Rare-event analysis of circulating human dendritic cell subsets and their presumptive mouse counterparts.

Background: Considerable interest has focused recently on murine CD8alpha- and CD8alpha+ dendritic cell (DC) subsets, because of their roles in initiating and regulating immune responses. Attention has also centered on their presumed human counterparts, DC1 and DC2, respectively, and their precursors. Identification and quantification of these subsets in the blood may be crucial to understanding and monitoring of their immunologic significance, particularly in humans, where blood may be the only tissue readily or routinely available.

Methods: Leukocytes were isolated from anticoagulated human or mouse (C57BL/10J) blood using conventional procedures. Four-color, rare-event, flow cytometric analysis was used to identify DC1 precursors (pDC1; lineage [lin]- CD4+ CD11c+ HLA-DR+) or DC2 precursors (pDC2; lin- CD4+ CD11c- CD123(hi) [IL-3Ralpha(hi)] HLA-DR+) in normal humans. In mice, CD8alpha+ (CD11b(lo), CD11c+) and CD8alpha- (CD11b(hi), CD11c+) DC subsets were identified both in normal animals and after administration of the potent DC growth factor, fms-like tyrosine kinase 3 ligand (Flt3L).

Results: All human subjects examined had discrete populations of pDC1 and pDC2 comprising approximately 0.6% and 0.1% of blood mononuclear cells. CD8alpha- and CD8alpha+ DC constituted approximately 0.75% and 0.2%, respectively, of blood mononuclear cells in normal mice, and 12% and 0.5%, respectively, in Flt3L-treated animals. Flt3L administration substantially increased the absolute numbers of circulating CD11c+ DC by approximately 200-fold.

Conclusions: In addition to pDC1 and CD8alpha- DC, pDC2 and CD8alpha+ DC can be identified in normal human or mouse blood, respectively. Monitoring and isolation or characterization of these cells may provide novel insights into their functional significance in transplantation and other clinical conditions.