Full CD34 Donor Chimerism Following CD34 Chimerism Directed Donor Lymphocyte Infusion Results in Reduced Relapse and Superior Overall Survival Compared with Mixed Chimerism Status in Allogeneic Haemopoietic Stem Cell Transplant Recipients

Introduction Peripheral blood CD34 donor chimerism (DC) is a clinically valuable method for detecting relapse of myeloid malignancies following allogeneic hematopoietic stem cell transplantation (alloHSCT). Our institution has recently shown that CD34 DC ≤80% was superior to CD3 DC for predicting relapse (1). While pre-emptive DLI has demonstrated efficacy in preventing relapse in the setting of falling whole unfractionated and CD3 DC, little has been reported about its utility in falling CD34 DC. Methods We conducted a retrospective analysis of consecutive patients who received DLI at the Alfred Hospital in Melbourne, Australia between January 2012 and January 2023. Peripheral blood CD34 DC was monitored in alloHSCT patients transplanted for myeloid malignancies or acute lymphoblastic leukaemia (ALL). CD34+ cells were enriched using magnetic beads and DC measured using an STR-PCR method, which had a sensitivity of 1%. DLI was administered to patients with CD34 ≤80% that did not respond to immunosuppression taper. DLI doses were based on donor type and recipient weight, with subsequent doses administered incrementally according to a predefined protocol. Starting doses were 3x10 6/kg for sibling donors, 1x10 6/kg for unrelated donors and 0.5x10 6/kg for haploidentical donors. DLI was discontinued upon achieving full DC (FDC) defined as CD34 DC > 95%, clinically significant graft-versus-host disease (GVHD), disease progression, or lack of available donor cells. Results One hundred and six patients received DLI between 2012 and 2023. Twenty-five of these patients received DLI for CD34 DC ≤80% without morphologic relapse. Eleven of 25 (44%) patients received DLI alone, while 14 (56%) patients received it in combination with other therapies, primarily a hypomethylating agent. At the time of DLI, 5 (20%) of the 25 patients had concurrent decreased CD3 DC ≤ 80%. The median follow-up period was 1.84 years (range 0.58 to 8.43 years). Sixteen of 25 (66.7%) patients achieved CD34 FDC while 8 (33.3%) had persistent CD34 mixed DC (CD34 MDC). One patient could not be assessed due to death from infection. Patient characteristics and outcomes according to response to DLI are presented in Table 1. The cumulative incidence of relapse was significantly lower in the CD34 FDC group (12.5% vs 75% at 2 years, p