Donor T Cells Require Notch Signals but Not Alloantigen Presentation from Specialized Secondary Lymphoid Organ Fibroblasts to Drive Graft-Versus-Host Disease

Notch signals drive pathogenic T cell alloreactivity in mouse models of allogeneic bone marrow transplantation (allo-BMT) leading to lethal graft-versus-host disease (GVHD). In a mouse model of MHC-mismatched allo-BMT with high intensity whole body conditioning (11 Gy), residual recipient or donor hematopoietic cells were dispensable sources of Notch signals to drive T cell alloreactivity. Instead, abrogating the expression of Delta-like1 (Dll1) and Delta-like4 (Dll4) Notch ligands in Ccl19-Cre⁺ secondary lymphoid organ (SLO) fibroblastic stromal cells protected mice from GVHD (Chung et al., JCI 2017). In this study we sought to 1) understand the expression and regulation of Notch ligands in fibroblastic stromal cells during allo-BMT; 2) test whether fibroblastic stromal cells are still critical sources of Notch signals in models of allotransplantation with reduced or no conditioning (when other radiation-sensitive sources of Notch ligands are better preserved); and 3) assess whether SLO fibroblastic stromal also present alloantigens to T cells during GVHD.

To characterize stromal sources of Notch ligands early after allo-BMT, we combined a Ccl19-Cre transgene with a ROSA26-YFP Cre-activated allele in Dll4-mCherry BAC reporter mice. We observed combined Ccl19-Cre activity and Dll4-mCherry reporter expression in CD157^hi fibroblastic reticular cells, MAdCAM1⁺ marginal reticular cells, CD21⁺ follicular dendritic cells, and a subset of MAdCAM1⁺ lymphatic endothelial cells. 12 hours after high intensity conditioning and allo-BMT, Dll4 mRNA and cell surface expression increased in fibroblastic but not in hematopoietic or endothelial cellular compartments. Dll4 upregulation required transfer of alloreactive T cells and was potentiated by the intensity of myeloablative conditioning, suggesting that fibroblastic stromal cells have increased availability of Notch ligands to drive GVHD after high-intensity conditioning.

To assess the functional impact of conditioning on the cellular sources of Notch ligands, we studied a parent-to-F1 allo-BMT model in which recipients are tolerant to donor-derived T cells, and the conditioning intensity can be titrated (C57BL/6 [H-2b] or BALB/c [H-2d] into BALB/c x C57BL/6 F1 [H-2d/b]). Cell-specific genetic loss of function systems combining floxed alleles for Dll1 and Dll4 with Ccl19-Cre⁺ and Mx-Cre⁺ transgenes allowed conditional deletion in SLO fibroblastic stromal cells or hematopoietic cells, respectively. Across all conditioning intensities tested, upregulation of core-2 O-glycosylation machinery in alloreactive T cells was critically regulated by Notch signals. Thus, induction of the core-2 glycoform of CD43 could be used as a sensitive surrogate readout of Notch signals received by alloreactive T cells. In models of allotransplantation with high-intensity (11 Gy), low-intensity (3 Gy), and no conditioning, fibroblastic stromal cells remained essential and hematopoietic cells were dispensable as sources of Notch signals driving both T cell core-2 O-glycosylation and lethal alloimmunity.

Because previous studies indicated that non-hematopoietic cells can serve as antigen-presenting cells in CD4⁺ T cell mediated GVHD, we tested if fibroblastic stromal cells could present MHC class II alloantigens in addition to Notch ligands. We used Ccl19-Cre⁺ crossed to mice with a floxed I-A allele to delete the cell-intrinsic ability of fibroblastic stromal cells to present MHC class II alloantigens. In multiple models of high-intensity conditioned CD4⁺ T cell mediated GVHD (BALB/c [H-2d] into BALB/c x C57BL/6 F1 [H-2d/b] and TEa TCR Tg into BALB/c x C57BL/6 F1 [H-2d/b]), fibroblastic stromal cells were dispensable as sources of alloantigen to drive T cell alloreactivity and lethal GVHD. Together, these results indicate that across varying conditioning intensities, specialized subsets of SLO fibroblastic stromal cells critically regulate T cell alloreactivity through presentation of Notch ligands independently of antigen presentation. Selective targeting of Delta-like Notch ligand expression on SLO fibroblastic cells may prevent T cell immunopathology while preserving other beneficial antigen-dependent T cell responses after both high-intensity conditioned allo-BMT and allotransplantation with reduced intensity conditioning.