SHS causes a broad spectrum of highly prevalent adverse respiratory health outcomes (1
). A large body of evidence supports a causal role for SHS in allergic airway disease occurrence (1
). We have previously shown that SHS can interact with allergen to exacerbate acute and immediate allergic responses in both animal models and human exposure trials (6
). Here, we demonstrate that susceptibility to SHS may be controlled by functional variation in airway antioxidant defenses. In the present randomized exposure trial, we show that common variants in GSTM1
, which have been associated with atopy (allergy, asthma, and atopic dermatitis), mediate the enhancement of allergic responses by SHS at levels commonly encountered in indoor environments. We also observed that individual responses to SHS and DEPs are correlated across the endpoints measured in this study. Our results provide further evidence that the severity of common allergic airway diseases, such as allergic rhinitis and asthma, is a consequence of the interplay between common genetic variants and environmental exposures (9
). The correlated responses to different exposures suggest that some individuals are at higher risk than others for allergic symptoms when exposed to a spectrum of environmental stressors.
The importance of these results, from a clinical and public health perspective, is reflected in the high frequency of variants in these genes in most populations, as well as the ubiquitous exposure to SHS. The null allele variant of GSTM1
and Ile/Ile genotype for GSTP1
are present in approximately 50 and 40% of individuals, respectively (38
). Although expression of GSTM1
is highest in the liver, it has also been identified in lung and nasal tissue (22
). Individuals who have the null genotype completely lack class μ GST isoenzyme activity. Similarly, GSTP1
is the major GST gene expressed in human airways, and the different GSTP1
codon 105 variants have different enzymatic activities. Based on our findings on the joint effects of GSTM1
, we estimate that 15–20% of the general population may be unprotected from the enhancement of allergic responses by SHS. Among allergic individuals, the proportion of the population that is highly susceptible to SHS enhancement is likely to be larger, because these variants are associated with increased risk for atopy (25
). This increased risk may arise because these polymorphisms are directly involved in atopy per se
, or because they modulate the role of pollutants on atopy development.
The findings on the role of GSTM1
in allergic responsiveness in the present study are consistent with those of our previous report that GSTP1
play important roles in susceptibility to the adjuvant effects of DEPs (9
). In the presence of DEPs, individuals with GSTM1
-null or the GSTP1
Ile105 wild-type genotypes showed enhanced nasal allergic responses. Compared with subjects with a functional GSTM1
genotype, subjects with the GSTM1
-null genotype had a significantly greater increase in IgE and histamine after DEP plus allergen challenge. The Ile105 GSTP1
genotype was associated with an increase in IgE and histamine after DEP plus allergen challenge. As observed after SHS exposure, the DEP enhancement was greatest in the subjects with both the GSTM1
-null and GSTP1
Ile/Ile genotypes. IgE production is regulated by IL-4 and the formation of a Th2 cytokine milieu. It is, therefore, somewhat surprising that we failed to find statistically significant differences in IL-4 and IFN-γ elicited by SHS. One likely explanation is that this is due to a type 2 error. Another is that very small changes in these cytokines can result in large changes in IgE. Alternatively, because oxidant pollutants, such as DEP, can directly target B cells to enhance class switching, it is possible that the effect of SHS on IgE production may be independent of IL-4. It is also possible that DEPs and tobacco smoke inherently differ in their allergy-modulating pathways. However, the correlation of responses for SHS and DEPs argues against different mechanisms.
We have previously shown that SHS acts as an adjuvant by interacting with allergens to affect the immune system and enhance allergic responses (6
). In mice, SHS exposure has an adjuvant effect, characterized by an increase in antigen-specific IgE, elevated Th2 responses, and influx of eosinophils into the lungs (7
). It is unclear which of the more than 6,000 compounds in sidestream tobacco smoke act as adjuvants; however, other particulate pollutants resulting from incomplete combustion of organic materials, such as ambient particulate matter and diesel exhaust, contain many of the constituents of SHS, such as polyaromatic hydrocarbons, suggesting that a spectrum of xenobiotics in the environment can contribute to enhancement of responses to allergens (9
). Human and murine in vitro
and in vivo
studies have demonstrated that DEP and polyaromatic hydrocarbons can also induce IgE production, increase Th2 cytokine production, select against Th1 cytokines, and augment histamine release, and that these effects are blocked by GSTM1 and GSTP1 (reviewed in References 42 and 43). Many of the proximal mechanisms underlying the effects of SHS, such as enhanced inflammation, formation of a Th2 cytokine environment, and the production of allergen-specific IgE antibodies, are believed to be driven by oxidative stress responses (10
). Inflammation itself is an oxidative event; SHS, DEPs, and a variety of other air pollutants can increase oxidative stress levels by stimulating the production of ROS directly and indirectly through enhanced airway inflammation. Small molecular and enzymatic antioxidants, such as the GSTs, can reduce the formation and effects of ROS. Here, we suggest that members of the GST family may play a key role in controlling the responses to a wide range of xenobiotics, in addition to SHS and DEPs, by detoxifying xenobiotic-derived ROS and oxidation products. We speculate that combustion products, as a class of complex mixtures, are likely to enhance allergic responses to allergens, and that variants in genes, including GSTM1
, may play an important role in determining who is protected from the adverse effects of these mixtures.
A growing number of epidemiologic studies have shown that GSTP1
variants are associated with airway hyperresponsiveness and asthma, especially in relation to secondhand smoke or other xenobiotic exposure (25
). Although we investigated inflammatory responses in the upper airway, the genetic susceptibility to SHS and DEPs is likely to apply to the lower airway, as exposures, such as to SHS, can enhance airway hyperreactivity in humans and murine models. The GSTP1
gene product provides more than 90% of the GST activity in the lung, suggesting that lower airway as well as upper airway effects of SHS may be strongly modulated by GSTP1
function. Further support for common genetic effects over the upper and lower airways is provided by evidence that the GSTP1
Ile105/Ile105 genotype is associated with IgE levels and increased risk for atopy, which affects both the upper and lower airways (25
Because the respiratory tract has multiple mechanisms to defend against insults from airborne toxins, it is unlikely that GSTM1
are the only loci that confer protection from airborne pollutants, such as SHS and DEP, or that they have a unique protective mechanism (21
). The variation in responses among subjects who are GSTM1
null suggests that other antioxidant genes with common functional polymorphisms exist that could contribute to susceptibility to more severe allergic responses on exposure to SHS and DEPs. For example, reduced nicotinamide adenine dinucleotide phosphate:quinone oxidoreductase, a flavoenzyme that can detoxify quinones and other oxidant chemicals, has also been reported to be protective in pollutant-induced asthma. Small molecule antioxidants and dietary intake may also contribute to antioxidant defenses directly, or by increasing expression of antioxidant genes (21
). Additional studies in larger populations are needed to discover additional protective loci with lower frequency variants, as well as to explore the role of dietary intake of antioxidants. Including genetic characterization of subjects in the design and analysis of inhalation challenge studies has great potential to advance our understanding of the role of the environment and susceptibility factors in allergic responses and other respiratory conditions.
In summary, SHS exposure is common in human populations, with estimates that up to 40% of children in the United States are exposed daily to SHS. Secondhand smoke causes a huge public health and clinical burden of disease. Our results provide important evidence that common genetic variants may determine who is protected and who is at risk for the adverse effects of SHS. Individuals with allergic airway diseases are recognized to be sensitive to the effects of SHS. We show that GSTM1 and the GSTP1 Val105 variant reduce the adjuvant effects of SHS on allergic responses to a common allergen. The elevated genetic susceptibility for a large number of individuals indicates great potential benefits of a public health intervention that targets reduction in SHS exposure. Although reductions in exposure are being achieved, more effective treatment of allergic airway disease may be possible via approaches that induce expression of GSTs.