The acquisition of dendritic cell (DC) molecules by T cells has been previously reported. However, it remains unclear whether the transfer is only mono- or bidirectional. In this study, we incubated CMFDA-labeled ovalbumin (OVA)-pulsed DC2.4 (DC2.4(OVA)) cells with Dil-labeled OT II CD4(+) T cells and analyzed the potential bidirectional molecule transfer. We also assessed the distribution of internalized membrane using two engineered DC2.4/la(b) GFP and MF4/TCRCFP DC lines. Our findings showed that membrane molecule transfer is bidirectional. CD4(+) T cells acquired Ia(b), CD11c , CD40, and CD80 from DC2.4(OVA) cells, and conversely DC2.4(OVA) cells took up CD4, CD25, CD69, and T cell receptor from T cells. The internalized molecules acquired by T cells and DCs mostly localized in endosomes and lysosomes, respectively. Taken together, this study demonstrated a novel phenomenon of bidirectional membrane molecule transfer between DCs and T cells. (c) 2007 Elsevier Inc. All rights reserved.