Diesel exhaust and asthma: hypotheses and molecular mechanisms of action.

Several components of air pollution have been linked to asthma. In addition to the well-studied critera air pollutants, such as nitrogen dioxide, sulfur dioxide, and ozone, diesel exhaust and diesel exhaust particles (DEPs) also appear to play a role in respiratory and allergic diseases. Diesel exhaust is composed of vapors, gases, and fine particles emitted by diesel-fueled compression-ignition engines. DEPs can act as nonspecific airway irritants at relatively high levels. At lower levels, DEPs promote release of specific cytokines, chemokines, immunoglobulins, and oxidants in the upper and lower airway. Release of these mediators of the allergic and inflammatory response initiates a cascade that can culminate in airway inflammation, mucus secretion, serum leakage into the airways, and bronchial smooth muscle contraction. DEPs also may promote expression of the T(subscript)H(/subscript)2 immunologic response phenotype that has been associated with asthma and allergic disease. DEPs appear to have greater immunologic effects in the presence of environmental allergens than they do alone. This immunologic evidence may help explain the epidemiologic studies indicating that children living along major trucking thoroughfares are at increased risk for asthmatic and allergic symptoms and are more likely to have objective evidence of respiratory dysfunction.

Medical treatment of asthma and knowledge about asthma's biologic mechanisms have improved in recent years. Yet asthma prevalence, hospitalization rates, and mortality rates continue to rise internationally in both adults and children (1)(2)(3)(4)(5). According to the Centers for Disease Control and Prevention, the number of individuals with self-reported asthma increased by 75% in the United States from 1980 to 1994 (6). The increase was seen in all races, both sexes, and all age groups, but nonwhite children have been particularly affected. The prevalence of pediatric asthma increased by 160% during the same time period in children under 4 years of age and by 74% in children over age 4 (7). Not only is the prevalence of asthma rising in industrialized countries, but also the severity among those afflicted has increased. A recent cross-sectional study found that the odds of an adverse outcome (i.e., intubation, cardiopulmonary arrest, or death) among children hospitalized for asthma in California doubled between 1986 and 1993 (8).
Asthma is more prevalent in the urbanized areas of industrialized countries (9). Numerous studies have demonstrated that specific components of air pollution may be associated with exacerbations of asthma (10)(11)(12)(13)(14). Although the levels of coarse particulate matter in the atmosphere have decreased over recent decades, the levels of fine particulate matter smaller than 2.5 µm in size (PM 2.5 ), such as diesel exhaust particles (DEPs), remain an ongoing problem (15). Ambient air pollution has been associated with hospitalizations and deaths due to exacerbations of cardiovascular and respiratory diseases (15). Particulate air pollution has also been linked more specifically to asthma (16,17).
Some of the evidence linking particulate air pollution and asthma is indirect. For instance, several studies found that children raised in more polluted regions of a country are more likely to develop respiratory diseases and allergies compared with children raised in "cleaner" regions (18,19). Within communities, children living on busy streets have a higher likelihood of developing chronic respiratory symptoms than those living on streets with lower traffic volume (10,20). When exposed to similar levels of Japanese cedar pollen (a standard allergen), people who live in highly trafficked areas have enhanced allergic reactions compared with people who live in rural areas. This suggests the possibility of a synergistic effect between air pollution and aeroallergens (21).
Diesel exhaust and DEPs have previously been associated with asthma (22)(23)(24). Current evidence supports the hypothesis that components of diesel exhaust worsen respiratory symptoms in individuals with preexisting asthma or allergies, and offers some support for the hypothesis that diesel exhaust and DEPs may play a role in causing asthma (25)(26)(27)(28). This paper critically analyzes the research relevant to the question of whether diesel exhaust exposure is associated with asthma. We also review molecular mechanisms by which particulate matter in diesel exhaust may facilitate and promote asthmatic symptoms.

Molecular Basis for the Inflammatory Events in Asthma
Asthma is a chronic respiratory disease manifested by bronchial hyperresponsiveness, reversible bronchial constriction, airway inflammation, and respiratory symptoms such as wheezing, dyspnea, coughing, and chest tightness (29,30). A complex immunologic cascade, including recruitment of inflammatory cells from the bloodstream to the bronchial mucosa, is characteristic of asthma (31).
During asthma attacks, both inflammatory and structural cells of the respiratory tract are activated. Activated cells include T cells, mast cells, eosinophils, macrophages, epithelial cells, fibroblasts, and bronchial smooth muscle cells. By releasing proinflammatory and cytotoxic mediators and cytokines, these cells are all involved in a cascade that leads to the acute and chronic symptoms of asthma (30). Figure 1 summarizes the immunologic events involved in asthma.
T lymphocytes appear to play a particularly important role in airway inflammation. T cells have been demonstrated in the airways of patients with fatal asthma (32) and appear to be vital for regulating the immune pathways that control allergic immune responses (31). In general, T cells can be classified into two major subsets consisting of CD4 + or CD8 + cells. CD4 + T cells differentiate into several phenotypes of T cells, including T helper 1 (T H 1) and T helper 2 (T H 2) (33). A shift in the predominant T-cell population from the T H 1 type to the T H 2 type has been associated with asthma (34).
Immunoglobulins, cytokines, and chemokines appear to play important roles in the inflammatory foundation of asthma. For example, IL-5 promotes the development and survival of eosinophils, the cells that help drive the chronic asthmatic response. IL-8 is a potent chemoattractant for neutrophils and primes eosinophil responses. IL-10 builds and prolongs the immune response by stimulating production of more T H 2 cells. IL-4 and IL-13 act on B cells to stimulate production of antigen-specific immunoglobulin E (IgE), and GM-CSF is an important growth and survival factor for neutrophils, eosinophils, and macrophages. The relationship between these molecules and the eventual clinical symptomatology of asthma is illustrated in Figure 1.

Theories on the Etiology of Asthma
Genetic and environmental factors interact to cause asthma (1). There is substantial epidemiologic evidence, supported by clinical and toxicologic data, regarding a variety of asthma risk factors. Atopy is a major heritable risk factor for asthma and involves the familial tendency to develop immediate-type hypersensitivity (i.e., IgE-mediated) immune responses to specific allergens (34). Although genetic predisposition may be important in the development of asthma, recent increases in the prevalence and severity of asthma seem to have occurred too rapidly to be mediated solely by genetic shifts (35).
Environmental factors that have been associated with adult and childhood asthma include allergen exposure, environmental tobacco smoke, socioeconomic status, nutrition, family size, history of infections, and ambient levels of air pollution (2,7). Although no consensus exists on the relative importance of each of these factors, the development of asthma is clearly multifactorial. Some scientists have hypothesized that fetuses and infants may take the first steps toward sensitization to environmental allergens during critical windows of susceptibility during early life, perhaps because of an environment that encourages dominance of the T H 2 phenotype beyond fetal life (9,34). Because components of diesel exhaust have been shown to affect numerous inflammatory and immunologic pathways in the respiratory tract, including promoting induction of a T H 2 phenotypic response, some researchers hypothesize that exposures to diesel exhaust may play a role in the development or exacerbation of asthma and allergic disorders (36).

Composition of Diesel Exhaust
Arising from the combustion of diesel fuel in compression-ignition engines, diesel exhaust consists of a complex mixture of particulate matter, including elemental carbon and polycyclic aromatic hydrocarbons (PAHs; i.e., phenanthrene, fluorenes, naphthalenes, pyrenes, fluoranthrenes), as well as acid aerosols, volatile organic compounds, various hydrocarbons (including highly reactive quinones), and gases, including carbon dioxide (CO 2) , carbon monoxide (CO), nitric oxide (NO), nitrogen dioxide (NO 2 ), and sulfur dioxide (SO 2 ) (37). After combustion of diesel fuel, the exhaust components tend to aggregate into discrete, spherical, respirable particles approximately 0.1-0.5 µm   in diameter (38). These particles consist of an inert carbonaceous core with a large surface area, ideal for adsorbing heavy metals and organic compounds such as PAHs. The PAHs are small compounds of three to five benzene rings that can easily diffuse through cell membranes and bind to receptors within the cytoplasm. One such receptor is the aromatic hydrocarbon receptor complex (36). In addition, diesel exhaust contains many substances that are listed as toxic air pollutants by the State of California and as hazardous air pollutants by the U.S. Environmental Protection Agency (37,39). Buses, trucks, and other heavy industrial transport vehicles are major sources of ambient diesel exhaust pollution. Utilization of diesel fuel has steadily increased in the United States over the past several decades: the number of miles traveled by commercial trucks in the United States has increased by 235% between 1950 and 1985, and cargo tonnage carried by trucks has increased by 169% (40).
DEPs are major sources of ambient PM 2.5 (41). In California, an estimated 26% of all particulate matter from fuel combustion sources arises from the combustion of diesel engines (41). In 1996, diesel exhaust also comprised a quarter of the NO smog precursors released nationally in the United States (39).

Epidemiologic Studies Linking Diesel Exhaust and Asthma
There is some epidemiologic evidence associating exposure to high levels of diesel exhaust with asthma. Wade and Newman (42) describe three railroad workers who traveled in locomotive units directly behind the lead diesel-powered locomotive engine and eventually developed acute or subacute onset of respiratory symptoms. They demonstrated symptoms consistent with asthma, including hyperreactive airways, airflow limitation, and reversibility with bronchodilators. None of these workers had any known preexisting respiratory conditions. Numerous components within diesel exhaust are respiratory irritants (38), including some of the acid aerosols, volatile organic compounds, and gases in the mixture. The irritant effect alone could potentially trigger asthmatic symptoms at sufficiently high exposure levels.
Although exposure to acutely high levels of diesel exhaust can produce respiratory symptoms, there is also epidemiologic evidence that chronic exposure to diesel exhaust at lower environmental levels may also be associated with increased levels of respiratory symptoms. For instance, children living near busy diesel trucking routes have decreased lung function in comparison with children living near roads with mostly automobile traffic (10). A population-based survey of more than 39,000 children living in Italy found that children living on streets with heavy truck traffic were 60-90% more likely to report acute and chronic symptoms such as wheeze, phlegm, and diagnoses such as bronchitis, bronchiolitis, and pneumonia (43). A German study of over 3,700 adolescent students found that those living on streets with "constant" truck traffic were 71% more likely to report symptoms of allergic rhinitis and more than twice as likely to report wheezing (44).

Diesel Exhaust Gases and Potential Adverse Respiratory Effects
Diesel exhaust contains many well-known air pollutants that have been associated with asthma exacerbations (45), including SO 2 , NO 2 , and fine particulate matter smaller than 10 µm in size (PM 10 ), which are all criteria air pollutants (39).
Several studies have found temporal associations between ambient particulate levels (PM 10 ) and emergency department admissions for exacerbations of asthma (16,17,46). Some recent studies have also shown relationships between both daily and long-term levels of SO 2 and child hospital visits for respiratory diseases (11,47). SO 2 causes bronchoconstriction in asthmatics during exercise. These effects are above and beyond the effects of exercise alone. Adult asthmatic subjects exposed to ambient concentrations (0.5 ppm SO 2 ) during just a few minutes of moderate exercise experienced significant drops in forced expiratory volume in 1 sec (FEV 1 ) (48,49). There is also evidence that shortterm exposure of asthmatics to NO 2 at ambient atmospheric levels may increase airway responsiveness to SO 2 (50). Therefore, it is possible that some of the gases related to diesel exhaust may trigger exacerbations of asthmatic and allergic symptoms in already asthmatic subjects (51)(52)(53).
Several epidemiologic studies have reported associations between daily and chronic levels of NO 2 and exacerbations of asthma (12,24,26,54). Toxicologic evidence indicates that NO 2 is directly harmful to the respiratory system. Normal healthy subjects exposed for 2 hr to 2 ppm NO 2 demonstrated increases in IL-8 and neutrophils (55). An in vitro study exposed human nasal mucosal tissues to NO 2 and ozone and reported elevated histamine levels (56). Another study that exposed mildly asthmatic human subjects to 260 ppb (500 µg/m 3 ) NO 2 for 30 min found that the response to an inhaled allergen was enhanced after the NO 2 exposure (57).
Acute exposures to diesel exhaust, even at low concentrations, have been shown to elicit inflammatory responses. There is some evidence to suggest that the inflammatory response from diesel exhaust may not simply be due to SO 2 and NO 2 exposures. Fifteen nonasthmatic volunteers exposed for 1-hr periods to diesel exhaust (at PM 10 concentrations of 300 µg/m 3 and NO 2 concentrations of 1.6 ppm) developed elevated levels of neutrophils, macrophages, B cells, mast cells, T lymphocytes, histamine, endothelial adhesion molecules, and lactate dehydrogenase in their airways at 6 hr postexposure (58). Such effects do not occur in nonasthmatics exposed to NO 2 alone at comparable concentrations, making the particles the more likely culprit. An in vitro study found that exposure of human bronchial epithelial cells to unfiltered diesel exhaust released inflammatory cytokines, whereas diesel exhaust that was filtered (and therefore contained gases but no particulate matter) did not have this effect (59). These studies suggest that the particulate components of diesel exhaust may play a more significant role in triggering airway inflammation than the gaseous components.

Molecular Mechanisms of Action of DEPs in the Respiratory Tract
It is not entirely clear which DEP components produce toxicity. Some studies suggest that the majority of the toxicity is attributable to the adsorbed organic compounds (38,60,61), whereas others conclude that the most toxic portion of a DEP is the carbonaceous core (15). Regardless of which specific components of DEPs are most toxic, it appears that DEPs may be associated with both early and late phases of the inflammatory response in asthma.
Typically, the early asthmatic phase is predominantly IgE mediated, whereas the late phase involves complex networks of inflammatory mediators, including eosinophils, T cells, cytokines, chemokines, and immunoglobulins (30).
There are numerous hypothesized interactions of DEPs with the immune and respiratory systems. DEPs may act directly to alter specific immunologic pathways that may precipitate acute exacerbations of asthma. Direct effects of DEPs include stimulation of IgE production, eosinophilic degranulation, augmentation of cytokine and chemokine production and release, free radical formation, and effects on production of NO in the airways (62). As an adjuvant with environmental allergens, DEPs appear to enhance the differentiation of CD4 + T lymphocytes into the T H 2 phenotype and enhance allergen-specific IgE and IgG production. The potential pathways by which DEPs may promote asthma are summarized in Table 1.

Reviews, 2002 • Diesel exhaust and asthma
Environmental Health Perspectives • VOLUME 110 | SUPPLEMENT 1 | February 2002 Animal studies Increase in GM-CSF mRNA expression in the lungs of mice in cells involved in asthmatic activity intranasally exposed to DEPs (66) Increase in GM-CSF in mice exposed to DEPs and allergen (

Direct Immunologic Effects of DEPs Enhanced IgE Production by Effects on B Lymphocytes
DEPs consistently enhance the production of IgE in the airways (63)(64)(65). IgE is produced by activated B cells in response to a specific allergen. Once produced, IgE attaches to mast cells and, when cross-linked by allergen, induces mast cells to release histamine and leukotrienes. The chemicals released from mast cells cause constriction of bronchial smooth muscle, mucus secretion, and serum leakage into the airways and result in acute asthma symptoms (30). The mast cell is often considered the central cell type in the acute asthmatic response, and IgE is the critical immunoglobulin driving the mast cell response.
In a study of eleven nonsmoking, nonallergic volunteers, Diaz-Sanchez et al. (65) showed that exposure to DEPs significantly increases IgE levels in nasal fluids by greatly increasing the numbers of IgE-secreting cells and by altering the expression of IgE mRNA isoforms. In comparison, there was no effect on IgG, IgA, or IgM antibody production. This suggests that DEP exposure in vivo induces both a quantitative increase in IgE production and a shift in the type of IgE that is produced. Although most studies support the finding that DEPs increase IgE synthesis (63)(64)(65), one study in mice failed to find an increase in IgE synthesis from DEPs alone (66).
In vitro evidence suggests that IgEsecreting B cells may be directly stimulated by DEPs. For instance, PAHs from DEPs were able to induce production of IgE in purified human B cells treated with IL-4 and CD-40 (67). Another study demonstrated that phenanthrene, a major PAH in DEPs, increased IgE in human B cells transformed by Epstein-Barr virus (64). The IgE stimulation by phenanthrene was accompanied by an increased expression of total IgE mRNA. In addition, several studies have found that the DEP-mediated increase in IgE synthesis may be amplified when DEPs act as an adjuvant to environmental allergens (68)(69)(70)(71)(72)(73).

Stimulation of Eosinophils
Diesel exhaust may also stimulate the proliferation of eosinophils. Eosinophil production is regulated by IL-3, IL-5, and GM-CSF. The granules of mature eosinophils contain chemokines, leukotrienes, and toxic proteins. Degranulation of eosinophils in mucosal tissues results in bronchial inflammation and contributes to asthmatic symptoms (74). Just as mast cells are regarded as the central cell for the acute asthmatic response, eosinophils are often regarded as the critical cell type in chronic asthma.
DEPs may enhance eosinophilic infiltration into the respiratory tract and subsequent degranulation. Healthy human volunteers exposed to diesel exhaust had increased eosinophils and other inflammatory molecules on bronchial biopsies 6 hr after exposure (58). However, a similar study did not detect increased eosinophils in induced sputa 4 hr after exposure to DEPs (75). Induced sputa are less sensitive than bronchial biopsies at detecting subtle inflammatory changes in the lower airway. Eosinophils incubated with DEPs had enhanced adherence to human nasal epithelial cells and enhanced levels of degranulation (76). In animal assays, the DEP-induced eosinophilia is enhanced in the presence of allergens such as ovalbumin (OVA) and is accompanied by enhanced airway hyperresponsiveness to acetylcholine challenge (68,77).

Influence on Cytokine Expression
Exposure to DEPs may augment levels of many different cytokines (soluble protein immune mediators such as interleukins) and chemokines (attractant proteins that induce migration of different cell types). These molecules are key chemical messengers in the inflammatory processes of asthma. Various interleukins stimulate T-cell switching between T H 1 and T H 2 subtypes, stimulate B cells, attract and prolong the survival of eosinophils, and play other roles orchestrating the immunologic cascade that results in an allergic or asthmatic response.
Augmentation of interleukin levels. DEPs and associated polyaromatic hydrocarbons may increase levels of some interleukins. For example, healthy humans exposed nasally to 0.15 mg of DEPs suspended in 200 µL of saline expressed T H 2type cytokines (i.e., IL-4, IL-5, IL-6, IL-10) in their nasal mucosal cells 18-24 hr after exposure (65).
IL-4 production may be enhanced by pyrene, a PAH found in DEPs (78). The molecular mechanism of this effect may be upregulation of IL-4 mRNA transcription. IL-4 is a T H 2-type cytokine that induces isotype switching in B cells to alter antibody production from the IgM to IgE isotype and is also central to the production of IgE (79). DEPs may enhance IL-4 production more effectively with allergen than it does alone. Mice injected intratracheally with DEPs plus Japanese cedar pollen manifested an IL-4 production about twice as high as that seen in mice injected with Japanese cedar pollen alone (80). This enhancement in IL-4 production increased to an 8-fold level in mice injected with OVA and DEPs compared with mice receiving only OVA. A later study examining cytokine production in DEPexposed and control mice sensitized with OVA found that IL-4 and IL-10 production in spleen cells was significantly increased in the group of DEP-exposed mice (69). In addition, humans challenged with DEPs plus ragweed antigen had enhanced local IgE, IL-4, and IL-13 production accompanied by isotype switching from IgM or IgD to IgE antibody in nasal lavage cells (71). In comparison, isotype switching did not occur

Reviews, 2002 • Diesel exhaust and asthma
Environmental Health Perspectives • VOLUME 110 | SUPPLEMENT 1 | February 2002 Levels of IL-5 are also increased after DEP exposure. IL-5 is an important factor for the proliferation and activation of eosinophils after exposure to certain allergens such as OVA and pollen (81,82). A recent study found that mRNA expression for IL-5 was significantly lower in patients who had no nasal symptoms when compared with those who required medicines to control allergic symptoms during pollen season (83). Two human studies found that exposure to DEPs resulted in increased levels of IL-5 (65,84). However, other human, animal, and in vitro studies found that diesel exhaust alone did not result in any IL-5 response (38,66,81,82,85).
Despite the conflicting results about the effect of DEPs alone on IL-5, DEPs consistently increase IL-5 levels in the presence of environmental allergens. For instance, healthy human subjects exposed to DEPs with ragweed antigen had significantly increased levels of IL-5 and other T H 2 cytokines in nasal lavage fluid (86). Mice exposed to diesel exhaust combined with OVA sensitization had increased expression of IL-5 in lung tissue and developed airway inflammation and hyperresponsiveness (77,81,82,87). Instillation of OVA and DEPs together produced a 3-to 4-fold increase in IL-5 in mouse lung tissue compared with the levels in mice exposed to OVA or DEPs alone (77). DEPs may enhance the symptoms of allergic rhinitis by a synergistic effect with pollen to increase IL-5 secretion (86).
DEPs also increase the presence of IL-8, a member of the CXC chemokine family. Produced primarily by macrophages, IL-8 is one of the most important mediators in the recruitment of neutrophils to the respiratory tract (88). Neutrophils appear to be important inflammatory leukocytes in airway secretions of patients with acute severe asthma (89). IL-8 appears to play an important role in augmenting the numbers of activated eosinophils in asthmatic patients (90).
Increased IL-8 levels are found in bronchial washings and bronchial tissues of healthy humans exposed to diesel exhaust levels similar to those in the ambient air of many cities (84). In vitro exposure to DEPs has also been found to enhance the release of IL-8 from various types of airway cells, including human bronchial epithelial cells (38,60,(91)(92)(93), human mucosal microvascular endothelial cells (94), and human nasal epithelial cells (38,94).
Effect on other inflammatory mediators. DEPs may enable the release of several additional molecules involved in airway inflammation. In animal and in vitro models, DEPs increase GM-CSF. In both animals and humans, GM-CSF is thought to sustain the asthmatic response by prolonging the survival of eosinophils and neutrophils (95). Mice intranasally exposed to DEPs developed bronchial constriction associated with increased levels of GM-CSF in bronchial epithelial cells; blocking the GM-CSF response abolished the DEP-evoked airway hyperresponsiveness (66). DEP-induced increases in GM-CSF were also shown in vitro in exposed human bronchial epithelial cells (60,93,96), human mucous membrane epithelial cells, and human nasal epithelial cells (38,94). However, no effect on GM-CSF levels in bronchial cells was found in one study of human volunteers exposed to diesel exhaust (84).
Proposed mechanisms by which DEPs may increase GM-CSF include increased expression of the histamine H 1 receptor (94) and free radical production, which may independently elevate GM-CSF levels (97). A recent study demonstrated that free radical scavengers inhibit the DEP-mediated GM-CSF release in airway epithelial cells (97), providing some support for the latter hypothesis. Free radical production is part of the inflammatory pathway discussed in more detail below.

Expression of Chemokines
DEPs have been shown to increase the expression of RANTES (regulated upon activation, normal T-cell expressed and secreted), a chemokine that is central to the delivery of eosinophils to the airway (30). RANTES also plays a role in attracting leukocytes during the inflammatory response (98). Upon exposure to DEPs, expression of the gene for RANTES was increased in the bronchial epithelial cells of asthmatic (96) and nonasthmatic individuals (92). Although DEPs enhance both IL-8 and RANTES, an inhibitor of p38 mitogenactivated protein (MAP) kinase apparently prevents these effects. p38 MAP kinase is thought to be important in the signal transduction pathway leading to upregulation of nuclear factors (e.g., activator protein 1 [AP-1] and nuclear factor kappa B [NFκB]) that activate the transcription of genes for IL-8 and RANTES. Thus, DEPs may enhance IL-8 and RANTES through activation of the p38 MAP kinase pathway in human bronchial epithelial cells, which leads to upregulation of nuclear transcription factors AP-1 and NFκB (92).

Inflammatory Effects of DEPs
Although DEPs may have numerous effects on the immunologic cascade involved in allergy and asthma, there is also some evidence that these particles may have a more direct irritant or cytotoxic effect in the respiratory tract. Although there is overlap between the two pathways, this inflammatory mode of action is somewhat distinct from the more immunologic effects described above. The inflammatory pathway in asthma is shown in Figure 2.

Enhanced Superoxide Production
DEPs may induce the production of oxidants such as superoxide (O 2 -) and hydroxyl radical (OH -), reactive compounds that can cause direct damage to the pulmonary epithelium (99). Superoxides appear to be part of a cellular response against the adsorbed organic molecules on DEPs and may promote apoptosis in macrophages (100), thereby causing release of more inflammatory and cytotoxic molecules. Intratracheal DEP exposure in mice enhances the activity of P450 reductase, an enzyme that increases production of superoxide. This provides a possible mechanism by which DEPs may stimulate superoxide production (101). While increasing superoxide production, DEPs may also reduce the superoxide scavenging activities of superoxide dismutase (SOD) and glutathione in vitro. For example, when the antioxidant catalase was exposed to the oxidant stress of hydrogen peroxide (H 2 O 2 ) in the presence of DEPs and chlorine, the activity of the catalase was inhibited dose dependently (102).
This type of inhibitory activity by DEPs can reduce the capacity of the body to counteract oxidants (e.g., H 2 O 2 ), thereby providing another mechanism for cellular injury. Lim et al. (101) provide evidence for this by demonstrating that the activity of CuZn-superoxide dismutase (SOD) and Mn-SOD was decreased after intratracheal exposure to DEPs in mice. DEPs may also inhibit the activity of antioxidants through a deactivating reaction between SOD and quinones, which are present on the surface of DEPs (103). Therefore, DEPs appear to increase the superoxide load yet decrease the body's innate superoxide scavenging activity, which leads to potentially higher levels of cytotoxicity.
Increases in superoxides may be a key factor in asthmatic and allergic responses. For instance, pretreatment with polyethyleneglycol-conjugated SOD suppressed DEP-related airway alterations in mice, including infiltration of inflammatory cells, mucus hypersecretion, and airway constriction (99,104). This illustrates that direct cellular toxicity by superoxides may play a role in asthma. Superoxides may also activate intracellular signaling pathways, including those involving NFκB and AP-1, that upregulate chemokine and cytokine expression. This may help mediate and sustain inflammatory responses in asthma.

Effect on the Nitric Oxide Pathway
DEPs may be capable of influencing NO production. NO is elevated in asthmatic patients and has been proposed as a biologic marker for airway inflammation (105,106). NO is synthesized from the amino acid arginine by the enzyme nitric oxide synthase (NOS). NO is normally released constitutively by one isoform of NOS, but NO may also be produced from augmented expression of inducible forms of NOS by various stimuli. However, the precise role of NO in asthma is not clear. Interestingly, it appears that NO produced by constitutive NOS may have anti-inflammatory effects, whereas NO produced from inducible forms of NOS may have proinflammatory effects (105).
DEPs may affect both the constitutive and inducible NOS pathways. Intratracheal exposure of mice to DEPs increased production of both the constitutive and inducible NOS isoforms (101). However, another study found that DEP-induced airway inflammation was aggravated by NO generated from the inducible form of NOS (105). This study suggested that DEPs may aggravate airway inflammation by inhibition of NO production by the constitutive form of NOS.
Although DEPs may alter the NO pathway, the implications for asthma are not clear. One theory is that NO may react with superoxide to form a compound called peroxynitrite that may play a key role in the development of airway inflammation and hyperresponsiveness (101).

Adjuvant Immunologic Effects of DEPs
Although DEP exposure alone can elicit adverse biologic effects in the airway, the effect of DEPs has been repeatedly shown to be even greater in conjunction with allergens (82,87). For example, mice exposed intranasally to DEPs and OVA have far greater levels of anti-OVA IgE than mice exposed solely to DEPs or OVA alone (73). Guinea pigs exposed for 4 weeks to diesel exhaust and challenged with histamine experienced nasal mucosal hyperresponsiveness, sneezing, and nasal secretion, while those exposed to either diesel exhaust or histamine alone had far weaker responses (107).
An innovative study of 10 nonsmoking atopic human subjects tested the potential for DEPs to create a brand new immune response to an allergen. The investigators exposed the atopic subjects on three occasions to the neoantigen keyhole limpet hemocyanin (KLH), a compound to which humans are not normally sensitized. Twenty-four hours prior to each exposure to the new antigen, the subjects were exposed nasally to a concentration of DEPs roughly equivalent to 1-3 days of breathing Los Angeles air. Subjects exposed to KLH alone did not develop IgE antibodies to this compound, whereas subjects exposed to DEPs followed by KLH developed KLHspecific IgE and mounted a T H 2-type cytokine response with increased levels of IL-4. This important study indicates that DEPs may promote new allergic sensitization to antigens in addition to aggravating existing allergic diseases (108).
Theories as to how DEPs may have adjuvant effects include stimulation of a T H 2type immune response, by acting as delivery agents for coallergens, and by increasing allergen-specific IgE and IgG production.

DEPs and Induction of a T H 2 Phenotypic Response
Exposure to diesel exhaust may induce T cells to differentiate into a T H 2 phenotype (34). Rather than a direct effect of DEPs alone, this shift toward a T H 2 phenotype seems to occur as an adjuvant effect of DEPs with allergens. In the presence of allergen, DEPs stimulate the release of T H 2-specific cytokines (i.e., IL-4, IL-5, IL-6, IL-10, and IL-13). These cytokines appear to play a major role in the molecular pathophysiology underlying the clinical manifestations of asthma and allergies. Increased levels of T H 2-type cytokines have stimulatory effects on B cells, enhancing IgE production, as discussed above. In a study of 13 nonsmoking volunteers, Diaz-Sanchez et al. (86) found that exposure to DEPs plus ragweed results in increased expression of all of the T H 2-type cytokines in nasal lavage fluid and decreased expression of T H 1-type cytokines. A study of 27 nonsmoking volunteers with known allergies found that intranasal coadministration of DEPs and an allergen to which the subjects are sensitized stimulates a dramatic increase over 18 hr of T H 2-type cytokines such as IL-4 and IL-6. The initial production of these cytokines appears to derive from mast cells in the mucosa (79).
The precise mechanism of how DEPs stimulate the T H 2 pathway has not been determined. However, the time during development when an organism is exposed to DEPs may be vital in priming the immune system for development and maintenance of the T H 2 pattern. Exposure to DEPs and environmental allergens during early life may predispose individuals to asthma and allergic disorders later in life by promoting the expression of T H 2 phenotypic responses (34,109).

Physical Interactions between DEPs and Allergens
DEPs may enhance the immune response to allergens by physically binding with them. By this mechanism, DEPs may be transported with allergens such as pollen grain fragments into human airways, where both agents may be deposited on the mucosa at the same location. This proximity may facilitate synergistic immunologic responses and respiratory symptoms. DEPs bind strongly with certain allergens. For instance, a study that incubated DEPs with purified natural grass pollen allergen, Lol p 1, for 30 min found that this compound was bound to DEPs with sufficient strength that it could not be removed by washing methods (110). Another study used immunogold labeling to demonstrate the presence of the allergens Can f 1 (dog) and Bet v 1 (birch pollen) on the surface of suspended particulate matter, similar to DEP, which was collected from the indoor environment. In addition, the allergens Fel d 1 (cat) and Der p 1 (house dust mite) both attached to DEP when incubated with DEP in vitro (111).
However, actual binding of DEP to allergen does not appear to be necessary to the immune response. For instance, pollen grains from timothy grass do not adhere significantly to DEP in vitro, but the combination does induce synergistic inflammatory changes (i.e., influx of macrophages, eosinophilic granulocytes, and granuloma formation) in the lungs of rats (112). Another study demonstrated that the capacity of a particle to adsorb antigens was not related to its ability to enhance allergic responses (113). Thus, the binding or adsorption of DEP to antigen may be less important than the physical proximity of the two agents on the mucosal surface.

Enhancement of IgE and IgG Production
Exposure to DEP and many environmental allergens has been shown to augment both IgE and IgG production. Both IgE and IgG 1 antibodies are the result of T H 2 cytokine environments. Research in mice has demonstrated that DEP produces allergen-specific  (85). Production of IgG 1 antibodies is dependent on T H 2 lymphocytes in mice, and has been linked in humans to delayed asthmatic reactions. Human nasal instillation studies involving exposures to 0.30 mg DEP (equivalent to total exposure on 1-3 average days in Los Angeles) along with a ragweed antigen challenge showed that ragweed-specific IgE levels peaked far higher in the presence of DEP, with a maximum level 4 days postexposure. The levels of ragweedspecific IgG 4 (an isoform of IgG that is linked to IgE expression) also increased in these studies, although other forms of IgG were not affected (45,86).
Adjuvant IgE antibody responses were observed in mice exposed by intraperitoneal injection to OVA and DEPs (114). However, another study measured IgE and IgG responses to intratracheal instillation of diesel exhaust and OVA sensitization in strains of mice that were either high IgG responders or high IgE responders. In contrast to the previous study, IgE production did not change in either strain, but the combined exposure dramatically increased IgG 1 production and IL-2 and IL-5 levels in the high IgG responders (85). Similar studies (81,82,87) found that inhaled exposure to diesel exhaust with OVA sensitization for 5-6 weeks increased both IgG 1 and IgE levels. Studies in mice using other allergens such as house dust mite antigen and Japanese cedar pollen were consistent with the literature using OVA. Mice immunized with either of these antigens mounted a much greater IgG 1 response with exposure to DEPs than mice exposed to the same level of allergen without DEPs. A similar response was found for IgE synthesis, indicating that both antibodies play a role in the adjuvant effects of DEPs on the immune response (72,113). Guinea pigs exposed to DEPs for 5 weeks with OVA sensitization once per week developed 7-fold greater anti-OVA IgG antibody than guinea pigs exposed only to filtered air, indicating that the response is not specific to mice. The exposed guinea pigs also experienced slight concentrationdependent increases in IgE antibody (115). Similar results have been seen in rats, where intranasal or intratrachial co-exposure to DEPs and pollen grains resulted in a much greater serum level of specific IgE and IgG 1 antibodies than exposure to either alone. Electron microscopy revealed pollen grains in the alveoli surrounded by DEP-loaded macrophages (116). One interesting study examined the effects of oral ingestion of DEPs in mice because it is known that airborne particulate reaches not only the lung but also the mucosa of the gastrointestinal tract. DEPs in the gut mucosa also appear to act as an adjuvant, enhancing both T H 1and T H 2-type responses to allergen and enhancing production of allergen-specific IgG 1 (117).

Conclusions and Considerations for Further Research
Rising rates of asthma and allergies create a public health imperative to identify any modifiable environmental factors that may cause or contribute to these diseases. Abundant evidence suggests that components of diesel exhaust can cause biologic responses that are related to asthma. Although evidence from research cited in this article indicates that exposures to diesel exhaust and DEPs are associated with the inflammatory and immune responses involved in asthma, some questions remain regarding the underlying molecular mechanisms.
DEPs alone may augment levels of IgE, trigger eosinophil degranulation, and stimulate release of numerous cytokines and chemokines. DEPs also may play a role in unleashing the cytotoxic effects of free radicals in the airways. All of these cellular mechanisms would be expected to produce airway inflammation, bronchial smooth muscle contraction, serum leakage, and mucus production, thereby resulting in the clinical symptoms of asthma. Interestingly, DEPs appear to have a far greater impact as an adjuvant with allergens than it has alone.
The immune events leading to the asthmatic response are intertwined, and DEPs likely act at numerous points on the pathway. Stimulation of the T H 2-type pathway and increase in IgE production are two of the most important and likely mechanisms by which DEPs may generate and sustain an asthmatic response. The timing of exposure to air pollutants such as DEPs during early life may also be critical in fostering the persistence of the T H 2 phenotype.
DEPs also have other biologic effects, such as increasing superoxide and NO levels. However, the evidence for these effects is currently found only in a few animal or in vitro studies, and key questions remain. Although exposure to diesel exhaust appears capable of inducing inflammatory changes in the respiratory tract, this area is poorly understood. Most important, the epidemiologic evidence linking diesel exhaust and asthma is distressingly sparse because of a paucity of studies that have collected relevant exposure data.
More research is needed to investigate the mechanism and the clinical relevance of the observed adjuvant effect of co-exposure to DEPs and allergens. One study demonstrated that this adjuvant effect results in increased respiratory resistance in mouse airways after acetylcholine challenge (118). This line of research will help to link the observed immunologic alterations with clinical relevance. The question of windows of vulnerability in early life and the induction of an allergic phenotype also requires further investigation. Research is needed to demonstrate more clearly the effect of DEPs on reactive oxygen species, superoxide, and NO production. Epidemiologic research on allergic and/or asthmatic human populations would be particularly valuable. Observational studies of children, including quantitative assessment of DEP exposure and airway function, would remove some of the uncertainties associated with the epidemiologic research to date.
Despite the need for further research, it is biologically plausible that diesel exhaust and associated particles are associated with asthma and other allergies in humans. In light of these findings, public health efforts to reduce exposures to diesel exhaust are warranted. In particular, reducing the exposure of infants and children should be a priority as part of a coordinated effort to improve the prevention and management of childhood asthma.