Liver Lymphopoiesis And Liver-Resident NK Cells In Adult Mice

Liver is a unique immunological site, to which dual blood supply brings immune stimulants from gastrointestinal tract, and features by complex repertoires of immune cells capable of effective defense. The structural organization of sinusoids may selectively retain immune cells from circulating blood. Meanwhile, hematopoiesis of adult liver also probably influences the inclusion of hepatic immune system. Fetal liver is one of the hematopoietic places during ontogeny. Though HSCs is generally considered to migrate from fetal liver to BM, the hematopoietic capacity of liver is still found in adults. Thus, the contribution of liver hematopoiesis to liver immune system is of great interest.The liver is an organ with predominant innate immunity. Besides large population of cells with phagocytosis, NK and NKT cells are in high frequency comparing liver with other organs. The mechanism underlying innate immune domination as well as the different between innate cells in liver and other organs attracts many attentions. Different phenotypes were described in liver NK cells and NK cells in other organs. It has been reported that nearly half of liver NK cells possessed DX5" immature profile, with a weaker secretion of cytokines and cytotoxicity than DX5+classical NK cells, but could kill target cells by TRAIL related pathway. Moreover, memory NK cells in hapten induced contact hypersensitivity and some virus infections were liver resident, implying liver DX5" NK cells might be another NK cell subset rather than a stage during NK cell development.In view of the above point, this study contains the following two aspects:I. Liver lymphopoiesis in adult mice1. Hematopoietic progenitors existed in adult liver.We analyzed the surface marker on liver mononuclear cells (MNCs) and found cells with the phenotypes of hematopoietic progenitors (HPCs) in them. Further study showed that liver MNCs transfer could rescue lethally irradiated mice, demonstrating hematopoietic capacity of adult liver.2. Liver MNCs transfer rebuilt immune system predominated by T cells.Myeloid and lymphoid lineages were slowly rebuilt after liver MNCs transfer. Interestingly, CD3+CD19-T cells were major population in recipients3weeks after liver MNCs transfer, while CD3-CD19+B cells were major population in BM transfer groups.3. Liver MNCs contained HPCs functionally similar to BM cells.Purified liver HPCs similarly reconstituted T and B cells as BM cells. Contrast to T cell predominance after liver MNCs transfer, liver HPCs regenerated T cells later than B cells.4. Liver MNCs contained liver-resident memory CD8+T cells.Hepatic mature T cells recruited to the liver after transfer. They proliferated and maintained population for long time. Most of them were CD8T cells with memory phenotypes.Conclusion Ⅰ:Adult liver contained both HPCs and T cells with lymphopoietic capacity. T and B cells were similarly reconstituted by HPCs in the liver and BM, but NK cell recovery was quite different. Liver resident memory CD8T cells also rapidly proliferated and long-lived after transfer with liver specific retention.Ⅱ. The features of liver unique NK cells1. Liver contained a unique NK cell subset.Nearly half of hepatic NK1.1+NK cells were DX5-NK cells, which were rare in other organs and phenotypically distinct from BM immature NK cells. They maintained DX5-phenotype and did not convert to DX5+NK cells after adoptive transfer, implying they were NK cell subset different from classical NK cells rather than immature NK cells. Besides, we found CD49a was a specific marker for them by genechip.2. Hepatic unique NK cells conferred CHS responses.In hapten induced contact hypersensitivity (CHS), CD49a+DX5-NK cells isolated from sensitized mice liver delivered antigen specific memory to naive mice, while CD49a DX5+NK cells could not.3. Memory NK cells were liver resident and priming in the liver. Liver unique NK cells naturally possessed memory capacity. They resided in the liver and acquired memory ability after hapten sensitization. We traced hapten delivery by FITC, and found hepatocytes could absorb haptens after sensitization. Monocytes and DCs also took part in this process. Thus, liver might be a site where naive NK cells were primed.4. Liver-resident HPCs differentiated into CD49a+DX5-NK cells.Very few DX5-NK cells were detected in the livers of BM transplanted mice, suggesting that DX5-NK cells were not predominantly derived from BM. We transferred purified hepatic precursors and found they could give rise to liver unique NK cells.Conclusion Ⅱ:We identified a unique NK cell subset in the liver characterized by the CD49a+DX5-phenotype. This subset possessed memory potential and conferred hapten-specific CHS responses upon hapten challenge. Importantly, CD49a+DX5-NK cells were liver resident and appeared to originate from hepatic HPCs but not from the BM.