Epigenetic propagation of CD4 expression is established by theCd4proximal enhancer in helper T cells

Abstract

The stability of a lineage program (cellular memory) is dependent on mechanisms that epigenetically maintain active or repressed states of gene expression (transcriptional memory). Although epigenetic silencing of genes has been clearly demonstrated from yeast to mammals, heritable maintenance of active transcription has been less clearly defined. To investigate the potential role of active transcriptional memory during lineage diversification, we employed targeted mutation of a positive-acting cis element in the Cd4 locus to determine the impact on CD4 expression and the differentiation of CD4+ helper T cells in mice. We show that the proximal enhancer (E4 P ) of Cd4 is essential for CD4 expression in immature CD4+ 8 + thymocytes. Futhermore, its loss resulted in reduced and unstable expression of CD4 in mature T cells. However, if the enhancer was deleted after cells had already committed to the helper T-cell lineage, CD4 expression remained high and was stable upon cell division. ''Active'' histone modifications, once initiated by E4 P , were also propagated independently of the enhancer. Thus, E4 P is responsible for establishing an epigenetically inherited active Cd4 locus in the helper T-cell lineage. To our knowledge, this is the first genetic demonstration of active transcriptional memory in mammalian cells. The formation of diverse tissues and organ systems during development requires the heritable propagation of distinct programs of gene expression in each type of differentiated cell. These programs are generated and maintained by epigenetic mechanisms that are only partially understood. The differentiation from multipotent thymic progenitors of T lymphocytes with distinct phenotypes and immune system functions has provided a valuable developmental system for studying how heritable gene expression is initiated and maintained. CD4 + helper T cells and CD8 + cytotoxic T cells are each derived from progenitors that express both the CD4 and CD8 coreceptors (termed ''double-positive'' or DP cells). Investigation of coreceptor expression, which is tightly linked to the functional program of the mature cells, has provided valuable insight into transcriptional mechanisms involved in the lineage bifurcation. Because of the convenient phenotypic characterization of T-cell subsets, they have also proved to be ideal to study mechanisms of heritability of transcription states, or transcriptional memory, during cell lineage diversification. The most immature thymocytes, double-negative (DN) cells, express neither CD4 nor CD8. Rearrangement of the Tcrb locus that results in expression of the b subunit of the T-cell antigen receptor (TCR) is followed by upregulation of both CD4 and CD8 at the DP stage, during which the Tcra locus is rearranged (Berg and Kang 2001). The few DP thymocytes that express an ab TCR of appropriate avidity for self-peptide-major histocompatibility complexes (MHC) undergo positive selection, whereupon they down-regulate CD8 expression to exhibit the CD4 + 8 lo phenotype. These transitional cel...

Cd8 enhancer E8 I and Runx factors regulate CD8α expression in activated CD8 + T cells

“…E4p has been shown to be essential for CD4 expression in DP thymocytes, and E4p is required to establish an epigenetic pattern at the Cd4 locus that allows the propagation of CD4 expression in dividing mature CD4 + T cells, even in the absence of E4p (26). E8 I appears to have a very similar function at the Cd8a locus, with respect to its role in the establishment of CD8 expression during TCR stimulation.…”

Memories of lost enhancers: Figure 1.

“…A new definitive study A recent study in Genes & Development from the Littman laboratory (Chong et al 2010) addresses the initiation versus maintenance question in the most definitive way currently available. They analyzed the role of the T-lymphocyte-specific enhancer (E4p) that is located 13 kb 59 of the murine Cd4 gene.…”

The Epigenetic Landscape of Lineage Choice: Lessons From the Heritability of Cd4 and Cd8 Expression

“…independently of the genetic element that initially activates Cd4 transcription ( Figure 1C). As mentioned earlier, E4 P is required for CD4 expression in DP thymocytes (Chong et al, 2010). However, positive selection of E4 P −/− thymocytes activates CD4 expression in T helper lineage cells, possibly through a putative "maturation enhancer", leading to a reduced population of CD4 + helper T cells.…”

“…The variant H2A.Z has been shown to be required for memory (faster reactivation) of genes in yeast (Brickner et al, 2007). While no difference in H2A.Z occupancy was found at the Cd4 promoter between WT and E4 P −/− CD4 + cells (Chong et al, 2010), it remains possible that H2A.Z underlies stable Cd4 expression as it is also thought to play critical roles in enhancer accessibility (He et al, 2010). Thus E4 P -dependent H2A.Z deposition at an unidentified enhancer element could promote stable Cd4 expression.…”

“…Germline and conditional targeting of two of these enhancers, E4 P and E4 T , has helped to determine their relevant functions (Chong et al, 2010) (Figure 1C). E4 T was found to be dispensable for Cd4 expression in TCRαβ T cells, but required for expression on a subset of lymphoid tissue inducer (LTi) cells, now commonly referred to as innate lymphoid cells, in the small intestine lamina propria.…”