Alterations in Immune Cell Composition during First-Line Therapy with Mosunetuzumab for Follicular or Marginal Zone Lymphoma

Background: Bispecific CD20xCD3 antibodies (BiAbs) that engage T-cells to eliminate CD20+ B-cells have emerged as a potent immunotherapy for the treatment of follicular (FL) and marginal zone (MZL) lymphomas. BiAbs have primarily been studied in patients with extensive prior immunosuppressive and lymphodepleting therapies, so their effects on the immune system without such exposures are not well understood. Unlike cellular therapies, BiAbs are administered continuously over time, spanning from 6 months to an indefinite period, and recent studies have raised concerns about T-cell exhaustion due to continuous BiAb exposure in B-lymphoblastic leukemia (Philipp et al, Blood 2022). Mosunetuzumab is a BiAb approved for treatment of relapsed/refractory FL. The ongoing phase 2 BrUOG-401 trial (NCT04792502) is investigating mosunetuzumab as first-line therapy for FL or MZL. Here, we present preliminary results focusing on treatment-induced changes in circulating immune cells.