IMMUNOBIOLOGYLymphoid precursors are directed to produce dendritic cells as a result of TLR9ligation during herpes infection*Robert S. Welner,1*Rosana Pelayo,1Yoshinori Nagai,1Karla P. Garrett,1Todd R. Wuest,2Daniel J. Carr,2Lisa A. Borghesi,3Michael A. Farrar,4and Paul W. Kincade11Immunobiology and Cancer Program, Oklahoma Medical Research Foundation, Oklahoma City;2Department of Ophthalmology and Department ofMicrobiology and Immunology, University of Oklahoma Health Sciences Center, Oklahoma City;3Department of Immunology, University of Pittsburgh, PA; and4Center for Immunology and Department of Laboratory Medicine and Pathology, University of Minnesota, MinneapolisHematopoietic stem and progenitor cellswere previously found to express Toll-like receptors (TLRs), suggesting thatbacterial/viral products may influenceblood cell formation. We now show thatcommon lymphoid progenitors (CLPs)from mice with active HSV-1 infection arebiased to dendritic cell (DC) differentia-tion, and the phenomenon is largely TLR9dependent. Similarly, CLPs from micetreated with the TLR9 ligand CpG ODNhad little ability to generate CD19ⴙB lineage cells and had augmented com-petence to generate DCs. TNF␣ mediatesthe depletion of late-stage lymphoid pro-genitors from bone marrow in many in-flammatory conditions, but redirection oflymphopoiesis occurred in TNF␣ⴚ/ⴚmicetreated with CpG ODN. Increased num-bers of DCs with a lymphoid past wereidentified in Ig gene recombination sub-strate reporter mice treated with CpGODN. TLR9 is highly expressed on lym-phoid progenitors, and culture studiesrevealed that those receptors, rather thaninflammatory cytokines, accounted for theproduction of several types of functionalDCs. Common myeloid progenitors arenormally a good source of DCs, but thispotential was reduced by TLR9 ligation.Thus, alternate differentiation pathwaysmay be used to produce innate effectorcells in health and disease. (Blood. 2008;112:3753-3761)IntroductionHematopoietic stem cells (HSCs) give rise to progenitors withpotential to produce blood cell types with remarkably stablecharacteristics. Although this process is tightly controlled, recentfindings suggest that hematopoiesis is dynamic and also responsiveto environmental factors.1The loss of differentiation options isgradual, and T lymphocytes, natural killer (NK) cells, and dendriticcells (DCs) can each be made from multiple progenitors underexperimental circumstances.1,2Indeed, apparently similar DCsarise from distinct myeloid or lymphoid progenitors.3This newperspective raises the possibility that choices are made betweenmultiple pathways to replenish effectors of the immune system.Thus, it is important to learn what normal and disease conditionsfavor particular differentiation routes.Several major categories of DCs have been found in murine bonemarrow (BM). Conventional DCs (cDCs) are competent to presentantigens, whereas plasmacytoid dendritic cells (pDCs) are potentproducers of type I interferon.3The pDCs are divisible into 2 subtypes(pDC1 and pDC2) on the basis of RAG-1 expression and patterns ofcytokine production.4Under experimental conditions, DCs are pro-duced from stem cells, as well as lymphoid and myeloid progenitors.3-5Flk-2/flt-3 ligand and the associated Stat3 signaling pathway areimportant for DC differentiation; consequently, efficient progenitorsbear the Flk-2/flt-3 receptor.3In our experience, the highest yields ofpDCs are obtained from the primitive Lin⫺c-KithiSca-1⫹(LSK) fractionof murine BM.4Two recent reports identified a Lin⫺Flt3⫹c-Kitlo-CD1 15⫹pro-DC population capable of generating pDCs and at least2 categories of DCs.6,7However , greater yields of DCs were producedfrom more primitive progenitors, and some of those are alreadyrestricted to particular DC pathways.7Common lymphoid progenitors(CLPs) represent the main pathway to B lineage cells and include mostprogenitors destined for the NK lineage.8,9CLPs also appear to makesome contribution to DC production.10It is unclear if DCs have variousorigins, depending on environmental conditions and demands.A subset of BM with hybrid characteristics of NK cells and DCswas recently discovered4,11,12and designated interferon-producingkiller dendritic cells (IKDCs). Like NK cells, IKDCs are develop-mentally dependent on Id-2, independent of Notch signals andthrive in interleukin-15 (IL-15).11,13Whereas IKDCs can be madefrom activated NK cells,14IKDC regeneration from transplantedprogenitors did not closely parallel the formation of NK cells.13Recent studies concluded that IKDCs are like NK cells in beingrestricted to type 2 interferon secretion.15,16Thus, IKDCs appear torepresent specialized NK cells, and it remains to be seen whetherthey have any unique functions.Additional perspective on environmental cues for dif ferentiation hascome from the discovery that stem and progenitor cells expressfunctional Toll-like receptors (TLRs 2 and 4), and TLR signals alterlympho-hematopoiesis.17HSCs were stimulated to enter cycle andacquire lineage markers by exposure to the TLR4 ligand lipopolysaccha-ride (LPS). LPS also reduced the differentiation requirements formyeloid progenitors and caused lymphoid progenitors to produce cDCs.This new mechanism represents a potential means for pathogenSubmitted April 11, 2008; accepted May 23, 2008. Prepublished online asBlood First Edition paper, June 13, 2008; DOI 10.1182/blood-2008-04-151506.*R.S.W. and R.P. contributed equally to this work.An Inside Blood analysis of this article appears at the front of this issue.The online version of this article contains a data supplement.The publication costs of this article were defrayed in part by page chargepayment. Therefore, and solely to indicate this fact, this article is herebymarked ‘‘advertisement’’ in accordance with 18 USC section 1734.© 2008 by The American Society of Hematology3753BLOOD, 1 NOVEMBER 2008䡠VOLUME 112, NUMBER 9 For personal use only. at UNIV OF PITTSBURGH HSLS on October 23, 2008. www.bloodjournal.orgFromproducts to signal the rapid generation of innate immune cells within theBM or other tissues.18However , its physiologic relevance in infectiousdiseases had not been explored.TLRs participate in pathologic changes associated with herpesvirus infections.19-21We have now found that lymphoid progenitorsassume other fates after HSV-1 inoculation of normal but notTLR9-deficient mice. Thus, unique differentiation pathways can beused to generate cells of the