Murine thymocytes possess specific cell surface-associated exoaminopeptidase activities: preferential expression by immature CD4-CD8- subpopulation.

Murine thymocytes are shown to possess at least three well-defined exo-N-aminopeptidase activities on their surface. One of them cleaves the prolyl bond in the synthetic dipeptide nitroanilide Gly-Pro-pNA (Km 0.95 mM and Vmax 8 nmol/h at pH 7.4 and 37 degrees C) and is specifically inhibited by phenylmethane sulfonyl fluoride, diprotin A, Gly-Pro-Ala and Gly-Pro-Gly-Gly. These data further support identification of this enzyme with a serine exopeptidase dipeptidyl peptidase IV (DPP IV), previously reported to be specific for collagen. The two other forms of N-exopeptidase activities are detected when Ala-pNA and Leu-pNA are used as substrates. Leu-aminopeptidase activity (Km 1.4 mM, Vmax 15 nmol/h) and Ala-aminopeptidase activity (Km 4.0 mM, Vmax 20 nmol/h) are inhibited by inhibitors for thiol- and trypsin-like proteinases, i.e. tosyl lysyl chloromethyl ketone, leupeptin and N-ethylmaleimide. Addition inhibition of Leu-aminopeptidase activity by peptstatin, a known inhibitor of carboxyl proteases, suggests that aminopeptidase activity detected with Leu-pNA is different in part from Ala-aminopeptidase activity. Among the various lymphoid cell populations tested, the three aminopeptidase activities are increased by three- to fourfold in the immature CD4-CD8- thymocyte subset as well as in the thymoma BW5147. In contrast, cortisone-resistant thymocytes, lymph node and spleen cells exhibit levels of activities almost similar to that of unfractionated thymocytes. During ontogeny, the levels of these activities are increased four- to sevenfold on fetal thymocytes (from days 14 to 16). Finally, when thymocytes or spleen cells are cultured with a mitogenic concentration of concanavalin A, their proliferative responses are correlated with an enhancement of the aminopeptidase activities (1.3- to 5-fold). From these results, a correlation between the presence of these peptidases on the cell surface of immature and mature lymphoid cells and biological responsiveness is suggested.