Ventilator-associated lower respiratory tract infections (VA-LRTIs) are less common in immunocompromised patients than in general ICU patients, so host response might be less important than anticipated in the pathophysiology of VA-LRTIs http://ow.ly/1V6Y30ih8yS
In this issue of the European Respiratory Journal, Moreau et al.  describe the impact of immunosuppression on incidence, aetiology and outcome of ventilator-associated lower respiratory tract infections (VA-LRTIs). The authors report a lower incidence of VA-LRTIs in immunosuppressed patients, with an odds ratio of 0.64, which is in contradiction to their hypothesis and common belief.
It seems that the presumed positive association between immunosuppression and the development of VA-LRTI is mostly based on “common sense” and not on empirical data; no large study has previously quantified this relationship reliably. In their prospective observational cohort of almost 3000 intensive care unit (ICU) patients at risk for VA-LRTI, 22% were defined as “immunosuppressed” due to the presence of a solid malignancy, haematological malignancy, AIDS or a history of allogeneic stem cell transplant, the use of immunosuppressive drugs or status after organ transplant. VA-LRTI was defined according to the currently accepted definitions . Since immunosuppressed patients have an increased risk of decease during their stay on the ICU, and a deceased patient cannot develop VA-LRTI, competing risk analysis is necessary to quantify the risk of infection, and Moreau et al.  adequately dealt with this type of bias [3–5]. They are to be applauded for their meticulous methodological approach. Thus, based on the available empirical evidence, we must accept that an immunosuppressed state does not increase the risk of VA-LRTI but might actually protect against VA-LRTI.
Why did we believe the contrary and what are the consequences of these findings?
Lower respiratory tract infections are the result of immigration of pathogenic bacteria into the lungs (figure 1). In ventilated patients, immigration into the lungs is increased through the presence of the endotracheal tube, positive pressure ventilation and an increased concentration of potential pathogenic bacteria in the stomach and oro-pharynx [6, 8]. At the same time, elimination is decreased due to a reduced cough reflex and mucociliary clearance. As the host response is also an important factor in elimination of bacteria , especially in the absence of physical clearance of bacteria from the lower respiratory tract, it makes perfect pathophysiological sense that immunosuppressed patients more frequently develop VA-LRTIs. In fact, it was such a no-brainer that no one dared to question it.