High mobility group 1 (HMG1) protein is an abundant component of all mammalian nuclei, and related proteins exist in all eukaryotes. HMG1 binds linear DNA with moderate affinity and no sequence specificity, but bends the double helix significantly on binding through the minor groove. It binds with high affinity to DNA that is already sharply bent, such as linker DNA at the entry and exit of nucleosomes; thus, it is considered a structural protein of chromatin. HMG1 is also recruited to DNA by interactions with proteins required for basal and regulated transcriptions and V(D)J recombination. Here we generate mice harbouring deleted Hmg1. Hmg1-/- pups are born alive, but die within 24 hours due to hypoglycaemia. Hmg1-deficient mice survive for several days if given glucose parenterally, then waste away with pleiotropic defects (but no alteration in the immune repertoire). Cell lines lacking Hmg1 grow normally, but the activation of gene expression by the glucocorticoid receptor (GR, encoded by the gene Grl1) is impaired. Thus, Hmg1 is not essential for the overall organization of chromatin in the cell nucleus, but is critical for proper transcriptional control by specific transcription factors.

HMG boxes are DNA binding domains present in chromatin proteins, general transcription factors for nucleolar and mitochondrial RNA polymerases, and gene‐ and tissue‐specific transcriptional regulators. The HMG boxes of HMG1, an abundant component of chromatin, interact specifically with four‐way junctions, DNA structures that are cross‐shaped and contain angles of approximately 60 and 120 degrees between their arms. We show here also that the HMG box of SRY, the protein that determines the expression of male‐specific genes in humans, recognizes four‐way junction DNAs irrespective of their sequence. In addition, when SRY binds to linear duplex DNA containing its specific target AACAAAG, it produces a sharp bend. Therefore, the interaction between HMG boxes and DNA appears to be predominantly structure‐specific. The production of the recognition of a kink in DNA can serve several distinct functions, such as the repair of DNA lesions, the folding of DNA segments with bound transcriptional factors into productive complexes or the wrapping of DNA in chromatin.

The receptor TLR9, recognizing unmethylated bacterial DNA (CpG), is expressed by B cells and plays a role in the maintenance of serological memory. Little is known about the response of B cells stimulated with CpG alone, without additional cytokines. In this study, we show for the first time the phenotypic modification, changes in gene expression, and functional events downstream to TLR9 stimulation in human B cell subsets. In addition, we demonstrate that upon CpG stimulation, IgM memory B cells differentiate into plasma cells producing IgM Abs directed against the capsular polysaccharides of Streptococcus pneumoniae. This novel finding proves that IgM memory is the B cell compartment responsible for the defense against encapsulated bacteria. We also show that cord blood transitional B cells, corresponding to new bone marrow emigrants, respond to CpG. Upon TLR9 engagement, they de novo express AID and Blimp-1, genes necessary for hypersomatic mutation, class-switch recombination, and plasma cell differentiation and produce Abs with anti-pneumococcal specificity. Transitional B cells, isolated from cord blood, have not been exposed to pneumococcus in vivo. In addition, it is known that Ag binding through the BCR causes apoptotic cell death at this stage of development. Therefore, the ability of transitional B cells to sense bacterial DNA through TLR9 represents a tool to rapidly build up the repertoire of natural Abs necessary for our first-line defense at birth.

CD40 is a member of the tumor necrosis factor receptor superfamily, expressed on a wide range of cell types including B cells, macrophages, and dendritic cells. CD40 is the receptor for CD40 ligand (CD40L), a molecule predominantly expressed by activated CD4 ؉ T cells. CD40͞CD40L interaction induces the formation of memory B lymphocytes and promotes Ig isotype switching, as demonstrated in mice knocked-out for either CD40L or CD40 gene, and in patients with X-linked hyper IgM syndrome, a disease caused by CD40L͞TNFSF5 gene mutations. In the present study, we have identified three patients with an autosomal recessive form of hyper IgM who fail to express CD40 on the cell surface. Sequence analysis of CD40 genomic DNA showed that one patient carried a homozygous silent mutation at the fifth base pair position of exon 5, involving an exonic splicing enhancer and leading to exon skipping and premature termination; the other two patients showed a homozygous point mutation in exon 3, resulting in a cysteine to arginine substitution. These findings show that mutations of the CD40 gene cause an autosomal recessive form of hyper IgM, which is immunologically and clinically undistinguishable from the X-linked form.

The ATPase ISWI is a subunit of several distinct nucleosome remodeling complexes that increase the accessibility of DNA in chromatin. We found that the isolated ISWI protein itself was able to carry out nucleosome remodeling, nucleosome rearrangement, and chromatin assembly reactions. The ATPase activity of ISWI was stimulated by nucleosomes but not by free DNA or free histones, indicating that ISWI recognizes a specific structural feature of nucleosomes. Nucleosome remodeling, therefore, does not require a functional interaction between ISWI and the other subunits of ISWI complexes. The role of proteins associated with ISWI may be to regulate the activity of the remodeling engine or to define the physiological context within which a nucleosome remodeling reaction occurs.

The mammalian nuclear protein HMG1 contains two segments that show a high sequence similarity to each other. Each of the segments, produced separately from the rest of the protein in Escherichia coli, binds to DNA with high specificity: four-way junction DNA of various sequences is bound efficiently, but linear duplex DNA is not. Both isolated segments exist as dimers in solution, as shown by gel filtration and chemical crosslinking experiments. HMG1-like proteins are present in yeast and in protozoa: they consist of a single repetition of a motif extremely similar to the DNA binding segments of HMG1, suggesting that they too might form dimers with structural specificity in DNA binding. Sequences with recognizable similarity to either of the two DNA binding segments of HMG1, called HMG boxes, also occur in a few eukaryotic regulatory proteins. However, these proteins are reported to bind to specific sequences, suggesting that the HMG box of proteins distantly related to HMG1 might differ significantly from the HMG box of HMG1-like proteins.

A.Eberharter and S.Ferrari contributed equally to this workThe chromatin accessibility complex (CHRAC) was originally de®ned biochemically as an ATP-dependent nucleosome remodelling' activity. Central to its activity is the ATPase ISWI, which catalyses the transfer of histone octamers between DNA segments in cis. In addition to ISWI, four other potential subunits were observed consistently in active CHRAC fractions. We have now identi®ed the p175 subunit of CHRAC as Acf1, a protein known to associate with ISWI in the ACF complex. Interaction of Acf1 with ISWI enhances the ef®ciency of nucleosome sliding by an order of magnitude. Remarkably, it also modulates the nucleosome remodelling activity of ISWI qualitatively by altering the directionality of nucleosome movements and the histone`tail' requirements of the reaction. The Acf1±ISWI heteromer tightly interacts with the two recently identi®ed small histone fold proteins CHRAC-14 and CHRAC-16. Whether topoisomerase II is an integral subunit has been controversial. Re®ned analyses now suggest that topoisomerase II should not be considered a stable subunit of CHRAC. Accordingly, CHRAC can be molecularly de®ned as a complex consisting of ISWI, Acf1, CHRAC-14 and CHRAC-16. Keywords: CHRAC/chromatin structure/ISWI/ nucleosome remodelling IntroductionThe packaging of the eukaryotic genome into chromatin has important implications for fundamental nuclear processes such as DNA replication (DePamphilis, 1999), transcription (Pollard and Peterson, 1998;Wolffe and Hayes, 1999;Strahl and Allis, 2000), repair (Schlissel, 2000) and recombination (Moggs and Almouzni, 1999;Smerdon and Conconi, 1999). Repressive chromatin structure can be rendered permissive for interaction of regulatory factors through the action of remodelling machines that modulate the structure of the nucleosome (Kadonaga, 1998;Kingston and Narlikar, 1999;Vignali et al., 2000). One type of`r emodelling' machine requires continuous ATP hydrolysis for its activity. Central to their function are ATPases of the SWI2/SNF2 superfamily that are found in all eukaryotes, from yeast to man (Eisen et al., 1995). The currently characterized enzymes de®ne three distinct families of nucleosome remodelling machines: SWI2/ SNF2-like, ISWI-like and Mi2-like (Boyer et al., 2000;Brehm et al., 2000;Guschin et al., 2000). Nucleosome remodelling factors driven by the ATPase ISWI have ®rst been isolated by biochemical fractionation of Drosophila embryo extracts. Three distinct ISWI-containing complexes with different protein composition and functional characteristics have been isolated: the nucleosome remodelling factor (NURF; Tsukiyama and Wu, 1995), the chromatin accessibility complex (CHRAC; Varga-Weisz et al., 1997) and the ATP-dependent chromatin assembly and remodelling factor (ACF; Ito et al., 1997). For the NURF complex, so far two ISWI-associated proteins have been identi®ed: the WD repeat protein NURF-55, which can also be found in the chromatin assembly factor CAF-1 (Martinez-Balbas et al., 1998), and the inorganic pyrop...

CD40 is a member of the tumor necrosis factor receptor family, which is expressed by a variety of cells including B cells, macrophages, dendritic cells, and other nonimmune cell types. CD40 activation is critical for B-cell proliferation, immunoglobulin (Ig)-isotype switching, and germinal center formation. In physiological conditions, the activation of CD40 occurs by binding to its natural ligand, CD154, which is expressed on activated T cells. The in vivo critical role of CD40-CD154 interaction on B-cell differentiation and isotype switching is provided by the discovery that mutations in either CD40 or CD154 gene cause the hyper IgM syndrome, termed HIGM3 or HIGM1, respectively, characterized by very low levels of serum IgG, IgA, and IgE, with normal or elevated IgM, associated with a defective germinal center formation. Originally considered humoral primary immunodeficiencies, the clinical features and the defect of T-cell priming, resulting from a defective T-B cell or dendritic cell interaction, is now considered as combined immunodeficiencies. In this article, we present a comprehensive overview of the clinical, genetic, and immunological features of patients with hyper IgM syndrome due to CD40 mutations.