MHC Class II Deficiency

Introduction Primary human immunodeficiencies constitute a heterogenous group of inborn errors of the immune system due to a variety of genetic abnormalities. The study of patients with primary immunodeficiency diseases has expanded our understanding of immunity. Recent progress in immunobiology and genetics has identified with increasing precision the causes of many primary immunodeficiencies; diagnosis and therapy, as a result, can be more specific and effective (26,87). Among the primary immunodeficiencies, MHC class II deficiency (MHC-II deficiency) is caused by the absence of MHC-II expression on the cell surface. Patients suffering from this disease were first identified in the late 1970s and early 1980s (31,35,36,51,55,90,101,102). Since then, more than 70 patients have been clearly described. The lack of MHC-II expression results in a severe defect in both cellular and humoral immune responses to foreign antigens and is consequently characterized by an extreme susceptibility to viral, bacterial, fungal, and protozoal infections, primarily of the respiratory and gastrointestinal tracts. Severe malabsorption with failure to thrive ensues, often leading to death in early childhood (see below and references 32, 50). The disease was formally named “major histocompatibility complex class II deficiency” by the World Health Organization (87). It is also frequently referred to as the “bare lymphocyte syndrome” (BLS). However, the term BLS was first used to describe a defect in MHC class I (MHC-I) expression in patients in whom MHC-II expression was not examined (102), and it is now often used synonymously for all defects involving expression of MHC-I (BLS type I), MHC-II (BLS type II), or both (BLS type III) (103). Only the immunodeficiency syndrome associated with a constant and profound defect of MHC-II expression will be discussed here, and the term “MHC-II deficiency” will therefore be used. Structure, Function, and Regulation of MHC Class II Molecules MHC-II molecules, also called human leukocyte antigens (HLA) in humans, are heterodimeric transmembrane glycoproteins consisting of α and β chains. There are 3 different human MHC-II isotypes (HLA-DR, -DP and -DQ) encoded by distinct α chain (A) and β chain (B) genes that are clustered on the short arm of chromosome 6. MHC-II genes are characterized by a high level of polymorphism. For example, more than 160 alleles have been described for the HLA-DRB1 locus. MHC-II molecules are expressed at the cell surface of various different cell types and they play a pivotal role in 4 processes of the immune response (Figure 1):Fig. 1: Structure and function of MHC class II molecules. MHC-II molecules are composed of 2 chains (α and β). They present peptides derived from exogenous antigens (Ag) to CD4 + T cells, which recognize the MHC-II/peptide complex via their T-cell receptors (TCR). They control antigen-specific activation of CD4 + T-helper lymphocytes in the periphery and are thus crucial for the initiation and propagation of immune responses (15). They are essential for the positive and negative selection that generate the mature CD4 + T-lymphocyte repertoire in the thymus (45,46). They participate in the activation of antigen presenting cells (APCs) on which they are expressed (88). They drive T-lymphocyte activation induced by superantigens (2,41). Two modes of MHC-II expression are generally recognized: constitutive and inducible (60,99). Constitutive expression of MHC-II molecules is largely restricted to epithelial cells in the thymus and to professional antigen presenting cells, such as B lymphocytes and dendritic cells. The majority of other cell types are MHC-II negative. In many of these, however, expression of the MHC-II genes can be induced by certain stimuli, particularly by interferon gamma (IFN-γ) (4,29,60,80), and other cytokines such as IL-4, TNF-α, prostaglandins, and glucocorticoids (29,33,60,80,104). The 3 isotypes DR, DP, and DQ are coordinately regulated. Considering the key function of MHC-II molecules, it is not surprising that correctly regulated MHC-II expression is important for the control of the immune response. This is clearly demonstrated by the fact that a lack of MHC-II expression severely cripples the immune system (50,60,80), while aberrant or inappropriate expression may be implicated in certain autoimmune diseases (1,6,33). For over a decade, there has been, therefore, a considerable amount of interest in the molecular mechanism controlling the expression of MHC-II genes. Both the constitutive and inducible modes of MHC-II expression are controlled primarily at the level of their transcription by a short promoter proximal enhancer region consisting of 4 highly conserved cis-acting DNA sequences referred to as the S, X, X2, and Y boxes. These 4 regulatory sequences are well conserved in all MHC-II genes in human and other species (4,29,60,80) and are sufficient for MHC-II gene expression. Different studies also suggest that polymorphisms within this well-conserved promoter can affect the level of MHC-II gene expression (57,58,93). A considerable amount of effort has been devoted to the identification and isolation of DNA-binding proteins that control transcription of MHC-II genes by interacting with the S, X, X2, and Y boxes. Three of the MHC-II promoter binding complexes identified by biochemical methods, RFX, NF-Y, and X2BP, have turned out to be particularly relevant to the regulation of MHC-II genes (Figure 2).Fig. 2: MHC-II promoters. Many different proteins can bind to the X, X2, and Y cis-acting elements of MHC-II promoters. The names of these proteins are indicated. However, it is now believed that the RFX, X2BP, and NF-Y complexes, which bind together cooperatively (double-headed arrow), are the major DNA-binding proteins implicated in MHC-II gene regulation. RFX (regulatory factor X) is a ubiquitously expressed heteromeric nuclear complex that binds to the X box of MHC-II promoters (81). It can be recruited to all MHC-II promoters by cooperative interactions with NF-Y and X2BP (66,79,82). Direct functional evidence for a crucial role of RFX in MHC-II gene expression has been provided by in vitro transcription experiments (22) and by elucidation of the molecular defects affecting MHC-II regulatory mutants, namely cell lines derived from patients suffering from MHC-II deficiency (23,63,97). RFX belongs to a novel family of related DNA-binding proteins, of which the other members (24,78,85) are probably not involved in the control of MHC-II gene transcription. The Y box binding complex NF-Y (also called YEBP or CBF) is a CCAAT binding protein of which 3 subunits have been cloned in various species (43,61,96,105). NF-Y is a heterotrimeric transcription factor consisting of 3 subunits, NF-YA, NF-YB, and NF-YC. NF-Y is ubiquitously expressed and has been reported to be involved in the regulation of a wide variety of eukaryotic genes, including MHC-II genes (17,61,92). Domains required for DNA binding and transcriptional activation have been mapped within these NF-Y subunits (54,95,114). Evidence for the role of NF-Y in the expression of MHC-II genes was derived from in vitro binding studies, classical transactivation assays, and in vitro transcription experiments (61,62,112,113). The X2 box binding complex X2BP is a nuclear (39,66) that was recently shown to contain CREB (65). RFX binds cooperatively with X2BP (39,66) and NF-Y (82) to generate a very stable higher order protein-DNA complex and this stable binding is required for promoter activation (22,59,66,79,82). Formation of this higher order complex is clearly required for occupation of MHC class II promoters in vivo, because these promoters remain bare (47,48) in cells in which formation of the higher order complex is abrogated by mutations destroying RFX (60,97) or by mutations in the Y box of MHC-II promoters (112). In addition to the RFX complex, another protein called CIITA (class II transactivator) is essential for the MHC-II molecule expression (Figure 3). CIITA is a non-DNA-binding transactivator that represents the “master regulator” for the expression of MHC-II genes (38,64,80). Recent experiments have demonstrated that CIITA functions as a coactivator. Although CIITA activates transcription of MHC-II genes via the promoter proximal region, it does not seem to bind directly to the DNA. This implicated that CIITA is recruited to MHC-II promoters by protein-protein interactions with DNA-bound factors. We and others have recently been able to demonstrate these interactions (20,37,38,70,115). Chromatin immunoprecipitation experiments have revealed that CIITA is indeed physically associated in vivo with MHC-II promoters and with the promoters of the Ii and HLA-DM genes. Furthermore, in vitro recruitment experiments have demonstrated that the association of CIITA with MHC-II promoters is mediated by multiple synergistic protein-protein interactions with components of the MHC-II enhanceosome (37,64).Fig. 3: Molecular defects in MHC-II deficiency. MHC-II promoter occupancy and transcription status of MHC-II genes (MHC-II, invariant chain [Ii] and DM) in normal MHC-II-positive B cells and in B cells from complementation groups A, B, C, and D. The gray boxes represent the S, X, X2, and Y regulatory DNA sequences present in the promoter proximal regions of all MHC-II genes. The protein complexes RFX, X2BP, and NF-Y bind, respectively, to the X, X2, and Y boxes. In addition, the protein CIITA is essential for the MHC-II gene expression. CIITA is mutated in complementation group A; mutations in CIITA do not modify promoter occupation in vivo. In complementation groups B, C, and D, the RFXANK, RFX5, or RFXAP subunits of RFX are mutated, respectively. A deficiency in RFX leads to a bare promoter in vivo.MHC-II Deficiency Much of what we know today about the highly complex molecular mechanisms regulating MHC-II gene transcription derives from a detailed molecular dissection of the genetic defects responsible for primary MHC-II deficiency. MHC-II deficiency is a severe autosomal recessive immunodeficiency disease resulting from a selective lack of MHC-II expression, and an absence of CD4 + T-cell-dependent cellular and humoral immune response (60,80,83,84). MHC-II deficiency is a rare disease; only approximately 70 patients from 57 unrelated families have been reported worldwide. The majority of patients are of North African origin (Algeria, Tunisia, Morocco) (32,50,56). The remaining patients are of diverse ethnic backgrounds including Spain, Italy, Turkey, France, the Netherlands, the United States, Israel, Saudi Arabia, and Pakistan (9,14,34–36,44,51,56,71). As expected for a rare disease, there is a high incidence of consanguinity in the affected families (56). The disease has an autosomal recessive mode of inheritance. A comparison between the pattern of inheritance of the disease and the MHC genotype in affected families has demonstrated that the genetic lesions responsible for the MHC-II deficient phenotype lie outside of the MHC locus (18,32). This represented the first indication that the disease is due to defects in transacting regulatory factors required for expression of MHC-II genes. The affected genes are now indeed known to encode for at least 4 distinct regulatory factors controlling transcription of MHC-II genes. Clinical manifestations Clinical manifestations include primarily septicemia and recurrent infections of the gastrointestinal, pulmonary, upper respiratory, or urinary tracts. Patients are prone to bacterial, fungal, viral, and protozoal infections. These start within the first year of life, and subsequent evolution of the disease is characterized by an inexorable progression of the infectious complications until death ensues. Few children reach puberty; the majority die between the ages of 6 months and 5 years (Table 1).TABLE 1: Clinical manifestations of MHC-II deficiency *Bacterial infections in various locations are These include infections, and The frequently include and and species have also been In all infections of the together with or are responsible for and failure to and due to to be more in MHC-II deficiency than it is in other The a of and an by and cells. is the manifestations are not Many patients of The of is with associated with and due to and infections have been in a of infections have been in all The infectious identified include respiratory and manifestations due to infections have been in a of and were frequently responsible for Two patients with of with with patients a severe autoimmune In addition to severe and recurrent infections due to a wide variety of which are to all has been reported in MHC-II deficiency patients not in other patients of the manifestations associated with MHC-II deficiency have also been The manifestations of MHC-II deficiency have been described in and are in of the manifestations can be for by the absence of antigen via MHC-II molecules. The and constant of the defect in MHC-II expression as the absence of cellular and humoral immune responses to foreign patients are to immune responses to specific as by type This with an absence of T cell responses in vitro in the of antigens with which the patients been or to by infections. with the absence of MHC-II expression is the that lymphocytes from the patients have a to lymphocytes in the lymphocyte 2: of MHC-II deficiency is severely The majority of patients are or have a in or 2 responses to and infections by are generally or It is that associated with autoimmune have been in Patients have normal of T and B In the majority of however, CD4 + T cell are while + T is in of This selection and of CD4 + T cells resulting from a lack of MHC-II expression in the The considerable from to It is however, that this does not with the genetic in the of the and in MHC Class II Deficiency cell studies have the of MHC-II deficiency patients 4 distinct complementation groups B, C, and that the disease is In addition to the certain in regulatory have also been complementation groups Three vitro DNA-binding studies, of I and in vivo have revealed the of 2 distinct biochemical in MHC-II deficiency A X box binding complex, the RFX complex, was first identified in nuclear from MHC-II-positive B cell lines on the of to bind in vitro to the X box of MHC-II promoters. This binding was to be deficient in the majority of MHC-II deficiency patients 2 I the MHC-II promoter in MHC-II-positive cells were to be in MHC-II deficiency cell lines The lack of RFX binding and the revealed by I studies were shown by in vivo experiments to be with an promoter phenotype MHC-II deficiency patients in complementation groups B, C, and group the deficiency in RFX binding the absence of I and a bare promoter In MHC-II deficiency patients from complementation group A a biochemical phenotype (see 3: and molecular in MHC-II for MHC Class II Deficiency the gene affected in complementation group A A genetic on expression was to the genes affected in MHC-II deficiency. this complementation of a cell from group A to the isolation of CIITA (class II transactivator) CIITA is a protein to other known proteins (Figure It functions as a non-DNA-binding that both constitutive and expression of MHC-II genes The CIITA gene represents the “master regulator” for the expression of MHC-II genes (see in the expression of CIITA and MHC-II is For example, the constitutive expression of MHC-II genes in B cells is by constitutive expression of The of B cells mature cells is by of the CIITA resulting in the of MHC-II expression The majority of cell types do not of and are consequently MHC-II negative. In such cells, the expression of and thus of MHC-II genes, can generally be induced by with This has been demonstrated in a wide variety of different cell including cells, and CIITA is a protein composed of CIITA an an 3 and a region is a an region and an region to be a is a a DNA-binding and 2 regions of RFXAP is a RFXAP an in and a and a nuclear is regulated and distinct and promoters control transcription of the CIITA promoters and have been identified in the human CIITA gene Three of these and are conserved in the The study of these promoters has demonstrated that it is their activation that the and of MHC-II gene expression Two of the promoters constitutive expression in professional antigen presenting cells. I is highly specific for dendritic cells while promoter is used primarily in B cells. A promoter expression. The is by in a wide variety of professional and antigen presenting cells, including and cells. promoter is the major promoter in many cell of promoter has also been The on promoters and for expression may as a function of the cell patients and in cell have been in complementation group A (Table CIITA mutations in patients and a cell in complementation group the gene affected in group B The gene that is in patients in complementation group B was identified by a biochemical The RFX complex was and sequences derived from the were used to and the gene was called because the protein it a protein-protein consisting of 3 (Figure The is in and it has also been reported to a are in many proteins that have short or not this region of functions as a is The gene has also been called to that it is mutated in complementation group B of the cell lines from patients in complementation group B expression of all MHC-II isotypes B for of all known and mutations in thus represent the molecular defect in the different mutations of the gene have been characterized in unrelated patients of these mutations affect the of the is a within the The remaining 6 are or mutations leading to proteins all or of the patients have the the of a (Table this group a certain of was Two affected for this have to and years of respectively, can be between the genotype and the of the disease mutations in patients in complementation group the gene affected in group The complementation used to CIITA was to cell lines a defect in binding of the RFX complex group B, C, and of a cell from a in complementation group to the isolation of RFX5, the of RFX was to be mutated in all MHC-II deficiency patients and an in in complementation group (Figure belongs to a novel family of related DNA-binding other members of this family are not involved in the control of the MHC-II gene expression RFX proteins have been conserved evolution in a wide variety of They have been recruited in a diverse of unrelated including regulation of the cell in the expression of various genes including MHC-II genes, and the control of genes such as of the highly B members of the family are characterized by a known to encode the DNA-binding The for this to other known DNA-binding play a key role in the cooperative binding between the RFX complex and the NF-Y complex of MHC-II genes is on the region of which was also to with CIITA cell lines derived from MHC-II deficiency patients and in cell have been in complementation group (Table mutations in patients and a cell in complementation group the gene in group The gene that is in patients in complementation group was identified by a biochemical The RFX complex was and sequences derived from the were used to and the The gene was called protein The primary of RFXAP an region, a region, and a nuclear (Figure RFXAP was to with both and and to and In RFXAP may the RFX complex in with the box the of patients were in complementation group D. The identification of RFXAP patients to be in group and to that mutations in RFXAP to the of immunodeficiency as in in the other groups Three different mutations the RFXAP gene have been identified in 6 unrelated families in complementation group D. 3 of these mutations to the of severely affecting the RFXAP gene in an in have also been in this cell a resulting from the of a (Table RFXAP mutations in patients and a cell in complementation group of MHC-II deficiency and an and severe of MHC-II deficiency have been described In to the in the of MHC-II expression in the patients does not all MHC-II genes and does not affect all cell types to the In B cells from these the and genes are while the and genes are In cells, on the other there is also a level of and expression. of the immune response is than in classical MHC-II deficiency. In of immune functions in these patients the of MHC-II-positive antigen presenting cells As expected from their and represent a complementation group that is distinct from groups of RFX and I has not been in the patients to complementation group and However, in vivo experiments have demonstrated that these patients an promoter occupancy of genes that are a bare while of genes that are expressed are The affected genes have not been In addition to and other patients with an have been identified the In 5 patients from group A, 3 from group B, and 2 as manifestations were severe or and was to be In patients this be by of the responsible for the MHC-II expression in 2 families mutations of the CIITA gene and do not transactivation as shown both in vivo and in vitro and Among 3 patients from group B, now and respectively, 2 the of the gene that has been in other patients at These 2 to with infections on and that remain to be In the the gene a leading to a for MHC-II transactivation These patients demonstrate that with the now be able to severe It is that such patients will not the used for of MHC-II deficiency. for MHC-II Deficiency and of infections and other complications can at the and of the associated with MHC-II deficiency. of the of and However, in of the these do not and generally between the ages of 5 and As for other immunodeficiency is the of for MHC-II deficiency. an is the of are now The is is with unrelated or with related The 2 are infections and failure or patients have now been by with such the than in other immunodeficiency (Table in MHC-II deficiency can be from the with in MHC-II deficiency. the lack of MHC-II expression, the of disease is to that in patients with other of CD4 + T cell remain in This is probably due to MHC-II expression by the epithelial cells of the the lack of MHC-II expression in cells does not to be for patients that the 4 genes affected in MHC-II deficiency have been gene a to Introduction of the RFX5, and genes cells of patients in complementation groups A, B, C, and D, respectively, represent a The and of this type of has recently been by in mutations in the cells were from these vivo with a the and functions in the patients were by the cells MHC-II expression is controlled in a and and or of MHC-II expression induced by the therefore be in the This not represent a major in the of gene with RFX5, RFXANK, and which are expressed ubiquitously at constant in all cell of on the other is regulated by promoters that in their and by expression of a CIITA will thus be to of the disease by gene can be in 3 as an to which until now the only therapy, to early and thus to of the in of is an an that be and to the patients of the key is the of the defects in a of the Two of MHC-II deficiency and both are very to the human These are the CIITA and which will be for the of gene for MHC-II deficiency Primary human immunodeficiencies are a heterogenous group of inborn errors of the immune Among MHC class II deficiency (MHC-II deficiency) is caused by the absence of MHC-II expression on the cell surface. The disease was named “major histocompatibility complex class II deficiency” it is also frequently referred to as the “bare lymphocyte syndrome” (BLS). The disease is due to defects in transacting regulatory factors required for expression of MHC-II genes. The affected genes are now known to encode at least 4 distinct regulatory factors controlling transcription of MHC-II genes. These transacting called RFXANK, RFX5, and are essential and specific for the MHC class II gene expression. while all patients have very the genetic defects are with mutations in of these 4 transacting factors. we the and molecular that MHC-II deficiency and