The T-cell-independent immune response to the hapten NP uses a large repertoire of heavy chain genes

Critical roles of the E3 ubiquitin ligase FBW7 in B-cell response and the pathogenesis of experimental autoimmune arthritis

Proper regulation of B-cell function is essential for effective humoral immunity and maintenance of immune tolerance. Here, we found that FBW7 (F-box/WD40 repeat-containing protein 7) is highly expressed in germinal centre B and B1 cells, and confirmed that it has an intrinsic role in maintaining homeostasis of mature B cells and B-1 cells. FBW7 deletion led to an impairment of antibody response, and although germinal centre formation was not affected, antibody class-switch recombination and affinity maturation processes were defective. Likewise, memory immune response was severely impaired. Moreover, FBW7 ablation ameliorated the pathogenesis of an autoimmune disease model, collagen-induced arthritis, by reducing the production of anti-collagen II autoantibodies. Taken together, these data suggest that FBW7 may be an attractive target for developing new therapeutics for the treatment of autoimmune diseases.

Immune Cell Number, Phenotype, and Function in the Elderly with Sepsis

Sepsis is a form of life-threatening organ dysfunction caused by dysregulated host responses to an infection that can be partly attributed to immune dysfunction. Although sepsis affects patients of all ages, elderly individuals display increased susceptibility and mortality. This is partly due to immunosenescence, a decline in normal immune system function associated with physiological aging that affects almost all cell types in the innate and adaptive immune systems. In elderly patients with sepsis, these alterations in immune cells such as endothelial cells, neutrophils, monocytes, macrophages, natural killer cells, dendritic cells, T lymphocytes, and B lymphocytes, are largely responsible for their poor prognosis and increased mortality. Here, we review recent studies investigating the events affecting both innate and adaptive immune cells in elderly mice and patients with sepsis, including alterations in their number, phenotype, and function, to shed light on possible new therapeutic strategies.

Myeloid-derived suppressor cells as a potential therapy for experimental autoimmune myasthenia gravis

We recently demonstrated that hepatic stellate cells induce the differentiation of myeloid-derived suppressor cells (MDSCs) from myeloid progenitors. In this study, we found that adoptive transfer of these MDSCs effectively reversed disease progression in experimental autoimmune myasthenia gravis (EAMG), a T cell-dependent and B cell-mediated model for myasthenia gravis. In addition to ameliorated disease severity, MDSC-treated EAMG mice showed suppressed acetylcholine receptor (AChR)-specific T cell responses, decreased levels of serum anti-AChR IgGs, and reduced complement activation at the neuromuscular junctions. Incubating MDSCs with B cells activated by anti-IgM or anti-CD40 Abs inhibited the proliferation of these in vitro-activated B cells. Administering MDSCs into mice immunized with a T cell-independent Ag inhibited the Ag-specific Ab production in vivo. MDSCs directly inhibit B cells through multiple mechanisms, including PGE2, inducible NO synthase, and arginase. Interestingly, MDSC treatment in EAMG mice does not appear to significantly inhibit their immune response to a nonrelevant Ag, OVA. These results demonstrated that hepatic stellate cell-induced MDSCs concurrently suppress both T and B cell autoimmunity, leading to effective treatment of established EAMG, and that the MDSCs inhibit AChR-specific immune responses at least partially in an Ag-specific manner. These data suggest that MDSCs could be further developed as a novel approach to treating myasthenia gravis and, even more broadly, other diseases in which T and B cells are involved in pathogenesis.

Restricted VH/VL usage and limited mutations in gluten-specific IgA of coeliac disease lesion plasma cells

Coeliac disease (CD), an enteropathy caused by cereal gluten ingestion, is characterized by CD4(+) T cells recognizing deamidated gluten and by antibodies reactive to gluten or the self-antigen transglutaminase 2 (TG2). TG2-specific immunoglobulin A (IgA) of plasma cells (PCs) from CD lesions have limited somatic hypermutation (SHM). Here we report that gluten-specific IgA of lesion-resident PCs share this feature. Monoclonal antibodies were expression cloned from single PCs of patients either isolated from cultures with reactivity to complex deamidated gluten antigen or by sorting with gluten peptide tetramers. Typically, the antibodies bind gluten peptides related to T-cell epitopes and many have higher reactivity to deamidated peptides. There is restricted VH and VL combination and usage among the antibodies. Limited SHM suggests that a common factor governs the mutation level in PCs producing TG2- and gluten-specific IgA. The antibodies have potential use for diagnosis of CD and for detection of gluten.

Extension of the germinal center stage of B cell development promotes autoantibodies in BXD2 mice

OBJECTIVE

Regulator of G protein signaling (RGS) proteins inhibit chemokine signaling by desensitizing G protein-coupled receptor signals. This study was undertaken to determine the mechanisms by which RGS13 promotes the generation of pathogenic autoantibodies in germinal centers (GCs), using BXD2-Rgs13-/- mice.

METHODS

Confocal and light microscopy imaging techniques were used to determine the location of cells that express RGS13 and activation-induced cytidine deaminase (AID) in the mouse spleen, and the number of plasmablasts. The levels of GC and plasma cell program transcripts in GC B cells were determined by real-time quantitative polymerase chain reaction (qPCR). Differential interleukin-17 (IL-17)-mediated expression of RGS13 in GC versus non-GC B cells was analyzed using A20 and 70Z/3 B cells.

RESULTS

In the spleens of BXD2 mice, RGS13 was mainly expressed by GC B cells and was stimulated by IL-17 but not IL-21. IL-17 up-regulated RGS13 in A20 GC cells but not 70Z/3 non-GC B cells. BXD2- Rgs13-/- mice exhibited smaller GCs and lower AID levels, suggesting lower somatic hypermutation and affinity maturation. However, GC B cells from BXD2- Rgs13-/- mice showed increased levels of IgMbright plasmablasts, up-regulation of the genes encoding plasma program, including interferon regulatory factor 4, B lymphocyte-induced maturation protein 1, and X-box binding protein 1 and the p-CREB target genes Fosb and Obf1, and down-regulation of the GC program genes Aid, Pax5, and Bach2 compared to BXD2 mice. BXD2-Rgs13-/- mice had lower titers of IgG autoantibodies and IgG deposits in the glomeruli, suggesting reduced autoantibody pathogenicity.

CONCLUSION

RGS13 deficiency is associated with a reduction in GC program genes and the exit of fewer pathogenic IgM plasmablasts in BXD2 mice. Our findings indicate that prolonged GC program, mediated by up-regulation of RGS13, enhances AID expression and enables the generation of pathogenic autoantibodies in autoreactive GCs.

Immunoglobulin class switching appears to be regulated by B-cell antigen receptor-specific T-cell action

Antigen affinity is commonly viewed as the driving force behind the selection for dominant clonotypes that can occur during the T-cell-dependent processes of class switch recombination (CSR) and immune maturation. To test this view, we analyzed the variable gene repertoires of natural monoclonal antibodies to the hapten 2-phenyloxazolone (phOx) as well as those generated after phOx protein carrier-induced thymus-dependent or Ficoll-induced thymus-independent antigen stimulation. In contrast to expectations, the extent of IgM heterogeneity proved similar and many IgM from these three populations exhibited similar or even greater affinities than the classic Ox1 clonotype that dominates only after CSR among primary and memory IgG. The population of clones that were selected during CSR exhibited a reduced VH/VL repertoire that was enriched for variable domains with shorter and more uniform CDR-H3 lengths and almost completely stripped of variable domains encoded by the large VH1 family. Thus, contrary to the current paradigm, T-cell-dependent clonal selection during CSR appeared to select for VH family and CDR-H3 loop content even when the affinity provided by alternative clones exhibited similar to increased affinity for antigen.

Limiting CDR-H3 diversity abrogates the antibody response to the bacterial polysaccharide α 1→3 dextran

Anti-polysaccharide Ab responses in mice are often oligoclonal, and the mechanisms involved in Ag-specific clone production and selection remain poorly understood. We evaluated the relative contribution of D(H) germline content versus N nucleotide addition in a classic oligoclonal, T-independent Ab response (α 1→3 dextran [DEX]) by challenging adult TdT-sufficient (TdT(+/+)) and TdT-deficient (TdT(-/-)) gene-targeted mice, limited to the use of a single D(H) gene segment (D-limited mice), with Enterobacter cloacae. D-limited mice achieved anti-DEX-specific levels of Abs that were broadly comparable to those of wild-type (WT) BALB/c mice. Sequence analysis of the third CDR of the H chain intervals obtained by PCR amplification of V(H) domain DNA from DEX-specific plasmablasts revealed the near universal presence of an aspartic acid residue (D99) at the V-D junction, irrespective of the composition of the D(H) locus. Although WT mice were able to use germline D(H) (DQ52, DSP, or DST) gene segment sequence, TdT activity, or both to produce D99, all three D-limited mouse strains relied exclusively on N addition. Additionally, in the absence of TdT, D-limited mice failed to produce a DEX response. Coupled with previous studies demonstrating a reduced response to DEX in TdT(-/-) mice with a WT D(H) locus, we concluded that in the case of the anti-DEX repertoire, which uses a short third CDR of the H chain, the anti-DEX response relies more intensely on sequences created by postnatal N nucleotide addition than on the germline sequence of the D(H).

IL-7-dependent B lymphocytes are essential for the anti-polysaccharide response and protective immunity to Streptococcus pneumoniae

Young children are impaired in their response to T cell-independent (TI) Ags, such as pneumococcal polysaccharide (PPS). B lymphopoeisis early in life is IL-7 independent, whereas in adults it is IL-7 dependent. Therefore, we hypothesized that IL-7-driven B lymphopoiesis plays a critical role in promoting Ab responses to TI Ags. Young but not adult mice are impaired in responses to PPS vaccination and to 4-hydroxy-3-nitrophenyl-acetyl-Ficoll, a widely studied model TI Ag, and B1b cells generate Ab responses to these Ags. In this paper, we show that, despite having B1b, B1a, and MZ B cells-all of which are involved in TI responses-young wild-type or adult mice deficient either in IL-7 or in IL-7Ralpha are severely impaired in anti-PPS responses and do not survive Streptococcus pneumoniae challenge, indicating IL-7-dependent B cells are required for TI immunity. Consistent with this, PPS immunization induced a robust TI response in young IL-7 transgenic mice that was comparable to adult wild-type responses. Moreover, immunized young or adult IL-7 transgenic mice were completely resistant to S. pneumoniae challenge. Our data indicate that activating the IL-7 signaling pathway could restore impaired TI responses in the young.

Deregulation of c-Myc Confers distinct survival requirements for memory B cells, plasma cells, and their progenitors

Deregulation of the c-Myc oncogene is tightly associated with human and murine plasma cell (PC) neoplasms. Through the analysis of Ag-specific B cell responses in mice where Myc is targeted to the Igh Calpha locus, we show here that c-Myc dramatically impairs the primary and secondary Ab response. This impairment is differentiation stage specific, since germinal center B cell formation, affinity maturation, and class switch recombination were intact. Examination of PC viability revealed that c-Myc triggered apoptosis only upon final maturation when Ab is secreted and is resistant to the survival factor BAFF (B cell-activating factor belonging to the TNF family). In contrast, PC precursors (PC(pre)) that ultimately give rise to mature PCs survived normally and vigorously expanded with BAFF signaling. We further show that c-Myc also facilitates the apoptosis of memory B cells. Thus, Calpha-Myc controls both cellular arms of long-lived B cell immunity than previously anticipated. Only when deregulation of c-Myc was combined with enforced Bcl-x(L) expression were mature PCs able to survive in response to BAFF. These data indicate that the survival requirements for tumor-susceptible PC(pre) and PCs are distinct and that tumor progression likely develops as PC(pre) transition to functional PCs when apoptotic pathways such as members of the Bcl-2 family are disabled.

A distinct role for B1b lymphocytes in T cell-independent immunity

Pathogenesis of infectious disease is not only determined by the virulence of the microbe but also by the immune status of the host. Vaccination is the most effective means to control infectious diseases. A hallmark of the adaptive immune system is the generation of B cell memory, which provides a long-lasting protective antibody response that is central to the concept of vaccination. Recent studies revealed a distinct function for B1b lymphocytes, a minor subset of mature B cells that closely resembles that of memory B cells in a number of aspects. In contrast to the development of conventional B cell memory, which requires the formation of germinal centers and T cells, the development of B1b cell-mediated long-lasting antibody responses occurs independent of T cell help. T cell-independent (TI) antigens are important virulence factors expressed by a number of bacterial pathogens, including those associated with biological threats. TI antigens cannot be processed and presented to T cells and therefore are known to possess restricted T cell-dependent (TD) immunogenicity. Nevertheless, specific recognition of TI antigens by B1b cells and the highly protective antibody responses mounted by them clearly indicate a crucial role for this subset of B cells. Understanding the mechanisms of long-term immunity conferred by B1b cells may lead to improved vaccine efficacy for a variety of TI antigens.

Germinal center-independent affinity maturation in tumor necrosis factor receptor 1-deficient mice

Germinal centers (GCs) have been identified as site at which the somatic mutation of immunoglobulins occurs.However, somatic mutations in immunoglobulins have also been observed in animals that normally do not harbor germinal centers. This clearly indicates that somatic mutations can occur in the absence of germinal centers. We therefore attempted to determine whether or not GCs exist in TNFR1-deficient mice, and are essential for the somatic mutation of immunoglobulins, using (4-hydroxy-3-nitropheny)acetyl-ovalbumin (NP-OVA). Both wild-type and TNFR1-deficient mice were immunized with NPOVA, and then examined with regard to the existence of GCs. No typical B-cell follicles were detected in the TNFR1-deficient mice. Cell proliferation was detected throughout all splenic tissue types, and no in vivo immunecomplex retention was observed in the TNFR1-deficient mice. All of these data strongly suggest that no GCs were formed in the TNFR1-deficient mice. Although TNFR1-deficient mice are unable to form GCs, serological analyses indicated that affinity maturation had been achieved in both the wild-type and TNFR1-deficient mice. We therefore isolated and sequenced several DNA clones from wild-type and the TNFR1-deficient mice. Eight out of 12 wild-type clones, and 11 out of 14 clones of the TNFR-1-deficient mice contained mutations at the CDR1 site. Thus, the wild-type and TNFR1-deficient mice were not extremely different with regard to types and rates of somatic mutation. Also, high-affinity antibodies were detected in both types of mice. Collectively, our data appear to show that affinity maturation may occur in TNFR1-deficient mice, which completely lack GCs.

Increased negative selection impairs neonatal B cell repertoire but does not directly lead to generation of disease-associated IgM auto-antibodies

To determine if increased negative B cell selection, due to lowered signaling threshold of responsiveness to a ligand as a result of SHP-1 deficiency, during ontogeny leads to the origin of disease-associated IgM auto-antibodies (AAbs), 47 V(H)J558+ VDJCmu rearrangements from SHP-1-deficient viable motheaten (me(v)/me(v)) and 24 J558+ VDJCmu rearrangements from normal me(v)/+ neonatal (<24 h post-birth) B cells were examined for their structural properties. None of the J558+ VDJCmu rearrangements from autoimmune-prone me(v)/me(v) had the characteristic CDR3H size restriction or arginine residues noted in disease-associated IgM AAbs. However, the MVAR2/10 genes are expressed at a high frequency in me(v)/me(v) (31.9%) as compared with me(v)/+ (16.7%), and pM11 gene expression is exclusively (14.9%) noted in me(v)/me(v) B cells. Clearly, there is a trend toward higher expression of pM11 genes (P-value < or = 0.09) in autoimmune-prone me(v)/me(v) strain. The CDR2H region of J558+ VDJCmu recombinations from me(v)/me(v) has increased hotspot triplets predisposing to mutations as compared with me(v)/+ (P-value < or = 0.01) mice. A higher DFL D-gene expression is noted in J558+ VDJCmu rearrangements from me(v)/me(v) (P-value < or = 0.1) in contrast to me(v)/+. The sophisticated logistic regression and odds ratio analysis of V-, D- and J-gene expressions in neonatal B cells from me(v)/me(v) and me(v)/+ mice demonstrates differential composition of the germ line IgM repertoire as a result of SHP-1 deficiency. These observations suggest that increased negative B cell selection during ontogeny impairs the developing IgM antibody repertoire but does not directly lead to generation of disease-associated IgM AAbs.

Novel Role of the Ras Cascade in Memory B Cell Response

Engagement of the B cell antigen receptor (BCR) triggers the Ras cascade, but the biological role of the latter in B cell response is unknown. Here, we report that in T cell-dependent response, the role of the Ras cascade is confined to memory B cells and possibly marginal zone B cells. When Ras-dependent BCR signaling was impaired, the generation of IgG germinal center B cells was unaffected but the recruitment of high-affinity cells into the memory compartment and terminal differentiation were inhibited. Furthermore, inhibition of MEK activity consistently impaired antibody production by IgG memory B cells (but not naïve B cells) in vitro. Notably, this impairment was countered by overexpression of Bcl-2. Thus, our data suggest that upon antigen stimulation, memory B cells are susceptible to apoptosis but can be rescued via an antiapoptotic effect mediated through the Ras cascade.

Identification of novel VH1/J558 immunoglobulin germline genes of C57BL/6 (Igh b) allotype

Although a rich database of Igh a allotype mouse immunoglobulin germline genes exists, current information on Igh b allotype immunoglobulin germline genes is limited. Among the immunoglobulin VH genes, single-cell amplified from six Igh b (C57BL/6 background) spleens in this study, 602 clonally independent immunoglobulin VH sequences belonging to the VH1/J558 family were identified. Whereas 335 of these sequences could be traced to have originated from 29 different VH1/J558 germline genes deposited in the NCBI Igblast database, the remaining 267 sequences appeared to have originated from 21 novel germline genes. Of the 50 VH1/J558 germline genes utilized in the peripheral repertoire of these Igh b allotype mice, the most frequently used genes included 45.21.2, V165.1, J558.6, J558.18A, and V23. Whereas the majority of the novel genes uncovered represented allelic counterparts of previously described Balb/c (Igh a allotype) genes, some appeared to represent truly novel germline genes. Collectively, the VH1/J558 germline genes exhibited high amino acid residue usage variability at the CDR1 positions, H31, H33, and H35, and the CDR2 positions, H50, H52, H53, H54, H56, and H58. The 50 VH1/J558 germline genes expressed in the peripheral Igh b repertoire also varied widely in the net charge of their CDR regions, raising the possibility that they may be differentially utilized to encode anti-nuclear autoantibodies.

Age-related changes in lymphocyte development and function

The effects of aging on the immune system are widespread and extend from hematopoietic stem cells and lymphoid progenitors in the bone marrow and thymus to mature lymphocytes in secondary lymphoid organs. These changes combine to result in a diminution of immune responsiveness in the elderly. This review aims to provide an overview of age-related changes in lymphocyte development and function and discusses current controversies in the field of aging research.

Cutting edge: gamma delta T cells provide help to B cells with altered clonotypes and are capable of inducing Ig gene hypermutation

It has not been resolved whether gammadelta T cells can collaborate with germinal center B cells and support Ig hypermutation during an Ab response to a truly defined T-dependent Ag. In this study, we show that in the absence of alphabeta T cells, immunization with the well-defined T-dependent Ag, (4-hydroxy-3-nitrophenyl) acetyl (NP) conjugate, was able to induce Ig hypermutation. However, the clonotypes of B cells responding to NP were dramatically altered in TCR beta(-/-) mice. Unlike B cells in wild-type mice that use canonical VDJ rearrangements, most NP-responding B cells in mutant mice use analog genes of the J558 gene family. In addition, the majority of anti-NP Abs produced in mutant mice use kappaL chain instead of lambda1L chain, which dominates in mice of Igh(b) background. Thus, the B cell population that collaborates with gammadelta T cells is distinct from B cells interacting with conventional alphabeta Th cells.

Plasmodium chabaudi chabaudi infection in mice induces strong B cell responses and striking but temporary changes in splenic cell distribution

B cells and Abs play a key role in controlling the erythrocytic stage of malaria. However, little is known about the way the humoral response develops during infection. We show that Plasmodium chabaudi chabaudi causes major, but temporary changes in the distribution of leukocytes in the spleen. Despite these changes, an ordered response to infection develops, which includes vigorous extrafollicular growth of plasmablasts and germinal center formation. Early in the response, the lymphocytes in the T zone and follicles become widely spaced, and the edges of these compartments blur. This effect is maximal around the peak of parasitemia. Germinal centers are apparent by day 8, peak at day 20, and persist through day 60. Extrafollicular foci of plasmablasts are visible from day 4 and initiate a very strong plasma cell response. Initially, the plasma cells have a conventional red pulp distribution, but by day 10 they are unconventionally sited in the periarteriolar region of the white pulp. In this region they form clusters occupying part of the area normally filled by T cells. B cells are absent from the marginal zone for at least 30 days after the peak of infection, although flow cytometry shows their continued presence in the spleen throughout infection. Relatively normal splenic architecture is regained by day 60 of infection. These results show that the changes in splenic cell distribution are linked to the presence of parasites and do not seem to interfere with the development of the humoral response.

Immunity in the absence of CD28 and CD137 (4-1BB) molecules

We report the generation and immune regulation of mice that are deficient in CD28 and 4-1BB (CD137) genes. These mice were viable, fertile and did not display any overt abnormalities and had a normal T cell phenotype in thymus and spleen. Proliferative responses to anti-CD3 and ConA were enhanced in 4-1BB-/- but not in either CD28-/- or double mutant mice, while levels of interleukin-2 were decreased in all mutant mice. Although the 4-1BB-/- mice displayed increased basal levels of most immunoglobulin isotypes tested, the plateau levels of immunoglobulin G2a, immunoglobulin G2b and immunoglobulin A were particularly high compared to wild type controls. The immunoglobulin class switch to T-dependent antigen was normal in 4-1BB-/- mice but was greatly affected in both CD28-/- and 4-1BB-/- CD28-/- mice. Vesicular stomatitis virus-specific cytotoxic T lymphocyte responses and plaque reduction neutralizing ability was differentially reduced in all mutant mice. Contact sensitivity to allergens showed marginal but not significant change in ear thickness in 4-1BB-/- mice, but an ability to mount contact hypersensitivity to the same antigens was greatly curtailed in CD28-/- and double mutant mice.

Low-level hypermutation in T cell-independent germinal centers compared with high mutation rates associated with T cell-dependent germinal centers

Exceptionally germinal center formation can be induced without T cell help by polysaccharide-based antigens, but these germinal centers involute by massive B cell apoptosis at the time centrocyte selection starts. This study investigates whether B cells in germinal centers induced by the T cell-independent antigen (4-hydroxy-3-nitrophenyl)acetyl (NP) conjugated to Ficoll undergo hypermutation in their immunoglobulin V region genes. Positive controls are provided by comparing germinal centers at the same stage of development in carrier-primed mice immunized with a T cell-dependent antigen: NP protein conjugate. False positive results from background germinal centers and false negatives from non-B cells in germinal centers were avoided by transferring B cells with a transgenic B cell receptor into congenic controls not carrying the transgene. By 4 d after immunization, hypermutation was well advanced in the T cell-dependent germinal centers. By contrast, the mutation rate for T cell-independent germinal centers was low, but significantly higher than in NP-specific B cells from nonimmunized transgenic mice. Interestingly, a similar rate of mutation was seen in extrafollicular plasma cells at this stage. It is concluded that efficient activation of hypermutation depends on interaction with T cells, but some hypermutation may be induced without such signals, even outside germinal centers.

VH gene analysis in sporadic Burkitt's lymphoma: somatic mutation and intraclonal diversity with special reference to the tumor cells involving germinal center

We analyzed the immunoglobulin heavy chain variable region (V(H)) gene in seven cases of sporadic Burkitt's lymphoma (sBL) to elucidate their cell of origin. In particular, we focused on the V(H) gene status of tumor cells involving adjacent germinal center (GC) by microdissecting histological sections. Among the seven V(H) genes V(H)1 family was found in two, V(H)3 in four, and V(H)4 in one. All rearranged V(H) genes demonstrated somatic mutations at percentages ranging from 1.4 to 7.5% (mean, 4.2%), which is a similar level to that seen in IgM-only B cells. Three out of four V(H) genes with more than 2% sequence difference from their corresponding germline counterpart showed evidence of antigen selection in their framework region 3. Three cases demonstrated signs of intraclonal diversity with a mutational frequency of 0.47-0.98%, which was 13.5-28.8 times as great as the Taq infidelity in our experimental conditions. However the level of somatic mutation and the effect of antigen selection on V(H) gene were diverse in these three cases, and the relationship between V(H) gene somatic mutation status and intraclonal diversity was unclear in sBL. In the analysis of microdissected tissues, all 20 tumor clones in the adjacent GCs showed additional replacement mutations in complementarity determining region 3, suggesting a role of antigen in tumor progression. This finding resembles the phenomenon that memory B-cells reenter into GC to undergo further affinity maturation. In contrast, 7/11 V(H) gene sequences irrelevant to GC were identical to those of the major tumor clone. Thus our findings suggested that sBL is derived from memory B-cells rather than GC B-cells, and that antigen stimulation is involved in the clonal expansion of a proportion of sBL.

Isotype-specific selection of high affinity memory B cells in nasal-associated lymphoid tissue

Mucosal immunoglobulin (Ig)A dominance has been proposed to be associated with preferential class switch recombination (CSR) to the IgA heavy chain constant region, Calpha. Here, we report that B cell activation in nasal-associated lymphoid tissue (NALT) upon stimulation with the hapten (4-hydroxy-3-nitrophenyl)acetyl (NP) coupled to chicken gamma globulin caused an anti-NP memory response dominated by high affinity IgA antibodies. In the response, however, NP-specific IgG(+) B cells expanded and sustained their number as a major population in germinal centers (GCs), supporting the view that CSR to IgG heavy chain constant region, Cgamma, operated efficiently in NALT. Both IgG(+) and IgA(+) GC B cells accumulated somatic mutations, indicative of affinity maturation to a similar extent, suggesting that both types of cell were equally selected by antigen. Despite the selection in GCs, high affinity NP-specific B cells were barely detected in the IgG memory compartment, whereas such cells dominated the IgA memory compartment. Taken together with the analysis of the V(H) gene clonotype in GC and memory B cells, we propose that NALT is equipped with a unique machinery providing IgA-specific enrichment of high affinity cells into the memory compartment, facilitating immunity with high affinity and noninflammatory secretory antibodies.

Mechanisms of chromosomal translocations in B cell lymphomas

Reciprocal chromosomal translocations involving the immunoglobulin (Ig) loci are a hallmark of most mature B cell lymphomas and usually result in dysregulated expression of oncogenes brought under the control of the Ig enhancers. Although the precise mechanisms involved in the development of these translocations remains essentially unknown, a clear relationship has been established with the mechanisms that lead to Ig gene remodeling, including V(D)J recombination, isotype switching and somatic hypermutation. The common denominator of these three processes in the formation of Ig-associated translocations is probably represented by the fact that each of these processes intrinsically generates double-strand DNA breaks. Since isotype switching and somatic hypermutation occur in germinal center (GC) B cells, the origin of a large number of B cell lymphomas from GC B cells is likely closely related to aberrant hypermutation and isotype switching activity in these B cells.

A pivotal role for DNase I-sensitive regions 3b and/or 4 in the induction of somatic hypermutation of IgH genes

Chimeric mice were prepared from embryonic stem cells transfected with IgH genes as transgenes and RAG-2-deficient blastocysts for the purpose of identifying the cis-acting elements responsible for the induction of somatic hypermutation. Among the three transgene constructs used, the V(H) promoter, the rearranged V(H)-D-J(H), an intron enhancer/matrix attachment region, and human Cmu were common to all, but the 3'-untranslated region in each construct was different. After immunization of mice with a T cell-dependent Ag, the distribution and frequency of hypermutation in transgenes were analyzed. The transgene lacking the 3' untranslated region showed a marginal degree of hypermutation. Addition of the 3' enhancer resulted in a slight increase in the number of mutations. However, the transgene containing DNase I-sensitive regions 3b and 4 in addition to the 3' enhancer showed more than a 10-fold increase in hypermutation, reaching levels comparable to those observed in endogenous V(H)186.2 genes of C57BL/6 mice.

Cutting edge: germinal centers can be induced in the absence of T cells

Immunization of mice containing mutations that inactivate the TCR Cbeta and Cdelta genes with the T cell-independent (TI) type 2 Ag (4-hydroxy-3-nitrophenyl)acetyl-Ficoll induces clusters of peanut agglutinin-binding B cells in the spleen. These clusters are histologically indistinguishable from germinal centers (GCs) typical of T cell-dependent immune responses. They are located in follicles, and contain mature follicular dendritic cells, immune complex deposits, and B cells that display the phenotypic qualities of conventional GC B cells. However, the kinetics of this TI GC response differ from T cell-dependent GC responses in being rapidly induced and of short duration. Moreover, the Ab V genes expressed in TI GCs have not undergone somatic hypermutation. Therefore, T cells may be required for B cell differentiation processes associated with the intermediate and latter stages of the GC reaction, but they are dispensable for the induction and initial development of this response.

Canonical germinal center B cells may not dominate the memory response to antigenic challenge

Spleen and bone marrow (BM) are the major sites of antibody production and anamnestic response in systemically immunized mice. We examined the VDJ segment repertoire of antibody plaque-forming cells (APFC) in those two sites in the course of antibody responses to the hapten nitrophenyl (NP). Individual IgG APFC expressed any one of 10 V(H) segments of the V186.2/V3 (J558) gene family: 186.2, 102, 23, C1H4, 165.l, CH10, 3, 593.3, 24.8 and 671.5. The majority of cells in both spleen and BM expressed the V186.2 gene joined to a D segment with Tyr95. During a 2-month period after a single immunization, the V186.2(+) APFC in BM accumulated 3 times as many somatic mutations than splenic APFC (average 8.5 versus 3 mutations/V(H)); this process was T(h) dependent as shown by in vivo depletion of CD4(+) lymphocytes. However, the V186.2(+) APFC in both spleen and BM shared a recurrent W33L replacement, indicating their common origin from germinal centers. The APFC expressing the other (analogue) V(H) segments were evenly represented in the spleen and BM, but they accumulated few, if any, mutations. The anamnestic V186.2(+) APFC were highly mutated both in the spleen and BM; they represented a new and unexpected clonotype. The V/D segments were joined by Gly95 instead of Tyr95, the W33L was absent and a new shared K58R replacement appeared. The APFC expressing the 'analogue' V(H) genes comprised approximately 20% of the anamnestic response and did not accumulate more mutations, but their affinities were in the range of the memory V186.2(+) cells. These data suggest that the late primary and secondary responses to a hapten may be born by different B cell lineages, and that some clonotypes may reach the memory pool without an extensive mutation and expansion.

Fas is required for clonal selection in germinal centers and the subsequent establishment of the memory B cell repertoire

In T cell-dependent immune responses, high-affinity B cells are selected and differentiate into memory cells; however, the mechanism behind this process remains largely unknown. Here, we report that the selection of high-affinity B cells within germinal centers (GCs) is impaired in Fas-deficient lpr mice in the primary response, probably owing to inefficient negative selection. The memory compartment in control mice is mostly established by precursors generated from the early GCs, whereas the lpr defect expands the memory compartment by the increased recruitment of newly generated precursors from the late GCs, resulting in the accumulation of heavily mutated memory B cells at high frequency. These results suggest that Fas is required for clonal selection within GCs and the establishment of the memory B cell repertoire.

B cell immunopoiesis: visualizing the impact of CD40 engagement on the course of T cell-independent immune responses in an Ig transgenic system

This study tracks the fate of antigen-reactive B cells through follicular and extrafollicular responses and addresses the function of CD40 in these processes. The unique feature of this system is the use of transgenic B cells in which the heavy chain locus has been altered by site-directed insertion of a rearranged V(H) DJ(H) exon such that they are able to clonally expand, isotype-switch and follow a normal course of differentiation upon immunization. These Ig transgenic B cells when adoptively transferred into non-transgenic (Tg) mice in measured amounts expanded and differentiated distinctively in response to T cell-independent (TI) or T cell-dependent (TD) antigens. The capacity of these Tg B cells to faithfully recapitulate the humoral immune response to TI and TD antigens provides the means to track clonal B cell behavior in vivo. Challenge with TI antigen in the presence of agonistic anti-CD40 mAb resulted in well-defined alterations of the TI response. In vivo triggering of Tg B cells with TI antigen and CD40 caused an increase in the levels IgG produced and a broadening of the Ig isotype profile, characteristics which partially mimic TD responses. Although some TD characteristics were induced by TI antigen and CD40 triggering, the Tg B cells failed to acquire a germinal center phenotype and failed to generate a memory response. Therefore, TD-like immunity can be only partially reconstituted with CD40 agonists and TI antigens, suggesting that there are additional signals required for germinal center formation and development of memory.

Regulation of VH gene repertoire and somatic mutation in germinal centre B cells by passively administered antibody

Immunization with T-dependent antigens induces a rapid differentiation of B cells to plasmacytes that produce the primary immunoglobulin M (IgM) and IgG antibodies with low affinities for the immunogen. It is proposed that the IgG antibody forms immune complexes with the residual antigen which provide an important stimulus for the formation of germinal centres (GC) and the activation of somatic mutation. This hypothesis was tested by passive administration of hapten-specific antibody into mice shortly after the immunization with nitrophenyl (NP) coupled to chicken gamma globulin (NP-CGG) in an environment of limited T-cell help. Athymic mice that received normal T helper cells at 72 hr after the administration of antigen produced low levels of anti-NP antibody and the splenic GC formation was delayed until day 12 after the antigen administration. The analysis of VDJ segments from NP-reactive GC B cells showed very few mutations in the VH genes. Passive injection of anti-NP IgG1 monoclonal antibody - but, not IgM - stimulated the GC formation up to normal levels and the somatic mutation activity in the GC B cells was fully restored. In addition, GC B cells in the recipients of IgG1 antibody demonstrated a change in the usage of germline-encoded VH genes which was not apparent among the primary antibody-forming cells. These results suggest the existence of a specific feedback mechanism whereby the IgG antibody regulates the GC formation, clonotypic repertoire and somatic mutation in GC B cells.

Severe attenuation of the B cell immune response in Msh2-deficient mice

Recently, results obtained from mice with targeted inactivations of postreplication DNA mismatch repair (MMR) genes have been interpreted to demonstrate a direct role for MMR in antibody variable (V) gene hypermutation. Here we show that mice that do not express the MMR factor Msh2 have wide-ranging defects in antigen-driven B cell responses. These include lack of progression of the germinal center (GC) reaction associated with increased intra-GC apoptosis, severely diminished antigen-specific immunoglobulin G responses, and near absence of anamnestic responses. Mice heterozygous for the Msh2 deficiency display an "intermediate" phenotype in these regards, suggesting that normal levels of Msh2 expression are critical for the B cell response. Interpretation of the impact of an MMR deficiency on the mechanism of V gene somatic hypermutation could be easily confounded by these perturbations.

Somatic hypermutation in T-independent and T-dependent immune responses to Haemophilus influenzae type b polysaccharide

Secondary immune responses to T-independent antigens are characterized by little or no affinity maturation, a phenomenon attributed to limited somatic hypermutation. In the human immune response to Haemophilus influenzae type b capsular polysaccharide, however, there are numerous differences between rearranged heavy chain variable region gene segments and candidate germline genes, irrespective of antigen presentation in a T-independent or T-dependent form. To determine the characteristics of somatic hypermutation in this response, we analyzed rearranged heavy chain variable region segments and associated 3' untranslated JH4-JH5 introns from monoclonal anti-Hib PS antibodies. Mutation of untranslated introns and heavy chain variable segments in both T-independent and T-dependent responses resembles that described in murine and unselected human immune responses. Although mutation is frequent in both T-independent and T-dependent anti-Hib PS responses, there is little evidence of antigen-driven selection, suggesting that ongoing pressure to conserve the variable segment germline configuration limits affinity maturation in this immune response.

Localization of Splenic B Cells Activated for Switch Recombination by In Situ Hybridization with Iγ1 Switch Transcript and Rad51 Probes

B cells are activated for switch recombination by signals from Th cells, but the site at which this first occurs in vivo has yet to be identified. By in situ hybridization of splenic sections using riboprobes specific for the Iγ1 switch transcript and Rad51 mRNA, we have visualized B cells that are newly activated for switch recombination and characterized the spatial and temporal patterns of Iγ1 and Rad51 mRNA expression. Within 2 days after immunization with (4-hydroxy-3-nitrophenyl)acetyl-chicken gamma-globulin, expression of Iγ1 switch transcripts and Rad51 mRNA was evident and was localized to B220+ B cells clustered within the T cell-rich periarteriolar lymphoid sheath (PALS) and surrounding follicles. By Ab staining, we have shown previously that cells switching from IgM to IgG expression can be visualized at 3 to 5 days postimmunization and colocalize to clusters of Rad51+ cells. Hybridization of adjacent sections with probes for Cμ and Cγ1 mRNA now shows that switching from μ to γ expression occurs within Rad51+Iγ1+ regions of the PALS and peaks between days 3 and 5. Colocalized expression of Iγ1 and Rad51 transcripts was observed from days 2 through 12 of the immune response. Iγ1 and Rad51 transcripts were down-regulated but still detectable at 12 days postimmunization, when they were evident in peanut agglutinin-positive germinal center B cells. Taken together, these observations show that B cells are first activated for switch recombination in the T cell-rich PALS.

Immunoglobulin double isotype-producing hybridomas isolated from an autoimmune (NZB x NZW)F1 mouse

In the sera of (NZB x NZW)F1 (B/W) mice that develop a lupus-like syndrome, increased levels of IgG antibodies (Ab) reacting with TNP have been detected before the appearance of IgG anti-DNA Ab and clinical symptoms. A single injection of trinitrophenyl-bovine serum albumin (TNP/BSA) in physiological saline into a young B/W mouse (3 months old), followed by fusion of its splenocytes 3 days later, gave rise to hybridomas simultaneously secreting IgM and IgG Ab with anti-TNP reactivity. Both mu and gamma chains were detected in culture supernatants by ELISA, and double isotype-producing cells were labeled by immunofluorescence. Molecular analysis of two of these double isotype-producing hybridomas showed the presence of mRNA coding for both mu and gamma chains of Ig, and this gamma mRNA could be translated in vitro into a gamma heavy (H) chain. Comparison of the H chain variable-region sequences of IgM and IgG revealed 100% homology between mu and gamma V(H) genes in one clone, while mu and gamma V(H) genes showed only 80% homology in the other clone. Both clones produced a single kappa light (L) chain. These two hybridomas, isolated from a B/W mouse, thus represent two different mechanisms of double isotype expression: the first one corresponds to an IgM to IgG switch, while the second one reflects a lack of allelic exclusion.

Insight into the origin and clonal history of B-cell tumors as revealed by analysis of immunoglobulin variable region genes

Recombination of VH, DH and JH genes is a unique first step in normal B-cell development. Subsequent differentiation to a mature plasma cell is accompanied by further events in the Ig genes, including VL-JL joining, somatic hypermutation and isotype switching. Chromosomal changes leading to B-cell tumors can occur at many points in this sequence, and may be partly a consequence of the genetic mobility and mutability permitted in order to generate a diverse antibody repertoire. V genes of neoplastic B cells may reflect the point of maturation reached by the B cell of origin, prior to transformation. Analysis of tumors therefore provides useful information on V-gene patterns in normal B cells, and may add another dimension to classification of B-cell tumors. Transformation may also preserve cell populations normally destined to die by apoptosis. Tumor cells arrested in the site where somatic hypermutation and isotype switch are occurring can still be subject to these processes, and could be influenced by persisting antigen. However, mutation is silenced at the point of exit to the periphery, leading to fixed mutational patterns in tumors of mature B cells. V-gene analysis provides an invaluable tool for understanding the genesis of neoplastic change. It also has a clear clinical relevance in tracking tumor cells, measuring residual disease, and finally in offering the opportunity of developing vaccines for treatment.

Age-related changes in antibody repertoire: contribution from T cells

The immune system of aged mice produces antibodies that are characterized by low affinity, diminished protection against infections and autoreactivity. It has been shown that these antibodies may be encoded by different immunoglobulin V genes and that the mechanism of somatic hypermutation in the V genes is inefficient. Studies on scid mice reconstituted with B and T cells from donors of different ages suggested that both lymphocyte subsets may contribute to the age-related changes in antibody repertoire. With help provided by T cells from young mice, the response to a hapten, nitrophenyl(acetyl), became gradually dominated by B-cell clones that rearranged a particular germline VH gene (V186.2). However, help from the aged T cells resulted in a heterogeneous response of B cells expressing many different V segments. Analysis of discrete foci of primary antibody-forming cells suggested that the aged T-helper cells are unable to govern the normally-occurring competition between the B-cell clones that have different affinities for the hapten. It is proposed that a signaling disequilibrium from the aged T cells, which provide less efficient help in quantitative terms, supports the growth of low-affinity B cells. This process may be exacerbated due to the apparent hyperactivity of aged B cells to CD40-mediated mitogenic signal.

Immunosenescence and germinal center reaction

Dysfunction of the immune system in aged individuals includes at least two important factors: accumulation of immunocytes with reduced function and accumulation of lymphocyte clones with self-reactive potential. Coincidently, there is a profound reduction of the germinal center reaction in the aged. While this reduction is likely the result of age-associated impairment in lymphocyte function (e.g. diminished response to costimulus, altered lymphokine production etc.), the reduction of germinal centers may itself make an important contribution to further immunological dysfunction.

Immunization with immune complex alters the repertoire of antigen-reactive B cells in the germinal centers

The differentiation of memory B cells in germinal centers (GC) is selectively enhanced upon administration of antigen-antibody complexes. To characterize the repertoire of this response, we examined the rearranged immunoglobulin heavy chain variable (V(H)) genes from mouse splenic GC after a single immunization with either antigen, nitrophenyl (NP) hapten coupled to keyhole limpet hemocyanin, or with a preformed complex of antigen with a monoclonal anti-NP antibody of gamma1 isotype. Among antigen-immunized mice, NP-reactive GC B cell populations in the antigen-induced GC consisted mostly of cells expressing the canonical V186.2 gene which contained, on average, 0.8 point mutations/V(H) gene by day 8 after immunization. These results are indicative of the beginning of somatic hypermutation and consistent with previously published analyses of NP antigen-driven GC. In contrast, the NP-specific B cells in GC that were elicited by administration of immune complex represented a heterogeneous cell population expressing nine different germ-line segments of the V186.2/V3 (J558) gene family, i.e. V23, V24.8, C1H4, V3, CH10, V165.1, V102, V671.5 and V186.2. Moreover, the average frequency of mutations in these genes was 1.7, reaching up to 4 mutations/V(H) in some GC. Administration of the antigen NP in complex with specific antibody apparently alters the process of interclonal competition in the GC and results in loss of dominance by V186.2+ cells and nearly stochastic representation of diverse clonotypes. These results suggest an important feedback regulation of the B cell repertoire by antibody and indicate a role for immune complexes in the activation of somatic hypermutation.

Germinal center development

Using a set of surface markers including IgD and CD38, human tonsillar B cells were classified into discrete subpopulations. Molecular and functional analysis allowed us to identify: i) two sets of naive B cells (Bm1 and Bm2); ii) germinal center founder cells (Bm2'); iii) an obscure population of germinal center B cells, displaying a high load of somatic mutations in IgV genes, C mu to C delta switch and preferential Ig lambda light chain usage: these cells may represent the precursors of normal and malignant IgD-secreting plasma cells; iv) the centroblasts (Bm3) in which somatic mutation machinery is activated; v) the centrocytes (Bm4) in which isotype switch occurs; vi) the memory B cells. The characterization of these subpopulations showed that: i) programmed cell death is set before somatic mutations, possibly providing an efficient way for affinity maturation; ii) only high affinity centrocytes are allowed to switch isotype; iii) CD40-ligation inhibits plasmacytic differentiation of mature B lymphocytes; iv) memory B cells preferentially differentiate into plasma cells; v) IgD isotype switch occurs in normal B cells; vi) receptor editing may be induced by somatic mutations in germinal centers. We also characterized two types of antigen-presenting cells in germinal centers: follicular dendritic cells that select high affinity B cells, and a new subset of germinal center dendritic cells that activate germinal center T cells.

Mechanism of antigen-driven somatic hypermutation of rearranged immunoglobulin V(D)J genes in the mouse

Available data relevant to the mechanism of somatic hypermutation have been critically evaluated in the context of alternative models: (i) error-generating reverse transcription (RT) followed by homologous recombination; and (ii) error-prone DNA replication/repair. A set of basic principles concerning somatic hypermutation has also been formulated and a revised and expanded "RT-Mutatorsome" concept (analogous to telomerase) is presented which is consistent with these principles and all data on the distribution of somatic mutations in normal and Ig transgenic mice carrying particular V(D)J and flanking region constructs. It is predicted that in the mouse VH and Vk loci. the J-C intronic Enhancer-Nuclear Matrix Attachment Region (Ei/MAR) contains a unique sequence motif or secondary structure which ensures that only V(D)J sequences mutate whilst other regions of the genome are not mutated.

Molecular characterization of VDJ transcripts from a newborn piglet

The sequences of 42 transcripts, expressed with IgM, IgG and IgA and cloned from the mesenteric lymph node of a newborn piglet, are described. Forty transcripts used either DHA and DHB and their FR4 were identical to the single swine germline JH. The low frequency of somatic mutation made it possible to identify 35/41 as originating from five putative germline VH genes, of which VHA, VHB and VHC accounted for > 85%. The remaining six transcripts were hybrids of these five germline genes. The most 3' functional VH gene (VH2 = VHB) was the only one exclusively expressed with IgM although VHA, of unknown location in the genome, accounted for half of all transcripts. Junctional diversity in CDR3 was extensive and asymmetrical, in that D-J joining contributed more diversity than V-DJ joining. Reading frame II was used twice as frequently as frame III and the CDR3 generated using the former would have a higher expected R/S ratio. This study indicated that the expressed VH repertoire of the newborn piglet is restricted and nearly germline although junctional diversity is mature and better developed than in fetal mice. The hybrid clones suggest that swine compensate for their < 20 VH genes and single JH by using somatic gene conversion. There was no evidence for exclusive or preferential expression of the most 3' VH gene as occurs in chickens and rabbits respectively, and switching to downstream constant regions probably occurs in utero, even in the theoretical absence of environmental antigens and maternal regulatory molecules. Preferential VHA expression is probably a selection phenomenon.

Within germinal centers, isotype switching of immunoglobulin genes occurs after the onset of somatic mutation

Human tonsillar B cells were separated into naive IgD+CD38-CD23- (Bm1) and IgD+CD38-CD23 (Bm2), germinal center IgD-CD38+CD23- centroblasts (Bm3) and IgD-CD38+CD77- centrocytes (Bm4) and memory IgD-CD38- (Bm5) subsets. Previous IgVH sequence analysis concluded that the triggering of somatic mutations occurs during the transition from Bm2 subset into the Bm3 subset. To determine the initiation of isotype switching, sterile transcript expression was analyzed by amplification, cloning, and sequencing. A selective sterile I gamma, I alpha, and I epsilon expression was observed at centrocyte (Bm4) stage, suggesting that isotype switch is triggered within germinal centers, after somatic mutation is initiated with centroblasts (Bm3). Finally, the high level of 5'S gamma-S mu 3' DNA switching circles observed in germinal center B cells indicates that within human tonsils, germinal center is a major location for isotype switching.

Relative contribution of T and B cells to hypermutation and selection of the antibody repertoire in germinal centers of aged mice

The immune system of aged individuals often produces antibodies that have lower affinity and are less protective than antibodies from young individuals. Recent studies in mice suggested that antibodies produced by old individuals may be encoded by distinct immunoglobulin (Ig) genes and that the somatic hypermutation process in these individuals is compromised. The present study employed Ighb scid mice reconstituted with normal lymphocytes from young (2-3-mo-old) and aged (20-25-mo-old) donors and immunized with a protein conjugate of the hapten (4-hydroxy-3-nitrophenyl)acetyl (NP) to determine whether the molecular changes in antibody repertoire reflect senescence in the B cells or whether they are mediated by the aging helper T lymphocytes. The NP-reactive B cells from splenic germinal centers (GC) were recovered by microdissection of frozen tissue sections and their rearranged Ig heavy chain variable region (VH) genes of the V186.2/V3 families were sequenced. It was found that the VH gene repertoire of the GC B cells was strongly influenced by the source of the CD4+ T cells. When T cells were donated by young mice, the anti-NP response in GC was dominated by the canonical V186.2 gene, even if the responder B cells came from aged donors. However, when the mice were reconstituted with T cells from aged donors, the expression of the V186.2 gene by young B cells was diminished and the response was dominated by the C1H4 gene, another member of the V186.2/V3 family. In contrast, the somatic hypermutation process in the GC B cells followed a different pattern. The mutation frequencies in the animals that were reconstituted with both B and T cells from young donors (1/50 to 1/150 bp) were comparable to the frequencies previously reported for NP-immunized intact young/adult mice. However, when either lymphocyte subset was donated by the aged mice, the mutation frequencies declined. Thus, mice reconstituted with T cells from the aged and B cells from the young had severely compromised mutational mechanism. Likewise, the recipients of aged B and young T cells had diminished mutations even though the repertoire of their anti-NP response was dominated by the canonical V186.2 gene. It appears that the change in germine-encoded repertoire and the decrease of somatic hypermutation represent distinct mechanisms of immunosenescence and that the aging of helper T cells plays a pivotal role in both of these processes.

V-gene repertoire and hypermutation of rheumatoid factors produced in rheumatoid synovial inflammation and immunized healthy donors

We have compared RF from normal, immunized donors and RF derived from the synovial tissues of RA patients. We have found a difference in the preferential use of VL and VH gene families. In both conditions, RFs were found to have accumulated somatic mutations. However, there was a striking difference in the patterns of mutation. RFs from normals were characterized by a very low R:S ratio in the CDR1+2, considerably lower than seen among the RARFs. In addition, there was little increase in affinity with increasing numbers of mutations in a group of clonally related RFs from an immunized normal. This contrasts with RF from RA, where there is evidence of both affinity maturation and class switching. Together these data suggest that in healthy persons there is a controlling mechanism to limit the affinity of RF autoantibodies, and that this is lost in RA. The higher affinity of the RA-derived RF may be of significance in the pathology of the disease.

Control of autoantibody affinity by selection against amino acid replacements in the complementarity-determining regions

Rheumatoid factor (RF) autoantibodies can be produced in healthy individuals after infections or immunizations and thus escape normal tolerization mechanisms. It has not been clear whether such autoantibodies can undergo somatic hypermutation and affinity maturation similar to antibodies to exogenous antigens. We have investigated how these autoantibodies are regulated in normal individuals by analyzing the sequences of monoclonal IgM RFs obtained as hybridomas from donors after immunization. The variable regions undergo extensive hypermutation, but in contrast to antibodies against exogenous antigens, there is a strong selection against mutations that result in replacement of amino acids in the hypervariable, or complementarity-determining, regions. Furthermore, we found no increase in affinity of these RFs with the accumulation of mutations. This suggests that high-affinity variants are tolerized during the hypermutation process and there is a peripheral mechanism operating on certain autoreactive B cells that, while not deleting or anergizing all autoreactive cells, prevents the generation of high-affinity autoantibodies. Comparison of RFs by using the VH1 DP-10 heavy chain variable region segment from both normal individuals and rheumatoid arthritis (RA) patients suggests that RF from RA patients may not be subject to such a controlling mechanism.

Molecular characterization of J558 genes encoding tight-skin mouse autoantibodies: identical heavy-chain variable genes code for antibodies with different specificities

Tight-skin mouse, a mutant strain with a single gene defect, develops cutaneous hyperplasia and specific autoantibodies, like humans affected by scleroderma. The autoantibodies produced in the tight-skin mouse are encoded primarily by heavy-chain variable (VH) genes from the J558 family. To understand the genetic basis of production of autoantibodies, we have analyzed the structure of J558 genes encoding these autoantibodies. The results showed that J558 genes encoding these antibodies were not derived from a selected germ-line gene(s) or a single subfamily but were derived from genes belonging to diverse J558 subfamilies. However, two prototype VH genes representing two new subfamilies were found to be repeatedly expressed in their germ-line form in eight independent clones. Autoantibodies with distinct specificities appear to be generated by pairing of similar/identical VH genes with different V kappa genes derived from the same or different families. Fourteen of 18 autoantibodies shared a conserved heptapeptide sequence motif, YNEKFKG, in the second complementarity-determining region of heavy chains. Usage of germ-line genes from diverse J558 subfamilies bearing a common motif to encode autoantibodies suggests a regulatory role for this motif. Thus, selection and expansion of the autoreactive B-cell repertoire in the tight-skin mouse appear to be VH-gene mediated. The frequency of N nucleotide addition at diversity-joining (D-JH) junctions was lower, whereas the frequency of usage of the DFL16 segment was higher. Finally, in contrast to normal and other autoimmune mouse strains, the frequencies of D-D fusions and D inversions were higher in tight-skin mouse total immunoglobulin as well as autoantibody repertoires.

A lambda 1 transgene under the control of a heavy chain promoter and enhancer does not undergo somatic hypermutation

To identify cis-acting elements responsible for targeting somatic hypermutation to immunoglobulin variable regions, we generated transgenic mice which carry a rearranged lambda 1 gene regulated by the heavy chain intron enhancer, E mu, and the heavy chain promoter PH186.2 from the VH186.2 variable region. C57BL/6 x SJL founders were bred with C57BL/6 mice to establish a line carrying a single copy of the transgene. Somatic hypermutation was studied by generating hybridoma cell lines from mice immunized with the hapten (4-hydroxy-3-nitrophenyl)acetyl (NP) coupled to chicken gamma globulin. The immune response in this transgenic line was dominated by the endogenous VH186.2 heavy chain variable region and the transgenic lambda 1 light chain, and the transgene was actively expressed in all hybridomas analyzed. In this work we show that the transgenic V lambda 1 regions do not undergo hypermutation, despite high levels of expression, while the expressed heavy chain V regions accumulate mutations at a rate typical of the NP response in C57BL/6 mice. Thus, within the same B cell, the PH186.2 promoter in connection with E mu drives efficient expression of both a VH and a V lambda region, but only the VH is a target for somatic hypermutation. Our observations show that cis-acting sequences that activate immunoglobulin gene transcription are not sufficient to target somatic hypermutation.