Two promoters for the CD5 gene: one operating in T cells and activated B cells and another restricted to resting B cells

The CD5 T cell marker is present on a minute fraction of B cells. These B lymphocytes produce multispecific autoantibodies and generate most of the B chronic lymphocytic leukemias (CLL). However, very little is known regarding the regulation of the gene activity in B cells, compared with T cells.

Material and methods

B cells were isolated from tonsils, CLL blood and the control Daudi cell line cells, while T cells were obtained from normal peripheral blood (PB) and the control Jurkat cell line. Conventional and quantitative RT-PCR, 5' rapid amplification of cDNA ends (RACE), sequencing, Southern blot and in situ hybridization were required in this study.

Results

CD5 transcripts were identified in tonsil and CLL B cells, as well as PB and Jurkat T cells. Using the RACE technique, the 5' region of CD5 cDNA was amplified through an adaptor-ligated 5' primer coupled with a 3' end-specific primer. In these conditions, the conventional exon 1 was not identified in the mRNA from resting B cells. An alternative exon 1 was identified and its transcription confirmed using RT-PCR with appropriate primers and Southern blot. Importantly, the CD5 5'-flanking region contains TATA and CAAT boxes, and recognition sites for Ikaros in the B cells, but not in T cells. Furthermore, after a 48-hour stimulation with PMA, the conventional exon 1 was used in activated B- as in the T-cells.

Conclusions

Alternative exon 1 structures may initiate transcription of CD5 using two different promoters, one being operative in the resting B cells and the other in any T cells and B cells only when activated. Such a finding substantiates our hypothesis of innate (resting ?) and acquired (activated ?) CD5⁺ B cells, which might be relevant to the pathogenesis of nonorgan-specific autoimmune disorders.