Dendritic cells are known to be activated by a wide range of microbial
products, leading to cytokine production and increased levels of membrane
markers such as major histocompatibility complex class II molecules. Such
activated dendritic cells possess the capacity to activate na\"ive T cells. In
the present study we demonstrated that immature dendritic cells secrete both
the YM1 lectin and lipocalin-2. By testing the ligands of these two proteins,
chitosan and siderophores, respectively, we also demonstrated that chitosan, a
degradation product of various fungal and protozoal cell walls, induces an
activation of dendritic cells at the membrane level, as shown by the
up-regulation of membrane proteins such as class II molecules, CD80 and CD86
via a TLR4-dependent mechanism, but is not able to induce cytokine production.
This led to the production of activated dendritic cells unable to stimulate T
cells. However, costimulation with other microbial products overcame this
partial activation and restored the capacity of these activated dendritic cells
to stimulate T cells. In addition, successive stimulation with chitosan and
then by lipopolysaccharide induced a dose-dependent change in the cytokinic
IL-12/IL-10 balance produced by the dendritic cells