The Liver Can Host Donor Hematopoiesis and Maintains a Strong Population of Innate Immune Cells That Contributes to Host Protection After Transplantation of Purified Hematopoietic Stem Cells.

Abstract 3545 Poster Board III-482 Standing in the line of first defense, the liver is a critical immunocompetent organ. It is armed with lymphocytes, including T cells (TC), natural killer (NK) cells, NK T cells, and a variety of antigen-presenting cells (APC), such as dendritic cells and resident macrophages (Mph), called Kupffer cells. Because it is exposed to large amounts of toxins and antigens, both destructive and harmless, liver immunity must provide immunogenic and tolerogenic mechanisms. Moreover, as the organ of fetal blood production the liver can, if required, resume its hematopoietic function. Here, we studied the role of the liver as a hematopoietic and lymphatic organ after hematopoietic cell transplantation (HCT). Lethally irradiated BALB.K and BALB.B mice were given MHC-matched, FACS purified hematopoietic stem cells (HSC; cKit+Sca1+Thy1.1loLin-) from AKR/J and C57BL/6 donors, respectively, alone or supplemented with 10 ∧ 7 splenocytes (SP) for GVHD induction. Mononuclear cells (MNC) were Ficoll-separated from flushed livers 1 to 6 weeks (w) post transplant (pTX) and FACS analyzed. In recipients of TC-containing grafts, the liver was a major target organ of acute graft-vs-host disease (GVHD) with prominent donor lymphocyte expansion causing destruction of the hepatic portal morphology. Rare HSC-derived cells were observed in the livers. In contrast, mice given purified HSC showed no clinical or histological signs of GVHD, yet early pTX a high proportion of donor HSC-derived MNC was observed within the livers, comprising ∼75% of the MNC at 2w. Phenotype analysis revealed that these HSC-derived MNC were primarily NK cells (DX5+CD122+) or Mph (Mac1+F4/80+). In fact, amongst all nucleated cells, NK cells represented >10% and were mixed donor/host type. Interestingly, the Mph were all donor derived. This observation of over-representation by cells of innate immunity (including NK cells and Mph) in livers of recipients of HSC alone led us to hypothesize that these cells might exert protective functions against increased amounts of pathogens and toxins entering the circulation from irradiation-damaged intestines. Thus, to suppress donor Mph reconstitution pTX, silica was injected intraperitoneally on d-1, and every 3d thereafter. All recipients of HSC alone recovered rapidly after irradiation (d5-7), while at this time point recipients of HSC plus silica showed severe weight loss, hunched posture, ruffled fur, diarrhea, with ∧ 7) was injected simultaneously with HSC, or with a delay of 7d or 9d. All mice given SP on d0 died within 9d and 3/5 of those receiving SP on d7 died by d12. However, all mice given SP on d9 recovered fully and showed no signs of GVHD, despite the lymphopenic host environment that usually promotes homeostatic expansion of mature donor TC. In conclusion, the role of the liver as an immunologically active organ after ‘conventional’ HCT is often masked by donor TC expansion with subsequent GVHD. Here, we provide evidence that if grafts are devoid of mature lymphoid cells, innate immunity recovers rapidly, and in fact exceeds unmanipulated controls. Donor Mph may protect the host from pathogens and endotoxemia. Moreover, they may neutralize activated donor TC and thereby mediate tolerance between donor and host. Likewise, the elevated proportion of donor and host NK cells, which is lacking in GVHD affected mice, suggest another beneficial mechanism of protection, as NK cells have been reported to be capable of reducing GVHD. Immunohistochemical studies for a better quantitative assessment of resident immune cells in the liver pTX are underway. Disclosures: No relevant conflicts of interest to declare.