Pseudomonas aeruginosa infection is a hallmark of
lung disease in
cystic fibrosis. Acute
infection with P. aeruginosa profoundly inhibits alveolar macrophage clearance of apoptotic cells (efferocytosis) via direct effect of
virulence factors. During
chronic infection, P. aeruginosa evades host defense by decreased virulence, which includes the production or, in the case of mucoidy, overproduction of
alginate. The impact of
alginate on innate immunity, in particular on macrophage clearance of apoptotic cells is not known. We hypothesized that P. aeruginosa strains that exhibit reduced virulence impair macrophage clearance of apoptotic cells and we investigated if the
polysaccharide alginate produced by mucoid P. aeruginosa is sufficient to inhibit alveolar macrophage efferocytosis. Rat alveolar or human peripheral blood monocyte (THP-1)-derived macrophage cell lines were exposed in vitro to exogenous
alginate or to wild type or
alginate-overproducing mucoid P. aeruginosa prior to challenge with apoptotic human Jurkat T-lymphocytes. The importance of LPS contamination and that of structural integrity of
alginate polymers was tested using
alginate of different purities and
alginate lyase, respectively.
Alginate inhibited alveolar macrophage efferocytosis in a dose- and time-dependent manner. This effect was augmented but not exclusively attributed to
lipopolysaccharide (LPS) present in
alginates.
Alginate-producing P. aeruginosa inhibited macrophage efferocytosis by more than 50%. A mannuronic-specific
alginate lyase did not restore efferocytosis inhibited by exogenous guluronic-rich marine
alginate, but had a marked beneficial effect on efferocytosis of alveolar macrophages exposed to mucoid P. aeruginosa. Despite decreased virulence, mucoid P. aeruginosa may contribute to chronic airway
inflammation through significant inhibition of alveolar clearance of apoptotic cells and debris. The mechanism by which mucoid bacteria inhibit efferocytosis may involve
alginate production and synergy with LPS, suggesting that
alginate lyase may be an attractive therapeutic approach to airway
inflammation in
cystic fibrosis and other
chronic obstructive pulmonary diseases characterized by P. aeruginosa colonization.