Ate na�ve CD8+ T cytotoxic i and na�ve CD4+ T helper lymphocytes, respectively [7, 10, i 69]. For a na�ve T lymphocyte to become an effector cell i different signals are required. The first signal comes from the direct interaction of the T cell receptor (TCR) of the na�ve T lymphocyte with the peptide bound to the MHC i molecule (Signal 1). The second signal required for na�ve T i cell activation