Toll-like receptors: function and roles in lung disease

Toll-like receptor (TLR) proteins have been shown to play a pivotal role in both innate and adaptive immune responses in higher vertebrates. TLR proteins enable the host to recognize a large number of pathogen-associated molecular patterns such as bacterial lipopolysaccharides, viral RNA, CpG-containing DNA, and flagellin, among others. Engagement of TLR proteins leads to the upregulation of costimulatory molecules and proinflammatory cytokines, as well as reactive nitrogen and oxygen products. The role of TLR proteins in lung-associated pathologies such as airway hyperreactivity, allergic asthma, and tuberculosis is being intensively studied. This review summarizes many of the findings made to date on the roles of TLR proteins in a variety of lung diseases. Generally, TLR proteins serve a protective role in infectious diseases, such as tuberculosis. The progression of chronic inflammatory lung diseases, such as allergic asthma, can also be influenced by TLR-dependent responses.