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Protein kinase C (PKC) plays an essential role in intracellular signal transduction for various cell functions, including concanavalin A (Con A)-induced cap formation. This enzyme is known to be proteolysed by calpain, which is a Ca2(+)-dependent thiol proteinase. As reported previously, in polymorphonuclear leukocytes (PMNs) from beige mouse, the model of Chediak-Higashi syndrome, Con A-induced cap formation significantly increased compared with that in normal mouse. However, after pretreatment of beige PMNs with the thiol proteinase inhibitors leupeptin or E-64, the capping decreased to normal levels. Meanwhile, Con A-induced the translocation of PKC from the cytosolic to membrane fraction within 5 min in both mice, which is essential to the activation of this enzyme. However, after the translocation, an abnormal rapid decline in membrane-bound PKC activity was noted in beige mouse PMNs. Both leupeptin and E-64 also corrected the rapid decline in PKC activity observed in the beige mouse. These findings suggest that the normalization of Con A cap formation in beige mouse PMNs by the thiol proteinase inhibitors is associated with the correction of abnormality in PKC activity.

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We studied the effect of low-density lipoprotein (LDL) oxidized by opsonized zymosan-stimulated polymorphonuclear leukocytes (PMN) on natural killer (NK) cell activity. Oxidized LDL inhibited NK cell activity in a dose-dependent manner, whereas normal LDL left it unaffected. However, oxidized LDL did not inhibit antibody-dependent cell-mediated cytotoxicity (ADCC). Moreover, a positive correlation was observed between the amount of thiobarbituric acid-reacting substances (TBARS) on the sample of oxidized LDL and the degree of inhibition of NK cell activity. We also showed that oxidized LDL suppressed the binding capacity of purified large granular lymphocytes (LGL) to target cells without changing the lytic activity. These results therefore suggest that activated PMN can modulate NK cell activity by oxidizing LDL.