HETEROGENEOUS EXPRESSION OF RECOMBINATION ACTIVATING GENES AND SURFACE CD5 IN CD3(LOW) CD4(+) CD8(+) THYMOCYTES

Clonal selection in the thymus occurs mostly in the CD4(+) CD8(+) (dou ble positive; DP) stage. Within DP thymocytes, cells in the CD3(low) s ubset are believed to be involved in clonal selection, and this subset is generally considered as a homogeneous population. T-cell antigen-r eceptor (TCR) signals on DP thymocytes are known to (i) down-regulate recombination activating gene (RAG) expression; and (ii) up-regulate t he expression of T-cell function-associated molecules, including the c ell surface glycoprotein CD5. The present study examined the expressio n of RAG and CD5 molecules among the subpopulations of normal adult DP thymocytes. DP thymocytes were fractionated according to their cell s urface expression levels of TCR-CD3 complex into CD3(dim), CD3(lo), CD 3(med), and CD3(hi) cells, since TCR expression is known to increase d uring thymocyte maturation. Down-regulation of RAG mRNA was located be tween the CD3(low) and CD3(med) DP populations. However, within the DP CD3(low) subpopulation, we found that CD5 varies from low to high exp ression levels. Upon fractionation of DP CD3(low) thymocytes into CD5( low) and CD5(high) subsets, we were able to detect down-regulation of RAG transcripts within the CD3(low) subpopulation. Thus, by the criter ia of CD5 surface expression and RAG mRNA expression levels, DP CD3(lo w) thymocytes can be considered a heterogeneous thymocyte subpopulatio n.