The mechanism of antigenic stimulation of primary and secondary clonal precursor cells

Original Antigenic Sin: How Original? How Sinful?

We review the phenomenon of "original antigenic sin" (OAS) in antibody responses to influenza A virus (IAV) infection or vaccination. OAS refers to the preferential induction of antibodies with higher affinity to priming versus boosting immunogens. We emphasize its mechanistic basis and origins in the basic immunobiology of B-cell responses to myriad immunogens. We tabulate 23 studies in animals and humans to show that the magnitude of OAS depends on many variables. We discuss a number of misconceptions about OAS, examine the extent to which OAS is sinful, and argue that OAS is evolutionary selected and not a deleterious by-product of selection for other features of the immune response. We end by raising questions regarding the mechanistic basis of OAS whose answers could contribute to improving influenza virus vaccines on the road to the holy grail of a "universal" influenza vaccine.

Modeling human adaptive immune responses with tonsil organoids

Most of what we know about adaptive immunity has come from inbred mouse studies, using methods that are often difficult or impossible to confirm in humans. In addition, vaccine responses in mice are often poorly predictive of responses to those same vaccines in humans. Here we use human tonsils, readily available lymphoid organs, to develop a functional organotypic system that recapitulates key germinal center features in vitro, including the production of antigen-specific antibodies, somatic hypermutation and affinity maturation, plasmablast differentiation and class-switch recombination. We use this system to define the essential cellular components necessary to produce an influenza vaccine response. We also show that it can be used to evaluate humoral immune responses to two priming antigens, rabies vaccine and an adenovirus-based severe acute respiratory syndrome coronavirus 2 vaccine, and to assess the effects of different adjuvants. This system should prove useful for studying critical mechanisms underlying adaptive immunity in much greater depth than previously possible and to rapidly test vaccine candidates and adjuvants in an entirely human system.

Reactivation of IgG-switched memory B cells by BCR-intrinsic signal amplification promotes IgG antibody production

Secondary antibody responses are marked by faster kinetics, improved antibody affinity and a switch from IgM to other immunoglobulin isotypes, most notably IgG, compared with primary responses. These changes protect from reinfection and represent the principle of most vaccination strategies. Yet, the molecular mechanisms that underlie B-cell memory responses are unclear. Here we show, by inactivating the immunoglobulin tail tyrosine (ITT) signalling motif of membrane-bound IgG1 in the mouse, that the ITT facilitates maintenance and reactivation of IgG-switched memory B cells in vivo. The ITT motif equips IgG-switched cells with enhanced BCR signalling capacity, which supports their competitiveness in secondary immune reactions and drives the formation of IgG-secreting plasma cells even in the absence of T-cell help. Our results demonstrate that ITT signalling promotes the vigorous production of IgG antibodies and thus provide a molecular basis for humoral immunological memory.

The Memory Function of the B Cell Antigen Receptor

Activated B lymphocytes preserve their antigen experience by differentiating into long-lived pools of antibody-secreting plasma cells or various types of memory B cells (MBCs). The former population constantly produces serum immunoglobulins with sufficient specificity and affinity to thwart infections with recurrent pathogens. By contrast, memory B cell populations retain their antigen receptors on the cell surface and hence need pathogen-induced differentiation steps before they can actively contribute to host defense. The terminal differentiation of MBCs into antibody-secreting plasma cells is hallmarked by the absence of the lag phase characteristic for primary antibody responses. Moreover, secondary antibody responses are predominantly driven by MBCs that bear an antigen receptor of the IgG class on their surface although IgM-positive memory populations exist as well. These fundamental principles of B cell memory were enigmatic for decades. Only recently, we have begun to understand the underlying mechanisms. This review summarizes our current understanding of how different subpopulations of MBCs are generated during primary immune responses and how their functional heterogeneity on antigen recall is controlled by different signaling capabilities of B cell antigen receptor (BCR) isotypes and by the nature of the antigen.

Tumor-Associated Glycans and Immune Surveillance

Changes in cell surface glycosylation are a hallmark of the transition from normal to inflamed and neoplastic tissue. Tumor-associated carbohydrate antigens (TACAs) challenge our understanding of immune tolerance, while functioning as immune targets that bridge innate immune surveillance and adaptive antitumor immunity in clinical applications. T-cells, being a part of the adaptive immune response, are the most popular component of the immune system considered for targeting tumor cells. However, for TACAs, T-cells take a back seat to antibodies and natural killer cells as first-line innate defense mechanisms. Here, we briefly highlight the rationale associated with the relative importance of the immune surveillance machinery that might be applicable for developing therapeutics.

The role of activation-induced deaminase in lupus nephritis

High affinity autoreactive IgG antibodies have been implicated in the development of lupus nephritis and other autoimmune disorders. With the discovery of activation-induced deaminase (AID), this question could be finally tested by examining the impact of AID deficiency in autoimmune-prone mice like the MLR/lpr strain. We have recently shown that AID-deficient MRL/lpr mice experienced a complete abrogation of lupus nephritis, and increased survival despite a dramatic increase in autoreactive IgM. Subsequent studies demonstrated that anti-dsDNA IgM is not pathogenic and in fact protects MRL/lpr from glomerulonephritis. AID-deficiency is also associated with decreased antibody-independent B cell-mediated autoimmunity likely through the loss of high affinity receptors through somatic hypermutation. Combined these results directly implicate AID in the development of B cell mediated autoimmunity. However, studies with hyper IgM AID-deficient patients indicate an increase in the incidence of certain autoimmunities. These results, likely the result of the immunodeficiency associated with AID deficiency, suggest caution in therapeutic approaches based in AID inhibition.

Altered pattern of immunoglobulin hypermutation in mice deficient in Slip-GC protein

We recently identified a novel germinal center GTPase, SLIP-GC, that localizes to replication factories in B cells and that, when reduced, induces DNA breaks in lymphoma B cell lines in an activation-induced deaminase (AID)-dependent manner. Herein, we generated mice deficient in SLIP-GC and examined the impact of SLIP-GC deficiency in immunoglobulin hypermutation and class switch recombination, both AID-dependent mechanisms. SLIP-GC-deficient mice experienced a substantial increase in mutations at G:C base pairs at the region downstream of JH4 in the immunoglobulin heavy chain locus. This change was reflected in the overall mutation frequency, and it was associated with an increase in transitions from G:C base pairs, a hallmark of AID-mediated deamination during replication. In addition, G:C transitions at non-immunoglobulin loci also increased in these mice. Given the intracellular localization of SLIP-GC to sites of replicating DNA, these results suggest that SLIP-GC protects replicating DNA from AID-mediated deamination of cytosines in both strands.

Activation-induced deaminase contributes to the antibody-independent role of B cells in the development of autoimmunity

B cells contribute to autoimmunity both as secretors of pathogenic antibodies and through the activation of autoreactive T cells. B cells and antibodies acquire higher affinity to self-antigen through a process known as immunoglobulin hypermutation or SHM. The contribution of SHM to pathogenic antibody development in lupus has been established in various autoimmune mouse models and by examining antibodies from patients. However, its role in the antibody-independent contribution of B cells to autoimmunity has not been examined. Herein, we generate lupus-prone MRL/lpr mice with a limited IgM-only B cell repertoire, no secreted antibodies and no SHM. This enabled us to isolate the role of somatic hypermutation in B cell-mediated autoimmunity. We found that SHM-deficiency correlated with a reduction in autoreactive B cells, a decrease in T cell activation and a decrease in kidney lymphocytic infiltration. These data establish AID as an important contributor to the antibody-independent role of B cells in autoimmunity.

Altered Ig hypermutation pattern and frequency in complementary mouse models of DNA polymerase ζ activity

To test the hypothesis that DNA polymerase ζ participates in Ig hypermutation, we generated two mouse models of Pol ζ function: a B cell-specific conditional knockout and a knock-in strain with a Pol ζ mutagenesis-enhancing mutation. Pol ζ-deficient B cells had a reduction in mutation frequency at Ig loci in the spleen and in Peyer's patches, whereas knock-in mice with a mutagenic Pol ζ displayed a marked increase in mutation frequency in Peyer's patches, revealing a pattern that was similar to mutations in yeast strains with a homologous mutation in the gene encoding the catalytic subunit of Pol ζ. Combined, these data are best explained by a direct role for DNA polymerase ζ in Ig hypermutation.

The signaling tool box for tyrosine-based costimulation of lymphocytes

Triggering lymphocyte effector functions is controlled by a diverse array of immune cell coreceptors that dampen or potentiate the primary activation signal from antigen receptors. Attenuation of lymphocyte activation has been shown to be accomplished by immunoreceptor tyrosine-based inhibition motifs that upon phosphorylation recruit protein or lipid phosphatases. By contrast, a general concept of signal amplification and/or diversification is still out. However, the recent discovery of antigen receptor-intrinsic costimulation by membrane-bound immunoglobulins in class-switched memory B cells identified a consensus phosphorylation motif that can boost antigen-induced signal chains and is also employed by costimulatory receptors on T and Natural Killer cells to provide secondary signals for cellular activation. Here we define a common basis of tyrosine-based lymphocyte costimulation comprising immunoglobulin tail tyrosine (ITT)-like phosphorylation motifs and their proximal effectors, growth factor receptor-bound protein (Grb) 2 and phosphatidylinositol-3 kinase (PI3K) enzymes of class IA.

Receptors and signaling mechanisms for B-lymphocyte activation, proliferation and differentiation--insights from both in vivo and in vitro approaches

During the last three decades, a number of B-lymphocyte specific surface antigens have been defined some of which may also show activation/differentiation specific expression. Here, we review the various signaling events and the receptor-ligand interactions for B-cell development, activation and differentiation. Our discussion and presentation include reviewing the in vivo and in vitro mechanisms. Focus is on the experiments that give us valuable insights into the B cell signaling mechanisms in vitro. Three significant pathways in B-cell development - c-Kit, FLT-3 and IL-7 signaling pathways are elucidated upon. Both antigen dependent and antigen independent mechanisms of B cell stimulation are also reviewed.

Effect of CpG oligonucleotides on vaccine-induced B cell memory

Adding synthetic oligodeoxynucleotides containing unmethylated CpG motifs to Anthrax vaccine adsorbed (AVA, the licensed human vaccine) increases the speed and magnitude of the resultant Ab response. Ab titers persist in the protective range for >1 year, significantly longer than in animals vaccinated with AVA alone. Unexpectedly, a majority of mice immunized with CpG-adjuvanted AVA maintained resistance to anthrax infection even after their Ab titers had declined into the subprotective range. The survival of these animals was mediated by the de novo production of protective Abs by high affinity memory B cells re-stimulated immediately after challenge. Thus, a previously unrecognized benefit of CpG oligodeoxynucleotides adjuvants is their ability to expand the long-lived memory B cell population. Current findings demonstrate that CpG-adjuvanted AVA mediates protection both by stimulating a strong/persistent serum Ab response and by generating a high-affinity long-lived pool of memory B cells.

Activation-induced deaminase, AID, is catalytically active as a monomer on single-stranded DNA

Hypermutation and class switch recombination of immunoglobulin genes are antigen-activated mechanisms triggered by AID, a cytidine deaminase. AID deaminates cytidine residues in the DNA of the variable and the switch regions of the immunoglobulin locus. The resulting uracil induces error-prone DNA synthesis in the case of hypermutation or DNA breaks that activate non-homologous recombination in the case of class switch recombination. In vitro studies have demonstrated that AID deaminates single-stranded but not double-stranded substrates unless AID is in a complex with RPA and the substrate is actively undergoing transcription. However, it is not clear whether AID deaminates its substrates primarily as a monomer or as a higher order oligomer. To examine the oligomerization state of AID alone and in the presence of single-stranded DNA substrates of various structures, including loops embedded in double-stranded DNA, we used atomic force microscopy (AFM) to visualize AID protein alone or in complex with DNA. Surprisingly, AFM results indicate that most AID molecules exist as a monomer and that it binds single-stranded DNA substrates as a monomer at concentrations where efficient deamination of single-stranded DNA substrates occur. The rate of deamination, under conditions of excess and limiting protein, also imply that AID can deaminate single-stranded substrates as a monomer. These results imply that non-phosphorylated AID is catalytically active as a monomer on single-stranded DNA in vitro, including single-stranded DNA found in loops similar to those transiently formed in the immunoglobulin switch regions during transcription.

Naive CD4(+) T cell frequency varies for different epitopes and predicts repertoire diversity and response magnitude

Cell-mediated immunity stems from the proliferation of naive T lymphocytes expressing T cell antigen receptors (TCRs) specific for foreign peptides bound to host major histocompatibility complex (MHC) molecules. Because of the tremendous diversity of the T cell repertoire, naive T cells specific for any one peptide:MHC complex (pMHC) are extremely rare. Thus, it is not known how many naive T cells of any given pMHC specificity exist in the body or how that number influences the immune response. By using soluble pMHC class II (pMHCII) tetramers and magnetic bead enrichment, we found that three different pMHCII-specific naive CD4(+) T cell populations vary in frequency from 20 to 200 cells per mouse. Moreover, naive population size predicted the size and TCR diversity of the primary CD4(+) T cell response after immunization with relevant peptide. Thus, variation in naive T cell frequencies can explain why some peptides are stronger immunogens than others.

Abrogation of lupus nephritis in activation-induced deaminase-deficient MRL/lpr mice

We generated MRL/lpr mice deficient in activation-induced deaminase (AID). Because AID is required for Ig hypermutation and class switch recombination, these mice lack hypermutated IgG Abs. Unlike their AID wild-type littermates, AID-deficient MRL/lpr mice not only lacked autoreactive IgG Abs but also experienced a dramatic increase in the levels of autoreactive IgM. This phenotype in AID-deficient mice translated into a significant reduction in glomerulonephritis, minimal mononuclear cell infiltration in the kidney, and a dramatic increase in survival to levels comparable to those previously reported for MRL/lpr mice completely lacking B cells and well below those of mice lacking secreted Abs. Therefore, this study wherein littermates with either high levels of autoreactive IgM or autoreactive IgG were directly examined proves that autoreactive IgM Abs alone are not sufficient to promote kidney disease in MRL/lpr mice. In addition, the substantial decrease in mortality combined with a dramatic increase in autoreactive IgM Abs in AID-deficient MRL/lpr mice suggest that autoreactive IgM Abs might not only fail to promote nephritis but may also provide a protective role in MRL/lpr mice. This novel mouse model containing high levels of autoreactive, unmutated IgM Abs will help delineate the contribution of autoreactive IgM to autoimmunity.

Decreased frequency of somatic hypermutation and impaired affinity maturation but intact germinal center formation in mice expressing antisense RNA to DNA polymerase zeta

To examine a role of DNA polymerase zeta in somatic hypermutation, we generated transgenic mice that express antisense RNA to a portion of mouse REV3, the gene encoding this polymerase. These mice express high levels of antisense RNA, significantly reducing the levels of endogenous mouse REV3 transcript. Following immunization to a hapten-protein complex, transgenic mice mounted vigorous Ab responses, accomplished the switch to IgG, and formed numerous germinal centers. However, in most transgenic animals, the generation of high affinity Abs was delayed. In addition, accumulation of somatic mutations in the V(H) genes of memory B cells from transgenic mice was decreased, particularly among those that generate amino acid replacements that enhance affinity of the B cell receptor to the hapten. These data implicate DNA polymerase zeta, a nonreplicative polymerase, in the process of affinity maturation, possibly through a role in somatic hypermutation, clonal selection, or both.

3',5'-cyclic adenosine monophosphate regulates expression of synaptotagmin in neonatal sympathetic ganglia in vitro

The expression of the synaptic vesicle protein, synaptotagmin, in developing rat superior cervical ganglia is influenced by transsynaptic factors associated with membrane depolarization. The present study examines the role of cyclic AMP in the regulation of synaptotagmin in neonatal superior cervical ganglia maintained in explant culture. Ganglia were treated for 48 h in vitro with the Na+-channel ionophore, veratridine, or with pharmacological agents that alter cyclic AMP levels. Levels of cyclic AMP and synaptotagmin were determined by radioimmunoassay. Veratridine treatment significantly increased cyclic AMP in cultured ganglia, with a long time course, and also increased synaptotagmin levels. Drugs that elevate cyclic AMP levels significantly increased synaptotagmin levels, with similar magnitude to that produced by veratridine treatment. These pharmacological agents did not alter neuron survival or total ganglionic protein content. No additive effects were observed after combined treatment with veratridine and pharmacological agents that increased cyclic AMP. Agents that blocked adenylyl cyclase blocked the veratridine-induced increase in synaptotagmin levels. The results suggest that regulation of expression of synaptotagmin in neonatal sympathetic neurons is mediated partially by cyclic AMP.

Antigen Drives Very Low Affinity B Cells to Become Plasmacytes and Enter Germinal Centers

In the first week of the primary immune response to the (4-hydroxy-3-nitrophenyl)acetyl (NP) hapten, plasmacytic foci and germinal centers (GCs) in C57BL/6 mice are comprised of polyclonal populations of B lymphocytes bearing the λ1 L-chain (λ1+). The Ig H-chains of these early populations of B cells are encoded by a variety of VH and D exons undiversified by hypermutation while later, oligoclonal populations are dominated by mutated rearrangements of the VH186.2 and DFL16.1 gene segments. To assess directly Ab affinities within these defined splenic microenvironments, representative VDJ rearrangements were recovered from B cells participating in the early immune response to NP, inserted into Ig H-chain expression cassettes, and transfected into J558L (H−; λ1+) myeloma cells. These transfectoma Abs expressed a remarkably wide range of measured affinities (Ka = 5 × 104-1.3 × 106 M−1) for NP. VDJs recovered from both foci and early GCs generated comparable affinities, suggesting that initial differentiation into these compartments occurs stochastically. We conclude that Ag normally activates B cells bearing an unexpectedly wide spectrum of Ab affinities and that this initial, promiscuous clonal activation is followed by affinity-driven competition to determine survival and clonal expansion within GCs and entry into the memory and bone marrow plasmacyte compartments.

B Cell Tolerance to a Minor, But Not to a Major, Antigenic Surface of the Self Antigen, Cytochrome c

To study B cell tolerance to the mitochondrial protein cytochrome c (CYT), the B cell response to pigeon CYT (PCC) was examined in mice transgenic for PCC. PCC was coupled to OVA to provide T cell help, since PCC-specific T cells in PCC-transgenic mice are deleted in the thymus. The frequency of secondary B cells responding to the minor antigenic surface around residue 44 on PCC was decreased about 10-fold in native PCC-transgenic mice compared with that in control mice or in transgenic mice expressing an altered form of PCC that lacked the heme and had a different amino acid sequence at the N-terminus. A similar decrease has been observed in the frequency of B cells in normal mice recognizing the site around residue 44 on mouse CYT compared with the frequency of B cells recognizing the corresponding site on foreign CYT. There were no major decreases but apparently were compensatory increases in the frequencies of B cells recognizing other sites on PCC in the native PCC-transgenic mice compared with those in other mice. These results indicate that B cells in mice are only partially tolerant to self CYT. A possible basis for this partial tolerance relating to the fate of CYT in cell death is discussed. This may be the first example of the use of a transgenic system to study B cell tolerance to a homologous self Ag.

A Periarteriolar Lymphoid Sheath-Associated B Cell Focus Response Is Not Observed During the Development of the Anti-Arsonate Germinal Center Reaction

The behavior of p-azophenylarsonate (Ars)-specific B cell clones during the primary T cell-dependent splenic response of A/J mice was investigated using an immunohistochemical approach. The earliest Ars-specific B cells were observed as isolated cells in the red pulp by day 3 after immunization with Ars-keyhole limpet hemocyanin, (KLH) and at day 6, large clusters of Ars-specific B cells were first detected in germinal centers, which continued to be observed for an additional 8 to 15 days. Surprisingly, no Ars-specific B cell foci were observed in or near the CD4 T cell-rich periarteriolar lymphoid sheath (PALS) during the entire primary response. Nevertheless, A/J mice immunized with (4-hydroxy-3-nitrophenyl)acetyl-chicken gamma globulin (NP-CGG) or Ars-CGG mounted robust splenic (4-hydroxy-3-nitrophenyl)acetyl or CGG-specific PALS-associated focus reactions, respectively. In contrast, no Ars-specific PALS B cell foci were detected in A/J mice immunized with Ars-CGG. These data add to a growing body of evidence indicating that B cell proliferation and differentiation in CD4 T cell-rich microenvironments are not prerequisites for the GC reaction. Taken together with previous results obtained using other model Ags, the data suggest that the specificity of the B cell Ag receptor may strongly influence the lymphoid microenvironment in which a B cell clone first undergoes Ag-driven clonal expansion and differentiation.

DNA vaccines: safety and efficacy issues

DNA technology has been harnessed to produce a variety of plasmid-based vaccines designed to prevent viral, bacterial and parasitic infections. The rapid adoption and implementation of this novel vaccine strategy carries with it important safety and efficacy concerns. This review will focus on whether DNA vaccines (1) are likely to induce systemic or organ-specific autoimmune disease, (2) have the potential to induce tolerance rather than immunity, and (3) are as effective in individuals with depressed immune function as they are in healthy adults.

The B-cell biology of aging

Although both the number and responsiveness of peripheral B cells in aged mice remain relatively intact, there are dramatic changes in B-cell generation. Alterations in B-cell development include both a skewing of V-gene utilization, especially in cells responsive to phosphorylcholine, and a decrease in the generation of various developmental B-cell subsets. The altered representation of these subsets appears to be the consequence of two developmental blocks. The first developmental block occurs during the maturation of pro-B cells and is evidenced by a decrease in the number of pre-B cells. The second developmental block occurs at the earliest stage of sIg(+)-cell maturation (sIgMvery lo). Because of this block in B-cell maturation, in spite of a decrease in incoming pre-B cells, the number of sIgMvery lo cells appears to increase in aged mice. Additionally, the time of residence of cells within this maturational stage increases dramatically, while the proportion of cells in more mature (sIgMhi) stages of bone marrow development are decreased. In addition to the decreased number of maturing bone marrow B cells, the population of splenic B cells that represent recent bone marrow émigrés (HSAvery hi) is markedly decreased. In the face of this decrease in newly emerging cells from the bone marrow, the population of mature splenic B cells is maintained by their increased longevity.

Long-term effect of primary immunization on subsequent immune responsiveness

Specific antigen/adjuvant combinations preferentially induce type 1 or type 2 cytokine responses. For example, BALB/c mice primed with TNP-ovalbumin in complete Freund's adjuvant (TNP-OVA/CFA) produce a type 2-dominated response characterized by the activation of IL-4-secreting cells and the production of IgG1 and IgE anti-TNP antibodies. In contrast, mice primed with TNP conjugated to Brucella abortus (TNP-BA) produce a type 1 response dominated by the secretion of IFN-gamma and IgG2a anti-TNP antibodies. We examined whether treating young mice with these antigen/adjuvant combinations altered the cytokine profile of their subsequent immune responses. Mice immunized with TNP-BA and boosted several months later with TNP-OVA/CFA developed a cytokine and antibody profile similar to the priming rather than boosting antigen. This was also observed in mice immunized with TNP-OVA/CFA and boosted with TNP-BA. Both the ratio of IL-4:IFN-gamma-secreting cells and the isotype of antibodies produced by these mice were altered by primary immunization. Analysis of Con A-responsive cells from these animals showed that long-lived changes in the frequency of T lymphocytes available to secrete type 1 versus type 2 cytokines were induced by strong primary immunogens.

Induction of neonatal tolerance by plasmid DNA vaccination of mice

Plasmid DNA vaccines capable of preventing viral, bacterial, and parasitic infections are currently under development. Our labs have shown that a plasmid DNA vaccine encoding the circumsporozoite protein of the malaria parasite elicits protective immunity against live sporozoite challenge in adult BALB/c mice. We now find that the same DNA vaccine induces tolerance rather than immunity when administered to 2-5 d-old mice. Neonatally tolerized animals were unable to mount antibody, cytokine or cytotoxic responses when rechallenged with DNA vaccine in vitro or in vivo. Tolerance was specific for immunogenic epitopes expressed by the vaccine-encoded, endogenously produced antigen. Mice challenged with exogenous circumsporozoite protein produced antibodies against a different set of epitopes, and were not tolerized. These findings demonstrate important differences in the nature and specificity of the immune response elicited by DNA vaccines versus conventional protein immunogens.

Limiting dilution analysis of virus-specific memory B cells by an ELISPOT assay

Many acute viral infections induce long-term, sometimes even life-long humoral immunity. To characterize this immune response, accurate quantitation of memory B cells and plasma cells is essential. Plasma cells can be quantitated directly ex vivo by virtue of their ability to spontaneously secrete antibody. Memory B cells on the other hand, do not spontaneously secrete antibody but require antigenic stimulation in order to proliferate and differentiate into antibody secreting cells (ASC). In this study, an ELISPOT-based limiting dilution assay (LDA) was developed for quantitating virus-specific B cell memory following acute lymphocytic choriomeningitis virus (LCMV) infection of adult mice. Antiviral memory B cell precursor (MBCp) frequencies were calculated from in vitro stimulated cultures using either a conventional ELISA-based LDA to measure accumulated virus-specific antibody in the culture medium or a new ELISPOT-based LDA that identifies the antibody-secreting daughter cells directly. In terms of sensitivity, the ELISPOT-based LDA and the ELISA-based LDA both calculated LCMV-specific MBCp frequencies to be approximately 1/2 x 10(4) spleen cells. However, compared to the 12 days of in vitro stimulation required to estimate MBCp frequencies by the ELISA-based LDA, the ELISPOT-based LDA required only 3-6 days of stimulation to quantitate MBCp frequencies. If cell division was blocked by gamma-irradiation or treatment with mitomycin C, the MBCp frequency dropped below detection (< 1 MBCp/10(6) cells), indicating that virus-specific B cells quantitated by this assay must both proliferate and differentiate into antibody secreting cells in order to be detected. Naive, uninfected mice did not have LCMV-specific memory B cells, demonstrating that only in vivo-generated antiviral B cells from LCMV-immune mice were detected by this assay. chi 2 analysis of the ELISPOT-based LDA showed that the MBCp frequency data fit a linear regression model (p = 0.0137), indicating single-hit kinetics in which only one cell type was limiting. These results indicate that the ELISPOT-based LDA provides a rapid and statistically accurate method to quantitate virus-specific B cells.

Induction of cell cycle regulatory proteins in anti-immunoglobulin-stimulated mature B lymphocytes

Progression through the cell cycle is a tightly controlled process that integrates signals generated at the plasma membrane with the proteins that form the cell cycle machinery. The current study chronicles the induction of cyclins, cyclin-dependent kinases (cdk), and cdk inhibitors in low density primary mouse B lymphocytes after anti-immunoglobulin plus interleukin 4 (IgM + IL-4) stimulation. In this system, > 85% of cells remain in the G0/G1 phase of cell cycle for an initial 24-h period, followed by entry of up to 50% of the cells into S phase, commencing around 30 h and peaking at 48 h. Extensive time course analyses of these anti-IgM + IL-4-stimulated B cells revealed that the G1-associated D-type cyclins D2 and D3 were induced by 3 h after stimulation, and that cyclins E, A, and B were subsequently induced sequentially, beginning at mid-G1, G1/S transition, and S phase, respectively. The G1-associated cyclin D1 was not expressed at any stage of the anti-Ig + IL-4-induced B cell cycle. cdk2, cdk4, and cdk6 were induced during G1, whereas cell division cycle-2 (cdc2) was induced concomitantly with S phase. Irrespective of their expression, the kinases cdk2 and cdc2 were only active from S phase onwards, suggesting that productive cyclin/kinase complex formation did not occur until that time. Cell cycle inhibitors p21 and p19 were induced by anti-Ig + IL-4, peaking in expression at mid-G1 and S phase, respectively. Stimulation of low density B cells with anti-Ig + IL-4 caused rapid down regulation of the p27 inhibitor, however this protein was reexpressed at 54-96 h after stimulation. In contrast, B cells stimulated with anti-CD40, a stimulus which induces long-term B cell proliferation, permanently down regulated p27. These findings are consistent with the concept that p27 reexpression contributes to the G1 arrest that follows antigen receptor crosslinking. Low density B cells cultured in the viability-enhancing cytokine IL-4 alone also showed induction of D2 and D3 cyclin expression. However, the D2 expression was transient, and the D3 expression was substantially lower than that observed in B cells induced to proliferate by anti-Ig + IL-4. This partial induction of D2 and D3 expression may explain IL-4's ability to promote B cell entry into G1 but not S phase of cell cycle, and furthermore, its ability to truncate G1 progression when B cells are subsequently stimulated with anti-Ig.

A new bell-shaped function for idiotypic interactions based on cross-linking

Most recent models of the immune network are based upon a phenomenological log bell-shaped interaction function. This function depends on a single parameter, the "field," which is the sum of all ligand concentrations weighted by their respective affinities. The typical behavior of these models is dominated by percolation, a phenomenon in which a local stimulus spreads globally throughout the network. The usual reason for employing a log bell-shaped interaction function is that B cells are activated by cross-linking of their surface immunoglobulin receptors. Here we formally derive a new phenomenological log bell-shaped function from the chemistry of receptor cross-linking by bivalent ligand. Specifying how this new function depends on the ligand concentrations requires two fields: a binding field and a cross-linking field. When we compare the activation functions for ligand-receptor pairs with different affinities, the one-field and the two-field functions differ markedly. In the case of the one-field activation function, its graph is shifted to increasingly higher concentration as the affinity decreases but keeps its width and height. In the case of the two-field activation function, the graph of a low-affinity interaction is nested within the graphs of all higher-affinity interactions. We show that this difference in the relations among activation functions for different affinities radically changes the network behavior. In models that described B cell proliferation using the one-field activation function, network behavior was dominated by low-affinity interactions. Conversely, in our new model, the high-affinity interactions are the most significant. As a consequence, percolation is no longer the only typical network behavior.

Cross-linking reconsidered: binding and cross-linking fields and the cellular response

We analyze a model for the reversible cross-linking of cell surface receptors by a collection of bivalent ligands with different affinities for the receptor as would be found in a polyclonal anti-receptor serum. We assume that the amount of cross-linking determines, via a monotonic function, the rate at which cells become activated and divide. In addition to the density of receptors on the cell surface, two quantities, the binding field and the cross-linking field, are needed to characterize the cross-linking curve, i.e., the equilibrium concentration of cross-linked receptors plotted as a function of the total ligand site concentration. The binding field is the sum of all ligand site concentrations weighted by their respective binding affinities, and the cross-linking field is the sum of all ligand site concentrations weighted by the product of their respective binding and cross-linking affinity and the total receptor density. Assuming that the cross-linking affinity decreases if the binding affinity decreases, we find that the height of the cross-linking curve decreases, its width narrows, and its center shifts to higher ligand site concentrations as the affinities decrease. Moreover, when we consider cross-linking-induced proliferation, we find that there is a minimum cross-linking affinity that must be surpassed before a clone can expand. We also show that under many circumstances a polyclonal antiserum would be more likely than a monoclonal antibody to lead to cross-linking-induced proliferation.

Organ culture of human lymphoid tissue. I. Characteristics of the system

The major aim of three-dimensional tissue culture is to preserve the natural architecture of the tissue and thereby allow the cells to retain their original functions during in vitro cultivation. Here we describe a method for the rapid preparation of three-dimensional tissue explants from human lymphoid organs. The precision-cut tissue slices are of uniform size and thickness and can be cryopreserved and stored in liquid nitrogen without substantial loss of viability or functionality of the cells. Upon in vitro culture, cells within the explants survived as well as their counterparts cultured in single cell suspension. However, spontaneous immunoglobulin (Ig) production in explants started more promptly and often reached considerably higher levels than that in suspension cultures run in parallel. Lymphocytes within the slices could be activated by polyclonal stimuli such as PHA, as shown by the upregulation of the activation markers CD23 and CD25 on B and T cells, respectively. However, approximately five-fold higher concentrations of mitogen than those used for suspension cultures were needed. Taken together, the system presented here constitutes a potent tool for the investigation of the complex interactions leading to activation and differentiation of B and T cells in lymphoid organs.

Altering the antibody repertoire via transgene homologous recombination: evidence for global and clone-autonomous regulation of antigen-driven B cell differentiation

Antibody VH transgenes containing small amounts of natural 5' and 3' flanking DNA undergo nonreciprocal homologous recombination with the endogenous Igh locus in B cells. The resulting "hybrid" heavy chain loci are generated at a low frequency but are fully functional, undergoing somatic hypermutation and isotype class switching. We have used this recombination pathway to introduce a somatically mutated variable (V) region with an unusually high affinity for the hapten p-azophenylarsonate (Ars) into the preimmune antibody repertoire. The affinity of this V region for Ars is 100-fold higher than any unmutated anti-Ars antibody previously characterized. Expression of the transgene-encoded V region did not affect many aspects of antigen-driven B cell differentiation, including somatic hypermutation, in either Ars-specific transgene- or endogenous V gene-expressing clones. Thus, the regulation of these processes appears to operate in a "global" fashion, in that the mechanisms involved are imperceptive of the relative affinities for antigen of the antibodies expressed by B cell clones participating in the immune response. In contrast, the selection of V region mutants leading to affinity maturation and memory cell formation was found to be strongly influenced by the transgenic V region, but only in clones expressing this V region. Hybridomas derived from transgene- and endogenous V region-expressing memory cells were isolated at similar frequencies from individual transgenic mice. The V regions expressed by hybridomas in both of these groups had 2- to 30-fold greater affinity for Ars than their unmutated precursors, despite the fact that the transgene-encoded precursors had 100-fold higher affinity than their endogenous counterparts. These results show that the criterion for entry into the memory compartment is established not by the affinity of a B cell's V region relative to all other V regions expressed during the response, but by the affinity of this V region relative to its unmutated precursor. Thus, the development of B cell memory is regulated in a "clone-autonomous" fashion.

Transient dominance of the early primary immune response by a highly conserved B-cell clone that is distinguished by its lack of memory, high threshold of activation, and a high affinity

We establish here that the very early primary response to the hapten phthalate (Xmp) is distinguished by a restricted heterogeneity with over 80% of the anti-Xmp antibodies expressing a single well-defined cross-reactive idiotype (CRIXmp-1) associated with a previously described highly conserved clonotype that is expressed by most inbred strains of mice and many outbred mouse populations as well. The characteristic early dominance of this one clonotype in the primary response is transient. While the CRIXmp-1 clonotype is present later in the primary and throughout the secondary response, it represents only a very small proportion of the total anti-Xmp antibody population at these times. The early dominance of the single clonotype is rapidly replaced by a heterogeneous population of antibodies. Differential activation thresholds for the primary response clonotype (CRIXmp-1) and secondary response clonotypes, and the failure of the dominant primary response clonotype to expand in the secondary response (i.e., absence of memory) suggest the presence of two distinct B-cell lineages.

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.

A natural IgM antibody does inhibit polyclonal and antigen-specific IgM but not IgG B-cell responses

Since a B-cell growth-inhibitory natural IgM antibody was identified in the culture supernatants of LPS-stimulated murine splenic B lymphocytes [11], attempts have been made to define other possible functional role(s) of this antibody. Here we show that this regulatory IgM is able to inhibit not only the proliferation of splenic B cells, but also their IgM secretion during LPS-induced polyclonal, as well as antigen (FITC-KLH)-specific antibody responses. In contrast, IgG1 production of hapten (FITC)-specific B cells neither during restimulation with LPS nor in the presence of carrier-specific T lymphocytes in vitro was affected by regulatory IgM. Therefore, whereas newly emerging naive B cells are highly susceptible, IgG-secreting B cells appear to be completely resistant to inactivation by the regulatory IgM autoantibody.

Three-dimensional structure of the human class II histocompatibility antigen HLA-DR1

The three-dimensional structure of the class II histocompatibility glycoprotein HLA-DR1 from human B-cell membranes has been determined by X-ray crystallography and is similar to that of class I HLA. Peptides are bound in an extended conformation that projects from both ends of an 'open-ended' antigen-binding groove. A prominent non-polar pocket into which an 'anchoring' peptide side chain fits is near one end of the binding groove. A dimer of the class II alpha beta heterodimers is seen in the crystal forms of HLA-DR1, suggesting class II HLA dimerization as a mechanism for initiating the cytoplasmic signalling events in T-cell activation.

Peptide-major histocompatibility complex class II complexes with mixed agonist/antagonist properties provide evidence for ligand-related differences in T cell receptor-dependent intracellular signaling

Clonal activation of CD4+ and CD8+ T lymphocytes depends on binding of peptide-major histocompatibility complex (MHC) molecule complexes by their alpha/beta receptors, eventually resulting in sufficient aggregation to initiate second messenger generation. The nature of intracellular signals resulting from such T cell receptor (TCR) occupancy is believed to be independent of the specific structure of the ligand being bound, and to vary quantitatively, not qualitatively, with the concentration of ligand offered and the affinity of the receptor for the peptide-MHC molecule complex. In contrast to the expectations of this model, the analysis of the response of a T helper type 1 clone to mutant E alpha E beta k molecules in the absence or presence of a peptide antigen revealed that peptide inhibited the interleukin 2 (IL-2) response to an otherwise allostimulatory mutant form of this MHC class II molecule. The inhibition was not due to competition for formation of alloantigen, it required TCR recognition of peptide-mutant MHC molecule complexes, and it decreased IL-2 production without affecting receptor-dependent IL-3, IL-2 receptor alpha, or size enlargement responses. This preferential reduction in IL-2 secretion could be correlated with the costimulatory signal dependence of this cytokine response, but could not be overcome by crosslinking the CD28 molecule on the T cell. These results define a new class of TCR ligands with mixed agonist/antagonist properties, and point to a ligand-related variation in the quality of clonotypic receptor signaling events or their integration with other signaling processes. It was also found that a single TCR ligand showed greatly different dose thresholds for the elicitation of distinct effector responses from a cloned T cell population. The observations that changes in ligand structure can result in qualitative alterations in the effects of receptor occupancy and that quantitative variations in ligand density can be translated into qualitative differences in T cell responses have important implications for models of intrathymic selection and control of the results of active immunization.

Age-dependent effects of deafferentation of the rat superior cervical ganglion on expression of P65 (synaptotagmin) during postnatal development

Previous studies have shown that deafferentation of the rat superior cervical ganglion (SCG) alters the levels of p65 (synaptotagmin), a synaptic vesicle integral membrane protein, within the ganglion. Neonatal deafferentation blocks normal postnatal increases in p65, while deafferentation in adult animals produces a transient increase in p65 expression. The present study examines the time course of the shift from the neonatal to adult pattern of response to deafferentation. Neonatal and 7 day old rats showed the neonatal response to deafferentation. Ganglia from rats aged 14 days or older at deafferentation exhibited the transient increase in p65 at 7 days after surgery. The shift from the neonatal to adult response occurs during the second postnatal week. The change in response to deafferentation may be associated with refinement of synaptic function in a manner yet to be determined.

Pattern formation in one- and two-dimensional shape-space models of the immune system

A large-scale model of the immune network is analyzed, using the shape-space formalism. In this formalism, it is assumed that the immunoglobulin receptors on B cells can be characterized by their unique portions, or idiotypes, that have shapes that can be represented in a space of a small finite dimension. Two receptors are assumed to interact to the extent that the shapes of their idiotypes are complementary. This is modeled by assuming that shapes interact maximally whenever their coordinates in the space-space are equal and opposite, and that the strength of interaction falls off for less complementary shapes in a manner described by a Gaussian function of the Euclidean "distance" between the pair of interacting shapes. The degree of stimulation of a cell when confronted with complementary idiotypes is modeled using a log bell-shaped interaction function. This leads to three possible equilibrium states for each clone: a virgin, an immune, and a suppressed state. The stability properties of the three possible homogeneous steady states of the network are examined. For the parameters chosen, the homogeneous virgin state is stable to both uniform and sinusoidal perturbations of small amplitude. A sufficiently large perturbation will, however, destabilize the virgin state and lead to an immune reaction. Thus, the virgin system is both stable and responsive to perturbations. The homogeneous immune state is unstable to both uniform and sinusoidal perturbations, whereas the homogeneous suppressed state is stable to uniform, but unstable to sinusoidal, perturbations. The non-uniform patterns that arise from perturbations of the homogeneous states are examined numerically. These patterns represent the actual immune repertoire of an animal, according to the present model. The effect of varying the standard deviation sigma of the Gaussian is numerically analyzed in a one-dimensional model. If sigma is large compared to the size of the shape-space, the system attains a fixed non-uniform equilibrium. Conversely if sigma is small, the system attains one out of many possible non-uniform equilibria, with the final pattern depending on the initial conditions. This demonstrates the plasticity of the immune repertoire in this shape-space model. We describe how the repertoire organizes itself into large clusters of clones having similar behavior. These results are extended by analyzing pattern formation in a two-dimensional (2-D) shape-space.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Tolerance to liver-specific antigens

We have described a TG model for peripheral tolerance of alloreactive CTL. Expression of Q10/L on hepatocytes renders mice functionally tolerant, although in vitro we observe that TG animals have normal numbers of CTL.Pf directed against this antigen. The basis for the tolerance presumably resides in the fact that the TG mice are lacking a subpopulation, either through deletion or anergy, that is responsible for recognition of the antigen on hepatocytes in vivo. The data are consistent with a tolerance model where cells with high affinity receptors are silenced. The presumed low affinity antigen-specific cells remaining in TG mice cannot be primed in vivo when immunized with antigen on spleen cells. Further, these CTL generate poor lytic activity in vitro. This failure to prime TG CTL in vivo could be attributed to primed cells traveling to the liver where they become tolerized when exposed to antigen on hepatocytes. However, we show that TG cells, after transfer to non-TG recipients, cannot be primed in vivo, indicating that the presumed low-affinity cells remaining in TG mice are not readily activable in this milieu. These data also indicate that this tolerance is not readily reversible during a 10- to 17-d time interval.

B cells in autoimmune (NZB x NZW)F1 mice show altered IgG isotype switching upon T cell-dependent antigenic stimulation in vitro

Humoral immune responses of (NZB x NZW)F1 (BWF1) autoimmune mice to T cell-dependent antigens often exhibit a predominance of IgG2 antibodies, while normal mice produce IgG1 antibodies. In order to determine whether this results from differences in properties of the B cells or the T cells involved, the responses of both primary and secondary BWF1 B cells to the antigen DNP-hemocyanin (Hy) were measured in limiting dilution splenic fragment cultures in the presence of normal T cell help. Furthermore, the capacity of Hy-primed lymph node T cells from BWF1 mice to provide help to BALB/c nu/nu B cells was determined in modified splenic fragment cultures. These experiments indicated that (a) stimulation of primary BWF1 B cells with DNP-Hy and normal T cell help failed to yield significant numbers of clones which produced any of the IgG isotypes; (b) antigenic stimulation of BWF1 secondary B cell clones also demonstrated a paucity of IgG1, but elevated production of IgG2 isotypes; and (c) Hy-primed BWF1 lymph node T cells were comparable to those derived from BALB/c mice in their capacity to provide both help for nu/nu B cell responses and modulation of IgG isotype switching. BWF1 B cells apparently differ from normal murine B cells in their capacity to produce IgG antibodies upon T cell-dependent antigenic stimulation.

The isotype potential of B cells present in BALB/c mice chronically infected with Mesocestoides corti

Infection of BALB/c mice with Mesocestoides corti results in a chronic infection with a pronounced splenomegaly and hypergammaglobulinemia. A prominent feature of this infection is that the vast majority of serum immunoglobulin produced is restricted to IgG1 and IgM. As much as 30-fold increases in serum IgG1 levels have been noted. To ascertain whether, as a result of infection, the resident B cell pool is committed to IgG1, B cells from infected animals were tested for their ability to produce various isotypes after stimulation. In one series of experiments, B cells from normal and infected animals were used as donor cells in the splenic fragment assay. The results show that the frequency of 2,4-dinitrophenyl-specific and phosphorylcholine-specific B cells remains unaltered in infected animals compared to controls. Importantly, the hapten-specific B cell clones induced were found to express multiple isotypes. These results demonstrate that the nonactivated B cell pool in spleens of infected mice is not committed to IgG1 and IgM production.

Control of primary and secondary antibody responses by cytotoxic T lymphocytes specific for a soluble antigen

BALB/c mice immunized with keyhole limpet hemocyanin (KLH) develop Thy-1+ and CD8+, KLH-specific cytotoxic T cells (Tc). Such Tc cells can lyse TNP-specific B cells activated with TNP-KLH, but not with TNP-ovalbumin. Cytotoxicity was inhibited by anti-H-2K/D antibodies, but not by anti-Ia antibodies, suggesting a major histocompatibility complex class I-restricted killing. Selective enrichment for virgin and memory TNP-specific B cells revealed that the latter cells were relatively resistant to lysis by KLH-specific Tc cells. Depletion of CD8+ T cells from cultures of TNP-specific virgin B cells activated with TNP-KLH and KLH-primed T cells, increased the titer of anti-TNP antibodies in the culture supernatants. This increase was reduced if KLH-primed CD8+ T cells were added to the culture 1 day before its termination. Anti-TNP antibody secretion by memory B cells activated in the same manner was not affected by depletion or addition of CD8+ cells. These results suggest that Tc cells are generated following immunization with a soluble antigen which may participate in the down-regulation of primary B cell responses.