Transcriptional regulation and co-stimulatory signaling in antiviral T cell immunity
AffiliationMicrobiology & Immunology
Document TypePhD thesis
Access StatusThis item is embargoed and will be available on 2021-05-17.
© 2018 Dr. Simone Nüssing
Special AT-rich binding protein-1 (SATB1) is a global chromatin organizer, promoting or repressing gene transcription in mice and human. In this PhD thesis, SATB1 expression was examined in humans across ages and tissues (Chapter 3). The molecular (Chapter 4) and functional (Chapter 5) role of SATB1 was investigated during anti-viral immunity in mice using an influenza (IAV) infection model. Additionally, the effect of CD27-mediated co-stimulation was studied in the context of HIV-1 infection (Chapter 6). SATB1 has pivotal roles during T cell development and maturation, with lineage fate decision in hematopoietic stem cells and gradual changes in SATB1 expression contributing to T cell development in the thymus in mice. In Chapter 3, SATB1 expression was analyzed across lymphocyte compartments from different human tissues and correlated with PD-1 expression in virus-specific CD8+ T cells. SATB1 expression in pediatric and adult donors showed that SATB1 expression was highest in the human thymus with differential expression levels from DN to DP thymocytes and down-regulation of SATB1 in peripheral T cells. Chapter 3 shows that SATB1 expression in the periphery is not static but follows fine-tuned expression dynamics with downregulation from naïve to antigen-specific CD8+ T cells, likely to be antigen- and tissue-dependent. These data led to the hypothesis that fine-tuned SATB1 expression is necessary for maintaining fate-potential in developing and mature, peripheral T cells. Several molecular mechanisms have been identified for gene regulation by SATB1 with wide-range impacts on the overall chromatin landscape. Previous studies in our laboratory showed that SATB1 mRNA levels are high in naïve, but low in effector CD8+ T cells. The impact of SATB1 in repressing transcriptional programs in naïve CD8+ T cells, prior to its downregulation in effector T cells, was addressed in Chapter 4 of this study. ChIP-Sequencing analysis was performed to decipher genomic binding sites of SATB1 in naïve and effector CD8+ T cells. SATB1 ChIP-Seq data demonstrated that SATB1 binding sites were predominately distal to transcriptional start sites, likely to harbor transcriptional enhancer sites, with reduced SATB1 binding sites in effector over naïve CD8+ T cells. To understand the effects of SATB1 on the transcriptional regulation in naïve and IAV-specific CD8+ T cells, SATB1 imposter mice (SATB1imp/imp) were used in this PhD study. In these mice, Satb1 contains a point mutation in the DNA-binding domain encoding position. SATB1 protein expression in SATB1imp/imp mice persists but is dysfunctional with reduced DNA-binding capability. CD8+ T cells from SATB1imp/imp mice showed up-regulation of certain gene profiles, especially at the naïve stage, such as Pdcd1, Ctla4 and Ccl5, characteristic of activated or exhausted T cells. In Chapter 5, an IAV infection model was used, to examine the effects of dysfunctional SATB1 in IAV-specific CD8+ T cell response generation. CD8+ T cell numbers were consistently reduced in SATB1imp/imp mice with significantly reduced IAV-specific CD8+ T cell numbers in lungs on d10 post-infection. SATB1imp/imp CD8+ T cells exhibited an early overexpression of PD-1 from the naïve stage and reduced polyfunctionality within IAV-specific SATB1imp/imp CD8+ T cells. Using a bone marrow chimera approach, in which mice were reconstituted with a mixture of wildtype and SATB1imp/imp-derived lymphocytes, data showed that reduced T cell numbers and PD-1 overexpression are T cell intrinsic in SATB1imp/imp mice. Immunotherapies, including anti-PD-1, anti-CD27 and histone deacetylase inhibitors, are often used in clinical trials to manipulate activation of T cells. In Chapter 6, we used CD27-mediated stimulation to understand the effect on CD4+ T cells with and without HIV-1 infection. CD27 is a co-stimulatory receptor of the TNF-family, expressed on naïve and central memory T cells. Non-permanent stimulation via CD27 leads to increased primary and memory antiviral CD8+ T cell responses in mice. Here, in humans, CD27-mediated stimulation of CD4+ T cells via its ligand CD70 exhibited profound activation potential in vitro, with high CD4+ T cell proliferation and GzmB production. To examine whether this high activation potential could trigger re-activation of viral transcription in latently infected CD4+ T cells, we re-stimulated CD4+ T cells with conventional α-CD28 or CD27-mediated co-stimulation in an in vitro latency model. Unexpectedly, re-stimulation via CD27 of CD4+ T cells led to reduced viral reactivation compared to α-CD28 stimulation of CD4+ T cells. However, similar transcriptional reactivation levels were obtained when CD4+ T cells isolated from HIV+ individuals on ART were re-stimulated with the two protocols. Strikingly, pre-stimulation of CD4+ T cells prior to in vitro HIV-1 infection showed a trend towards reduced HIV-DNA integration and overall infection. This suggests that CD27-mediated stimulation could lead to activation of antiviral mechanisms that reduces CD4+ T cells HIV-1 infection. Overall, this PhD study provides an in-depth understanding of the transcriptional and co-stimulatory regulations of T cell differentiation in response to viral infections. SATB1’s ability to regulate immune checkpoint molecules, such as PD-1 by its DNA-binding capability in antiviral immunity highlights its significance in future PD-1-related cancer and HIV-1 immunotherapy trials used to reverse T cell exhaustion.
KeywordsSATB1; T cells; immunity; influenza; HIV
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