Novel Tools Reveal Niche Availability and Inflammatory State Determines the Cellular Origin of Kupffer Cells.

The study of Kupffer cells (KCs) remains in its infancy due to the lack of KC-specific tools. By comparing the KC gene expression with those of other mfs, we identified a number of KC-specific genes. Using Technitium99-labeled nanobodies, whole body imaging and analysis of liver homogenates we found one of these genes to be solely expressed in the liver by KCs and thus this gene was used to construct KC-DTR knock-in mice. Administration of a single dose of DT is sufficient to completely and specifically eliminate KCs, without inflammation. Despite their ablation, KC-like cells were found in the liver one-week later. By generating bone marrow chimeras in which the livers are not irradiated, we found the returning KCs derived from the bone marrow while adoptive transfer studies revealed Ly6C^hi monocytes to be the progenitor. Importantly, these monocyte-derived KCs (mo-KCs) self-maintain for at least 4 months without any new input from circulating monocytes and acquire a functional KC phenotype. As monocytes do not contribute to KCs in adulthood in either parabiosis or bone marrow chimera studies, we sought to evaluate whether complete depletion of the KC niche is required to allow mo-KC generation. Thus, we administered a suboptimal dose of DT resulting in a partial depletion of KCs. Despite the presence of embryonically-derived KCs, mo-KCs were still identified. Interestingly, KC depletion as mediated by acetaminophen overdose did not result in the generation of mo-KCs. Taken together our results demonstrate that niche availability and inflammation state determines the cellular origin of KCs.