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Antigen-presenting cells (APC), like dendritic cells (DC), are essential for T-cell activation, leading to immunity or tolerance. Multiple DC subsets each play a unique role in the immune response. Here, a novel splenic dendritic-like APC has been characterized in mice that has immune function and cell surface phenotype distinct from other, described DC subsets. These were identified as a cell type continuously produced in spleen long-term cultures (LTC) and have an in vivo equivalent cell type in mice, namely ‘L-DC’. This study characterizes LTC-DC in terms of marker phenotype and function, and compares them with L-DC and other known splenic DC and myeloid subsets. L-DC display a myeloid dendritic-like phenotype equivalent to LTC-DC as CD11cloCD11bhiMHC-II−CD8α− cells, distinct by high accessibility and endocytic capacity for blood-borne antigen. Both LTC-DC and L-DC have strong antigen cross-presentation ability leading to strong activation of CD8+ T cells, particularly after exposure to lipopolysaccharide. However, they have weak ability to stimulate CD4+ T cells in antigen-specific responses. Evidence is presented here for a novel DC type produced by in vitro haematopoiesis which has distinct antigen-presenting potential and reflects a DC subset present also in vivo in spleen.

Murine splenic stroma has been found to provide an in vitro niche for hematopoiesis of dendritic-like APC. Two distinct cell types have been characterized. The novel “L-DC” subset has cross-presenting capacity, leading to activation of CD8+ T cells, but not activating CD4+ T cells, which is consistent with their CD11cloCD11bhiMHC-II− phenotype. For L-DC, an equivalent tissue-specific APC has been found only in spleen. A second population of CD11chiCD11bloMHC-II+ cells resembling conventional dendritic cells (cDC) can activate both CD4 and CD8 T cells. Production of L-DC but not cDC-like cells is now shown to be dependent on contact between the L-DC progenitor and stroma such that the presence of a Transwell membrane can prevent L-DC development. Since L-DC can be produced continuously in vitro in stromal co-cultures overlaid with bone marrow (BM) progenitors, it was hypothesized that L-DC progenitors are self-renewing. The L-DC progenitor is shown here to be defined by the Flt3−c-kit+Lin−Sca-1+ (F−KLS) subset of adult BM which contains primitive HSC. Since the less primitive F+KLS HSC subset also contains L-DC progenitors, Flt3 does not appear to be a defining marker for this progenitor. Precursors of the cDC-like subset are found only within the F+KLS subset and seed production of a transient population of APC. All data identify differentiation of L-DC from HSC, and of cDC-like cells from DC precursors, which occurs independently of inflammatory signals and is dependent on a splenic stromal microenvironment.

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IntroductionDendritic cells (DCs) are specialized antigen-presenting cells, which capture and process peripheral antigens for presentation to naive T cells. 1 They exist in peripheral tissues as immature cells, displaying characteristics optimal for antigen uptake and surveillance of local environments. Under these conditions, they present apoptotic self cells to T cells to maintain peripheral tolerance. 2,3 On exposure to pathogens, DCs can also become immunogenic, adopting a mature, activated phenotype, a state distinct by additional secretion of proinflammatory cytokines, such as interleukin-12, which promote effector T-cell differentiation. 4 Multiple DC subsets are present in tissues around the body, and in spleen the main subsets include conventional DCs (cDCs) and plasmacytoid DCs (pDCs). cDCs are represented by 2 main populations of CD8␣ Ϫ and CD8␣ ϩ cDC. Morphologically, cDCs are small, nongranular cells 5 that express high levels of CD11c and major histocompatibility complex class II (MHC-II) markers. 6 pDCs are a distinct subset characterized by a small and plasmacytoid morphology, and a pronounced capacity to secrete high levels of IFN-␣ on viral stimulation. 7,8 Isolating rare DC subsets for experimentation is a difficult task, and the isolation and study of DC precursors represent an even more challenging area. It is, however, necessary to delineate precursors to perform lineage experiments to define their downstream progeny. Indeed, examination of DC precursors from early granulocyte-macrophage colony-stimulating factor/interleukin-4 cytokine culture systems has not been possible because of the enforced maturation of precursors. 9,10 Recently, improved cultures of bone marrow (BM) cells with Fms-like tyrosine kinase 3 ligand (Flt3-L) have facilitated the study of DC development, 11,12 with in vitro identification of 2 sequential cDC precursors CD11c int CD115 ϩ c-kit Ϫ pre-DCs and CD11c Ϫ CD115 ϩ c-kit int proDCs. 13 Furthermore, in vivo counterparts of pre-DCs have been identified in spleen as CD11c int CD43 int SIRP-␣ int cells, 14 and counterparts to pro-DC have been identified as Lin Ϫ c-kit lo Flt3 ϩ cells, present in BM but not spleen. 15 Success in these studies highlights the value of in vitro culture systems for investigating DC development from precursors.Previously, development of dendritic-like cells in spleen longterm cultures (LTCs) was demonstrated, which was unique in relation to other culture systems producing DCs. Cultures are continuous, maintaining 2 cell populations distinct in size as "small" and "large" cells. 16 Large cells were previously characterized as a homogeneous population of immature myeloid dendriticlike CD11c lo CD11b hi CD8 Ϫ MHC-II Ϫ/lo cells, named LTC-DCs. These were highly endocytic and weakly able to present antigen to CD4 ϩ T cells. 17,18 The identity of a precursor within the small cell population was resolved by sorting and coculturing small cells above a spleen stromal cell line, which supports LTC-DC development. 16 This resulted in development of larg...