Differential alveolar epithelial injury and protein expression in pneumococcal pneumonia

Abstract

RATIONALE: Streptococcus pneumoniae is a major cause of community- acquired pneumonia and its damaging effects on lung tissue can be extensive. The alveolar epithelium consists of alveolar type I (ATI) cells, involved in gas exchange, ATII cells, which are progenitor cells which secrete surfactant. Cell- specific marker proteins can be used to look at targeted epithelial injury by measurement in broncho -alveolar lavage (BAL) fluid, and expression of cell- specific proteins in lung tissue can be used to identify changes in cell population and functionality. The initial purpose of this thesis was to examine differential alveolar epithelial injury in pneumococcal pneumonia.RESULTS: Non-lethal rat models of acute (24 hours), resolving (72 hours) and recovering (3 weeks) pneumococcal pneumonia were set up, by way of intratracheal inoculation of wildtype D39 S. pneumoniae. Models were characterised by bacterial clearance with an acute inflammatory response and damage to the air-blood barrier that was resolving by 72 hours. Lungs were histologically normal after 3 weeks. Assessment of ATI cell- specific protein RTI40 demonstrated possible regulation in protein expression in both BAL fluid and lung tissue, rendering it inappropriate as a marker of injury in this model, and expression differed from that of other ATI cell- specific proteins. A new method of quantifying damaged ATII cells, using cell- specific proteins APN /MMC4, RTH70 and Pro -SP -C, showed targeted injury to these cells by 24 hours, which was shown to be resolving by 72 hours. Differential expression of ATII cell- specific surfactant proteins cytoplasmic Pro -SP -C and secreted SP -D was identified. Subsequent study to elucidate the mechanism of this differential protein expression compared the initial study with other models of lung infection. To determine the effect of pneumococcal toxin pneumolysin, PLN -A pneumolysin- deficient bacteria were used. Results showed that changes in expression of RTI40 and SP -D were not dependent on pneumolysin production. Further investigation to determine the extent that differential protein expression was characteristic of other Gram -positive infections S. aureus, wildtype 8325 -4 strain was used. Results demonstrated that differential expression of RTI40 was specific to pneumococcal ii infection but that expression of SP -D was common to other Gram -positive infections. To further explore the mechanism, in vitro bacterial co- culture experiments were carried out using S. pneumoniae D39 and the SV40 -T2 alveolar epithelial cell line, to establish if the observation was caused by a direct effect by bacteria or bacterial products. These showed that expression of RTI40 and SP -D were not altered by S. pneumoniae alone.CONCLUSIONS: Pneumococcal pneumonia induces specific injury to ATII cells and differential protein expression in both ATI and ATII cells. Expression of RTI40 is specific to the pneumococcus but expression of SP -D is common to other Gram -positive bacteria. Expression of neither protein was mediated by pneumolysin or by direct interaction of bacteria or bacterial products. These results provide valuable insight into host response to bacterial pathogens.