Antithymocyte globulin (ATG) has been shown to reduce chronic graft-versus-host disease (GVHD) particularly in allogeneic peripheral blood stem cell transplantation (PBSCT) from unrelated donors; however, anti-GVHD effects of lower doses of ATG remains to be elucidated. We conducted a nationwide retrospective study to compare the outcomes of unrelated PBSCT with or without rabbit ATG (thymoglobulin) in 287 patients. A median ATG dose was 2.0 mg/kg. The primary endpoint, the cumulative incidence of moderate–severe chronic GVHD at 2 years was 22.1% in the ATG group, which was significantly less than that in the non-ATG group (36.3%, P = 0.025). The ATG group had a higher incidence of immunosuppressant discontinuation, GVHD-free, relapse-free survival, and moderate–severe chronic GVHD-free, relapse-free survival at 2 years compared to the non-ATG group. The incidences of grade III–IV aGVHD and moderate–severe chronic GVHD were significantly higher in patients with high absolute lymphocyte count (ALC) before the administration of ATG, whereas relapse rate was significantly higher in patients with low ALC before ATG. In conclusion, low-dose ATG effectively suppresses chronic GVHD in unrelated PBSCT, and ALC before ATG may be a potential predictor for GVHD and relapse.
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