A soluble form of ICAM-1 (sICAM-1) have been observed in normal human serum (Roth-lein et al., J. Immunol. 147, 3788-3793) and at elevated levels in inflammatory and tumor bearing status (Seth et al., Lancet, 338, 83-84; Giavazzi et al., Cane. Res. 52, 2628-2630; Harning et al., Cane. Res., 51, 5003-5005). However, the mechanism to produce the sICAM-1 has been still unknown. In this report we presented evidence for the presence of the mRNA specifically encoding sICAM-1, which is probably generated by alternative splice donor site selection. A 19-base deletion occurred right upstream of the transmembrane region gave rise to reading frameshift and eliminate the entire transmembrane and cytoplasmic domains, resulting in incapability of ICAM-1 molecules to reside in the membrane. A reverse transcription-polymerase chain reaction (RT-PCR) using a primer pair specific to sICAM-1 revealed a positive expression in all tissues analyzed, though the amount and the ratio to the conventional species varied slightly from tissue to tissue. Inflammatory cytokines displayed a complex pattern in the ICAM-1 mRNA expression depending on the combination of cytokines and the cultured cell lines used.
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