ALTERNATIVELY SPLICED FORMS OF MICA AND MICB LACKING EXON 3 IN A HUMAN CELL LINE AND EVIDENCE OF THEIR PRESENCE IN HUMAN PERIPHERAL BLOOD MONONUCLEAR CELLS

2.1
TITLE: ALTERNATIVELY SPLICED FORMS OF MICA AND MICB LACKING EXON 3 IN A HUMAN CELL LINE AND EVIDENCE OF THEIR PRESENCE IN HUMAN PERIPHERAL BLOOD MONONUCLEAR CELLS

Y. Zou,¹ P. Stastny.¹

¹Department of Internal Medicine, University of Texas Southwestern Medical Center, Dallas, TX, USA

MICA and MICB genes encode MHC class I chain–related proteins, which are polymorphic, do not appear to present peptides or associate with &bgr;2–microglobulin, and are expressed predominately in epithelial cells, endothelial cells, fibroblasts and several cultured cell lines. Alternatively spliced isoforms are known to exist for HLA–A and B, as well as HLA–G and the MHC class I–related gene, MR1. In the course of cloning MICA and MICB cDNA from the colon carcinoma cell line HCT116, it was observed that two kinds of cDNAs were obtained: a 1161 bp cDNA representing full length MICA or MICB and a shorter variant of 873 bp. The sequences of these short cDNAs were those of the correct MICA or MICB alleles but lacking exon 3. They were found in 7/72 clones examined or about 10% and were called MICA2 and MICB2. MICA1 and MICA2 were transfected into Chinese hamster ovary (CHO) cells and found to be expressed both in the cells and on their surface. PCR with a primer based on a sequence formed by the joining of exons 2 and 4 allowed detection of the isoforms in different cells. The alternatively spliced cDNA was found in the monocyte line U937, the B cell lines YAR and BM15, and in freshly prepared normal PBMC, in addition to the cell line HCT116. The functional significance of these isoforms of MICA and MICB is at present unknown.