Natural Interferon Producing Cells Develop from Murine CD31^+(high)/Ly6C^- Marrow Progenitors.

Dendritic cells (DC) play a critical role in the initiation and regulation of immune responses. Based on immunophenotypic and functional charcateristics, murine CD11c+ DC can be classified into CD11c⁺, CD11b⁺, and B220⁻ “conventional” DC and CD11c⁺, CD11b^−/dim, B220⁺, and Gr-1⁺ interferon-producing cells (IPC) or plasmacytoid dendritic cells (pDC). The developmental origin of IPC remains controversial. In this study, we investigated the capacity of different murine bone marrow subpopulations to acquire immunophenotypic, morphologic and functional characteristics of IPC after in vitro culture with Flt-3L. Total bone marrow (BM) from 129/SvJ mice was sorted into six distinct BM subsets based on surface expression of the myeloid markers CD31 and Ly6C and subsequently cultured with Flt-3L for 8 days. Purified CD31^high/Ly6C⁻ BM cells were the only subset that consistently developed characteristics closely resembling that of IPC. Specifically, these cells expressed high levels of CD11c, low levels of CD11b, the lymphoid marker B220, and the IPC-specific marker 440C. Furthermore, these cells displayed the typical plasmacytoid morphology with eccentrically located nucleus showing mature lymphoid chromatin and lack of nucleoli. Functionally, these cells showed a high proliferation rate as demonstrated by the [³H] thymidine release assay and secreted detectable levels of IFN-alpha when stimulated with CPG ODN 2216. In contrast, all other sorted subsets failed to proliferate in the presence of Flt-3L. Similar results were obtained using BM from B6-RAG^−/− mice. These findings indicate that, although both the CD31^high/Ly6C⁻ and CD31^int/Ly6C⁻ BM subsets contain early progenitor cells corresponding to “cobblestone area forming cells”, only the CD31^high/Ly6C⁻ is able to give rise to IPC when cultured with Flt-3L. Interestingly, CD31^high/Ly6C⁻ BM cells have been previously shown to generate also conventional DC if cultured with GM-CSF. Thus, IPC and DC may derive from common or closely related progenitors. Moreover, our results demonstrate that IPC development, in contrast to that of lymphocytes, does not require RAG proteins.