Mutation in the class II trans-activator leading to a mild immunodeficiency.

The expression of MHC class II molecules is essential for all Ag-dependent immune functions and is regulated at the transcriptional level. Four trans-acting proteins control the coordinate expression of MHC class II molecules: class II trans-activator (CIITA), regulatory factor binding to the X box (RFX)-associated protein; RFX protein containing ankyrin repeats, and RFX5. In humans, defects in these genes result in MHC class II expression deficiency and cause combined immunodeficiency. Most patients with this deficiency suffer from severe recurrent infections that frequently lead to death during early childhood. We investigated three sisters, now ages 21, 22, and 24 years, in whom MHC-II deficiency was detected. Even though the eldest sibling was asymptomatic and the other two had only mild immunodeficiency, none of the three class II isotypes was expressed on T cell blasts, fibroblasts, EBV B cell lines, or epidermal dendritic cells. Residual HLA-II expression was detected in fresh PBMC. Somatic complementation identified the disease as CIITA deficiency. A homozygous T1524C (L469P) substitution was found in the coding region of the CIITA cDNA and was shown to be responsible for the defect in MHC-II expression. This missense mutation prevents the normal functioning of MHC-II but does not lead to the nuclear exclusion of the L469P CIITA. Transfection experiments demonstrated that the CIITA L469P mutant had residual MHC class II trans activation activity, which might explain the unusual clinical course of the patients studied. This study shows that an attenuated clinical phenotype or an asymptomatic clinical course can be observed in patients despite a profound defect in the expression of MHC class II genes. The frequency of the inherited MHC class II deficiency might thus be underestimated.

CIITA leucine-rich repeats control nuclear localization, in vivo recruitment to the major histocompatibility complex (MHC) class II enhanceosome, and MHC class II gene transactivation.

The major histocompatibility complex (MHC) class II transactivator CIITA plays a pivotal role in the control of the cellular immune response through the quantitative regulation of MHC class II expression. We have analyzed a region of CIITA with similarity to leucine-rich repeats (LRRs). CIITA LRR alanine mutations abolish both the transactivation capacity of full-length CIITA and the dominant-negative phenotype of CIITA mutants with N-terminal deletions. We demonstrate direct interaction of CIITA with the MHC class II promoter binding protein RFX5 and could also detect novel interactions with RFXANK, NF-YB, and -YC. However, none of these interactions is influenced by CIITA LRR mutagenesis. On the other hand, chromatin immunoprecipitation shows that in vivo binding of CIITA to the MHC class II promoter is dependent on LRR integrity. LRR mutations lead to an impaired nuclear localization of CIITA, indicating that a major function of the CIITA LRRs is in nucleocytoplasmic translocation. There is, however, evidence that the CIITA LRRs are also involved more directly in MHC class II gene transactivation. CIITA interacts with a novel protein of 33 kDa in a manner sensitive to LRR mutagenesis. CIITA is therefore imported into the nucleus by an LRR-dependent mechanism, where it activates transcription through multiple protein-protein interactions with the MHC class II promoter binding complex.

CIITA is a transcriptional coactivator that is recruited to MHC class II promoters by multiple synergistic interactions with an enhanceosome complex.

By virtue of its control over major histocompatibility complex class II (MHC-II) gene expression, CIITA represents a key molecule in the regulation of adaptive immune responses. It was first identified as a factor that is defective in MHC-II deficiency, a hereditary disease characterized by the absence of MHC-II expression. CIITA is a highly regulated transactivator that governs all spatial, temporal, and quantitative aspects of MHC-II expression. It has been proposed to act as a non-DNA-binding transcriptional coactivator, but evidence that it actually functions at the level of MHC-II promoters was lacking. By means of chromatin immunoprecipitation assays, we show here for the first time that CIITA is physically associated with MHC-II, as well as HLA-DM, Ii, MHC-I, and beta(2)m promoters in vivo. To dissect the mechanism by which CIITA is recruited to the promoter, we have developed a DNA-dependent coimmunoprecipitation assay and a pull-down assay using immobilized promoter templates. We demonstrate that CIITA recruitment depends on multiple, synergistic protein-protein interactions with DNA-bound factors constituting the MHC-II enhanceosome. CIITA therefore represents a paradigm for a novel type of regulatory and gene-specific transcriptional cofactor.

Quantitative control of MHC class II expression by the transactivator CIITA.

Activation of T lymphocytes is quantitatively controlled by the level of expression of MHC class II molecules. Both constitutive and inducible expression of MHC class II genes is regulated by the transactivator CIITA, which is itself tightly regulated. Since the level of MHC class II molecules expressed is a functionally essential parameter, it was of interest to explore whether MHC class II expression is quantitatively controlled by the level of the transactivator. This report shows that in a variety of experimental conditions one does indeed observe, in both mouse and man, a quantitative control of MHC class II expression by the level of CIITA. This relationship between the regulator gene, which behaves as a rate-limiting factor, and its target genes clarifies our understanding of the quantitative modulation of MHC class II expression, and thus of T lymphocyte activation.

Efficient repression of endogenous major histocompatibility complex class II expression through dominant negative CIITA mutants isolated by a functional selection strategy.

Major histocompatibility complex class II (MHC-II) molecules present peptide antigens to CD4-positive T cells and are of critical importance for the immune response. The MHC-II transactivator CIITA is essential for all aspects of MHC-II gene expression examined so far and thus constitutes a master regulator of MHC-II expression. In this study, we generated and analyzed mutant CIITA molecules which are able to suppress endogenous MHC-II expression in a dominant negative manner for both constitutive and inducible MHC-II expression. Dominant negative CIITA mutants were generated via specific restriction sites and by functional selection from a library of random N-terminal CIITA deletions. This functional selection strategy was very effective, leading to strong dominant negative CIITA mutants in which the N-terminal acidic and proline/serine/threonine-rich regions were completely deleted. Dominant negative activity is dependent on an intact C terminus. Efficient repression of endogenous MHC-II mRNA levels was quantified by RNase protection analysis. The quantitative effects of various dominant negative CIITA mutants on mRNA expression levels of the different MHC-II isotypes are very similar. The optimized dominant negative CIITA mutants isolated by functional selection should be useful for in vivo repression of MHC-II expression.

Correction of defective expression in MHC class II deficiency (bare lymphocyte syndrome) cells by retroviral transduction of CIITA.

Retrovirus-mediated gene transfer was used to restore expression to MHC class II-negative patient cells from complementation group A(II) of MHC class II immunodeficiency or bare lymphocyte syndrome (BLS). The cells of these patients do not transcribe MHC class II genes due to a defect in the trans-acting factor, CIITA. We constructed a vector, pGAG/Ii-CIITA, with the MHC class II-associated invariant chain promoter driving CIITA expression. Cocultivation with the virus producer line was consistently shown to be the optimal method for infection of all cell types. The induction of MHC class II expression after virus infection was rapid, and high levels of expression were achieved in cell lines within 1 wk of infection. In addition, expression was easily detectable even in peripheral blood cells of a BLS patient within a few days. Cell lines maintained in vitro for several months remained positive, and the proportion of cells with surface expression of DR was correlated with the number of integrated proviruses. Moreover, transduced B lymphoblastoid cell lines readily established tumors in CB17-scid/scid mice, and the MHC class II-positive cells demonstrated a clear competitive advantage in vivo. Ultimately, we hope to use this transduction system to restore normal immune function to a BLS patient for which no other therapeutic option currently exists.

Expression of MHC class II molecules in different cellular and functional compartments is controlled by differential usage of multiple promoters of the transactivator CIITA.

The highly complex pattern of expression of major histocompatibility complex class II (MHC-II) molecules determines both the immune repertoire during development and subsequently the triggering and the control of immune responses. These distinct functions result from cell type-restricted expression, developmental control and either constitutive or inducible expression of MHC-II genes. Yet, in these various situations, MHC-II gene expression is always under the control of a unique transactivator, CIITA. Here we show that the CIITA gene is controlled by several distinct promoters, two of which direct specific constitutive expression in dendritic cells and B lymphocytes respectively, while another mediates gamma-interferon-induced expression. Thus the cellular, temporal and functional diversity of MHC-II expression is ultimately controlled by differential activation of different promoters of a single transactivator gene. This provides novel experimental tools to dissect compartment-specific gain or loss of MHC-II function in vivo.

Two novel mutations in the MHC class II transactivator CIITA in a second patient from MHC class II deficiency complementation group A.

Congenital MHC class II deficiency or bare lymphocyte syndrome (BLS; McKusick 209920) is caused by defects in trans-acting regulatory factors that control MHC class II expression and is therefore a disease of gene regulation. There are at least four complementation groups and the genetic and molecular dissection of this rare disease has contributed considerably to our current understanding of the molecular mechanisms governing MHC class II expression. Identification of the gene that is defective in BLS complementation group A, CIITA (MHC class II transactivator), has led to the discovery that CIITA acts as a master control factor of MHC class II expression. We have identified the CIITA mutations in a second patient from BLS group A. Two novel mutations abolish CIITA function, as shown by transfection experiments. Molecular analysis of these two novel mutations, together with the one described earlier in the first patient, is informative in terms of CIITA structure-function relationships.

Regulation of MHC class II genes: lessons from a disease.

Precise regulation of major histocompatibility complex class II (MHC-II) gene expression plays a crucial role in the control of the immune response. A major breakthrough in the elucidation of the molecular mechanisms involved in MHC-II regulation has recently come from the study of patients that suffer from a primary immunodeficiency resulting from regulatory defects in MHC-II expression. A genetic complementation cloning approach has led to the isolation of CIITA and RFX5, two essential MHC-II gene transactivators. CIITA and RFX5 are mutated in these patients, and the wild-type genes are capable of correcting their defect in MHC-II expression. The identification of these regulatory factors has furthered our understanding of the molecular mechanisms that regulate MHC-II genes. CIITA was found to be a non-DNA binding transactivator that functions as a molecular switch controlling both constitutive and inducible MHC-II expression. The finding that RFX5 is a subunit of the nuclear RFX-complex has confirmed that a deficiency in the binding of this complex is indeed the molecular basis for MHC-II deficiency in the majority of patients. Furthermore, the study of RFX has demonstrated that MHC-II promoter activity is dependent on the binding of higher-order complexes that are formed by highly specific cooperative binding interactions between certain MHC-II promoter-binding proteins. Two of these proteins belong to families of which the other members, although capable of binding to the same DNA motifs, are probably not directly involved in the control of MHC-II expression. Finally, the facts that CIITA and RFX5 are both essential and highly specific for MHC-II genes make possible novel strategies designed to achieve immunomodulation via transcriptional intervention.

Molecular defects in the bare lymphocyte syndrome and regulation of MHC class II genes.

The complex pattern of expression of major histocompatibility complex (MHC) class II molecules plays an essential role in the control of the immune response. Our understanding of the molecular mechanisms controlling this expression has benefited greatly from the identification of the regulatory factors defective in two forms of a hereditary disease of MHC class II regulation: bare lymphocyte syndrome. This has also provided new tools for the experimental modulation of MHC class II expression.

Complementation cloning of mammalian transcriptional regulators: the example of MHC class II gene regulators.

Cloning by complementation of mutant cell lines is a powerful way in which to identify and isolate important regulatory genes on the basis of functional assays. The recent cloning of two essential regulators of major histocompatibility complex (MHC) class II gene expression has not only advanced our understanding of the complex mechanisms controlling these genes, but also helps to illustrate the feasibility of this approach for the study of mammalian gene regulation.

RB and a novel E2F-1 binding protein in MHC class II deficient B-cell lines and normal IFN-gamma induction of the class IL transactivator CIITA in class II non-inducible RB-defective tumor lines.

The major histocompatibility (MHC) class II genes encode cell surface proteins that bind antigenic peptide for presentation to T-cells. The class II proteins are expressed constitutively on B-cells and EBV-transformed B-cells, and are inducible by IFN-gamma on a wide variety of cell types. Retinoblastoma protein (RB) is a tumor suppressor and functions as a transcriptional repressor by binding and inactivating the transactivator E2F-I. RB-defective tumor lines are non-inducible for MHC class II by IFN-gamma, or very weakly inducible, but transfection of 2 different lines with RB expression vectors re-establishes or substantially enhances class II inducibility. Therefore, we examined the RB status of a series of B-cell mutants that are defective in class II expression, generated either in vitro or derived from Bare Lymphocyte Syndrome (BLS) patients. Nuclear matrix-bound RB was detectable in all cases, indicating that loss of RB is not responsible for decreased class II expression in these lines. A second E2F-I binding protein, most likely DP-I, was also apparently normal in both class II-positive and -negative B-cell lines. We also examined the IFN-gamma induction of CIITA in RB-defective lines. CIITA is a class II gene transactivator known to be defective in one form of BLS and to be required for the induction of MHC class II by IFN-gamma. CIITA mRNA is normally inducible by IFN-gamma in class II non-inducible, RB-defective lines, and in one line, re-expression of RB has no effect on CIITA mRNA induction levels. Thus, the block in MHC class II inducibility in RB-defective cells is not due to a block in CIITA inducibility.

The two novel MHC class II transactivators RFX5 and CIITA both control expression of HLA-DM genes.

MHC-encoded HLA-DMA and -DMB molecules are atypical MHC chains that play an essential role in antigen presentation by MHC class II molecules. They resemble both MHC class I and II molecules but are not expressed at the cell surface. From the study of MHC class II regulatory mutants, it was found recently that two novel transactivators, CIITA and RFX5, are essential for the control of MHC class II gene expression. We report here that CIITA and RFX5, although operating at different levels of transcriptional control, are also both essential regulators of HLA-DMA and -DMB genes. This is true for both the constitutive and the inducible mode of DM gene expression. Indeed, both CIITA and RFX5 cDNA can correct the HLA-DMA and -DMB gene expression defect in the respective regulatory mutants. The involvement of these two transcription factors accounts for the coordinate expression of MHC class II and HLA-DM, two sets of molecules that perform quite different functions in the overall process of antigen presentation.

A novel DNA-binding regulatory factor is mutated in primary MHC class II deficiency (bare lymphocyte syndrome).

Regulation of MHC class II gene expression is an essential aspect of the control of the immune response. Primary MHC class II deficiency is a genetically heterogeneous disease of gene regulation that offers the unique opportunity of a genetic approach for the identification of the functionally relevant regulatory genes and factors. Most patients exhibit a characteristic defect in the binding of a nuclear complex, RFX, to the X box motif of MHC class II promoters. Genetic complementation of a B-lymphocyte cell line from such a patient with a cDNA expression library has allowed us to isolate RFX5, the regulatory gene responsible for the MHC class II deficiency. This gene encodes a novel DNA-binding protein that is indeed a subunit of the RFX complex. Mutations in the RFX5 gene have been characterized in two patients. Transfection of the patient's cells with the RFX5 cDNA repairs the binding defect and fully restores expression of all the endogenous MHC class II genes in vivo.

Identification of cytosolic proteins that bind to the Fanconi anemia complementation group C polypeptide in vitro. Evidence for a multimeric complex.

The oligomeric structure of Fanconi anemia complementation group C (FACC) was investigated in mammalian cell lysates. Using an affinity-purified polyclonal antibody, FACC was immunoprecipitated from radiolabeled cell lysates and shown to form monomers of 63 kDa. Association of FACC with heterologous proteins was investigated by co-precipitation of radiolabeled proteins with a recombinant chimeric FACC molecule fused to the constant portion of the human IgG1 heavy chain (FACC gamma 1). Expression of FACC gamma 1 in FACC-deficient Fanconi anemia (FA) lymphoblasts corrected the hypersensitivity of these cells to mitomycin C. Binding of FACC gamma 1 to protein A-agarose and incubation with radiolabeled cell lysates identified three polypeptides with molecular masses of 65, 50, and 35 kDa that were also detected on immunoblots probed with the purified FACC gamma 1 polypeptide. FACC, as well as the three FACC-binding polypeptides, co-fractionated with cytosolic and membrane extracts. Binding was specific for the FACC moiety of FACC gamma 1 and was detected in cytosolic extracts of a number of FA and non-FA mammalian cells. These results demonstrate that FACC binds directly to a family of ubiquitous cytosolic proteins and is conserved in a wide range of mammalian cells.

Developmental extinction of major histocompatibility complex class II gene expression in plasmocytes is mediated by silencing of the transactivator gene CIITA.

Constitutive major histocompatibility complex (MHC) class II gene expression is tightly restricted to antigen presenting cells and is under developmental control. Cells of the B cell lineage acquire the capacity to express MHC class II genes early during ontogeny and lose this property during terminal differentiation into plasma cells. Cell fusion experiments have suggested that the extinction of MHC class II expression in plasma cells is due to a dominant repression, but the underlying mechanisms are not understood. CIITA was recently identified as an MHC class II transactivator that is essential for MHC class II expression in B lymphocytes. We show here that inactivation of MHC class II genes in plasmocytes is associated with silencing of the CIITA gene. Moreover, experimentally induced expression of CIITA in plasmocytes leads to reexpression of MHC class II molecules to the same level as that observed on B lymphocytes. We therefore conclude that the loss of MHC class II expression observed upon terminal differentiation of B lymphocytes into plasmocytes results from silencing of the transactivator gene CIITA.

An HLA-DRB alpha-helix motif shared by DR11 and DR8 alleles is implicated in the pluriallelic restriction of peptide-specific T-cell lines.

The T-cell recognition of HLA-DR-peptide complexes is generally restricted by the polymorphism of the DRB molecules but pluriallelic restriction has been described. The molecular basis of restriction and promiscuity of such peptide-specific responses is poorly understood. We isolated a panel of T-cell lines specific for the tetanus toxin peptide p2 (TT830-843) exhibiting pluriallelic restriction by DR11 and DR8 alleles. Fine restriction specificity of the T-cell lines was examined in functional assays against DR oligotyped APCs expressing different variants of DR11 and DR8 alleles. Our results show that (a) polymorphisms between serologically related alleles are relevant in terms of restriction of the peptide-specific T-cell response; in some instances, a single amino acid substitution can determine the restriction of a T-cell line; (b) different patterns of restriction are not the result of specific differences in DR-p2 binding as p2 peptide binds to all DR11 and DR8 alleles tested (DRB1* 1101, -1102, -1103, -1104, 110X, -0801, -0802, -0803, and -0806); and (c) pluriallelic restriction of the peptide-specific T-cell response correlates with the presence of a DRB1 alpha-helix motif (67-71-86) shared by some DR11 and DR8 alleles. Possible implications of pluriallelic restriction of peptide-specific T-cell response in autoimmune disorders associated with DR11 and DR8 are discussed.

Regulation of MHC class II expression by interferon-gamma mediated by the transactivator gene CIITA.

Major histocompatibility complex (MHC) class II genes are expressed constitutively in only a few cell types, but they can be induced in the majority of them, in particular by interferon-gamma (IFN-gamma). The MHC class II transactivator gene CIITA is defective in a form of primary MHC class II deficiency. Here it is shown that CIITA expression is controlled and induced by IFN-gamma. A functional CIITA gene is necessary for class II induction, and transfection of CIITA is sufficient to activate expression of MHC class II genes in class II-negative cells in the absence of IFN-gamma. CIITA is therefore a general regulator of both inducible and constitutive MHC class II expression.