A Synthetic Peptide Homologous to Retroviral Transmembrane Envelope Proteins Depresses Protein Kinase C Mediated Lymphocyte Proliferation and Directly Inactivated Protein Kinase C: A Potential Mechanism for Immunosuppression

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Abstract
CKS‐17, an immunosuppressive peptide homologous to certain retroviral transmembrane envelope protein, has been shown to inhibit lymphocyte proliferation in response to mitogens or alloantigens when covalently attached to bovine serum albumin (CKS‐17‐BSA). To define its site of action, we determined if CKS‐17 conjugated to human serum albumin (CKS‐17‐HSA) could block the direct activation of lymphocytes by phorbol‐12‐myristate‐13‐acetate (PMA) or by a synthetic diacylglycerol, dioctanoylglycerol (DiCs). CKS‐17‐HSA inhibited lymphocyte proliferation in response to PMA and ionomycin in a dose‐dependent manner with up to 88% inhibition occurring with 15μM CKS‐17‐HSA. The conjugated peptide also inhibited the proliferation of lymphocytes in response to DiC8 and ionomycin by up to 57% at 15 μM CKS‐17‐HSA. Based on these findings we investigated the effect of CKS‐17‐HSA on the activity of protein kinase C (PKC), an enzyme directly activated by PMA and DiC8. PKC was isolated chromatographically from the cytosol of human neutrophils or the human lymphoblastoid cell line Jurkat. CKS‐17‐HSA caused a dose‐dependent enzyme inhibition with a concentration giving half‐maximal inhibition (IC50) of ca.3 μM and >95% inhibition at 15 μM CKS‐17‐HSA. Inhibition of PKC by the conjugated peptide was not reversed by increasing concentrations of Ca2+, Mg2+, phosphatidylserine, diolein, or adenosine triphosphate (ATP), indicating that the conjugated peptide did not function as a chelator or competitive inhibitor. In contrast to its effects on PKC, CKS‐17‐HSA did not inhibit the activity of adenosine 3′: 5′‐cyclic monophosphate (cyclic AMP)‐dependent protein kinase (PK‐A) nor the calcium and phospholipid‐independent form of PKC (PK‐M). Moreover the peptide inhibited in vivo PKC activity in cytosol of intact cells and in membrane of PMA‐stimulated cells. These results suggest that the inhibition of lymphocyte proliferation by CKS‐17‐HSA may be due to the direct inactivation of PKC.