A novel RIPK1 inhibitor attenuates GVHD

In this issue of Blood, Yu et al identify the role of enterocyte RIPK1/RIPK3 in the generation of graft-versus-host disease (GVHD) and generate a potent RIPK1 inhibitor, Zharp1-211, which inhibits GVHD without impacting graft-versus-leukemia (GVL) in a murine model.¹ 

Acute GVHD in the gastrointestinal (GI) tract is a critical determinant of mortality and morbidity after allogeneic stem cell transplantation.² Intestinal epithelial cells (IECs) have been emerging as a pivotal lineage for the initiation and amplification of acute GVHD, while also representing a major target.³ The characteristic pathological feature of acute gut GVHD is the apoptosis of IECs. More recently, multiple nonapoptotic gene-regulated cell death pathways have been identified including necroptosis.⁴ In necroptosis, receptor-interacting serine/threonine-protein kinase 1 (RIPK1) and RIPK3 recruit mixed lineage kinase domain-like (MLKL) which executes cell death by forming membrane pores. Autophagy is a pivotal pathway by which IECs are protected from necroptosis. Thus, if the autophagic protein ATG16L1 is abrogated in IECs, RIPK3 overtly promotes MLKL phosphorylation, necroptotic cell death, and GVHD.⁵ However, necroptosis independent effects of the RIPK1/RIPK3 complex during GVHD have not been described.

Yu et al demonstrate that inactivation or depletion of RIPK1 or RIPK3 specifically in IECs profoundly reduces GVHD mortality and attendant visceral GVHD histopathology in autophagy-competent recipients.¹ This effect occurred in association with reduced levels of circulating cytokines and chemokines. Intriguingly, the protective effects of RIPK1 and RIPK3 inactivation were not dependent on the necroptotic or caspase-8 dependent apoptotic programmed cell death pathways. Indeed, MLKL deletion in recipient IECs delayed the onset of GVHD but did not prevent lethality. Rather, the authors found that RIPK1/RIPK3 complexes bind JAK1 to phosphorylate and activate STAT1 in IECs, resulting in enhanced chemokine (CCL9 and CCL10) secretion at multiple time points after bone marrow transplantation. Donor T-cell migration to the GI tract during GVHD is known to be dependent on multiple chemokine receptors such as CXCR3,⁶ which is the receptor for CCL9 and CCL10, and CCR5.⁷ RIPK1/RIPK3 complexes also enhanced systemic cytokine (interferon gamma [IFNγ] and tumor necrosis factor [TNF]) generation and major histocompatibility complex (MHC) class II expression by IECs at late time points during the effector phase of acute GVHD. IFNγ and TNF are known to be T-cell derived cytokines that are important effectors of GVHD tissue injury after BMT⁸ and the reduction seen in the absence of RIPK1 presumably reflects inhibition of T-cell migration and activation in the gut to attenuate this distal proinflammatory response in GVHD. IFNγ-dependent MHC class II expression by IECs early after BMT has been demonstrated to mediate donor CD4⁺ T-cell expansion and activation.⁹ However, in this setting (late after BMT), the increase in MHC class II expression on IECs is typically downstream of donor T-cell derived IFNγ⁹ and most likely serves to enhance IEC targeting by donor CD4⁺ effector T cells. Thus, RIP1K inhibition-dependent protection of IECs from GVHD-mediated injury serves as a powerful means to interrupt the feedforward cascade characteristic of severe gut GVHD.

Importantly, the authors have also synthesized a novel selective RIPK1 kinase inhibitor, Zharp1-211, that prevented murine GVHD while maintaining GVL effects. They also confirmed the effects of Zharp1-211 on chemokine transcription and MHC class II expression in human intestinal organoids. JAK1/2 inhibitors such as ruxolitinib, which are currently used in the treatment of GVHD generate immunosuppression via effects on the proliferation and activation of donor T cells and antigen presenting cells.² They also cause bone marrow suppression and associated cytopenia.^2,10 In contrast, Zharp1-211 did not appear to mediate effects on hematopoietic cells, and thus did not impair engraftment and count recovery. Thus, Zharp1-211 may possibly avoid the immune and marrow suppression seen after classical nonselective JAK1/2 inhibition. This new small molecule inhibitor of RIPK1 therefore represents a novel and promising agent to treat systemic GVHD and could be tested in well-designed clinical trials.

Conflict-of-interest disclosure: G.R.H. has consulted for Generon Corporation, NapaJen Pharma, iTeos Therapeutics, Neoleukin Therapeutics and has received research funding from Compass Therapeutics, Syndax Pharmaceuticals, Applied Molecular Transport, Serplus Technology, Heat Biologics, Laevoroc Oncology, Genentech, and iTeos Therapeutics. M.K. declares no competing financial interests.