IL-12 reduces the severity of Sendai virus-induced bronchiolar inflammation and remodeling☆
Introduction
Interleukin-12 (IL-12) is a heterodimeric cytokine that is important in resistance to intracellular infectious agents. It acts through direct up-regulation of IFN-γ and, thus, the T helper-1 (Th-1) immune response [1], [2]. IL-12 also acts to prevent the development of the T helper-2 (Th-2) cells and Th-2 cytokines [2]. Coexpression and covalent linkage of both of the IL-12 chains (IL-12 p40 and IL-12 p35) are required for the generation of the functionally active heterodimer, IL-12 p70 [3].
The differential expression of IL-12 in rodents has been shown to be a critical controller of bacterial, fungal, protozoal, and viral infections [4]. Listeriosis resistant mouse strains, such as C57BL/6, have macrophages and dendritic cells with higher IL-12 producing capabilities than susceptible strains during acute infection [4]. Administration of recombinant IL-12 increases resistance to this bacterium in susceptible mouse strains, and the antibody neutralization of IL-12 leads to increased susceptibility [4]. In mice infected with murine cytomegalovirus (MCMV), respiratory syncytial virus (RSV), influenza virus, and herpes simplex virus (HSV), increases in IL-12 expression are critical to the early activation of NK cells and the establishment of a Th-1 antiviral immune response [4], [5], [6], [7]. During MCMV and RSV infections, IL-12 p70 levels increase in serum (50% and 4-fold, respectively) acutely after infection [5], [6].
Parainfluenza virus type I (Sendai) infection in weanling rats produces pulmonary structural and functional abnormalities with many features that resemble asthma including; episodic, reversible airway obstruction, airway hyperresponsiveness to methacholine, chronic airway wall inflammation, and airway wall remodeling [8]. Brown Norway (BN) rats are susceptible to virus-induced chronic structural and functional abnormalities, whereas Fischer (F344) rats are highly resistant to these virus-induced effects [8], [9]. Early after infection, BN rats have greater pulmonary expression of the Th-2 cytokines interleukin-4 (IL-4) and interleukin-5 (IL-5), less IFN-γ production, and fewer CD8+ T lymphocytes than F344 rats [10]. BN rats are high IgE producers and respond to virus infection with enhanced airway recruitment of mast cells and eosinophils and with prolonged viral clearance as compared to F344 rats [10], [11]. Production of the fibrogenic cytokines transforming growth factor-β (TGF-β) and tumor necrosis factor-α (TNF-α) is greater and more prolonged in BN rats during the viral repair process [12], [13]. Airway dysfunction in rats following virus infection may be under the polygenic control and manifested by cytokine dysregulation, as asthma appears to be the case in children with asthma [14], [15].
The goal of this research was to determine the role of IL-12 in the development of resistance to chronic airway disease induced by Sendai virus infection during early life. We hypothesized that F344 rats are more resistant to virus-induced airway damage and fibrosis because they produce high levels of IL-12 in the acute response to virus that up-regulates Th-1 responses, antiviral immunity, and reduces airway fibrosis. The objectives of this study were (1) to determine the levels of IL-12 mRNA and IL-12 protein expressed in the lungs of both rat strains after viral inoculation and (2) to determine if IL-12 p70 treatment in virus-susceptible BN rats would abrogate or lessen the severity of Sendai virus-induced bronchiolar inflammation and fibrosis that are associated with virus-induced bronchiolar dysfunction.
Section snippets
Animals
Twenty-two-day-old weanling, male, pathogen-free BN/RijHsd and F344/NHsd rats were purchased from Harlan Sprague Dawley, Inc., Madison, WI, and Indianapolis, IN, respectively. The control and virally infected animals were housed separately in adjacent, identical, and individually ventilated and HEPA filtered cages (Micro-Isolator VCL-HD™ system, Lab Products, Inc., Seaford, DE). The University of Florida Animal Care and Use Committee approved all procedures.
Viral procedures and sample collection
The rats were inoculated with
IL-12 expression in F344 and BN rats after Sendai virus infection
F344 rats had significantly higher pulmonary levels of IL-12 p40 mRNA detected by real-time PCR than BN rats at zero day (uninfected controls), one, two and three days after virus inoculation (Fig. 1). The largest difference in IL-12 p40 levels between strains was at two days following virus inoculation when 3.4-fold higher levels were detected in F344 rats. BN rats had virus-induced increases in IL-12 p40 mRNA at three and five days following inoculation. Virus infection did not induce an
Discussion
We hypothesized that resistance to virus-induced chronic airway inflammation and remodeling during early life was controlled at least in part by early high IL-12 gene expression in response to infection. The strongest data consistent with our hypothesis were that weanling F344 rats resistant to virus-induced chronic airway inflammation and remodeling produce higher levels of pulmonary IL-12 mRNA and protein than virus-susceptible BN rats. Furthermore, treatment of virus-susceptible young BN
Acknowledgements
We wish to thank Karen R. Dukes and Christy L. Voakes for their technical assistance.
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This study was supported by NIH grants HL61018 and HL56396.