Recirculating bone marrow B cells in C57BL/6 mice are more tolerant of highly hydrophobic and highly charged CDR-H3s than those in BALB/c mice

Peripheral apoptosis and limited clonal deletion during physiologic murine B lymphocyte development

Self-reactive and polyreactive B cells generated during B cell development are silenced by either apoptosis, clonal deletion, receptor editing or anergy to avoid autoimmunity. The specific contribution of apoptosis to normal B cell development and self-tolerance is incompletely understood. Here, we quantify self-reactivity, polyreactivity and apoptosis during physiologic B lymphocyte development. Self-reactivity and polyreactivity are most abundant in early immature B cells and diminish significantly during maturation within the bone marrow. Minimal apoptosis still occurs at this site, however B cell receptors cloned from apoptotic B cells show comparable self-reactivity to that of viable cells. Apoptosis increases dramatically only following immature B cells leaving the bone marrow sinusoids, but above 90% of cloned apoptotic transitional B cells are not self-reactive/polyreactive. Our data suggests that an apoptosis-independent mechanism, such as receptor editing, removes most self-reactive B cells in the bone marrow. Mechanistically, lack of survival signaling rather than clonal deletion appears to be the underpinning cause of apoptosis in most transitional B cells in the periphery.

Immunodominant surface epitopes power immune evasion in the African trypanosome

The African trypanosome survives the immune response of its mammalian host by antigenic variation of its major surface antigen (the variant surface glycoprotein or VSG). Here we describe the antibody repertoires elicited by different VSGs. We show that the repertoires are highly restricted and are directed predominantly to distinct epitopes on the surface of the VSGs. They are also highly discriminatory; minor alterations within these exposed epitopes confer antigenically distinct properties to these VSGs and elicit different repertoires. We propose that the patterned and repetitive nature of the VSG coat focuses host immunity to a restricted set of immunodominant epitopes per VSG, eliciting a highly stereotyped response, minimizing cross-reactivity between different VSGs and facilitating prolonged immune evasion through epitope variation.

Special AT-Rich Sequence-Binding Protein 1 Supports Survival and Maturation of Naive B Cells Stimulated by B Cell Receptors

Epigenetic mechanisms underpin the elaborate activities of essential transcription factors in lymphocyte development. Special AT-rich sequence-binding protein 1 (SATB1) is a chromatin remodeler that orchestrates the spatial and temporal actions of transcription factors. Previous studies have revealed the significance of SATB1 in T cell lineage. However, whether and how SATB1 controls B cell lineage development is yet to be clarified. In this study, we show that SATB1 is an important factor during splenic B cell maturation. By analyzing SATB1/Tomato reporter mice, we determined the dynamic fluctuation of SATB1 expression in the B cell lineage. Although SATB1 expression decreased to minimal levels during B cell differentiation in the bone marrow, it resurged markedly in naive B cells in the spleen. The expression was dramatically downregulated upon Ag-induced activation. Splenic naive B cells were subdivided into two categories, namely SATB1^high and SATB1^-/low, according to their SATB1 expression levels. SATB1^high naive B cells were less susceptible to death and greater proliferative than were SATB1^-/low cells during incubation with an anti-IgM Ab. Additionally, SATB1^high cells tended to induce the expression of MHC class II, CD86, and CD83. Accordingly, naive B cells from B lineage-specific SATB1 conditional knockout mice were more susceptible to apoptosis than that in the control group upon anti-IgM Ab stimulation in vitro. Furthermore, conditional knockout mice were less capable of producing Ag-specific B cells after immunization. Collectively, our findings suggest that SATB1 expression increases in naive B cells and plays an important role in their survival and maturation.

Preimmune Control of the Variance of TCR CDR-B3: Insights Gained From Germline Replacement of a TCR Dβ Gene Segment With an Ig D H Gene Segment

We have previously shown that the sequence of the immunoglobulin diversity gene segment (D H ) helps dictate the structure and composition of complementarity determining region 3 of the immunoglobulin heavy chain (CDR-H3). In order to test the role of germline D sequence on the diversity of the preimmune TCRβ repertoire of T cells, we generated a mouse with a mutant TCRβ DJC locus wherein the Dβ2-Jβ2 gene segment cluster was deleted and the remaining diversity gene segment, Dβ1 (IMGT:TRDB1), was replaced with DSP2.3 (IMGT:IGHD2-02), a commonly used B cell immunoglobulin D H gene segment. Crystallographic studies have shown that the length and thus structure of TCR CDR-B3 places amino acids at the tip of CDR-B3 in a position to directly interact with peptide bound to an MHC molecule. The length distribution of complementarity determining region 3 of the T cell receptor beta chain (CDR-B3) has been proposed to be restricted largely by MHC-specific selection, disfavoring CDR-B3 that are too long or too short. Here we show that the mechanism of control of CDR-B3 length depends on the Dβ sequence, which in turn dictates exonucleolytic nibbling. By contrast, the extent of N addition and the variance of created CDR3 lengths are regulated by the cell of origin, the thymocyte. We found that the sequence of the D and control of N addition collaborate to bias the distribution of CDR-B3 lengths in the pre-immune TCR repertoire and to focus the diversity provided by N addition and the sequence of the D on that portion of CDR-B3 that is most likely to interact with the peptide that is bound to the presenting MHC.

The sequences encoded by immunoglobulin diversity (DH ) gene segments play key roles in controlling B-cell development, antigen-binding site diversity, and antibody production

Although at first glance the diversity of the immunoglobulin repertoire appears random, there are a number of mechanisms that act to constrain diversity. For example, key mechanisms controlling the diversity of the third complementarity determining region of the immunoglobulin heavy chain (CDR-H3) include natural selection of germline diversity (D_H ) gene segment sequence and somatic selection upon passage through successive B-cell developmental checkpoints. To test the role of D_H gene segment sequence, we generated a panel of mice limited to the use of a single germline or frameshifted D_H gene segment. Specific individual amino acids within core D_H gene segment sequence heavily influenced the absolute numbers of developing and mature B-cell subsets, antibody production, epitope recognition, protection against pathogen challenge, and susceptibility to the production of autoreactive antibodies. At the tip of the antigen-binding loop (PDB position 101) in CDR-H3, both natural (germline) and somatic selection favored tyrosine while disfavoring the presence of hydrophobic amino acids. Enrichment for arginine in CDR-H3 appeared to broaden recognition of epitopes of varying hydrophobicity, but led to diminished binding intensity and an increased likelihood of generating potentially pathogenic dsDNA-binding autoreactive antibodies. The phenotype of altering the sequence of the D_H was recessive for T-independent antibody production, but dominant for T-cell-dependent responses. Our work suggests that the antibody repertoire is structured, with the sequence of individual D_H selected by evolution to preferentially generate an apparently preferred category of antigen-binding sites. The result of this structured approach appears to be a repertoire that has been adapted, or optimized, to produce protective antibodies for a wide range of pathogen epitopes while reducing the likelihood of generating autoreactive specificities.

Conditional Deletion of Sost in MSC-Derived Lineages Identifies Specific Cell-Type Contributions to Bone Mass and B-Cell Development

Sclerostin (Sost) is a negative regulator of bone formation and blocking its function via antibodies has shown great therapeutic promise by increasing both bone mass in humans and animal models. Sclerostin deletion in Sost KO mice (Sost^-/- ) causes high bone mass (HBM) similar to sclerosteosis patients. Sost^-/- mice have been shown to display an up to 300% increase in bone volume/total volume (BV/TV), relative to age-matched controls. It has been postulated that the main source of skeletal sclerostin is the osteocyte. To understand the cell-type specific contributions to the HBM phenotype described in Sost^-/- mice, as well as to address the endocrine and paracrine mode of action of sclerostin, we examined the skeletal phenotypes of conditional Sost loss-of-function (Sost^iCOIN/iCOIN ) mice with specific deletions in (1) the limb mesenchyme (Prx1-Cre; targets osteoprogenitors and their progeny); (2) midstage osteoblasts and their progenitors (Col1-Cre); (3) mature osteocytes (Dmp1-Cre); and (4) hypertrophic chondrocytes and their progenitors (ColX-Cre). All conditional alleles resulted in significant increases in bone mass in trabecular bone in both the femur and lumbar vertebrae, but only Prx1-Cre deletion fully recapitulated the amplitude of the HBM phenotype in the appendicular skeleton and the B-cell defect described in the global KO. Despite WT expression of Sost in the axial skeleton of Prx1-Cre deleted mice, these mice also had a significant increase in bone mass in the vertebrae, but the sclerostin released in circulation by the axial skeleton did not affect bone parameters in the appendicular skeleton. Also, both Col1 and Dmp1 deletion resulted in a similar 80% significant increase in trabecular bone mass, but only Col1 and Prx1 deletion resulted in a significant increase in cortical thickness. We conclude that several cell types within the Prx1-osteoprogenitor-derived lineages contribute significant amounts of sclerostin protein to the paracrine pool of Sost in bone. © 2018 The Authors. Journal of Bone and Mineral Research Published by Wiley Periodicals, Inc.

In the Absence of Central pre-B Cell Receptor Selection, Peripheral Selection Attempts to Optimize the Antibody Repertoire by Enriching for CDR-H3 Y101

Sequential developmental checkpoints are used to “optimize” the B cell antigen receptor repertoire by minimizing production of autoreactive or useless immunoglobulins and enriching for potentially protective antibodies. The first and apparently most impactful checkpoint requires μHC to form a functional pre-B cell receptor (preBCR) by associating with surrogate light chain, which is composed of VpreB and λ5. Absence of any of the preBCR components causes a block in B cell development that is characterized by severe immature B cell lymphopenia. Previously, we showed that preBCR controls the amino acid content of the third complementary determining region of the H chain (CDR-H3) by using a VpreB amino acid motif (RDR) to select for tyrosine at CDR-H3 position 101 (Y101). In antibodies bound to antigen, Y101 is commonly in direct contact with the antigen, thus preBCR selection impacts the antigen binding characteristics of the repertoire. In this work, we sought to determine the forces that shape the peripheral B cell repertoire when it is denied preBCR selection. Using bromodeoxyuridine incorporation and evaluation of apoptosis, we found that in the absence of preBCR there is increased turnover of B cells due to increased apoptosis. CDR-H3 sequencing revealed that this is accompanied by adjustments to D_H identity, D_H reading frame, J_H, and CDR-H3 amino acid content. These adjustments in the periphery led to wild-type levels of CDR-H3 Y101 content among transitional (T1), mature recirculating, and marginal zone B cells. However, peripheral selection proved incomplete, with failure to restore Y101 levels in follicular B cells and increased production of dsDNA-binding IgM antibodies.

Massively Parallel Sequencing of Peritoneal and Splenic B Cell Repertoires Highlights Unique Properties of B-1 Cell Antibodies

B-1 cells are a unique subset of B cells that are positively selected for expressing autoreactive BCRs. We isolated RNA from peritoneal (B-1a, B-1b, B-2) and splenic (B-1a, marginal zone, follicular) B cells from C57BL/6 mice and used 5'-RACE to amplify the IgH V region using massively parallel sequencing. By analyzing 379,000 functional transcripts, we demonstrate that B-1a cells use a distinct and restricted repertoire. All B-1 cell subsets, especially peritoneal B-1a cells, had a high proportion of sequences without N additions, suggesting predominantly prenatal development. Their transcripts differed markedly and uniquely contained V_H11 and V_H12 genes, which were rearranged only with a restricted selection of D and J genes, unlike other V genes. Compared to peritoneal B-1a, the peritoneal B-1b repertoire was larger, had little overlap with B-1a, and most sequences contained N additions. Similarly, the splenic B-1a repertoire differed from peritoneal B-1a sequences, having more unique sequences and more frequent N additions, suggesting influx of B-1a cells into the spleen from nonperitoneal sites. Two CDR3s, previously described as Abs to bromelain-treated RBCs, comprised 43% of peritoneal B-1a sequences. We show that a single-chain variable fragment designed after the most prevalent B-1a sequence bound oxidation-specific epitopes such as the phosphocholine of oxidized phospholipids. In summary, we provide the IgH V region library of six murine B cell subsets, including, to our knowledge for the first time, a comparison between B-1a and B-1b cells, and we highlight qualities of B-1 cell Abs that indicate unique selection processes.

Alterations in B cell development, CDR-H3 repertoire and dsDNA-binding antibody production among C57BL/6 ΔD-iD mice congenic for the lupus susceptibility loci sle1, sle2 or sle3

Systemic lupus erythematosus (SLE) is an autoimmune disease that reflects a failure to block the production of self-reactive antibodies, especially those that bind double-stranded DNA (dsDNA). Backcrossing the lupus-prone NZM2410 genome onto C57BL/6 led to the identification of three genomic intervals, termed sle1, sle2 and sle3, which are associated with lupus susceptibility. We previously generated a C57BL/6 strain congenic for an immunoglobulin DH locus (ΔD-iD) that enriches for arginine at dsDNA-binding positions. We individually introduced the ΔD-iD allele into the three sle strains to test whether one or more of these susceptibility loci could affect the developmental fate of B cells bearing arginine-enriched CDR-H3s, the CDR-H3 repertoire created by the DH and the prevalence of dsDNA-binding antibodies. We found that the combination of the ΔD-iD allele and the sle1 locus led to a decrease in mature, recirculating B cell numbers and an increase in marginal zone cell numbers while maintaining a highly charged CDR-H3 repertoire. ΔD-iD and sle2 had no effect on peripheral B cell numbers, but the CDR-H3 repertoire was partially normalized. ΔD-iD and sle3 led to an increase in marginal zone B cell numbers, with some normalization of hydrophobicity. Mice with ΔD-iD combined with either sle1 or sle3 had increased production of dsDNA-binding IgM and IgG by 12 months of age. These findings indicate that the peripheral CDR-H3 repertoire can be categorically manipulated by the effects of nonimmunoglobulin genes.

VpreB serves as an invariant surrogate antigen for selecting immunoglobulin antigen-binding sites

Developmental checkpoints eliminate B cells synthesizing defective immunoglobulin heavy (HC) and light (LC) chains. The first checkpoint tests for formation of a VpreB/λ5/µHC-containing preB-cell receptor (preBCR) and predicts whether µHCs will bind conventional LCs to form membrane IgM. VpreB and λ5 also create a sensing site that interacts with µHC antigen-binding region CDR-H3, but whether it plays a role in immunoglobulin repertoire selection and function is unknown. On a position-by-position basis, we analyzed the amino acid content of CDR-H3s from H chains cloned from living and apoptotic preB cells and from IgG:Antigen structures. Using a panel of DH gene-targeted mice, we show that progressively reducing CDR-H3 tyrosine content increasingly impairs preBCR checkpoint passage. Counting from cysteine at Framework 3 position 96, we found that VpreB particularly selects for tyrosine at CDR-H3 position 101, and that Y101 also binds antigen in IgG:Antigen structures. VpreB thus acts as an early invariant antigen. It selects for particular CDR-H3 amino acids and shapes the specificity of the IgG humoral response. This helps explain why some neutralizing antibodies against pathogens are readily produced while others are rare.

Central nervous system infiltrates are characterized by features of ongoing B cell-related immune activity in MP4-induced experimental autoimmune encephalomyelitis

In multiple sclerosis (MS) lymphoid follicle-like aggregates have been reported in the meninges of patients. Here we investigated the functional relevance of B cell infiltration into the central nervous system (CNS) in MP4-induced experimental autoimmune encephalomyelitis (EAE), a B cell-dependent mouse model of MS. In chronic EAE, B cell aggregates were characterized by the presence of CXCL13(+) and germinal center CD10(+) B cells. Germline transcripts were expressed in the CNS and particularly related to TH17-associated isotypes. We also observed B cells with restricted VH gene usage that differed from clones found in the spleen. Finally, we detected CNS-restricted spreading of the antigen-specific B cell response towards a myelin and a neuronal autoantigen. These data imply the development of autonomous B cell-mediated autoimmunity in the CNS in EAE - a concept that might also apply to MS itself.

Differences in the composition of the human antibody repertoire by B cell subsets in the blood

The vast initial diversity of the antibody repertoire is generated centrally by means of a complex series of V(D)J gene rearrangement events, variation in the site of gene segment joining, and TdT catalyzed N-region addition. Although the diversity is great, close inspection has revealed distinct and unique characteristics in the antibody repertoires expressed by different B cell developmental subsets. In order to illustrate our approach to repertoire analysis, we present an in-depth comparison of V(D)J gene usage, hydrophobicity, length, D_H reading frame, and amino acid usage between heavy chain repertoires expressed by immature, transitional, mature, memory IgD⁺, memory IgD⁻, and plasmacytes isolated from the blood of a single individual. Our results support the view that in both human and mouse, the H chain repertoires expressed by individual, developmental B cell subsets appear to differ in sequence content. Sequencing of unsorted B cells from the blood is thus likely to yield an incomplete or compressed view of what is actually happening in the immune response of the individual. Our findings support the view that studies designed to correlate repertoire expression with diseases of immune function will likely require deep sequencing of B cells sorted by subset.