Alveolar type II epithelial cells present antigen to CD4(+) T cells and induce Foxp3(+) regulatory T cells.

RATIONALE: Although the contribution of alveolar type II epithelial cells (AECIIs) in respiratory immunity has become increasingly appreciated, their precise function in the induction and regulation of T-cell reactivity to self-antigen remains poorly understood.
OBJECTIVES: To investigate the role of AECII in the initiation of T-cell reactivity to alveolar self-antigen, and to clarify their function in the peripheral induction of Foxp3(+) regulatory CD4(+) T cells.
METHODS: To dissect the complex cellular and molecular functions of AECIIs in lung inflammation and immune regulation, we use a transgenic mouse model for CD4(+) T-cell-mediated pulmonary inflammation.
MEASUREMENTS AND MAIN RESULTS: Here we report that AECIIs present endogenously expressed antigen on major histocompatibility complex class II molecules to CD4(+) T cells. Epithelial antigen display was sufficient to induce primary T-cell activation and pulmonary inflammation. Upon inflammation, AECIIs induce the differentiation of Foxp3(+) regulatory T cells by a mechanism involving antiproliferative soluble factors, including transforming growth factor (TGF)-beta. Whereas, in acute inflammation, TGF-beta appears to be the dominant factor to induce regulatory T cells, other AECII-derived factors can substitute for and/or synergize with TGF-beta in chronic pulmonary inflammations.
CONCLUSIONS: AECIIs are capable of priming naive CD4(+) T cells, demonstrating an active participation of these cells in respiratory immunity. Moreover, AECIIs display as yet unrecognized functions in balancing inflammatory and regulatory T-cell responses in the lung by connecting innate and adaptive immune mechanisms to establish peripheral T-cell tolerance to respiratory self-antigen.