We investigated the regulation of and the intracellular trafficking involved in the membrane expression of CD1c antigen on activated mature T cells. Membrane expression of this glycoprotein was highly regulated and dependent on the activation state of the cells. The presence of the CD1c antigen on activated peripheral blood mononuclear cells (PBMCs) was confirmed by flow cytometry, reverse transcriptase-PCR (RT-PCR), and immunoperoxidase staining. The RT-PCR analysis of the alpha3- and 3'-untranslated regions of CD1C showed that phytohemagglutinin (PHA) activation induced expression of transcripts that encode the three isoforms (soluble, membrane, and cytoplasmic/soluble). Immunocytochemical studies showed a specific association of CD1c with the cell membrane and a cytoplasmic, perinuclear distribution. Although flow-cytometric staining confirmed the intracellular presence of CD1c, membrane expression on PHA blast cells was not detected. We found that membrane detection of CD1c antigen was temperature dependent. Cell surface binding of the anti-CD1c monoclonal antibody (mAb) was consistently negative at 4 and 37 degrees C but was detected at room temperature (18-22 degrees C). At physiologic temperatures, activated PBMCs showed intracellular accumulation of the anti-CD1c mAbs, indicating that CD1c cycled between cell surface and intracellular compartments. The CD1c exocytosis pathway was sensitive to Brefeldin A, cytochalasin B, and chloroquine.