The Biology of Chronic Graft-versus-Host Disease: A Task Force Report from the National Institutes of Health Consensus Development Project on Criteria for Clinical Trials in Chronic Graft-versus-Host Disease

Abstract

Chronic graft-versus-host disease (GVHD) is the leading cause of late, nonrelapse mortality and disability in allogeneic hematopoietic cell transplantation recipients and a major obstacle to improving outcomes. The biology of chronic GVHD remains enigmatic, but understanding the underpinnings of the immunologic mechanisms responsible for the initiation and progression of disease is fundamental to developing effective prevention and treatment strategies. The goals of this task force review are as follows: This document is intended as a review of our understanding of chronic GVHD biology and therapies resulting from preclinical studies, and as a platform for developing innovative clinical strategies to prevent and treat chronic GVHD.

Keywords: Blood and marrow transplantation; Chronic graft-versus-host disease; Clinical manifestations; Immune mechanisms.

Figure 1. Pathways to functional tolerance or chronic GVHD

Several factors significantly influence the immunologic landscape that evolves after allogeneic HCT and is ultimately responsible for 1) normal immune reconstitution, including the restoration of protective, anti-infective host immunity and the reestablishment of expanded T- and B-cell repertoires; 2) functional tolerance with preservation of graft-versus-tumor effects; or 3) immune dysregulation and alloreactivity that drives the development of chronic GVHD. These factors include, but are not limited to the following: conditioning regimen intensity, donor:host parameters including graft source, donor type, HLA match, age, and gender, the contribution of both mature lymphocytes infused at the time of HCT and those that are generated from the donor stem cell graft and educated in thymic remnants of the host, and the efficiency (or lack thereof) of early and late regulatory mechanisms. MA = myeloablative conditioning; RIC = reduced intensity conditioning; NMA = nonmyeloablative conditioning; +GVL = presence of graft-vs.-leukemia activity.

Figure 2. Biologic phases of chronic GVHD

A three-step model for the initiation and development of chronic GVHD is proposed that involves: early inflammation and tissue injury (phase 1), dysregulated immunity (phase 2), and aberrant tissue repair often with fibrosis (phase 3)*. In phase 1, numerous soluble, inflammatory, proteins including cytokines and TLR agonists are released in response to cytotoxic agents, infections, and acute GVHD. Together with cellular components of the innate immune system, these inflammatory stimuli result in diffuse, non-specific damage to numerous organs and the vascular endothelium. Endothelial cell activation and injury set the stage for the migration of donor immune cells into secondary lymphoid organs including the spleen and lymph nodes and subsequently into GVHD target tissues. Phase 2 is characterized by activation of effector populations in the adaptive immune system including T cells, B cells, antigen presenting cells and NK cells with compensatory inhibition by regulatory populations including Tregs, Bregs, NKregs, and possibly Tr1 cells. The response appears to be both antigen-specific (major and minor histocompatibility antigens) and non-antigen specific. Thymic injury and dysfunction engendered during phase I and phase II has deleterious effects on pathways of central tolerance. In Phase 3 is the propagation of tissue injury by dysregulated donor lymphocyte populations in the context of aberrant repair mechanisms, which sets the stage for the release of pro-fibrotic mediators leading to macrophage and fibroblast activation, collagen deposition, fibrosis, and irreversible end-organ dysfunction. *It should be noted that whereas these usually are sequential events, inpatients phase 1 can often go both to phase 2 and phase 3 simultaneously or sometimes only to phase 2 without phase 3.

Figure 3. Phase one: Early inflammation and tissue injury

Diagram of damage-induced activation of the innate immune system resulting in recruitment of Th1/Tc1 and Th17 cells to a tissue site. Ongoing damage to epithelial and connective tissue releases damage associated molecular patterns (DAMPs) including RNA, DNA, chromosomal HMGB1, extracellular matrix materials, ATP, and uric acid. RNA and DNA can be taken up into endosomes as part of immune complexes with anti-nuclear material auto-antibodies (triggering TLR3, TLR7, and TLR9). ECM and HMGB1 bind to plasma membrane TLR2, TLR4, and RAGE complexes. All of these TLR pathways trigger IRF transcription factors, inducing IFNα, and TNFα and IL-6 through NFkB. NLRP3 inflammasome formation can be triggered by ATP (via P2XR7), resulting in IL-1β production. IFNα, and IL-1β plus IL-6, can induce T-cell differentiation into Th1/Tc1 and Th17. IFNα and IL-17 also induce chemokines (CXCL9/10 and CCL20) that recruit Th1 and Th17 cells into tissues from the blood. Cytolytic attack by these effector T cells continues a cycle of tissue damage and release of DAMP molecules.

Figure 4. Phase Two: Chronic inflammation and dysregulated immunity

The production and release of inflammatory stimuli enhance interactions between antigen presenting cells and donor-derived lymphocyte populations including both CD4⁺ T cells, CD8⁺ T cells, and B cells. Some of the responses are antigen-specific, and some derive from non-specific, inflammatory pathways. Activation of donor lymphocytes results in the generation of effector populations. The production of signature cytokines, B cell activating factors (sBAFF) and auto-antibodies, along with chemokine receptor-ligand interactions, are part of the aberrant immune response contributing to inflammation and the recruitment of effector and regulatory cells into peripheral target tissues. The difference between the presence and absence of chronic GVHD appears to be due to a predominance of either dysregulated effector mechanisms when active chronic GVHD is present versus regulatory populations when chronic GVHD is absent or has resolved. The relative contributions of each remain to be determined.

Figure 5. Phase Three: Aberrant tissue repair and pathways of antibody-mediated fibrosis

The contribution of B cells to the development of chronic GVHD has been recently underscored in several experimental systems. One pathway emphasizes interactions between donor-derived T cells and B cells in secondary lymphoid tissues, including the spleen peripheral lymph nodes. The generation of allo-/auto-reactive B cells and the dysregulated production of allo-/auto-antibodies initiates a cascade of events that involves activation of monocytes and macrophages along with endothelial and epithelial injury. The release of soluble factors including TGFβ and fibroblast stimulation characteristic of aberrant tissue repair results in collagen and matrix production and deposition culminating in target organ fibrosis and dysfunction.

Figure 6. Pulmonary dysfunction and the tri-phasic model of chronic GVHD

Conceptually, the tri-phasic model of chronic GVHD can be applied to the development of lung dysfunction after allogeneic HCT. In phase one, acute lung injury occurs as a consequence of an allogeneic immune response and results in the influx of donor immune cells into an inflamed pulmonary parenchyma (A). Persistence of an inflammatory signal in the setting of dysregulated immunity promotes the transition from acute to chronic injury in phase two (B and D). If the inciting injurious event predominantly involves bronchiolar epithelial cells, phase II is associated with the development of chronic bronchiolitis (B). In the context of aberrant repair, chronic inflammation proceeds to phase three. Lung fibroblasts increase dramatically in number and contribute to the enhanced deposition of collagen and granulation tissue in and around bronchial structures, ultimately resulting in complete obliteration of small airways and fixed obstructive lung disease (OLD) characteristic of bronchiolitis obliterans (BrOb) (C). If, by contrast, the principal target of chronic inflammation is the alveolar epithelium, leukocyte recruitment and matrix deposition during phase two contribute to interstitial pneumonitis (D). Fibroblast proliferation and intra-septal collagen deposition during phase three ultimately results in interstitial thickening, septal fibrosis, significant volume reduction, and the development of severe restrictive lung disease (RLD) and interstitial fibrosis (E).