Therefore, to delineate the tumor-dependent mechanism by which Tregs developed and activated in the TME, we established in vitro co-culture systems where CD4+naïve T cells were either directly or indirectly co-cultured with the NPC cell line C666 or normal nasopharyngeal epithelial (NPE) cell lines, NP460 and NP69 (Fig. 1I). Here, CD4 is linked to nasopharyngeal carcinoma.