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Characterization of Vβ‐bearing cells in athymic (nu/nu) mice suggests an extrathymic pathway for T cell differentiation

In the present article, the expression of the T cell receptor (TcR) β chain and other T cell molecules was evaluated in surface immunoglobulin‐negative spleen cell populations of young and old BALB/c and C57BL/6 nude mice, using a panel of monoclonal antibodies. The results obtained show that in young nude mice, most Thy‐1high cells do not express other T cell markers. These mice have, however, a sizable population of Thy‐1low cells with the same phenotype of α/β+, CD4−CD8− thymocytes or MRL/lpr peripheral T cells, expressing predominantly genes of the Vβ8 family. The evolution of α/β+ cells in aging nudes is strongly suggestive of an extrathymic pathway of differentiation of these cells since (a) the acquisition of high density TcR and CD3, as well as Thy‐1 or CD4CD8 antigens at the cell surface of nude Vβ+ T cells is not simultaneous; (b) α/β+ cells in nude mice co‐express other T cell markers at random and, even in old mice, they never completely resemble to the predominant high Thy‐1+ CD3+ TcR α/β+, CD4+CD8+ cells of euthymic controls; and (c) BALB/c nude T cells express Vβ11 genes, that are deleted in euthymic BALB/c mice. This latter finding may also indicate differences in the mechanisms of selection of T cell specificities in the thymus vs. the peripheral pools.