Immune homeostasis is a host characteristic that maintains biological balance within a host. Humans have evolved many host defence mechanisms that ensure the survival of individuals upon encountering a pathogenic infection, with recovery or persistence from a viral infection being determined by both viral factors and host immunity. Chronic viral infections, such as hepatitis B virus, hepatitis C virus and HIV, often result in chronic fluctuating viraemia in the face of host cellular and humoral immune responses, which are dysregulated by multi-faceted mechanisms that are incompletely understood. This review attempts to illuminate the mechanisms involved in this process, focusing on immune homeostasis in the setting of persistent viral infection from the aspects of host defence mechanism, including interferon-stimulated genes, apolipoprotein B mRNA editing enzyme catalytic polypeptide 3 (APOBEC3), autophagy and interactions of various immune cells, cytokines and regulatory molecules.

TREX2, a 3’-5’ exonuclease, plays a pivotal role in cleaving nucleoside monophosphates at the 3’ terminus of ssDNA. Despite previous insights into its biochemical functions—homodimerisation, DNA binding, and enzymatic activity—the precise biological significance of TREX2 remains elusive. The broad objective of this thesis was to elucidate the physiological function of the 3’-5’ DNA exonuclease TREX2, aiming to deepen our understanding of the cellular mechanisms governed by TREX2. Considering the implications of DNA exonuclease dysregulation in autoimmune disorders, we aimed to explore the potential implications of TREX2 deficiency on immune function. Due to the lack of anti-TREX2 antibodies recognising endogenous TREX2, we worked on overexpression systems. During interphase, GFP-tagged TREX2 predominantly localised to the cytoplasm; however, throughout all stages of mitosis, it accumulated within the nucleus, showing significant colocalisation with chromatin. This suggests a multifaceted role for TREX2 in maintaining chromatin integrity during cell division. Utilising the CRISPR-Cas9 knockout technique, we targeted the TREX2 gene in wildtype HaCaT cells. Our unexpected findings revealed that TREX2 deficiency led to reduced basal type I IFN activity, contrary to the anticipated effect observed with TREX1 deficiency. This suggests that TREX2 plays a crucial role in maintaining the homeostasis of the type I IFN pathway. Moreover, upon stimulation with cGAS- specific agonists, TREX2 knockout HaCaT cells exhibited diminished responsiveness, supporting a positive regulatory function of TREX2 in the cGAS-sensing pathway. Live imaging further demonstrated colocalisation of TREX2 and cGAS with mitotic chromatin, underscoring their collaborative role in cellular dynamics. In conclusion, TREX2 emerges as a crucial player in regulating the type I IFN pathway, influencing immune homeostasis in association with cGAS. These findings pave the way for comprehending the intricate interplay of TREX2 in cellular processes and its implications for immune modulation.