Class II Major Histocompatibility Complex Transactivator (CIITA): A Master MHC-II Regulator Impacting Cancer and Beyond.

The class II major histocompatibility complex transactivator (CIITA) is a non-DNA-binding master regulator of major histocompatibility complex class II (MHC-II) gene expression, essential for antigen presentation and adaptive immunity. Functioning as a scaffold, CIITA recruits chromatin remodelers and transcription coactivators to form the MHC-II enhanceosome, facilitating transcriptional activation. CIITA expression is tightly regulated through four promoters and is subject to both cytokine-induced and epigenetic control. Dysregulation of CIITA underpins several immune-related disorders. Its deficiency results in bare lymphocyte syndrome, a severe immunodeficiency. Variants in the CIITA gene have been implicated in autoimmune diseases, graft rejection, and immune dysregulation. Chromosomal translocations involving CIITA are among the most common genomic alterations in some B-cell lymphomas. Additionally, pathogens such as cytomegalovirus (CMV), Epstein-Barr virus (EBV), human immunodeficiency virus (HIV), and hepatitis B virus (HBV) exploit CIITA suppression to evade immune surveillance. In oncology, epigenetic silencing of CIITA contributes to MHC-II downregulation and tumor immune evasion. Restoration of CIITA expression enhances tumor immunogenicity, T cell infiltration, and responsiveness to immunotherapy. CIITA also modulates the tumor microenvironment, influences prognosis, and has therapeutic relevance in hematologic and solid tumors. Its multifunctional role positions CIITA as a critical immune regulator and a promising therapeutic target in cancer immunotherapy, antiviral strategies, and immune modulation.