Key Points• Mice overexpressing T-bet in T cells show aberrant hematopoiesis of myeloid cells and functional conversion of regional macrophages.• The mice developed a severe PAP-like disease with a hematopoietic disorder resembling the human disease.Although overexpression of T-bet, a master transcription factor in type-1 helper T lymphocytes, has been reported in several hematologic and immune diseases, its role in their pathogenesis is not fully understood. In the present study, we used transgenic model mice (T-bet tg/wt and T-bet tg/tg ) to investigate the effects of T-bet overexpression selectively in T lymphocytes on the development of hematologic and immune diseases. The results showed that T-bet overexpression in T cells spontaneously induced maturation arrest in the mononuclear phagocyte lineage, as well as spontaneous dermatitis and pulmonary alveolar proteinosis (PAP)-like disease in T-bet tg/wt and T-bet tg/tg mice, respectively. T-bet tg/tg alveoli with the PAP phenotype showed remarkable reorganization of alveolar mononuclear phagocyte subpopulations and impaired function, in addition to augmented T-cell infiltration. In addition, PAP development in T-bet tg/tg mice was found to be associated with increased migration of myeloid cells from the bone marrow into the peripheral blood. These findings reveal an unexpected link between T-bet overexpression in T lymphocytes and the development of PAP caused by reorganization of mononuclear phagocytes in the lung, and provide new insight into the molecular pathogenesis of secondary PAP accompanied by hematologic disorders. (Blood. 2015;125(2):370-382)