Gamma 2b transgenic mice as a model for the role of immunoglobulins in B cell development

Flexible ordering of antibody class switch and V(D)J joining during B-cell ontogeny

Omenn syndrome is a severe immunodeficiency disease commonly arising from hypomorphic RAG recombinase gene mutations. RAG recombinase mediates V(D)J joining during early B-lymphocyte development in the bone marrow (BM). V(D)J recombination and class switch recombination are thought to partition between the BM and secondary lymphoid organs. Kenter and colleagues show that V(D)J joining and switching are interchangeably inducible in the BM. This study has important implications for the development of Omenn syndrome, autoimmune diseases, and leukemia.

V(D)J joining is mediated by RAG recombinase during early B-lymphocyte development in the bone marrow (BM). Activation-induced deaminase initiates isotype switching in mature B cells of secondary lymphoid structures. Previous studies questioned the strict ontological partitioning of these processes. We show that pro-B cells undergo robust switching to a subset of immunoglobulin H (IgH) isotypes. Chromatin studies reveal that in pro-B cells, the spatial organization of the Igh locus may restrict switching to this subset of isotypes. We demonstrate that in the BM, V(D)J joining and switching are interchangeably inducible, providing an explanation for the hyper-IgE phenotype of Omenn syndrome.

Spontaneous class switch recombination in B cell lymphopoiesis generates aberrant switch junctions and is increased after VDJ rearrangement

Mature B cells replace the mu constant region of the H chain with a downstream isotype in a process of class switch recombination (CSR). Studies suggest that CSR induction is limited to activated mature B cells in the periphery. Recently, we have shown that CSR spontaneously occur in B lymphopoiesis. However, the mechanism and regulation of it have not been defined. In this study, we show that spontaneous CSR occurs at all stages of B cell development and generates aberrant joining of the switch junctions as revealed by: 1) increased load of somatic mutations around the CSR break points, 2) reduced sequence overlaps at the junctions, and 3) excessive switch region deletion. In addition, we found that incidence of spontaneous CSR is increased in cells carrying VDJ rearrangements. Our results reveal major differences between spontaneous CSR in developing B cells and CSR induced in mature B cells upon activation. These differences can be explained by deregulated expression or function of activation-induced cytidine deaminase early in B cell development.

Scarcity of lambda 1 B cells in mice with a single point mutation in C lambda 1 is due to a low BCR signal caused by misfolded lambda 1 light chain

The presence of valine-154 instead of glycine in the constant region of lambda1 causes a severe lambda1 B cell defect in SJL and lambda1-valine knock-in mice with a compensatory increase in lambda2,3 B cells. The defect is due to low signaling by the lambda1-valine BCR. lambda1-Valine B cells deficient in the SHP-1 phosphatase survive better than lambda2,3 B cells in these mice, or lambda1 B cells in lambda1 wildtype mice. Low signaling is apparently due to misfolding of the lambda1-valine light chain as demonstrated by the absence of a regular beta-sheet structure determined by circular dichroism, the sedimentation of the light chain in solution, and the association of valine-valine constant regions in a yeast two-hybrid assay. lambda1-Valine B cells that survive apparently have a higher BCR signal, presumably because of their specific lambda1-heavy chain combination or having encountered a high-affiniy antigen. lambda1-Valine mice have increased B1 cells which were shown by others to have a higher signaling potential. Valine mice crossed with non-conventional gamma2b transgenic mice, in which B cell development is accelerated and in which B1 cells and high signaling cells are greatly reduced, have essentially no, lambda2,3 B cells, but increased numbers of lambda1-valine B cells. This supports the conclusion that the major defect in lambda1-valine mice is the inability of valine-preB cells to produce a threshold signal for B cell development. The reduction of lambda2,3 B cells in valine mice with a gamma2b transgene shows that the majority of their compensatory increase is almost entirely of the B1 cell type.

A fail-safe mechanism for negative selection of isotype-switched B cell precursors is regulated by the Fas/FasL pathway

In B lymphocytes, immunoglobulin (Ig)M receptors drive development and construction of naive repertoire, whereas IgG receptors promote formation of the memory B cell compartment. This isotype switching process requires appropriate B cell activation and T cell help. In the absence of T cell help, activated B cells undergo Fas-mediated apoptosis, a peripheral mechanism contributing to the establishment of self-tolerance. Using Igμ-deficient μMT mouse model, where B cell development is blocked at pro-B stage, here we show an alternative developmental pathway used by isotype-switched B cell precursors. We find that isotype switching occurs normally in B cell precursors and is T independent. Ongoing isotype switching was found in both normal and μMT B cell development as reflected by detection of IgG1 germline and postswitch transcripts as well as activation-induced cytidine deaminase expression, resulting in the generation of IgG-expressing cells. These isotype-switched B cells are negatively selected by Fas pathway, as blocking the Fas/FasL interaction rescues the development of isotype-switched B cells in vivo and in vitro. Similar to memory B cells, isotype-switched B cells have a marginal zone phenotype. We suggest a novel developmental pathway used by isotype-switched B cell precursors that effectively circumvents peripheral tolerance requirements. This developmental pathway, however, is strictly controlled by Fas/FasL interaction to prevent B cell autoimmunity.

Unexpected autoantibody production in membrane Ig-mu-deficient/lpr mice

In the B lymphocyte lineage, Fas-mediated cell death is important in controlling activated mature cells, but little is known about possible functions at earlier developmental stages. In this study we found that in mice lacking the IgM transmembrane tail exons (muMT mice), in which B cell development is blocked at the pro-B stage, the absence of Fas or Fas ligand allows significant B cell development and maturation, resulting in high serum Ig levels. These B cells demonstrate Ig heavy chain isotype switching and autoimmune reactivity, suggesting that lack of functional Fas allows maturation of defective and/or self-reactive B cells in muMT/lpr mice. Possible mechanisms that may allow maturation of these B cells are discussed.

Developmental regulation of B lymphocyte immune tolerance compartmentalizes clonal selection from receptor selection

B lymphocyte development is a highly ordered process that involves immunoglobulin gene rearrangements, antigen receptor expression, and a learning process that minimizes the development of cells with reactivity to self tissue. Two distinct mechanisms for immune tolerance have been defined that operate during early bone marrow stages of B cell development: apoptosis, which eliminates clones of cells, and receptor editing, which spares the cells but genetically reprograms their autoreactive antigen receptors through nested immunoglobulin L chain gene rearrangements. We show here that sensitivity to antigen-induced apoptosis arises relatively late in B cell development and is preceded by a functionally distinct developmental stage capable of receptor editing. This regulation compartmentalizes clonal selection from receptor selection.

Self-antigen does not accelerate immature B cell apoptosis, but stimulates receptor editing as a consequence of developmental arrest

In pre-B lymphocytes, productive rearrangement of Ig light chain genes allows assembly of the B cell receptor (BCR), which selectively promotes further developmental maturation through poorly defined transmembrane signaling events. Using a novel in vitro system to study immune tolerance during development, we find that BCR reactivity to auto-antigen blocks this positive selection, preventing down-regulation of light chain gene recombination and promoting secondary light chain gene rearrangements that often alter BCR specificity, a process called receptor editing. Under these experimental conditions, self-antigen induces secondary light chain gene rearrangements in at least two-thirds of autoreactive immature B cells, but fails to accelerate cell death at this stage. These data suggest that in these cells the mechanism of immune tolerance is receptor selection rather than clonal selection.

BCR ligation induces receptor editing in IgM+IgD- bone marrow B cells in vitro

The ability of BCR cross-linking to stimulate receptor editing was analyzed in vitro using bone marrow B cells from immunoglobulin (Ig) transgenic (Tg) and non-Tg mice. In cultured Ig-Tg cells, BCR ligation induced receptor editing as measured by up-regulation of RAG gene expression, light chain gene DNA rearrangements, and expression of lambda-light chain protein in cells that previously expressed kappa. In the culture conditions used, BCR ligation induced light chain rearrangements in most immature IgM+IgD- bone marrow B cells in the absence of significant cell death or cell growth. Receptor editing in non-Tg B cells was also documented in cultures treated with anti-immunoglobulin. These results provide direct evidence for the ability of BCR ligation to stimulate immunoglobulin light chain gene rearrangements in immature B cells.