Ectopic expression of human thymosin β4 confers resistance to Legionella pneumophila during pulmonary and systemic infection in mice

초록

Thymosin beta-4 (T beta 4) is an actin-sequestering peptide that plays important roles in regeneration and remodeling of injured tissues. However, its function in a naturally occurring pathogenic bacterial infection model has remained elusive. We adopted T beta 4-overexpressing transgenic (Tg) mice to investigate the role of T beta 4 in acute pulmonary infection and systemic sepsis caused by Legionella pneumophila. Upon infection, T beta 4-Tg mice demonstrated significantly lower bacterial loads in the lung, less hyaline membranes and necrotic abscess, with lower interstitial infiltration of neutrophils, CD41, and CD81 T cells. Bronchoalveolar lavage fluid of T beta 4-Tg mice possessed higher bactericidal activity against exogenously added L. pneumophila, suggesting that constitutive expression of T beta 4 could efficiently control L. pneumophila. Furthermore, qPCR analysis of lung homogenates demonstrated significant reduction of interleukin 1 beta (IL-1 beta) and tumor necrosis factor alpha (TNF-a), which primarily originate from lung macrophages, in Tb4-Tg mice after pulmonary infection. Upon L. pneumophila challenge of bone marrowderived macrophages (BMDM) in vitro, secretion of IL-1b and TNF-a proteins was also reduced in T beta 4-Tg macrophages, without affecting their survival. The anti-inflammatory effects of BMDM in T beta 4-Tg mice on each cytokine were affected when triggering with tlr2, tlr4, tlr5, or tlr9 ligands, suggesting that anti-inflammatory effects of T beta 4 are likely mediated by the reduced activation of Toll-like receptors (TLR). Finally, T beta 4-Tg mice in a systemic sepsis model were protected from L. pneumophila-induced lethality compared to wild-type controls. Therefore, Tb4 confers effective resistance against L. pneumophila via two pathways, a bactericidal and an anti-inflammatory pathway, which can be harnessed to treat acute pneumonia and septic conditions caused by L. pneumophila in humans.

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