Our understanding of the basic immunobiological properties of DC has been significantly advanced over the years. This has not only provided good explanations for the problems encountered, but also stimulated many new

ideas regarding the potential ways forward aimed to improve DC therapy in a more fundamental way. The important issues lie within DC heterogeneity and functional plasticity, and hence their immunogenic versus tolerogenic properties or potentials. LBH589 It has gradually become clear that DC are not a homogeneous population, and questions have also been raised about the origin and nature of the monocyte-derived, DC-like cells generated in vitro 27. The ability of these cells to provide activation signals, of both antigen-specific and non-specific triggers, can vary vastly among DC subsets or lineages, and depends on their functional status 28–31. Among them, a unique human DC subset (CD11c+CD141+), with superior antigen cross-presentation capacity and expressing the XC chemokine

receptor 1 (XCR1+), has recently been identified by several groups as the homologue of mouse CD8α+ DC 32–35. As with their murine counterparts, this type of DC was found to be effective activators of CD8+ cytotoxic T cells, which selleck products may have important implications in the design of new human DC vaccines. Moreover, in addition to subset-dependence, the functional properties of DC are also associated with the maturation status of the cell. Immature DC are in a so-called “antigen-uptake mode”, with low cell surface expression of MHC class I and class II molecules, which

can be rapidly enhanced upon exposure to maturation or activation signals, acquiring subsequently the “antigen-presenting mode”. The low MHC expression may therefore affect the ability of immature DC to present antigen to T cells. Under certain conditions, DC can even exert tolerogenic effects by producing immunosuppressive molecules, HAS1 or by inducing regulatory T cells, to inhibit the immune system 1, 8, 24, 36. The concept of tolerogenic DC has become far more appreciated. It is now recognised that while immunogenic DC play an important role in host defence, their tolerogenic counterparts are crucial for the maintenance of self-tolerance, being part of a built-in mechanism to avoid autoimmunity 37. It has been demonstrated that, under the tumourigenic microenvironment, the host DC possessed a typical tolerogenic, or regulatory, phenotype 38. DC, as a double-edged sword, can therefore induce either active immunity or tolerance depending on their functional conditions. The types and functional status of DC, hence the immunogenic “quality” or nature of the cell vectors employed for tumour vaccine delivery, are therefore of critical importance. Various attempts have subsequently been made in order to generate DC with a highly immunogenic phenotype.