Abstract
Introduction: B-cell maturation antigen (BCMA)-directed chimeric antigen receptor T-cell (CAR T) therapies such as ciltacabtagene autoleucel (cilta-cel) induce deep and durable responses in relapsed/refractory multiple myeloma (MM). Because BCMA is expressed on normal and malignant plasma cells, on-target/off-tumor activity contracts the normal B-cell compartment, leading to profound, often protracted hypogammaglobulinemia.
Next generation sequencing (NGS)-based measurable residual disease (MRD) assays such as clonoSEQ® not only detect malignant clones to sensitivities of 10-6 but also enumerate unique immunoglobulin heavy (IgH) and light chain (IgK, IgL) sequences arising from normal B-cells. We hypothesized that early post-infusion IgH/IgK/IgL metrics reflect B-cell reconstitution kinetics and may confer prognostic information after BCMA CAR T-cell therapy.
Methods: This single-center retrospective study included all MM patients aged ≥18 years treated with commercial cilta-cel at the University of Chicago who were MRD negative by NGS (clonoSEQ®) at the 10-5 threshold within 100 days post-infusion. From each clonoSEQ® report, we abstracted total nucleated cells and total and unique IgH, IgK, and IgL sequences. B cell abundance (total IgH/total nucleated cells) and repertoire diversity (unique/total IgH, IgK, and IgL, respectively) were calculated for each sample. Survival curves were estimated with the Kaplan–Meier method and compared with the log-rank test after dichotomizing B-cell metrics as indicated below. Hazard ratios (HRs) were calculated using Cox proportional-hazards models.
Results: Thirty-seven patients met inclusion criteria. Median age was 66 years (range 41-85); 27/37 (73%) were male. High-risk cytogenetic abnormalities (HRCA), defined as t(4;14), t(14;16), t(14;20), deletion 17p/tp53 deletion, 1q gain, or 1q amp, were present in 25/37 (68%). The cohort had received a median of 4 prior lines of therapy (range 2-8). All patients achieved MRD negativity <10-5 within 100 days post-infusion and 36/37 (95%) had MRD<10-6.
With a median follow-up of 15.5 months from CAR T-cell infusion, the 1-year PFS was 75% (95% CI 55-88%); 1-year OS was 89% (95% CI 69-96%).
The median IgH-based B-cell abundance was 2.7% (range 0.05-32.7%, interquartile ranges (IQR) 0.5%-5.4%). Neither low B-cell abundance (below median or in first quartile) nor high B-cell abundance (fourth quartile) was associated with PFS or OS.
As for repertoire diversity, the median unique/total IgH, IgK, and IgL sequence percentages were 81.7% (range 38.6-100%, IQR 72-94%), 67.3% (range 11.5-100%, IQR 38-100%), and 83.1% (range 20-100%, IQR 53-100%), respectively. There were no statistically significant associations between PFS and the lowest quartile for unique IgH percentage (HR 0.56, 95% CI 0.1-2.5, p=0.45), IgK percentage (HR 0.29, 95% CI 0.04-2.3, p=0.24), or IgL percentage (HR 2.05, 95 CI 0.6-7.6, p=0.28). No B-cell diversity metric was associated with OS at this follow-up.
Eight patients exhibited low diversity in at least 2 loci (IgH, IgK, IgL). None of these 8 have progressed or died. One-year PFS was 100% vs 69% for others; 1-year OS was 100% vs 86% for others.
Conclusions: Although post-CAR T bone marrow NGS is primarily deployed for MRD assessment alone, this study highlights its additional utility for profiling B-cell reconstitution. We provide early benchmarks for ‘normal’ B-cell recovery kinetics in deep responders and, preliminarily, observe that oligoclonal (low diversity) reconstitution across multiple Ig loci after cilta-cel could complement MRD status as a prognostic tool. Validation in larger cohorts with longer follow-up is required to confirm these exploratory signals and to define diversity thresholds that could inform immune-reconstitution–guided monitoring after BCMA-CAR T therapy.
