B‐cell activating factor receptor expression in B‐lymphoblastic leukaemia: Flow cytometric analysis and implications for targeted therapy

Cluster of differrentiation 19 (CD19)-directed therapies have significantly improved outcomes in B-cell malignancies; however, relapse driven by antigen escape remains a major clinical challenge, underscoring the need for alternative therapeutic targets. The B-cell activating factor receptor (BAFF-R), a key regulator of B-cell survival, is highly expressed in mature B-cell neoplasms, but its relevance in B-lymphoblastic leukaemia (B-ALL) remains unclear. We assessed BAFF-R expression in diagnostic and relapsed fresh B-ALL samples using multi-parameter flow cytometry. BAFF-R was consistently expressed in most cases, albeit at lower levels than in mature B cells, and expression was retained in the majority of CD19-negative relapses following CD19-directed therapy. Notably, tumor protein p53 (TP53) alterations were significantly enriched in CD19-negative immune escape disease (68.8% vs. 21.7%; p = 0.0006; OR = 7.9), yet BAFF-R expression remained preserved in this high-risk group. These findings establish BAFF-R as a stable immunotherapeutic target in B-ALL, including in CD19-negative and TP53-altered relapsed disease, and underscore its relevance amid ongoing early-phase clinical trials of BAFF-R-directed chimeric antigen receptor (CAR) T-cell therapy.