ACTIVATION INDUCES OVEREXPRESSION OF HLA CLASS I MOLECULES IN T LYMPHOCYTES.
G. Frumento, S. Franchello, D.E. Geraghty* and G.B. Ferrara,  Immunogenetics Laboratory, National Institute for Cancer Research c/o Advanced Biotechnology Center, Genova, Italy and *Human Immunogenetics Program, Fred Hutchinson Cancer Research Center, Seattle, WA.


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Activation by mitogens, but also by MoAbs recognizing CD molecules, like CD2, CD3 and CD28, induces both in CD4+ and CD8+ T lymphocytes a significant increase in membrane Class I expression. The phenomenon peaks at 24 hrs after activation, when levels three times higher than those of resting cells are achieved, and lasts for several days. All the three subclasses A, B and C are equally increased, and there are no differences in Class I overexpression whether cells have naive or memory phenotype. Finally, the overexpression can be prevented by Brefeldin A, suggesting that it is the outcome of massive arrival at the cell membrane of newly synthesized Class I molecules. This phenomenon causes the rapid and almost complete replacement of preexisting peptide-Class I complexes with complexes assembles after activation, possibly resulting in presentation of peptides derived from activation induced proteins.