HIGH EXPRESSION OF MEMBRANE COFACTOR PROTEIN OF COMPLEMENT (CD46) IN HUMAN LEUKEMIA-CELL LINES - IMPLICATION OF AN ALTERNATIVELY SPLICED FORM CONTAINING THE STA DOMAIN IN CD46 UP-REGULATION

Human membrane cofactor protein (MCP, CD46) is a receptor for the meas les virus and serves as a complement regulator which protects host cel ls from autologous complement attack. MCP is highly polymorphic due to a variety of mRNA splice products. The levels of MCP expression on T and myeloid cell lines are usually two-eightfold higher than those on their normal counterparts, whereas Burkitt's lymphoma B cell lines exp ress less MCP than B cell lineages carrying no EB virus. The molecule has a Ser/Thr-rich (ST) domain adjacent to the functional domain, name ly short consensus repeats (SCR). The ST domain and a cytoplasmic tail (CYT) contribute to the MCP polymorphism. The ST domain is encoded by three exons (A, B and C) and major ST isoforms are STABC, STBC and ST C. The authors investigated the relationship between the expression le vels and isoform usage of MCP by flow cytometry using specific antibod ies against STA and STC, by reverse transcriptase-polymerase chain rea ction (RT-PCR) with size markers for each splice variant, and by RT-PC R/Southern blotting using a specific probe for STA. The results were ( 1) the profiles of mean shifts of myeloid and T cell lines were STC < STA on flow cytometry while those of B cell lines and normal blood cel ls were STA < STC; (2) all cell lines tested by RT-PCR expressed the m essages for the isoforms STBC/CYT1, STC/CYT1, STBC/CYT2, and STC/CYT2. The band for STABC/CYT2 overlapped that for STC/CYT1, and the band fo r STABC/CYT1 was marginal in all cell lines examined; (3) semi-quantit ative analysis of the STABC isoforms by Southern blotting indicated th e presence of high levels of the STABC messages in myeloid and T-cell lines in comparison with B lymphoid cells and normal leucocytes. Thus, the quantity of MCP expressed parallels the STABC message level, whic h is up-regulated in T and myeloid leukaemia cell lines.