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Abstract

Epigenetic regulation governs T-lineage differentiation including T-helper polarization. Unlike the immunocompetent adults, neonatal naïve CD4⁺ T cells display reduced capacity to polarize into proinflammatory T-helper (Th) lineages (e.g., Th1, Th17), rendering newborn vulnerable to intracellular infections. However, the epigenetic mechanism underlying the skewed neonatal T-helper polarization remains elusive. On the other hand, thymocytes are subject to developmental arrest and malignant transformation due to mutations, leading to T-cell acute lymphoblastic leukaemia (T-ALL), an aggressive blood cancer. While epigenetic reprogramming has been implicated in the initiation of T-ALL the lack of an in vitro model and appropriate normal controls has hampered the quantitative measurements of epigenetic signatures associated with T-ALL. To search for the age-dependent epigenetic differences attributable for the neonatal Th deficiency, the epigenome of neonatal naïve CD4⁺ T cells was compared to their adult counterpart, showing that the adult-unique enhancers are specifically enriched with binding motifs of IRF family. TBK1, encoding a kinase that activates IRF3, was found to be silenced via a bivalent promoter only in the neonatal naïve CD4⁺ T cells, which could rein in the downstream IRF3 phosphorylation and nuclear translocation, resulting in the neonatal deficiency of Th17 polarization. I further identified the bivalent genes specific to one age group or shared by both age groups and assessed their expression trajectory during in vitro Th17 polarization. Next, to delineate the epigenetic dysregulation associated with T-ALL initiation, I utilized a T-ALL model synthesized by transducing cord blood derived CD34⁺ (CB CD34⁺) cells with a constitutively active NOTCH1 allele, LMO2, TAL1 and BMI1 (collectively abbreviated as NLTB) in an in vitro condition promoting T-cell differentiation. In comparison to the non-transduced control cells, a marked increase in the genomic occupancy of H3K4me3 was observed in the NLTB-transduced CB CD34⁺ cells. Surprisingly, the unique genomic regions consistently marked by H3K4me3 in the NLTB-transduced cells were associated with gene bodies and intergenic regions rather than gene promoters as might be expected for this mark, from which many putative targets associated with T-ALL initiation were identified. My thesis revealed putative targets for therapeutic avenues for the skewed neonatal Th polarization and T-ALL, respectively.