Ectopic Expression of Human Thymosin ß4 Confers Resistance to Legionella pneumophila during Pulmonary and Systemic Infection in Mice.

ABSTRACT

Thymosin beta-4 (Tß4) 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 Tß4-overexpressing transgenic (Tg) mice to investigate the role of Tß4 in acute pulmonary infection and systemic sepsis caused by Legionella pneumophila Upon infection, Tß4-Tg mice demonstrated significantly lower bacterial loads in the lung, less hyaline membranes and necrotic abscess, with lower interstitial infiltration of neutrophils, CD4+, and CD8+ T cells. Bronchoalveolar lavage fluid of Tß4-Tg mice possessed higher bactericidal activity against exogenously added L. pneumophila, suggesting that constitutive expression of Tß4 could efficiently control L. pneumophila Furthermore, qPCR analysis of lung homogenates demonstrated significant reduction of interleukin 1 beta (IL-1ß) and tumor necrosis factor alpha (TNF-α), which primarily originate from lung macrophages, in Tß4-Tg mice after pulmonary infection. Upon L. pneumophila challenge of bone marrow-derived macrophages (BMDM) in vitro, secretion of IL-1ß and TNF-α proteins was also reduced in Tß4-Tg macrophages, without affecting their survival. The anti-inflammatory effects of BMDM in Tß4-Tg mice on each cytokine were affected when triggering with tlr2, tlr4, tlr5, or tlr9 ligands, suggesting that anti-inflammatory effects of Tß4 are likely mediated by the reduced activation of Toll-like receptors (TLR). Finally, Tß4-Tg mice in a systemic sepsis model were protected from L. pneumophila-induced lethality compared to wild-type controls. Therefore, Tß4 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.