Heterogeneous expression of recombination activating genes and surface CD5 in CD3low CD4+ CD8+ thymocytes

Clonal selection in the thymus occurs mostly in the CD4+ CD8+ (double positive; DP) stage. Within DP thymocytes, cells in the CD3low subset are believed to be involved in clonal selection, and this subset is generally considered as a homogeneous population. T-cell antigen-receptor (TCR) signals on DP thymocytes are known to (i) down-regulate recombination activating gene (RAG) expression; and (ii) up-regulate the expression of T-cell function-associated molecules, including the cell surface glycoprotein CD5. The present study examined the expression of RAG and CD5 molecules among the subpopulations of normal adult DP thymocytes. DP thymocytes were fractionated according to their cell surface expression levels of TCR-CD3 complex into CD3dim, CD3lo, CD3med, and CD3hi cells, since TCR expression is known to increase during thymocyte maturation. Down-regulation of RAG mRNA was located between the CD3low and CD3med DP populations. However, within the DP CD3low subpopulation, we found that CD5 varies from low to high expression levels. Upon fractionation of DP CD3low thymocytes into CD5low and CD5high subsets, we were able to detect down-regulation of RAG transcripts within the CD3low subpopulation. Thus, by the criteria of CD5 surface expression and RAG mRNA expression levels, DP CD3low thymocytes can be considered a heterogeneous thymocyte subpopulation.