Purpose: The retinal pigment epithelium (RPE) is a barrier to Klebsiella pneumoniae infection in endogenous endophthalmitis. Nevertheless, the inflammatory response of RPE cells upon interaction with this pathogen has not been studied. Here we tested the hypothesis that K. pneumoniae induces an inflammatory response in human retinal epithelial cells. Methods: In this study we set out to investigate the effects of whole K. pneumoniae and of its lipopolysaccharide on RPE cells in vitro using bacterial invasion and cytotoxicity assays, fluorescent microscopy and ELISA. For that, we utilized K. pneumoniae strain ATCC 43816 and the continuous human retinal-pigmented epithelial cell line ARPE-19. Results: Stimulation of ARPE-19 with live K. pneumoniae for 24 h induced a 31.5-fold (p = 0.0132) increase in IL-6 and 6.5-fold (p = 0.0004) increase in MCP-1 levels compared to the non-infected control cells. Purified K. pneumoniae LPS (1 mu g ml(-1)) also induced cytokine levels, MCP-1 (1.7-fold upregulation; p = 0.0006) and IL-6 (1.3-fold upregulation, p = 0.065). The tested K. pneumoniae strain ATCC 43816 did not have a significant effect on the viability of ARPE-19 cells (11% decrease, p = 0.096) and showed a low ability to invade the cells. Conclusions: Both whole live K. pneumoniae and K. pneumoniae LPS exert a strong pro-inflammatory effect on retinal pigmented epithelial cells, consistent with clinical manifestations of disease. Bacterial pro-inflammatory effects are not likely related to host cell invasion. This is the first investigation of the interactions of a major endogenous endophthalmitis pathogen, K. pneumoniae with human retinal pigmented epithelial cells. (C) 2011 Elsevier Inc. All rights reserved.