Protein polymorphism of human IL-18 identified by monoclonal antibodies

Six mAbs were raised against human 'functionally inactive' recombinant IL-18, ELISA for determination of 'functionally inactive' forms of IL-18 were established using two of these mAbs (#21 and #132), and inactive species of IL-18 protein were examined with human blood plasma and macrophages (Mp). In 6-day GM-CSF-treated monocytes, namely Mp, the mAb #21 recognized the IL-18 proform (24 kDa) and a 48 kDa dimer by immunoblotting. In contrast, only the 24 kDa species was detected as a relatively faint band with a commercial mAb against 'active' IL-18. No IL-18 species was detected in premature monocytes. Thus, the dimeric IL-18 was produced in Mp and detectable with the mAb we established. In blood plasma of normal subjects and patients, the #21-recognizable IL-18 was also detected by ELISA, the levels of which were not consistent with those obtained with the commercially available kit for determination of 'functionally active' IL-18. We designated the former as type 2 and the latter as type 1. Strikingly, IL-18 type 1 was detected in all volunteers while type 2 was detected in similar to 30% of healthy subjects, and the levels of type 2 were high (10-100 ng/ml) compared to those of type 1 (0.02-0.55 ng/ml) in their blood plasma. In patients with atopic dermatitis, the mean value of type 1 was high (200 ng/ml) compared to those of normal subjects (0.122 ng/ml) and patients with lung cancer (0.113 ng/ml). Production of high type 1 may be associated with an immunomodulatory state in atopic dermatitis. The levels and frequencies of IL-18 type 2 were not significantly changed among these populations. Hence, large amounts of type 2 species are produced in monocyte-Mp differentiation, and their levels and frequencies are unchanged in blood plasma irrespective of the levels of type 1.