Infections and asthma pathogenesis: a critical role for dendritic cells?

Respiratory viral infections can influence the course of asthma at different time points. Severe respiratory viral infections at early age might be associated with a higher prevalence of asthma in later childhood. In established asthma, viral infections are a frequent cause of asthma exacerbation. Epidemiological and experimental animal data can illuminate the mechanisms by which viral infections can lead to sensitization to antigen and exacerbate ongoing allergic airway inflammation. In experimental rodent models of asthma, respiratory viral infection at the time of a first inhaled antigen exposure is described to induce Th2 sensitization and to enhance the allergic response to a second encounter with the same antigen. Virus infections can modulate airway dendritic cell function by up-regulation of costimulatory molecule expression, enhanced recruitment, and by inducing an inflammatory environment, all leading to an enhanced antigen presentation and possibly changing the normal tolerogenic response to inhaled antigen into an immunogenic response. In established asthma, respiratory viral infections attract several inflammatory cells, alter receptor expression on airway smooth muscle and modulate neuroimmune mechanisms, possibly leading to exacerbation of disease. Animal data suggest that the link between respiratory viral infections and increased asthma is causally related, the viral infection acting on the immune and structural cells to enhance antigen presentation and inflammatory cell recruitment.