Allogeneic hematopoietic cell transplantation (allo-HCT) is an essential therapeutic modality for patients with hematological malignancies and other blood disorders. Unfortunately, acute graft-versus-host disease (aGVHD) remains a major source of morbidity and mortality following allo-HCT, which limits its use in a broader spectrum of patients. Chronic graft-versus-host disease (cGVHD) also remains the most common long-term complication of allo-HCT, occurring in reportedly 30-70% of patients surviving more than 100 days. Chronic GVHD is also the leading cause of non-relapse mortality (NRM) occurring more than 2 years after HCT for malignant disease. Graft versus tumor (GVT) is a major component of the overall beneficial effects of allogeneic HCT in the treatment of hematological malignancies. Better understanding of GVHD pathogenesis is important to identify new therapeutic targets for GVHD prevention and therapy. Emerging data suggest opposing roles for different T cell subsets, e.g., IFN-γ producing CD4+ and CD8+ T cells (Th1 and Tc1), IL-4 producing T cells (Th2 and Tc2), IL-17 producing T cells (Th17 and Tc17), IL-9 producing T cells (Th9 and Tc9), IL-22 producing T cells (Th22), T follicular helper cells (Tfh), regulatory T-cells (Treg) and tissue resident memory T cells (Trm) in GVHD and GVT etiology. In this review, we first summarize the general description of the cytokine signals that promote the differentiation of T cell subsets and the roles of these T cell subsets in the pathogenesis of GVHD. Next, we extensively explore preclinical findings of T cell subsets in both GVHD/GVT animal models and humans. Finally, we address recent findings about the roles of T-cell subsets in clinical GVHD and current strategies to modulate T-cell differentiation for treating and preventing GVHD in patients. Further exploring and outlining the immune biology of T-cell differentiation in GVHD that will provide more therapeutic options for maintaining success of allo-HCT.

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Prognostic biomarkers used to identify likelihood of disease progression have not been identified for chronic graft-versus-host disease (cGVHD), the leading cause of late non-relapse mortality (NRM) in survivors of allogenic hematopoietic cell transplantation. Gastrointestinal cGVHD (GI-cGVHD) has been particularly challenging to classify. Here, we analyzed three proteomics markers [Regenerating-islet-derived-3-alpha (Reg3α), C-X-C-motif-ligand (CXCL9) and Stimulation-2 (ST2)] in two independent cohorts of patients with cGVHD totaling 289 patients. Plasma concentrations of Reg3α were significantly increased in patients with GI-cGVHD compared to those without (p=0.0012, p=0.01 respectively), CXCL9 and ST2 were not. Patients with high Reg3α (≥72ng/mL) vs. low Reg3α had higher NRM (23% vs. 11%, p=0.015). Since Reg3α has been identified as a lower GI-tract marker in acute GVHD, we correlated Reg3α with lower acute-like GI-cGVHD vs. classical fibrotic-like esophageal manifestations and found Reg3a did not differ between the subtypes. No difference was observed between upper and lower subtypes. Patients with extremely high Reg3α (≥180 ng/mL) had higher GI-scores but not higher lower-GI-scores. In multivariate Cox regression model, patients with high Reg3α were 1.9 times more likely to die without relapse. Our findings demonstrate the utility of Reg3α as a prognostic marker for GI-cGVHD. These data warrant prospective biomarker validation studies.

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