A clinicopathological study on correlation between CD74+ fibroblasts and the formation of immunosuppressive microenvironment in oral squamous cell carcinoma

[Objective:] To investigate the expression of CD74+ fibroblasts in oral squamous cell carcinoma (OSCC) and their correlation with the immunosuppressive microenvironment of OSCC. [Methods:] The expression of CD74 of fibroblasts in OSCC tumor tissues and adjacent non-cancerous tissues will be detected using immunohistochemistry. The correlation between the expression of CD74 in cancer-associated fibroblasts (CAFs) and patients' clinicopathological parameters, prognosis, and immune cell infiltration will be further investigated. Primary CAFs, paired normal fibroblasts (NFs), and human peripheral blood mononuclear cells (PBMCs) from non-OSCC patients will be extracted to compare the differences in the expression of CD74 between the two types of fibroblasts. Subsequently, these two types of fibroblasts will be co-cultured with PBMCs, and the concentrations of interleukin-10 (IL-10) and IL-35 in the culture supernatant will be measured by enzyme-linked immunosorbent assay (ELISA). The effects of CAFs with different CD74 expression levels on the migration of PBMCs were compared by Transwell assay and flow cytometry. [Results:] The number of CD74+ fibroblasts in OSCC tumor tissues was significantly higher than that in adjacent non-cancerous tissues (P<0.001). Moreover, OSCC patients with high CD74 expression had a significantly lower 5-year overall survival rate (P<0.001). Immunohistochemical results showed that in tumor tissues with high CD74+ fibroblast expression, the ratio of FOXP3+ cells to CD4+ cells was significantly higher (P<0.001), and there was a significant positive correlation between the number of CD74+ fibroblasts and FOXP3+ cells (r=0.439 3). In vitro experiments confirmed that the expression of CD74 in primary CAFs from OSCC tumor tissues was higher than that in the paired NFs. After co-culturing with PBMCs for 12 h, the concentrations of IL-10 and IL-35 in the culture medium of the CAFs CD74 high-expression group were significantly higher than those in the NFs group (P<0.05). Additionally, the concentrations of IL-35 in the culture medium of the CAFs CD74 high-expression group were higher than those in the CAFs CD74 low-expression group (P<0.05). The results of flow cytometry showed that CAFs with high CD74 expression had a stronger chemotactic ability toward CD4+ FOXP3+ cells (P<0.05). [Conclusion:] CD74+ fibroblasts are associated with poorer prognosis in OSCC patients, and may be involved in the recruitment of CD4+ FOXP3+ cells.