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

Abstract

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.

INTRODUCTION

The class II major histocompatibility complex transactivator (CIITA) is the master regulator of major histocompatibility complex (MHC) class II gene expression, a key element in adaptive immunity. It initiates transcription of MHC class II genes, enabling antigen presentation to T cells. Unlike typical transcription factors, CIITA does not bind DNA directly; instead, it recruits transcription factors and cofactors to form complexes on MHC promoters, coordinating gene activation. It acts as both a transcriptional coactivator through the formation of an enhanceosome and as a general transcription factor that occasionally substitutes for core transcription machinery components. Although it primarily regulates MHC class II, CIITA can also enhance MHC class I gene transcription, broadening its role in immune regulation. CIITA gene mutations cause bare lymphocyte syndrome type II, a severe immunodeficiency marked by absent MHC class II expression. Additionally, CIITA dysregulation is linked to cancer where reduced MHC expression aids tumor immune evasion.[1,2] However, the potential of CIITA as a unified therapeutic target across the spectrum of human disease remains largely underexplored. Although its silencing drives immune evasion in cancer and chronic infections, its dysregulation is equally critical in autoimmunity and allograft rejection. Therefore, this review aims to synthesize the biology of CIITA, positioning it not merely as a transcription factor but as a master switch of immune visibility that can be therapeutically tuned across diverse clinical landscapes, whether upregulated to restore host defense or downregulated to induce tolerance.

A literature search was conducted using the PubMed and Google Scholar databases to identify relevant studies published through October 31, 2025. Search strategies employed “CIITA” as the primary keyword, in combination with domain-specific terms including “oncology,” “cancer,” “infection,” and “autoimmunity.” Both preclinical and clinical reports were reviewed to ensure a thorough synthesis of the current landscape.

MECHANISM OF ACTION OF CIITA AS AN IMMUNOREGULATORY MOLECULE

CIITA is a member of the mammalian nucleotide-binding and leucine-rich-repeat (NLR) protein family. It is the master regulator of MHC class II gene expression, including human leukocyte antigen (HLA) genes (HLA-DR, HLA-DQ, HLA-DP), the accessory genes (HLA-DM, HLA-DO), and invariant chain.[1,2] As a distinct member of the NLR transcription factor family, CIITA not only regulates innate immunity like most other NLR proteins, but also plays a critical role in adaptive immunity by controlling MHC II expression and antigen presentation. Constitutive expression of CIITA is seen in antigen-presenting cells (APCs), including macrophages, dendritic cells (DCs), B lymphocytes, activated T lymphocytes, and thymic epithelial cells. CIITA expression can also be induced by cytokine interferon gamma (IFN-γ) in non-APCs.[2,3]

Upon stimulation by IFN-γ released from immune cells such as natural killer (NK) cells, IFN-γ binds to its receptor (IFNγR) on the target cell membrane, initiating a signaling cascade that activates CIITA. CIITA does not directly bind DNA but instead acts as a scaffold that orchestrates transcription regulatory protein recruitment to the MHC-II gene promoter, forming the MHC-II enhanceosome. MHC-II enhanceosome binds various transcription factors (regulatory factor X [RFX], cAMP response element–binding protein [CREB], nuclear transcription factor Y [NFY], TBP-associated factor [TAF], TATA-binding protein [TBP]) to the SXY regulatory module of MHC-II promoters (W/S, X1, X2, and Y motifs). CIITA also coordinates the recruitment of transcriptional coactivators, including histone acetyltransferases (HATs), chromatin remodeling protein (SMARCA4/BRG1), and transcriptional elongation factor complexes such as positive transcription elongation factor b (p-TEFb; cyclin T1 and cyclin-dependent kinase [CDK] 9) and CDK7. HATs (SRC-1, p300/CREB-binding protein [CBP]-associated factor [PCAF], and p300/CBP) acetylate chromatin to enable transcriptional activation. Engagement of transcriptional elongation complexes subsequently promotes RNA polymerase II–mediated transcription of MHC-II genes.[2,4] The resulting MHC-II molecules are expressed on the cell surface and engage CD4+ T cells for antigen presentation, facilitating adaptive immune responses that have important clinical implications in immune epigenetics, infections, and cancer immunology (Fig. 1).

Figure 1.

IFN-γ–induced activation of CIITA and MHC-II gene expression. Upon binding to its receptor (IFNγR), IFN-γ initiates a signaling cascade that activates CIITA. Acting as a non–DNA-binding scaffold, CIITA nucleates the MHC-II enhanceosome on promoter SXY modules by recruiting transcription factors (RFX, CREB, NFY) and chromatin remodelers (BRG1). To drive transcription, CIITA further engages HATs (SRC-1, PCAF, p300/CBP) for chromatin opening and elongation complexes (p-TEFb, CDK7) to activate RNA polymerase II, resulting in surface MHC-II expression and subsequent CD4+ T cell engagement.

CBP: CREB-binding protein; CDK7: cyclin-dependent kinase 7; CDK9: cyclin-dependent kinase 9; CIITA: class II transactivator; CREB: cAMP response element–binding protein; HAT: histone acetyltransferase; IFN-γ: interferon gamma; IFNγR: interferon gamma receptor; MHC-II: major histocompatibility complex class II; NFY: nuclear transcription factor Y; NK: natural killer; p-TEFb: positive transcription elongation factor b; PCAF: p300/CBP-associated factor; RFX: regulatory factor X; RNA Pol II: RNA polymerase II; SRC-1: steroid receptor coactivator-1; TAF: TBP-associated factor; TATA: TATA box; TBP: TATA-binding protein; W/S, X1, X2, Y: conserved cis-regulatory elements of the SXY module.

Created in BioRender: https://BioRender.com/5krhjni.

CIITA expression is regulated through both epigenetic and posttranslational modifications. At the transcriptional level, CIITA is controlled by epigenetic modulation of four independent promoters, with each generating transcripts with distinct first exons that encode different CIITA isoforms. CIITA promoter I (pI) regulates constitutive expression of CIITA type 1 isoform in DCs. The role of CIITA promoter II (pII) remains unknown. CIITA promoter III (pIII) serves as the main regulatory element for constitutive CIITA expression in B lymphocytes and activated T cells. CIITA promoter IV (pIV) is the main CIITA promoter that is inducible by IFN-γ.[5] Induction of CIITA by IFN-γ is dependent on transcriptional coactivator BRG, encoded by SMARCA4 gene, and specific transcription factors like STAT1 and interferon regulatory factor (IRF)1.[2,4]

At the posttranslational level, CIITA is further regulated by a signaling cascade that couples extracellular cues to antigen presentation via phosphorylation- and ubiquitin-dependent mechanisms.[6] Extracellular signal–regulated kinase (ERK) 1/2-mediated phosphorylation of CIITA within its P/S/T-rich regulatory domain primes CIITA for ubiquitination, promoting mono-ubiquitination that enhances transcriptional activity and CIITA-mediated MHC class II gene expression. [7] This same regulatory domain also serves as the binding interface for FBXO11, which is an E3 ubiquitin ligase that recognizes CIITA through the P/S/T-rich region and mediates its poly-ubiquitination and proteasomal degradation, thereby downregulating CIITA activity.[8–10]

THE IMPACT OF CIITA ON HUMAN DISEASE

CIITA, by modulating MHC class II gene transcription and hence acting as a master regulator of immune system coordination, impacts diverse human diseases. These include cancer, infections, immunodeficiency, autoimmunity, and transplantation tolerance.

Role of CIITA in Cancer

The anticancer immune response against mutated peptides of potential immunological relevance (neoantigens) is attributed to MHC-I–restricted cytotoxic CD8+ T cell responses as well as to MHC-II–restricted CD4+ T cells.[11] CIITA plays a pivotal role in antigen presentation, directly impacting tumor immunogenicity and immune evasion. Tumor cells may either inherently possess or gradually acquire the ability to modulate CIITA expression, leading to significant heterogeneity between tumor types. Many tumors, like colon cancer, primarily use the CIITA pIV promoter to regulate MHC-II expression. In plasmacytoma cells, for example, MHC class II expression is predominantly driven by CIITA pIII.[4,12]

One of the principal mechanisms by which tumor cells evade immune surveillance involves suppression of MHC-II expression, which is regulated by CIITA. This suppression is frequently driven by epigenetic silencing of CIITA promoters though DNA methylation and posttranslational modifications of histone.[4] Loss of CIITA disrupts tumor-immune interactions by abolishing tumor-intrinsic MHC class II expression, which is essential for direct antigen presentation to and activation of CD4+ T cells. This loss impairs Th1 effector differentiation, reduces production of key T cell chemokines like CXCL9, and alters the functional composition of the tumor immune infiltrate, collectively fostering an immunosuppressive tumor microenvironment (TME). Consequently, CIITA-deficient tumors exhibit diminished T cell infiltration, blunted antitumor immunity, and potential resistance to immune checkpoint blockade.[13]

Epigenetic silencing of CIITA through DNA methylation downregulates MHC-II expression and renders it unresponsive to IFN-γ stimulation. This mechanism has been reported in glioblastoma, ocular melanoma, hepatocellular carcinoma (HCC), gastric cancer, and colorectal cancer.[2,14–16] Histone deacetylation represents another epigenetic mechanism that suppresses CIITA-mediated MHC-II expression, observed in colon cancer, plasmacytomas, squamous cell carcinomas, rhabdomyosarcoma, and diffuse large B-cell lymphoma (DLBCL).[2,3,12,17,18] In small cell lung cancer and neuroblastoma, suppression of MHC-II expression can occur through epigenetic silencing of CIITA promoters driven by overexpression of HASH-1, L-myc, and N-myc.[19]

Chromosomal translocations involving CIITA are among the most frequent genomic abnormalities observed in B-cell lymphomas, particularly in primary mediastinal B-cell lymphoma (PMBCL). Comprehensive genomic and transcriptomic analyses show that coding sequence mutations, deletions, and chromosomal rearrangements affecting CIITA occur in approximately 71% of PMBCL cases, resulting in loss of CIITA function and reduced MHC-II expression. Consequently, CIITA disruption is associated with diminished tumor immunogenicity, reflected by decreased HLA-DR/DP/DQ expression and reduced T cell infiltration, thereby facilitating immune escape. In addition, CIITA structural rearrangements can drive overexpression of immune checkpoint ligands such as PD-L1 through activation of partner genes, creating a “double-hit” immune-evasive phenotype characterized by impaired antigen presentation and enhanced T cell exhaustion. Detection of CIITA translocations by fluorescence in situ hybridization is a useful diagnostic adjunct to support the diagnosis of PMBCL. Although highly suggestive of PMBCL, CIITA rearrangements are not disease specific and can also be observed in mediastinal gray-zone lymphoma and classical Hodgkin lymphoma.[20,21]

Multiple negative regulators of CIITA have been identified across distinct regulatory layers in different cancers. For example, expression of FBXO11, which promotes proteasomal degradation of CIITA via ubiquitination, is inversely correlated with CIITA-mediated MHC-II expression and survival in patients with breast cancer.[9] FOXP1 is a transcriptional repressor of CIITA target genes that reduces tumor immunogenicity in DLBCL and is associated with MYC expression and more aggressive lymphoma subtypes.[22] FOXP1 serves as an oncogene in B-cell lymphomas and ovarian cancer and as a tumor suppressor gene in breast cancer, prostate cancer, and RCC.[23] Decoy receptor 3 (DcR3), a member of the tumor necrosis factor (TNF) receptor superfamily implicated in tumor progression, has been shown to downregulate MHC class II expression on tumor-associated macrophages in pancreatic cancer by promoting histone deacetylation at CIITA promoters.[4]

CIITA-mediated MHC class II gene expression has prognostic implications in many cancers, including lymphoma, and colorectal and hepatobiliary cancers. Low expression level of CIITA is often associated with inferior survival.[4,21] However, an opposite trend was reported in pancreatic cancer, in which high CIITA expression was associated with poorer prognosis, though limited sample size precludes definitive conclusions.[4] Furthermore, CIITA has been implicated in multiple myeloma–induced bone disease, where it upregulates the secretion of osteolytic cytokines from osteocytes, exacerbating bone loss.[24]

CIITA-driven MHC-II expression and NLRC5-mediated MHC-I antigen presentation pathways represent interconnected determinants of immune checkpoint inhibitor (ICI) response, operating within a unified IFN-γ–STAT1 axis that simultaneously governs antigen presentation capacity and PD-L1–mediated immune evasion.[25] High tumor mutational burden (TMB), which is a biomarker predictive of response to immune checkpoint blockade, generates abundant neoantigens whose immunogenicity depends critically on both MHC-I–mediated presentation to CD8+ T cells and CIITA-driven MHC-II presentation to CD4+ helper T cells.[26–29] Although both MHC-I and MHC-II genotypes are predictors of response to immune checkpoint blockade, MHC-I diversity is more closely associated with tumor susceptibility than with mutation selection pressure. MHC-II genotype constrains the mutational landscape during tumorigenesis in a manner complementary to MHC-I. MHC-II exerts stronger selective pressure on driver mutations than MHC-I, as evidenced by the observation that mutations poorly bound to MHC-II are positively selected during tumorigenesis at higher rates than MHC-I–invisible mutations, reflecting the critical role of CD4+ T cell–mediated immune surveillance in constraining the tumor mutational landscape.[11,30] Dual loss of MHC-I and MHC-II expression occurring in 27% of PD-1–resistant melanomas through B2M gene disruption or CIITA epigenetic silencing represents a critical ICI resistance mechanism that necessitates concurrent restoration of both antigen presentation pathways, with CIITA reactivation strategies showing promise for tumors with epigenetically silenced CIITA rather than genetic B2M alterations.[31] PD-L1 and PD-1 expression are among the key biomarkers of ICI responsiveness; tumor cell–expressed PD-L1 engages PD-1 on infiltrating T cells to suppress antitumor immunity, whereas PD-1 expression on tumor-infiltrating lymphocytes (TILs) reflects prior T cell activation and predicts sustained response to checkpoint blockade. Although PD-L1 overexpression enhances tumor cell response to immune checkpoint blockade, it is not an absolute requirement, as some PD-L1–negative tumors still respond.[32] PD-1 expression on TILs is similarly associated with improved survival after anti–PD-1/PD-L1 therapy, independent of TMB. Patients with both TIL PD-1 positivity and high TMB demonstrate the most pronounced and durable survival benefit, supporting a multifactorial biomarker approach.[33] Although CIITA does not directly regulate PD-L1 transcription, CIITA-driven MHC-II expression indirectly orchestrates PD-L1 upregulation through a positive feedback loop. MHC-II–expressed cancer cells activate tumor-specific CD4+ T cells, triggering local IFN-γ production. This IFN-γ independently induces PD-L1 expression via the STAT1-IRF1 axis while simultaneously driving CXCL9 production, promoting further T cell recruitment and creating a self-amplifying cycle that transforms the TME into a T cell-inflamed, immunotherapy-sensitive phenotype.[13,34]

In summary, the anticancer immune response depends on both MHC-I–restricted CD8+ T cells and MHC-II–restricted CD4+ T cells, with the MHC-II genotype shaping the tumor’s mutational landscape and influencing neoantigen selection. Tumor cells frequently evade immune detection by silencing CIITA, the master regulator of MHC-II expression, through epigenetic mechanisms such as DNA methylation and histone deacetylation. Restoring CIITA function—via HDAC or DNMT inhibitors, gene therapy, or vaccines—can reinstate MHC-II expression, enhance immune infiltration, and improve responses to immune checkpoint blockade.

Role of CIITA in Infections

CIITA plays a central role in host defense by coordinating MHC class II expression and supporting CD4+ T cell-mediated immune responses against a broad range of infectious pathogens. Beyond its primary function in antigen presentation, CIITA also exerts direct antimicrobial effects that limit pathogen replication, transcription, and entry, particularly in viral infections.[3,35]

CIITA restricts viral replication by functioning as a transcriptional restriction factor against several human retroviruses. In human T-lymphotropic virus types 1 and 2 (HTLV-1 and HTLV-2), CIITA interacts with the respective viral transactivators Tax-1 and Tax-2, thereby inhibiting activation of the nuclear factor-κB (NF-κB) pathway, which is required for HTLV-1–driven oncogenesis in adult T cell leukemia/lymphoma. Similarly, in human immunodeficiency virus (HIV) infection, CIITA interferes with viral transcription by competing with the viral transactivator Tat for binding to cyclin T1 within the p-TEFb complex, effectively limiting transcriptional elongation and viral replication.[2]

In addition to these transcriptional effects, CIITA contributes to innate antiviral defenses that operate independently of antigen presentation against pathogens such as severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and Ebola virus. CIITA promotes the expression of the p41 isoform of CD74, which disrupts cathepsin-mediated processing of viral glycoproteins, thus inhibiting viral entry and fusion.[36] Reduced CIITA levels have been associated with more severe COVID-19 outcomes, highlighting its critical role in antiviral immunity.[38] Given the multifaceted antimicrobial activities of CIITA, it is not surprising that many pathogens have evolved strategies to disrupt CIITA expression or activity as a means of immune evasion. One frequently targeted axis is IFN-γ signaling, which is essential for CIITA induction. Cytomegalovirus (CMV) blocks both constitutive and IFN-γ–induced CIITA expression by interfering with the JAK/STAT pathway, thereby lowering HLA-DR surface levels on infected cells. Similar attenuation of IFN-γ–mediated CIITA induction has been observed in infections with human parainfluenza virus type 3, varicella-zoster virus, Toxoplasma gondii, and Mycobacterium tuberculosis, highlighting IFN-γ signaling as a common target exploited by pathogens for immune evasion.[3]

CIITA suppression can also arise from interference with its transcriptional regulation. Chlamydia species suppress CIITA expression by promoting degradation of upstream stimulatory factor (USF-1), a transcription factor required for IFN-γ–induced expression of CIITA.[3] Epstein-Barr virus (EBV) uses a lytic-phase transcription factor Zta, which inhibits CIITA-mediated MHC class II expression.[36] Similarly, human herpesvirus 8 (HHV-8), also known as Kaposi sarcoma–associated herpesvirus, suppresses CIITA transcription through increased expression of latency associated nuclear antigen (LANA), which interferes with transcription factor IRF4 binding at CIITA promoters, leading to reduced expression of MHC class II, particularly HLA-DQ.[35]

Certain viruses can directly interfere with CIITA functional activity, as illustrated by hepatitis B virus (HBV) and hepatitis C virus (HCV) infections. Disruption of CIITA contributes to immune evasion and viral persistence, features that underlie chronic infection and are linked to HCC. HCV suppresses CIITA promoter activity and diminishes HLA-DR expression on infected cells, thereby reducing immunogenicity and promoting chronic infection. CIITA exerts antiviral effects against HBV by activating ERK signaling, leading to decreased viral transcription and replication; however, HBV counteracts this restriction through the HBx protein, which binds to and inhibits CIITA function.[35]

In summary, CMV, EBV, HHV-8, and other pathogens inhibit CIITA through different mechanisms, disrupting IFN-γ signaling or blocking transcription factors. HBV and HCV use CIITA suppression to promote chronic infection and liver cancer, and CIITA itself counters HBV through ERK signaling. CIITA also restricts retroviruses like HIV and HTLV and aids innate defense against SARS-CoV-2 and Ebola virus by blocking viral entry and fusion.

Role of CIITA in Immunodeficiency, Autoimmune Disease, and Transplantation

CIITA deficiency causes a rare inherited form of primary immunodeficiency disease known as bare lymphocyte syndrome (BLS). This syndrome is characterized by recurrent respiratory and gastrointestinal infections, poor nutrient absorption, slow growth, and skin ulcers.[2]

HLA proteins encoded by MHC class II genes play an important role in autoimmune diseases and organ transplantation. CIITA, which controls the expression of the key autoimmune risk genes HLA-DRB1 and HLA-DQB1, is a potential genetic risk locus for autoimmune diseases. The CIITA gene is located at chromosome 16p13—a region linked to autoimmune conditions like multiple sclerosis and rheumatoid arthritis (RA). Polymorphisms within the CIITA gene have been implicated in susceptibility to multiple autoimmune disorders, although these associations remain contested with conflicting evidence due to potential confounding epistatic and gene-environment interactions. For instance, a meta-analysis found no clear association between a specific CIITA variant and RA, but a few more recent studies observed significant associations between certain CIITA variants with autoimmune conditions such as RA, type 1 diabetes mellitus, multiple sclerosis, and systemic lupus erythematosus (SLE).[38–42] The mechanism of the CIITA risk allele involves a blunted transcriptional response to proinflammatory stimuli like IFN-γ, leading to suboptimal MHC-II expression and compromised antigen presentation required for the thymic negative selection of autoreactive clones and peripheral activation of regulatory T cell (Treg) populations, which are essential mediators of immune tolerance and tissue repair.[43]

HLA polymorphism is the primary cause of allograft rejection. The role of CIITA in antigen presentation renders it a promising target for enhancement of allograft survival. Preclinically, CIITA blockade prevents acute allograft rejection and prolongs allograft survival.[44]

Therapeutic Strategies for CIITA Modulation

Therapeutic modulation of CIITA has emerged as a promising strategy to enhance tumor immunogenicity and improve responses to immunotherapy. Restoration of CIITA expression in tumor cells enhances MHC class II–mediated antigen presentation and promotes immune infiltration by CD4+ helper T cells, DCs, and CD8+ cytotoxic T lymphocytes.[45] Preclinical studies demonstrate that epigenetic therapies can reverse CIITA silencing, particularly through inhibition of histone deacetylases (HDACs) and DNA methyltransferases (DNMTs).[4,12,17,18,46,47] Treatment with valproic acid, which has HDAC inhibitory activity, restored MHC class II expression and reduced tumor growth in DcR3-overexpressing pancreatic cancer.[4] Restoration of CIITA-driven MHC-II expression with HDAC inhibitors has also been shown to overcome anti–PD-1 resistance in B-cell lymphoma.[47] Combination of CIITA-targeted epigenetic modulators with other therapies is another potential therapeutic strategy. Preclinical evidence demonstrates that combining DNMT inhibitors with anti–PD-1 overcomes checkpoint resistance by restoring CD8+ T cell infiltration and type I interferon signaling through endogenous retroviral demethylation-induced viral mimicry, achieving 47.6% complete response in previously immunotherapy-refractory NK/T cell lymphoma.[48] HDAC inhibitor depsipeptide has been shown to potentiate antineoplastic effect of chemotherapy such as 5-fluorouracil in colon cancer.[4] A recent phase 2 clinical trial demonstrated that combining the HDAC inhibitor vorinostat with the PD-1 inhibitor pembrolizumab yields clinically significant response rates across multiple squamous cell cancer types.[49] The clinical success of ICI and epigenetic modifier combinations provides a compelling rationale for incorporating CIITA reactivation into multimodal immunotherapy strategies.

Emerging strategies include gene-based delivery of CIITA and vaccine platforms designed to exploit CIITA-driven antigen presentation. Mounting an efficacious antitumor immune response requires coordinated CD8+ cytotoxic T lymphocyte and CD4+ helper T cell activation. MHC class I–based vaccine constructs are often limited by inadequate priming of CD4+ helper T cells. Enforced MHC class II expression on neoplastic cells through CIITA-mediated transcriptional reprogramming reconstitutes their capacity to function as APCs and promotes Th1 polarized TME, leading to antitumor immunity. Although direct CIITA transfection relies on MHC-II–dependent antigen presentation to activate T cell responses, adenoviral vector–mediated CIITA delivery engages additional immunostimulatory mechanisms that function independently of MHC-II expression.[50–52] CIITA expression can be stimulated by IFN-γ, but CIITA-transfected head and neck tumor cells exhibit stronger MHC-II expression than that achieved by IFN-γ stimulation alone.[53] In pancreatic cancer, epigenetically modified vaccines combining HDAC and DNMT inhibitors reverse CIITA silencing and enhance antitumor immunity.[4] Enforced CIITA expression in lung cancer improves T cell recruitment and sensitizes tumors to anti–PD-1 checkpoint blockade.[13]

Sipuleucel-T, the sole U.S. FDA-approved DC vaccine, demonstrates the clinical feasibility of ex vivo–engineered autologous DC therapy by pulsing patient-derived DCs with tumor antigen to enhance T cell priming. Emerging messenger RNA (mRNA)–based DC vaccines offer superior flexibility by enabling ex vivo transfection with tumor antigen–encoding mRNA, achieving robust tumor-specific T cell responses with favorable safety profiles. Overexpression of mRNA-mediated CIITA in DC vaccines represents a promising strategy, as forced MHC-II expression would amplify CD4+ helper T cell priming capacity and enhance antitumor immunity while maintaining the safety profile of autologous cell-based approaches.[54] Lipid-nanoparticle–formulated mRNA technology also provides an emerging delivery platform for CIITA-based therapeutics, enabling efficient transfection of immune cells, as demonstrated by dose-dependent suppression of HLA-DR expression following CIITA or HLA-DRA–targeting mRNA delivery in human whole blood ex vivo models.[55] Beyond conventional gene delivery, precision epigenome editing via CRISPR/dCas9-based platforms represents a transformative approach that selectively reactivates CIITA while minimizing off-target transcriptional effects. CRISPR/Cas9-mediated targeting of the CIITA locus efficiently ablates HLA-II expression in primary immune cells, establishing CIITA as a target that could modulate immune responses, which is particularly important in chimeric antigen receptor T cell therapy. Although these studies used CIITA gene disruption rather than activation, they established the technical framework required for future applications of dCas9-based selective CIITA reactivation (Fig. 2).[56,57]

Figure 2.

Mechanisms of CIITA suppression and therapeutic restoration of MHC-II expression in cancer. Epigenetic silencing (histone deacetylation, DNA methylation) and oncogenic repression of CIITA drive MHC-II downregulation, leading to tumor immune evasion (top). Therapeutic interventions using HDAC/DNMT inhibitors or precision gene therapies (CRISPR/Cas9, vaccines) restore CIITA promoter activity and surface MHC-II expression, reinvigorating T-cell recognition and promoting antitumor immunity (bottom).

Cas9: CRISPR-associated protein 9; CIITA: class II transactivator; CRISPR: clustered regularly interspaced short palindromic repeats; DNMT: DNA methyltransferase; HDAC: histone deacetylase; MHC-II: major histocompatibility complex class II; TCR: T cell receptor.

Created in BioRender: https://BioRender.com/bkkkvu8.

In parallel, pharmacologic suppression of CIITA represents an alternative therapeutic strategy. Statins, which are widely used for cardiovascular disease prevention, suppress IFN-γ–induced MHC class II expression by inhibiting the CIITA transcriptional program through blockade of the mevalonate pathway. Beyond their immunomodulatory effects, which are relevant to various autoimmune diseases, statins disrupt oncogenic Ras- and Rho GTPase signaling and activate antiproliferative and proapoptotic pathways, potentially sensitizing melanoma cells to NK cell–mediated cytotoxicity. Collectively, these findings position CIITA as a versatile therapeutic target, with both restoration and controlled inhibition strategies offering distinct opportunities to recalibrate antitumor immunity and enhance treatment outcomes.[58,59]

Challenges and Future Perspectives

Despite their therapeutic promise, the clinical utility of HDAC inhibitors in restoring CIITA-driven antigen presentation is limited by their nonselective mechanism of action. These broad-spectrum agents simultaneously target multiple HDAC isoforms, generating widespread histone acetylation and chromatin remodeling that reactivates CIITA expression but simultaneously engages immunosuppressive pathways, particularly STAT3-dependent JAK signaling, which amplifies antiapoptotic gene expression and confers therapeutic resistance. HDAC inhibitors also carry risks of off-target immune dysregulation, including autoimmune toxicity and excessive cytokine release, that paradoxically undermine the intended antitumor immune response. These limitations underscore the need for more selective HDAC isoform–specific inhibitors capable of achieving promoter-targeted CIITA reactivation while minimizing genome-wide chromatin alterations and unwanted immunological effects.[60,61] On the other hand, sequential combination strategies pairing selective HDAC inhibitors with DC vaccines or ICIs have demonstrated synergistic antitumor activity in preclinical models and early-phase clinical trials, suggesting that optimized HDAC inhibitor treatment sequencing may overcome current limitations of nonselective HDAC inhibition.[63]

DNMT inhibitors also present a unique therapeutic paradox; they effectively reverse CIITA promoter methylation to reactivate HLA-II transcription and simultaneously demethylate the FOXP3-TSDR (Treg-specific demethylated region), enhancing FOXP3 expression and Treg differentiation and suppressive function. This expansion of Tregs can paradoxically suppress the antitumor CD8+ T cell response even as CIITA-driven antigen presentation capacity is being restored.[63] FOXP3E2+ Tregs in the TME represent an independent prognostic biomarker with enhanced immunosuppressive capacity across breast cancer subtypes.[65]

Furthermore, TME often exhibits substantial temporal and spatial heterogeneity in MHC-II expressions. This variability can significantly influence localized antigen presentation, CD4+ T cell engagement, the formation of tertiary lymphoid structures, and response to ICI.[65,66] This heterogeneity of CIITA-driven MHC-II expression within the TME necessitates combination approaches that simultaneously restore antigen presentation capacity and reduce immunosuppressive cell populations, a challenge that may be addressable through biomarker-driven patient stratification.[62]

CONCLUSIONS

CIITA plays a central role in coordinating immune responses through its regulation of MHC class II gene expression, with implications spanning oncology, infections, immunodeficiency, autoimmunity, and transplantation. Its function as a master regulator of antigen presentation enables effective immune surveillance, and its dysregulation, whether via genetic alterations, epigenetic silencing, or viral interference, can lead to immune evasion. CIITA signaling suppression facilitates tumor escape from immune detection, but emerging strategies, including HDAC and DNMT inhibitors or gene therapy, may potentially restore CIITA activity and enhance tumor immunogenicity. Understanding the diverse regulatory mechanisms and context-specific roles of CIITA not only advances our knowledge of immune modulation but also opens new avenues for therapeutic intervention across a broad spectrum of diseases.