Peripheral CD8⁺ T-cell clonality and NK-cell expansion mark early response to mosunetuzumab in indolent B-cell lymphoma

Mosunetuzumab is highly effective against many B-cell lymphomas, but longitudinal immune changes induced by intermittently dosed CD20×CD3 bispecific antibodies remain incompletely characterized. We performed immune profiling of peripheral blood samples from patients receiving fixed-duration (6 months) subcutaneous mosunetuzumab for previously untreated follicular or marginal zone lymphoma on a phase 2 clinical trial (NCT04792502). Analyses included flow cytometry, cytokine profiling, and RNA-seq of isolated CD8+ T-cells with TCR repertoire inference. Early (at 3 weeks) on-treatment samples showed broad cytokine induction, CD8+ T-cell redistribution with decreased TEMRA cells and increased CD27+CD62L+ transitional effector memory-like cells, increased PD-1 and LAG-3 expression, and induction of CD8+ transcriptional programs consistent with activation, proliferation, exhaustion priming, and SREBF2-regulated cholesterol metabolism. By mid-treatment (at 3 months), activation-associated features contracted toward baseline, with no evidence of phenotypic exhaustion, though with persistent overexpression of LAG3, HAVCR2, and LAYN on RNA-seq, as well as sustained SREBF2 activation. CD8+ T-cell clonality increased at mid-treatment compared with baseline and higher clonality was associated with early clinical complete response. Mosunetuzumab also induced indirect NK-cell activation and expansion, including skewing toward CD56dimCD16bright effector state. HLA-DR upregulation on NK cells correlated with early increases in NK-activating plasma cytokines, and higher NK-cell count at mid-treatment was associated with complete response. Together, these findings define a CD8+ effector-remodeling state on mosunetuzumab therapy, characterized by clonal expansion, selective inhibitory receptor expression, sustained activation of cholesterol biosynthesis, and cytokine-linked engagement of innate immune mechanisms, providing a framework for potential strategies to support T-cell function and enhance bispecific antibody efficacy.