Selective Disruption of PU.1 in Mature Dendritic Cells Affects Their Tissue Distribution and T Cell Homeostasis

Abstract 518

PU.1, a hematopoietic transcription factor, is indispensable for development of conventional dendritic cells (cDCs) from hematopoietic stem cells. However, the function of PU.1 in mature cDC remains unclear. To test the possible role of PU.1 in mature cDCs, we developed mice lacking PU.1 selectively in mature cDCs (DC-PU.1^D/D mice) by crossing a PU.1^flox mouse line with a transgenic Itgax (CD11c)-Cre strain. In these mice, cDCs were dramatically reduced in spleen, thymus, lymph node, and skin, down to <40%, <25%, <10% and <5% of DCs in control mice respectively, whereas bone marrow cDCs and common dendritic cells progenitors (CDPs) were not affected. Surprisingly, T cell numbers were significantly decreased in DC-PU.1^D/D mice, whereas thymic T cell development was normal, suggesting that maintenance of mature T cell pool might be impaired, presumably by dysfunction of PU.1^D/D cDCs. In fact, PU.1^D/D cDCs failed to efficiently induce ovalbumin-specific T cell response and to produce inflammatory cytokines in response to Toll like receptor (TLR) stimulation both in vitro and in vivo. The intravenous transfer of spleen PU.1^D/D cDCs failed to repopulate the spleen of recipient mice, suggesting their poor survival in vivo. Furthermore, the expression of critical molecules for inflammatory responses was downregulated in PU.1^D/D cDCs as compared to normal cDCs. These molecules included Myd88 and NFkB that are downstream molecules of TLR signaling, CD86 that is required for T cell stimulation, and CCR7 that is required for cDC migration. These results clearly show that PU.1 is required for development of the functional cDC pool, and the cDC pool plays a critical role in T cell homeostasis.