Targeting ST2 Alleviates Graft-Versus-Host Disease Mortality and Maintains Graft-Versus-Leukemia

Graft-versus-host disease (GVHD) remains the major barrier to the success of allogeneic stem cell transplantation (SCT). We have reported that soluble suppression of tumoreginicity 2 (ST2) measured in plasma of post-SCT patients is a marker for risk of therapy-resistant GVHD and death. ST2 is a member of the IL-1 receptor family whose sole known ligand is IL-33. Soluble ST2 acts as a decoy receptor for IL-33. Thus, we hypothesized that blockade of soluble ST2 will increase the amount of free IL-33 that will be able to bind to membrane ST2 on CD4+ T cells, driving them toward a Th2 phenotype, which will alleviate GVHD. First, we found that similar to plasma ST2 course in SCT patients, plasma ST2 was markedly increased prior to and at the onset of GVHD symptoms in multiple clinically relevant GVHD murine models (B6 to C3H.SW shown, Fig. 1A). Based on this observation, anti-ST2 antibody given with a prophylactic schedule (one dose before SCT and one dose at day+1 post-SCT) significantly reduced GVHD severity and mortality (Fig. 1B). Pathology analysis indicated that anti-ST2 treated recipients showed lower histopathologic score in liver and intestine (Fig. 1C). Strikingly, anti-ST2 significantly increased plasma IL-33 (Fig. 1D). Whole transcriptome analysis of mesenteric lymph node T cells showed that anti-ST2 modulated gene expression of helper T cell (Th1/Th2) related cytokines (Fig. 1E), suggesting that ST2 blockade may affect the helper T cell compartment. To further assess the effects of ST2 blockade on helper T cells, we examined the Th1/Th2 balance by flow cytometry. Administration of anti-ST2 shifted Th1/Th2 balance toward a Th2 phenotype (Fig. 2). In addition, ST2 blockade also induced expansion of regulatory T cells (Tregs) (Fig. 2).

Given that anti-ST2 administration also up-regulated expression of genes, such as Granzyme A, that can mediated graft vs. leukemia (GVL) responses (Fig. 1E), we postulated that ST2 blockade would not affect therapeutic GVL activity. To confirm this, we developed a clinically relevant model of leukemia, in which C3H.SW recipients were challenged with syngeneic GFP+ MLL-AF9 induced acute myeloid leukemia cells. Our results indicated that administration of anti-ST2 preserved substantial GVL activity and resulted in significantly improved leukemia-free survival (Fig. 3), suggesting that anti-ST2 ameliorated GVHD and maintained GVL response.

In summary, we found that prophylaxis with anti-ST2 antibody could alleviate GVHD severity and mortality while preserving GVL effect. ST2 blockade increased the level of plasma IL-33, skewed the Th1/Th2 balance toward a Th2 phenotype, and induced Foxp3+Tregs [(with preserved expression of membrane ST2 (not shown)]. Our findings suggest that ST2 is a novel therapeutic target to ameliorate GVHD.