The delivery of an antigen from the endocytic compartment into the cytosol for cross-presentation is restricted to early immature dendritic cells.

Dendritic cells (DCs) are the only antigen-presenting cell population having a cross-presentation capacity. For cross-presentation, however, the intracellular antigen-processing pathway and its regulatory mechanism have not been defined. Here we report the differences in cross-presentation ability among murine bone marrow-derived immature DC, early immature day8-DC and late immature day10-DC, and fully mature day10 + lipopolysaccharide DC. Day8-DCs and day10-DCs show an immature phenotypic profile but are different in morphology. Day8-DCs can internalize an abundant volume of exogenous soluble ovalbumin (OVA) and result in cross-presentation. In contrast, day10-DCs are not able to cross-present, although they maintain efficient macropinocytosis. Exogenously internalized OVA antigens are stored in the endocytic compartments. The endocytic compartments are temporarily maintained at mildly acidic pH in day8-DCs and are rapidly acidified in day10-DCs after uptake of antigens. We show that OVA antigens accumulated in the endocytic compartments move into the cytosol in day8-DCs but do not in day10-DCs. NH(4)Cl-treatment, which neutralizes the acidic endocytic compartments and/or delays endosomal maturation, restores day10-DCs for transport the stored OVA antigens from the endocytic compartments into the cytosol. Diphenyleneiodonium chloride-treatment, which acidifies the endocytic compartments, decreases an amount of transported OVA antigen into the cytosol in day8-DCs. These data indicate that only the early immature stage of DC interferes with endosomal maturation, even after uptake of exogenous antigens, and then transports the antigens into the cytosol.