Impact of Klebsiella pneumoniae on efferocytosis of polymorphonuclear cells

Abstract

Abstract Efficient clearance of invading pathogen and the infiltrating cells by phagocytic cells are two key events that are required for successful elimination of a pathological insult. Clearance of the pathogen is usually achieved by phagocytosis and dead cells are cleared by a process called efferocytosis. While there is substantial information on various strategies employed by a number of pathogens to subvert phagocytosis, our knowledge about inhibition of efferocytosis by microbes is extremely limited. In that regard, we observed that, polymorphonuclear cells (PMNs) infected with Klebsiella pneumoniae (KPn), an opportunistic pathogen and a frequent cause of nosocomial infections leading to sepsis, were efferocytosed less than their uninfected counterparts. This inhibition did not correlate with the infection induced effect on phosphatidylserine (PS) exposure, a well-studied “eat me” signal recognized by efferocytic cells or p53 expression, as a marker of apoptosis in PMNs. We further observed that bacterial replication is required for the full inhibitory effect on efferocytosis since heat-killed KPn were not as effective in inhibiting efferocytosis of PMNs. Next we examined the possible role of LPS in diminishing effecrocytosis, given that KPn is a Gram-negative bacterium. By utilizing cells deficient in TLR-4 (a receptor for LPS) and direct stimulation with LPS we determined that LPS is able to directly inhibit efferocytosis of PMNs. To our knowledge, this study is the first to show a modulatory effect of KPn infection and LPS on PMNs leading to inhibition of their efferocytic uptake by macrophages. This could have implications in acute and chronic airway inflammatory diseases such as pneumonia, sepsis, cystic fibrosis, and COPD.