Int J Mol Sci

T-cell/histiocyte-rich large B-cell lymphoma (THRLBCL) is a rare, aggressive subtype of diffuse large B-cell lymphoma characterized by a profoundly immunosuppressive tumor microenvironment. PD-L1 overexpression by tumor cells is a recognized immune escape mechanism and may underlie resistance to cellular therapies, including CAR T-cell therapy. We report a case of a 29-year-old woman with refractory stage IV-B THRLBCL treated with anti-CD19 CAR T-cell therapy (varnimcabtagene autoleucel), who achieved an initial response (day +28) but experienced disease progression by day +100 despite robust CAR T-cell expansion. Peripheral blood analysis revealed persistent absolute B-cell aplasia, while bone marrow biopsy confirmed CD19-positive disease. Comparative immunohistochemistry demonstrated markedly increased PD-L1 expression in post-CAR T-cell samples, suggesting adaptive immune resistance via PD-1/PD-L1-mediated CAR T-cell inhibition. Nivolumab was initiated at month +4 to overcome this checkpoint-mediated resistance. Notably, a complete metabolic response was documented on PET/CT after four doses of nivolumab (month +6). The patient remains in sustained remission, with persistent B-cell aplasia, four years post-intervention. This case provides clinical and pathological evidence supporting the use of immune checkpoint blockade to rescue CAR T-cell efficacy, highlighting the potential of this synergistic approach in THRLBCL and possibly other B-cell malignancies exhibiting similar immune evasion.