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

SummaryDendritic 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. NH4Cl‐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.