An antibody VH gene that promotes marginal zone B cell development and heavy chain allelic inclusion

A Hard(y) Look at B-1 Cell Development and Function

A small population of B cells exists in lymphoid tissues and body cavities of mice that is distinct in development, phenotype, and function from the majority (B-2) B cell population. This population, originally termed "Ly-1" and now "B-1," has received renewed interest as an innate-like B cell population of fetal-derived hematopoiesis, responsible for natural Ab production and rapid immune responses. Molecular analyses have begun to define fetal and adult hematopoiesis, while cell-fate mapping studies have revealed complex developmental origins of B-1 cells. Together the studies provide a more detailed understanding of B-1 cell regulation and function. This review outlines studies that defined B-1 cells as natural Ab- and cytokine-producing B cells of fetal origin, with a focus on work conducted by R.R. Hardy, an early pioneer and codiscoverer of B-1 cells, whose seminal contributions enhanced our understanding of this enigmatic B cell population.

Positive selection of natural poly-reactive B cells in the periphery occurs independent of heavy chain allelic inclusion

Natural autoreactive B cells are important mediators of autoimmune diseases. Receptor editing is known to play an important role in both central and peripheral B cell tolerance. However, the role of allelic inclusion in the development of natural autoreactive B cells is not clear. Previously, we generated μ chain (TgV_H3B4I) and μ/κ chains (TgV_H/L3B4) transgenic mice using transgene derived from the 3B4 hybridoma, which produce poly-reactive natural autoantibodies. In this study, we demonstrate that a considerable population of B cells edited their B cells receptors (BCRs) via light chain or heavy chain allelic inclusion during their development in TgV_H3B4I mice. Additionally, allelic inclusion occurred more frequently in the periphery and promoted the differentiation of B cells into marginal zone or B-1a cells in TgV_H3B4I mice. B cells from TgV_H/L3B4 mice expressing the intact transgenic 3B4 BCR without receptor editing secreted poly-reactive 3B4 antibody. Interestingly, however, B cell that underwent allelic inclusion in TgV_H3B4I mice also produced poly-reactive autoantibodies in vivo and in vitro. Our findings suggest that receptor editing plays a minor role in the positive selection of B cells expressing natural poly-reactive BCRs, which can be positively selected through heavy chain allelic inclusion to retain their poly-reactivity in the periphery.

Excessive amounts of mu heavy chain block B-cell development

Antigen-independent B-cell development occurs in several stages that depend on the expression of Ig heavy and light chain. We identified a line of mice that lacked mature B cells in the spleen. This mouse line carried approximately 11 copies of a transgene of the murine heavy chain constant region locus, and B-lineage cells expressed excessive amounts of the intracellular μ heavy chain. B-cell development failed in the bone marrow at the pro/pre B-cell transition, and examination of other lines with various copy numbers of the same transgene suggested that deficiencies in B-cell development increased with increased transgene copy number. Expression of a transgenic (Tg) light chain along with the Tg μ heavy chain led to minimal rescue of B-cell development in the bone marrow and B cells in the spleen. There are several potential mechanisms for the death of pro/pre B cells as a consequence of excess heavy chain expression.

Reactivity profiles of broadly neutralizing anti-HIV-1 antibodies are distinct from those of pathogenic autoantibodies

OBJECTIVE

Broadly neutralizing antibodies (bNt Abs) against HIV-1 are rarely produced during natural infection, and efforts to induce such Abs by vaccination have been unsuccessful. Thus, elucidating the nature and cellular origins of bNt Abs is a high priority for vaccine research. As the bNt monoclonal Abs (MAbs) 2F5, 4E10 and 2G12 have been reported to bind select autoantigens, we investigated whether these MAbs display a broader range of autoreactivity and how their autoreactivity compares with that of pathogenic autoAbs.

METHODS

An autoantigen microarray comprising 106 connective tissue disease-related autoantigens and control antigens was developed and used, in combination with ELISAs, to compare the reactivity profiles of MAbs 4E10, 2F5 and 2G12 to those of four pathogenic autoAbs derived from patients with antiphospholipid-syndrome (APS), and to serum from a patient with systemic lupus erythematosus (SLE).

RESULTS

The APS MAbs and SLE serum reacted strongly with multiple autoantigens on the microarray, whereas anti-HIV-1 MAb reactivity was limited mainly to HIV-1-related antigens. The APS autoAbs reacted strongly with CL, yet only 4E10 bound CL at high concentrations; both 2F5 and 4E10 bound their HIV-1 epitopes with a 2-3-log higher apparent affinity than CL. Moreover, the polyreactivity of 4E10, but not CL15, could be blocked with dried milk.

CONCLUSION

The reactivity profiles of bNt anti-HIV-1 MAbs are fundamentally distinct from those of pathogenic autoAbs that arise from dysregulated tolerance mechanisms. This suggests that the limited polyreactivity observed for the bNt MAbs, and for HIV-1-Nt Abs in general, may arise through alternative mechanisms, such as extensive somatic mutation due to persistent antigen selection during chronic infection.

Antibodies in a heavy chain knock-in mouse exhibit characteristics of early heavy chain rearrangement

Studies in autoantibody transgenic mice have demonstrated receptor editing rearrangements at Ab H and L chain loci. However, the physiologic role of H chain editing (V(H) replacement and rearrangement on the second allele) has been called into question. It is unclear if additional rounds of H chain rearrangement are driven by BCR specificity. In this study, we analyze the manner in which B cells undergo additional H chain rearrangements in an anti-DNA H chain knock-in mouse, B6.56R. We find that rearrangements in 56R(+) B cells tend to involve the D gene locus on both alleles and the most J(H)-proximal V(H) gene segments on the endogenous allele. As a result, some B cells exhibit V(D)J rearrangements on both H chain alleles, yet allelic exclusion is tightly maintained in mature 56R B cells. As B cells mature, a higher proportion expresses the nontransgenic H chain allele. Rearrangements on both H chain alleles exhibit junctional diversity consistent with TdT-mediated N-addition, and TdT RNA is expressed exclusively at the pro-B cell stage in B6.56R. Collectively, these findings favor a single, early window of H chain rearrangement in B6.56R that precedes the expression of a functional BCR. B cells that happen to successfully rearrange another H chain may be favored in the periphery.

Influence of B cell antigen receptor expression level on pathways of B cell tolerance induction

We have described an Ig-transgenic, autoreactive B cell clonotype that undergoes a novel tolerance pathway. Early in development this clonotype expresses average BCR levels, but these levels are progressively down-regulated as development proceeds efficiently to the mature, follicular compartment. This clonotype does not display conventional features of anergy and can be induced to undergo apoptosis and receptor editing in in vitro bone marrow cultures, but these pathways are not taken in vivo. These data suggested that autoantigen-driven down-regulation of BCR levels and, hence, avidity for autoantigen allows this clonotype to bypass conventional tolerance mechanisms. To test this idea, we enforced elevated levels of expression of BCR in this clonotype by making the transgenic Igh locus homozygous. This resulted in retarded clonotype development and L chain receptor editing in vivo. These data support a pivotal role for adaptive, autoantigen-induced adjustment of BCR expression levels in the regulation of primary B cell development and tolerance.

B cell lineage contributions to antiviral host responses

B cell responses are a major immune protective mechanism induced against a large variety of pathogens. Technical advances over the last decade, particularly in the isolation and characterization of B cell subsets by multicolor flow cytometry, have demonstrated the multifaceted nature of pathogen-induced B cell responses. In addition to participation by the major follicular B cell population, three B cell subsets are now recognized as key contributors to pathogen-induced host defenses: marginal zone (MZ) B cells, B-1a and B-1b cells. Each of these subsets seems to require unique activation signals and to react with distinct response patterns. Here we provide a brief review of the main developmental and functional features of these B cell subsets. Furthermore, we outline our current understanding of how each subset contributes to the humoral response to influenza virus infection and what regulates their differential responses. Understanding of the multilayered nature of the humoral responses to infectious agents and the complex innate immune signals that shape pathogen-specific humoral responses are likely at the heart of enhancing our ability to induce appropriate and long-lasting humoral responses for prophylaxis and therapy.

Autoantigen-B cell antigen receptor interactions that regulate expression of B cell antigen receptor Loci

Levels of AgR (BCR) expression are regulated during B cell development, activation, and induction of tolerance. The mechanisms responsible for and consequences of this regulation are poorly understood. We have described a class of DNA-based autoantigen-reactive B cell that down-regulates BCR expression during development to mature follicular phenotype. In this study, we show that at immature stages of primary differentiation, individual B cells of this type can dynamically modulate levels of expression of BCR in inverse proportion to degree of autoantigen engagement and induced BCR signaling. These adjustments in BCR expression are not associated with cell death, BCR revision, or altered development, and do not require TLR 9. Strikingly, modulation of BCR subunit gene RNA levels and transcription parallels these changes in BCR expression, indicating a direct link between autoantigen-BCR interactions of this type and regulation of transcription of BCR-encoding loci. We propose that this adaptive process allows this class of autoreactive B cell to avoid conventional tolerance pathways and promotes development to mature phenotype.

Quantitatively Reduced Participation of Anti-Nuclear Antigen B Cells That Down-Regulate B Cell Receptor during Primary Development in the Germinal Center/Memory B Cell Response to Foreign Antigen

The peripheral B cell compartment contains high levels of "polyreactivity" including autospecificities. We have described a pathway that certain autoreactive B cells may take in gaining stable access to the foreign Ag-responsive peripheral compartment. This pathway was revealed in mice expressing a targeted Ig H chain transgene encoding BCRs with "multireactivity" for the hapten arsonate and DNA-based autoantigens. B cells expressing such BCRs develop to mature follicular phenotype and locale, and are not short-lived. These B cells express very low levels of BCR, indicating that they are not "ignorant" of self Ag, but do not display features of anergy in in vitro assays. Nonetheless, a variety of states of lymphocyte anergy has been described, and some may only be manifested in vivo. As such, we analyzed the ability of these B cells to participate in a T cell-dependent immune response to arsonate in vivo. These B cells mount an early primary response similar to control B cells, including homing to follicles, migration to the T-B interface, and induction of costimulatory molecules, proliferation, differentiation to AFCs, class switching, and entry into GCs and somatic hypermutation. Nonetheless, these B cells display reduced participation in the latter stages of the GC response and in the anamnestic AFC response. In total, these data suggest that while the autoreactivity of this type of B cell does not result in anergy, the ability of such B cells to participate in a cross-reactive immune response to foreign Ag is compromised.

B-1 B cells: development, selection, natural autoantibody and leukemia

B-1 (CD5+) B cells constitute a phenotypic and functionally distinct population of B cells in mouse that show enriched expression of autoreactive B-cell antigen receptors and that produce several types of natural autoantibodies. Recently, there has been much progress in this field of research. Evidence has appeared for the existence of distinctive B-cell precursors that preferentially generate B-1 B cells, and the crucial requirement for strong B-cell antigen receptor signaling in the maturation of B-1 B cells has been established. Other work focuses on a phenotypically similar population that lacks CD5, termed 'B-1b', which shows similarities and differences from most CD5+ B cells in both development and function. The relationship of normal B-1 cells with B-cell lymphomas and leukemias continues to be a subject of interest and debate.