bluuebunny/crossref_metadata_2025 huggingface dataset:
T‐cell fate
Authors: Hedrick Stephen M. , Sharp Leslie L. , Hedrick Stephen M. , Sharp Leslie
Published: April 2006

Summary: Recent studies suggest that lineage commitment steps, which occur during T‐cell differentiation, follow principles in common with fate specification in simple invertebrates. Here we review T‐cell development from the perspective of developmental biology. We present models for αβγσ Y5 and CD4 vs CD8 lineage commitment that are consistent with previously published and newly presented experiments.
Accessibility control of variable region gene assembly during T‐cell development
Authors: Sleckman Barry P. , Bassing Craig H. , Bardon Christiana G. , Okada Ami , Khor Bernard , Bories Jean‐Christophe , Monroe Robert , Alt Frederick W.
Published: April 2006

Summary: T‐cell development is a complex and ordered process that is regulated in part by the progressive assembly and expression of antigen receptor genes. T cells can be divided into two lineages based on expression of either an αβ or γδ T‐cell antigen receptor (TCR), The genes that encode the TCR β and y chains lie in distinct loci, whereas the genes that encode the TCR a and S chains he in a single locos (TCR α/δ locus). Assembly of TCR variable region genes is mediated by a site‐specific recombination process that is common among all lymphocytes. Despite the common nature of this process, recombination of TCR genes is tightly regulated within the context of the developing T cell. TCR β, γ and δ variable region genes are assembled prior to TCR α variable region genes. Furthermore, assembly of TCR β variable region genes is regulated within the context of allelic exclusion. The regulation of rearrangement arid expression of genes within the TCR α/δ locus presents a complicated problem. TCR α and δ variable region genes are assembled at different stages of T‐cell development, and fully assembled TCR α and δ variable region genes must be expressed in distinct hneages of T cells, αβ and γδ. respectively We have developed several experimental approaches lo assess the role of cis‐acting elements in regulating recombination and expression of TCR genes. Here we describe these approaches and discuss our analyses of the regulation of accessibility of the TCR β and TCR α/δ foci during T‐cell development.
The Syk family of protein tyrosine kinases in T‐cell activation and development
Authors: Chu David H. , Morita Craig T , Weiss Arthur
Published: April 2006

Summary: The processes of T‐cell development and activation employ similar immature and mature receptors as well as similar signal transduction pathways to achieve different outcomes. Many signaling molecules are shared between [he receptor signaling pathways, including two families of cytoplasmic protein tyrosine kinases, the Src family and the Syk family. The two Syk family members expressed in T cells, Syk and ZAP‐70, are structurally similar but are expressed at different times during rhymic development and during T‐cell activation. These two kinases, althongh they share many physical features, differ in terms of biochemical activity and regulation. We discuss the overlapping and distinct characteristics of Syk and ZAP‐70 in T‐cell signaling and the potential biological importance of their differences.
CD4/CD8 lineage commitment: matching fate with competence
Authors: Chan Susan , Correia‐Neves Margarida , Benoist Christophe , Mathis Diane
Published: April 2006

Summary: The outcome of positive selection of T lymphocytes is that there is a close match between the lineage adopted by a particular cell (CD4+ or CD8+) and the specificity of the T‐cell receptor for the class of Major Histocompatibility Complex molecule recognized. How this match is obtained has been a matter of debate. We review the evidence, from recent and older experiments, that indicates that the process follows a selective logic, rather than an instructive one.
Probing the activation requirements for naive CD8+ T cells with Drosophila cell transfectants as antigen presenting cells
Authors: Cai Zeling , Brunmark Anders B. , Luxembourg Alain T , Garcia K. Christopher , Degano Massimo , Teyton Luc , Wilson Ian , Peterson Per A. , Sprent Jonathan , Jackson Michael R.
Published: April 2006

Summary: Activation of T cells involves multiple receptor‐ligand interactions between T cells and antigen presenting cells (APC), At least two signals are required for T‐cell activation: Signal 1 results from recognition of MHC/peptide complexes on the APC by cell surface T‐cell receptors (TCR). whereas Signal 2 is induced by the interactions of co‐stimulatory molecules on APC with their complementary receptors on T cells. This review focuses on our attempts to understand these various signals in a model system involving the 2C TCR. The structural basis of Signal 1 was investigated by determining the crystal structure of 2C TCR alone and in complex with MHC/peptide. Analysis of these structures has provided some basic rules for how TCR and MHC/peptide interact; however, the critical question of how this interaction transduces Signal I to T cells remains unclear. The effects of Signal 1 and Signal 2 on T‐cell activation were examined with naive T cells from the 2C TCR transgenic mice, defined peptides as antigen and transfected Drosophila cells as APC. The results suggest that, except under extreme conditions, Signal I alone is unable to activate naive CD8 T cells despite the induction of marked TCR downregulation. Either B7 or intercellular adhesion molecule (ICAM)‐l can provide the second signal for CD8 T‐cell activation. However, especially at low MHC/peptide densities, optimal activation and differentiation of CD8 T cells required interaction with both B7 and [CAM‐1 on the same APC. Thus, the data suggest that at least two qualitatively different co‐stimulation signals are required for full activation of CD8 T cells under physiological conditions.
Induction of peripheral CD8+ T‐cell tolerance by cross‐presentation of self antigens
Authors: Miller J. F. A. P. , Kurts C. , Allison J. , Kosaka H. , Carbone F. , Heath W. R.
Published: April 2006

Summary: There is now convincing evidence that CD8+ T cells can be activated by professional antigen‐presenting ceils which present antigens derived from non‐lymphoid tissues in association with MHC class I molecules in the draining lymph nodes. This mechanism, referred to as cross‐presentation, enables the immune system to respond to those microorganisms that infect only non‐lymphoid tissues. Consistent with this view, cross‐presentation was found to focus on antigens expressed in high concentrations and those released from dying cells, which can be expected to result from viral infections. Recent evidence, however, demonstrates that high dose self antigens can be cross‐presented constitutively, resulting m the activation of autoreactive CD8+ T cells. This does not lead to auto‐immunity under physiologic conditions, but to CD95‐mediated deletion of the T cells. Cross‐presentation can thus engage a well‐defined pathway of antigen‐induced T‐cell death and purge the immune system of autoreactive CD8+ T cells. Low dose self antigens are not cross‐presented and are consequently ignored. The immune system therefore uses two strategies to avoid CD8+ T‐cell‐mediated autoimmunity in the periphery: deletion of autoreactive CD8+ T cells responding to high dose self antigens and ignorance of self antigens expressed at low concentrations.
Molecular regulation of interleukin‐2 expression by CD28 co‐stimulation and anergy
Authors: Powell Jonathan D. , Ragheb Jack A. , Kitagawa‐Sakakida Satoru , Schwartz Ronald H.
Published: April 2006

Summary: The consequences of T‐cell receptor engagement (signal I) are profoundly affected by the presence or absence of co‐stimulation (signal 2). T‐cell receptor (TCR) stimulation in the absence of CD 28‐mediated co‐stimulation not only results in little interleukin (IL)‐2 production, but induces a long lasting hyporesponsive state known as T‐cell clonal anergy The addition of CD 28 ligation to signal 1, on the other hand, results in the production of copious amounts of IL‐2. Our laboratory has utilized CD4+ Th1 clones in an effort to understand the molecular events resulting in enhanced IL‐2 production by co‐stimulation and the inhibition of IL‐ 2 production in anergy Our current studies have focused on defining the post‐transcriptional effects of CD28‐enhanced IL‐2 production The data suggest that a major component of CD28′s ability to regulate IL‐2 production occurs at the level of message stability and involves the 3′‐untranslated region of the message. In terms of anergy, our recent studies support the notion that it is not the result of TCR engagement in the absence of co‐stimulation. but rather signal 1 in the absence of IL‐2 receptor signaling and proliferation. Furthermore, T‐cell anergy appears to be an active negative state in which IL‐2 production is inhibited both at the level of signal transduction and by cis‐dominant repression at the level of the IL‐2 promoter.
The role of clonal anergy in the avoidance of autoimmunity: inactivation of autocrine growth without loss of effector function
Authors: Malvey Enka‐Nell , Telander David G. , Vanasek Tracy L. , Mueller Daniel L.
Published: April 2006

Summary: Exposure of mainre CD4+ T cells in the peripheral immune system to peptide‐antigen/MHC complexes in the absence of a threat of infection indnces tolerance to the antigen as a result of both a decreased clonal frequency (peripheral deletion) and the induction of proliferative unresponsiveness (clonal anergy) in the survivors. Interestingly, Th1‐like effector functions are not automatically blocked after the development of clonal anergy. Thus, anergic T cells have the capacity to mediate Th I ‐like helper activities if allowed to accumulate to high frequency hi this article, we examine those factors important to the development it of tolerance versus immunity against protein antigen, and speculate on the relationship that exists between effective peripheral tolerance induction and the avoidance of autoimmune disease.
The phylogeny and evolution of the thioester bond‐containing proteins C3, C4 and α2–macroglobulin
Authors: Dodds Mister W. , Alex Law S. K.
Published: April 2006

Summary: The complement system is an effector of both the acquired and innate immune systems of the higher vertebrates. It has been traced back at least as far as the echinoderms and so predates the appearance of the antibodies, T‐cell receptors and MHC molecules of adaptive immunity. Central to the function of complement is the reaction of the thioester bond located within the structure of complement components C3 and C4, The structural thioester first appeared in a protease inhibitor, α2‐macroglobulin, in which it is involved in the immobilisation and entrapment of proteases. An important development in the C3 molecule has been the acquisition of a catalytic His residue which greatly increases the rate of reaction of the thioester with hydroxyl groups and with water.
Structural and functional identification of complement components of the bony fish, carp (Cyprinus carpio)
Authors: Nakao Miki , Yano Tomoki
Published: April 2006

Summary: Complement is a humoral factor of innate immunity and plays important roles in immune surveillance and clearance of invading pathogens. Mammalian complement consists of the classical (antibody‐dependent), the alternative (antibody‐independent) and the lectin (triggered by mannose‐binding lectin) pathways of activation, and of the lytic pathway Phylogenetically, bony fish are one of the lowest groups of vertebrates with serum that shows hemolytic activity indistinguishable from that of mammalian complement. In our series of trials to address the question “How many components constitute the bony fish complement?” Functional analyses, protein isolation, and molecular cloning revealed the presence of major components constituting each pathway, the similarity between bony fish and mammalian complement being considerable. Recent investigations on carp (Cyprinus carpio)and other fish species revealed striking features unique to bony fish complement, including a remarkable diversity in structure and function of the third component, C3 and the presence of a newly identified lineage in evolution of the factor B and C2 family. For a significant insight into the evolution of complement systems and clinical applications to aquaculture industry, further extended studies are warranted.
Molecular genetics of the complement C3 convertases in lower vertebrates
Authors: Nonaka Masaru , Kuroda Noriyuki , Naruse Kiyosi , Shima Akihiro , Nonaka Masaru , Kuroda Noriyuki , Naruse Kiyosi , Shima Akihiro
Published: April 2006

Summary: Evolution of the two gene families of the complement system involved in the formation of the C3 convertases, B/C2 andC3/C4/C5, was studied at the cDNA level in lower vertebrates. Cyclostomes, the most primitive extant vertebrates, seem to possess only one member each of these families, indicating chat gene duplication between B and C2 or among C3, C4 and C5 occurred in the lineage of jawed vertebrates. Typical C3 and C4 cDNAs were identified in both amphibian (Xenopus) and teleost (medaka fish), locating the C3 /C4 gene duplication before the divergence of ray funned fish and lobe‐finned Fish, On the other hand, typical B cDNA was identified in Xenopus, whereas teleost counterparts from three species all showed intermediate character between B and C2, suggesting the possibility that the B/C2 gene duplication occurred in die tetrapod lineage. Genetic linkage between these two family genes within the MHC was observed in Xenopus but not in medaka fish.
Shark complement: an assessment
Authors: Smith Sylvia L
Published: April 2006

Summary: The classical (CCP) and alternative (ACP) pathways of complement activation have been established for the nurse shark (Ginglymostoma cirratum). The isolation of a cDNA done encoding a mannan‐binding protein‐associated serine protease (MASP)‐1‐like protein from the Japanese dogfish (Triakis scyllia) suggests the presence of a lectin pathway. The CCP consists of six functionally distinct components: C1n, C2n, C3n, C4n, C5n and C9n, and is activated by immune complexes in the presence of Ca++ and Mg++ ions. The ACP is antibody independent, requiring Mg++ ions and a heat‐labile 90 kDa factor B‐like protein for activity. Proteins considered homologues of C1q, C1 and C4 (C2n) of the mammalian complement system have been isolated from nurse shark serum. Shark C1q is composed of at least two chain types each showing 50% identity to human C1q chains A and B. Partial sequence of the globular domain of one of the chains shows it to be C1q like rather than the mannan‐binding protein. N‐terminal amino acid sequences of the α and β chain of shark C3 and C4 molecules show significant identity with corresponding human C3 and C4 chains. A sequence representing shark C4γ chain, shows little similarity to human C4γ chain. The terminal shark components C8n and C9n are functional analogues of mammalian C8 and C9. Anaphylatoxin activity has been demonstrated in activated shark serum, and porcine C5a desArg Induces shark leucocyte chemotaxis. The deduced amino acid sequence of a partial C3 cDNA clone from the nurse shark shows 50%, 30% and 24% homology with the corresponding region of mammalian C3, C4 and al‐macroglobulin. Deduced amino acid sequence data from partial Bf/C2 cDNA clones, two from the nurse shark and one from the Japanese dogfish, suggest that at least one species of elasmobranch has two distinct Bf/C2 genes.
Antibodies of sharks: revolution and evolution
Authors: Marchalonis John J. , Schluter Samud F. , Bernstein Ralph M. , Hohman Voleric S.
Published: April 2006

Summary: The combinatorial immune response is restricted to jawed vertebrates with cartilaginous fishes being the lowest extant species to have the mechanism for diversification and an extensive panoply of immuno‐globulins, T‐cell receptors and MHC products. Here, we review the molecular events of the “big bang” or rapid evolutionary appearance of the functionally complete combinatorial immune system coincident with the appearance of ancestral jawed vertebrates, suggesting that this event was catalyzed by horizontal transfer of DNA processing systems. We analyze the nature and extent of variable and constant domain diversity among the distinct immunoglobulin sets of carcharhine sharks focusing upon the X‐like light chains and the u and CO heavy chains. The detection and isolation of natural antibodies from the blood of unimmunized sharks illustrates a surprising ratige of recognition specificities and the existence of polyspecificity suggests that the antibody‐forming system of sharks offers unique opportunities for studies of immunological regulation. Although the homologies between shark and mamma an immunoglobulins are unequivocal, major differences in segmental gene organization present challenges co our understanding of basic immunological phenomena such as clonal restriction.
Evolution of immune surveillance and tumor immunity: studies in Xenopus
Authors: Robert Jacques , Cohen Nicholas
Published: April 2006

Summary: We have developed a novel experimental model of cancer immunity in the frog, Xenopus, which may provide a useful alternative to murine tumor models and a way to assess whether the control of tumor development is a fundamental function of the immune system of vertebrates. In Xenoptis, tumor immunity can be studied in two developmentally distinct immune systems. The larval immune system reflects characteristics of an ancestral system that appears to function without classical MHC class I antigen presentation and an efficient effector mechanism. The adult system appears more highly evolved in that it is remarkably similar to that of mammals and is able to generate a potent antitumor response. This amphibian model also provides a unique system with which to investigate a postulated role of heat shock proteins as components of an ancestral system of antigen presentation and/or immune surveillance that predates the antigen presentation pathway that exclusively involves MHC molecules.
T‐cell and natural killer cell development in thymectomized Xenopus
Authors: Horton John , Horton Trudy , Dzialo Robin , Gravawr Ian , Minter Ralph , Ritchie Pamela , Ganluna Lanier , Watson Martin , Caopet Max
Published: April 2006

Summary: The Xenopus early‐thymectomy model system is used to investigate the extent to which the thymus controls T‐cell development and to probe the evolution of natural killer (NK) ceils. Loss of T‐cell function following thymectomy, together with the paucity of cells expressing monoclonal antibody‐defined T‐cell surface markers, and greatly reduced expression of T‐cell receptor β transcripts in spleen, ever and intestine, indicate that T‐cell development is minimal in the absence of the thymus. Our findings therefore mitigate against the idea that a substantial extrathymic pathway of T‐cell development exists in early vertebrate evolution. Rather, they suggest that in this amphibian representative T cells are predominately thymus dependent. In vitro studies with control and thymectomized Xenopus splenocytes reveal that a non‐T/non‐B population and also two T‐cell subsets all display natural cytotoxicity towards allogeneic thymus lymphoid tumour cells (which are deficient in MHC antigen expression). Since Xenopus thymectomized early in larval development are permanently deficient in T cells, they may provide a useful phylogenetic model for the study of NK cells.
Expressed major histocompatibility complex class II loci in fishes
Authors: McConnell Thomas J. , Godwin Ulla B. , Cuthbertson Brandon J.
Published: April 2006

Summary: Peptides derived from parasites are presented to T helper cells by major histocompatibility complex (MHC) class II αβ heterotrimeric cell‐surface molecules. In mice and humans, the genes encoding these antigen‐presenting molecules are known to be polymorphic and poly‐genic. Multiple loci for MHC class II A and E genes are proposed to allow for an increased peptide‐binding repertoire. The multigenic nature of expressed MHC class II loci and the differences between these loci in fishes are the focus of this review, Particular emphasis is placed on an evolutionary comparison of class II B loci, especially two class 11 B loci that have undergone dramatic changes from one another suggesting an ancestral gene duplication event that took place at an early stage in the evolution of teleosts, The number of functional class II αβ loci heterotrimers may have a profound impact on the organisms ability to battle constantly evolving parasitic infections.
Major histocompatibility genes in cyprinid fishes: theory and practice
Authors: Stet René J. M. , Kruiswijk Corine R , Saeij Jeroen P. J. , Wiegertjes Geert F.
Published: April 2006

Summary: The first teleostean MHC sequences were described for carp. Subsequent studies in a number of cyprinid fishes showed that the class I sequences of these fishes are of particular interest. Two distinct lineages (Cyca‐Z and Cyca‐U) are found in the common and ginbuna crucian carp, but only the U lineage is present in zebrafish and other non‐cyprinid species. The presence of the Z lineage is hypothesised to be the result of an allotetraploidisation event. Both phylogenetic analyses and amino acid sequence comparisons suggest that Cyca‐Z sequences are non‐classical class I sequences, probably similar to CD I. The comprehensive phylogenetic analyses of these sequences revealed different phylogenetic histories of the exons encoding the extracellular domains. The MHC genes were studied in laboratory and natural models. The natural model addressed the evolution of MHC genes in a Barbus species flock. Sequence analysis of class I and class II supported the species designation of the morphotypes present in the lake, and as a consequence the trans‐species hypothesis of MHC polymorphism. The laboratory model involves the generation of gynogenetic clones, which can be divergently selected for traits such as high and low antibody response. The role of MHC molecules can be investigated further by producing a panel of isogenic lines.
Rainbow trout cytokine and cytokine receptor genes
Authors: Secombes Christopher , Zou Jun , Daniels Garry , Cunningham Charles , Koussounadis Antonis , Kemp Graham
Published: April 2006

Summary: Transforming growth factor‐β (TGF‐β) and interleukin‐1β (IL‐1β) have been cloned from rainbow trout (Oncorhynchus mykiss) in our laboratory. The trout TGF‐β is typical of members of the cysteine knot cytokine family, and has highest homology to TGF‐β1, TGF‐β4 and TGF‐βs The gene organisation is different to other TGF‐β genes despite die presence of seven exons. Trout IL‐1β has less homology lo known IL‐1βs (49–56% amino acid similarity) but shows dear conservation of the secondary structure of the “mature peptide”. An interleukin‐converting enzyme cut site is not apparent however, and it remains to be determined whether fish IL‐1β must be cleaved for biological activity. The trout IL‐1β gene consists of six exons, one less than in mammals, with the missing exon at the 51 end of the gene. Two trout chemokine receptors have also been isolated in our laboratory, having high homology to CXC‐R4 and CC‐R7 (67% and 56% nucleotide identity respectively). With a view to studying the biological effects of fish cytokines we have also cloned the inducible nitric oxide (iNOS) and cyclooxygenase (COX‐2) genes in trout. Both have high homology to known mammalian and chicken genes, and have been shown to be inducible in trout.
CK‐1, a putative chemokine of rainbow trout (Oncorhynchus mykiss)
Authors: Dixon Brian , Shum Benny , Adams Erin J. , Magor Katharine , Hedrick Ronald P. , Muir David G. , Parham Peter
Published: April 2006

Summary: Chemokines arc small inducible proteins that direct the migration of leukocytes. While chemokines are well characterised in mammals, they have yet to be identified in fish. We have isolated a cDNA clone from rainbow trout (Oncorhynchus mykiss) which encodes a protein (CK‐1) having structural features typical of chemokines. Amino‐acid residues that define the β‐chemokines of mammals are conserved in CK‐1, including the paired cysteine motif, CC. Further similarities are shared with the C6 subfamily of β‐chemokines. In contrast, the organisation of the CK‐f gene is closer to that of mammalian α‐chemokine genes than β‐chemokine genes. The CK‐1 gene is present in all four salmonid species examined and the nucleotide sequences of the exons are highly conserved. CK‐1 has characteristics in common with mammalian α and β‐chemokine suggesting that this salmonid chemokine gene preserves traits once present in the ancestral chemokine gene from which modern mammalian chemokine genes evolved.
Interferon‐inducible Mx proteins in fish
Authors: Leong Jo‐Ann C. , Trobridge Grant D. , Kim Carol H. Y. , Johnson Marc , Simon Ben
Published: April 2006

Summary: Mx proteins are members of a family of hiterferon‐inducible genes expressed when cells are treated with double‐stranded RNA or virus infection. These proteins are important components of the antiviral response and form the first line of the body's defense against virus infections. The exact mechanism of action for these proteins has not been discovered, but mice missing the Mx genes are extremely sensitive to influenza virus infection. Mammals have between two and three Mx genes whose functions may vary with regard to the inhibition of a specific virus, cellular localization, and activity. The cDNA of three rainbow trout Mx proteins has been cloned and a comparison of their sequences with that of avian and mammalian species reveals striking conservation of domains. They all maintain the tripartite ATP/GTP binditig domain and the dynamin family signature in the amino terminal half of the protein. In the carboxyl terminal half of the Mx proteins are che Iocalization signals and the leucine zipper motifs which account for the trimerization of Mx in the cell. Like the rat and human Mx proteins, the different trout Mx proteins exhibit distinctly different immunohistochemical staining patterns in cells transfected with plasmids expressing RBTMx1, RBTMx2, or RBTMx3, To date, the antiviral function of the trout Mx proteins has not been satisfactorily established.
Ichthyophthirius multifiliis: a model of cutaneous infection and immunity in fishes
Authors: Dickerson Harry , Clark Theodore
Published: April 2006

Summary: The parasitic ciliate Ichthyophthirius multifilus offers a useful system for the study of cutaneous immunity against an infectious microorganism. Naive fish usually die following infection, but animals surviving sublethal parasite exposure become resistant to subsequent challenge. This resistance correlates with the presence of humoral antibodies in the sera and cutaneous mucus of immune fish. A mechanism of immunity has recently been elucidated that involves and body binding to surface proteins (referred to as immobilization antigens or i‐antigens) located on the parasite cell and ciliary membranes. Antibody‐mediated cross‐linking of i‐antigens triggers a response by the parasite resulting in its exit from the host. These effects can be observed directly on the surface of live fish. In addition to allowing the observation of effector responses in vivo, Ichthyophthirius also provides a means to study the ontogeny of the mucosal immune response. The sites of antigen capture and presentation, and the sites of antibody production, are unknown with regard to cutaneous immunity. Because the external epithelial surfaces of fish are often the points of pathogen entry, a basic understanding of the inductive immune mechanisms and immune cell interactions in the skin and gills is extremely important with regard to vaccine development. The development of Ichthyophthirius as an experimental system and how it might be used to address these issues are discussed in this review.
Progress in understanding the mechanism and consequences of somatic hypermutation
Authors: Storb Ursula
Published: April 2006

This volume is dedicated lo the somatic hypermutation of immunoglobulin (Ig) genes. There has been considerable progress in the past few years in our understanding of the molecular mechanism of this process, the cellular events, and the diseases in which somatic hypermutation seems to play a major role. Most of the major issues are discussed in the chapters of this book or alluded to in this Introduction, and future challenges are outlined. Many mysteries remain on all the issues.
Mutation, selection, and memory in B lymphocytes of exothermic vertebrates
Authors: Hsu Ellen
Published: April 2006

Summary: Unlike mammals, cold‐blooded vertebrates produce antibodies of low heterogeneity that show little increase in binding affinity with time after immunization. In secondary responses, antibody titers and affinities are often Uttlc, if any, higher than in primary responses. That is, specificity, diversity, and memory ‐ the hallmarks of the immune system ‐ are rather meager in the humoral immune responses of exothermic vertebrates. As the genetic components of the immunoglobulin (Ig) gene systems in fishes, amphibians or reptiles are not deferent ii number or diversity, their responses probably do not stern from restrictions in the primary antibody repertoire. Somatic hypermutation at the Ig locus, which generates diversity and higher affinity antibodies in mammals, is not lacking in the South African frog Xenopus or in tbe sbark. However, the Ig mutants recovered are strongly biased toward alterations at GC pairs, an indication that they have not undergone effective selection. While cells resembling follicular dendritic cells are present in cold‐blooded vertebrates, germinal centers do not form. U is suggested that this absence of germinal centers, the site of selection for the mutants with higher affinity receptors and of differentiation into memory B cells in mammals, may explain the principal differences between cold and warm‐blooded vertebrates.
Somatic diversification of IgH genes in rabbit
Authors: Knight Katherine L. , Barrington Robert A.
Published: April 2006

Summary: Rabbits have helped elucidate one of the major immunologic puzzles, namely the genetic control of antibody diversity. The primary IgH antibody repertoire in rabbits is dominated by B cells that use the same germline VH‐gene segment in VDJ gene rearrangements. The VDJ genes of essentially all B lymphocytes undergo somatic diversification within the first few weeks of the rabbit's life. Such diversification occurs both by a somatic gene conversion‐like mechanism as well as by somatic hyper‐pointmutation. The diversification that occurs early in ontogeny takes place in gut‐associated lymphoid tissues and potentially depends on external factors such as microbial antigens. Few, if any. new B lymphocytes develop in adult rabbits and we discuss how the antibody repertoire is maintained throughout life. Finally, we discuss the molecular mechanism of somatic gene conversion of Ig genes, including die possibility that this involves the use of RAD5 1, an enzyme required for gene conversion‐mediated mating type switch in yeast.
Diversification of rabbit VH genes by gene‐conversion‐like and hypermutation mechanisms
Authors: Mage Rose G.
Published: April 2006

Summary: Where, when and how does Vu diversification occur in the rabbit? Early diversification by gene‐conversion and somatic hypermutation in rabbit appendix and chicken bursa of Fabricius are similar processes; the chicken bursa and the rabbit appendix have homologous functions. However, diversification in bursa starts during embryonic development whereas it starts in rabbit appendix about 2 weeks after birth in the presence of antigens and superantigens that ma) contribute to positive and negative selection, affect B‐cell expansion and mold the repertoire. The biochemical steps leading to diversification by gene conversion are unknown. However elevated levels of RAD51 mRNA in both chicken bursa and young rabbit appendix suggest that repair of double stand breaks may be involved. The base changes found in expressed rabbit VH sequences derived from rearrangement of known germline VH genes followed by one or more gene conversions occur with frequencies similar to those found in analyses of somatic hypermutation. The Ser codons in CDR1 and CDR2 of rabbit VHl I genes are all AGY rather than TCN, suggesting that they may represent intrinsic hotspots for hypermutation comparable to those described in human and mouse VHr. Somatic hypermutation may further refine antibody affinities in rabbit germinal centers.
Immunoglobulin hypermutation in cultured cells
Authors: Green Nancy S. , Lin Mark M. , Schorff Mctthew D.
Published: April 2006

Summary: Studies of endogenous and engineered Ig genes in mice have begun to reveal some of the cis‐acting regions that are involved in the somatic hypermutation of variable regions in vivo. These studies suggest that the initiation of transcription plays a role in this process. However, it will be difficult to identify and manipulate the individual genetic elements and the trans‐acting proteins that regulate and target the mucational events using solely in vivo assays. These studies would be greatly facilitated if constructs containing the genetic elements that are essential for V‐region mutation could be transfected into cultured cells and undergo high rates of V‐region mutation in vitro, and if permissive and non‐permissive cell lines could be identified. Such in vitro systems would also allow a detailed molecular and biochemical analysis of this process. Here, we discuss some of the in vitro systems that have been developed and use data from our own studies in cultured cells to illustrate the potential benefits of studying V‐region hypermutation in model in vitro systems.
Monitoring and interpreting the intrinsic features of somatic hypermutation
Authors: Neuberger Michael S. , Ehrenstein Michael K , Klix Norman , Jolly Christopher J. , Yélamos José , Rada Cristina , Milstein César
Published: April 2006

Summary: We have used both normal and transgenic mice to analyse the recruitment and targeting of somatic hypermutation to the immunoglobulin loci. We compare methods for analysing hypermutation and discuss how large databases of mutations can be assembled by PCR amplification of the rearranged V‐gene flanks from the germinal centre B cells of normal mice as well as by transgene‐specific amplification from transgenic B cells. Such studies confirm that hypermutation is preferentially targeted to the immunoglobulin V gene with the bcl6 gene, for example, escaping this intense mutational targeting in germinal centre B cells. We review our data concerning the nature of the hypermutation domain and the targeting of hotspots within that domain. We consider how enhancer‐mediated recruitment of hypermutation to the immunoglobulin loci operates in a clonally maintained fashion and illustrate how both the degree of expression and demethylation of the transgene broadly correlate with its mutability.
Cis‐acting sequences that affect somatic hypermutation of Ig genes
Authors: Slorb Liquid , Peters Andrew , Klotz Emily , Kirn Nayun , Shen Hong Ming , Hackett John , Rogerson Brian , Martin Terence E.
Published: April 2006

Summary: We review our studies on the mechanism of somatic hypermutation of immunoglobulin genes. Most experiments were carried out using Ig transgenes. We showed in these experiments that all required cisacting elements are present within the 10–16 kb of a cransgene. Only the Ig variable region and its proximate flanks are mutated, not the constant region. Several Ig gene enhancers are permissive for somatic mutation. Association of the enhancer with its natural Ig promoter is not necessary. However, the mutation process seems specific for Ig genes. No mutations were found in housekeeping genes from cells with high levels of somatic hypermutation of their Ig genes. The Ig enhancers may provide the Ig gene specificity. An exception may he the BCL6 gene, which was mutated in but not hut not in mouse B cells

Transcription of a region is required for its mutability When the transcriptional promoter located upstream of the variable region is duplicated upstream of the constant region, this region also becomes mutable. This suggests a model in which a mutator factor associates with the RNA polymerase at the promoter, travels with the polymerase during elongation, and causes mutations during polymerase pausing. The DNA repair systems, nucleotide excision repair and DNA mismatch repair, are not required. Our recent data with an artificial substrate of somatic mutation suggest that pausing may be due to secondary structure of the DNA or nascent RNA, and the specific mutations to preferences of the mutator factor.
Analysis of the targeting of the hypermutational machinery and the impact of subsequent selection on the distribution of nucleotide changes in human V rearrangements
Authors: Dörner Thomas , Foster Sandra J. , Brezinschek Hans‐Peter , Lipsky Peter E. , Dörner Thomas , Fustcr Sandra J. , Brtzinschtk Hons‐Peltr , Lipsky Peter E.
Published: April 2006

Summary: B cells are unique in that they generate and tolerate a high rate of mutations in their antigen receptor genes and employ these mutations as a basis of avidity maturation. The precise role of the mutational machinery versus subsequent selection in determining the frequency and distribution of mutations has not been fully analyzed. To address these issues, the influence of the intrinsic mutational machinery and subsequent selection on the frequency and distribution of mutations in the expressed human immunoglobulin repertoire was analyzed. Analysis of non‐productively rearranged vH genes from individual human B cells provided an opportunity to examine the immediate impact of somatic hypermtitation without superimposed selective influences. Comparison with the frequency and distribution of mutations in the productively rearranged human VH genes permitted an estimate of the influences of subsequent selection.
Affinity selection and repertoire shift: paradoxes as a consequence of somatic mutation?
Authors: Andersson Kerstin , Wrammert Jens , Leanderson Tomas
Published: April 2006

Summary: Affinity selection of antibodies during immune responses relies on two mechanisms, one molecular that involves the targeted introduction of somatic mutations into rearranged immunoglobulin genes and one cellular that involves the clonal expansion of B cells expressing a surface immunoglobulin with a higher affinity for antigen compared to their competitors. In this review we focus on the conditions for affinity selection during the establishment, expansion and memory phases of the immune response. We postulate that somatic mutation evolved prior to affinity selection and we present a model for selection of B cells in germinal centres. We also discuss the possibility that antibody repertoire shift occurs during the memory maintenance phase. Finally, we argue that a significant affinity selection and a selection for polyclonality of immune responses occur during this stage of the immune response.
The roles of antibody variable region hypermutation and selection in the development of the memory B‐cell compartment
Authors: Manser Tim , Tumns‐Btundage Kathleen M. , Casson Lawrence P. , Giusti Angela M. , Hande Shailaja , Notidis Evongelia , Vora Kalpit A.
Published: April 2006

Summary: Somatic hypermutation and selection of immunoglobulin (Ig) variable (V)‐region genes, working in concert, appear to be essential for memory B‐cell development in mammals. There has been substantial progress on the nature of the cis‐acting DNA elements that regulate hypermutation. The data obtained suggest that the mechanisms of Ig gene hypermutation and transcription are intimately intertwined. While it has long been appreciated that stringent phenotypic selection forces are imposed on the somatically mutated Ig V regions generated during a T‐cell dependent B‐cell response, the mechanisms involved in this selection have remained enigmatic. Our studies have questioned the role of foreign antigen deposited on follicular dendritic cells in affinity‐based positive selection of V regions, and have shown that this selection takes place in a “clone‐autonomous”’ fashion. In addition, our data strongly suggest that affinity for antigen alone is not the driving force for selection of B‐cell clones into the memory compartment. In contrast, we suggest that a combination of positive selection for increased foreign antigen binditig, and negative selection of antibody V regions that are autoreactive at the onset of the response, or have acquired autoreactivity via hypermutation, results in the “specificity maturation” of the memory B‐cell response
Somatic hypermutation in antoimmune thyroid disease
Authors: Mclntosh Richard , Watson Philip , Weetman Anthony
Published: April 2006

Summary: Autoimmune thyroid disease is one of the most common autoimmune diseases. There is typically patient antibody (Ab) reactivity to one or more of the antigens thyroglobulin (Tg), thyroid peroxidase (TPO) and the thyroid stimulating hormone receptor (TSFlr). With the advent of combinatorial library technology, there has been an enormous increase in the number of sequences from Ab to Tg and TPO, The repertoire of both Tg and TPO Ab is restricted and indicates the importance of somatic hypermutation in the development of the high affinity, Ab response. However, there are still too few sequences to determine patterns in which the mutation occurs, which residues are introduced during substitution and how individual substitutions affect the affinity of the Ab, Ab to the TSHr are of far greater pathological significance than those to Tg and TPO, but the current repertoire of Ab to the TSHr has yet to include the high affinity IgG Ab characteristic of patient serum Ab, Instructive analysis of the role of somatic hypermutation in the development of TSHr Ab therefore still awaits the isolation of the pathologically active repertoire. Despite this, the Ab response in thyroid autoimmunity remains one of the best characterised of human autoimmune diseases.
Somatic hypermutation of immunoglobulin variable region genes: focus on follicular lymphoma and multiple myeloma
Authors: Kosmas Christos , Stamatopoulos Kostas , Papndoki Theodora , Belessi Chrysoula , Yataganas Xenophon , Anagnostou Dimitra , Loukopoulos Dimitris
Published: April 2006

Summary: Analysis of the rearranged immunoglobulin variable region gene hypermutation has provided important information concerning the clonal history and ontogenetic origin of various B‐cell lymphoproliferative disorders. Under the selective pressure of antigen, mutational events in immunoglobulin genes will fine tune survival of B‐cell clones bearing immunoglobulin with high affinity for antigen. Our studies aimed at analyzing neoplastic disorders originating from germinal and post‐germinal center B‐cells: follicular lymphoma and multiple myeloma. respectively. Despite the already acknowledged evidence for a selectable distribution of mutations within the clonal immunoglobulin variable heavy chain genes, very little is known about the contribution of light chains in the process of antigen selection. In follicular lymphoma. a more limited pattern of somatic mutation with less evidence of antigen selection was observed in variable K light chain genes (40%) than in their partner heavy chain genes (80%). In myeloma, hypermutation of variable light chain genes, with a distribution suggestive of antigen selection, was frequently observed. Based on these data and recent reports it appears that the light chain expressed by the clonogenic myeloma B‐cells plays a pivotal role in the antigen selection process. Additionally, abortive K light chain variable region genes in X‐expressing myeloma carried a significant number of somatic mutations indicating that the cell of origin is open to the hypermutation machinery at that particular developmental stage irrespective of antigen selection.
The immunology of AIDS‐associated lymphomas
Authors: Ng Valerie L. , McGrath Micahoel S.
Published: April 2006

Summary: Lymphomas that occur in patients with human immunodeficiency virus (HIV) infection are predominantly of B‐cell origin and subsets show evidence for Epstein‐Barr virus (EBV) infection for chromosomal translocations in the C‐mye locus. The only subject of lymphoma clearly related to the immunodeficiency caused by HIV infection (similar to transplantation‐associated lymphomas) is the EBV primary central nervous system lymphoma. The systemic AIDS‐related lymphomas (ARIs) represent a complex set of disease processes histologically categorized as large cell or small non‐cleaved (Burkitt's‐like) lymphomas as likely early representatives of monoclonal immunoglobulin (Ig)‐expressing B‐cell lymphomas. Variable region analysis of lymphoma‐associated Ig has shown evidence for extensive somatic mutation with little evidence for appropriate affinity maturation. These observations suggest that abnormal control of B‐cell maturation in response to polyclonal antigenic stimulation may play a central roles in this disease. Overall. ARLs generally appear to be outgrowths vided by abnormal T‐cell with significant growth control influence provided by abnormal T‐cell and antigen‐presenting cell processes.
Ecto‐phosphodiesterase/pyrophosphatase of lymphocytes and non‐lymphoid cells: structure and function of the PC‐1 family
Authors: Goding James W , Terkeltaub Robert , Maurice Michele , Deterre Philippe , Sali Adnan , Belli Sabina I.
Published: April 2006

Summary: Many developmentally regulated membrane proteins of lymphocytes are ecto‐enzymes, with their active sites on the external surface of the cell. These enzymes commonly have peptidase, phosphodiesterase or nucleotidase activity. Their biological roles are just beginning to be discovered. Although their expression is usually associated with particular stages of lymphoid differentiation, the same gene products are often expressed on the surface of certain non‐lymphoid cell types outside the immune system, indicating that their functions cannot be unique to lymphocytes, nor can they be ubiquitous. The plasma cell membrane protein PC‐1 (phosphodiesterase I; EC 3.1.4.1/nucleotide pyrophosphatase; EC 3.6.1.9), which was one of the first serological markers for lymphocyte subsets to be discovered, is a typical example. Within the immune system, PC‐1 is confined to plasma cells, which represent about 0.1% of lymphocytes. However, PC‐1 is also expressed on cells of the distal convoluted tubule of the kidney, chondrocytes, osteoblasts, epididymis and hepatocytes. Recent work has shown that PC‐1 is a member of a multigene family of ecto‐phosphodiesterases that currently has two other members, PD‐1α (autotaxin) and PD‐1β (B10). Within this family, the extracellular domains are highly conserved, especially around the active site. In contrast, the transmembrane and cytopiasmic domains are highly divergent. Individual members of the ecto‐phosphodiesterase family have distinct patterns of distribution in different cell types, and even within the same cell. For example, PC‐1 is present only on the basolateral surface of hepatocytes, while B10 (PD‐1β) is confined to the apical surface. Analysis of conservation and differences in the sequence of their cytoplasmic tails may illuminate intracellular tar‐getting signals. Ecto‐phosphodiesterases may play a part in diverse activities in different tissues, including recycling of nucleotides. They may also regulate the concentration of pharmacologically active extracellular compounds such as adenosine or its derivatives and ceil motility. Some members may modulate local concentrations of pyrophosphate, and hence influence calcification in bone and cartilage.