Analysis of mutations and chromosomal localisation of the gene encoding RFX5, a novel transcription factor affected in major histocompatibility complex class II deficiency

MHC class II deficiency is a severe primary immunodeficiency characterised by the absence of major histocompatibility complex class II (MHC-II) gene expression. It is genetically heterogeneous and can result from defects in at least four different trans-acting regulatory genes required for transcription of MHC-II genes. One of these genes has recently been shown to encode a novel DNA binding protein called RFX5, which is one subunit of a heteromeric protein complex (RFX) that binds to the promoters of MHC-II genes. We have characterised the mutations in all four patients known to harbour a defect in the RFX5 gene and have mapped this new human disease gene to chromosome 1 band q21, a region frequently exhibiting chromosomal aberrations in a variety of preneoplastic and neoplastic diseases.

Defective MHC class II expression in an MHC class II deficiency patient is caused by a novel deletion of a splice donor site in the MHC class II transactivator gene

MHC class II deficiency patients are mutated for transcription factors that regulate the expression of major histocompatibility complex (MHC) class II genes. Four complementation groups (A-D) are defined and the gene defective in group A has been shown to encode the MHC class II transactivator (CIITA). Here, we report the molecular characterization of a new MHC class II deficiency patient, ATU. Cell fusion experiments indicated that ATU belongs to complementation group A. Subsequent mutation analysis revealed that the CIITA mRNA lacked 84 nucleotides. This deletion was the result of the absence of a splice donor site in the CIITA gene of ATU. As a result of this novel homozygous genomic deletion, ATU CIITA failed to transactivate MHC class II genes. Furthermore, this truncated CIITA of ATU did not display a dominant negative effect on CIITA-mediated transactivation of various isotypic MHC class II promoters.

Transcriptional control of MHC genes and T cell development in MHC class II deficiency

MHC class II deficiency has proven to be an excellent model to study transcription regulation of MHC genes and T cell development. Cell lines established from MHC class II deficient patients have been of great value for the identification of proteins necessary for MHC expression and their study has resulted in the identification of a common regulatory pathway for MHC class II and class I genes. The lack of MHC class II expression was found to have a profound effect on the development of the CD4+ T cell lineage, in particular on the composition of the T cell receptor repertoire, revealing aberrant thymic selection processes in these patients. Here, we will discuss several aspects of the transcriptional regulation of MHC genes and the impact of deficient MHC class II expression on T cell development.

Discoordinate expression of invariant chain and MHC class II genes in class II transactivator-transfected fibroblasts defective for RFX5

MHC class II deficiency or bare lymphocyte syndrome is a severe combined immunodeficiency caused by defects in MHC-specific transcription factors. In the present study, we show that fibroblasts derived from a recently identified bare lymphocyte syndrome patient, SSI, were mutated for RFX5, one of the DNA-binding components of the RFX complex. Despite the lack of functional RFX5 and resulting MHC class II-deficient phenotype, transfection of exogenous class II transactivator (CIITA) in these fibroblasts can overcome this defect, resulting in the expression of HLA-DR, but not of DP, DQ, and invariant chain. The lack of invariant chain expression correlated with lack of CIITA-mediated transactivation of the invariant chain promoter in transient transfection assays in SSI fibroblast cells. Consequently, these CIITA transfectants lacked Ag-presenting functions.

Molecular analysis of an MHC class II deficiency patient reveals a novel mutation in the RFX5 gene

Patients suffering from major histocompatibility complex (MHC) class II deficiency, a rare primary immunodeficiency, are characterized by a lack of MHC class II expression which is the result of defects in trans-acting factors. At least four complementation groups, A, B, C, and D, can be discerned. The gene affected in group C patients is known to be RFX5 and encodes one of the subunits of the multimeric phosphoprotein complex, RFX. In the present study we fused fibroblasts of a recently identified MHC class II deficiency patient, OSE, with fibroblasts derived from patients representative of each of the four complementation groups. Transient heterokaryon analysis indicated that OSE belonged to complementation group C. Furthermore, transfection of wild-type RFX5 cDNA into OSE fibroblasts resulted in restoration of the defect. Mutation analysis revealed that the RFX5 mRNA lacked four nucleotides and that this deletion was the consequence of a G to A transition in a splice acceptor site. Genomic oligotyping demonstrated that OSE was homozygous for the splice site mutation.

Intrauterine transfusions affect fetal T-cell immunity

Intrauterine transfusion (IUT) therapy is the treatment of choice in severe hemolytic disease of the fetus. This treatment automatically implies the introduction of alloantigens in the fetal circulation, which might potentially influence the unprimed fetal immune system. The present study provides evidence that the fetal immune system is indeed prone to modulations of the T-cell receptor BV (TCRBV) repertoire as a result of IUT treatment. Most notably, IUT therapy affects the composition of the CD4+ repertoire, whereas this effect may be obscured in the CD8+ subset. The CD8+ subset was found to be influenced by alterations of the TCRBV repertoire both in IUT patients and controls, suggesting that modulations in this subset could be the result of developmental influences. A more detailed analysis on the composition of the individual TCRBV families was performed by evaluating the distribution of the complementarity determining region 3 (CDR3) size lengths of [32P]-radiolabeled TCRBV transcripts. Using this technique, referred to as spectratyping, only marginal changes were observed in the CD4+ and CD8+ subset during the course of treatment and gestational development of both IUT-treated patients and controls. Therefore, the alterations in the overall TCRBV repertoire were of a quantitative rather than a qualitative nature. To evaluate whether the observed alterations in TCRBV usage-frequencies were a reflection of an allo-reactive response, a primed lymphocyte test (PLT) was performed in 3 IUT-treated patients. We observed that IUT, performed as early as 23 weeks of gestation, may induce the establishment of memory T cells against the IUT donor. However, there was no association between the observed changes in TCRBV repertoire and the magnitude of the secondary allo-reactive response.

Antigen processing and presentation by human trophoblast-derived cell lines

The trophoblast-derived choriocarcinoma cell lines JEG-3 and JAR express the nonclassical MHC class I molecules HLA-G (JEG-3) or, at a low level, HLA-E (JAR), but lack expression of the classical MHC class I molecules HLA-A and HLA-B. Expression of these nonclassical MHC class I genes was found to coincide with expression of the genes encoding the peptide transporter associated with Ag processing (TAP). In immunoprecipitation studies, a physical interaction between the TAP complex and HLA-G or HLA-E could be demonstrated. To investigate whether trophoblast-derived cell lines were capable of peptide processing, transport, and loading of MHC class I molecules, HLA-A*0201-expressing transfectants of JEG-3 and JAR were used for functional studies. These transfectants were recognized by both allospecific cytotoxic T cell clones and, after viral infection, by an influenza A matrix peptide-specific cytotoxic T cell clone, indicating that these trophoblast-derived cell lines were capable of presenting endogenously derived peptides in the context of HLA-A*0201. From these observations, it can be inferred that the TAP complex and other molecules involved in Ag processing and presentation by MHC class I molecules are functionally active in these trophoblast-derived cell lines. This implies that trophoblasts are able to provide antigenic peptides for presentation by nonclassical MHC class I molecules that are naturally expressed by this cell type.

Antigen processing and presentation by human trophoblast-derived cell lines

The trophoblast-derived choriocarcinoma cell lines JEG-3 and JAR express the nonclassical MHC class I molecules HLA-G (JEG-3) or, at a low level, HLA-E (JAR), but lack expression of the classical MHC class I molecules HLA-A and HLA-B. Expression of these nonclassical MHC class I genes was found to coincide with expression of the genes encoding the peptide transporter associated with Ag processing (TAP). In immunoprecipitation studies, a physical interaction between the TAP complex and HLA-G or HLA-E could be demonstrated. To investigate whether trophoblast-derived cell lines were capable of peptide processing, transport, and loading of MHC class I molecules, HLA-A*0201-expressing transfectants of JEG-3 and JAR were used for functional studies. These transfectants were recognized by both allospecific cytotoxic T cell clones and, after viral infection, by an influenza A matrix peptide-specific cytotoxic T cell clone, indicating that these trophoblast-derived cell lines were capable of presenting endogenously derived peptides in the context of HLA-A*0201. From these observations, it can be inferred that the TAP complex and other molecules involved in Ag processing and presentation by MHC class I molecules are functionally active in these trophoblast-derived cell lines. This implies that trophoblasts are able to provide antigenic peptides for presentation by nonclassical MHC class I molecules that are naturally expressed by this cell type.

Lack of T cell oligoclonality in enzyme-digested synovial tissue and in synovial fluid in most patients with rheumatoid arthritis

The dominant presence of specific T-cell populations in the rheumatoid joint as detected by Southern blot analysis of T cell receptor (TCR) gene rearrangements would indicate local antigen recognition and T cell proliferation. We therefore studied TCR beta chain gene rearrangements using a C beta 2 probe in paired samples of T cell populations from synovial tissue and peripheral blood (n = 6) as well as synovial fluid (n = 16) and peripheral blood (n = 18) of patients with rheumatoid arthritis (RA). Peripheral blood mononuclear cells from healthy donors (n = 7) served as a control. T cells were studied directly after isolation or after non-specific expansion with OKT3 monoclonal antibody (MoAb) and T cell growth factor (TCGF). DNA samples were digested with EcoRI and HindIII to detect rearrangements to C beta 1 and C beta 2, respectively. Extra bands were detected in all EcoRI-digested DNA samples prepared from both freshly isolated and non-specifically expanded T cell populations of patients and healthy donors, possibly representing 'common' (V-) D-J rearrangements. Dominant rearrangements were found in only two out of 16 synovial fluid T cell populations (one freshly isolated and one expanded) and not in peripheral blood or synovial tissue derived T cell populations. No extra bands were detected in HindIII-digested DNA samples. To investigate the effect of in vitro culture techniques on rearrangement patterns we studied DNA samples prepared from synovial tissue T cells obtained both by outgrowth from tissue with TCGF or by enzyme digestion and subsequent expansion either with TCGF or with OKT3 MoAb and TCGF. Whereas the latter T cell population yielded 'common' rearrangements, the former T cell populations yielded different dominant rearrangements. These data indicate that oligoclonality of the T cell populations in synovial tissue and synovial fluid of patients with RA is a rare event. The data also show the influence of in vitro culture techniques on the result of TCR gene rearrangement analysis.