Lung Inflammatory Response in Pneumonia

In normal conditions, alveolar macrophages (AMs) are the main cells that respond to bacteria that reach lower airways. However, if the microbial inoculum is too high or too virulent to be stopped by AM alone, these cells recruit polymorphonuclear neutrophils (PMN) into the alveoli from the vascular compartment. Cytokines, such as tumour necrosis factor-alpha (TNF-alpha), interleukin-1-beta (IL-beta), interleukin-6 (IL-6), and interleukin-8 (IL-8), secreted by the AM are able to attract PMN enhanced for phagocytosis, ready to destroy the invading pathogens. However, excessive cytokine production has deleterious effects, with a systemic inflammatory response (sepsis) that can lead to multiorganic failure and death. Other cytokines, such as interleukin-10 (IL-10) balance this response, attenuating several inflammatory mechanisms. The inflammatory lung response in pneumonia has been well studied in animals, and more recently in humans, using bronchoalveolar lavage to measure some inflammatory mediators (TNF-alpha, IL-1 beta, IL-6, IL-8). From these studies, it seems that: 1) the inflammatory response to pneumonia is compartmentalized for most cytokines (in contrast to adult respiratory distress syndrome (ARDS)), except for IL-6 which is a general marker of inflammation. On the other hand, C-reactive-protein is an acute-phase protein synthesized by the liver through the stimulus of IL-6 that may also be an easy-to-measure marker of inflammation that is directly related to IL-6; 2) some of these cytokines may be useful as prognostic indices; 3) there is no clear relationship between the local lung bacterial burden and the intensity of the inflammatory response; and 4) the administration of granulocyte colony-stimulating factor (G-CSF) is a promising therapeutic approach that is still under clinical investigation. In the future, it is probable that the therapeutic goal in severe pneumonia will be to find the exact point at which inflammation is beneficial but not deleterious. The measurement of the inflammatory response may serve for this purpose.