MCL Cells in Tumor Microenvironment Impair T-Cell Metabolic Fitness and Effector Function

Background: Mantle cell lymphoma (MCL) is an aggressive, incurable B cell malignancy. Major advances have been achieved with the development of clinically effective targeted agents including BTK and BCL-2 inhibitors. However, patients often relapse after these treatments. The emerging immune therapies combining immune checkpoint blockade and adoptive T-cell transfer are becoming increasingly desirable, but patient responses are heterogeneous, with a significant rate of failure. Tumor microenvironment (TME) plays an important role by inducing immune checkpoint and immune suppressive signaling or creating a metabolic barrier to antitumor immunity, particularly effector T-cells. To evade immune defenses, tumor and stromal cells cooperate and tactically express inhibitory ligands, such as PD-L1, while upregulating immune checkpoint receptors on effector T-cells and CAR T cells, resulting in negative signaling that leads to T-cell exhaustion, one of the major challenges in CAR T-cell therapy. Limited nutrient accessibility and a hostile immunosuppressive TME posts a strong barrier to effective immune therapies, with T-cell immunity being mostly affected. Impaired metabolism may also inhibit the function of effector T-cells while promoting suppressive activity of regulatory T-cells.

Results: Our recent transcriptomic analysis of primary MCL clinical specimens comprised of microenvironment cell populations, revealed that genes related to immune function were dysregulated, particularly the genes responsible for IFNG-mediated anti-tumor immunity. Proliferation in response to TCR stimulation is an important measurement for T-cell effector function and potency. Reduced proliferation upon T-cell activation denotes T-cell dysfunction. Indeed, T-cells isolated from MCL specimens especially CAR T non-response patients exhibit diminished cell division upon stimulation with anti-CD3/CD28 beads (25-75% less expansion). To recapitulate the interplay between T-cells and TME, MCL cells were co-cultured with the non-MCL fraction from the purification, and T-cells were then phenotypically profiled. Intriguingly, MCL-derived T-cells mostly displayed exhaustion phenotypes, indicated by decreased expression of T-cell activation markers (CD25, CD28, CD38, and CD71), but increased expression of exhaustion and senescence molecules (CTLA4, LAG3 and PD-1). Moreover, MCL-derived T cells failed to induce the expression of cytokine, such as INF-γ (<1-1.8 vs. 7.1 fold induction) and TNF-α (<1-1.3 vs. 10.2 fold induction), upon TCR stimulation, in contrast to T-cells derived from healthy donors, implicative of reduced cytotoxicity towards tumor cells. The results led us to further attest our hypothesis that MCL cells induce T-cell exhaustion by modulating the expression of inhibitory molecules or immune checkpoint molecules. Metabolic fitness is crucial for T-cell survival and effector function upon activation. A stable mitochondria membrane potential (ΔΨm) is required to maintain metabolic fitness of effector T-cells. The rate of glucose uptake is considered a prominent indicator of T-cell activation. However, MCL derived T-cells displayed remarkable decreased glucose uptake (fold MFI <1-1.21 vs. 1.95), mitochondria mass (p = 0.011) and ΔΨm (fold MFI <1-1.28 vs. 1.9), compared to those from healthy donors, indicating T-cell mitochondrial biogenesis and fitness are impaired in the MCL cell dominated TME. To assess the anti-tumor immunity of T-cells in MCL, we also examined the cytotoxicity of MCL-derived tumor specific cytotoxic T-lymphocytes (CTL) and observed compromised effector activity compared to that from healthy donors.

Conclusion: We presented evidence that MCL cancer cells suppress immune cell function by upregulating immunosuppressive signaling and forging a metabolic barrier to effector T-cells and immunotherapy. Targeting the MCL TME while enhancing T-cell metabolic fitness would dramatically advance current immunotherapy. Despite the success of checkpoint blockade and adoptive cell transfer therapy, complimentary approaches to overcome the metabolic barrier of immunosuppressive TME holds promise to dramatically enhance the efficacy of immunotherapies. Further, identification of the precise pathways and targets that determine T-cell metabolic fitness would facilitate developing new approaches to bolster current immunotherapy.