Allele-specific expression of the mouse B-cell surface protein CD72 on T cells

CD72 is a 45 000 Mr mouse B-cell surface glycoprotein involved in B-cell proliferation and differentiation. Expression of mouse CD72 is thought to be restricted to the B-cell lineage. We recently demonstrated that the monoclonal antibodies K10.6 and B9.689, previously defined as recognizing the mouse lymphocyte alloantigens Ly-19.2 and Ly-32.2, respectively, recognize specific alleles of CD72. Early studies using antibody-mediated cytotoxicity assays demonstrated that K10.6 and B9.689 react with B cells, several T-cell lines, and a subset of peripheral T cells. These findings led us to consider the possibility that CD72 might also be expressed on a subset of T cells. In this report we demonstrate that CD72 is constitutively expressed on a fraction of peripheral T cells isolated from strains of mice expressing the CD72(b) allele, but not the CD72(a) or CD72(c) alleles. Three days after activating T cells with concanavalin A or plate-bound CD3-specific mAb, CD72 is expressed on a larger fraction of peripheral T cells as well as a fraction of thymocytes from mouse strains expressing the CD72(b) allele. CD72 is expressed on both the CD4(+) and CD8(+) thymocyte and peripheral T-cell subsets. No CD72 expression is detected on activated thymocytes or peripheral T cells from mouse strains expressing the CD72(a) or CD72(c) alleles. Expression of CD72(b) on peripheral T cells was confirmed by northern blot analysis demonstrating CD72 mRNA expression. These results demonstrate that CD72 expression is not restricted to B lineage cells in mouse strains expressing the CD72(b) allele; instead, a population of T lineage cells in these mice also expresses CD72.