The level of expression of mu heavy chain modifies the composition of peripheral B cell subpopulations

Multiple factors influence the contribution of individual immunoglobulin light chain genes to the naïve antibody repertoire

Background

The naïve antibody repertoire is initially dependent upon the number of germline V(D)J genes and the ability of recombined heavy and light chains to pair. Individual VH and VL genes are not equally represented in naïve mature B cells, suggesting that positive and negative selection also shape the antibody repertoire. Among the three member murine Vκ10 L chain family, the Vκ10C gene is under-represented in the antibody repertoire. Although it is structurally functional and accessible to both transcriptional and recombination machinery, the Vκ10C promoter is inefficient in pre-B cell lines and productive Vκ10C rearrangements are lost as development progresses from pre-B cells through mature B cells. This study examined VH/Vκ10 pairing, promoter mutations, Vκ10 transcript levels and receptor editing as possible factors that are responsible for loss of productive Vκ10C rearrangements in developing B cells.

Results

We demonstrate that the loss of Vκ10C expression is not due to an inability to pair with H chains, but is likely due to a combination of other factors. Levels of mRNA are low in sorted pre-B cells and undetectable in B cells. Mutation of a single base in the three prime region of the Vκ10C promoter increases Vκ10C promoter function in pre-B cell lines. Pre-B and B cells harbor disproportionate levels of receptor-edited productive Vκ10C rearrangements.

Conclusions

Our findings suggest that the weak Vκ10C promoter initially limits the amount of available Vκ10C L chain for pairing with H chains, resulting in sub-threshold levels of cell surface B cell receptors, insufficient tonic signaling and subsequent receptor editing to limit the numbers of Vκ10C-expressing B cells emigrating from the bone marrow to the periphery.

Role of the Igh intronic enhancer Eμ in clonal selection at the pre-B to immature B cell transition

We previously described a checkpoint for allelic exclusion that occurs at the pre-B cell to immature B cell transition and is dependent upon the IgH intronic enhancer, Eμ. We now provide evidence that the breach in allelic exclusion associated with Eμ deletion results from decreased Igμ levels that make it difficult for emerging BCRs to reach the signaling threshold required for positive selection into the immature B cell compartment. We show that this compartment is smaller in mice carrying an Eμ-deficient, but functional, IgH allele (VHΔ(a)). Pre-B cells in such mice produce ≈ 50% wild-type levels of Igμ (mRNA and protein), and this is associated with diminished signals, as measured by phosphorylation of pre-BCR/BCR downstream signaling proteins. Providing Eμ-deficient mice with a preassembled VL gene led not only to a larger immature B cell compartment but also to a decrease in "double-producers," suggesting that H chain/L chain combinations with superior signaling properties can overcome the signaling defect associated with low Igμ-chain and can eliminate the selective advantage of "double-producers" that achieve higher Igμ-chain levels through expression of a second IgH allele. Finally, we found that "double-producers" in Eμ-deficient mice include a subpopulation with autoreactive BCRs. We infer that BCRs with IgH chain from the Eμ-deficient allele are ignored during negative selection owing to their comparatively low density. In summary, these studies show that Eμ's effect on IgH levels at the pre-B cell to immature B cell transition strongly influences allelic exclusion, the breadth of the mature BCR repertoire, and the emergence of autoimmune B cells.

Signals from a self-antigen induce positive selection in early B cell ontogeny but are tolerogenic in adults

Positive and negative signals from self-Ags shape the B cell repertoire and the development of distinct B cell subsets, but little is known about what distinguishes these signals. To address this question, we have studied the development of anti-hen egg lysozyme MD4 Ig transgene B cells while systematically varying the level, distribution, and timing of exposure to different forms of hen egg lysozyme as a self-Ag. This process has allowed us to explore the effects of Ag independent of BCR specificity. Our findings show how the selection of autoreactive B cells is a competitive process involving immunogenic and tolerogenic forms of self-Ags. Due to a developmental switch during B cell ontogeny, autoreactive anti-hen egg lysozyme MD4 Ig transgene B cells are negatively selected by self-Ags in adult bone marrow but susceptible to positive selection by some of the same self-Ags in fetal and neonatal life. However, the persistence of B1 cells and IgM autoantibodies from early ontogeny enables autoreactive B cells from the adult bone marrow to escape negative selection. Our data suggest that this rescue may be due to the clearance or masking of self-Ag by IgM autoantibody. We discuss the implications of these findings in terms of B cell selection and the maintenance of self-tolerance during early and adult life.

Analysis of marginal zone B cell development in the mouse with limited B cell diversity: role of the antigen receptor signals in the recruitment of B cells to the marginal zone

The quasimonoclonal (QM) mouse provides an intelligible model to analyze the B cell selection as the competition between two major 4-hydroxy-3-nitrophenylacetyl-specific B cell populations whose BCR are comprised of the knockin V(H)17.2.25 (V(H)T)-encoded H chain and the lambda1 or lambda2 L chain. In this study, we show the QM system is useful to examine how BCR signals guide a subset of B cells to the marginal zone (MZ). Compared with the control C57BL/6 mice, the QM mice had approximately 2.7-fold increased number of B cells exhibiting the MZ B cell phenotype and a larger MZ area in the spleen. Interestingly, V(H)T/lambda2 B cells significantly predominated over V(H)T/lambda1 B cells in MZ-(V(H)T/lambda1:V(H)T/lambda2 approximately 3:7) and transitional 2-B cell subsets, while these two populations were comparable in immature, transitional 1, and mature counterparts. Thus, the biased use of lambda2 in the MZ B cells may be the result of selection in the periphery. The enlargement of MZ B cell compartment and the preferred recruitment of the V(H)T/lambda2 B cells were further augmented by doubling the V(H)T gene, but dampened by the dysfunction of Bruton's tyrosine kinase, suggesting a positive role of BCR signaling in this selection. Comparison of Ag specificity between V(H)T/lambda1 and V(H)T/lambda2 IgM mAbs revealed a polyreactive nature of the V(H)T/lambda2 BCR, including the reactivity with ssDNA. Taken together, it is suggested that polyreactivity (including self-reactivity) of BCR is crucial in driving B cells to differentiate into the MZ phenotype.

B Cell Selection and Susceptibility to Autoimmunity

Autoreactive B cells arise routinely as part of the naive B cell repertoire. The immune system employs several mechanisms in an attempt to silence these autoreactive cells before they achieve immunocompetence. The BCR plays a central role in B cell development, activation, survival, and apoptosis, and thus is a critical component of the regulation of both protective and autoreactive B cells. The strength of signal mediated by the BCR is determined by numerous factors, both B cell intrinsic and B cell extrinsic. Perturbations in the molecules that regulate the BCR signal strength or that activate pathways that engage in cross talk with the BCR-mediated signaling pathways can lead to the aberrant survival and activation of autoreactive B cells. In this review, we will discuss the some newly identified genetic loci and factors that modulate the BCR signal transduction pathway and, therefore, the regulation of autoreactive B cells. We will also provide evidence for a model of autoreactivity in which a reduction in the strength of the BCR signal allows the survival and the modulation of a naive B cell repertoire replete with autoreactivity.

Positive selection of the peripheral B cell repertoire in gut-associated lymphoid tissues

Gut-associated lymphoid tissues (GALTs) interact with intestinal microflora to drive GALT development and diversify the primary antibody repertoire; however, the molecular mechanisms that link these events remain elusive. Alicia rabbits provide an excellent model to investigate the relationship between GALT, intestinal microflora, and modulation of the antibody repertoire. Most B cells in neonatal Alicia rabbits express V_Hn allotype immunoglobulin (Ig)M. Within weeks, the number of V_Hn B cells decreases, whereas V_Ha allotype B cells increase in number and become predominant. We hypothesized that the repertoire shift from V_Hn to V_Ha B cells results from interactions between GALT and intestinal microflora. To test this hypothesis, we surgically removed organized GALT from newborn Alicia pups and ligated the appendix to sequester it from intestinal microflora. Flow cytometry and nucleotide sequence analyses revealed that the V_Hn to V_Ha repertoire shift did not occur, demonstrating the requirement for interactions between GALT and intestinal microflora in the selective expansion of V_Ha B cells. By comparing amino acid sequences of V_Hn and V_Ha Ig, we identified a putative V_H ligand binding site for a bacterial or endogenous B cell superantigen. We propose that interaction of such a superantigen with V_Ha B cells results in their selective expansion.

Regulation of B-cell development by antibody specificity

Although autoantigen-induced negative selection plays an important role in shaping the mature B-cell repertoire, studies in recent years have suggested that differentiation into any of the three mature B-cell subsets (marginal zone B cells, follicular B cells and B-1 B cells) is not a passive product of autoreactive B-cell elimination. Instead, evidence suggests that entry into a mature subset involves active B-cell receptor signaling and self-antigen-mediated positive selection.

Signaling in transitional type 2 B cells is critical for peripheral B-cell development

Splenic peripheral B-cell development and the events regulating this functionally significant but relatively poorly defined developmental process have become a major focus in recent studies in B-cell immunology. Following the exit from the bone marrow, peripheral B cells develop through transitional type 1 (T1) and transitional type 2 (T2) B-cell stages. Emerging data suggest that the T2 subset is the immediate precursor of the mature B-cell populations present in the spleen. In this review, we first elaborate on the evidence describing the unique properties of CD21hiCD24hiCD23hiIgMhiIgDhi T2 B cells. T2 cells uniquely activate a proliferative, pro-survival, and differentiation program in response to B-cell antigen receptor (BCR) engagement. The potential mechanisms leading to the differential BCR responsiveness of T1 versus T2 B cells are discussed. We also review evidence that distinguishes key BCR-dependent signaling pathways operative in T2 and mature B cells. These signaling cascades include a protein kinase Cbeta (PKCbeta)-dependent cell-survival pathway and a second PKCbeta-independent pathway essential for BCR-driven differentiation. Finally, we discuss recent intriguing results suggesting that the type of signal(s) encountered by T2 cells leads to their differential maturation toward the follicular mature versus marginal zone mature B-cell populations. These combined observations suggest important implications with regard to B-cell selection and tolerance, potential novel therapeutic targets for B-cell lymphomas, and how the intricate balance of commensal organisms and other microenvironmental signals interact to promote the generation of 'innate-like' versus adaptive effector B-cell populations.

Human peripheral B cells: a different cytometric point of view

BACKGROUND

Human peripheral B lymphocytes, analyzed by current flow cytometers, frequently show complex patterns of morphological and fluorescence signals. However, fluorescence intensity values are commonly reported without any correlation to the cell surface area. We propose a different approach, based on the evaluation of the ratio of phenotype fluorescence intensity to forward scatter intensity, to determine the apparent fluorescence density of surface molecules.

METHODS

Starting from list mode acquired data, and after logical gating of live B cells, the analytical procedure suggests a serial scanning of the FSC versus SSC plot to obtain apparent fluorescence density of progressively larger cells.

RESULTS

This method, applied to normal human peripheral B lymphocytes, was able to detect the presence of steady and modulated (with respect to cell size) fluorescence densities for a variety of surface molecules. B cells from patients with B cell disorders displayed interesting alterations of the phenotype density values and distributions.

CONCLUSIONS

Our preliminary data show that, in human B cell cytometry, the apparent fluorescence density based method allows one to recognize variations in fluorescence intensities solely due to cell size differences and to disclose patterns of expression not detectable by the conventional intensity based approach.

Level of B cell antigen receptor surface expression influences both positive and negative selection of B cells during primary development

To examine the effect of B cell Ag receptor (BCR) surface density on B cell development, we studied multiple lines of mice containing various copy numbers of an IgH micro delta transgene. The V(H) gene in this transgene encodes multireactive BCRs with low affinity for self Ags. These BCRs promote differentiation to a B cell subpopulation that shares some, but not all of the properties of marginal zone (MZ) B cells. Surface BCR level was found to be related to transgene gene copy number in these mice. In mice containing 1-15 copies of the transgene, elevated surface BCR levels were correlated with increased numbers of B cells in the MZ-like subset. However, in mice containing 20-30 copies of the transgene, massive clonal deletion of B cells was observed in the bone marrow, few B cells populated the spleen, and B cells were essentially absent from the lymph nodes. These data support the idea that autoantigens mediate not only negative, but positive selection of developing B cells as well. More importantly, they illustrate the profound influence of BCR surface density on the extent to which either of these selective processes take place.