Isolation of a Common Natural Type-I Interferon Producing Cell and Dendritic Cell Progenitor Population in Mouse Bone Marrow.

Most type-I interferon producing cells (IPCs) and dendritic cells (DCs) are non-dividing cells with a short in vivo half-live of several days, and thus need to be continuously replaced. A common differentiation pathway for IPCs and DCs, and accordingly, the existence of common IPC and DC progenitors remains controversial. Flt3-ligand (Flt3L) is a non-redundant cytokine for in vivo IPC and DC development: IPC and DC differentiation potential is confined to Flt3⁺-hematopoietic progenitors; Flt3L KO mice show massively reduced IPCs and DCs. In contrast to Flt3, the “myeloid” cytokines GM-CSF and M-CSF seem to be less relevant in steady-state IPC and DC differentiation, however, they might be critically important in inflammatory conditions.

To identify a candidate common IPC and DC progenitor population, we evaluated Flt3 and “myeloid” cytokine receptor expression in mouse bone marrow. We found that c-kit^intlin⁻ cells contained a Flt3⁺M-CSFR⁺ fraction that in Flt3L supplemented cultures gave rise to about 95% pure CD11c⁺MHC class II⁺ cells, consisting of both CD11c⁺B220⁺ IPCs and CD11c⁺B220⁻ DCs, at a efficiency comparable to that of hematopoietic stem cells. In the presence of GM-CSF, Flt3⁺M-CSFR⁺c-kit^intlin⁻ cells gave rise to CD11c⁺CD11b⁺ DCs but not CD11c⁻CD11b⁺ macrophages/monocytes. Furthermore, Flt3⁺M-CSFR⁺c-kit^intlin⁻ cells possessed very poor, if any activity in myeloid colony forming assays, and lacked pre-B cell colony forming activity.

In both, lethally and sub-lethally irradiated mice, transferred Flt3⁺M-CSFR⁺c-kit^intlin⁻ cells differentiated into CD11c⁺B220⁺ IPCs, CD11c⁺CD8α⁺, and CD11c⁺CD8α⁻ conventional DC subsets, while no other hematopoietic cells were detectable. In vivo reconstitution and CFSE-labeling experiments showed that Flt3⁺M-CSFR⁺c-kit^intlin⁻ cells extensively proliferate in the lethally irradiated mice, reaching peak progeny levels of IPC and DC at day 10 after transplantation, indicating high proliferative, but limited self-renewal capacity of these cells. Quantitative RT-PCR analysis revealed high expression of DC and IPC-development affiliated genes (such as PU.1, STAT3, GM-CSFR, and CX₃CR1), but no lymphoid- and erythroid-development affiliated gene transcription. These data suggest the existence of common developmental intermediates for both IPCs and DCs in mouse bone marrow, and thus might provide new insights into the regulation of IPC and DC differentiation in steady-state and inflammation.