Clonal analysis of the anti-DNA repertoire of murine B lymphocytes

Humoral autoimmunity and transplant vasculopathy: when allo is not enough

For several decades, allograft rejection was believed to be mediated almost exclusively by cellular immune responses, but it is now realized that humoral responses also play a major role. Although directed typically against donor human leukocyte antigen, it is becoming increasingly evident that the antibody response can also target autoantigens that are shared between donor and recipient and that this autoantibody may contribute to graft rejection. Many aspects of transplant-induced humoral autoimmunity remain poorly understood and key questions persist; not least what triggers the response and how autoantibody causes graft damage. Here, we collate results from recent clinical and experimental studies in transplantation and autoimmune diseases to propose answers to these questions.

A new site-directed transgenic rheumatoid factor mouse model demonstrates extrafollicular class switch and plasmablast formation

The AM14 rheumatoid factor (RF) transgenic (Tg) mouse has been valuable for studying how self-reactive B cells are regulated beyond central tolerance, because they remain ignorant in normal mice. AM14 B-cell activation can be studied on autoimmune-prone strains or by inducing activation with IgG2a anti-chromatin antibodies (Abs). Despite the utility of conventional Ig-Tg mice, site-directed Ig-Tg (sd-Tg) mice provide a more physiological model for B-cell responses, allowing class switch and somatic hypermutation. We report here the creation of an AM14 sd-Tg mouse and describe its phenotype on both normal and autoimmune-prone backgrounds. AM14 sd-Tg B cells develop normally but remain unactivated in the BALB/c background, even after significant aging. In contrast, in the autoimmune-prone strain MRL/lpr, AM14 sd-Tg B cells become activated and secrete large amounts of IgG RF Ab into the serum. Class-switched Ab-forming cells were found in the spleen and bone marrow. IgG RF plasmablasts were also observed in extrafollicular clusters in the spleens of aged AM14 sd-Tg MRL/lpr mice. Class switch and Ab secretion were observed additionally in AM14 sd-Tg BALB/c B cells activated in vivo using IgG2a anti-chromatin Abs. Development of IgG auto-Abs is a hallmark of severe autoimmunity and is related to pathogenesis. Using the AM14 sd-Tg, we now show that switched auto-Ab-forming cells develop robustly outside germinal centers, further confirming the extrafollicular expression of activation induced cytidine deaminase (AID). This model will allow more physiological studies of B-cell biology in the future, including memory responses marked by class switch.

Association of alpha-actinin-binding anti-double-stranded DNA antibodies with lupus nephritis

OBJECTIVE

Anti-double-stranded DNA (anti-dsDNA) antibodies may contribute to the pathogenesis of glomerulonephritis (GN) by cross-reacting with alpha-actinin in murine models and in some patients with systemic lupus erythematosus (SLE). We therefore sought to determine possible disease associations with serologic and clinical features and to characterize this new autoantibody specificity.

METHODS

One hundred patients with SLE were recruited into this multicenter study, as well as 100 rheumatic disease controls and 2,100 healthy blood donors. Clinical disease was evaluated by the SLE Disease Activity Index (SLEDAI; excluding the anti-DNA component). Anti-dsDNA antibodies were detected by conventional enzyme-linked immunosorbent assay (ELISA) and by a commercial enzyme immunoassay (EIA). Anti-alpha-actinin antibodies were detected by ELISA, and their specificity was confirmed by Western blotting and by indirect immunofluorescence using rat kidney sections and mesangial cells as substrates. Highly positive sera were selected for absorption experiments and were affinity-purified for cross-reactivity studies and measurement of antibody avidity.

RESULTS

Sera from 62 of the SLE patients had anti-dsDNA antibodies; 21 of these sera also had anti-alpha-actinin antibodies, as compared with 1 of the 38 sera without anti-dsDNA antibodies. Of the 22 patients with anti-alpha-actinin antibodies, 10 had GN, as compared with 14 of the 78 without anti-alpha-actinin antibodies (P < 0.01). In patients with GN, anti-alpha-actinin, but not anti-dsDNA, antibodies correlated with the SLEDAI score (minus the anti-DNA component) and with treatment. The fraction of serum anti-dsDNA antibodies that cross-reacted with alpha-actinin exhibited high avidity for dsDNA, as determined using a commercial EIA for high-avidity anti-dsDNA antibodies and an in-house conventional ELISA.

CONCLUSION

The alpha-actinin-binding antibodies are significantly associated with GN in SLE. Whether such autoantibodies may anticipate the development of this complication of SLE remains to be verified.

T-helper cell intrinsic defects in lupus that break peripheral tolerance to nuclear autoantigens

Special populations of T helper cells drive B cells to produce IgG class switched, pathogenic autoantibodies in lupus. The major source of antigenic determinants (epitopes) that trigger interactions between lupus T and B cells is nucleosomes of apoptotic cells. These epitopes can be used for antigen-specific therapy of lupus. Secondly, the autoimmune T cells of lupus are sustained because they resist anergy and activation-induced programmed cell death by markedly upregulating cyclooxygenase (COX) 2 along with the antiapoptotic molecule c-FLIP. Only certain COX-2 inhibitors block pathogenic anti-DNA autoantibody production in lupus by causing death of autoimmune T helper cells. Hence COX-2 inhibitors may work independently of their ability to block the enzymatic function of COX-2, and structural peculiarities of these select inhibitors may lead to better drug discovery and design.

Major peptide autoepitopes for nucleosome-centered T and B cell interaction in human and murine lupus

The potential cross-reactivity of normal T and B cells to nuclear antigens is vast, probably due to their "education" by apoptotic cell antigens in generative lymphoid organs. Despite this "nucleocentric repertoire," as we call it, the peripheral immune system normally remains tolerant or ignorant of the products of apoptosis. However, the T helper (Th) cells, and also B cells of lupus, have a regulatory defect in the expression of CD40 ligand (CD40L). A sustained hyper-expression of CD40L by lupus T cells can be triggered by sub-threshold stimuli, and is associated with impaired phosphorylation of Cbl-b, a critical downregulatory molecule in T cell signal transduction. This CD40L hyper-expression abnormally prolongs co-stimulatory signals to autoimmune B cells, and it probably instigates APC (dendritic cells, resting anti-DNA B cells, and macrophages) to present apoptotic cell autoantigens in an immunogenic fashion. We have identified the dominant nucleosomal epitopes that are critical for cognate interactions between autoimmune Th cells and anti-DNA B cells in lupus. By scanning of overlapping synthetic peptides, and by mass spectrometry of naturally processed peptides, five major epitopes in nucleosomal histones were localized, namely H1'(22-42), H2B(10-33), H3(85-105), H4(16-39), and H4(71-94). The autoimmune T cells as well as B cells of lupus recognize these epitopes, and with age, autoantibodies against the peptide epitopes cross-react with nuclear autoantigens. Moreover, the peptide autoepitopes can be promiscuously presented and recognized by lupus T cells in the context of diverse MHC alleles. This cross-reactivity opens up the possibility of developing "universally" tolerogenic peptides for therapy of lupus in humans despite their MHC diversity. Indeed, tolerogenic therapy with a single histone peptide epitope can halt the progression of established glomerulonephritis in lupus-prone mice by "tolerance spreading" that inactivates a broad spectrum of autoimmune T and B cells in concert.

From T to B and back again: positive feedback in systemic autoimmune disease

Systemic lupus erythematosus, a prototypical systemic autoimmune disease, is the result of a series of interactions within the immune system that ultimately lead to the loss of self-tolerance to nuclear autoantigens. Here, we present an integrated model that explains how self-tolerance is initially lost and how the loss of tolerance is then amplified and maintained as a chronic autoimmune state. Key to this model are the self-reinforcing interactions of T and B cells, which we suggest lead to perpetuation of autoimmunity as well as its spread to multiple autoantigen targets.

Early cellular events in systemic autoimmunity driven by chromatin-reactive T cells

In vivo exposure of the thymus of normal mice to procainamide-hydroxylamine, a lupus-inducing drug, causes development of chromatin-reactive T cells. Autoantibodies subsequently appear, but their origin and significance are unknown. The current studies were undertaken to determine the specificities of B cells that respond to chromatin-reactive T cells at the initiation of this autoimmune process. Three days after adoptive transfer of 6 x 10(6) chromatin-reactive T cells, B cells with the capacity to secrete IgM anti-chromatin antibodies were detected in 1/10(6) splenocytes, and these became 10- to 50-fold more numerous if either the donor T cells or the recipient had defective Fas due to the lpr allele. Five days later these mice developed IgG anti-chromatin-secreting B cells at a precursor frequency of 3-6 x 10(-5). B cells with dDNA-binding activity isolated from mice primed in vivo to a complex of methylated pigeon cytochrome c and dDNA could stimulate naive, cytochrome c-reactive T cells in vitro, demonstrating that B cells can internalize dDNA-bound proteins through their dDNA immunoblobulin receptor and can functionally present a T cell epitope. However, no capacity of chromatin for binding anti-dDNA antibodies was detected, and IgM dDNA-specific B cells did not expand when challenged with chromatin-reactive T cells in vivo. The rapid and robust expansion of anti-chromatin-secreting B cells indicates that the normal immune repertoire includes nontolerant autoreactive B cells that respond to strong T cell drive and are readily manifested if Fas-mediated activation-induced cell death is inhibited.

Persistence of Autoreactive T Cell Drive Is Required to Elicit Anti-Chromatin Antibodies in a Murine Model of Drug-Induced Lupus

Long-term treatment with procainamide and numerous other medications is occasionally associated with the development of drug-induced lupus. We recently established a murine model for this syndrome by disrupting central T cell tolerance. Two intrathymic injections of procainamide-hydroxylamine (PAHA), a reactive metabolite of procainamide, into (C57BL/6 × DBA/2)F1 mice resulted in the appearance of chromatin-reactive T cells and anti-chromatin autoantibodies. The current study explores in this model the role of autoreactive T cells in autoantibody production and examines why autoantibodies after a single intrathymic drug injection were much more limited in isotype and specificity. Injection of as few as 5000 chromatin-reactive T cells into naive, syngeneic mice induced a rapid IgM anti-denatured DNA response, while injection of at least 100-fold greater number of activated T cells was required for induction of IgG anti-chromatin Abs, suggesting that small numbers of autoreactive T cells can be homeostatically controlled. Mice subjected to a single intrathymic PAHA injection after receiving splenic B cells from an intrathymic PAHA-injected syngeneic donor also developed anti-chromatin Abs, but adoptive transfer of similarly primed T cells or of B cells without intrathymic PAHA injection of the recipient failed to produce an anti-chromatin response. However, anti-chromatin Abs developed after a single intrathymic PAHA injection in Fas-deficient C57BL/6-lpr/lpr mice, suggesting that activation-induced cell death limited autoimmunity in normal mice. Taken together, these results imply that chromatin-reactive T cells produced by intrathymic PAHA created a B cell population primed to somatically mutate and Ig class switch when subjected to a heavy load or second wave of autoreactive T cells.

Autoreactive B cell regulation: peripheral induction of developmental arrest by lupus-associated autoantigens

Anti-Sm and anti-ssDNA transgenic (Tg) mice were generated using the VH-D-JH rearrangement of an anti-Sm hybridoma of MRL/Mp-lpr/lpr origin. B cells of each specificity account for 15%-35% of the splenic repertoire, but no circulating anti-Sm or anti-ssDNA antibodies are detected. Most autoreactive cells exhibit an immature B cell phenotype and have short half-lives equivalent to those of non-Tg immature B cells. However, at least some anti-Sm B cells are functional, because immunization with murine snRNPs induces anti-Sm secretion. We propose that anti-Sm and anti-ssDNA are eliminated during the transition to mature B cells and that this late stage of tolerance induction is consequential to their spontaneous activation in murine lupus.

A disease-related rheumatoid factor autoantibody is not tolerized in a normal mouse: implications for the origins of autoantibodies in autoimmune disease

We have analyzed B cell tolerance in a rheumatoid factor (RF) transgenic mouse model. The model is based on AM14, a hybridoma, originally isolated from an autoimmune MRL/lpr mouse that has an affinity and specificity typical of disease-related RFs from this strain. AM14 binds to immunoglobulin (Ig)G2a of the "a" allotype (IgG2aa) and not to IgG2ab. Thus, by crossing the transgenes onto an IgHa (BALB/c) background or to a congenic IgHb (CB.17) background, we could study the RF-expressing B cells when they were self-specific (IgHa) or when they were not self-specific (IgHb). These features make the AM14 model unique in focusing on a true autoantibody specificity while at the same time allowing comparison of autoreactive and nonautoreactive transgenic B cells, as was possible in model autoantibody systems such as anti-hen egg lysozyme. Studies showed that AM14 RF B cells can make primary immune responses and do not downregulate sIgM, indicating that the presence of self-antigen does not induce anergy of these cells. In fact, IgHa AM14 transgenic mice have higher serum levels of transgene-encoded RF than their IgHb counterparts, suggesting that self-antigen-specific activation occurs even in the normal mouse background. Since AM14 B cells made primary responses, we had the opportunity to test for potential blocks to self-reactive cells entering the memory compartment. We did not find evidence of this, as AM14 B cells made secondary immune responses as well. These data demonstrate that a precursor of a disease-specific autoantibody can be present in the preimmune repertoire and functional even to the point of memory cell development of normal mice. Therefore, immunoregulatory mechanisms that normally prevent autoantibody production must exert their effects later in B cell development or through T cell tolerance. Conversely, the data suggest that it is not necessary to break central tolerance, even in an autoimmune mouse, to generate pathologic, disease-associated autoantibodies.

Constitutive expression of bcl-2 in B cells causes a lethal form of lupuslike autoimmune disease after induction of neonatal tolerance to H-2b alloantigens

The bcl-2 protooncogene has been shown to provide a survival signal to self-reactive B cells, but it fails to override their developmental arrest after encounter with antigen. Furthermore, constitutive expression of bcl-2 in B cells does not promote the development of autoimmune disease in most strains of mice, indicating that signals other than those conferred by bcl-2 are required for long-term survival and differentiation of self-reactive B cells in vivo. To further examine the factors that are required for the pathogenesis of autoimmune disease, we have assessed the effect of bcl-2 overexpression on the development of host-versus-graft disease, a self-limited model of systemic autoimmune disease. In this model, injection of spleen cells from (C57BL/6 x BALB/c)F1 hybrid mice into BALB/c newborn parental mice induces immunological tolerance to donor tissues and activation of autoreactive F1 donor B cells through interactions provided by allogeneic host CD4+ T cells. BALB/c newborns injected with spleen cells from (C57BL/6 x BALB/c)F1 mice expressing a bcl-2 transgene in B cells developed high levels of anti-single-stranded DNA and a wide range of pathogenic autoantibodies that were not or barely detectable in mice injected with nontransgenic spleen cells. In mice injected with transgenic B cells, the levels of pathogenic autoantibodies remained high during the course of the study and were associated with long-term persistence of donor B cells, development of a severe autoimmune disease, and accelerated mortality. These results demonstrate that bcl-2 can provide survival signals for the maintenance and differentiation of autoreactive B cells, and suggest that both increased B cell survival and T cell help play critical roles in the development of certain forms of systemic autoimmune disease.

Involvement of B-1 cells in mucosal immunity and autoimmunity

B-1 cells are distinguished from conventional B cells by their anatomical localization, surface phenotypes and functional characteristics. The physiological functions and pathological roles of these cells remain controversial. In this review, Masao Murakami and Tasuku Honjo summarize recent evidence for the involvement of B-1 cells in mucosal immunity and autoimmunity, and discuss the relationship between these phenomena.

Breakdown of B cell tolerance in a mouse model of systemic lupus erythematosus

Anti-DNA antibodies, specifically those that stain nuclei in a homogenous nuclear (HN) fashion, are diagnostic of systemic lupus erythematosus (SLE) and the MRL-lpr/lpr SLE murine model. We have used a heavy chain transgene that increases the frequency of anti-HN antibodies to address whether their production in SLE is the consequence of a defect in B cell tolerance. Anti-HN B cells were undetectable in nonautoimmune-prone transgenic mice, but in MRL-lpr/lpr transgenic mice their Ig was evident in the sera and they were readily retrievable as hybridomas. We conclude that nonautoimmune animals actively delete anti-HN-specific B cells, and that MRL-lpr/lpr mice are defective in this process possibly because of the lpr defect in the fas gene.

Human CD5-positive B cells in lymphoid malignancy and connective tissue diseases

The current literature on human CD5-positive B cells (CD5 + B cells) has been analysed, with a special emphasis on non organ-specific auto-immune diseases. Malignant cells of most of the chronic lymphoid leukaemias of the B cell lineage express the CD5 molecule. Antibodies of the IgM class produced by leukaemic B cells are multispecific auto-antibodies. The CD5 + B cell subset may be expanded in non organ-specific autoimmune diseases, such as rheumatoid arthritis, primary Sjögren's syndrome, systemic lupus erythematosus. This holds true for various conditions, including organ-specific auto-immune diseases. Since auto-immune features are common in lymphoproliferative disorders, and the latter be a complication in non organ-specific auto-immune diseases, CD5 + B cells may represent an intermediary between these auto-immune diseases and B cell lymphoproliferations. Studies on the regulation of CD5 + B cell production and function are likely to shed light on the aetiology of, and pathogenetic mechanisms operating in the different disease states.

Differential sensitivity to interleukins of CD5+ and CD5- anti-DNA antibody-producing B cells in murine lupus

We studied the effects of interleukins (IL) on in vitro IgM and IgG anti-DNA antibody production by splenic B cells from autoimmune disease-prone NZBxNZW (NZB/W) F1 mice. It was found that different interleukins regulate phenotypically distinct B cells producing separate isotype of anti-DNA antibodies. IL-2 slightly but significantly inhibited the production of IgM anti-DNA antibodies. IL-4 and IL-6 significantly enhanced the antibody production, but the effects were not so marked and inconsistent, particularly with respect to IL-6. By contrast, the effects of IL-5 were remarkable, particularly on splenic B cells from young mice. As for IgG anti-DNA antibodies, IL-6, but not other interleukins, markedly up-regulated the antibody production by splenic B cells from mice over 6 months of age, in a dose dependent fashion. Thus, the ability of B cells to produce IgG anti-DNA antibodies appears to be dependent on the surface expression of IL-6 receptor (IL-6R) at the ages when the mice begin to develop the disease. Studies of the surface phenotypes showed that while the IL-5-sensitive major IgM anti-DNA producers were CD5+Lp-3(CD43)-sIgM+, the IL-6-sensitive major IgG anti-DNA producers were CD5-Lp-3+sIgM-. However, significant amounts of IgG antibodies were also produced, in the presence of IL-6, by CD5+Lp-3+sIgM+, but not by CD5-Lp-3+sIgM+ B cells from 6-month-old mice. We suggest that surface phenotypes of anti-DNA antibody producers change from CD5+Lp-3-sIgM+IL-5R+, CD5+Lp-3+sIgM+IL-6R+ and subsequently to CD5-Lp-3+sIgM-(sIgG+)IL-6R+ in NZB/W F1 mice with aging.

Regulatory aspects of clonally expanded B-1 (CD5+ B) cells

B-1 (CD5+ B) cells appear early in ontogeny, produce mainly unmutated polyreactive antibodies, and are capable of self-renewal. B-1 cells clonally expand with age and are the malignant cell in chronic lymphocytic leukemia. In this report immunological analysis of B-1 malignancies in NZB mice, a murine model of chronic lymphocytic leukemia, is related to current information on B-1 cells. B-1 clones from NZB mice produce high levels of interleukin-10, detected at the RNA level by semi-quantitative polymerase chain reaction. In addition, the B-1 malignant clones in NZB mice and their hybrids, are negative for B220/6B2 expression, the B-specific antigenic form of CD45 which is a membrane-associated phosphatase involved in lymphocyte activation. Both the autocrine production by B-1 cells of interleukin-10 and altered CD45 expression may be responsible for the clonal expansion of these cells, as well as the accompanying T cell expansion. We report the establishment of an in vitro cytotoxic CD8+ T cell line derived from an NZB with a B-1 malignancy. The effect of B-1 cell-derived interleukin-10 on subsets of T lymphocytes may account for the immunoregulatory properties of B-1 cells. In addition, the NZB malignancies were also characterized for immunoglobulin variable region sequence and antigen specificity. The B-1 malignancies produced immunoglobulin derived from unmutated germline sequences with no N base substitutions. It appears that both the immunoglobulin and interleukin-10 produced by the B-1 malignant cell in NZB mice may have immunoregulatory properties. A study of B-1 malignancies may shed light on the immunoregulatory properties of non-clonally expanded normal B-1 cells.

Correlation of antibody multireactivity with variable region primary structure among murine anti-erythrocyte autoantibodies

A high proportion of the antibodies in the preimmune repertoire bind to several unrelated antigens and are considered to be multireactive. This property is reportedly associated with the antibodies produced by CD5+ B lymphocytes. Because many antibodies specific for bromelain-treated mouse red blood cells (BrMRBC) derive from CD5+ B cells, we tested monoclonal antibodies of this specificity for multireactivity. Two variable region combinations, VH11/V kappa 9 and VH12/V kappa 4, account for greater than 80% of this repertoire, but none of these antibodies exhibited a multireactive phenotype. In contrast, three anti-BrMRBC binding antibodies belonging to the J558 family (BrM1, BrM8, and CH12) showed varying degrees of multireactivity, and bound both highly negatively and positively charged antigens. The amino acid sequences of the VH regions of these antibodies are highly homologous (greater than 85% identical) and they possess large VH-D-J junctions with extensive N-region insertions. The kappa chains of two of these antibodies utilize an identical V kappa gene segment, while the third uses a very different V kappa with only 50% homology. The entire H chain V regions of these antibodies are unusually basic, with isoelectric points of 9.5-10, a feature which might be important in promoting interactions with acidic epitopes. The multireactive antibodies also contain regions with a high concentration of hydroxylside chain amino acids, especially in their VH-D-J junctions. This region also contains acidic amino acid residues, which may be important in binding of positively charged epitopes. We propose that an open, accessible binding site and a charge polarity may be features which facilitate the binding of charged epitopes, providing a structural basis for multireactivity of at least some antibodies.

Spectrotypes of anti-DNA antibodies show that anti-DNA-secreting B-cell clones of SLE patients are restricted in number, stable and long lived

We have investigated the number of B-lymphocyte clones secreting anti-ssDNA antibodies in SLE patients and a chronic active hepatitis patient by isoelectric focusing and reverse immunoblotting of serum antibodies. Individual clones can be identified by the unique pattern of bands produced by their antibodies (the clonotype). Using this technique, we have shown that the anti-DNA response of the majority of SLE patients is clonally restricted, in many cases only a single B-cell clone responding. We have also measured qualitative and quantitative changes in expression of B-cell clones and shown that these clones are remarkably stable with lifespans of up to six years or more. These results are in agreement with previous observations of clonal restriction of the anti-DNA response in three mouse models of SLE and in addition show that, unlike the mouse models, human anti-DNA-secreting B-cell clones are extremely stable and long-lived. The implications of these results for models of initiation and regulation of the autoimmune response are discussed.

Qualitative difference of anti-DNA antibody-producing cell precursors in the pre-immune B cell repertoire between normal and lupus-prone mice

The precursor frequency for anti-DNA antibody-producing cells in the pre-immune B cell repertoire was investigated in young female BALB/c and NZW mice, and in young and aged female NZB x NZWF1 (B/WF1) mice. Spleen cells from these mice were diluted serially and stimulated polyclonally in vitro with lipopolysaccharide (LPS) and IL-4 to induce both IgM and IgG1 production. The results demonstrated that there existed virtually no difference in precursor frequency for IgM anti-DNA antibody-producing cells between normal and lupus mice, confirming previous observations made by other investigators. In contrast, the number of precursors for IgG1 anti-DNA antibody-producing cells was much higher in young and old B/WF1 mice than in normal mice. These results suggest that the high frequency of precursors for IgG1 anti-DNA antibody-producing cells in the pre-immune B cell repertoire of B/WF1 mice is a crucial factor for the pathogenesis of systemic lupus erythematosus.

Identification of components of the endoplasmic reticulum and Golgi complex by murine autoreactive monoclonal antibodies

A number of autoreactive monoclonal antibodies have been produced by the fusion of spleen cells from unprimed BALB/c mice. The specificities of two of these monoclonal autoantibodies, MUI 38 and MUI 100, have been further examined. By indirect immunofluorescence, monoclonal antibody MUI 38 showed discrete perinuclear staining of acetone-fixed murine 3T3 fibroblasts, which was similar to that obtained with the Golgi vital stain, C6-NBD-ceramide, and with rhodamine-labelled wheat germ agglutinin. Furthermore, the staining pattern with antibody MUI 38 in cells treated with either monensin, taxol or nocodazol was altered in a manner consistent with the known effects of these drugs on Golgi morphology. In contrast, monoclonal antibody MUI 100 showed a diffuse cytoplasmic staining pattern, similar to FITC-Con A, indicative of reactivity with the endoplasmic reticulum. At high dilutions antibody MUI 100 showed only a perinuclear staining pattern, indicating that MUI 100 reacted with the Golgi as well as the endoplasmic reticulum. Both monoclonal antibodies are IgM kappa class and both showed reactivity with acetone-fixed fibroblasts from a number of species, indicating that the antigens are highly conserved. By immunoblotting with total membranes of murine 3T3 cells under either reducing or non-reducing conditions, monoclonal antibody MUI 100 reacted with a number of components with apparent molecular weights (MW) from 27,000 to 63,000. This reactivity was abolished when the 3T3 membranes were treated with sodium periodate, indicating antibody MUI 100 may be specific for carbohydrate. In addition, MUI 100, but not MUI 38, possessed rheumatoid factor activity, reacting with IgG from normal sera of a number of different species. Furthermore, monoclonal antibody MUI 100 was shown to be specific for the Fc domain of IgG. Absorption of MUI 100 antibody with normal rabbit IgG-Sepharose reduced the anti-endoplasmic reticulum reactivity, therefore both activities are attributable to the same antibody.

Evidence for regulation of naturally activated autoreactive B cells

A large fraction of naturally activated B cells in the neonate displays degenerate specificity, including reactivity with autoantigens. Transgenic mouse models of autoreactive B cells are mainly concerned with monospecific B cells of high avidity, and the fate of naturally activated autoreactive B cells is still a matter of debate. To pursue this question further, we chose an IgM autoantibody with a recurrent idiotype (Id), i.e. Sp6, because transgenic mice expressing this IgM also were available. In a first approach monoclonal antibodies (mAb) derived from untreated, antigenically stimulated and transgenic mice were used to test whether there were indications for deletion or for Id regulation of naturally activated autoreactive B cells. Over 90% of thymus and spleen cell derived hybridomas from 6-day-old Sp6-transgenic mice were trinitrophenyl (TNP) reactive, carried the Sp6-Id and bound to a panel of self antigens, including mouse albumin. We failed to obtain B cell hybridomas from the thymus of 28-day-old Sp6-transgenic mice. Furthermore, we could not detect any mAb carrying an anti-Sp6 Id, but Sp6 did weakly bind to itself. About 25% of mAb derived from control mice displayed degenerate specificity, the majority of them also were TNP reactive. The Sp6 Id was found at a low frequency and a comparable number of mAb carried an anti-Sp6 Id. Prenatal manipulation at the antigen level (trinitrobenzenesulfonic acid treatment) led to a transient expansion of TNP- and autoreactive mAb. The number of mAb carrying the Sp6 Id was not increased, but mAb carrying an anti-Sp6 Id were observed at high frequency. Those mAb also displayed degenerate specificity. Since Sp6-transgenic mice were perfectly healthy, it is concluded that this particular autoreactive antibody of degenerate specificity cannot be harmful for the developing organism, which may possibly be due to its self-binding capacity. Furthermore, some process of down-regulation was indicated by the absence of B cells expressing the transgene in the thymus of young adult mice. Autoreactivity of untreated and prenatally antigen-treated mice was, in addition, regulated at the Id level. In particular, mAb recognizing the Id of Sp6 were significantly expanded in antigenically stimulated mice. The data were interpreted in the sense that autoreactive B cells appearing early during ontogeny were rather strictly controlled either by (functional) clonal deletion or by idiotypic connectivity.

The contribution of I-Abm12 to the production of autoantibodies to dsDNA

The development of IgG autoantibodies to dsDNA in NZBxNZW F1 (NZB/W) and NZBxSWR F1 (SNF1) mice have been linked to specific alleles of MHC class II genes contributed by the NZW and SWR parents respectively. Recently, our laboratory has shown that the introduction of the bm12 mutation into NZB mice (NZB.H-2bm12) results in mice which are phenotypically similar to NZB/W F1 mice and, in particular, develop IgG anti-dsDNA antibodies. A variety of immune abnormalities have been described in autoimmune NZB (H-2d) mice. It is, however, unclear at present, whether all these abnormalities are due to the influence or effect of a single set of linked genes or due to multiple genes. It was reasoned that NZB.H-2bm12 mice provide a unique opportunity to examine this issue. Specifically, we bred a series of five different F1 colonies of mice: (a) NZB.H-2bm12/b F1; (b) NZB.H-2bm12/d F1; (c) NZB-H-2b/d F1; (d) NZB-H-2bm12 x B6.C-2bm12 F1 (NZB/B6.H-2bm12 F1); and (e) NZB x B6.C-H-2bm12 F1 (NZB/B6.H-2d/bm12 F1) mice. All groups of mice were serially followed for the appearance of IgM and IgG anti-ssDNA and anti-dsDNA antibodies, splenic CFU-B, spontaneous secretion of IgM, FMF analysis, proteinuria and survival. We report herein that H-2bm12 genes have a dominant influence on the appearance of IgG anti-dsDNA antibodies. In contrast, antibodies to ssDNA, IgM secreting cells, CFU-B and Ly-1 B cells are linked to genes from the NZB background. Finally, we particularly note an absence of IgG antibodies to dsDNA in NZB-H-2b/d F1 mice.

Idiotypic profile of natural autoantibodies in newborn and young adult BALB/c mice

Idiotypic profiles of autoreactive monoclonal antibodies (MoAb) were evaluated by their reactivity with a panel of alkaline phosphatase (AP)-coupled detector MoAb derived from the same fusions. Attention was given to the question of whether differences exist between MoAb derived from spleen cells (SC) or thymocytes (TC) and whether ID profiles would change during post-natal development. In the newborn, natural autoantibodies and MoAb which did not react with any one of eight autoantigens displayed different ID profiles, autoreactive MoAb being characterized by the expression of a restricted pattern of ID. During post-natal development, changes of ID expression were only observed with autoreactive MoAb. Many ID which were detected on MoAb derived from 6-day-old mice were not detected on SC-derived MoAb from young adults, while a few ID were significantly over-represented. Furthermore, especially with TC-derived MoAb, a clear linkage between certain idiotypes and autoantigen specificities could be demonstrated. Thus, in contrast to non-autoreactive MoAb, natural autoantibodies in the young adult were characterized by expressing only a selected number of ID at high frequency. Furthermore, the B-cell environment apparently played a role, since there were marked differences between ID profiles of TC- versus SC-derived MoAb. The data are interpreted in the sense that expansion and maturation of naturally activated autoreactive B cells are controlled rather than being random processes.

Autoreactive antibodies in thymus and spleen of neonatal and young adult BALB/c mice: influence of prenatal tolerization

The repertoire of autoantibody-producing B cells was evaluated in a collection of spleen- and thymus-derived hybridomas from 6- and 28-day-old BALB/c mice, which were untreated or prenatally tolerized with trinitrobenzenesulphonic acid (TNBS). MoAb were tested for their reactivity with TNP-BSA and the autoantigens thyroglobulin (TG), myoglobin (MG), actin (AC), cytochrome C (CY), collagen (CO), transferrin (TF), single-stranded DNA (ssDNA), double-stranded DNA (dsDNA), and bromelain-treated mouse red blood cells (BrMRBC). More than 10% of spleen cell (SC)-derived MoAb from 6- and 28-day-old control mice did bind to AC, ssDNA, dsDNA, MY, and TG, the frequency of MoAb reacting with MY, TG, and BrMRBC increasing with age. Thymus cell (TC)-derived hybridomas contained autoreactive clones too, but only few of them produced multireactive MoAb. MoAb from prenatally TNBS-treated mice were more frequently autoreactive than MoAb from control mice, especially if derived from TC hybridomas. The most remarkable difference in the reactivity pattern as compared with MoAb from untreated mice consisted of a significant increase in the frequency of TG-, My-, ssDNA- and above all dsDNA-reactive MoAb, all TC-derived multireactive MoAb binding to dsDNA. The differences in autoreactivity between MoAb from prenatally untreated and TNBS-treated mice as well as age- and organ-related variations support the interpretation that part of the repertoire of naturally activated B cells is not random but is influenced by and responding to the available panel of self antigens.

Frequency and phenotypic feature of autoantibody-producing cell precursors in the preclinical stage of murine lupus

The present study addresses the question of whether there is a difference in the frequencies of autoantibody-producing B-cell precursors in healthy compared with lupus-prone mouse strains. Spleen mononuclear cells (MNC) from 4-week-old (i.e. at the preclinical stage of lupus) mice were activated in vitro for 3 and 6 days with lipopolysaccharides (LPS) and the numbers of IgG, IgA and IgM autoantibody-producing cells were analysed by the ELISPOT assay. The results indicate a high frequency of IgM autoantibody-secreting cells after both 3 and 6 days in vitro stimulation. In spite of high frequencies of IgG-producing cells appearing late during the course of LPS stimulation, no IgG or IgA autoantibody producing cells were detected. No significant differences in the autoantibody repertoire were noted between healthy and lupus-prone mice, indicating that independent of the genetic background the immune system has the capacity to react with autoantibody production. Phenotypic analysis of LPS-induced, IgM-secreting B cells showed clearly that the majority of them were surface IgM+, CD5+ but Thy-1-.

Lipopolysaccharide and dexamethasone induce mouse mammary tumor proviral gene expression and differentiation in B lymphocytes through distinct regulatory pathways

Endogenous mouse mammary tumor virus (MMTV) proviral transcripts are up regulated during the normal course of B-lymphocyte differentiation. We report here that the regulatory mechanisms which lead to increased levels of MMTV transcripts in differentiating, lipopolysaccharide (LPS)-stimulated normal B cells and in the inducible B-cell lymphoma line CH12 are at least partially distinct from those controlling increases in immunoglobulin and J-chain gene expression. In studies designed to characterize the stimulatory pathways leading to MMTV expression in CH12 cells, we found that stimulation with either LPS or dexamethasone (Dex), a transcriptional activator of MMTV genes, induced not only MMTV expression but also differentiation to antibody secretion. Only Dex-induced and not LPS-induced MMTV expression and differentiation were inhibited by the glucocorticoid antagonist RU486, demonstrating that Dex and LPS stimulate B cells by distinct molecular pathways. Therefore, in B cells, MMTV expression can be regulated via either the conventional hormone receptor-dependent pathway or a hormone receptor-independent pathway. Furthermore, these results suggest that steroid stimulation of B cells can lead to alterations in the expression of other results suggest that steroid stimulation of B cells can lead to alterations in the expression of other steroid-responsive genes that can become involved in the process of B-cell differentiation.

Lipopolysaccharide and dexamethasone induce mouse mammary tumor proviral gene expression and differentiation in B lymphocytes through distinct regulatory pathways

Endogenous mouse mammary tumor virus (MMTV) proviral transcripts are up regulated during the normal course of B-lymphocyte differentiation. We report here that the regulatory mechanisms which lead to increased levels of MMTV transcripts in differentiating, lipopolysaccharide (LPS)-stimulated normal B cells and in the inducible B-cell lymphoma line CH12 are at least partially distinct from those controlling increases in immunoglobulin and J-chain gene expression. In studies designed to characterize the stimulatory pathways leading to MMTV expression in CH12 cells, we found that stimulation with either LPS or dexamethasone (Dex), a transcriptional activator of MMTV genes, induced not only MMTV expression but also differentiation to antibody secretion. Only Dex-induced and not LPS-induced MMTV expression and differentiation were inhibited by the glucocorticoid antagonist RU486, demonstrating that Dex and LPS stimulate B cells by distinct molecular pathways. Therefore, in B cells, MMTV expression can be regulated via either the conventional hormone receptor-dependent pathway or a hormone receptor-independent pathway. Furthermore, these results suggest that steroid stimulation of B cells can lead to alterations in the expression of other results suggest that steroid stimulation of B cells can lead to alterations in the expression of other steroid-responsive genes that can become involved in the process of B-cell differentiation.

Autoantibodies, LY-1, and immunoglobulin V gene expression in hybridomas obtained from young and from old New Zealand black mice

We obtained a large number of hybridomas from 1-month-old and 16-month-old New Zealand black mice to study the fine specificities of the autoantibodies produced, the expression of Ly-1, and the expression of the immunoglobulin V gene families in this autoimmune strain. Analysis of the autoantibody specificities yielded 2 major classifications: those specific for a single autoantigen and those that exhibited multispecific binding. Among the multispecific antibodies, 2 categories were found: an antigen-inhibitable group and an antigen-noninhibitable group. A large proportion of VHJ558 and VH7183 gene families was observed in hybridomas obtained from 1-month-old mice, and in hybridomas obtained from 16-month-old mice, there was a large proportion of VHJ558 and VH36-60 gene families. Among the autoantibody kappa chains secreted by the hybridomas, there was a higher frequency of the V kappa 1, V kappa 8, and V kappa 9 gene families. Autoantibodies were produced by both the Ly-1+ and the Ly-1- B cell subsets.

In vitro autoantibody production by normal adult and cord blood B cells

To investigate the repertoire of autoantibodies in humans, anti-DNA and rheumatoid factor (RF) production in vitro was assessed in cultures of adult peripheral blood B cells and neonatal umbilical venous blood B cells. B cells were stimulated under various culture conditions, using an immobilized monoclonal anti-CD3 antibody and adult T cells or Staphylococcus aureus (SA) in the presence or absence of adult T cells or factors derived from mitogen-stimulated adult T cells as polyclonal B cell activators. Total IgM, as well as IgM anti-DNA and RF, were assessed by ELISA. Total IgM production was induced from adult and neonatal B cells with SA plus T cell factors, as well as anti-CD3-stimulated T cells. RF was induced from adult and cord blood B cells by either mode of stimulation, whereas significant anti-DNA production was observed only when B cells were stimulated with anti-CD3-activated T cells. These results confirm the presence of B cell precursors for autoantibodies in the preimmune as well as normal adult repertoire, and indicate that the production of anti-DNA and RF appears to be regulated independently.

CD5 mRNA expression and auto-antibody production in early human B cells immortalized by EBV

A number of studies have suggested that lymphocytes producing polyreactive antibodies belong to the CD5+ B-cell subset. In this study we have examined CD5 at the cell surface and mRNA levels in EBV-driven cord blood and fetal liver clones previously characterized in terms of their antibody specificities. We show that EBV-immortalized cells can express surface CD5, and that some of the clones not expressing surface CD5 express it at the mRNA level. The complete absence of CD5 mRNA in some polyreactive clones is consistent with the proposition that the production of auto-antibodies and multispecific antibodies is not restricted to the CD5+ B-cell subset.

Distinct surface phenotypes of B cells responsible for spontaneous production of IgM and IgG anti-DNA antibodies in autoimmune-prone NZB x NZW F1 mice

Autoimmune-prone NZB x NZW F1 (B/W F1) mice produce a high titer of anti-DNA antibodies, In vivo and in vitro studies showed that in the early life of these mice, the immunoglobulin isotype of these antibodies almost exclusively belongs to IgM class, however, IgG anti-DNA antibodies begin to develop when the mice are about 5-6 months old and the titer exceeds that of IgM antibodies from age 7 months on. We asked whether or not the B cell population responsible for IgM and IgG antibody production belongs to the same lineage. The surface phenotypes of B cell populations responsible for the spontaneous production of either IgM or IgG anti-DNA antibodies were examined using panning and sorting methods with several monoclonal antibodies to B cells, including CD5 (Ly-1) and Lp-3; the latter defines a unique B cell differentiation antigen. We obtained evidence that surface phenotypes of B cells secreting IgM anti-DNA antibodies belong to CD5+ Lp-3- and those of B cells secreting IgG anti-DNA antibodies which occur only in old B/W F1 mice belong to CD5- Lp-3+ subpopulations. The majority of peritoneal B cells were CD5+ Lp-3+ throughout the life span of the mice and anti-DNA antibody production was never evidenced. These findings were discussed in relation to age-associated changes of B cell populations in the spleen of this strain of mice.

Anti-DNA antibodies from autoimmune mice arise by clonal expansion and somatic mutation

The proximate cause of autoantibodies characteristic of systemic autoimmune diseases has been controversial. One hypothesis is that autoantibodies are the result of polyclonal nonspecific B cell activation. Alternatively, autoantibodies could be the result of antigen-driven B cell activation, as observed in secondary immune responses. We have approached this question by studying monoclonal anti-DNA autoantibodies derived from unmanipulated spleen cells of the autoimmune MRL/lpr mouse strain. This analysis shows that anti-DNAs, like rheumatoid factors (19), are the result of specific antigen-driven stimulation. In addition, correlation of sequences with fine specificity shows that: (a) somatic mutations can cause specificity for dsDNA and that such mutations are selected for; (b) arginine residues play an important role in determining specificity; and (c) anti-idiotypes that recognize the majority of anti-DNA are probably not specific for any one family of V regions.

Immunologic tolerance to DNA in B cell lines from both normal and autoimmune mice

We examine whether B cell lines enriched for DNA specificity from either autoimmune (BWF1) or normal mice (Balb/c) can be rendered unresponsive to autoantigen in terms of the specific suppression of direct antibody-forming cells to DNA. These B cell lines were both Lyt-1 positive and negative. Preincubation with oligonucleotide, covalently linked to mouse gamma-globulin, specifically suppressed the antigen-driven response elicited by DNA horse red blood cells in B cell lines from both strains of mice. There is a 5-fold difference in susceptibility to DNA-specific tolerance induction between B cell lines of BWF1 and Balb/c mice. Thus, B cells from autoimmune mice do not appear to have an inherent absolute defect in being rendered tolerant to autoantigen, but are relatively less susceptible to DNA-specific tolerance than nonautoimmune cell lines.

Autoantibody repertoire analysis in normal and lupus-prone mice

We have analyzed at the clonal level (limiting dilution assay) the repertoire of lipopolysaccharide (LPS)-responsive murine B cells committed to the production of autoantibodies characteristic of systemic lupus erythematous (SLE), i.e. anti-single-stranded DNA (ssDNA), anti-double-stranded DNA, anti-Sm and rheumatoid factors (RF). Our results demonstrated that: (1) the frequency of precursor B cells producing each lupus autoantibody (approximately 1 in every 100-400 LPS-responding B cell) was similar in two non-autoimmune (C57BL/6 and BALB/c) and four SLE-prone (NZB, (NZB x NZW)F1, MRL/MpJ and BXSB/MpJ) mice despite the marked differences in autoimmune responses in the different SLE-prone mice, and (2) the relative frequency of autoantibody-secreting precursor B cells was constant throughout life, and equally distributed among activated and resting B-cell populations and among B cells from the peritoneal cavity and spleen. The lack of association of anti-ssDNA secretion with anti-Sm or RF secretion in cultures set up with a smaller number of B cells ruled out the possibility that the similar frequency of different autoantibody-secreting cell precursors is due to the poly-specificity of IgM autoantibodies. Notably, the frequencies of autoantibody-secreting precursor cells were significantly lower, approximately 4 and 10 times, than those of anti-tetanus toxoid and anti-dinitrophenyl antibody-producing precursor B cells, respectively. The similar frequency of precursor B cells producing four different lupus autoantibodies on the one hand and the considerable variation in each autoimmune response among SLE-prone mice on the other, support the hypothesis that specific stimulatory mechanisms may govern each autoimmune response in different SLE strains of mice.

Ontogenic development of autoantibody repertoires in spleen and peritoneal cavity of normal mice: examples of T cell-dependent and -independent reactivities

The ontogenic development of B cell clonal precursors (BCP) reactive to bromelain-treated, syngeneic erythrocytes (BrMRC) and to single-stranded DNA has been studied by limiting dilution of both spleen and peritoneal cells. It was found that the frequency of anti-BrMRC BCP in the spleen is very low up to 4 weeks of age and slowly increases thereafter, to reach adult levels by 6-10 weeks. In the peritoneal cavity, no such BCP can be found before 2 weeks, but they occur at a very high frequency already by 3 weeks of age. Injection of adult, normal syngeneic T cells at birth has no apparent effect on the representation of anti-BrMRC BCP in the peritoneal cavity, but brings these to adult levels or even higher in the spleen already at 3 weeks of age. Accordingly, adult athymic (nude) mice contain normal frequencies of BrMRC-specific BCP in the peritoneal cavity but are devoid of such clones in the spleen. In contrast, the frequency of anti-DNA BCP is very high throughout postnatal development in both spleen and peritoneal cavity, of normal and athymic mice, in both resting and naturally activated splenic B cell compartments, and it is independent of T cell transfers into nude animals. These results indicate the role of T cells in the establishment of some clonal specificities in the adult, splenic autoreactive B cell repertoire.

Sudden appearance of anti-protein IgG1-forming cell precursors early during primary immunization

The anti-keyhole limpet hemocyanin (KLH) B-cell repertoire of unimmunized adult mice was examined by culture of splenocytes (generally 100-3000) at limiting dilution. Cells were polyclonally stimulated with Escherichia coli lipopolysaccharide (LPS) and an interleukin-4-containing lymphokine mixture in the presence of 3T3 fibroblast filler cells. After 7 days of culture, supernatants were examined for their content of anti-KLH IgM and IgG1 antibody by an enzyme-linked immunosorbent assay (ELISA). Parallel cultures of smaller numbers (generally 1-15) of splenocytes were examined to determine the cloning efficiency of B cells in terms of total IgM and IgG1 production. Whereas one spleen cell in 370 produced clones secreting anti-KLH IgM, only 1% of these produced IgG1 that could bind to KLH, despite the fact that about half of the clones switched to IgG1 production with these stimuli. In mice immunized with KLH, this situation did not change until day 5, when there was a sudden, explosive emergence of B cells that could form clones secreting anti-KLH IgG1. The absolute number of such cells in the spleen was found to rise by a factor of 350 between days 3 and 7 of immunization. Moreover, the median amount of IgG1 antibody formed per clone and binding to KLH also rose markedly. In contrast, neither the numbers nor the median KLH-binding antibody content of anti-KLH IgM clones changed significantly after immunization. The results show that the repertoire of anti-protein B cells detected through IgM formation in ELISA consists chiefly of cells producing antibody of low avidity and of doubtful in vivo significance. Assuming that the small proportion of these cells making antibody that is of sufficient avidity to bind as the IgG1 isotype are the ancestors of the many such cells found on day 7 of the primary immune response, one would have to postulate a very high recruitment and/or division rate to account for the increase in numbers and avidity that occurs. It is possible that the anti-KLH IgG1 precursors that suddenly emerge are the results of early variable region gene (V) mutations in B cells. Moreover, it is not excluded that they represent products of a subset of B cells different from those that give rise to the primary in vitro anti-KLH IgM response. The findings have implications for theories of B-cell tolerance.

Characterization of a presecretory phase in B-cell differentiation

We have identified and characterized an inducible in vitro subclone of the CH12 B-cell lymphoma, CH12-LBK, which appears to represent a transitional phase in the B-cell differentiation pathway. This phase, which we call the "presecretory" phase, falls between replicating B cells that are not secreting antibodies and B cells that secrete antibody at a high rate. Presecretory cells are characterized by abundant steady-state levels of immunoglobulin and joining (J) chain transcripts and of protein but low levels of mouse mammary tumor virus envelope transcripts and low rates of immunoglobulin secretion. Additional stimulation is required for presecretory cells to differentiate into cells that secrete antibodies at a high rate. The existence of cells with this phenotype suggests that high-level expression of immunoglobulin and J-chain protein does not necessarily commit a B cell to polymerize and secrete multimeric immunoglobulin. Rather, other gene products, expressed after immunoglobulin and J-chain transcripts have been upregulated late in B-cell differentiation, appear responsible for inducing high rates of antibody secretion.

Immunochemistry of DNA

Since the first reports of anti-DNA antibodies in sera of patients with systemic lupus erythematosus (SLE) in 1957, studies of nucleic acid immunochemistry have grown in two directions. One has been the analysis of the specificity, the nature and the origins of these autoantibodies. The second has been exploration of anti-nucleic acid antibodies that can be induced experimentally, their specificities, and their application as biochemical reagents. Although the properties of autoantibodies and experimentally induced antibodies differ in certain respects, these two lines of research are complementary and provide important information for each other. For example, the production of autoantibodies by adjuvant-stimulated B cells yields a background that has to be considered in evaluating the specificity of weak responses to experimental nucleic acid immunogens: in turn, the possibilities and limitations of experimental immunization should be considered in evaluating possible stimuli for autoantibody production. Several aspects of nucleic acid immunochemistry have been described and evaluated in previous reviews. Following some general statements of historical perspective, this review will emphasize questions addressed and findings of about the last five years.

Anti-DNA antibodies, autoimmunity and the immunoglobulin repertoire

The advent of hybridoma and recombinant DNA technology about a decade ago has allowed a detailed analysis the structure, properties and molecular genetics of antibodies. These techniques, combined with studies of idiotypes and of Abelson-transformed and other cell lines, have resulted in major findings which are of particular importance to both the normal immune system and to autoimmunity. The rearrangement and expression of antibody genes in the normal immune system are discussed first, as a background for an appreciation of the significance of the molecular genetics of autoantibodies. We then turn to autoantibody genes, with an emphasis on anti-DNA antibodies and their role in the autoimmune disease, systemic lupus erythematosus. A model for the genetics of lupus which includes a possible role for Ig genes is considered.

The significance of natural autoantibodies

Since Burnet first introduced his "forbidden clones" theory, the discrimination between self and non-self and the physiologic mechanisms of avoiding autoimmunity remained an enigma. The realization in the past two decades that autoantibodies reacting with various self antigens are common in normals has led to intensive research on the origin and physiologic role of these "natural autoantibodies". After reviewing the extensive literature on the appearance of natural autoantibodies in normal animals and humans, and the studies proving unequivocally that natural autoantibodies are coded by germ line genes, we will discuss the current hypotheses explaining their appearance and physiologic role. Despite the fact that numerous hypotheses explaining the origin of natural autoantibodies have been postulated only the two important ones will be discussed. The first, proposed by Cunningham, holds that clonal deletion as viewed by Burnet operates in early life; however, later in life all autoreactive B cells not eliminated during ontogeny are prevented from expanding and secreting anti-self antibodies by a compensatory suppressor mechanism. Therefore, natural autoantibodies are postulated to be autoantibodies which are produced only in minute quantities allowed by the suppressor mechanism. The second hypothesis views autoantibody formation as a result of cross reaction between foreign and self determinants. It is suggested that the part of the B cell population which gives rise to autoantibodies carries a polyspecific receptor; fixation of a foreign antigen to this receptor induces the B cell to undergo a series of divisions and mutations, which under the selective pressure of the antigen leads to production of a highly specific antibody. Thus natural autoantibodies may constitute the antibodies secreted by these B cells prior to encountering foreign antigens. The biologic role of natural autoantibodies is also elusive. The common denominator to all the theories dealing with that puzzling question is the view that natural autoantibodies have a positive role in normal immune reactions, perhaps even an essential role without which normal immune function would be disrupted. Grabar suggested that natural autoantibodies are part of a physiologic mechanism for cleansing the organism of self and non-self products in which classical antibodies serve to clear the body of foreign invading agents, while natural autoantibodies rid the organism of its own catabolic products.(ABSTRACT TRUNCATED AT 400 WORDS)

Comparative analysis of natural antibody specificities among hybridomas originating from spleen and peritoneal cavity of adult NZB and BALB/c mice

A preliminary experiment showed that the supernatants of in vitro cultured peritoneal cells (rich in Ly-1 B cell subset shown to secrete most IgM autoantibodies against bromelain-treated mouse red blood cells (BrMRBC) and DNA) from different mouse strains did not contain any significant antibody activity against DNA and cytoskeleton proteins, although the presence of anti-BrMRBC antibodies was clearly evident. Therefore, we investigated comparative natural antibody (NAb) specificities against an antigen panel (DNA, cytoskeleton proteins, IgG, bovine serum albumin (BSA), BrMRBC, trinitrophenyl (TNP), and trimethylammonium (TMA) haptens) among Ig-secreting hybridoma collections from the splenic (158) and peritoneal (230) immune compartments of autoimmune New Zealand black (NZB) and lipopolysaccharide (LPS)-stimulated BALB/c mice. The data showed: (i) isotypic restriction (mu and gamma 3 only), predominance of TMA ion-reactive (including BrMRBC) but negligible anti-DNA-reactive antibody specificities, and lack of simultaneous polyspecific widespread reactivity (i.e. at least four or more antigens) against DNA and cytoskeleton proteins in the peritoneal cavity; (ii) predominance of simultaneous widespread polyspecific reactivity against DNA and cytoskeleton proteins but negligible or no TMA hapten-reactive antibody specificities in the spleen. These observations reflect certain differences in the B cell repertoire of peritoneal cavity (rich in Ly-1 B cells) compared with spleen. The NAb against BrMRBC and those reactive with DNA and cytoskeleton proteins, which have been suggested to be secreted by the Ly-1 B cell subset, are distinguishable on the basis of the presence of separate recurrent idiotypes and preferential localization of B lymphocytes directed against these autoantigens.

Somatic mutations in B lymphocytes: new perspectives in tolerance research?

This paper extends the concept of clonal anergy developed in the author's laboratory. It has been shown that the primary population of B lymphocytes induced into clonal expansion and IgM antibody formation by mitogens contains many cells capable of autoantibody synthesis, but the affinity of binding to the self constituents, or indeed to foreign antigens, is low. The creation of high affinity antibody, which will still register strongly in an ELISA as an IgG molecule, demands not only the addition of lymphokines to cause isotype switching, but also intentional immunization of the donor mice to permit mutations in V region genes and selection of higher affinity B memory cells. This process appears to begin about 6 days after in vivo immunization. It is postulated that these mutational events occur primarily in germinal centres, and that there must be mechanisms to prevent escape of cells which, by chance, mutate not to higher affinity against an immunogen, but to higher affinity against a self constituent. If such mutants were allowed to enter the long-lived, recirculating pool of B lymphocytes, they might pose a graver threat of autoimmune disease than the low-affinity anti-self cells of the primary repertoire. Therefore, it is suggested that recently mutated germinal centre B cells represent a pool of 'pre-memory' cells, which are immature in the sense of displaying the same kind of sensitivity to negative signalling by antigen that immature B cells from newborn spleen or adult bone marrow display. If so, then the earliest phases of memory generation represent a second window of opportunity for tolerance induction within the B lymphocyte compartment.

Expression of anti-DNA clonotypes and the role of helper T-lymphocytes during the autoimmune response in mice tolerant to alloantigens

BALB/c mice neonatally injected with semiallogeneic (C57BL/6 x BALB/c) F1 spleen cells become tolerant to C57BL/6 alloantigens and exhibit chimaerism due to persistence of F1 lymphocytes. Such mouse chimaeras develop an autoimmune (lupus-like) disease characterised by hypergammaglobulinaemia with production of autoantibodies against DNA, Sm antigen and other self-antigens characteristic of SLE in addition to circulating immune complexes and glomerular deposition of immunoglobulins. We have studied the autoimmune response by analysing the isoelectric focusing (IEF)+ patterns (spectrotypes) of anti-ss and anti-dsDNA antibodies produced by these animals. The results show that the anti-DNA response is remarkably restricted, only a very small number of lymphoid cell clones responding in the majority of animals. The behaviour of these clones has been followed during the development of the autoimmune response by analysis of their individual IEF patterns (clonotypes). The first appearance of clones secreting anti-DNA autoantibodies was observed in 3-4 week old mice. Changes in spectrotype occurred during the course of the response but they remained restricted to a very small number of clones in almost all the animals studied. Changes in clonotype consistent with somatic mutation in committed, anti-DNA-secreting clones were also observed. Helper T-lymphocytes of host origin are shown to be required for the development of an autoimmune response.

The CD5+ B cell: a B cell lineage with a central role in autoimmune disease?

It is apparent that B cells are heterogeneous with respect to, for example, the antigens they express on their surface, and the stimuli to which they can respond. It is still unclear to what extent these differences relate to the stage of differentiation (eg. virgin B cells differing from activated B cells or memory cells), or whether distinct developmental lineages might exist. It has been proposed by some authors that, in the mouse, B cells expressing the ly-1 antigen constitute a separate lineage. In man also, a minor population of B cells expresses detectable levels of the CD5 antigen, but far less information is available about these cells. Interest in the CD5+ and ly-1+ B cell subpopulations has been further stimulated by the suggestion that these cells might play a special role in autoimmune disease. Although, in mouse, ly-1+ B cells differ in several respects from ly-1- B cells, the main evidence that they form a separate lineage derives from experiments in which ly-1+ B cells could not be reconstituted with adult bone marrow. It should be borne in mind that the situation is quite different in humans where, following bone marrow transplantation, CD5+ B cells are rapidly restored. Moreover, in the irradiated mice, at least in some of the experiments ly-1+ B cells were in fact reconstituted by adult bone marrow. Furthermore, at least in humans, expression of CD5 can sometimes be induced. There is, as yet, no good evidence that human CD5+ B cells form a distinct lineage, and it is possible that CD5 expression depends upon microenvironmental influences acting on the B cell during its differentiation. Several interesting properties have been attributed to ly-1+ B cells, including the ability to provide help to other B cells, and the secretion of autocrine factors. However there is also evidence that these features are not exclusive to B cells expressing ly-1. It has also been suggested that ly-1+ B cells might be long-lived. It is not yet known whether some of the properties of ly-1+ B cells might be a direct result of their expressing this antigen; this may become more clear when the function of CD5 is elucidated. The suggestion that the repertoire of ly-1+ B cells might be biased towards the expression of certain V genes is very interesting. Many of the hybridomas from neonatal mice produce antibodies which are multi-specific, and therefore well suited to form a first line of defence against potential pathogens.(ABSTRACT TRUNCATED AT 400 WORDS)