When early acute rejection is adequately controlled, transplanted organs enter a regenerative phase characterized by the presence of recipient monocyte-derived, tissue-reparative macrophages. These cells are responsible for tissue remodeling by removing necrotic cells, stimulating revascularization, secreting tissue-trophic factors, and suppressing intragraft allogeneic T cell reactions.1 Ideally, transplanted tissues would then return to normal homeostasis and stable allograft acceptance would be achieved. However, nonresolving inflammation is the more usual outcome, leading to progressive organ fibrosis, parenchymal cell death, and late formation of donor-specific antibodies. This process, which is called chronic rejection, is currently the leading cause of kidney, lung, and heart transplant failure. Although T cell alloimmunity is an important driver of nonresolving inflammation, there is now a growing appreciation for the pathological importance of dysregulated tissue-repair processes in chronic transplant injury.2 With few drugs on the horizon, finding new therapies to regulate the innate switch between resolution of inflammation and chronic rejection is a priority. Extracorporeal photopheresis (ECP) is a long-established and safe therapeutic procedure. ECP was originally developed for the management of cutaneous T-cell lymphoma.3 Later, it was introduced as an empiric therapy for various autoimmune diseases and inflammatory conditions. Today, ECP is used for special indications in solid organ transplantation, notably retarding bronchiolitis obliterans syndrome (BOS) in lung transplant recipients4 and treating heart transplant rejection.5 Although these established applications show that ECP is clinically valuable in cardiothoracic transplant recipients, we lack a proper understanding of its immunological actions. Consequently, many clinicians have been reluctant to explore other applications, especially in abdominal organ transplantation. ECP has nevertheless recently resurfaced for niche indications, such as minimizing immunosuppression during viral infections or malignancy, suppressing DSA levels after kidney transplantation, and avoiding calcineurin inhibitors early after liver transplantation in patients with preoperative renal impairment. The ECP procedure involves collecting autologous mononuclear cells from blood by apheresis, which are then exposed to 8-methoxypsoralen (8-MOP) and ultraviolet A (UVA) irradiation to induce apoptotic cell death.6 The resulting suspension, which is known as a photopheresate, is then returned to the patient by intravenous infusion. Of note, this procedure not only induces cell death in leukocytes, but also brings blood components into contact with plastic surfaces and anticoagulants, activating cells and modifying plasma proteins. Hence, photopheresates not only contain dead or dying leukocytes but also a living cell fraction and acellular components. The immunomodulatory effects of ECP are generally attributed to apoptotic cells or subcellular particles liberated from apoptotic cells. It has been known since the late 1990s how peripheral tolerance against self-antigens is maintained by a steady-state turn-over of dendritic cells.7,8 Nonimmunogenic death of parenchymal cells releases self-antigens that are taken up by tissue-resident dendritic cells (DCs). These DCs then migrate in a semimatured state to secondary lymphoid organs, where they contribute to self-reactive T cell deletion or anergy, creating an essential niche for regulatory T cells (Treg) survival. This fundamental mechanism has been exploited in transplant experiments to establish donor-specific tolerance. Intravenous injection of apoptotic splenocytes prolongs allogeneic cardiac graft survival in immunocompetent mice through T cell-suppressive effects mediated by splenic red-pulp macrophages and CD8+ marginal zone DC.9 Receptors that recognize apoptotic cells, either directly or via opsonins, are well described; in particular, how their engagement upregulates anti-inflammatory and tissue-reparative functions in macrophages and DC is now understood in molecular detail.10 Crucially, myeloid antigen presenting cells exposed to apoptotic cells control interleukin (IL)-17-producing CD4+ T cells and interferon (IFN)-γ-producing CD8+ T cells, while also supporting FoxP3+ Treg differentiation and survival. Therefore, the therapeutic benefits of ECP in transplantation, including its antifibrotic effects, were thought to be primarily mediated by myeloid regulatory cells suppressing alloreactive T cells, which secondarily deprives B cells of T cell help. This concept was recently revised after Zhiyi Liu et al11 published game-changing work in the Journal of Clinical Investigation, which showed that ECP suppressed airway fibrosis in a mouse lung transplant model by restricting transforming growth factor-β (TGF-β) bioavailability through induced secretion of decorin from alveolar macrophages. In this study, bronchiolar injury leading to severe obliterative bronchiolitis lesions was triggered in fully allogeneic (H-2q into H-2b) transplanted lungs by deleting club cells that inducibly expressed DT receptor (Figure 1). Recipients subsequently treated with photopheresates generated from 8-MOP/UVA-treated recipient-strain splenocytes had reduced peribronchiolar inflammation and fewer obliterative bronchiolitis lesions than saline-treated controls. Furthermore, ECP promoted regeneration of club cells, limited graft infiltration by neutrophils, and suppressed both IL-17-producing CD4+ T cells and IFN-γ-producing CD8+ T cells. Importantly, ECP reduced the number of autoreactive CD4+ T cells directed against collagen V and k-α tubulin, which are self-antigens associated with BOS, as well as reducing serum DSA levels. Hence, ECP specifically controlled allogeneic T and B cell responses.FIGURE 1.: ECP suppresses lung transplant fibrosis by inducing decorin expression in alveolar macrophages. Zhiyi Liu et al investigated the effects of photopheresates on airway inflammation and development of bronchiolitis obliterans (OB) lesions in a mouse lung transplant model. C57BL/6 (B6; H-2b) mice received a left lung transplant from allogeneic 3T-FVB (H-2q) donors. Graft acceptance was induced with 250 μg anti-CD40L (MR1) i.p. on day 0 and 200 μg mouse CTLA4-Ig i.p. on day 2. 3T-FVB mice carry transgenes for a reverse tetracycline activator under the club cell secretory protein (CCSP) promoter, Cre under control of the reverse tetracycline activator and lox-P-activated diphtheria toxin A. On day 8, deleting club cells by applying DOX caused bronchiolar injury leading to severe OB lesions in NaCl control-treated mice. ECP therapy was modeled by treating splenocytes from B6 mice with 8-MOP and UVA irradiation to induce apoptotic cell death. ECP-treated mice received i.p. injections of photopheresates equivalent to 107 mononuclear cells on days 9, 12, and 15 posttransplantation. Figure adapted from Zhiyi Liu et al.11 CD, cluster of differentiation; DOX, doxycycline; ECP, extracorporeal photopheresis; FVB, Friend leukemia virus B mouse strain; IFN, interferon; IL, interleukin; i.p., intraperitoneal; MOP, methoxypsoralen; NaCl, saline; TGF, transforming growth factor; UVA, ultraviolet A.TGF-β is a pleiotropic growth factor whose immunological activities are modulated by many cofactors, resulting in both immunoregulatory and immunogenic effects within transplanted organs. On the one hand, TGF-β suppresses tissue inflammation and helps to restore immune homeostasis, partly through stabilization of FoxP3+ Treg; on the other hand, TGF-β promotes Th9 and Th17 cell differentiation, and potently stimulates fibrogenesis. In their study, Zhiyi Liu and colleagues measured the amount and activity of TGF-β isoforms in bronchiolar lavage and serum. Remarkably, although TGF-β1 levels were not different in ECP-treated and control mice, the bioactivity of TGF-β was significantly less in ECP-treated recipients. Profiling expression of cofactors involved in TGF-β signaling led to the discovery that decorin, an antagonist of TGF-β activity, was strongly upregulated in alveolar macrophages after engulfing 8-MOP/UVA-treated leucocytes. Comparing lung transplants from mice with decorin-deficient alveolar macrophages to wild-type donors showed that ECP acts primarily by inducing alveolar macrophages to secrete decorin, limiting TGF-β bioavailability. Infiltration of lung allografts by CCR2-expressing blood monocytes drives pulmonary fibrosis. In their study, Zhiyi Liu and coworkers showed that TGF-β signaling upregulates CCL2 expression in alveolar macrophages, which promotes accumulation of CCR2+ monocyte-derived macrophages within allografts. These macrophages cross-presented donor MHC class I molecules (specifically, H-2Kq) and responded to TGF-β by enhancing their capacity for activating tissue-resident memory CD8+ T cells (TRM cells). Expression of Granzyme B by activated PD-1+ CD49a+ CD8+ TRM cells led to further airway epithelial damage and promoted development of BOS. Thus, TGF-β was identified as the primary driver of monocytic infiltration, CD8+ TRM cell activation, and fibrosis after lung transplantation. In this scenario, ECP prevents the development of BOS by limiting TGF-β bioavailability through induced expression of decorin. Accordingly, ECP not only suppresses T cell-dependent graft injury but also regulates TGF-β-dependent, T cell-independent fibrogenesis. The surprising mechanisms unveiled by Zhiyi Liu and colleagues point toward new pharmacological targets, as well as opening many potential indications for ECP in other transplant settings. Whether reduced transplant vasculopathy in ECP-treated cardiac transplant could be explained by similar mechanisms must now be investigated. The possible implications for managing chronic rejection in kidney transplant recipients are remarkable. ECP has perhaps finally found its purpose in organ transplantation as a direct antifibrotic therapy.