The impact of successive infections on the lung microenvironment.

The effect of infection history on the immune response is ignored in most models of infectious disease and in preclinical vaccination studies. No one, however, is naive and repeated microbial exposure, in particular during childhood, shapes the immune system to respond more efficiently later in life. Concurrent or sequential infections influence the immune response to secondary unrelated pathogens. The involvement of cross-reactive acquired immunity, in particular T-cell responses, is extensively documented. In this review, we discuss the impact of successive infections on the infected tissue itself, with a particular focus on the innate response of the respiratory tract, including a persistent alteration of (1) epithelial or macrophage expression of Toll-like receptors or adherence molecules used by subsequent bacteria to invade the host, (2) the responsiveness of macrophages and neutrophils and (3) the local cytokine milieu that affects the activation of local antigen-presenting cells and hence adaptive immunity to the next infection. We emphasize that such alterations not only occur during coinfection, but are maintained long after the initial pathogen is cleared. As innate responses are crucial to the fight against local pathogens but are also involved in the maintenance of the homeostasis of mucosal tissues, dysregulation of these responses by repeated infections is likely to have a major impact on the outcome of infectious or allergic disease.