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*
Department of Immunology, Osaka Medical Center for Cancer and Cardiovascular Diseases, Osaka, Japan;
Department of Molecular Immunology, Nara Institute of Science and Technology, Nara, Japan;
Laboratory of Host Defenses, Institute for Advanced Medical Sciences, Hyogo College of Medicine, Nishinomiya, Japan;
Osaka Red Cross Blood Center, Osaka, Japan; and
¶
Research and Development Center, Fuso Pharmaceutical Industries, Osaka, Japan
Monoclonal Abs 21 and 132 were raised against human functionally
inactive rIL-18, and plasma IL-18 levels were determined by the
sandwich ELISA established with these mAbs. Plasma IL-18, designated
type 2, was detected by this ELISA, and the levels found were not
consistent with those obtained with the commercially available kit for
determination of functionally active IL-18 (type 1). Type 1 was
detected in all volunteers, whereas type 2 was detected in 
30% of
healthy subjects, and the levels of type 2 in their blood plasma were
high (25–100 ng/ml) compared with those of type 1 (0.05–0.3 ng/ml).
We purified IL-18 type 2 from blood plasma of volunteers with high
IL-18 type 2 concentrations, and its Mr was
determined to be 800 kDa by SDS-PAGE and molecular sieve HPLC. The
purified 800-kDa protein, either caspase-1-treated or untreated,
expressed no or marginal IL-18 function in terms of potentiation of
NK-mediated cytolysis and IFN-
induction, and it barely bound
IL-18R-positive cells. N-terminal amino acid analysis indicated that
the purified protein was IgM containing a minimal amount of
IL-18 proform and its fragment. Again, the purified IgM from IL-18
type2-positive volunteers exhibited cross-reaction with mAb 21 against
IL-18. This band was not detected with 125-2H, an mAb against
functionally active IL-18. Hence, human IgM carries functionally
inactive IL-18 forming a disulfide-bridged complex, and this IL-18
moiety is from 10- to 100-fold higher than the conventional type 1
IL-18 in blood circulation in 30% normal
subjects.
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