Genetic variability in the development of pulmonary tolerance to inhaled pollutants in inbred mice.

After repeated exposures, many individuals develop tolerance to the adverse health effects of inhaled pollutants. Pulmonary tolerance can be characterized as the ability of the lung to withstand the adverse actions of a toxic compound after repeated exposures. To determine whether genetic background is important to the development of pulmonary tolerance to inhaled pollutants, 11 inbred strains of mice were exposed once (1x) or for 5 consecutive days (5x) to 1.0 mg/m(3) of zinc oxide (ZnO). Development of pulmonary tolerance was assessed by measuring polymorphonuclear leukocyte and protein levels in bronchoalveolar lavage fluid and comparing the responses of the 1x and 5x groups. Significant interstrain variation in polymorphonuclear leukocyte and protein responses was observed between the groups with 1x and 5x exposures, which indicates that genetic background has an important role in the development of pulmonary tolerance. The BALB/cByJ strain and the DBA/2J strain were the most tolerant and nontolerant, respectively. The CByD2F1/J offspring were uniformly nontolerant. The development of tolerance was also investigated in BALB/cByJ and DBA/2J mice after 1x and 5x exposure to ozone and aerosolized endotoxin. Discordance in the phenotypic pattern of pulmonary tolerance among strains after exposure to ZnO, ozone, and endotoxin suggested that different mechanisms may be responsible for the development of pulmonary tolerance to these agents.

Development of pulmonary tolerance in mice exposed to zinc oxide fumes.

As a result of repeated exposures to inhaled toxicants such as zinc oxide (ZnO), numerous individuals acquire tolerance to the exposures and display reduced symptoms. To ascertain whether tolerance is developed in an animal model, NIH-Swiss mice were exposed to 1.0 mg/m(3) ZnO for 1, 3, or 5 days (1X, 3X, or 5X), and polymorphonuclear leukocyte (PMN) and protein levels in bronchoalveolar lavage (BAL) were measured. Mice acquired tolerance to neutrophil infiltration into the lungs, as total PMNs returned near baseline in 5X-exposed animals as compared to that of the 1X exposure group (1X = 2.7 +/- 0.4 x 10(4), 5X = 0.2 +/- 0.1 x 10(4), mean +/- SE, p < 0.05). Development of tolerance to changes in lavageable protein, however, was not observed (1X = 313 +/- 29 microg/ml, 5X = 684 +/- 71 microg/ml, p < 0.05). Tolerance to PMN influx did not persist following re-exposure to ZnO after 5 days of rest. In contrast to ZnO exposure, following single and repeated exposure to aerosolized endotoxin there was development of tolerance to protein in BAL (1X = 174 +/- 71 microg/ml, 5X = 166 +/- 14 microg/ml, p > 0.05), but not to PMN influx (1X = 5.5 +/- 1.7 x 10(4), 13.9 +/- 1.7 x 10(4), p < 0.05). Induction of lung metallothionein (MT) was also observed in mice exposed once or repeatedly exposed to ZnO, suggesting that MT may play a role in its molecular mechanism.

Functional genomics of oxidant-induced lung injury.

In summary, acute lung injury is a severe (>40% mortality) respiratory disease associated with numerous precipitating factors. Despite extensive research since its initial description over 30 years ago, questions remain about the basic pathophysiological mechanisms and their relationship to therapeutic strategies. Histopathology reveals surfactant disruption, epithelial perturbation and sepsis, either as initiating factors or as secondary complications, which in turn increase the expression of cytokines that sequester and activate inflammatory cells, most notably, neutrophils. Concomitant release of reactive oxygen and nitrogen species subsequently modulates endothelial function. Together these events orchestrate the principal clinical manifestations of the syndrome, pulmonary edema and atelectasis. To better understand the gene-environmental interactions controlling this complex process, we examined the relative sensitivity of inbred mouse strains to acute lung injury induced by ozone, ultrafine PTFE, or fine particulate NiSO4 (0.2 microm MMAD, 15-150 microg/m3). Measuring survival time, protein and neutrophils in bronchoalveolar lavage, lung wet: dry weight, and histology, we found that these responses varied between inbred mouse strains, and susceptibility is heritable. To assess the molecular progression of NiSO4-induced acute lung injury, temporal relationships of 8734 genes and expressed sequence tags were assessed by cDNA microarray analysis. Clustering of co-regulated genes (displaying similar temporal expression patterns) revealed the altered expression of relatively few genes. Enhanced expression occurred mainly in genes associated with oxidative stress, anti-proteolytic function, and repair of the extracellular matrix. Concomitantly, surfactant proteins and Clara cell secretory protein mRNA expression decreased. Genome wide analysis of 307 mice generated from the backcross of resistant B6xA F1 with susceptible A strain identified significant linkage to a region on chromosome 6 (proposed as Aliq4) and suggestive linkages on chromosomes 1, 8, and 12. Combining of these QTLs with two additional possible modifying loci (chromosome 9 and 16) accounted for the difference in survival time noted in the A and B6 parental strains. Combining these findings with those of the microarray analysis has enabled prioritization of candidate genes. These candidates, in turn, can be directed to the lung epithelium in transgenic mice or abated in inducible and constitutive gene-targeted mice. Initial results are encouraging and suggest that several of these mice vary in their susceptibility to oxidant-induced lung injury. Thus, these combined approaches have led to new insights into functional genomics of lung injury and diseases.

Pathogenomic mechanisms for particulate matter induction of acute lung injury and inflammation in mice.

To begin identifying genes controlling individual susceptibility to particulate matter, responses of inbred mouse strains exposed to nickel sulfate (NiSO4*) were compared with those of mice exposed to ozone (O3) or polytetrafluoroethylene (PTFE). The A strain was sensitive to NiSO4-induced lung injury (quantified by survival time), the C3H/He (C3) strain and several other strains were intermediate in their responses, and the C57BL/6 (B6) strain was resistant. The strains showed a pattern of response similar to the patterns of response to O3 and PTFE. The phenotype of A x B6 offspring (B6AF1) resembled that of the resistant B6 parental strain, with strains exhibiting sensitivity in the order A > C3 > B6 = B6AF1. Pathology was comparable for the A and B6 mice, and exposure to NiSO4 at 15 microg/m3 produced 20% mortality in A mice. Strain sensitivity for the presence of protein or neutrophils in lavage fluid differed from strain sensitivity for survival time, suggesting that they are not causally linked but are controlled by an independent gene or genes. In the B6 strain, exposure to nickel oxide (NiO) by instillation (40 to 1000 nm) or inhalation (50 nm) produced no changes, whereas inhalation of NiSO4 (60 or 250 nm) increased lavage proteins and neutrophils. Complementary DNA (cDNA) microarray analysis with 8,734 sequence-verified clones revealed a temporal pattern of increased oxidative stress, extracellular matrix repair, cell proliferation, and hypoxia, followed by a decrease in surfactant-associated proteins (SPs). Certain expressed sequence tags (ESTs), clustered with known genes, suggest possible coregulation and novel roles in pulmonary injury. Finally, locus number estimation (Wright equation) and a genomewide analysis suggested 5 genes could explain the survival time and identified significant linkage for a quantitative trait locus (QTL) on chromosome 6, Aliq4 (acute lung injury QTL4). Haplotype analysis identified an allelic combination of 5 QTLs that could explain the difference in sensitivity to acute lung injury between parental strains. Positional candidate genes for Aliq4 include aquaporin-1 (Aqp1), SP-B, and transforming growth factor-alpha (TGF-alpha). Transgenic mice expressing TGF-alpha were rescued from NiSO4 injury (that is, they had diminished SP-B loss and increased survival time). These findings suggest that NiSO4-induced acute lung injury is a complex trait controlled by at least 5 genes (all possibly involved in cell proliferation and surfactant function). Future assessment of these susceptibility genes (including evaluations of human synteny and function) could provide valuable insights into individual susceptibility to the adverse effects of particulate matter.

Genetic susceptibility to irritant-induced acute lung injury in mice.

Recent studies suggest that genetic variability can influence irritant-induced lung injury and inflammation. To begin identifying genes controlling susceptibility to inhaled irritants, seven inbred mouse strains were continuously exposed to nickel sulfate (NiSO(4)), polytetrafluoroethylene, or ozone (O(3)), and survival time was recorded. The A/J (A) mouse strain was sensitive, the C3H/He (C3) strain was intermediate, and the C57BL/6 (B6) strain was resistant to NiSO(4)-induced acute lung injury. The B6AF(1) offspring were also resistant. The strain sensitivity pattern for NiSO(4) exposure was similar to that of polytetrafluoroethylene or ozone (O(3)). Pulmonary pathology was comparable for A and B6 mice. In the A strain, 15 microg/m(3) of NiSO(4) produced 20% mortality. The strain sensitivity patterns for lavage fluid proteins (B6 > C3 > A) and neutrophils (A >/= B6 > C3) differed from those for acute lung injury. This phenotype discordance suggests that these traits are not causally linked (i.e., controlled by independent arrays of genes). As in acute lung injury, B6C3F(1) offspring exhibited phenotypes (lavage fluid proteins and neutrophils) resembling those of the resistant parental strain. Agreement of acute lung injury strain sensitivity patterns among irritants suggested a common mechanism, possibly oxidative stress, and offspring resistance suggested that sensitivity is inherited as a recessive trait.

Functional Genomics of Particle-Induced Lung Injury.

Currently, the biological mechanisms controlling adverse reactions to particulate matter are uncertain, but are likely to include oxidative lung injury, inflammation, infection, and preexisting pulmonary disease (e.g., chronic obstructive pulmonary diseaseJ. Each mechanism can be viewed as a complex trait controlled by interactions of host (genetic) and environmental factors. We propose that genetic factors play a major role in susceptibility to particulate matter because the number of individuals exposed (even in occupational settings) is often large, but relatively few people respond with increases in morbidity and even mortality. Previous clinical studies support this hypothesis, having discovered marked individual variation in diminished lung function following oxidant exposures. Advances in functional genomics have facilitated the examination of this hypothesis and have begun to provide valuable new insights into gene-environmental interactions. For example, genome-wide scans can be completed readily in mice that enable assessment of chromosomal regions with linkage to quantitative traits. Recently, we and others have identified linkage to oxidant-induced inflammation and mortality. Such linkage analysis can narrow and prioritize candidate gene(s) for further investigation, which, in turn, is aided by existing transgenic mouse models. In addition, differential expression (microarray) analysis enables simultaneous assessment of thousands of genes and expressed sequence tags. Combining genome-wide scan with microarray analysis permits a comprehensive assessment of adverse responses to environmental stimuli and will lead to progress in understanding the complex cellular mechanisms and genetic determinants of susceptibility to particulate matter.