Submitted March 28, 2006
Accepted July 31, 2006
Antigen recognition induces phosphatidylserine exposure on the cell surface of human CD8+ T cells
Karin Fischer*, Simon Voelkl, Jana Berger, Reinhard Andreesen, Thomas Pomorski, and Andreas Mackensen
Dept. of Hematology/Oncology, University of Regensburg, Regensburg, Germany
Institute of Biology, Cell Biophysics, Humboldt University Berlin, Berlin, Germany
* Corresponding author; email: karin.fischer{at}klinik.uni-regensburg.de.
In eukaryotic cells the phospholipid phosphatidylserine (PS) is restricted to the inner plasma membrane leaflet. This lipid asymmetry which is maintained by the concerted action of phospholipid transport proteins is mainly lost during apoptosis. Here, we demonstrate that primary human CD8+ cytotoxic T lymphocytes (CTL) expose PS upon T cell receptor (TCR)-mediated antigen (Ag) recognition. In contrast to PS externalisation on apoptotic cells, activation-induced PS exposure is less pronounced and reversible. Fluorescence microscopic analysis revealed that PS is distributed inhomogenously over the plasma membrane and concentrated in membrane lipid raft domains at the immunological synapse. By studying the activity of PS transport proteins using a fluorescence-labeled PS analogue, we found that activation of CTL inhibited the flippase-mediated inward directed PS transport without affecting the outward transport. Shielding of exposed PS by annexinV protein during Ag recognition diminished cytokine secretion, activation and cell-cell clustering of Ag-specific CTL. In summary, our data demonstrate for the first time that externalized PS on Ag-stimulated CTL is linked to T cell activation and probably involved in cell-cell contact formation at the immunological synapse.
