ANALYSIS OF THE HUMAN CD21 TRANSCRIPTION UNIT REVEALS DIFFERENTIAL SPLICING OF EXON-11 IN MATURE TRANSCRIPTS AND EXCLUDES ALTERNATIVE SPLICING AS THE MECHANISM CAUSING SOLUBILIZATION OF CD21

CD21 is found in a soluble form at low levels in normal human sera and at elevated levels in sera from patients with EBV-associated diseases and B-CLL. Ablation of complement, injection of recombinant soluble C D21 and knock-out of CD21 in mice by gene targeting interfere with T-c ell-dependent immune responses, suggesting that in vivo-generated solu ble CD21 may exert immunoregulatory functions. Soluble CD21 has a mole cular weight of 130 000/135 000, which is equivalent to the entire ext racellular domain. Soluble forms of membrane-anchored molecules may be generated by proteolytic cleavage of the extracellular portion or by the exclusion of the hydrophobic transmembrane region via alternative splicing. To delineate whether alternative splicing of CD21 mRNA creat es transcripts encoding for the soluble form of CD21 we analyzed by PC R CD21 expression in PBLs, spleen, tonsils, bone marrow and in various cell lines. We found that all CD21 mRNA species contained the transme mbrane exons, thus excluding alternative splicing as a factor contribu ting to the serum pool of soluble CD21. Differential splicing of the C D21 transcription unit has also been suggested for exon 11. Within the CD21 gene exons 3, 7 and 11 have a high degree of homology. However, we found in malignant human cell lines and primary lymphocytes from bl ood, bone marrow, tonsils and spleen that exon 11, but not exon 3 or 7 , is subject to alternative splicing. We cloned and sequenced the intr on preceding exon 11 and found that the surrounding splice sites match consensus splice sites. In conclusion, we show that human CD21 exon 1 1 is alternatively spliced in malignant cell lines of lymphoid origin and in purified lymphocytes from blood, tonsils, bone marrow and splee n. We found that both exon 11 lacking and exon 11 containing transcrip ts are always coexpressed and the ratio of of the two forms is > 1. Mo reover, we exclude the possibility that alternative splicing of the tr ansmembrane region is the mechanism leading to soluble CD21. (C) 1997 Elsevier Science Ltd. All rights reserved.