The role of innate immunity in the pathogenesis of asthma: evidence for the involvement of Toll-like receptor signaling.

Infectious diseases have a major impact on both the development and the severity of asthma. The rise in incidence of asthma in industrialized countries over the last decades has been attributed to increased hygiene standards as well as the concomitant usage of antibiotics, which together lower the incidence of infections. Although this point of view is supported by both clinical studies and experimental approaches in mice, an increasing body of evidence suggests that certain infectious diseases may predispose for the development of asthma, thus challenging the ;hygiene hypothesis' in its classical form. Toll-like receptors (TLRs) are centrally involved in orchestrating immune responses towards various micro-organisms. Because of this, it is tempting to speculate that signaling through TLRs may be involved in mechanisms provoking Th1- or Th2-biased immune responses and may, therefore, be an important factor in either preventing or promoting allergic airway disease. This review summarizes clinical and experimental data from mouse models focused on the impact of TLR-signaling on allergic asthma.