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

Thymosin beta-4 (Tβ4) is an actin-sequestering peptide that plays important roles in regeneration and remodeling of injured tissues. However, its function in naturally occurring pathogenic bacterial infection model has remained elusive. We adopted Tβ4-overexpressing transgenic (Tg) mice to investigate the role of Tβ4 in acute pulmonary infection and systemic sepsis caused by L. pneumophila Upon infection, Tβ4-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 of Tβ4-Tg mice possessed higher bactericidal activity against exogenousely added L. pneumophila, suggesting that constitutive expression of Tβ4 could efficiently control L. pneumophila. Furthermore, qPCR analysis of lung homogenates demonstrated significant reduction of IL-1β and TNF-α in Tβ4-Tg mice after pulmonary infection, which was primarily originated from lung macrophages. Upon L. pneumophila challenge of bone marrow derived macrophages (BMDM) in vitro, secretion of IL-1β and TNF-α proteins are also reduced in Tβ4-Tg macrophages, without affecting their survival. The anti-inflammatory features of BMDM in Tβ4-Tg mice on each cytokines was affected when triggering with tlr2, tlr4, tlr5 or tlr9 ligands, suggesting that anti-inflammatory effects of Tβ4 are likely mediated by the reduced activation of TLR receptors. Finally, Tβ4-Tg mice from systemic sepsis model were protected from L. pneumophila -induced lethality compared to wild-type controls. Therefore, Tβ4 confers effective resistance against L. pneumophila via two pathways: bactericidal and anti-inflammatory pathways, that can be harnessed to treat acute pneumonia and septic conditions caused by L. pneumophila in humans.