Neonatal thymectomy results in a repertoire enriched in T cells deleted in adult thymus

In B6AF1 mice, T lymphocytes that use the V beta 11-positive (and not V beta 6-positive or V beta 8-positive) segment in their receptor for antigen are greatly reduced in the thymus and peripheral lymphoid tissues, most likely as a result of clonal deletion. The relative number of V beta 11-positive cells in adult lymph nodes was ten times as high in B6AF1 mice thymectomized 1 to 4 days after birth as in normal mice. Moreover, for the first 10 days of life of B6AF1 mice, mature V beta 11-positive T cells were readily detected in the thymus and spleen. Thus neonatal thymectomy results in the maintenance of the receptor repertoire of early postnatal life, and this correlates with the subsequent development of organ-specific autoimmune diseases.     

Depletion of conventional mature B cells and compromised specific antibody response in bovine immunoglobulin μ heavy-chain transgenic mice

In this study, we introduced the bovine immunoglobulin μ heavy-chain gene (the orphaned gene on BTA11) into mouse germline cells. Bovine IgM was highly expressed in selected transgenic lines, and it largely inhibited rearrangements of the endogenous immunoglobulin heavy chain (IgH) genes in these lines. The forced expression of bovine IgM resulted in reduced numbers of pro- and pre-B cells but increased the number of immature B cells in the transgenic mice. Bovine IgM-expressing B cells can migrate from the bone marrow to the spleen, but most of the cells are arrested at the T1 transitional B cell stage, leading to a significantly lower number of T2 transitional and mature B cells in the spleen. Although the serum concentrations of endogenous IgM and IgG in the transgenic mice were significantly decreased, the IgA levels were slightly increased compared to the WT mice. The bovine IgM level in the serum was only one-tenth to one-fifth of that of endogenous mouse IgM, suggesting that most of the serum immunoglobulin were contributed by endogenous IgH gene-expressing B cells. These transgenic mice also exhibited a lower frequency of unique complementarity determining region 3 (CDR3) sequences in their VH repertoire and Vκ repertoire but exhibited an increased frequency of unique CDR3 in their Vλ repertoire. Compared to the WT mice, the transgenic mice had a significantly higher percentage of mouse IgM-expressing B cells that expressed λ chains. Finally, we showed that the transgenic mice were deficient in a specific antibody response to antigen stimulation.     

Influence of clonal selection on the expression of immunoglobulin variable region genes

The humoral immune response of the mouse to certain antigens is characterized by the dominant expression of a single or limited number of related, immunoglobulin variable region (V) structures by antibody-secreting lymphocytes. Such dominance could be due to preferred expression of these V regions in the B cell population prior to the immune response or could result from the action of selective or regulatory mechanisms during the immune response. Expression of a heavy chain variable region (VH) gene segment that partially encodes a V region structure that dominates the immune response to para-azophenylarsonate (Ars) in strain A mice was examined in the B cell population of Ars nonimmune mice. This VH gene segment participates in encoding several hundred thousand different V region structures expressed in this B cell population. The immune system is therefore capable of recurrently selecting a single V region structure from such a repertoire for dominant expression by antibody-secreting lymphocytes during an immune response.     

Structure and diversity of the human T-cell receptor beta-chain variable region genes

In order to characterize the variability of the expressed human T-cell receptor (TCR) beta-chain repertoire and contrast this variability to the known murine beta-chain repertoire, 15 independent complementary DNA (cDNA) clones containing TCR beta-chain variable region (V beta) genes were isolated from a human tonsil cDNA library. The nucleotide and derived amino acid sequences of these 15 V beta genes were analyzed together with 7 previously defined sequences. Fifteen different human V beta genes could be identified from 22 independent sequences. By means of DNA hybridization and sequence homology comparisons, it was possible to group these 15 genes into ten distinct V beta subfamilies, each containing from one to seven members. Minimal polymorphism was noted between individuals, except in multimember subfamilies. The amino acid sequences of these genes contain conserved amino acids that are also shared by murine TCR V beta genes and immunoglobulins; no features were found that distinguish human V beta genes from their murine counterparts. Evaluation of secondary structure showed that maximum variability coincides with generally hydrophilic portions of the amino acid sequence, while specific hydrophobic regions were conserved in all V beta genes examined.     

Structural insight into pre-B cell receptor function

The pre-B cell receptor (pre-BCR) serves as a checkpoint in B cell development. In the 2.7 angstrom structure of a human pre-BCR Fab-like fragment, consisting of an antibody heavy chain (HC) paired with the surrogate light chain, the "unique regions" of VpreB and lambda5 replace the complementarity-determining region 3 (CDR3) loop of an antibody light chain and appear to "probe" the HC CDR3, potentially influencing the selection of the antibody repertoire. Biochemical analysis indicates that the pre-BCR is impaired in its ability to recognize antigen, which, together with electron microscopic visualization of a pre-BCR dimer, suggests ligand-independent oligomerization as the likely signaling mechanism.     

Selective depletion in HIV infection of T cells that bear specific T cell receptor V beta sequences

The mechanism of T cell depletion during infection with the human immunodeficiency virus (HIV) is unclear. Examination of the repertoire of T cell receptor V (variable) regions in persons infected with HIV revealed the absence of a common set of V beta regions, whereas V alpha usage was normal. The lack of these V beta segments did not appear to correlate with opportunistic infections. The selective elimination of T cells that express a defined set of V beta sequences may indicate the presence of an HIV-encoded superantigen, similar to those encoded by the long terminal repeat of the mouse mammary tumor virus.     

Requirement for cotolerogenic gene products in the clonal deletion of I-E reactive T cells

T cells that express the T cell receptor V beta 5.2 domain react with the class II major histocompatibility complex (MHC) molecule I-E, and V beta 5.2+ T cells are deleted in mouse strains that express I-E glycoproteins. By examination of genetically defined recombinant inbred (RI) mouse strains, it was found that the deletion was dependent on the expression of I-E and one of a limited number of non-MHC gene products (cotolerogens). The gene encoding one of these cotolerogens maps to chromosome 12 and is linked to the endogenous provirus Mtv-9. These observations suggest that the I-E-mediated and minor lymphocyte-stimulating antigen (Mls)-mediated deletions of alpha beta T cells from the repertoire are similar; both require the expression of a class II MHC glycoprotein and a second non-MHC gene product.     

The chicken B cell compartment

A very unusual molecular mechanism is involved in generating the preimmune repertoire in the chicken bursa of Fabricius. A unique rearranged V gene is diversified through a program of segmental gene conversion with a pool of noncoding pseudogenes being used as donors. A specifically committed progenitor that originates in the embryonic bursa is responsible for long-term maintenance of the B cell population. Both these properties and the characteristics of the peripheral B cell compartment are discussed in terms of the evolution of the T and B immune systems.     

Contribution of receptor editing to the antibody repertoire

Receptor editing, clonal deletion, and anergy are the mechanisms by which B cells maintain tolerance to self antigens. To determine the extent to which receptor editing shapes the normal antibody repertoire, we generated an immunoglobulin kappa polymorphism that facilitates the detection of editing of immunoglobulin light chains in vivo. We found that B cells are targeted for editing during a 2-hour delay in development at the pre-BII cell stage, and that about 25% of all antibody molecules are produced by gene replacement. These results suggest that receptor editing represents a major force in shaping the antibody repertoire.     

The role of somatic hypermutation in the generation of antibody diversity

The immune system is capable of establishing an enormous repertoire of antibodies before its first contact with antigen. Most antibodies that express germ-line sequences are of relatively low affinity. Once antigen enters the system, it stimulates a somatic mutational mechanism that generates antibodies of higher affinity and selects for the expression of those antibodies to produce a more effective immune response. The details of the mechanism and regulation of somatic hypermutation remain to be elucidated.     

Crystal structure of a shark single-domain antibody V region in complex with lysozyme

Cartilaginous fish are the phylogenetically oldest living organisms known to possess components of the vertebrate adaptive immune system. Key to their immune response are heavy-chain, homodimeric immunoglobulins called new antigen receptors (IgNARs), in which the variable (V) domains recognize antigens with only a single immunoglobulin domain, akin to camelid heavy-chain V domains. The 1.45 angstrom resolution crystal structure of the type I IgNAR V domain in complex with hen egg-white lysozyme (HEL) reveals a minimal antigen-binding domain that contains only two of the three conventional complementarity-determining regions but still binds HEL with nanomolar affinity by means of a binding interface comparable in size to conventional antibodies.     

Development of the expressed immunoglobulin μ chain repertoire during maturation of mice B cells

In the bone marrow and spleen, the developing B cell populations undergo both negative and positive selections to shape their B cell receptor repertoire. To gain insight into the shift of the immunoglobulin heavy (IgH) chain repertoire during B cell development, we undertook large scale Ig μ chain repertoire analysis of pre-B, immature B and spleen B cell populations. We found that the majority of V_H gene segments, V_H families, J_H and D gene segments, were observed to have significantly different usage frequencies when three B cell populations were compared, but the usage profile of the V_H, D, and J_H genes between different B cell populations showed high correlations. In both productive and nonproductive rearrangements, the length of CDRH3 shortened significantly on average when B cells entered the periphery. However, the CDRH3 length distribution of nonproductive rearrangements did not follow a Gaussian distribution, but decreased successively in the order 3n-2, 3n-1 and 3n, suggesting a direct correlation between mRNA stability and CDRH3 length patterns of nonproductive rearrangements. Further analysis of the individual components comprising CDRH3 of productive rearrangements indicated that the decrease in CDRH3 length was largely due to the reduction of N addition at the 5′ and 3′ junctions. Moreover, with development, the amino acid content of CDRH3 progressed toward fewer positively charged and nonpolar residues but more polar residues. All these data indicated that the expressed Ig μ chain repertoire, especially the repertoire of CDRH3, was fine-tuned when B cells passed through several checkpoints of selection during the process of maturation.     

Human lymphocytes making rheumatoid factor and antibody to ssDNA belong to Leu-1+ B-cell subset

B lymphocytes bearing the Leu-1 cell-surface antigen (Leu-1+), the human equivalent of mouse Ly-1+ B lymphocytes, have been detected in human peripheral blood, but there is little information on their frequency and properties. Analysis by fluorescence-activated cell sorter and double immunofluorescence showed that Leu-1+ B cells are consistently present in the peripheral blood and spleens of healthy subjects and constitute 17.0 +/- 5.0% (mean value +/- standard deviation) and 17.3 +/- 3.9%, respectively, of total B cells. When purified Leu-1+ and Leu-1- B lymphocytes were transformed into immunoglobulin-secreting cells by infection with Epstein-Barr virus and the culture fluids were tested for reactivity with self-antigens, at least two important autoantibodies, antibody to the Fc fragment of human immunoglobulin G (rheumatoid factor) and antibody to single-stranded DNA, were found to be made exclusively by Leu-1+ B cells. It is concluded that the Leu-1+ lymphocytes represent a major subset of the normal human B cell repertoire and include the B cells capable of making autoantibodies similar to those found in systemic lupus erythematosus and rheumatoid arthritis.     

Antigen recognition by variable lymphocyte receptors

Variable lymphocyte receptors (VLRs) rather than antibodies play the primary role in recognition of antigens in the adaptive immune system of jawless vertebrates. Combinatorial assembly of leucine-rich repeat (LRR) gene segments achieves the required repertoire for antigen recognition. We have determined a crystal structure for a VLR-antigen complex, VLR RBC36 in complex with the H-antigen trisaccharide from human blood type O erythrocytes, at 1.67 angstrom resolution. RBC36 binds the H-trisaccharide on the concave surface of the LRR modules of the solenoid structure where three key hydrophilic residues, multiple van der Waals interactions, and the highly variable insert of the carboxyl-terminal LRR module determine antigen recognition and specificity. The concave surface assembled from the most highly variable regions of the LRRs, along with diversity in the sequence and length of the highly variable insert, can account for the recognition of diverse antigens by VLRs.     

Evidence for the effects of a superantigen in rheumatoid arthritis

While studying the alpha beta T cell receptor repertoire in rheumatoid arthritis (RA) patients, we found that the frequency of V beta 14+ T cells was significantly higher in the synovial fluid of affected joints than in the peripheral blood. In fact, V beta 14+ T cells were virtually undetectable in the peripheral blood of a majority of these RA patients. beta-chain sequences indicated that one or a few clones dominated the V beta 14+ population in the synovial fluid of individual RA patients, whereas oligoclonality was less marked for other V beta's and for V beta 14 in other types of inflammatory arthritis. These results implicate V beta 14-bearing T cells in the pathology of RA. They also suggest that the etiology of RA may involve initial activation of V beta 14+ T cells by a V beta 14-specific superantigen with subsequent recruitment of a few activated autoreactive v beta 14+ T cell clones to the joints while the majority of other V beta 14+ T cells disappear.     

转小粒野生稻基因组DNA水稻的产量与品质性状研究

野威B是将小粒野生稻(Oryza minuta)的基因组DNA通过穗茎注射法导入杂交水稻亲本V20B中培育出的转基因水稻新种质.与亲本V20B相比,野威B的有效穗数和每穗总粒数小于V20B,但每穗实粒数却高于V20B,平均结实率比V20B增加11.8%,千粒重比V20B的少6 g.野威B倒1叶和倒2叶叶鞘中可溶性糖含量...     

The need for central and peripheral tolerance in the B cell repertoire

The immune system normally avoids producing antibodies that react with autologous ("self") antigens by censoring self-reactive T and B cells. Unlike the T cell repertoire, antibody diversity is generated within the B cell repertoire in two phases; the first occurs by gene rearrangement in primary lymphoid organs, and the second phase involves antigen-driven hypermutation in peripheral lymphoid organs. The possibility that distinct cellular mechanisms may impose self tolerance at these two different phases of B cell diversification may explain recent findings in transgenic mouse models, in which self-reactive B cells appear to be silenced both by functional inactivation and by physical elimination.     

Influence of the major histocompatibility complex on positive thymic selection of V beta 17a+ T cells

A monoclonal antibody was used to show directly positive thymic selection of the T cell repertoire in mouse strains expressing the 17a beta-chain variable domain (V beta 17a) of the T cell receptor. In the absence of the potent tolerizing class II major histocompatibility complex (MHC) molecule, I-E, peripheral expression of V beta 17a+ T cell receptors varied with the MHC haplotype of the mouse strain. In the most extreme case, H-2q mice expressed high peripheral levels of CD4+ V beta 17a+ T cells (14 to 19 percent), whereas H-2b mice expressed low levels (3 to 4 percent). Analysis of (b x q)F1 mice and chimeric mice showed that these differences were determined by positive thymic selection and implicated the thymic epithelium as the controlling cell type.