Characterisation of innate immunity in patients with ventilator-associated pneumonia

Abstract

Ventilator- associated pneumonia (VAP) is the most common ICU -acquired infection in patients receiving mechanical ventilation, associated with excess mortality, morbidity and cost. The innate immune system is considered to be responsible for the rapid initial response in eradicating invading micro -organisms. Neutrophils are the professional phagocytes of innate immunity and are mobilised rapidly towards the site of inflammation where they will eliminate pathogens through phagocytosis and killing. Unregulated responses however can lead to neutrophil- mediated tissue injury. Secretory leukocyte protease inhibitor (SLPI) and elafin are considered to play a pivotal role in modulating innate responses whilst providing an antiprotease shield towards the influx of neutrophil- dependent proteases.

The hypothesis of this work was that an exaggerated pro- inflammatory response is present in patients with VAP while the local antiprotease /antimicrobial expression is impaired, potentially facilitating neutrophil- mediated tissue injury. In the same context, neutrophils in patients with VAP may have impaired capacity to eliminate pathogens whilst having the potential to cause tissue damage.

Healthy volunteers from primary care and patients from a general ICU with clinically suspected VAP were recruited in the study. Bronchoscopy and bronchoalveolar lavage (BAL) was performed and in the patients the presence or absence of pathogens at a concentration >104 cfu /ml of BAL fluid (BALF) defined the VAP group and the NON - VAP group respectively. Blood was also sampled and freshly prepared circulating neutrophils were used in in vitro assays. Serum and supernatants from BALF were used for quantification of cytokines and antiproteases.

BALF from patients with VAP displayed a trend towards a high number of neutrophils although the difference was not statistically significant. VAP was associated with a florid pro -inflammatory response as demonstrated by significantly elevated CXCL8 and interleukin -113 (IL -1 (3) levels.

Circulating neutrophils from both groups of critically ill patients exhibited significantly impaired phagocytic capacity and appeared to have the potential, upon contact with bacterial products, to cause damage to the membrane of epithelial cells.

BALF SLPI levels were similar in both groups of critically ill patients while elafin levels were significantly elevated in the BALF of VAP patients compared with healthy volunteers. However the excess of HNE does support an inherent imbalance in protease - antiprotease in the alveolar space in VAP.

The above findings provide more information in our understanding of the biology of VAP.A florid inflammatory response is present along with an impaired phagocytic capacity of neutrophils that simultaneously exhibit the potential to cause tissue damage. The antiprotease shield may also be impaired facilitating further host tissue injury.

Further research is needed to elucidate pathways that may govern the observed deficiencies and /or dysregulations and may provide the basis for novel therapeutic strategies targeting neutrophil- mediated lung injury.