Pseudomonas aeruginosa alginate is refractory to Th1 immune response and impedes host immune clearance in a mouse model of acute lung infection

Abstract

Pseudomonas aeruginosa is an opportunistic respiratory pathogen that accounts for most of the morbidity and mortality in cystic fibrosis (CF) patients. In CF-affected lungs, the bacteria undergo conversion from a non-mucoid to a non-tractable mucoid phenotype, due to overproduction of alginate. The effect of alginate production on pathogenicity was investigated by using an acute lung infection mouse model that compared a non-mucoid P. aeruginosa strain, PAO1, to its constitutive alginate-overproducing derivative, Alg þ PAOmucA22, and an alginate-defective strain, Alg À PAOalgD. Bacterial suspensions were instilled into the left bronchus and examined 24 and 48 h post-infection. The highest bacterial loads and the most severe lung pathology were observed with strain Alg À PAOalgD at 24 h post-infection, which may have been due to an increase in expression of bacterial elastase by the mutant. Significantly lower lung and spleen bacterial loads were found in the two non-mucoid (PAO1 and Alg À PAOalgD) groups, compared to the mucoid Alg þ PAOmucA22 group, between 24 and 48 h post-infection. The positive correlation between lung bacteriology and lung macroscopic pathology in the Alg þ PAOmucA22 group suggests that alginate production not only impedes pulmonary clearing, but also results in severe lung damage. Positive correlations between IL12 levels and lung macroscopic pathology, and between IL12 and IFN-ª levels in the Alg þ PAOmucA22 group, suggested a possible contribution of these pro-inflammatory cytokines to tissue damage. No significant differences were found between the three groups in lung cytokine responses at 24 or 48 h post-infection. However, on comparison within each group at 24 and 48 h post-infection, a significant increase in the pro-inflammatory cytokine IFN-ª was observed. Higher ratios of IFN-ª/IL4 and IFN-ª/IL10, but lower IL10 levels, were also found in all three groups. These results indicate a Th1-predominated immune response in these animals. Such cytokine responses could have aided the clearance of non-mucoid P. aeruginosa, but were not sufficient to alleviate infection by the mucoid variants. Alginate production may promote survival and persistence of this pathogenic micro-organism in the lung.

Importance of the Exopolysaccharide Matrix in Antimicrobial Tolerance of Pseudomonas aeruginosa Aggregates

“…Alginate has been shown to protect P. aeruginosa against human leukocyte killing (11), in addition to inhibiting pulmonary clearing and increasing bacterial load in a mucine lung infection model (19). Psl expression is also important for reduced phagocytosis (20).…”

Pseudomonas aeruginosa Flagellin and Alginate Elicit Very Distinct Gene Expression Patterns in Airway Epithelial Cells: Implications for Cystic Fibrosis Disease

“…Interestingly, our microarray analysis showed FRD1-dependent up-regulation of genes with antiapoptotic effect on epithelial and other cell types (63)(64)(65), and exposure to an alginate-producing strain results in diminished apoptosis in Calu-3 airway epithelial cells. Thus, the lack of the appropriate host defense and inflammatory milieu in the airways, and impaired bacterial clearance because of reduced epithelial cell apoptosis (59,66), may explain the increased persistence of these strains in animal models of acute infection (15)(16)(17)(18).…”

“…The subsequent intense neutrophil-dominated airway inflammation and progressive lung disease are major causes of morbidity and mortality in this disease (13,14). In vivo studies suggest that clearance of mucoid strains from murine lungs is diminished compared with nonmucoid strains, indicating improved survival of alginateproducing strains in the respiratory tract (15)(16)(17)(18). Alginate enhances mucin secretion by tracheal epithelial cells (19), and may inhibit neutrophil migration to the sites of infection (20).…”

The Exopolysaccharide Alginate Protects Pseudomonas aeruginosa Biofilm Bacteria from IFN-γ-Mediated Macrophage Killing

“…Interestingly, percent survival of PDO300 biofilm bacteria was similar to that seen with wild-type biofilms (PA01, PA14, and FRD1, see below), suggesting that the overproduction of alginate did not increase P. aeruginosa biofilm bacterial resistance to leukocyte killing. However, a recent work demonstrated that after 48 h of infection with this mutant strain, there was marked inflammation and high planktonic bacterial load in a variety of mouse organs vs the parent strain PA01 (46). Thus, the ability to overproduce alginate seems to have an early protective effect in acute infection models.…”