Regulation of CD4 and CD8 Coreceptor Expression and CD4 Versus CD8 Lineage Decisions

Takeshi Egawa1    Department of Pathology and Immunology, School of Medicine, Washington University in St. Louis, St. Louis, Missouri, USA
1 Corresponding author: email address: tegawa@wustl.edu

1 Introduction

Why is it important to study the regulation of CD4 and CD8 expression?

T lymphocytes are the major players in adaptive immunity. In immune responses against viral infection, innate immune cells rapidly react to pathogen invasion through recognition of pathogen-associated molecular patterns. Yet in many situations, pathogen eradication and the establishment of immunological memory require a greater magnitude of sustainable immune responses by adaptive immune cells. T cells are critical components of adaptive immune responses by functioning as effector cells to eliminate pathogen. T cells also provide adequate “help” to other cell types via expression of cytokines and adhesion molecules. The former function is mediated mainly by CD8+ cytotoxic T cells and the latter is mediated by CD4+ helper T cells.

T-cell development from multipotent progenitors (MPPs) or common lymphoid progenitors (CLPs) is initiated in the thymus (Rothenberg, 2014). Expression of CD4 and CD8 coreceptors defines four major populations of thymocytes at distinct developmental stages (Ellmeier, Sawada, & Littman, 1999). The most immature T-cell progenitors express neither CD4 nor CD8, thus called double negative (DN) thymocytes. DN thymocytes are further divided based on the expression of CD25 and CD44 into four subsets called DN1 through DN4 (Godfrey, Kennedy, Suda, & Zlotnik, 1993; Pearse et al., 1989; Rothenberg, 2014). MPPs or CLPs commit to the T-cell lineage in response to the signal through Notch1 and expression of the transcription factors Bcl11b and Tcf7 (also known as TCF1) at the CD44+CD25+ DN2 stage, and subsequently undergo selection for successful Tcrb rearrangements at the CD44−CD25+ DN3 stage (beta selection) (Ikawa et al., 2010; Li, Burke, et al., 2010; Li, Leid, & Rothenberg, 2010). After beta selection, thymocytes initiate proliferation as they transit to the CD44−CD25− DN4 stage and subsequently turn on expression of CD4 and CD8 (Ellmeier et al., 1999; Fehling, Krotkova, Saint-Ruf, & von Boehmer, 1995; Hoffman et al., 1996). In the C57BL6 genetic background, surface CD8 expression increases prior to CD4 (CD8 immature single positive, ISP) and the developing thymocytes subsequently become CD4+CD8+ double positive (DP) cells. DP thymocytes are finally selected for the expression of αβTCR with appropriate avidity to peptide–MHC complexes (pMHC) presented on cortical thymic epithelial cells to become mature thymocytes (Sawicka et al., 2014). Depending on the specificity of clonal TCR to MHC class I (MHC-I) or MHC class II (MHC-II), selected thymocytes differentiate into the CD8+ cytotoxic or CD4+ helper lineage, respectively. The selected mature thymocytes keep only one of the two coreceptors with the other repressed or permanently silenced at the transcriptional level. The specificity of TCR to MHC-I versus MHC-II, differentiation to the helper versus cytotoxic lineage, and the choice of coreceptor expression are all tightly linked.

This tight link makes the study of the regulation of CD4 and CD8 coreceptor expression unique and superb not only from the viewpoint of immunology but also from that of gene regulation. From the immunology aspect, helper and cytotoxic T-cell responses to TCR stimulation are distinct and the two cell types play nonredundant roles in immune responses, even though they are derived from a common precursor pool of DP thymocytes. Cytotoxic T cells are professional effectors to kill target cells, such as virally infected cells and cancer cells. Following activation through interaction with antigen-presenting dendritic cells, cytotoxic T cells undergo massive proliferation and produce IFN-γ and TNF. Ubiquitous expression of MHC-I allows cytotoxic T cells expressing CD8 and MHC-I-restricted TCR to recognize and kill a wide variety of cells that are infected by intracellular pathogens or transformed. In contrast, helper T cells that express MHC-II-restricted TCR and the CD4 coreceptor are functionally diverse and plastic. Activated helper T cells regulate cytotoxic T-cell responses, B-cell responses, and innate responses through production of various cytokines, depending on priming cytokine milieu provided by innate immune cells.

Such distinct properties of helper and cytotoxic T cells are programmed during a brief time window of thymic positive selection, in which continued expression or transcriptional repression of the CD4 or CD8 coreceptor is specifically determined. Because the helper versus cytotoxic lineage decision during positive selection is largely irreversible, it is reasonable to speculate that positive selection signals establish self-sustainable or imprinted genetic circuitry that establishes identities of helper and cytotoxic lineage cells. CD4 and CD8 coreceptors are not only markers for the distinct lineages of T cells. These molecules are involved in the initiation of the development of helper and cytotoxic T cells during positive selection, and continued expression of the same coreceptor used in the selection is essential for the functions of each subset. Therefore, it is possible that common genetic circuitry regulates the lineage identities and stable coreceptor expression in postselection thymocytes and mature T cells in the periphery. At the same time, mature SP thymocytes and T cells shut off expression of the coreceptor that is not associated with their TCR specificity to MHC. While CD8 repression in CD4+ T cells appears reversible, CD4 silencing in CD8+ T cells seems nearly irreversible and maintenance of the silencing by epigenetic mechanisms is implicated. While epigenetic gene silencing is an important mechanism to maintain stable gene expression signature in many cell types, the molecular mechanisms by which a gene is initially repressed by a transacting factor- and cis-element-dependent manner and subsequently becomes “epigenetically silenced” independent of the triggering transacting factors or its target cis-elements are poorly understood. From this point, study of Cd4 silencing serves as an excellent model system.

In this chapter, I will first summarize recent advances in genetic and epigenetic regulation of genes encoding CD4 and CD8 coreceptors and then discuss the roles of transcription factors in the regulation of coreceptor expression and helper versus cytotoxic lineage decisions.

2 CD4 and CD8 Coreceptors and Thymocyte Selection

2.1 Molecular timers defining the duration of positive selection signals

Based on the tight links between TCR specificity to MHC and coreceptor expression, several different models have been proposed to explain how positive selection signals regulate the helper versus cytotoxic lineage decision and experimentally validated. Two major classical models are the instructive model and the stochastic model. The instructive model proposed that the strength of positive selection signals resulting from different affinity of CD4 and CD8 cytoplasmic tails to the Src family tyrosine kinase Lck determines the outcome of the lineage choice of selected DP thymocytes (Hernandez-Hoyos, Sohn, Rothenberg, & Alberola-Ila, 2000; Itano et al., 1996; Seong, Chamberlain, & Parnes, 1992; Sohn, Forbush, Pan, & Perlmutter, 2001; Wiest et al., 1993). However, subsequent genetic studies using Lck-noninteracting CD4 demonstrated that CD4–Lck interactions are not absolutely essential for the helper lineage development (Killeen & Littman, 1993). Therefore, instructive signals, if any, are likely to be Lck-independent. Subsequent studies tested other...