Origin and maintenance of Kupffer Cells in homeostasis and liver injury

Abstract

Evaluation of liver resident Kupffer Cells (KC) has shown that in mouse these cells derive from a population of foetal liver monocytes that seed the tissues during embryogenesis and maintain themselves in steady state conditions independently of circulating monocytes, via local proliferation. The factors which regulate this KC autonomy have not yet been determined, but likely comprise both biochemical and physical factors which form their ‘niche’. Furthermore, their regulation, and specifically whether they retain total autonomy from monocyte-derived macrophages following liver injury has not been extensively studied. In this thesis I aimed to address the role of availability of the macrophage growth factor CSF1 in maintenance of KC and their autonomy from monocytes in homeostasis, and the roles and long-term fate of KC and monocyte-derived macrophages following acute and chronic liver injury. An optimal method for isolation and identification of F4/80hiCD11blo KC for flow cytometric analysis was first established. To explore the role of CSF1 in maintenance of KC, Csf1r-mApple mice were generated which revealed that KC expressed the highest levels of the csf1rmApple transgene compared with other leukocytes in the liver. In vivo administration of fluorescently-labelled CSF1-FcAF647 revealed that KC captured 10x more circulating CSF1 per cell than other myeloid cells in the liver and lung, suggesting efficient capture of CSF1 may be one mechanism that allows KC to regulate differentiation of monocytes in the liver. Delivery of CSF1-Fc to chimeric mice led to an increase in proliferation and accumulation of resident KC and a transient increase in the number of bone marrow-derived F4/80hiCD11blo cells in the liver, which were lost by 2 weeks following withdrawal of CSF1-Fc. These bone-marrow derived macrophages did not express the putative KC marker Tim4, indicating that they did not fully adopt a KC phenotype. Acute CCl4-driven liver injury resulted in a transient loss of approximately 50% of the resident KC population which recovered in number by 6 days post injury, through proliferation of the remaining KC. This was despite the fact that injury led to the massive recruitment of monocyte-derived macrophages some of which matured into an F4/80hiCD11blo population, but which also lacked Tim4 expression. These cells were spatially distinct from KC and were found clustered around blood vessels, presumed to be the region of necrotic damage. Comparison of gene expression was consistent with their involvement in tissue repair. Following CCl4-driven chronic liver injury, KC were depleted in number, but unlike in acute injury, the population did not return to normal numbers even 8 weeks following cessation of treatment. This time, the recruited macrophages these were not found in clusters but alongside resident KC in the parenchyma. However, in both acute and chronic injury, monocyte-derived macrophages could no longer be seen 8 weeks after injury, and did not mature into long-lived, self-renewing KC. In conclusion I found that CSF1 is one of the major limiting factors that dictates the size of the KC niche, but additional factors are required to fully support monocyte engraftment to the KC population. Following CCl4-driven acute liver injury, monocyte-derived macrophages do not contribute long-term to the KC population despite exhibiting a F4/80hiCD11blo phenotype during the latter stages of liver repair. KC are depleted long-term following CCl4- driven chronic liver injury, but monocyte-derived macrophages remain unable to replenish the population, indicating that damage to a physical component of the KC niche may have occurred