Abstract C009: Transcriptional regulation of CD19 by IKZF1 enhances CAR T-Cell immune targeting in mantle cell lymphoma

Abstract Mantle cell lymphoma (MCL) remains difficult to cure, and responses to chimeric antigen receptor (CAR) T-cell therapy vary in part due to modulation of target-antigen levels. We hypothesized that enhancing Ikaros transcription factor IKZF1, either directly by lentiviral overexpression or indirectly through inhibition of casein kinase 2 alpha (CK2α), a negative regulator of IKZF1 stability, would increase CD19 transcriptional output and thereby enhance CAR T-cell recognition and clearance of MCL cells. Human MCL cell lines JeKo-1 and Mino were engineered to overexpress IKZF1 using lentiviral vectors (controls: parental, empty-vector). Parallel experiments leveraged CK2α inhibition with silmitasertib to increase endogenous IKZF1 protein stability via reduced proteasomal turnover. CD19 surface density was quantified by flow cytometry with calibrated beads, CD19 transcripts by RT-qPCR, and promoter activity using CD19-luciferase reporters. Antitumor activity of CD19-directed CAR T cells was assessed in short-term co-cultures across effector:target (E:T) ratios, with target-cell viability (plate reader and flow cytometry) and cytokines (Lumit bioluminescence-based assay) collected at defined timepoints. For in vivo validation, luciferase-RFP-tagged JeKo-1/Mino xenografts were established in NSG mice followed by CAR T-cell infusion; tumor burden (bioluminescence) and survival were monitored longitudinally. IKZF1 overexpression increased CD19 mRNA and protein in JeKo-1 and Mino cells by ∼2.5- and 2.30-fold. CK2α inhibition produced a concordant phenotype, elevating endogenous IKZF1 and yielding proportional increases in CD19 surface density and transcript abundance. Promoter-reporter assays showed increased CD19 promoter activity. In vitro, CAR T-cell killing was enhanced by IKZF1 modulation: at E:T 1:1, viable target cells at hour 4 were 41.5% (IKZF1-OE) vs 63% (control), and the survival AUC decreased by 21.5%. CAR T-cell function mirrored antigen upregulation, with elevated IFN-γ, TNF, and IL-2 secretion (all p < 0.001) and reduced exhaustion-marker induction at 24 hours. CK2α-inhibited conditions similarly enhanced CAR T activation, supporting a mechanistic link between IKZF1 stabilization, heightened antigen availability and increased CAR T-cell target recognition and killing. In conclusion, increased IKZF1, either by direct overexpression or through CK2α inhibition, upregulates CD19 transcription and surface density in MCL, resulting in more rapid and durable CAR T-cell cytotoxicity in vitro and in vivo. These findings identify CK2α–IKZF1 as a druggable regulatory axis controlling antigen availability and highlight a complementary strategy to enhance CAR T-cell responses in MCL by maintaining or boosting CD19 expression. Ongoing CHIP-seq experiments will define IKZF1 occupancy at the CD19 locus and durability and safety across additional MCL in vivo models. Citation Format: Anya K. Sondhi, Tulasigeri M. Totiger, Stanzin Idga, Jonathan Schatz, Jay Spiegel, Justin Taylor. Transcriptional regulation of CD19 by IKZF1 enhances CAR T-Cell immune targeting in mantle cell lymphoma [abstract]. In: Proceedings of the AACR Immuno-Oncology Conference (AACR IO): Discovery and Innovation in Cancer Immunology: Revolutionizing Treatment through Immunotherapy; 2026 Feb 18-21; Los Angeles, CA. Philadelphia (PA): AACR; Cancer Immunol Res 2026;14(2 Suppl):Abstract nr C009.