Ageing and liver immune cells.

Ageing is associated with a dysregulated immune system that contributes to vulnerability in older adults to infection, malignancies, autoimmune diseases, and inflammatory disorders. This immune dysfunction can be categorised into two processes: progressive decline in immune responsiveness (immunosenescence) and chronic low-grade systemic inflammation (inflammaging). These processes perpetuate a cycle wherein persistent inflammation accelerates immune cell exhaustion and senescence, while diminished immune surveillance heightens inflammation, together promoting tissue damage and age-related disease. The liver, a crucial immune organ pivotal for maintaining systemic immune tolerance, assumes an increasingly prominent role in regulating peripheral immune tolerance as age-related thymic involution diminishes central tolerance. Ageing alters the liver's immune landscape, with diverse patterns of infiltration and structural remodelling marked by the emergence of ageing-related tertiary lymphoid-associated structures (ATLAS), enriched with focal clusters of inflammatory cells. These structures and associated fibrotic niches function as hubs for pro-inflammatory and pro-fibrotic signalling. Transcriptomic studies reveal consistent upregulation of inflammatory immune pathways and pro-inflammatory cytokines across the aged liver. Immune cells are dysregulated with liver macrophages shifting toward pro-inflammatory phenotypes, NK cells showing exhaustion with reduction in frequency and impaired senescent cell clearance. T and B cells accumulate exhausted phenotypes with expanding populations of senescence-associated T cells (SATs) and age-associated B cells (ABCs), respectively. Liver sinusoidal endothelial cells (LSECs) undergo pseudo-capillarization and defenestration, creating a physical barrier that impairs clearance of tissue-adjacent T cells by hepatocytes. Taken together, age-related immune changes in liver immune cells indicate that the liver plays a central role in systemic inflammation in old age.