FL is a kind of indolent lymphoma with heterogeneity. Previous studies focused on identifying related genes using differential expression analysis. In this study, we applied WGCNA analysis utilizing RNAseq data of FL and found three related co-expression gene modules, explored the gene network relationship, and identified eight key genes of the network. The prognostic value of eight key genes were validated in two independent FL patient cohorts and we identified that CCL19 was significantly associated with PFS and OS in FL patients receiving first-line chemoimmunotherapy, which indicated that CCL19 could be a potential prognosis biomarker for FL. To our best knowledge, this study is the first to report the potential prognostic value of CCL19 in FL.
In previous studies, different genes were reported to be related to the development of FL. About over 85% FL patients were characterized by t (14;18), which resulted in the overexpression of BCL2 protein, a family of protein that inhibits cell apoptosis[22]. Mutations of chromatin modifying genes, including EZH2, KMT2D and CREBBP, were reported to play a complex role in FL [23]. Mutations in linker histone genes (HIST1H1 B, C, D, and E; OCT2 [POU2F2]; IRF8; and ARID1A) were also reported to be associated with the pathogenesis of FL [24]. Besides, gene mutations including STAT6 and RRAGC were related to FL pathogenesis, which involving multiple pathways including mTOR and JAK/STAT pathways[25–27]. In recent studies, clinicogenetic risk models integrating genes for predicting prognosis of FL has been established[7, 8] and has showed the value as the prediction model in FL management. For germinal center-derived lymphoma, including FL, moving out of germinal center and local endothelial cells is essential for dissemination to other lymph nodes or tissues[28]. Previous studies showed that interaction of lymphoma cell with chemokine played an important role in the movement of lymphoma cell through endothelial cells[29–31] in chronic lymphocytic leukemia and classical Hodgkin lymphoma. Chemokines modulated chemotaxis in the migration and pathogenesis of FL cells was also presented in previous studies[32, 33]. In this study, among the three modules identified, the turquoise module showed significantly association with FL tumors. Functional analysis showed that genes in turquoise module was enriched in cell chemotaxis, extracellular matrix organization and extracellular structure organization. Results of our study further supported that pathway of cell interaction with chemokine may be closely related to the development of FL.
In this study, we identified that CCL19 was significantly associated with survival for FL patients treated with rituximab in combination with chemotherapy. CCL19 encoded cytokine that involved in immunoregulatory and inflammatory processes. CCL19 specifically binds to chemokine receptor CCR7. CCR7 and CCL19 played an important role in organizing thymic architecture and function, lymph-node homing of naive and regulatory T cell, as well as homeostasis and inflammation-induced lymph-node-bound migration of dendritic cells, which indicated that CCR7 and CCL19 involved in the homeostasis, immune surveillance, and tumor formation[34]. In previous studies, CCR7/CCL19 was reported to be associated with some types of cancer. CCR7 chemokine receptor binds to the ligand CCL19/CCL21 and promotes lymphogenesis and metastasis in breast cancer[35]. CCL19 overexpression significantly inhibited gastric cancer cell proliferation and tumor growth through CCL19/CCR7/AIM2 pathway[36]. Also, CCL19 overexpression is associated with malignant transformation in cervical cancer[37]. For lymphoma, O'Connor et al. reported that CCL19-CCR7 interactions may contributed to the increasing risk of age-related central nervous system lymphoma[38]. In T cell lymphoma, higher expression of CCR7 was associated with distant metastasis as well as tumor cell migration in vitro and the underlying mechanism might be associated with PI3K/AKT signaling pathway[39].
In our study, CCL19 was found to be overexpressed in different FL cell lines utilizing public database analysis. Previous studies have showed that strong in vitro chemotactic activities of CCL19 for T cells and DCs and might activate a LTα1β2-dependent pathway of normal and pathological lymphoid tissue formation[40]. In addition, CCL19 mRNA overexpression was related to higher survival rate. Also, elevated serum CCL19 concentration was associated with longer PFS and OS. The protective prognostic value of higher CCL19 expression level had been validated at both the transcription and protein concentration in two independent FL patient cohorts. In the analysis of immune cell infiltration in tumor microenvironment, the expression of CCL19 was associated with macrophages M1 and monocyte. The protective effect of CCL19 overexpression might be explained from the mechanism of CCL19 and the formation of FL. CCL19 was produced by T-zone fibroblastic reticular cells and are essential for the formation and maintenance of the T-cell zone in lymphoid organs, as well as T cells and DCs peripheral recruitment. Therefore, elevated CCL19 expression could induce the function of T-zone reticular cells and recruit T cells and DCs to tumor tissues[41]. Previous studies have reported the chemotaxis function of CCL19 for macrophages M1[42]. Also, decreased subpopulations of CD4+/CD8 + T cells, macrophages and dendritic cells in patients are associated with FL transformation and are predictors of worse survival[43, 44], which was consistent with the result of TIIC subtype analysis in our study.
In B cell lymphomas, including FL, DCs have showed to be correlated with better prognosis, suggesting that DCs may act against tumor cells in lymphoma [45–47]. Although in this study, the DC proportion has no significant correlation with the expression of CCL19, it is still worthwhile to exploring the function of CCL19 and DC cells for potential clinical treatment. In fact, new generation of CAR-T cell has been engineered to expressed IL-7 and CCL19 to elevate anti-tumor efficacy[48]. Moreover, dendritic cell-based active immunotherapies [49] and macrophage checkpoint inhibitor Hu5F9-G4 have showed efficacy in FL [50]. Result of this study indicated that CCL19 could act as a biomarker to identify FL patients who may have inferior efficacy derived from first-line immunotherapy. Considering the DC and macrophages recruitment capability of CCL19, it is worthwhile to explore whether these patients could benefit from dendritic cell-based active or macrophage-mediated immunotherapies for stimulating the innate immune system.
In this study, we sought to identify biomarkers for FL and further explore the prognostic role of CCL19 in FL. There are some limitations in our study. The mechanisms of immune reactions induced by CCL19 were not explored in our study and need to be further investigated to explore the underlying mechanism.