Current Concepts and Advances in Graft-Versus-Host Disease Immunology

Abstract

Worldwide, each year over 30,000 patients undergo an allogeneic hema-topoietic stem cell transplantation with the intent to cure high-risk hematologic malignancy, immunodeficiency, metabolic disease, or a life-threatening bone marrow failure syndrome. Despite substantial advances in donor selection and conditioning regimens and greater availability of allograft sources, transplant recipients still endure the morbidity and mortality of graft-versus-host disease (GVHD). Herein, we identify key aspects of acute and chronic GVHD pathophysiology, including host/donor cell effectors, gut dysbiosis, immune system and cytokine imbalance, and the interface between inflammation and tissue fibrosis. In particular, we also summarize the translational application of this heightened understanding of immune dysregulation in the design of novel therapies to prevent and treat GVHD.

Keywords: graft-versus-host disease; pathophysiology; prevention; regulation; therapy; tolerance.

Figure 1

aGVHD (acute GVHD) pathogenesis and treatment. Phase I: GVHD initiation. Pretransplantation conditioning causes barrier integrity loss, gut lumen pathogen-associated molecular pattern (PAMP) translocation, and dying host cell release of damage-associated molecular patterns (DAMPs) that activate host antigen-presenting cells (APCs). Host mucosal-associated invariant T (MAIT) cells produce IL-17A, which prevents intestinal dysbiosis. Activated APCs secrete IL-12, inducing host type 1 innate lymphoid cells (ILC1s) and T cells to release IFN-γ, which upregulates MHC-II on nonprofessional APCs (epithelial and stromal cells). APC activation is suppressed by IL-13-producing ILC2s, or by regulatory T cells (Tregs) utilizing secreted (IL-10) or contact-dependent (CTLA4, granzyme B, CD39) mechanisms, and modified by Siglec G-CD24 pathway agonists (e.g., CD24-Fc) or IL-12 inhibition (ustekinumab). Histone deacetylase inhibitor vorinostat and α−1-antitrypsin inhibit PAMP/DAMP-induced APC proinflammatory cytokine production and costimulation. Phase II: Donor T cell priming. Donor T cell activation follows interaction with recipient hematopoietic APCs [e.g. dendritic cells (DCs)] or nonhematopoietic APCs activated during phase I. Reconstituting donor CD103⁺ DCs migrate to draining lymph nodes, where they further prime donor T cells. Abatacept (CTLA4-Ig) disrupts Tcell:APC costimulation to inhibit aGVHD. Alloreactive donor T cell expansion is curtailed by early posttransplantation cyclophosphamide (PT-Cy). Phase III: effector responses. Primed T cells differentiate to type 1 T helper (Th1)/type 1 CD8⁺ T (Tc1) or Th17/Tc17 cells, directed by IL-12 or IL-6, respectively, and are susceptible to the Jak1/2 inhibitor ruxolitinib, IL-12/23p40 (ustekinumab), or IL-6R (tocilizumab) blocking monoclonal antibodies. Th1/Tc1 cells secrete IFN-γ, which is intestinal stem cell cytotoxic, while augmenting the negative regulatory indoleamine-2,3-dioxygenase (IDO) and PD-L1/PD-L2 pathways to restrain aGVHD. Th17/Tc17 cells secrete proinflammatory cytokines that mediate tissue pathology both directly and by recruitment of secondary effectors [e.g., neutrophils, Mφs (macrophages)]. IL-22-secreting ILC3s or exogenous IL-22 supplementation support intestinal stem cell regeneration. Myeloid-derived suppressor cells (MDSCs) suppress T effector functions via an arginase-1-dependent mechanism. Tregs [supported by IL-4-producing natural killer T (NKT) cells and regulatory DC cells (DCregs)] and regulatory type 1 T cells exert multiple pathways to control T effector cells. Abbreviations: cGVHD, chronic GVHD; GM-CSF, granulocyte-macrophage colony-stimulating factor; Tfh, T follicular helper; Tr1, regulatory type 1 T cell. Figure adapted

Figure 2

Chronic GVHD (cGVHD) pathogenesis and treatment. Phase I: Inflammation. Pretransplantation conditioning and acute GVHD (aGVHD) cause thymus injury. Phase II: T and B cell dysfunction. The resulting thymic dysfunction impairs donor regulatory T cell (Treg) generation and the negative selection of autoreactive donor T cell clones that escape into the periphery. Balanced Treg/T effector cell reconstitution can be restored by low-dose IL-2 treatment. Donor T follicular helper (Tfh) cells expand in secondary lymphoid organs and promote germinal center (GC) reactions and survival, expansion, and differentiation of donor B cells into antihost-immunoglobulin-producing plasma cells via IL-21 secretion. T follicular regulatory (Tfr) cells suppress Tfh cell:B cell interactions and antibody production. The Rho-associated coiled-coil kinase 2 (ROCK2) inhibitor, KD025, blocks Th17 differentiation and GC reactions by inhibiting Tfh cell generation. Donor B cells receive activating signals via multiple pathways during cGVHD. B cell receptor signaling activates the tyrosine kinase Syk, inhibited by fostamatinib and Entospletinib, and Bruton tyrosine kinase (BTK), inhibited by ibrutinib. IL-21 receptor (IL-21R) signals via the Jak1/2-STAT3 pathway and can be targeted by Jak1/2 inhibitor ruxolitinib or KD025. B cell-activating factor (BAFF) provides crucial prosurvival and mitogenic signals that can be blocked by anti-BAFF antibodies (e.g., belimumab). Donor B cell subsets can be depleted by the anti-CD20 antibody rituximab. Differentiated plasma cells secrete affinity-matured, antihost reactive immunoglobulin. Phase III: Fibrosis. Plasma cell-derived immunoglobulin is deposited in host tissues and stimulates donor CSF1-R⁺ macrophages, putatively via Fc receptor (FcR) ligation, to secrete profibrotic mediators. These include TGF-β that stimulates fibroblast proliferation and extracellular matrix production via the tyrosine-kinase receptor TGF-βR and is inhibitable by nilotinib and imatinib. The Hedgehog pathway inhibitor glasdegib and ROCK2 inhibitor KD025 each can directly ameliorate fibrosis. Abbreviations: Ab, antibody; CSF1, colony-stimulating factor 1; DC, dendritic cell; HSCT, hematopoietic stem cell transplantation; Mφ, macrophage; Th17, type 17 T helper. Figure adapted from images created with BioRender.com.

Figure 3

Failure of immune tolerance during GVHD. During acute GVHD (aGVHD) damage-associated molecular pattern (DAMP)/pathogen-associated molecular pattern (PAMP)-induced activation of host antigen-presenting cells (APCs) in secondary lymphoid organs (SLOs) results in switch from donor T cell anergy to activation and dysfunction or deletion of fibroblastic reticular cells (FRCs) and donor dendritic cells (DCs). Donor alloreactive T cells infiltrate the thymus, exerting cytotoxicity of thymic DCs, Aire-expressing medullar thymic epithelial cells (mTECs), and thymus type 3 innate lymphoid cells (ILC3s) that are already damaged by both the conditioning regimen and use of calcineurin inhibitors (CNIs). Thymic damage results in impaired negative selection, allowing escape of self-reactive T cell clones, and diminished thymic regulatory T cell (Treg) generation. Potentially self-reactive T cell clones escaping to the periphery fail to undergo peripheral tolerance in SLOs because of impaired antigen presentation on DCs and FRCs damaged during aGVHD. The outcome of impaired thymus function is the persistence of self-reactive lymphocytes. These defects in APCs within SLOs also impede peripheral Treg (pTreg) generation, causing profound Treg defects. Altogether, these mechanisms contribute to the failure of central and peripheral immune tolerance. Abbreviations: cGVHD, chronic GVHD; tTreg, thymic Treg. Figure adapted from images created with BioRender.com.