Claudia Giesecke

High-throughput sequencing of the paired human immunoglobulin heavy and light chain repertoire

Each B-cell receptor consists of a pair of heavy and light chains. High-throughput sequencing can identify large numbers of heavy- and light-chain variable regions (VH and VL) in a given B-cell repertoire, but information about endogenous pairing of heavy and light chains is lost after bulk lysis of B-cell populations. Here we describe a way to retain this pairing information. In our approach, single B cells (>5 × 104 capacity per experiment) are deposited in a high-density microwell plate (125 pl/well) and lysed in situ. mRNA is then captured on magnetic beads, reverse transcribed and amplified by emulsion VH:VL linkage PCR. The linked transcripts are analyzed by Illumina high-throughput sequencing. We validated the fidelity of VH:VL pairs identified by this approach and used the method to sequence the repertoire of three human cell subsets—peripheral blood IgG+ B cells, peripheral plasmablasts isolated after tetanus toxoid immunization and memory B cells isolated after seasonal influenza vaccination.

Mechanisms of B cell autoimmunity in SLE

Systemic lupus erythematosus (SLE) is a systemic autoimmune disease that is known to be associated with polyclonal B-cell hyperreactivity. The underlying causes of the diffuse B-cell over-reactivity are unclear, but potential candidates include (a) intrinsic hyper-reactivity leading to polyclonal B-cell activation with disturbed activation thresholds and ineffective negative selection; (b) lack of immunoregulatory functions; (c) secondary effects of an overactive inflammatory environment, such as overactive germinal center and ectopic follicular activity; and/or (d) disturbed cytokine production by non-B immune cells. These mechanisms are not mutually exclusive and may operate to varying extents and at varying times in SLE. Phenotypic and molecular studies as well as the results of recent clinical trials have begun to provide new insights to address these possibilities. Of importance, new information has made it possible to distinguish between the contribution played by abnormalities in central checkpoints that could lead to a pre-immune repertoire enriched in autoreactive B cells, on the one hand, and the possibility that autoimmunity arises in the periphery from somatic hypermutation and abnormal selection during T cell-dependent B-cell responses on the other. There is an intriguing possibility that apoptotic material bound to the surface of follicular dendritic cells positively selects autoreactive B cells that arise from non-autoreactive B-cell precursors as a result of somatic hypermutation and thereby promotes the peripheral emergence of autoimmunity.

Identification and characterization of the constituent human serum antibodies elicited by vaccination.

Significance Most vaccines confer immunity by eliciting long-term production of antibodies that bind to and neutralize the vaccine antigen. Remarkably, very little is known regarding the identities, sequence diversity, relative concentrations, or binding functionalities of the mAbs that comprise the serum repertoire elicited by vaccination. Here, we have delineated the constituent antibodies of the human serum IgG repertoire after vaccination and examined their relationship to the antibody V gene repertoire encoded by circulating B cells. The results detail the molecular composition and characteristics of the vaccine-specific serum antibody repertoire and demonstrate differences between the end-point response (the serum antibodies) and the peripheral B cells responding to the vaccine. Most vaccines confer protection via the elicitation of serum antibodies, yet more than 100 y after the discovery of antibodies, the molecular composition of the human serum antibody repertoire to an antigen remains unknown. Using high-resolution liquid chromatography tandem MS proteomic analyses of serum antibodies coupled with next-generation sequencing of the V gene repertoire in peripheral B cells, we have delineated the human serum IgG and B-cell receptor repertoires following tetanus toxoid (TT) booster vaccination. We show that the TT+ serum IgG repertoire comprises ∼100 antibody clonotypes, with three clonotypes accounting for >40% of the response. All 13 recombinant IgGs examined bound to vaccine antigen with Kd ∼ 10−8–10−10 M. Five of 13 IgGs recognized the same linear epitope on TT, occluding the binding site used by the toxin for cell entry, suggesting a possible explanation for the mechanism of protection conferred by the vaccine. Importantly, only a small fraction (<5%) of peripheral blood plasmablast clonotypes (CD3−CD14−CD19+CD27++CD38++CD20−TT+) at the peak of the response (day 7), and an even smaller fraction of memory B cells, were found to encode antibodies that could be detected in the serological memory response 9 mo postvaccination. This suggests that only a small fraction of responding peripheral B cells give rise to the bone marrow long-lived plasma cells responsible for the production of biologically relevant amounts of vaccine-specific antibodies (near or above the Kd). Collectively, our results reveal the nature and dynamics of the serological response to vaccination with direct implications for vaccine design and evaluation.

Epratuzumab targeting of CD22 affects adhesion molecule expression and migration of B-cells in systemic lupus erythematosus

IntroductionEpratuzumab, a humanized anti-CD22 monoclonal antibody, is under investigation as a therapeutic antibody in non-Hodgkins lymphoma and systemic lupus erythematosus (SLE), but its mechanism of action on B-cells remains elusive. Treatment of SLE patients with epratuzumab leads to a reduction of circulating CD27negative B-cells, although epratuzumab is weakly cytotoxic to B-cells in vitro. Therefore, potential effects of epratuzumab on adhesion molecule expression and the migration of B-cells have been evaluated.MethodsEpratuzumab binding specificity and the surface expression of adhesion molecules (CD62L, β7 integrin and β1 integrin) after culture with epratuzumab was studied on B-cell subsets of SLE patients by flow cytometry. In addition, in vitro transwell migration assays were performed to analyze the effects of epratuzumab on migration towards different chemokines such as CXCL12, CXCL13 or to CXCR3 ligands, and to assess the functional consequences of altered adhesion molecule expression.ResultsEpratuzumab binding was considerably higher on B-cells relative to other cell types assessed. No binding of epratuzumab was observed on T-cells, while weak non-specific binding of epratuzumab on monocytes was noted. On B-cells, binding of epratuzumab was particularly enhanced on CD27negative B-cells compared to CD27positive B-cells, primarily related to a higher expression of CD22 on CD27negative B-cells. Moreover, epratuzumab binding led to a decrease in the cell surface expression of CD62L and β7 integrin, while the expression of β1 integrin was enhanced. The effects on the pattern of adhesion molecule expression observed with epratuzumab were principally confined to a fraction of the CD27negative B-cell subpopulation and were associated with enhanced spontaneous migration of B-cells. Furthermore, epratuzumab also enhanced the migration of CD27negative B-cells towards the chemokine CXCL12.ConclusionsThe current data suggest that epratuzumab has effects on the expression of the adhesion molecules CD62L, β7 integrin and β1 integrin as well as on migration towards CXCL12, primarily of CD27negative B-cells. Therefore, induced changes in migration appear to be part of the mechanism of action of epratuzumab and are consistent with the observation that CD27negative B-cells were found to be preferentially reduced in the peripheral blood under treatment.

A unique population of IgG-expressing plasma cells lacking CD19 is enriched in human bone marrow.

Specific serum antibodies mediating humoral immunity and autoimmunity are provided by mature plasma cells (PC) residing in the bone marrow (BM), yet their dynamics and composition are largely unclear. We here characterize distinct subsets of human PC differing by CD19 expression. Unlike CD19(+) PC, CD19(-) PC were restricted to BM, expressed predominantly IgG, and they carried a prosurvival, distinctly mature phenotype, that is, HLA-DR(low)Ki-67(-)CD95(low)CD28(+)CD56(+/-), with increased BCL2 and they resisted their mobilization from the BM after systemic vaccination. Fewer mutations within immunoglobulin VH rearrangements of CD19(-) BMPC may indicate their differentiation in early life. Their resistance to in vivo B-cell depletion, that is, their independency from supply with new plasmablasts, is consistent with long-term stability of this PC subset in the BM. Moreover, CD19(-) PC were detectable in chronically inflamed tissues and secreted autoantibodies. We propose a multilayer model of PC memory in which CD19(+) and CD19(-) PC represent dynamic and static components, respectively, permitting both adaptation and stability of humoral immune protection.

Secondary Immunization Generates Clonally Related Antigen-Specific Plasma Cells and Memory B Cells

Rechallenge with T cell-dependent Ags induces memory B cells to re-enter germinal centers (GCs) and undergo further expansion and differentiation into plasma cells (PCs) and secondary memory B cells. It is currently not known whether the expanded population of memory B cells and PCs generated in secondary GCs are clonally related, nor has the extent of proliferation and somatic hypermutation of their precursors been delineated. In this study, after secondary tetanus toxoid (TT) immunization, TT-specific PCs increased 17- to 80-fold on days 6–7, whereas TT-specific memory B cells peaked (delayed) on day 14 with a 2- to 22-fold increase. Molecular analyses of VHDJH rearrangements of individual cells revealed no major differences of gene usage and CDR3 length between TT-specific PCs and memory B cells, and both contained extensive evidence of somatic hypermutation with a pattern consistent with GC reactions. This analysis identified clonally related TT-specific memory B cells and PCs. Within clusters of clonally related cells, sequences shared a number of mutations but also could contain additional base pair changes. The data indicate that although following secondary immunization PCs can derive from memory B cells without further somatic hypermutation, in some circumstances, likely within GC reactions, asymmetric mutation can occur. These results suggest that after the fate decision to differentiate into secondary memory B cells or PCs, some committed precursors continue to proliferate and mutate their VH genes.

Induction of Potent CD8 T Cell Cytotoxicity by Specific Targeting of Antigen to Cross-Presenting Dendritic Cells In Vivo via Murine or Human XCR1

Current subunit vaccines are incapable of inducing Ag-specific CD8+ T cell cytotoxicity needed for the defense of certain infections and for therapy of neoplastic diseases. In experimental vaccines, cytotoxic responses can be elicited by targeting of Ag into cross-presenting dendritic cells (DC), but almost all available systems use target molecules also expressed on other cells and thus lack the desired specificity. In the present work, we induced CD8+ T cell cytotoxicity by targeting of Ag to XCR1, a chemokine receptor exclusively expressed on murine and human cross-presenting DC. Targeting of Ag with a mAb or the chemokine ligand XCL1 was highly specific, as determined with XCR1-deficient mice. When applied together with an adjuvant, both vector systems induced a potent cytotoxic response preventing the outgrowth of an inoculated aggressive tumor. By generating a transgenic mouse only expressing the human XCR1 on its cross-presenting DC, we could demonstrate that targeting of Ag using human XCL1 as vector is fully effective in vivo. The specificity and efficiency of XCR1-mediated Ag targeting to cross-presenting DC, combined with its lack of adverse effects, make this system a prime candidate for the development of therapeutic cytotoxic vaccines in humans.

Tissue Distribution and Dependence of Responsiveness of Human Antigen-Specific Memory B Cells

Memory B cells (mBCs) are a key to immunologic memory, yet their distribution within lymphoid organs and the individual role of these for mBC functionality remain largely unknown. This study characterized the distribution and phenotype of human (Ag-specific) mBCs in peripheral blood (PB), spleen, tonsil, and bone marrow. We found that the spleen harbors most mBCs, followed by tonsils, BM, and PB, and we detected no major differences in expression of markers associated with higher maturity. Testing the distribution of tetanus toxoid–specific (TT+) mBCs revealed their presence in PB during steady state, yet absolute numbers suggested their largest reservoir in the spleen, followed by tonsils. To explore the role of both tissues in the maintenance of reactive B cell memory, we revaccinated controls and splenectomized and tonsillectomized individuals with TT. All donor groups exhibited comparable emergence of anti-TT IgG, TT+ plasma cells, and TT+ mBCs in the PB, together with similar molecular characteristics of TT+ plasma cells. In summary, human mBCs recirculate through PB and reside in different lymphoid organs that do not reflect different mBC maturity stages. The spleen and tonsil, although harboring the largest number of overall and TT+ mBCs, appear to be dispensable to preserve adequate responsiveness to secondary antigenic challenge.

The anti-CD74 humanized monoclonal antibody, milatuzumab, which targets the invariant chain of MHC II complexes, alters B-cell proliferation, migration, and adhesion molecule expression

IntroductionTargeting CD74 as the invariant chain of major histocompatibility complexes (MHC) became possible by the availability of a specific humanized monoclonal antibody, milatuzumab, which is under investigation in patients with hematological neoplasms. CD74 has been reported to regulate chemo-attractant migration of macrophages and dendritic cells, while the role of CD74 on peripheral naïve and memory B cells also expressing CD74 remains unknown. Therefore, the current study addressed the influence of milatuzumab on B-cell proliferation, chemo-attractant migration, and adhesion molecule expression.MethodsSurface expression of CD74 on CD27- naïve and CD27+ memory B cells as well as other peripheral blood mononuclear cells (PBMCs) obtained from normals, including the co-expression of CD44, CXCR4, and the adhesion molecules CD62L, β7-integrin, β1-integrin and CD9 were studied after binding of milatuzumab using multicolor flow cytometry. The influence of the antibody on B-cell proliferation and migration was analyzed in vitro in detail.ResultsIn addition to monocytes, milatuzumab also specifically bound to human peripheral B cells, with a higher intensity on CD27+ memory versus CD27- naïve B cells. The antibody reduced B-cell proliferation significantly but moderately, induced enhanced spontaneous and CXCL12-dependent migration together with changes in the expression of adhesion molecules, CD44, β7-integrin and CD62L, mainly of CD27- naïve B cells. This was independent of macrophage migration-inhibitory factor as a ligand of CD74/CD44 complexes.ConclusionsMilatuzumab leads to modestly reduced proliferation, alterations in migration, and adhesion molecule expression preferentially of CD27- naïve B cells. It thus may be a candidate antibody for the autoimmune disease therapy by modifying B cell functions.

Increased Frequency of a Unique Spleen Tyrosine Kinase Bright Memory B Cell Population in Systemic Lupus Erythematosus

Systemic lupus erythematosus (SLE) is characterized by B cell hyperactivity and autoantibody production. As spleen tyrosine kinase (Syk) is pivotal in B cell activation, these experiments aimed to examine the extent to which Syk was abnormally expressed in SLE B cells and the nature of the B cell subset that differently expressed Syk.