Conditional ablation of dendritic cells in transgenic mice.

Dendritic cells (DC) are professional Ag-presenting cells that play a major role in T cell-mediated immune responses and in thymocyte differentiation. To better analyze their physiological importance, we sought to generate transgenic mice presenting a conditional DC deficiency. We used a strategy based on the cell-specific expression of a suicide gene. The DC-targeted expression is obtained using HIV regulatory sequences; indirect evidence has suggested that these sequences control a preferential expression in DC. The suicide gene is the herpes simplex virus type 1 thymidine kinase (HSV1-TK) which allows conditional ablation of dividing HSV1-TK-expressing cells by converting nucleoside analogs such as ganciclovir (GCV) into toxic molecules. We generated transgenic mice expressing an HSV1-TK gene transcribed from HIV regulatory sequences. A low but significant HSV1-TK expression was observed in mature DC and DC precursors grown from granulocyte-macrophage colony-stimulating factor-supplemented bone marrow cultures. These HSV1-TK-expressing DC precursors are specifically killed by GCV. We next treated transgenic mice with GCV, and obtained a specific ablation of DC in spleen and thymus. Ninety percent of spleen DC could be depleted within a week, indicating a turnover rate of approximately 15% per day. Interestingly, this DC depletion always correlated with a major thymic atrophy and disappearance of CD4+CD8+ thymocytes. This animal model should help to assess the physiological role of DC in the immune response and in thymocyte differentiation. It should also help to appreciate the consequences of DC dysfunction in pathological situations, such as HIV-infection or allograft rejection.