Lung-resident tissue macrophages generate Foxp3+ regulatory T cells and promote airway tolerance

Airway tolerance is the usual outcome of inhalation of harmless antigens. Although T cell deletion and anergy are likely components of tolerogenic mechanisms in the lung, increasing evidence indicates that antigen-specific regulatory T cells (inducible T-reg cells [iT(reg) cells]) that express Foxp3 are also critical. Several lung antigen-presenting cells have been suggested to contribute to tolerance, including alveolar macrophages (Mempty sets), classical dendritic cells (DCs), and plasmacytoid DCs, but whether these possess the attributes required to directly promote the development of Foxp3(+) iT(reg) cells is unclear. Here, we show that lung-resident tissue Mempty sets coexpress TGF-beta and retinal dehydrogenases (RALDH1 and RALDH 2) under steady-state conditions and that their sampling of harmless airborne antigen and presentation to antigen-specific CD4 T cells resulted in the generation of Foxp3(+) T-reg cells. T-reg cell induction in this model depended on both TGF-beta and retinoic acid. Transfer of the antigen-pulsed tissue Mempty sets into the airways correspondingly prevented the development of asthmatic lung inflammation upon subsequent challenge with antigen. Moreover, exposure of lung tissue Mempty sets to allergens suppressed their ability to generate iT(reg) cells coincident with blocking airway tolerance. Suppression of T-reg cell generation required proteases and TLR-mediated signals. Therefore, lung-resident tissue Mempty sets have regulatory functions, and strategies to target these cells might hold promise for prevention or treatment of allergic asthma.