The NLR member CIITA: Master controller of adaptive and intrinsic immunity and unexpected tool in cancer immunotherapy

Abstract

Human nucleotide-binding oligomerization domain (NOD)-like receptors (NLR) include a large family of proteins that have important functions in basic physio-pathological processes like inflammation, cell death and regulation of transcription of key molecules for the homeostasis of the immune system. They are all characterized by a common backbone structure (the STAND ATPase module consisting in a nucleotide-binding domain (NBD), an helical domain 1 (HD1) and a winged helix domain (WHD), used by both prokaryotes and eukaryotes as defense mechanism. In this review, we will focus on the MHC class II transactivator (CIITA), the master regulator of MHC class II (MHC-II) gene expression and the founding member of NLR. Although a consistent part of the described NLR family components is often recalled as innate or intrinsic immune sensors, CIITA in fact occupies a special place as a unique example of regulator of both intrinsic and adaptive immunity. The description of the discovery of CIITA and the genetic and molecular characterization of its expression will be followed by the most recent studies that have unveiled this dual role of CIITA, key molecule in intrinsic immunity as restriction factor for human retroviruses and precious tool to induce the expression of MHC-II molecules in cancer cells, rendering them potent surrogate antigen presenting cells (APC) for their own tumor antigens.

Keywords: CIITA; Cancer; HTLV-1; NLR; Restriction factors.

Fig. 1

The MHC class II transactivator CIITA: schematic structure of the gene and function of the protein. (A) Expression of the CIITA gene is controlled by three independent promoters (pI, pIII and pIV) having different functions and expressed in specific cell types. pII is not associated to any transcript. The three types of mRNA encoding CIITA are derived from pI, pIII and pIV, and they encode three different protein isoforms (Type I, Type II and Type III), which differ only at their N-terminal ends. The region shared by all three isoforms contains an acidic activation domain (AD), a core signal transduction ATPase with numerous domains (STAND) and a leucin rich domain (LRR). The N-terminal region of the Type I isoform encodes an additional caspase-like activation and recruiting domain (CARD), not present in the Type II and III protein isoforms. (b) CIITA leads to the assembly of the MHC-II enhanceosome by interacting with CREB/CBP and the NF-Y and RFX transcription factors associated with W/SXY module. In addition, CIITA binds to chromatin modifying enzymes, as BRG1 and many transcriptional co-activators including histone deacetylases (HDACs), histone methyltransferases (HMTs) and histone acetyltransferases (HATs), as p300 and PCAF, thus contributing to chromatin remodeling and transcription activation. Furthermore, CIITA regulates the initiation of MHC-II transcription by recruiting several components of the canonical TFIID complex, as TAFs proteins and TBP. By interacting with Cyclin T1 and CDK9 of positive transcription elongation factor-b (P-TEFb) complex and CDK7, CIITA also controls the elongation of primary transcripts. Figure created with BioRender.com.

Fig. 2

CIITA is a restriction factor for human retroviruses: action on HIV. (A) The transcription of HIV-1 depends on a complex interplay between numerous viral and cellular factors. Cellular transcription factors such as nuclear factor kappa B (NF-κB), nuclear factor of activated T cells (NFAT), and specificity protein 1 (Sp1) bind to viral LTR and are crucial to initiate viral genome transcription. The viral transactivator Tat interacts with the Trans-Activation Response (TAR) region at the 5′ end of the nascent viral RNA and recruits the P-TEFb complex made by Cyclin T1 and CDK9 and RNA polymerase II (RNA Pol-II) to promote elongation of the primary transcript (B) CIITA binds to Cyclin T1 and displaces it from the P-TEFb complex. CIITA also binds another restriction factor, TRIM22, which drives the complex to specific nuclear bodies (TRIM22 NB) where CyclinT1 is also bound and further retained by promyelocytic protein PML. TRIM22 NB represent also a docking place for Sp1 whose detachment from the viral LTR due to interference mediated by TRIM22 contributes to drastically reduce basal HIV-1 transcription. The convergence of several restriction factors, particularly CIITA and TRIM22, acting in synergy to displace both basal transcription and transcript elongation factors in a single nuclear body represents an unprecedented finding in intrinsic immunity. Figure created with BioRender.com.

Fig. 3

CIITA is a restriction factor for human retroviruses: action on HTLV-1. Models of inhibition of Tax-1-mediated HTLV-1 transcription by CIITA. (A) Tax-1 activates proviral transcription by recruiting several cellular factors such as CREB, CBP, p300 and PCAF on HTLV-1 LTR promoter. (B) CIITA inhibits Tax-1-mediated HTLV-1 transcription by physically interacting with Tax-1 and preventing its association with CREB, CBP, p300 and PCAF and their recruitment on viral promoter. (C) In a non-mutually exclusive model, the interaction between CIITA and Tax-1 does not prevent the recruitment of the viral transactivator on viral LTR. However, in this case Tax-1 is not efficient at promoting HTLV-1 transcription because its association with PCAF is weak due to the steric hinderance caused by the Tax-1-CIITA and/or CIITA/PCAF interactions. Figure created with BioRender.com.

Fig. 4

CIITA may counteract the oncogenic properties of HTLV-1. The oncogenic potential of Tax-1 is mainly correlated to its ability to activate the NF-kB pathway, by acting both at cytoplasmic and nuclear level. In the cytoplasm Tax-1 binds to IKKγ subunit of the trimeric IKK complex, activating the kinase activity of IKKα and IKKβ subunits. The activation of IKK complex results in phosphorylation and proteasomal degradation of the cytoplasmic inhibitor (IκB) of NF-kB and the translocation of the NF-κB dimers (p50-p65/Rel (A) to the nucleus where it activates the transcription of NF-kB responsive genes. In the nucleus, Tax-1 binds to p65/RelA and stabilizes its association to the NF-kB responsive promoter, further promoting gene transcription. (B) By interacting with Tax-1, CIITA suppresses Tax-1 oncogenicity both at cytoplasmic and nuclear levels by (a) preventing its association and the subsequent activation of IKK complex, (b) impairing its nuclear translocation and thus its interaction with p65/RelA. Figure created with BioRender.com.

Fig. 5

CIITA and cancer. A) CIITA-modified cancer cells express functional MHC class II molecules and can function as surrogate antigen presenting to trigger and activate tumor specific CD4+ T helper cells, basic mediators of adaptive immunity. Downstream secretion of cytokines and particularly triggering of cytolytic T cells (CTL) leads to elimination of tumor cells. B) CIITA-modified cancer cells represent an unprecedented source of MHC-II bound tumor peptides that may be used for the formulation of novel anti-tumor vaccines. Tumor cells isolated from cancer patient are MHC-II-negative and express low levels of MHC-I molecules on cell surface. Tumor peptides potentially boundable to MHC-II molecules are hidden into cancer cells. Upon CIITA transfection, cancer cells express MHC-II molecules that can now bind hidden tumor specific peptides and present them on the cell surface. Both MHC-II and MHC-I bound peptides can be eluted, purified and selected on the basis of their exclusive expression in tumor but not in normal cells. After immunogenic validation, a selected group of tumor-specific peptides can be used to generate a vaccine cocktail to be tested in clinical setting as therapeutic vaccine in cancer patients. Perforin (PFN). Granzymes (GzmB). Figure created with BioRender.com.