Activated T cells transcribe an alternatively spliced mRNA encoding a soluble form of Qa‐2 antigen.

Among the best characterized non‐classical mouse major histocompatibility antigens are the Qa‐2 molecules. These proteins can serve as targets for allogenic cytotoxic T cells and as signal transducing molecules. They are structurally similar to H‐2 transplantation antigens in their N‐terminal and beta 2‐microglobulin binding domains but differ at their C‐termini. While the H‐2 antigens span the cell membrane, the Qa‐2 molecules are attached to the cell surface via phospholipid anchors. The genetic information encoding this attachment is contained in exon 5. In concanavalin A activated splenocytes the expression of membrane bound Qa‐2 antigens declines and, simultaneously, soluble forms of Qa‐2 molecules are secreted. We demonstrate here that the soluble Qa‐2 polypeptides are translated from alternatively spliced mRNAs lacking exon 5, while the membrane forms are encoded by the full‐size transcripts. In cultured cells the alternative splicing of the Qa‐2 message is induced by T‐cell activation splicing of the Qa‐2 message is induced by T‐cell activation with concanavalin A. The canonical mRNA encoding the membrane form of Qa‐2 predominates in unstimulated mouse tissues but the cultured cell lines, like activated T cells, express enhanced levels of the truncated mRNA. In some cell lines almost all Qa‐2 transcripts lack exon 5. For example, in L cells, mRNAs encoding soluble Qa‐2 molecules are at least 10 times more abundant than Qa‐2 transcripts encoding phospholipid anchored antigens. These findings are discussed in terms of potential functions of membrane bound and secreted Qa‐2 molecules.