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Introduction In Utero Hematopoietic Cell Transplantation (IUHCT) is a promising therapeutic strategy for congenital hematopoietic disorders. While mixed allogeneic hematopoietic chimerism with associated donor specific tolerance is routinely achieved by a predominant mechanism of central deletion, the critical events of donor and host thymocyte development have not been determined. In this study, we utilized the murine model of allogeneic IUHCT to analyze donor and host thymocyte development in the context of normal immune ontogeny. Methods Bone marrow (BM) cells (10x106) from C57/BL6 (B6, H2kb) mice were injected intravenously into Balb/c (H2kd) fetuses at embryonic day 14 (E14). E14 B6 fetuses injected with GFP B6 BM were used as congeneic controls. At indicated postnatal ages the thymocytes were delineated by multi-color flow cytometry. Cell apoptosis and proliferation were determined by Annexin V staining and in vivo BrdU incorporation, respectively. T cell alloreactivity was assessed by in vitro and in vivo MLR. Results Our findings demonstrate that the thymic processing of donor BM-derived thymocytes differs significantly from host thymocyte processing and from thymocyte development in normal control mice. While the phenotypic development of host thymocytes remained comparable with that of normal control Balb/c mice, the four major subsets of donor thymocytes showed altered distribution, with significantly higher proportions of single positive (SP) cells, and a dramatically lower proportion of CD4+CD8+ double positive (DP) cells, compared to their host-derived counterparts and B6 controls. The extent of the alteration is directly related to both BM chimerism levels and age. Higher levels of chimerism and/or older age are associated with more profound alterations in donor thymocyte distribution. Moreover, DP cells of donor origin show higher apoptosis and lower proliferation than those of the host. Donor TCR gamma/delta cells in DN cells which do not require positive selection based on MHC recognition are relatively increased compared to the host. Moreover, compared with the naive mice and congeneic chimeric mice, the donor BM-derived thymocytes in the allogeneic chimeric mice show increased proportion of DN3 and decreased proportion of DN4, but increased TCRβ+ proportions in both DN3 and DN4 cells, indicating that donor BM-derived thymocyte development is impeded during DN to DP transition, resulting from a MHC-restriction associated mechanism. In addition, in allogeneic chimeric mice, both host and donor BM-derived T cells are tolerant to allogeneic antigens in in vivo and in vitro MLR. Conclusion Our data suggests that in an allogeneic IUHCT system the immune reconstitution of the donor bone marrow-derived thymocytes differs from that of the host cells and that of normal mice. The data supports a mechanism of impaired MHC based positive selection of donor cells by the predominantly host thymic stroma resulting in lack of progression of a higher proportion of donor cells from the DN to DP stage of thymocyte development. Taken together, although donor BM-derived T cells undergo differential thymic development, permanent host-donor two-way tolerance could be achieved in the allogeneic IUHCT mouse model. These findings add to our understanding of the requirements for tolerance induction after IUHCT and have important clinical implications in choosing an optimal donor for IUHCT. Disclosures No relevant conflicts of interest to declare.

Introduction In Utero Hematopoietic Cell Transplantation (IUHCT) is a promising therapeutic strategy for congenital hematopoietic disorders. While mixed allogeneic hematopoietic chimerism with associated donor specific tolerance is routinely achieved by a predominant mechanism of central deletion, the critical events of donor and host thymocyte development have not been determined. In this study, we utilized the murine model of allogeneic IUHCT to analyze donor and host thymocyte development during postnatal ontogeny. Methods Bone marrow (BM) cells (10x106) from Foxp3GFP C57/BL6 (B6, H2kb) mice were injected intravenously into Foxp3GFP Balb/c (H2kd) fetuses at embryonic day 14 (E14). At indicated postnatal ages the thymocytes were delineated by multi-color flow cytometry. Cell apoptosis and proliferation were determined by Annexin V staining and in vivo BrdU incorporation, respectively. Results Our findings demonstrate that the thymic processing of donor BM-derived thymocytes differs significantly from host thymocyte processing and from thymocyte development in normal control mice. While the phenotypic development of host thymocytes remained comparable with that of normal control Balb/c mice, the four major subsets of donor thymocytes showed altered distribution, with significantly higher proportions of immature TCRβ-CD4+CD8- and TCRβ-CD4-CD8+ single positive (SP) cells, a slightly higher proportion of CD4-CD8- (DN) cells, and a dramatically lower proportion of CD4+CD8+ double positive (DP) cells, compared to their host-derived counterparts and B6 controls. The extent of the alteration is directly related to both BM chimerism levels and age. Higher levels of chimerism and/or older age are associated with more profound alterations in donor thymocyte distribution. Moreover, DP cells of donor origin show higher apoptosis and lower proliferation than those of the host. Donor DN cells and TCR gamma/delta cells which do not require positive selection based on MHC recognition are increased compared to the host. Donor T-regulatory cells were present at a higher frequency in the thymus and secondary lymphoid organs of chimeric animals compared to T-regulatory cells in host and normal strain controls. Conclusion Our data suggests that in an allogeneic IUHCT system the immune reconstitution of the donor bone marrow-derived thymocytes differs from that of the host cells and that of normal mice. The data supports a mechanism of impaired MHC based positive selection of donor cells by the predominantly host thymic stroma resulting in lack of progression of a higher proportion of donor cells from the DN to DP stage of thymocyte development. In contrast, generation of donor T-regulatory cells was not impaired and appeared to be increased in chimeric animals. The alloreactivity, regulatory suppression, and MHC restriction of the donor derived T cells, and the potential importance of these observations for clinical IUHCT remain to be determined. Disclosures: No relevant conflicts of interest to declare.

Abstract 1904 Introduction: In Utero Hematopoietic Cell Transplantation (IUHCT) is a potential treatment for congenital hematologic disorders. The rationale is to utilize normal events during hematopoietic and immunologic development to facilitate hematopoietic engraftment. While the risk of graft versus host disease (GVHD) after IUHCT would be anticipated to be high, based on Medawar's tenets, surprisingly little GVHD has been observed after experimental IUHCT and IUHCT induced GVHD has not been previously characterized. To better understand the potential risk of IUHCT induced GVHD prior to clinical application, we attempted to intentionally induce GVHD pathophysiology in the allogeneic murine model of IUHCT. Methods: Bone marrow (BM) cells (10×106) from C57/BL6 (B6, H2kb, GFP−) mice were co-injected with increasing doses of splenocytes from B6-GFP (H2kb, GFP+) mice intravenously into Balb/c (H2kd) fetuses at embryonic day 14 (E14). Control groups included: 1) 10×106 B6 BM cells alone; 2) same doses of B6-GFP splenocytes alone; and 3) IUHCT of congenic donor cell populations into B6 recipients. Surviving pups were assessed using a clinical GVHD scale and separate cohorts were harvested at pre- and post-natal time-points for analysis by fluorescence stereoscopic microscopy (FSM), flow cytometry for assessment of donor and host cell populations in hematopoietic tissues, and histologic assessment of target tissues for GVHD. Results: Prenatal Analysis - In the allogenic groups that received 10×106 BM cells plus increasing doses of splenocytes (0.1×106 to 10×106 splenocytes/fetus) a consistent distribution of GFP positive cells could be seen by whole body FSM in lymphohematopoietic sites (spleen, lymph nodes, thymus and fetal liver) at E16 that did not differ significantly from congenic controls. By E19 – E20 however, marked proliferation of the cells occurred with clear infiltration of GVHD target organs (skin, lungs, liver, intestine). This expansion was proportionate to the dose of splenocytes and was not seen in congenic or BM only controls. Increased mortality was not observed until doses greater than 5 × 106 splenocytes were given (approx. 5 × 1011 splenocytes/kg est. fetal wt.) with perinatal death likely attributable to respiratory insufficiency. Postnatal Analysis - In the allogeneic groups there was minimal clinical evidence of GVHD up to and inclusive of the 5×106 splenocyte dose. A few pups showed poor growth relative to the normal growth curve and mild fur ruffling, but these resolved by P15, and there was no further clinical evidence of GVHD and no postnatal deaths up to P60. In harvested tissues many GFP+ cells persisted in GVHD target organs, however, no GVHD histopathology was observed at any time. Flow cytometric analysis confirmed a proliferation of donor splenocytes that peaked around P5-P10. Lymphocyte subset analysis revealed an expansion of predominantly CD8+ T-cells. Alkaline phosphatase in the host liver, as a marker of GVHD was increased during the same time course and returned to normal by P15. An increase of host and donor BM derived CD4+ cells was also observed that peaked around P15-17. Further analysis confirmed an increase in both T-regulatory cells and activated CD4+ T-cells compared to normal or BM only controls. The most remarkable observation was a dramatic graft versus hematopoietic (GVH) effect with an increase in donor BM derived chimerism at P60 to > 80% with no clinical GVHD. These findings were seen only in the allogenic group that received both splenocytes and bone marrow cells, not in the other groups. In the group that received only bone marrow cells, the chimerism remained at low levels (Figure 1). Conclusion: Our data demonstrates that the fetus is in fact resistant to GVHD relative to myeloablated postnatal BMT recipients. While there is an aggressive alloimmune activation and expansion of donor T-cells, clinical manifestations are mild and are reversed by what appears to be a T-regulatory cell expansion. In our model, the alloimmune activation phase is accompanied by a potent GVH response providing a competitive advantage for donor cells and resulting in marked enhancement of donor cell engraftment without clinical evidence of GVHD. Further work in this model is directed toward defining regulatory and effector mechanisms involved in controlling GVHD and the GVH effect. Disclosures: No relevant conflicts of interest to declare.