Early Marrow Microenvironment Immune Patterns After Hematopoietic Stem Cell Transplant in Pediatric Acute Lymphoblastic Leukemia Are Associated with Later Development of Chronic GvHD and Relapse

Hematopoietic stem cell transplant (HSCT) is a curative therapy for acute lymphoblastic leukemia (ALL), but its success is limited by chronic graft-versus-host disease (cGvHD) and disease relapse. A central challenge is uncoupling the graft-versus-leukemia (GvL) effect from cGvHD. Early changes in the bone marrow microenvironment following HSCT may offer a predictive window into these divergent outcomes. We conducted a retrospective, single-center, exploratory study on 14 pediatric ALL HSCT patients. Applying single-cell antibody-sequencing (AbSeq) on archived bone marrow aspirates collected 60-100 days post-HSCT, we evaluated immune patterns associated with the development of cGvHD or ALL relapse after day 114. cGvHD after day 114 was associated with upregulation of the endoplasmic reticulum (ER) stress transcription factor <i>XBP1</i> in transitional B cell and IgM memory B cell populations, a mincle^highPD1^- neutrophil population, and exhausted <i>LAG3</i>⁺ effector memory T cells (T_EM). ALL relapse after day 114 was associated with higher <i>CD22</i>, <i>CD24</i>, and <i>ARG1</i> expression in M(IL-4)-like macrophages and exhausted <i>TIGIT</i>⁺ T_EM. Results from this exploratory study suggest that marrow immune signatures of B cell ER stress preceding later development of cGvHD and macrophage-mediated immune evasion preceding relapse may potentially be early biomarkers for separating GvL from cGvHD in ALL HSCT. Validation with larger cohorts is warranted.