Memory CD4+ T cells in man form two distinct subpopulations, defined by their expression of isoforms of the leucocyte common antigen, CD45.

The leucocyte common antigen, CD45 (T200, Ly-5), exists in a number of isoforms generated by differential usage of three exons that code for an extracellular region close to the NH₂ terminus. Use of antibodies to different isoforms of CD45 has lead to the identification of two subsets of CD4⁺ T cells in rat, man and mouse. Data on the functions of the two rat CD4⁺ T-cell subsets isolated on the basis of different levels of expression of exon B (and/or C) are largely concordant with those obtained from the two subsets of human CD4 ⁺ T cells defined by their levels of expression of exon A. However, results presented in this paper on the CD45 phenotype of rat T cells that produce interferon-gamma (IFN-γ) are not compatible with the human data, if it is assumed that there are only two functional subsets of CD4⁺ T cells. The data could, in principle, be reconciled if the expression of exons A and B defined three rather than two subsets, and this possibility has now been examined in man where monoclonal antibodies against both A and B exon products are available. The results show the presence of a third CD4⁺ T-cell subset and that this extra subset is contained within the population originally shown to provide memory T-cell function. [ABSTRACT FROM AUTHOR]