Expression of a novel isoform of CD45 by human CD4+ T lymphocytes in peripheral blood, synovial fluid, and cell lines

CD45 is a transmembrane glycoprotein, expressed as variably-spliced isoforms on most hematopoietic cells. The isoforms previously described in humans are generated by alternative splicing of exons 4-6. We have detected mRNA for a new isoform of CD45 expressed in a subset of T-helper cells by splicing out of exons 4-7,and designated CD45-f38. There is no known antibody specific to this isoform, and the stimulus for this alternative splicing is not known. Detection of f38 by RT-PCR was carried out using primers which span the region of interest.The amplified products were transferred to nylon and probed with an internal oligonucleotide probe crossing the exon 3-8 junction. The f38 isoform was not detectable in PBL from normal individu als, however it was detected in PBL of one individual with a recurrent viral infection, and in SF cells of a patient with T cell lymphoma and arthritis. Cell sorting demonstrated that in PBL CD45-f38 is expressed in the CD4+ CD8- population. The SF cells were homogeneous by VbTCR. analysis, made IL-2 message, and demonstrated surface expression of abTCR. CD4, CDlla, CD29, CD44, and CD45RO. In order to characterize expression of f38 we have developed a sensitive non-radioactive detection method and examined ten cell lines, finding message for f38 in the cell lineCEM which is CD4 + , abTCR-!-, CD45RO+. Lower molecular weight isoforms (including RO) are expressed on activated cells. These findings suggest preferential expression of f38 by activated CD4+, abTCR+, CD45RO+ T cells, and studies in this cell line will help identify conditions influencing CD45-f38 expression.