Identical expression of CD45R isoforms by CD45RC(+) 'revertant' memory and CD45RC(+) naive CD4 T cells

Naive and memory CD4 T cells are frequently defined by exon-specific monoclonal antibodies (mAb) which stain (or not) high- or low-molecular-weight (MW) isoforms of the leucocyte common antigen CD45. The link between isoform and the naive/memory designation is complicated by the fact that CD4 T cells with a 'memory' phenotype (CD45RA(-), RB-, RC-, or CD45RO(+)) may revert ('revertants') and re-express the high mw isoform (CD45RA(+), RB+, RC+). Isoform expression also changes during normal T-cell development. Furthermore, the picture may be incomplete since an exon-specific mAb will not detect all possible isoforms on a cell. We have used molecular techniques to determine whether revertant CD4 memory T cells were different from naive T cells with respect to CD45R isoform expression. Using the anti-CD45RC mAb OX22 to purify rat lymphocyte subsets, CD45R isoform expression was examined at the mRNA level in CD4 T cells at different stages of development and compared with that of B cells and unseparated lymphocytes. B cells contained abundant message for the highest MW 3-exon isoform ABC, the 2-exon isoforms AB and BC, and the null isoform O. Both immature CD45RC(-) (i.e. CD4(+)8(-) 'single positive' thymocytes, and peripheral Thy-1(+) recent thymic emigrants) and mature CD45RC(-) 'antigen-experienced' CD4 T cells had message for single-exons B, possibly C and for the O exon. In contrast, CD45RC(+) CD4 T cells contained mRNA coding for ABC (low level), AB, BC, B, C (low level) and O (low level), Importantly, there was no difference between CD45RC(+) T cells that had not seen antigen ('truly naive') and CD45RC(+) antigen-experienced revertant memory T cells. This observation has implications for understanding long-term immunological memory.