Alveolar macrophages stimulate enhanced cytokine production by pulmonary CD4+ T-lymphocytes in an exacerbation of murine chronic asthma

Abstract

The mechanisms underlying the exaggerated distal airway inflammation and hyperresponsiveness that characterize acute exacerbations of asthma are largely unknown. Using BALB/c mouse experimental models, we demonstrated a potentially important role for alveolar macrophages (AM) in the development of an allergen-induced exacerbation of asthma. To induce features of airway inflammation and remodeling characteristic of mild chronic asthma, animals were systemically sensitized and exposed to low mass concentrations (≈3 mg/m(3)) of aerosolized ovalbumin for 30 minutes per day, 3 days per week, for 4 weeks. A subsequent single moderate-level challenge (≈30 mg/m(3)) was used to trigger an acute exacerbation. In chronically challenged animals, cytokine expression by AM was not increased, whereas after an acute exacerbation, AM exhibited significantly enhanced expression of proinflammatory cytokines, including interleukin (IL) 1β, IL-6, CXCL-1, and tumor necrosis factor α. In parallel, there was a marked increase in the expression of several cytokines by CD4(+) T-lymphocytes, notably the Th2 cytokines IL-4 and IL-13. Importantly, AM from an acute exacerbation stimulated the expression of Th2 cytokines when cocultured with CD4(+) cells from chronically challenged animals, and their ability to do so was significantly greater than AM from either chronically challenged or naïve controls. Stimulation was partly dependent on interactions involving CD80/86. We conclude that in an acute exacerbation of asthma, enhanced cytokine expression by AM may play a critical role in triggering increased expression of cytokines by pulmonary CD4(+) T-lymphocytes.

Figure 1

Enhanced expression of mRNA for major proinflammatory cytokines by AM from the acute exacerbation group: TNF-α (A), IL-1β (B), IL-6 (C), and CXCL-1/KC (D). Data are the mean ± SEM (n = 6 samples per group) relative to HPRT. Fold change relative to the naïve group is shown in parentheses. Significant differences relative to the naïve group are as shown as **P < 0.01 and ***P < 0.001; relative to the acute exacerbation group are shown as ^†P < 0.05, ^††P < 0.01, and ^†††P < 0.001.

Figure 2

Heat maps illustrating expression of mRNA for various cytokines by CD4⁺ T-lymphocytes (A) and concentrations of various cytokines in BAL fluid (B). Levels in the acute exacerbation group, compared to the chronic challenge and single moderate challenge groups, are shown as log-transformed mean fold change relative to the naïve group, converted to increasingly darker colors.

Figure 3

Enhanced expression of mRNA for various cytokines by primed CD4⁺ T cells cocultured with AM from naïve animals, chronically challenged animals, or animals in which an acute exacerbation had been induced: A: IL-4, B: IL-5, C: IL-6, and D: IL-13. Data are the mean ± SEM (n = 3 samples per group) fold change relative to naïve AM. Significant differences relative to naïve AM are shown as **P < 0.01 and ***P < 0.001.

Figure 4

Inhibition of the enhanced expression of cytokine mRNA by primed CD4⁺ T cells, as a result of preincubation of cocultured AM from an acute exacerbation with CTLA-4 Fc chimera to prevent costimulation via CD80/86: A: IL-4, B: IL-5, C: IL-6, and D: IL-13. Data are the mean ± SEM (n = 6 samples per group) fold change relative to naïve AM. Significant differences relative to naïve AM are shown as *P < 0.05 and **P < 0.01; relative to non–CTLA-4-treated cultures are shown as ^†P < 0.05 and ^††P < 0.01.