A soluble form of CTLA-4 generated by alternative splicing is expressed bynonstimulated human T cells

CTLA-4, expressed by activated T cells, transduces an inhibitory signal. We show here that PCR amplification of the coding sequence of CTLA-4 in nonst imulated human T lymphocytes results in the amplification of two transcript s of 650 and 550 bp. Sequencing shows that the larger form codes for membra ne CTLA-4 and the 550-bp transcript is a spliced variant in which exon 2 co ding for the transmembrane region is deleted. This spliced cDNA has been na med CTLA-4delTM. The splicing induces a frame shift which results in the ad dition of 22 extra amino acids before a translational termination. Activati on of T cells with phorbol 12-myristate 13-acetate plus ionomycin or anti-C D3 plus anti-CD28 monoclonal antibodies induces a suppression of CTLA-4delT M mRNA expression associated with a preferential expression of the membrane CTLA-4 mRNA, showing that CTLA-4delTM mRNA expression is restricted to non activated T cells. A soluble immunoreactive form of CTLA-4 was detected in the serum of 14/64 healthy subjects. These results suggest that nonstimulat ed T cells may constitutively produce a soluble form of CTLA-4 which may ha ve an important role in the regulation of immune homeostasis.