Overexpression of the Runx3 transcription factor increases the proportion of mature thymocytes of the CD8 single-positive lineage

The Runx family of transcription factors is thought to regulate the differentiation of thymocytes. Runx3 protein is detected mainly in the CD4 ^-8⁺ subset of T lymphocytes. In the thymus of Runx3-deficient mice, CD4 expression is de-repressed and CD4^-8 ⁺ thymocytes do not develop. This clearly implicates Runx3 in CD4 silencing, but does not necessarily prove its role in the differentiation of CD4^-8⁺ thymocytes per se. In the present study, we created transgenic mice that overexpress Runx3 and analyzed the development of thymocytes in these animals. In the Runx3-transgenic thymus, the number of CD4^-8⁺ cells was greatly increased, whereas the numbers of CD4⁺8⁺ and CD4⁺8^- cells were reduced. The CD4^-8⁺ transgenic thymocytes contained mature cells with a TCR^highHSA^low phenotype. These cells were released from the thymus and contributed to the elevated level of CD4 ^-8⁺ cells relative to CD4⁺8^- cells in the spleen. Runx3 overexpression also increased the number of mature CD4 ^-8⁺ thymocytes in mice with class II-restricted, transgenic TCR and in mice with a class I-deficient background, both of which are favorable for CD4⁺8^- lineage selection. Thus, Runx3 can drive thymocytes to select the CD4^-8⁺ lineage. This activity is likely to be due to more than a simple silencing of CD4 gene expression.