CD4 + T cell lineage integrity is controlled by the histone deacetylases HDAC1 and HDAC2

Helper T cells (CD4+) and cytotoxic T cells (CD8+) are distinct lineages with specialized functions. While the transcription factor Th-POK is known to be a master regulator of CD4+ lineage commitment, the epigenetic mechanisms that maintain this lineage integrity throughout the life of a T cell remain poorly understood. This study investigates whether histone deacetylases (HDACs) 1 and 2 act as epigenetic gatekeepers to prevent CD4+ T cells from acquiring CD8+ effector characteristics.

The researchers generated mice with a T cell-specific, combined deletion of HDAC1 and HDAC2 (HDAC1-2 cKO) using the Cd4-Cre system. They analyzed the phenotype of these T cells using flow cytometry, gene expression arrays, and mass spectrometry. To understand the mechanism of lineage instability, they performed chromatin immunoprecipitation (ChIP) assays to examine the binding of HDACs and transcription factors to CD8-lineage gene promoters, and used retroviral transduction to test the functional interactions between HDACs, Runx3, and GATA-3.

The researchers found that HDAC1 and HDAC2 are crucial for suppressing CD8-lineage genes in CD4+ T cells. In the absence of these HDACs, CD4+ T cells spontaneously express CD8 markers and, upon activation, acquire a cytotoxic effector program characterized by the expression of Eomesodermin, T-bet, Granzyme B, and Perforin. This aberrant program is driven by the Runx-CBFbeta complex, which binds to CD8-lineage gene promoters and enhancers (such as E8I) when HDAC1 and HDAC2 are absent. Furthermore, the study suggests that while Th-POK expression remains largely normal in these cells, the loss of HDAC1 and HDAC2 leads to a failure in silencing the CD8-effector program, a process that is notably absent in TH2-polarized cells due to the inhibitory role of GATA-3.

This work identifies HDAC1 and HDAC2 as critical epigenetic regulators that enforce T cell lineage identity. By demonstrating that these enzymes repress the Runx-CBFbeta complex, the study provides a molecular explanation for how CD4+ T cells maintain their helper identity and avoid transdifferentiation into cytotoxic cells. These findings contribute to a deeper understanding of T cell plasticity and the epigenetic control of immune cell fate.