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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.

The expression of CD4 during T cell development is a highly regulated process. Numerous regulatory elements have been identified including a promoter, two distinct enhancers and a silencer. Here we report a methylation site in the first intron of the CD4 gene that is specifically demethylated in cells which have previously, or are currently expressing CD4. In addition, this site becomes progressively demethylated as T lymphocytes differentiate from double-negative to double-positive to CD4 single-positive thymocytes, and finally to CD4 single-positive peripheral T lymphocytes. This specific and progressive demethylation suggests that this site represents another potential control region for the regulation of CD4.

Lyb-2/CD72 is a 45-kDa mouse B cell surface protein that binds CD5 (Ly-1) and has been shown to induce B cell proliferation upon mAb binding. The serologically defined Ly-19.2 and Ly-32.2 lymphocyte alloantigens have mouse strain distribution patterns similar to that of the Lyb-2/CD72 alleles and map to the same region on chromosome 4 as Lyb-2/CD72. Our recent isolation of the Lyb-2a, -2b, and -2c cDNA has enabled us in this report to examine the relationship between Ly-19, Ly-32, and Lyb-2/CD72. A rat T cell line transfected with a mouse Lyb-2a cDNA is recognized by Ly-19.2-specific mAb, whereas transfectants expressing the Lyb-2b cDNA are recognized by both Ly-19.2 and Ly-32.2-specific mAb. Cell surface iodination immunoprecipitation analysis from Lyb-2a cDNA transfectants using Lyb-2a- and Ly-19.2-specific mAb as well as from Lyb-2b cDNA transfectants using Lyb-2b-, Ly-19.2-, and Ly-32.2-specific Ab, produced immunoprecipitates containing comigrating 45-kDa polypeptides. Preclearing studies with these transfectants indicate that the immunoprecipitated proteins represent the same polypeptide chain. These results demonstrate that the mouse Ly-19.2 and Ly-32.2 alloantigens are in fact the B cell differentiation Ag Lyb-2/CD72.

MRL lpr/lpr (lymphoproliferative, lpr) mice demonstrate an age-dependent lymphoproliferation and development of autoimmunity. Characteristic of the lymphoproliferation in these mice is the accumulation of large numbers of CD4-CD8-(CD4-8-),CD3+ T lymphocytes in their lymph nodes. The development of the CD4-8- cells, which also aberrantly express B220 and CD44 (Pgp-1) but are CD2-, has been shown to be thymus dependent. An unusual feature of lpr CD4-8-T lymphocytes is that although they appear unresponsive to stimulation, as defined by proliferation and IL-2 production, they have undergone thymic negative selection. As thymic deletion normally occurs at the CD4+CD8+ (CD4+8+) stage, this raises the dilemma that lpr CD4-8- T lymphocytes have either previously been CD4+8+, or they are able to undergo thymic selection as CD4-8- cells. We have addressed this question by examining the methylation status of the CD8 gene in MRL lpr CD4-8- lymph node cells. Demethylation of the CD8 gene has been shown to be an indicator of previous CD8 expression. We find that the CD8 gene in lpr CD4-8- lymph node cells, as well as in the abnormal B220+ CD4-8- lpr thymocytes, is demethylated, suggesting that these cells have previously expressed CD8. In addition, we find that the lpr CD4+8+ thymocyte population contains an increased percentage of atypical B220+, CD44+ cells that are virtually all CD2+. Taken together, these data are consistent with the lpr CD2-CD4-8- population of LNC having arisen from a CD2+ CD4+8+ thymic stage of differentiation.