Myelopoiesis in spleen-producing distinct dendritic-like cells

Dendritic cells (DC) represent a heterogeneous class of antigen presenting cells (APC). Previously we reported a distinct myeloid den-dritic-like cell present in spleen, as an in vivo counterpart to cells produced in murine spleen long-term cultures (LTC-DC). These cells, named ‘L-DC’, were found to be functionally and phenotypically distinct from conventional (c)DC, plasmacytoid (p)DC and monocytes. These results suggested that spleen may represent a niche for development of L-DC from endogenous progenitors. Adult murine spleen has now been investigated for the presence of L-DC progenitors. Lineage-negative (Lin)ckitlo and Linckithi progenitor subsets were identified as candidate populations, and tested for ability to produce L-DC; in vitro upon co-culture with the spleen stromal line STX3, and in vivo after adoptive therapy into mice. Both subsets colonized STX3 stroma in vitro for L-DC production, indicating that they con-tained either a common or two distinct progenitors for L-DC. However, only the Linckithi subset gave progeny cells after adoptive trans-fer into lethally irradiated mice. In vivo development was however multilineage and not restricted to L-DC development. Multilineage reconstitution reflects long-term reconstituting haematopoietic stem cells (LT-HSC), suggesting a close relationship between L-DC pro-genitors and LT-HSC. L-DC were however produced in vivo in much higher number than monocytes/macrophages and cDC, indicating the presence of a specific L-DC progenitor within the Linckithi subset. A model is advanced for development of L-DC directly from haematopoietic progenitors in spleen and dependent on the spleen microenvironment