A substantial proportion of the adult BALB/c available B cell repertoire consists of multireactive B cells

Do Antibodies Stimulate Myelin Repair in Multiple Sclerosis?

One of the major goals in the study of multiple sclerosis (MS) is to identify a beneficial therapeutic intervention that mimics the intrinsic reparative process and results in long-term clinical improvement. As yet, the therapeutic strategies tested in MS have failed to accomplish this task. However, one potential therapy that has shown some promise in rodent models of demyelination involves the administration of antibodies. Studies in various models of demyelination (virus-induced, autoimmune, and toxic) indicate that a subset of autoantibodies with reactivity to CNS antigens promote remyelination. We have identified a prototypic germline IgMk monoclonal antibody, designated SCH 94.03, with reactivity to a surface antigen on oligodendrocytes that promotes CNS remyelination. This antibody has the phenotypic features of polyreactive physiological natural autoantibodies. Additionally, treatment of MS patients with intravenous immunoglobulin, which contains these natural autoantibodies, may be efficacious in a subset of patients. We propose three mechanisms (direct stimulation of oligodendrocytes, immunomodulation, and opsonization of debris) by which polyreactive natural autoantibodies directed against CNS antigen may promote remyelination. Remyelination has the potential to not only improve conduction velocity but also may protect axons from injury and improve neurological function.

The lupus susceptibility locus Sle1 breaches peripheral B cell tolerance at the antibody-forming cell and germinal center checkpoints

We have described a line of V(H) knock-in mice termed HKIR in which the transgenic Igh locus partially encodes "dual-reactive" antichromatin and anti-p-azophenylarsonate (Ars) BCRs. HKIR B cells termed canonical, expressing a particular Vkappa L chain, evade central tolerance by down-regulating BCR levels. Canonical HKIR B cells can be recruited into the primary germinal center (GC) and Ab-forming cell (AFC) compartments via Ars immunization. However, their participation in the GC response rapidly wanes and they do not efficiently contribute to the memory compartment, indicating that they are regulated by a GC tolerance checkpoint. We analyzed the influence of the Sle1 genetic interval, shown to break tolerance of chromatin-reactive B cells, on the behavior of HKIR B cells during the anti-Ars response. Canonical B cells from congenic HKIR.Sle1 mice gave rise to elevated short and long-lived AFC responses, and the attenuated GC and memory responses characteristic of these B cells were relieved in adoptive, wild-type recipients. HKIR GC B cells containing Sle1 expressed increased levels of Bcl-2 and c-FLIP and decreased levels of Fas RNA compared with HKIR controls, suggesting direct alteration of the regulation of the GC response by Sle1. High titers of canonical and anti-dsDNA Abs spontaneously developed in many aged HKIR.Sle1 mice. Together, these data indicate that Sle1 perturbs the action of peripheral tolerance checkpoints operative on antinuclear Ag B cells in both the AFC and GC pathways in a cell autonomous fashion.

Autoimmunogenicity of the helix-loop-helix DNA-binding domain

Nonimmunogenic character of native DNA, and its high immunogenicity when presented in complex with the DNA-binding proteins indicate that the latter might contain molecular triggers of anti-DNA response. To find if this is the case, we have evaluated the autoimmunogenic potential of the main DNA-binding domain of HIV-1 reverse transcriptase that belongs to the canonical helix-loop-helix type. BALB/c mice were immunized with a peptide representing the domain, alone or in complex with the fragmented human DNA in the presence of an adjuvant. Mice were assessed for specific antibodies, autoantibodies against a panel of self-antigens; glomerular immunoglobulin deposition; and for the signs of autoimmune disease, such as proteinuria, and changes in the blood components. Immunization with the adjuvanted peptide-DNA complex induced autoantibodies against double-stranded DNA, histones, heterochromatin, and kidney proteins; glomerular IgG and IgA deposition; proteinuria; thrombocytopenia, and anemia. Altogether, this identifies the helix-loop-helix DNA-binding domain as one of the molecular triggers of autoimmunity to DNA and DNA-associated proteins. The experiments cast new light on the role of the DNA-binding retroviral proteins in the induction of autoimmunity, and on the origins of autoimmune complications in the microbial infections in general. It also implies that choosing the DNA-binding proteins as vaccine candidates should be done with precaution.

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

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

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

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

Dominant, hierarchical induction of peripheral tolerance during foreign antigen-driven B cell development

We created mice expressing transgene-encoded BCRs with "dual reactivity" for the hapten Ars and nuclear autoantigens. Expression of transgene-encoded BCRs was not evident in the memory compartment despite observation of transgene-expressing B cells in germinal centers following Ars immunization. In contrast, dual reactive mAbs were readily obtained from mice with enforced expression of Bcl-2 following secondary Ars immunization. However, while these mAbs were hypermutated and displayed increased affinity for Ars, all had reduced avidity for DNA and intracellular autoantigens. Thus, Bcl-2 alters dominant-negative selection of dual reactive B cells during the Ars response, but this is restricted to those with lowered autoreactivity, demonstrating a hierarchy of peripheral tolerance during memory B cell development.

Predominance of a novel splenic B cell population in mice expressing a transgene that encodes multireactive antibodies: support for additional heterogeneity of the B cell compartment

We generated IgHmudelta transgenic mice using a V(H) gene that in A/J mice encodes multireactive BCR in the preimmune B cell compartment and is predominantly expressed by a memory B cell subpopulation. Most primary splenic B cells in these mice have a size, cell-surface phenotype and in vitro response profile distinct from mature follicular (B2), marginal zone (MZ) or B1 B cells, but are long-lived and appear to be slowly cycling. They reside in conventional B cell areas of the spleen and mount robust foreign antigen-driven germinal center responses, but do not efficiently differentiate to secretory phenotype. We propose that these qualities result from ongoing, low-avidity BCR-self-ligand interactions and promote entry into the memory pathway. Given these data, and the enormous diversity and characteristic multireactivity of the preimmune antibody repertoire, we also suggest that it may be more appropriate to view the primary B cell compartment as a continuum of functional and phenotypic 'layers', rather than as a group of discrete B1, B2 and MZ subsets.

Self-limiting systemic autoimmune disease during reconstitution of T cell-deficient mice with syngeneic T cells: support for a multifaceted role of T cells in the maintenance of peripheral B cell tolerance

The T cell compartment can be partially reconstituted in mice with targeted inactivation of the TCR C(beta) and C(delta) genes by injection of mature, syngeneic T cells. Surprisingly, during this reconstitution high titers of IgG anti-nuclear antibodies and symptoms of systemic autoimmune disease develop. However, this autoimmune response is transient and aged, reconstituted mice show no overt signs of disease. The autoantibody response appears to be derived from a pre-existing population of host self-reactive B cells and requires CD40 ligand-mediated co-stimulation from donor cells. Diminution of this response is coincident with a vigorous germinal center reaction and the disappearance of a subpopulation of activated B cells that expresses elevated levels of Fas. Collectively, our data support the idea that T cells play a multifaceted role in the maintenance of peripheral B cell tolerance that includes mediating the activation-induced death of autospecific B cells.

Structural differences among monoclonal antibodies with distinct fine specificities and kinetic properties

The mAbs HyHEL-8, HyHEL-26 (HH8, and HH26, respectively) recognize epitopes on hen egg-white lysozyme (HEL) highly overlapping with the structurally defined HH10 epitope, while the structurally related XRPC-25 is specific for DNP and does not bind HEL. All four Abs appear to use the same Vk23 germ line gene, and all but HH8 use the same VH36-60 germ line gene. Of the three anti-HEL Abs, the sequences of HH26 variable regions are closest to those encoded by the respective germ line sequences. HH8 utilizes a different member of the VH36-60 gene family. Thus, the same Vk and VH genes, combined with somatically derived sequence differences, are used to recognize the unrelated Ags HEL and DNP. In contrast, different VH36-60 germ line genes are used to bind the same antigen (e.g. HH8 vs HH10 and HH26). While the affinities of HH10, HH8, and HH26 for HEL vary by less than 10-fold, their affinities for mutated Ag vary over several orders of magnitude. Analyses of Fab binding kinetics with natural species variants and site-directed mutants of lysozyme indicate that these cross-reactivity differences reflect the relative sensitivities of both the association and dissociation rates to antigenic mutation: HH8 has relatively mutation-insensitive association and dissociation rates, HH10 has a relatively mutation-sensitive association rate but more variable dissociation rates, and HH26 has variable association and dissociation rates. Only a few amino acid differences among the antibodies produce the observed differences in the robustness of the association and dissociation rates. Our results suggest that association and dissociation rates and mutation sensitivity of these rates may be independently modulated during antibody repertoire development.

Immunization through dermal delivery of protein-encoding DNA: a role for migratory dendritic cells

The early mechanisms by which DNA-dependent immunization occurs remain poorly understood. We determined whether intradermal injection of a cytomegalovirus (CMV) promoter-driven plasmid encoding hen egg lysozyme (pCMV:HEL) induced sensitization against the encoded protein, and whether cutaneous dendritic cells (DC) were involved in this sensitization. Both humoral and cellular responses to HEL were observed. DC that migrated from skin explant culture 3 days after injection of pCMV:HEL DNA contained mRNA encoding HEL. They induced a 3.5-7-fold increase in [3H]thymidine incorporation by HEL protein-primed CD4+ T cells compared to that induced by DC from mice injected with control plasmid. DC emigrating from skin explants recovered from pCMV:HEL injected mice also sensitized naive mice after adoptive transfer and induced the generation of CTL. Thus following DNA delivery within the dermis, DC can induce primary and secondary immune responses.

The roles of antibody variable region hypermutation and selection in the development of the memory B-cell compartment

Somatic hypermutation and selection of immunoglobulin (Ig) variable (V)-region genes, working in concert, appear to be essential for memory B-cell development in mammals. There has been substantial progress on the nature of the cis-acting DNA elements that regulate hypermutation. The data obtained suggest that the mechanisms of Ig gene hypermutation and transcription are intimately intertwined. While it has long been appreciated that stringent phenotypic selection forces are imposed on the somatically mutated Ig V regions generated during a T-cell dependent B-cell response, the mechanisms involved in this selection have remained enigmatic. Our studies have questioned the role of foreign antigen deposited on follicular dendritic cells in affinity-based positive selection of V regions, and have shown that this selection takes place in a "clone-autonomous" fashion. In addition, our data strongly suggest that affinity for antigen alone is not the driving force for selection of B-cell clones into the memory compartment. In contrast, we suggest that a combination of positive selection for increased foreign antigen binding, and negative selection of antibody V regions that are autoreactive at the onset of the response, or have acquired autoreactivity via hypermutation, results in the "specificity maturation" of the memory B-cell response.

Single amino acid substitutions in V(H) CDR2 are sufficient to generate or enhance the specificity of two forms of an anti-arsonate antibody variable region for DNA

We previously showed that a variety of amino acid substitutions at positions 58 and 59 in the V(H) CDR2 of an anti-arsonate (Ars) antibody Fab simultaneously resulted in increased or unaltered affinity for Ars and substantially enhanced affinity for DNA. To test the generality of these observations, we generated and characterized several antibody phage display libraries of this Fab containing random amino acid substitutions at V(H) CDR2 position 55. Position 55 was randomized in two contexts; in the unmutated V region, and in a previously isolated V(H) CDR2 position 58 and 59 mutant that displayed binding to both Ars and ssDNA. In the unmutated V region context, mutants that displayed strong binding to both Ars and DNA nearly exclusively contained Arginine residues at position 55. In the context of the 58 and 59 mutations, a variety of amino acid residues were observed at position 55 among mutants that bound strongly to both Ars and DNA, including Arginine, Lysine and Serine. None of these position 55 mutations measurable altered affinity for Ars. These data substantiate the view that "dual reactive" antibodies--specific for both a foreign and an autoantigen--are frequently generated in vivo via hypermutation during immune responses driven by the foreign antigen.

Signaling thresholds and interclonal competition in preimmune B-cell selection

The need to eliminate autoreactive B cells must be checked against the need for a diverse B-cell repertoire. Protein tyrosine phosphatases SHP1 and CD45 act antagonistically within B cells to set the threshold level of antigen-receptor engagement required for B-cell elimination. The fate of B cells binding weak autoantigens is also regulated by interclonal competition. In the presence of a normal diverse repertoire of competitor B cells, the autoantigen-binding cells are excluded from follicles in spleen and lymph nodes and undergo rapid cell death. In the absence of interclonal competition, the autoreactive cells enter the follicular microenvironment and survive. A model in which B cells compete for access to limiting follicular niches in order to survive is discussed.

Plasmacytoma-refractory BALB/cAnPt mice have naive T cell and highly specific B cell responses to antigen

Recently, a reduction in the incidence of pristane-induced plasmacytomas in BALB/cAnPt (BALB/c) mice that were kept in viral specific pathogen-free (SPF) conditions has been reported. Environmentally, these SPF-BALB/c mice differed from conventionally-housed (CON) mice only in viral exposure and diet (i.e. sterilization of mouse chow), since microbial colonization of the intestinal tract was seen to be equivalent. This report assessed the ability of SPF- and CON-BALB/c mice to respond to immunologic challenge with soluble antigen, i.e. hen egg white lyzosyme (HEL), as a means of evaluating differences in T and B cell function and, indirectly, evaluating the possible effects these differences might have on plasmacytoma development. When cultured in vitro for 5 days with HEL, HEL-primed lymph node cells (LNC) from SPF-BALB/c mice proliferated to a significantly lesser extent than HEL-primed CON-BALB/c LNC. Moreover, HEL-induced production of IFN-gamma and IL-5 was significantly lower in SPF LNC. Serum IgG1 levels were 10-fold lower in SPF-BALB/c mice with, or without prior immunization with HEL and were not reconstituted by repeated injections of HEL in adjuvant. Serum IgM levels of SPF- and CON-BALB/c mice were equivalent. This reduction in immune responses could not be attributed to a lack of colonization of secondary lymphoid organs, since flow cytometric analysis of LNC revealed no difference in the number of recoverable cells and the proportion of lymphocyte subsets (CD4+, CD8+ and CD45+ cells) obtained from SPF- and CON-BALB/c mice. However, only CON LNC were induced to increase surface expression of CD44 after antigenic or mitogenic stimulation in vitro. Antibody responsiveness to HEL, as evidenced by serum anti-HEL binding or splenic hybridoma studies, demonstrated higher levels of IgG1 antibodies in CON BALB/c mice than in SPF mice. However, a greater proportion of the SPF IgG1 antibodies present were specifically directed against HEL, so that specific activity was greater in SPF-BALB/c mice. Therefore, while SPF BALB/c mice have a more restricted response to HEL than CON-BALB/c mice, those antibodies that are produced are more specifically directed against HEL with very little apparent bystander/polyclonal activation of multireactive cells. Resistance to plasmacytomas in SPF-BALB/c mice, therefore, may stem from a reduced number of circulating memory T and B cells, which are capable of reacting and/or crossreacting with a chronic inflammatory stimulus.

Polyreactivity of human monoclonal antibodies: human IgM anti-rhesus monoclonal antibodies are frequently polyreactive

The aim of this study was to characterize human anti-Rhesus monoclonal antibodies cross-reacting with tissue antigens. Of the 155 monoclonal alloantibodies tested, 49 also reacted with intracellular antigens, as demonstrated by immunofluorescence assay on cryostat sections of animal and human tissues. This cross-reactivity was mainly a property of monoclonal alloantibodies belonging to the IgM isotype (among the 49 cross-reacting Mabs, 37 were IgM). The results confirm that during an immune response against a foreign antigen (alloantigen), B cells that produce polyreactive antibodies are not excluded from the pool of responding cells.

Increased utilization of polyreactive B cells during periods of generalized immune activation

This work examines the hypothesis that B cells secreting polyreactive antibodies (antibodies capable of binding to more than one self or foreign antigen) are preferentially utilized during periods of generalized immune stimulation. Four conditions characterized by such stimulation were examined: chronic virus infection, mitogen treatment, autoimmune disease and neonatal repertoire development. In normal adult mice, polyreactive IgM secreting lymphocytes constituted 8-9% of the actively expressed repertoire. Under conditions of generalized immune activation, this frequency increased to 13-19% (p. < .01). Polyreactive IgG secreting B cells, which were present at frequencies of < 0.5% in normal adult mice, were found at freqeuncies of 6-10% in mice with autoimmune disease, chronic virus infection or following mitogen treatment (p. < .001). We postulate that polyreactive lymphocytes are preferentially activated when the immune system is confronted with stimuli inadequately controlled by antigen-specific responses.

Negative selection of multireactive B cell clones in normal adult mice

In the absence of intentional immunizations, normal mice produce natural antibodies that react with a variety of self and foreign antigens. We have now addressed the putative physiological selection of such reactivities and some of their clonal characteristics, by analyzing antibodies produced by B cells at different stages of differentiation. Using an antigen-specific spot-enzyme-linked immunosorbent assay (ELISA) with a panel of self and foreign antigens, we found that newly formed B cells, either from adult bone marrow or from newborn spleen, contain the highest frequencies of IgM antibodies with reactivities towards the panel. Resting peripheral B cells show lower frequencies of such antibodies, that are lowest among naturally activated splenic plasma cells. Analyses of monoclonal IgM antibodies derived from lipopolysaccharide-stimulated bone marrow and spleen cell hybridomas in normal mice show that the majority of reactivities scored in spot-ELISA originate from multireactive IgM clones. In Western blots against a large number of self antigens, each multireactive IgM antibody studied shows a unique and specific pattern of reactivity. We conclude that multireactive B cell clones are very frequent in the emergent repertoires of newborns and adults, but are subsequently negatively selected from bone marrow to periphery, and from the available repertoire to that of natural plasma cells. It, thus, seems that multireactivity of natural antibodies is not a positively selected property, but represents the sum of unique multireactive clones that have escaped inactivation or deletion.

Induction of plasmacytomas that secrete monoclonal anti-peptide antibodies by retroviral transformation

ABL-MYC, a retrovirus that coexpresses v-abl and c-myc, was used to infect six BALB/c mice that had been immunized twice with a KLH-conjugated peptide that consisted of the 18 carboxyterminal amino acids of protein kinase C-eta (PKC-eta). All mice developed transplantable, monoclonal plasmacytomas, and five out of six plasmacytomas secreted antigen-specific antibodies, even after transplantation. All these antibodies recognized PKC-eta on Western blots of crude cell lysates and did not cross react with other isoforms of the PKC family.

Cross-reactivity of IgG anti-DNA-secreting B cells in patients with systemic lupus erythematosus

This study is the first to analyze the cross-reactivity of in vivo activated B cells from patients with systemic lupus erythematosus. A chamber ELIspot assay was used to determine whether lymphocytes secreting antibodies that bound to DNA or 2,4,6-trinitrophenol (TNP)-keyhole limpet-hemocyanin (KLH) could simultaneously bind to the unrelated antigens actin or ovalbumin. IgM anti-DNA-, IgM anti-TNP-KLH- and IgG anti-TNP-KLH-secreting B cells from patients and controls showed similar levels of cross-reactivity (ranging from 6% to 23%, depending upon the antibody isotype and antigen pair examined). In general, IgG-producing cells were less cross-reactive than IgM producers from the same individual (on the average threefold, p < 0.001). In contrast, IgG anti-DNA-secreting B cells from lupus patients (i) showed no decrease in cross-reactivity when compared to IgM anti-DNA-secreting cells and (ii) were significantly more cross-reactive than control IgG anti-DNA-secreting cells and IgG anti-TNP-KLH secreting cells from patients (p < 0.001). The degree of IgG anti-DNA cross-reactivity correlated with disease activity (r = 0.52, p < 0.02). The implications of these findings with respect to repertoire expression and disease pathogenesis are discussed.

Monoclonal IgG and IgM autoantibodies obtained after polyclonal activation, show reactivities similar to those of polyclonal natural autoantibodies

In this study, we described the properties of two groups of polyreactive IgG monoclonal antibodies (mAb) derived from splenocytes of a Plasmodium chabaudi-infected BALB/c mouse. These IgG mAb reacted with self antigens, but not with the parasite. Depending upon the antigen under study, they showed low (10(-5) M) to high (10(-8) M) affinities. When examined by immunoblotting using mouse organ extracts as the antigen source, each IgG mAb had a different reactivity profile. The IgG2b, but not the IgG2a mAb group, reacted with F(ab')2 fragments of IgG from normal mouse sera, as well as with F(ab')2 fragments of other mAb from the same fusion. Their anti-F(ab')2 activity was inhibited by various antigens, suggesting that the binding sites for these antigens and F(ab')2 fragments were close or overlapping. We also characterized two monoclonal IgM that were strongly and almost exclusively reactive with polyclonal F(ab')2 fragments of normal IgG, and that inhibited the binding of normal polyclonal IgG to self antigens. We previously described such properties for polyclonal IgM from normal mouse sera. The binding of the F(ab')2-reactive IgG mAb to self antigens was also inhibited by normal polyclonal IgM. These results indicate that the IgG and IgM obtained after polyclonal stimulation, exhibit characteristics similar to those of autoantibodies present in normal mouse sera. Furthermore, they confirm our previous studies showing the polyreactivity of polyclonal IgG. Finally, they also show that such IgG mAb exhibit properties similar to those of natural IgM, namely polyreactivity, affinity and connectivity.

The developmental patterns of B cell precursors distinguishing between environmental and nonenvironmental forms of phosphocholine

We have analyzed the developmental patterns of two groups of B cell precursors in nonimmunized BALB/c mice with respect to their relative proportions, absolute frequencies, V gene usage, fine specificity, and avidity for antigen. One group of B cells (group I) secretes antibodies specific for PC and PC-containing bacteria, whereas the other group (group II) produces antibodies recognizing only nonenvironmental PC-protein conjugates. A marked shift in the proportions of group I and group II occurs during ontogeny: while the group I B cells dominate (greater than 85%) the adult antibody repertoire, the group II B cells have equal representation in neonatal mice from Days 1 to 7, and remain as a significant portion until 2 weeks of age. Examination of the absolute frequencies of group I and group II B cells revealed that the frequency of group II B cells remained relatively stable throughout ontogeny, whereas group I B cells expanded rapidly after 7 days of age to predominate in the adult. Genetic analysis indicated that early group I antibodies were encoded by VH and VL genes different from adult group I antibodies which are mostly encoded by a single VH (S107) and VL (V kappa 22) gene combination (the T15 idiotype). On the other hand, early group II antibodies used VH genes comparable to their adult counterparts. The majority of early group I antibodies have lower avidity for PC than adult T15+ antibodies, whereas the avidity of neonatal group II antibodies varies considerably and is comparable with that of the adult group II antibodies. Our results suggest that the ontogeny of phosphocholine-specific B cells may be regulated according to their fine specificity rather than to their avidity or V gene usage.

Analysis of B lymphocyte cross-reactivity at the single cell level

A method is described for assaying whether individual in vivo activated B cells produce antibodies which cross-react with two distinct antigens. The technique utilizes a modification of the ELISA spot assay to determine the isotype and antigenic specificity(s) of these antibody secreting cells. To perform the assay, two plastic slides are coated with different antigens and secured together to form chambers capable of holding a monolayer of Ig secreting lymphocytes. The immunoglobulin secreted during culture binds to one or both sides of the antigen-coated chamber (depending upon the specificity and cross-reactivity of the antibody product). This technique was used to evaluate the cross-reactivity of hybridomas and freshly isolated IgM and IgG secreting B cells.

Autocrine regulation of murine B lymphocyte growth by an IgM antibody

Culture supernatants of LPS-stimulated murine B lymphocytes are able to inhibit the growth of freshly isolated splenic B cells via an IgM antibody. The binding specificity of this IgM is not yet defined, but appears to be a B lymphocyte surface structure distinct from membrane immunoglobulin, MHC class II antigen, transferrin and Fc gamma receptors, and B220. The regulatory autoantibody allows the normal progression of early, but not late steps in the cycle of polyclonally-stimulated B lymphocytes and does not affect the increased antigen-presenting capacity of activated B cells. Therefore, this autoregulatory cycle is apparently ubiquitous and may be a major component of B lymphocyte homeostasis under physiological, as well as pathological conditions. Moreover, these findings bring into focus a possible regulating role of B lymphocytes in the humoral immune response.

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.

Regulation of antibody production mediated by Fc gamma receptors, IgG binding factors, and IgG Fc-binding autoantibodies

Fc receptors (FcRs) are immunoglobulin-binding structures that enable antibodies to perform a variety of functions by forming connections between specific recognition and effector cells. Besides eliciting cytotoxicity, inducing secretion of mediators and endocytosis of opsonized particles, FcRs are involved in the regulation of antibody production, both as integral membrane proteins and as soluble molecules released from the cell surface. Most FcRs belong to the same family of proteins as their ligands (immunoglobulin superfamily). This review contains recent data obtained by use of monoclonal antibodies and cloning studies on FcRs and FcR-like molecules. The importance of fine specificity of receptor binding site(s)--that of the conformation of FcRs and their ligands in triggering signaling mechanisms--is analyzed. The regulatory function of membrane-bound and -released FcRs; the correlation between cell cycle, FcR expression, and release; as well as the possible mechanisms of these phenomena are discussed.

Regulation of an anti-self response: lack of influence of exogenous DNA on the in vitro anti-DNA response

Anti-DNA antibodies occur in outwardly normal individuals as well as in various forms of autoimmune disease. A number of publications have reported on the ability of added DNA to either induce or inhibit the in vitro production of anti-DNA antibody. In this study, the in vitro production of IgM anti-single stranded DNA (alpha ssDNA) antibody by spleen cells from normal or autoimmune mice neither depends upon, nor is inhibited by, the addition of high molecular weight DNA to the culture. The decrease in antibody forming cell plaques, reported previously, is due solely to the artifactual carryover of inhibitory material into the assay system, where it interferes with the expression of plaques by preventing anti-DNA antibody from reaching the DNA-coated erythrocytes. Similarly, plaque forming cell (PFC) methods have not detected alpha ssDNA antibody producing cells in murine spleen cells without culturing, but various other systems for measuring antibody normally detect anti-DNA antibodies in vivo. This discrepancy is also due to inadequate washing of freshly harvested cells to rid them of inhibitory substances which prevent them from registering as PFC. While S1 nuclease was able to prevent PFC interference by purified DNA, it did not remove the inhibitory substances from the culture supernatants; therefore substances other than ssDNA are able to interfere with alpha ssDNA PFC, suggesting that the alpha ssDNA PFC detected are polyspecific. Levels of alpha ssDNA PFC in spleen cells from non-autoimmune mice begin at one-quarter of the peak in vitro response, decrease to one-tenth in the first day and then reach peak values after 3 to 5 days of culture, suggesting that spleen cells are actively producing alpha ssDNA antibodies an in vivo and that then in vitro response is observed. Despite this evidence for an in vitro alpha ssDNA response, this response was not inhibited markedly by 1000 rad gamma-irradiation, while the response to sheep erythrocytes (SRBC) was profoundly suppressed. These findings suggest that anti-self B lymphocytes are resistant to interphase, possibly apoptotic, lymphocyte death due to gamma-irradiation, while anti-nonself B lymphocytes remain sensitive.

Polyreactivity of human monoclonal antibodies: human anti-Rh monoclonal antibodies of IgM isotype are frequently polyreactive

The specific aim of this study was to characterize human anti-Rh monoclonal antibodies cross-reacting with self-antigens. We studied supernatants from man-mouse hybridomas and from lymphoblastoid cell lines. Man-mouse hybridomas were established by fusion of peripheral blood lymphocytes from healthy individuals recently immunized against Rh alloantigens, with mouse myeloma (or man-mouse heteromyeloma) cell lines. Lymphoblastoid cell lines were produced by Epstein-Barr virus induction of lymphocytes from identical sources. Of the 55 monoclonal alloantibodies studied, 11 also reacted with intracellular self-antigens as demonstrated by immunofluorescence assay on cryostat sections of human tissues. This cross-reactivity was mainly a property of monoclonal alloantibodies belonging to the IgM isotype (among the 11 cross-reacting mAbs 10 were IgM). The cross-reactivities of these monoclonal antibodies were ascertained by absorption of alloreacting antibodies with red blood cells. Similar results were obtained on a panel of purified cellular antigens by ELISA. The results confirm that during an immune response against a foreign antigen (alloantigen), B cells that produce polyreactive antibodies are not excluded from the pool of responding cells. Therefore, polyreactive autoantibodies present in sera from healthy individuals may be the result of an immune response against foreign antigens.

Induction of plasmacytomas secreting antigen-specific monoclonal antibodies with a retrovirus expressing v-abl and c-myc

ABL-MYC, a recombinant murine retrovirus that expresses v-abl and c-myc, rapidly induces transplantable mono- or oligoclonal plasmacytomas in BALB/c mice. To determine if the targets for transformation of this retrovirus are antigen-committed B lymphocytes and to explore this system as an alternative technique for producing antigen-specific monoclonal antibodies, plasmacytomas were induced in mice that had been immunized with two different types of immunogens, hen egg white lysozyme and sheep red blood cells. The majority of these plasmacytomas secreted immunogen-specific antibodies. Plasmacytomas induced in unimmunized mice did not react with hen egg white lysozyme or sheep red blood cells. The specific antibodies were comparable in concentration, specificity, and affinity to monoclonal antibodies obtained with conventional hybridoma technology, but, in addition to IgGs and IgMs, they included specific IgA antibodies, which are rare among splenic-derived hybridomas. Our results demonstrate that a principal target for ABL-MYC is an antigen-committed B lymphocyte. In addition this procedure provides an alternative method for the production of monoclonal antibodies, without a requirement for hetero-caryon formation by cell fusion techniques.

Isolation and characterization of IgG2a-reactive autoantibodies from influenza virus-infected BALB/c mice

Repeated influenza virus infection induces the production of dominantly IgG2a-type virus-specific antibodies as well as the appearance of IgG2a-reactive autoantibodies in BALB/c mice characterized by low spontaneous rheumatoid factor-type autoantibody production. IgG2a-reactive autoantibody-producing hybridomas could be isolated from the spleen of influenza virus-infected BALB/c mice. The mAb produced by these clones represent not only IgM but also IgG and IgA isotypes and show strong isotype or isoallotype specificity. The common functional property of these autoantibodies is their preferential- and high-affinity binding to complexed, solid-phase-bound or heat-aggregated IgG2a when compared to native soluble or cell-bound IgG2a. The mechanism of induction and the possible biological function of these autoantibodies are discussed in the light of their fine specificity and functional properties.

In vivo manipulation of IgG2a production by isotype-specific autoantibodies

Repeated intranasal infection of Balb/c mice with A/PR/8 influenza virus induced an intense antiviral IgG response dominated by the IgG2a subclass, and accompanied by the appearance of IgG2a reactive autoantibodies. Cells producing IgG2a reactive autoantibodies could then be cloned as hybridomas from the virus infected animals. Monoclonal antibodies produced by selected hybridomas U28, Z26 and Z41 produced IgM-type antibodies with strong specificity for the IgG2a isotype bearing "a" allotypic determinants on the Fc region. These IgG2a specific autoantibodies showed highly preferred binding to solid phase bound or aggregated IgG2a, compared to soluble native IgG2a. Based on these characteristics they were classified as mono-reactive rheumatoid factor (RF)-like autoantibodies. Passive administration of IgM type IgG2a-specific autoantibodies to influenza virus infected animals resulted in a long-term reduction in the secondary antiviral response. This could be demonstrated by decreased virus neutralizing activity of the serum and diminished level of IgG2a-type anti-viral antibodies. A similar effect was observed in Balb/c mice contact sensitized with oxazolone: passive administration of RF-like antibodies resulted in reduced IgG2a response to oxazolone while the level of antibodies belonging to other isotypes was not influenced. These results suggest an isotype-specific regulatory function of these RF-like autoantibodies presumably acting via antigen-antibody complexes.