Mouse strain variation in Ly‐24 (Pgp‐1) expression by peripheral T cells and thymocytes: implications for T cell differentiation

The cell surface glycoprotein Ly‐24 has been proposed as a useful marker for the identification of in vivo‐primed T cells. Analysis of Ly‐24 surface expression by T cells from different mouse strains has shown variation in Ly‐24 expression that is not H‐2 linked; however, mice of the Ly‐24.1 allele (e.g. BALB/c) express relatively high amounts, whereas Ly‐24.2 strains (e.g. C57BL/6) are low expressors. In BALB/c (Ly‐24 high) and C57BL/6 (Ly‐24 low) mice, Ly‐24 was expressed by both CD4^‐CD8⁺ and CD4⁺CD8^‐ subpopulations of single‐positive T cells and thymocytes. Among CD4^‐CD8^‐ thymocytes, the overall expression of Ly‐24 was similar in both mouse strains. Analysis of CD4⁺ and CD8⁺ single‐positive thymocytes from newborn and adult BALB/c mice showed that the neonatal population contained fewer Ly‐24⁺ cells. However, using the cell surface markers J11d and CD3, neonatal single‐positive thymocytes were found to contain larger numbers of cells with the Ly‐24^‐J11d⁺CD3 low to negative phenotype. Taken together, these results show that in BALB/c (Ly‐24 high) mice, as soon as functional mature phenotype (CD3⁺) CD4⁺ and CD8⁺ single‐positive thymocytes are generated, they already express Ly‐24. These data cast doubt on the usefulness of Ly‐24 expression as a universal marker of in vivo‐primed T cells and suggest that in BALB/c mice thymus migrants may well be Ly‐24⁺. Expression of Ly‐24 by thymocytes is discussed in the context of current models of intrathymic T cell differentiation.