined CD34+ cell numbers
and found that they were increased in the mucosa of both
atopic, nonasthmatic, and atopic asthmatic subjects whereas cells positive for both CD34 and IL-5 receptor (IL-5R)
mRNA
indicative of IL-5 responsivenesswere present
only in subjects with asthma. The idea that differentiation of
progenitors could occur in situ at the site of allergic inflammation raises the possibility that local conditions in the lung
could play a role in the interaction of the allergic process and
smooth muscle reactivity.
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WHAT IS CONFLICTING |
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INTRODUCTION
WHAT IS KNOWN
WHAT IS CONFLICTING
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Are the Changes in Contractility, Cell Cycling, and Cytokine Production Due to the High Concentrations of IgE in the Allergic Serum?
Whether the changes in in vitro reactivity outlined above are
all due to the presence of high IgE is a matter of debate (26). Hakonarson and colleagues (9) have shown that passive sensitization is associated with upregulation and expression of the
low-affinity IgE receptor FcRII and that the application of
immune complexes mimicked this response. Others have
shown, however, that depletion of the allergic serum of its IgE
content (7) resulted in abolition of the contractile response to
allergen but retention of the sensitization induced hyperresponsiveness to histamine, and in addition that increased contractile responses to histamine were unaffected by a novel specific anti-IgE antibody, 17-9 (27). Furthermore, when a chimeric
humanized IgE was substituted for allergic serum, no increase
in response to histamine occurred (8). These results would
suggest that some serum-derived factor or factors that are unrelated to IgE are contributing to sensitization-induced hyperresponsiveness. The results of clinical trials examining the efficacy of anti-IgE therapy may shed some light on this conflict.
Are the Changes Induced by Passive Sensitization
the Result of the Actions of Cytokines such as
IL-1 and/or TNF-?
Again, although there is some evidence of a role for IL-1 in
mediating sensitization-induced changes in smooth muscle reactivity (10), Hirata and coworkers found that, whereas TNF-
is released on allergen challenge of sensitized tissue, IL-1
was not increased (18). For IL-1 to play the pivotal role that
has been suggested, exogenous IL-1 should potentiate histamine-induced contractile responses in human bronchus, and
as yet this evidence is not available.
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IMPORTANT QUESTIONS |
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TOP
INTRODUCTION
WHAT IS KNOWN
WHAT IS CONFLICTING
IMPORTANT QUESTIONS
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- What are the factors present in allergic serum that cause changes in smooth muscle contractile behavior, changes in cell cycling, and changes in cytokine message, protein release, and receptor expression?
This question needs to be answered by the use of careful fractionation of the serum and examination of all of the functional consequences of passive sensitisation. Logistical problems will be encountered, however, if combinations of factors present in separate fractions of the serum are producing all or some of the different cellular responses.
- Does passive sensitization produce changes in smooth muscle properties that represent a step in the spectrum of nonsensitized, actively sensitized, through to sensitized asthmatic smooth muscle?
By studying these different "degrees" of allergic response, can we gain valuable information about the interaction between the smooth muscle and the allergic response? We need to consider not only the interaction between allergic serum-derived factors and the smooth muscle cells but also the influence of resident or recruited inflammatory cell populations. These, although not present in primary cultures of human airway smooth muscle cells, may have produced changes in the muscle cell that alter its subsequent contractile and growth properties.
We need to know whether the increased numbers of mast cells present in the smooth muscle of sensitized patients have the potential to alter smooth muscle properties or if the increase in mast cells is the result of smooth muscle-induced differentiation of airway inflammatory cell progenitors.
It will be important to delineate the differences in the phenotype of muscle cells derived from nonsensitized, actively sensitized, and passively sensitized lungs.
- What are the signal transduction pathways leading to increased contraction, decreased relaxation, and increased cycling in passively sensitized airway smooth muscle?
Although there is an increasing body of evidence to suggest that sensitization-induced changes in contractility of human airway smooth muscle may be associated with alterations in calcium flux (1, 2), this may not necessarily underlie the upregulation of cytokines and the increased cycling of the cells. It is likely that these pathways will be complex and whether there is cross-over between the mechanisms leading to changes in contractility, in growth, and in cytokine production will require further study and specifically in human airway tissue.
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Footnotes |
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Correspondence and requests for reprints should be addressed to J. L. Black, Ph.D., Department of Pharmacology, University of Sydney, NSW 2006, Australia. E-mail: judblack{at}pharmacol.usyd.edu.au
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References |
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INTRODUCTION
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WHAT IS CONFLICTING
IMPORTANT QUESTIONS
REFERENCES
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1. Black, J. L., R. Marthan, C. L. Armour, and P. R. Johnson. 1989. Sensitization alters contractile responses and calcium influx in human airway smooth muscle. J. Allergy Clin. Immunol. 84: 440-447 [Medline].
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16. Johnson, P. R. A., S. Carlin, C. L. Armour, and J. L. Black. 1997. Antigen exposure increases growth and formation of c-Jun protein in human airway smooth muscle cells primed with atopic asthmatic serum (abstract). Am. J. Respir. Crit. Care Med. 155: A902 .
17. Hakonarson, H., N. Maskeri, C. Carter, and M. M. Grunstein. 1999. Regulation of TH1- and TH2-type cytokine expression and action in atopic asthmatic sensitized airway smooth muscle. J. Clin. Invest. 103: 1077-1087 [Medline].
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