Expression and function of transforming growth factor β in melioidosis

Abstract

Melioidosis, caused by the Gram-negative bacterium Burkholderia pseudomallei, is an important cause of community-acquired sepsis in Southeast Asia and northern Australia. An important controller of the immune system is the pleiotropic cytokine transforming growth factor β (TGF-β), of which Smad2 and Smad3 are the major signal transducers. In this study, we aimed to characterize TGF-β expression and function in experimental melioidosis. TGF-β expression was determined in 33 patients with culture-proven infection with B. pseudomallei and 30 healthy controls. We found that plasma TGF-β concentrations were strongly elevated during melioidosis. In line with this finding, TGF-β expression in C57BL/6 mice intranasally inoculated with B. pseudomallei was enhanced as well. To assess the role of TGF-β, we inhibited TGF-β using a selective murine TGF-β antibody. Treatment of mice with anti-TGF-β antibody resulted in decreased lung Smad2 phosphorylation. TGF-β blockade appeared to be protective: mice treated with anti-TGF-β antibody and subsequently infected with B. pseudomallei showed diminished bacterial loads. Moreover, less distant organ injury was observed in anti-TGF-β treated mice as shown by reduced blood urea nitrogen (BUN) and aspartate transaminase (AST) values. However, anti-TGF-β treatment did not have an effect on survival. In conclusion, TGF-β is upregulated during B. pseudomallei infection and plays a limited but proinflammatory role during experimental melioidosis.

Fig 1

TGF-β levels are elevated in melioidosis patients. (A) On admission, TGF-β levels in plasma of patients with culture-proven melioidosis (n = 33) were elevated compared to healthy controls (n = 30). (B) Plasma levels of TGF-β decreased in melioidosis patients 2 weeks after successful antibiotic treatment (n = 6). *, P < 0.05.

Fig 2

Increased expression of TGF-β in lung tissue of wild-type mice infected with B. pseudomallei. Elevated pulmonary TGF-β levels were observed in mice (n = 8) 72 h after intranasal infection with 7.5 × 10² CFU of B. pseudomallei compared to 24 h after infection. ***, P < 0.001.

Fig 3

Decreased expression of phosphorylated Smad2 in mice receiving anti-TGF-β treatment. TGF-β signaling was evaluated by quantifying the total levels of Smad2 and Smad3 and their phosphorylation in lung tissue by Western blot 48 h after intranasal infection with B. pseudomallei. The left 4 lanes show the expression of Smad2 (A) and Smad3 (B) in lung tissue of control mice and the right 4 lanes of mice that received anti-TGF-β treatment (2 × 200 μg TGFβ antibody). The bar graph shows the ratio of expressed phosphorylated Smad/total Smad corrected for actin loading levels. (C) A significant decrease in p-Smad2 is demonstrated in mice that were treated with anti-TGF-β antibody. *, P < 0.05.

Fig 4

Less organ damage in mice receiving anti-TGF-β treatment. Levels of (A) BUN, (B) creatinine, (C) AST, and (D) ALT were measured in B. pseudomallei-infected mice (n = 8/group) that were treated with anti-TGF-β antibody and their controls at 72 h as parameters for organ damage. Expression of AST and BUN were decreased, demonstrating less organ damage. *, P < 0.05. Data shown from single independent experiments.

Fig 5

Anti-TGF-β treatment results in diminished bacterial growth during experimental melioidosis. Mice (n = 8/group) were treated with anti-TGF-β antibody or control IgG antibody and inoculated with 7.5 × 10² CFU of B. pseudomallei intranasally. Bacterial numbers were quantified 72 h postinfection. Less growth was seen in the lungs (A) and spleens (B) of mice treated with anti-TGF-β antibody. *, P < 0.05. Data shown from single independent experiments.

Fig 6

No influence on survival of anti-TGF-β treatment during murine melioidosis. Mice (n = 12/group) received either 2× 200 μg control antibody (black squares) or 2× 200 μg anti-TGF-β antibody (white squares) and were infected with 7.5 × 10² CFU, a lethal dose, of B. pseudomallei intranasally, after which they were followed until death. Mortality was assessed every 4 h. No difference in survival was seen.