The T cell receptor repertoire of CD8(+)CD28(-) T lymphocytes is dominatedby expanded clones that persist over time

The costimulatory molecule CD28 is expressed on almost all CD4(+) T cells, but on only a portion of CD8(+) T cells in healthy human adults. alpha beta T cells may thus be divided into three phenotypically and functionally dif ferent subsets: CD4(+), CD8(+)CD28(+) and CD8(+)CD28(-). Using peripheral b lood lymphocytes from six healthy adults, we have studied the T cell recept or (TCR) repertoire within these subsets by analysis of the distribution of lengths of the complementarity determining region 3 (CDR3) of the beta var iable (BV) transcripts and flow cytometric analysis of TCR VP usage. Expand ed CDR3 lengths were identified in 86% of BV families within CD8(+)CD28(-) T cells, but in only 4% within CD4(+) T cells, and 35% within CD8(+)CD28(+) T cells (P , 0.01). When sequenced, the majority of expanded peaks were fo und to be dominated by single clones. Identical expanded clones were found within both CD8(+)CD28(+) and CD8(+)CD28(-) subsets, consistent with the be lief that CD8(+)CD28(-) T cells descend directly from CD8(+)CD28(+) T cells . Greatly expanded CD28(-) clones were found within both CD8(+) and CD4+ su bsets and persisted at the same magnitude for up to 4.5 years of observatio n. The finding of a small proportion of cells expressing Ki-67 showed that some of these clonally expanded cells were in the active stages of the cell cycle, but few of the cells expressed activation markers CD69, CD25, CD71 or CD122. One likely explanation for the persistence of expanded peripheral lymphocyte populations in healthy individuals is the presence of persisten t antigen.