Abstract
DAP12 is an adapter protein that associates with several receptors in macrophages. Little is known about the biological role of DAP12 in alveolar macrophages. In genome-wide profiling, we previously found that two DAP12-associated receptors, myeloid DAP12-associated lectin-1 and triggering receptor expressed on myeloid cells 2 (TREM2), were highly induced in alveolar macrophages from habitual smokers. Here, we found that transcript levels for these receptors in alveolar macrophages increased with packs per day of cigarettes smoked and expression of TREM2 protein was increased in lung macrophages of former smokers with emphysema compared with that in controls. In vitro, cigarette smoke directly induced expression of myeloid DAP12-associated lectin-1 and TREM2 and activation of DAP12 signaling in mouse macrophages. To determine whether DAP12 plays a role in cigarette smoke-induced pulmonary inflammation, we exposed wild-type and DAP12-deficient mice to chronic cigarette smoke and found significant reduction in recruitment of alveolar macrophages in DAP12-deficient mice. Because cigarette smoking induces the macrophage chemoattractant CCL2, we tested the chemotactic ability of DAP12-deficient macrophages and found abrogation of chemotaxis toward CCL2 in vitro. Airway administration of CCL2 also resulted in a significant reduction of macrophage recruitment to the lungs of DAP12-deficient mice compared with that in controls. DAP12 was also required for normal macrophage migration in a "scratch" assay. Reconstitution studies revealed that phosphorylation of the DAP12 ITAM was required for normal migration in vitro and association with TREM2 was sufficient for normal migration. These findings indicate that DAP12, possibly through association with TREM2, contributes to alveolar macrophage chemotaxis and recruitment to the lung and may mediate macrophage accumulation in lung diseases such as emphysema.