Tumor Microenvironment-Induced Interleukin-6 Regulates Proliferation and Survival in Patients with Mantle Cell Lymphoma (MCL) By Activating FGFR1 Signaling

Introduction: Mantle Cell Lymphoma (MCL) is a rare and aggressive subtype of B-cell lymphoma with a median survival of 4 years, although this has been improved over the past decade with the advent of small molecule inhibitors and cellular therapies. However, resistance eventually emerges, leading to disease progression and relapse. Recent studies have emphasized the importance and prospect of studying the interaction between MCL and the tumor microenvironment (TME) mainly through myeloid immune cells and/or stromal cells to promote pro-tumorigenic and drug-insensitivity properties in MCL by inducing specific pathways, including those dependent on Fibroblast Growth Factor Receptor 1 (FGFR1). Hence, more studies have shifted the focus to the crosstalk between MCL and the TME to understand fundamental cellular mechanisms and thus develop novel therapies.

Methods: We first studied whether MCL cells are more proliferative when cultured in the condition media (CM) supernatant derived from BM stromal cells (HS-5) and monocyte-derived macrophages (THP-1 CD14⁺) by WST-1 and MTT assay. We then utilized cytokine array and ELISA to identify and quantify what cytokines were secreted from the niche of BM and LN. To investigate the potential role of IL-6 on MCL progression and whether it is FGFR1-dependent, we next generated Cas9-mediated single and double KO clones in MCL cells (Jeko-1) Ctrl (WT), FGFR-1 knockout (KO), IL-6 receptor knockout (IL6-RKO), and FGFR-1 and IL6-R dual knockout (DKO) cells, in either standard RPMI media or in HS-5 CM or, in standard RPMI media with stimulation of recombinant IL-6, to confirm the FGFR-1 level and other proteins from multiple significant signaling pathways. IL-6 blocking antibody tocilizumab (TocZ) was utilized to test whether IL-6 blockade can abrogate the induction of specific pathways. We also injected NSG mice with WT or FGFR-1 KO MCL cells with or without HS-5 cells. We also injected humanized IL-6 (hIL-6) mice or NOG mice with WT or FGFR-1 KO MCL cells and monitored tumor development.

Results: Our WST-1 and MTT assay data showed that MCL cells were more proliferative when cultured in HS-5 CM or macrophage CM than in media alone. Our cytokine array and ELISA data revealed that high levels of IL-6 were secreted into the microenvironment from BM (500pg/ml) and LN (730pg/ml) stromal cells. We identified that IL-6 was significantly upregulated in the monocyte-derived macrophages compared to the MCL cells alone. We also detected higher basal and IL-6-induced FGFR-1 expression in only WT MCL cells but not in the KO lines when cultured in HS-5 CM. Direct coculture of MCL cells (JVM2, Jeko-1) with monocyte-derived macrophages increased the STAT3 tyrosine activation. Interestingly, the STAT3 signaling pathway was induced by IL-6 stimulation in WT and FGFR-1 KO cells but was abrogated by blocking with TocZ or when the IL-6RKO and DKO cell lines were used. Similarly, IL-6 stimulation induced FGFR-1 expression in only WT MCL cells but not FGFR-1 KO, IL-6RKO, and DKO MCL cells. TocZ abrogated IL6-induced phosphorylation of STAT-3 and downstream signaling. Direct coculture of MCL cells with monocyte-derived macrophages also increased the STAT3 tyrosine activation. NSG mice injected with both WT MCL cells and HS-5 cells developed tumors more quickly and had a median survival of 35 days than the ones injected with FGFR-1 KO MCL cells and HS-5 cells, which had a median survival of 80 days (pValue=0.001), even though both groups of mice eventually succumbed to tumor burden. Similarly, more aggressive tumor development was observed in humanized-IL-6 mice injected with WT MCL cells than the ones injected with FGFR-1 KO MCL cells ( n=10 each group; pValue=0.01), confirming that IL6-dependent and FGFR1 independent pathways contribute to relapse in preclinical models.

Discussion: We demonstrated that TME and its effect on tumor cell proliferation and survival through macrophages and stromal cells are critical factors in MCL progression. Our study identified a key cytokine, IL-6, derived from the BM and LN niche of TME that plays a crucial role in MCL growth via FGFR1. Moreover, we showed that IL-6 induces the JAK/STAT signaling pathway in an FGFR1-independent manner to promote tumor development. Our study provides novel therapeutic insights into MCL and TME interactions and suggests that inhibiting FGFR1 (MCL cells) and IL-6 (TME) axis could yield better overall survival among MCL patients.