Differentiation of early and late human hematopoietic progenitor cells into dendritic cells in vitro

Dendritic cells are specialized to capture antigens, process them and present them to T cells to initiate, regulate and fine tune immune responses towards pathogens and tumours. Therefore these cells play crucial roles in infection, cancer, allergy, autoimmunity and graft rejection. Besides immunity, they can cause anergy and trigger active tolerance, depending on the microenvironment conditions. Dendritic cells are heterogeneous for origin, anatomical localization, phenotype and function. Two subsets are recognized, myeloid and plasmacytoid dendritic cells. Myeloid cells include Langerhans cells, the dendritic cells of non-lymphoid connective tissue and those of lymphoid tissue and lymph. Langerhans cells are found in the epidermis and some mucosal epithelia and have an unique antigenic and morphological profile. They express langerin and contain special inclusions evident at the electron microscopy, i.e. Birbeck granules. The myeloid dendritic cells of non-lymphoid connective tissue express high levels of DC-SIGN/CD209. In vivo, myeloid dendritic cells differentiate from haematopoietic stem cells-derived precursors and localize in peripheral tissues, afterwards they migrate from sites of antigen uptake to lymphoid organs and during this process mature to antigen presenting cells capable of interacting with lymphocytes and stimulate both immune response and peripheral tolerance to self. Dendritic cells express peroxisome proliferator-activated receptors (PPAR)-γ. These receptors are involved in many regulatory pathways, such as those regarding lipid, lipoprotein and glucose metabolism, inflammation, endothelial function, cancer and bone morphogenesis. The stimulation of PPAR-γ on dendritic cells in vitro has been reported to have controversial effects. Knowledge of the stages and mechanisms of control of the differentiation of dendritic cells is a necessary premise to exercise control over these cells, useful for medical purposes. In this research the differentiation potential into myeloid dendritic cells in vitro and the effect of rosiglitazone, an agonist of PPAR-γ, on that differentiation have been evaluated for three different precursors collected from human peripheral blood, precisely CD14+ monocytes, CD34+ progenitors and CD133+ progenitors. Moreover, the cells differentiated from CD14+ monocytes were evaluated for their interaction with inorganic nanoparticles. The results of this work indicate that the differentiation potential of hematopoietic cells into myeloid dendritic cells with different phenotypes changes with the progression of spontaneous in vivo differentiation towards monocytes and that the specific orientation towards Langerhans cells begins very early. The differentiation degree achieved in vitro does not match entirely that seen in vivo, which indicates that more environmental factors are relevant in this respect than those that can be reproduced in vitro. CD133+ cells, which are the most immature progenitors, express PPAR-γ and, if transferred into culture, maintain this expression, that increases during differentiation. Therefore the possibility arises that PPAR-γ agonists or antagonists may be used to modulate this differentiation. Indeed, cells generated from all precursors tested in this research appeared to respond to the PPAR-γ agonist rosiglitazone at the morphological level, with signs of improved differentiation towards more mature dendritic cells. Immature myeloid dendritic cells proved able to incorporate inorganic nanoparticles without apparent damage, at least within the time of culture of the present experiments, which opens the way to explore the possible use of those particles in medicine. Since dendritic cells play crucial roles in infection, cancer, allergy, autoimmunity and graft rejection, thorough knowledge of their subsets differentiation and behaviour will open the path to tools allowing better control of those cells in clinical conditions.