Sca-1 expression defines developmental stages of mouse pDCs that show functional heterogeneity in the endosomal but not lysosomal TLR9 response

Plasmacytoid dendritic cells (pDCs) play an important role in innate and adaptive immunity and were shown to be identical to previously described natural interferon (IFN)-α-producing cells. Here, we describe two functionally distinct pDC subpopulations that are characterized by the differential expression of stem cell antigen-1 (Sca-1; Ly-6A/E). Sca-1^- pDCs are mainly found in the BM, appear first during development, show a higher proliferative activity, and represent the more precursor phenotype. Sca-1⁺ pDCs are mostly located in secondary lymphoid organs and represent a later developmental stage. Sca-1^- pDCs give rise to an Sca-1⁺ subset upon activation or in response to endogenous type I IFN. Interestingly, in contrast to Sca-1^- pDCs, Sca-1⁺ pDCs are defective in IFN-α production upon endosomal TLR9 stimulation, whereas lysosomal signaling via TLR9 is functional in both subsets. Gene expression analysis revealed that osteopontin is strongly upregulated in Sca-1^- pDCs. These data provide evidence for the molecular basis of the observed functional heterogeneity, as the intracellular isoform of osteopontin couples TLR9 signaling to IFN-α expression. Taken together, our results indicate that Sca-1^- pDCs are an early developmental stage of pDCs with distinct innate functions representing the true murine natural IFN-α-producing cells.