Acute and persistent alterations in pulmonary inflammatory cells and airway mast cells induced by Sendai virus infection in neonatal rats

Small Animal Models of Respiratory Viral Infection Related to Asthma

Respiratory viral infections are strongly associated with asthma exacerbations. Rhinovirus is most frequently-detected pathogen; followed by respiratory syncytial virus; metapneumovirus; parainfluenza virus; enterovirus and coronavirus. In addition; viral infection; in combination with genetics; allergen exposure; microbiome and other pathogens; may play a role in asthma development. In particular; asthma development has been linked to wheezing-associated respiratory viral infections in early life. To understand underlying mechanisms of viral-induced airways disease; investigators have studied respiratory viral infections in small animals. This report reviews animal models of human respiratory viral infection employing mice; rats; guinea pigs; hamsters and ferrets. Investigators have modeled asthma exacerbations by infecting mice with allergic airways disease. Asthma development has been modeled by administration of virus to immature animals. Small animal models of respiratory viral infection will identify cell and molecular targets for the treatment of asthma.

IL-12 reduces the severity of Sendai virus-induced bronchiolar inflammation and remodeling

The goal of this research was to determine whether differential pulmonary IL-12 gene expression controls susceptibility to Sendai virus-induced chronic airway inflammation and fibrosis in inbred rat strains. Sendai virus-resistant F344 rats and susceptible BN rats were studied from 1 to 14 days following virus inoculation. F344 rats had 3.4-fold higher IL-12 mRNA levels detected by real-time PCR in lung than BN rats as early as two days following inoculation. This increase in mRNA was associated at two days with increased total IL-12 protein and with a 2-fold increase in numbers of bronchiolar, OX-6-positive dendritic cells and an increased number of IL-12 p40-positive, bronchiolar macrophages and dendritic cells (p<0.05). Virus-susceptible BN rats treated with 3 mug of recombinant, mouse IL-12 intraperitoneally at the time of virus inoculation had a 22.1% decrease in severity of chronic bronchiolar inflammation and a 23.8% decrease in fibrosis compared to virus-inoculated BN rats treated with saline. IL-12 treatment induced increased IFN-gamma mRNA and protein expression after virus inoculation (p<0.05). The results demonstrate that there is differential pulmonary IL-12 gene expression between virus-susceptible and resistant rat strains and that IL-12 treatment can provide significant protection from virus-induced chronic airway inflammation and remodeling during early life.

Serum tryptase and the laboratory diagnosis of systemic mastocytosis

Total tryptase levels of 20 ng/mL or higher in a baseline serum sample when the ratio of total to beta-tryptase is 20 or greater strongly suggest underlying systemic mastocytosis. Whether these criteria prove to be more sensitive than a bone marrow biopsy will require further study. Although the absolute level of total tryptase does not predict disease severity, it may provide a practical method for assessing the efficacy of therapeutic interventions designed to reduce the mast cell burden.

The potential clinical utility of serum alpha-protryptase levels

BACKGROUND

Because biopsy criteria for diagnosing systemic mastocytosis are not precise, the value of serum alpha-protryptase levels in the work-up of suspected systemic mastocytosis should be considered.

OBJECTIVE

A retrospective analysis was performed on subjects with total tryptase serum levels that were high (>/=20 ng/mL), while beta-tryptase serum levels were normal (<1 ng/mL) or modestly elevated (1 to 5 ng/mL).

METHODS

Over a 3.5-year period, 52 qualifying specimens were identified from 1369 consecutive samples. The corresponding subjects were divided into those with suspected mastocytosis and those with suspected anaphylaxis. Subjects with suspected mastocytosis were subdivided into 3 subgroups on the basis of biopsy results (positive, negative, or not available). Subjects with suspected anaphylaxis were subdivided into living and deceased subgroups.

RESULTS

Among the 15 subjects who underwent biopsy, alpha-protryptase serum levels (the difference between directly-measured levels of serum total tryptase and beta-tryptase), when greater than 75 ng/mL (n = 9), were always associated with a positive biopsy result for systemic mastocytosis; levels from 20 to 75 ng/mL (n = 6) were associated with a positive biopsy result in 50% of subjects. alpha-Protryptase serum levels may be a more sensitive screening test than a bone marrow biopsy for this disorder. Also, elevated alpha-protryptase serum levels in some adult patients return to normal over time, suggesting that mast cell hyperplasia resolved in these patients. Finally, a high alpha-protryptase level may reveal anaphylaxis to be a presenting manifestation of systemic mastocytosis or mast cell hyperplasia.

CONCLUSION

Levels of serum alpha-protryptase, relative to those of beta-tryptase, appear to be useful in the diagnostic work-up and follow-up of subjects with suspected systemic mastocytosis.

Effects of dexamethasone on acute virus-induced airway dysfunction in adult rats

Respiratory viral infections have been associated with airway obstruction and hyperresponsiveness, and exacerbations of asthma. Although virus-induced asthma is thought to be precipitated by airway inflammation, the clinical efficacy and rationale for using antiinflammatory treatment during such exacerbations remains controversial. The purpose of this study was to use a well characterized animal model of respiratory viral illness to test the hypothesis that the inflammatory response to viral infection is responsible for the development of airway dysfunction. Adult rats were inoculated with either Sendai virus or sterile vehicle and treated with daily injections of dexamethasone or saline. At postinoculation d 4, 5, or 6, rats were evaluated for airway obstruction, hyperresponsivenes, inflammation, and lung viral titers. Saline-treated infected rats had significant airway obstruction (increased resistance, decreased dynamic compliance), hyperresponsiveness (i.v. methacholine), and inflammation (increased bronchoalveolar lavage leukocytes) compared with noninfected controls. In contrast, dexamethasone-treated infected rats had no increase in bronchoalveolar lavage leukocytes and significantly smaller changes in airway physiology, but had increased lung viral titers compared with saline-treated infected rats. We conclude that glucocorticoid suppression of the inflammatory response to respiratory viral infection largely prevents virus-associated airway dysfunction.

Virus-induced increases in airway mast cells in brown Norway rats are associated with enhanced pulmonary viral replication and persisting lymphocytic infiltration

Brown Norway (BN) rats are more susceptible than Fischer 344 (F344) rats to parainfluenza virus-induced lung injury and to bronchiolar mast cell increases that are associated with persistent airway hyperresponsiveness. In this study, pulmonary viral replication as well as immune, inflammatory, and airway mast cell responses to Sendai virus infection were compared between neonatal BN and F344 rats. BN rats supported prolonged viral replication, and viral titers in BN rats were 5-fold higher (p < .05) than in F344 rats at 7 days after inoculation. F344 rats had 18-fold higher (p < .06) numbers of lymphocytes in bronchoalveolar lavage fluid at 7 days after inoculation than did BN rats. Persisting bronchiolar aggregates of lymphocytes, plasma cells, and macrophages were more common, and increases in bronchiolar mast cells were greater in BN rats than in F344 rats. No strain differences were detected in bronchiolar intramural infiltrates of CD4 + or CD8 + cells. The greater susceptibility of BN rats to virus-induced increases in bronchiolar mast cells and airway responsiveness may be the result of their less efficient viral clearance mechanisms and more persistent bronchiole-centered inflammatory response.

Persistence of respiratory syncytial virus genome and protein after acute bronchiolitis in guinea pigs

Children with acute respiratory syncytial virus (RSV) bronchiolitis often develop sequelae of recurrent wheezing and asthma. To determine whether RSV persists within the lung after resolution of acute bronchiolitis, we examined the lungs of guinea pigs 60 days after intranasal inoculation with either human RSV (n = 10) or uninfected cell culture supernatant (n = 11). Evidence of viral persistence within the lung was determined by viral culture to test for replicating virus, immunohistochemistry to test for viral protein, and the reverse transcriptase-polymerase chain reaction (RT-PCR) to test for viral genomic RNA. Lungs were also examined histologically for evidence of bronchiolar inflammation or increased numbers of mast cells in the airway walls. All viral cultures were negative; however, there was positive immunohistochemical staining of occasional alveolar macrophages in six of ten RSV-inoculated guinea pigs while RT-PCR was positive in seven of ten RSV-inoculated animals. The six guinea pigs with evidence of RSV by immunohistochemistry and RT-PCR showed excess bronchiolar polymorphonuclear cell infiltrates (p < 0.005) but no increase in the number of airway wall mast cells. These results show that RSV protein and genomic RNA can persist in the lungs of experimentally inoculated guinea pigs for at least 60 days after infection and that persistence of the virus within alveolar macrophages might contribute to the pathogenesis of chronic bronchiolar inflammation.

Is asthma curable?

The information reviewed here supports the concept that asthma is potentially curable. Reports of complete, durable remission of asthma can no longer be regarded as fortuitous occurrences, unrepresentative of asthma in general. Systematic studies of anti-inflammatory drug therapy designed to explore possible induction or remission of asthma clearly are warranted. Studies of aggressive anti-inflammatory drug therapy of asthma at the onset, to avoid establishment of chronic asthma, also are desirable. The current goals of therapy of asthma have been revised to include reduction of airway hyperreactivity with topical anti-inflammatory drugs, in addition to relief of current symptoms. This approach may provide valuable resistance to exacerbations in response to antigen exposures, infections, exercise, or irritants. Pathophysiologic mechanisms apparently essential to the establishment and perpetuation of chronic asthma have been identified. These processes may be vulnerable to eradication by combination therapy with existing pharmacologic agents such as cyclosporin A or FK-506 (to suppress cytokine production), gold, methotrexate, and other anti-inflammatory drugs, alone or in combination. Equally important, the vigorous anti-inflammatory therapy may be necessary only long enough to achieve a resolution of the chronic pulmonary inflammation. Systematic studies of the use of these agents to induce partial, or complete, stable remissions of asthma should be performed. In the past, remissions of asthma in children with neoplasia and the other patients presented herein were complete, durable, and welcome, but they were largely unexpected and unpredictable. For the future, there is increasing reason to believe that predictable pharmacologically induced remission of asthma will be feasible.

Detection of enhanced neutrophil adhesion to parainfluenza-infected airway epithelial cells using a modified myeloperoxidase assay in a microtiter format

Despite growing evidence that respiratory virus infections precipitate episodes of airway obstruction and airway hyper-responsiveness in young children and in asthma, little information is available on the mechanisms by which virus infections alter the airway physiology. Airway inflammatory changes (including influx of inflammatory cells such as neutrophils) have been described during episodes of airway hyper-responsiveness in both animal models and human subjects. Neutrophil damage to several cell types has been shown to require adhesion as a primary step. In order to examine the potential interactions between virus-infected airway epithelial cells and neutrophils, we have studied the ability of neutrophils to adhere to virus-infected airway epithelial cell cultures. Neutrophil adherence was determined indirectly, using myeloperoxidase as a marker for adherent neutrophils in an assay system described here. Airway epithelial cell cultures (both primary human tracheal epithelial cells, and two permanent cell lines, A549 and BEAS-2B) were grown in 96-well tissue culture plates and infected with human parainfluenza virus type 2. Infected airway epithelial cell cultures supported significantly enhanced levels of neutrophil adherence (up to 50-75% of neutrophils added to the wells) compared to uninfected control cultures. Moreover, this adherence occurred in a virus dose-dependent fashion, with increasing levels of adherence noted at increasing viral multiplicities of infection. The assay system described allows the detection of small numbers of adherent neutrophils (as few as 1000 neutrophils) in a 96-well format.

Brown Norway rats are high responders to bronchiolitis, pneumonia, and bronchiolar mastocytosis induced by parainfluenza virus

The pathogenesis of parainfluenza 1 (Sendai) virus infection was compared among 25-day-old BN, F344, and LEW rats to identify a sensitive as well as a resistant inbred rat strain to Sendai virus-induced lung injury during early life. At 7 days after inoculation, BN rats had 65-fold higher (P less than .001) pulmonary viral titers and threefold higher (P less than .002) numbers of neutrophils in bronchoalveolar lavage fluid than did F344 rats. At 14 days after inoculation, when most virus-induced inflammation had been resolved, BN rats had a threefold greater (P less than .01) incidence of bronchioles with aggregates of lymphocytes and macrophages than did F344 rats. Control BN rats had higher numbers of bronchiolar eosinophils than did F344 or LEW rats. Although viral inoculation resulted in increased numbers of bronchiolar mast cells in all three strains at 14 days, bronchiolar mast cell density was greater (P less than .005) in virus-inoculated BN and LEW rats than in F344 rats. We conclude that BN rats are high responders and F344 rats are low responders to Sendai virus-induced bronchiolitis, pneumonia, and airway mastocytosis. These strain differences may be useful in elucidating important pathogenetic mechanisms in virus-induced airway injury and mastocytosis.