The developmental pathways of splenic dendritic cells

© 2011 Dr. Priyanka SatheDendritic cells (DC) are professional antigen presenting cells, specialised in the activation of naïve T-cells. DC can be further subdivided into circulating plasmacytoid DC (pDC), and conventional DC (cDC), which are functionally distinct subsets. cDC can be further subdivided into peripheral cDC, and lymphoid tissue resident cDC. In this thesis, we focus on the developmental pathways of murine splenic DC – that is, resident cDC and pDC. DC potential has been found in multiple early precursors, and an immediate resident cDC precursor has been described in the spleen. Two precursors have been described in the bone marrow that may form the developmental bridge between the upstream precursors, and the committed pre-cDC. We and others have described common DC progenitors (CDP), which are c-kitintflt3+M-CSFR+, and negative for lineage antigens (lin-). CDP are restricted to differentiation into cDC and pDC. Another precursor, the macrophage-DC progenitor (MDP) has been described as an intermediate stage on the pathways to macrophage and DC development. The definition of a macrophage-DC progenitor includes potential for both macrophages and steady state cDC within single progenitors. In these studies, the ability of a single progenitor to give rise to both macrophages and steady state DC, and thus the existence of a macrophage DC progenitor, has not been established. Here, we have here investigated the developmental potential of the putative MDP populations. Contrary to previous data, we find that the populations defined in the literature as ‘MDP’ are not restricted to the macrophage and DC lineages, but rather retain potential for other haematopoietic lineages. To detect single progenitors with both macrophage and steady state DC potential, we have developed a clonal assay using M-CSF and flt3-ligand, the cytokines that drive macrophage and steady state DC development respectively. We find no evidence of a progenitor with both macrophage and DC potential within the progenitors previously described as ‘MDP’. To determine whether an MDP exists outside of the populations described as such in the literature, we have examined alternative fractions of the bone marrow for candidate ‘MDP’. We find a population within the lin-M-CSFR+CD16/32lowc-kitint/highsca-1-flt3+ BM fraction gives rise to macrophages and steady state cDC on a clonal level. However, this population is not restricted to these lineages. Thus, we find no evidence for a macrophage DC progenitor as a common intermediate on the pathways to macrophage and DC development. In addition, we have investigated the pathways to plasmacytoid DC development. We find that flt3 ligand (FL)-driven differentiation of pDC occurs via multiple developmental pathways. We have described an intermediate precursor on the pathway from development of common lymphoid progenitors to pDC. Furthermore, we have established the validity of using a history of expression of recombinase-activating gene 1 and the presence of D – J rearrangements at the immunoglobulin heavy chain locus as a marker of developmental history. We here demonstrate that the presence of these markers in pDC indicates a developmental history distinct from those pDC that lack these markers. The description of pDC in vivo both with and without a history of RAG1 expression, or with and without D – J rearrangements thus indicates the operation of multiple developmental routes to pDC in vivo. We have also investigated the role of external factors in the pathways of DC development. We find that FL plays an important role in directing multipotent cells into the DC lineage. We further show that the cytokines interleukin-10 and granulocyte-macrophage colony stimulating factor promote DC development. In this thesis, we have clarified the pathways to the development of the resident dendritic cell subsets, and described some of the factors involved in regulating these pathways. The elucidation of the pathways to steady state resident DC development will form the basis for understanding the regulation of these pathways, and the adaptation of these pathways under conditions of infection