Dendritic cell (DC)-based cancer immunotherapy remains limited by heterogeneous immune responses and variable clinical efficacy. CD86, a key co-stimulatory molecule, is traditionally regarded as a marker of dendritic cell maturation; however, accumulating evidence suggests that CD86 expression is regulated by immune checkpoint interactions, inflammatory signaling, and tumor microenvironment-associated immune modulation. In this review, we summarize current evidence regarding the molecular mechanisms governing CD86 regulation, including MARCH1-mediated ubiquitination and CTLA-4-mediated trans-endocytosis, and discuss how suppressive cytokines, hypoxia, and metabolic stress influence dendritic cell function within the tumor microenvironment (TME). We further review the heterogeneity of CD86 regulation across dendritic cell subsets and immune contexts, as well as its potential relevance in secondary lymphoid organs and tumor-associated immune responses. In addition, we discuss current evidence regarding soluble CD86 (sCD86) and its reported associations with immune activation and dysregulated immune states in cancer. Current evidence supports that CD86 regulation is shaped by integrated co-stimulatory signaling, immune checkpoint interactions, and tumor microenvironment-associated suppression. Importantly, CD86 may function not only as a dendritic cell maturation marker but also as a dynamic immunoregulatory molecule with context-dependent implications in cancer immunity. However, substantial uncertainties remain regarding its mechanistic role, prognostic value, and therapeutic relevance across different tumor settings. Future mechanistic and translational studies are needed to clarify these unresolved issues.