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Proteasome activity is required for T lymphocyte aggregation after mitogen activation

The proteasome is a multicatalytic complex of proteases involved in T lymphocyte proliferation and activation through multiple mechanisms. In this study, we investigated its role in lymphocyte aggregation. We found that blocking proteasome activity by a proteasome‐specific inhibitor lactacystin (LAC) prevented clustering of T lymphocytes after stimulation with various mitogens. Expression of adhesion molecules ICAM‐1 and LFA‐1 at cell surfaces of activated T cells was decreased after treatment with LAC. Mechanisms by which the proteasome intervenes in the expression of these adhesion molecules were different. LAC inhibited ICAM‐1 expression at the mRNA level, whereas LFA‐1 inhibition was probably at a post‐translational level. Downregulation of these molecules after proteasome inhibition likely contributes to the observed repression of T cell aggregation. Our results show that the proteasome plays an important role in cell–cell interaction during T cell activation. J. Cell. Biochem. 81:347–356, 2001. © 2001 Wiley‐Liss, Inc.