Murine dendritic cells derived from myeloid progenitors of the thymus are unable to produce bioactive IL-12p70.

Dendritic cells (DC) are present at low density in the thymus where they mediate negative selection of self-reactive thymocytes. Previous reports suggest that thymic DC (TDC) are a single population of lymphoid-related DC. In this study, we documented the presence in the adult mouse thymus of an additional population of TDC exhibiting a myeloid phenotype (CD11c(+) CD8alpha(-) CD11b(+)). This population, which can be purified, represented approximately 20% of the total TDC and differs from the population of lymphoid TDC (CD11c(+) CD8(+) CD11b(-)) by its incapacity to produce IL-12p70 under double stimulation by LPS and anti-CD40. Furthermore, using an original culture system allowing expansion of DC from myeloid progenitors, we demonstrated that DC exhibiting a similar myeloid phenotype can be derived from a common DC/macrophage progenitor resident in the adult mouse thymus. We found that, in contrast with myeloid splenic DC expanded in the same conditions, these cultured TDC were unable to produce IL-12p70 under double stimulation by LPS and anti-CD40 or LPS and IFN-gamma. Thus, our results suggest that 1) adult mouse thymus contains at least two phenotypically and functionally distinct populations of DC; and 2) cultured myeloid DC derived from thymus and spleen differ by their ability to produce IL-12p70. The mechanisms underlying the differences in IL-12-secreting capacities of the cultured splenic and thymic DC are under current investigation.