Redemption of autoantibodies on anergic B cells by variable-region glycosylation and mutation away from self-reactivity

The best-understood mechanisms for achieving antibody self/non-self discrimination discard self-reactive antibodies before they can be tested for binding microbial antigens, potentially creating holes in the repertoire. Here we provide evidence for a complementary mechanism: retaining autoantibodies in the repertoire displayed as low levels of IgM and high IgD on anergic B cells, masking a varying proportion of autoantibody-binding sites with carbohydrates, and removing their self-reactivity by somatic hypermutation and selection in germinal centers (GCs). Analysis of human antibody sequences by deep sequencing of isotype-switched memory B cells or in IgG antibodies elicited against allogeneic RhD+ erythrocytes, vaccinia virus, rotavirus, or tetanus toxoid provides evidence for reactivation of anergic IgM(low) IgD+ IGHV4-34+ B cells and removal of cold agglutinin self-reactivity by hypermutation, often accompanied by mutations that inactivated an N-linked glycosylation sequon in complementarity-determining region 2 (CDR2). In a Hy10 antibody transgenic model where anergic B cells respond to a biophysically defined lysozyme epitope displayed on both foreign and self-antigens, cell transfers revealed that anergic IgM(low) IgD+ B cells form twice as many GC progeny as naïve IgM(hi) IgD+ counterparts. Their GC progeny were rapidly selected for CDR2 mutations that blocked 72% of antigen-binding sites with N-linked glycan, decreased affinity 100-fold, and then cleared the binding sites of blocking glycan. These results provide evidence for a mechanism to acquire self/non-self discrimination by somatic mutation away from self-reactivity, and reveal how varying the efficiency of N-glycosylation provides a mechanism to modulate antibody avidity.

Reproducible selection of high avidity CD8+ T-cell clones following secondary acute virus infection

The recall of memory CD8(+) cytotoxic T lymphocytes (CTLs), elicited by prior virus infection or vaccination, is critical for immune protection. The extent to which this arises as a consequence of stochastic clonal expansion vs. active selection of particular clones remains unclear. Using a parallel adoptive transfer protocol in combination with single cell analysis to define the complementarity determining region (CDR) 3α and CDR3β regions of individual T-cell receptor (TCR) heterodimers, we characterized the antigen-driven recall of the same memory CTL population in three individual recipients. This high-resolution analysis showed reproducible enrichment (or diminution) of particular TCR clonotypes across all challenged animals. These changes in clonal composition were TCRα- and β chain-dependent and were directly related to the avidity of the TCR for the virus-derived peptide (p) + major histocompatibility complex class I molecule. Despite this shift in clonotype representation indicative of differential selection, there was no evidence of overall repertoire narrowing, suggesting a strategy to optimize CTL responses while safeguarding TCR diversity.

Immune Response to Viruses: Antibody-Mediated Immunity

The notion about immunity to disease arose from the observation that those who recovered from an apparently contagious disease became resistant to a subsequent similar sickness. Much later it was shown that immunity is transferable by serum. The active serum components were identified to be immunoglobulins (Ig's), called antibodies. The enormous diversity of antibodies specific for distinct viruses, pathogens, and many other antigens is explained by clonal selection, whereby specific B-lymphocyte receptors recognize a particular antigen. The selected B-cells are triggered to undergo replication. A process of further cell differentiation and maturation ensues, leading to secretion of antibodies with high binding affinity toward the triggering antigen. Genes coding for the variable regions (involved in antigen binding) of Ig's are inherited as sets of gene fragments joined to form a complete gene in individual B-cells. This process and further hypermutations ensure the synthesis of diverse high affinity antibodies. The antibodies consist of pairs of light (L) and heavy (H) polypeptide chains. Variations in the constant portion of H-chains lead to production of Ig isotypes (IgM, IgA, IgD, IgE, and IgG (further subdivided into IgG1, IgG2, IgG3 and IgG4)), each having distinct effector functions. Host exposure to viruses leads to the production of antibodies with more than one specificity. Only some of these antibodies, recognizing so-called virus neutralization epitopes, diminish or eliminate virus infectivity. Other virus-specific antibodies play auxiliary roles or are ineffective. Sometimes antibodies cause enhancement of viral diseases or play a role in evasion of the immune system. Many antiviral immunoglobulins are being used for short-term pre-exposure prophylaxis or therapy.

Long-term protective effects can be accomplished only by antibodies elicited by successful vaccination relying on the phenomenon of immunological memory. T-lymphocytes play a major role in initiating and maintaining immunity against subsequent virus exposure. Antibodies are one of the essential features of antigen triggered adaptive immunity. Initial early defense against viruses is provided by components of innate immunity which evolutionarily precedes adaptive immunity, and remains an essential part of defense against pathogens in humans.

Death by a B cell superantigen: In vivo VH-targeted apoptotic supraclonal B cell deletion by a Staphylococcal Toxin

Amongst the many ploys used by microbial pathogens to interfere with host immune responses is the production of proteins with the properties of superantigens. These properties enable superantigens to interact with conserved variable region framework subdomains of the antigen receptors of lymphocytes rather than the complementarity determining region involved in the binding of conventional antigens. To understand how a B cell superantigen affects the host immune system, we infused protein A of Staphylococcus aureus (SpA) and followed the fate of peripheral B cells expressing B cell receptors (BCRs) with VH regions capable of binding SpA. Within hours, a sequence of events was initiated in SpA-binding splenic B cells, with rapid down-regulation of BCRs and coreceptors, CD19 and CD21, the induction of an activation phenotype, and limited rounds of proliferation. Apoptosis followed through a process heralded by the dissipation of mitochondrial membrane potential, the induction of the caspase pathway, and DNA fragmentation. After exposure, B cell apoptotic bodies were deposited in the spleen, lymph nodes, and Peyer's patches. Although in vivo apoptosis did not require the Fas death receptor, B cells were protected by interleukin (IL)-4 or CD40L, or overexpression of Bcl-2. These studies define a pathway for BCR-mediated programmed cell death that is VH region targeted by a superantigen.

Enforced expression of Bcl-2 selectively perturbs negative selection of dual reactive antibodies

We investigated the role of apoptosis in the development of B cell memory by analyzing the (p-azophenylarsonate) Ars response in a line of A strain mice in which expression of human Bcl-2 was enforced in the B cell compartment. Previous studies of the Ars immune response in these A. Bcl-2 mice, demonstrated that a large percentage of the antibodies expressed by the Ars induced memory B cell compartment had accumulated point mutations via somatic hypermutation that increased their affinity for both Ars and the autoantigen DNA ("dual reactive" antibodies). This was in sharp contrast to normal A strain mice which displayed no dual reactive B cells in their Ars induced memory B cell compartment. These data suggested that interference with apoptotic pathways regulated by Bcl-2 allows developing memory B cells that have acquired autoreactivity to bypass a peripheral tolerance checkpoint. Further studies of these mice, reported here, demonstrate that enforced expression of Bcl-2 does not alter serum antibody affinity maturation nor positive selection of B cells expressing somatically mutated antibody with an increased affinity for Ars. Moreover, the somatic hypermutation process was unaffected in A. Bcl-2 mice. Thus, enforced expression of Bcl-2 in A. Bcl-2 mice appears to selectively alter a negative selection process that operates during memory B cell differentiation.

T cell receptor (TCR)-mediated repertoire selection and loss of TCR vbeta diversity during the initiation of a CD4(+) T cell response in vivo

We recently described a novel way to isolate populations of antigen-reactive CD4(+) T cells with a wide range of reactivity to a specific antigen, using immunization with a fixed dose of nominal antigen and FACS((R)) sorting by CD4(high) expression. Phenotypic, FACS((R)), functional, antibody inhibition, and major histocompatibility complex-peptide tetramer analyses, as well as T cell receptor Vbeta sequence analyses, of the antigen-specific CD4(high) T cell populations demonstrated that a diverse sperm whale myoglobin 110-121-reactive CD4(+) T cell repertoire was activated at the beginning (day 3 after immunization) of the immune response. Within 6 d of immunization, lower affinity clones were lost from the responding population, leaving an expanded population of oligoclonal, intermediate affinity (and residual high affinity) T cells. This T cell subset persisted for at least 4 wk after immunization and dominated the secondary immune response. These data provide evidence that CD4(+) T cell repertoire selection occurs early in the immune response in vivo and suggest that persistence and expansion of a population of oligoclonal, intermediate affinity T cells is involved in CD4(+) T cell memory.

A B cell superantigen-induced persistent "Hole" in the B-1 repertoire

The bacterial toxin protein A from Staphylococcus aureus (SpA) interacts with B cell antigen receptors encoded by variable region heavy chain (V(H)) clan III genes via a V region framework surface that has been highly conserved during the evolution of the adaptive immune system. We have investigated the consequences of exposure to this prototypic B cell superantigen, and found that treatment of neonates or adults induces a T cell-independent deletion of a large supraclonal set of susceptible B cells that includes clan III/V(H) S107 family-expressing lymphocytes. In studies of different SpA forms, the magnitude of the induced deletion directly correlated with the V(H)-specific binding affinity/avidity. Upon cessation of SpA exposure, the representation of conventional splenic (B-2 subset) lymphocytes normalized; however, we found that the V(H) family-restricted deficit of peritoneal B-1 cells persisted. SpA treatment also induced a persistent loss of splenic S107-mu transcripts, with a loss of certain natural antibodies and specific tolerance to phosphorylcholine immunogens that normally recruit protective antimicrobial responses dominated by the S107-expressing B-1 clone, T15. These studies illustrate how a B cell superantigen can exploit a primordial Achilles heel in the immune system, for which B-1 cells, an important source of natural antibodies and host immune responses, have special susceptibility.

Relaxed negative selection in germinal centers and impaired affinity maturation in bcl-xL transgenic mice

The role of apoptosis in affinity maturation was investigated by determining the affinity of (4-hydroxy-3-nitrophenyl)acetyl (NP)-specific antibody-forming cells (AFCs) and serum antibody in transgenic mice that overexpress a suppressor of apoptosis, Bcl-xL, in the B cell compartment. Although transgenic animals briefly expressed higher numbers of splenic AFCs after immunization, the bcl-xL transgene did not increase the number or size of germinal centers (GCs), alter the levels of serum antibody, or change the frequency of NP-specific, long-lived AFCs. Nonetheless, the bcl-xL transgene product, in addition to endogenous Bcl-xL, reduced apoptosis in GC B cells and resulted in the expansion of B lymphocytes bearing VDJ rearrangements that are usually rare in primary anti-NP responses. Long-lived AFCs bearing these noncanonical rearrangements were frequent in the bone marrow and secreted immunoglobulin G(1) antibodies with low affinity for NP. The abundance of noncanonical cells lowered the average affinity of long-lived AFCs and serum antibody, demonstrating that Bcl-xL and apoptosis influence clonal selection/maintenance for affinity maturation.

Clonal dynamics of progressive neoplastic transformation

In a recent study, we found that newly isolated clones of NIH 3T3 mouse cells undergo neoplastic transformation more readily than uncloned cultures from which they were derived. After eleven low-density passages (LDPs), most of the 29 clones produced lightly stained early-stage transformed foci when grown to confluence in a primary assay for transformation, and one of them consistently produced a few tiny dense foci. In the present work, six of the clones were kept in LDPs for 56 passages and assayed for focus formation at confluence at six passage levels. The clone that produced tiny dense foci switched to light foci during the LDPs, four others produced light foci at different passage levels, and one progressed from light to dense foci after the last passage. By contrast, all the clones progressed to dense focus formation in five or fewer serial repetitions of the assay at confluence. Because all but one of the clones underwent about half as many total divisions at each LDP as they did when grown to the stationary state at confluence, the latter is more efficient in eliciting progression than the exponential growth of the LDPs. Extension of the period at confluence of uncloned cultures results in the appearance of dense foci within light foci. Because the latter are localized clonal populations, the intrafocal progression reinforces the conclusion that clonal expansion favors transformation. We discuss the significance of these results for the clonal origin of human cancer and the increased incidence of cancer with age.

The cellular ecology of progressive neoplastic transformation: a clonal analysis

A comparison was made of the competence for neoplastic transformation in three different sublines of NIH 3T3 cells and multiple clonal derivatives of each. Over 90% of the neoplastic foci produced by an uncloned transformed (t-SA') subline on a confluent background of nontransformed cells were of the dense, multilayered type, but about half of the t-SA' clones produced only light foci in assays without background. This asymmetry apparently arose from the failure of the light focus formers to register on a background of nontransformed cells. Comparison was made of the capacity for confluence-mediated transformation between uncloned parental cultures and their clonal derivatives by using two nontransformed sublines, one of which was highly sensitive and the other relatively refractory to confluence-mediated transformation. Transformation was more frequent in the clones than in the uncloned parental cultures for both sublines. This was dramatically so in the refractory subline, where the uncloned culture showed no overt sign of transformation in serially repeated assays but increasing numbers of its clones exhibited progressive transformation. The reason for the greater susceptibility of the pure clones is apparently the suppression of transformation among the diverse membership that makes up the uncloned parental culture. Progressive selection toward increasing degrees of transformation in confluent cultures plays a major role in the development of dense focus formers, but direct induction by the constraint of confluence may contribute by heritably damaging cells. In view of our finding of increased susceptibility to transformation in clonal versus uncloned populations, expansion of some clones at the expense of others during the aging process would contribute to the marked increase of cancer with age.

Mechanisms of relapse in acute leukaemia: involvement of p53 mutated subclones in disease progression in acute lymphoblastic leukaemia

Mutations of the p53 tumour suppressor gene are infrequent at presentation of both acute myeloblastic leukaemia (AML) and acute lymphoblastic leukaemia (ALL), being found in between 5-10% of AML and 2-3% of ALL. Here we have studied the frequency of detection of p53 mutations at relapse of both AML and B-precursor ALL. In those patients with detectable mutations at relapse we investigated whether the mutation was detectable at presentation and was thus an early initiating event or whether it had arisen as a late event associated with relapse. Bone marrow samples from 55 adults and children with relapsed AML (n = 41) or ALL (n = 14) were analysed for p53 gene alterations by direct sequencing of exons 5-9. For samples where a p53 mutation was found at relapse, analysis of presentation samples was carried out by direct sequencing of the exon involved, or by allele-specific polymerase chain reaction (PCR) if the mutation could not be detected using direct sequencing. A p53 mutated gene was found at relapse in seven out of 55 cases. The frequency was higher in relapsed ALL (four out of 14 cases; 28.6%) compared to AML (three out of 41 cases; 7.3%). In five out of the seven cases presentation samples were available to study for the presence of the mutation. In two out of two AML patients the p53 mutation was detectable in the presentation sample by direct sequencing. In three ALL patients analysis of presentation material by direct sequencing showed a small mutant peak in one case, the other two being negative despite the sample analysed containing > 90% blast cells. However in both of these patients, the presence of p53 mutation was confirmed in the presentation sample using allele-specific PCR. In one of these patients the emergence of a subclone at relapse was confirmed by clonality analysis using IgH fingerprinting. Our results confirm that in ALL p53 mutations are present in a proportion of patients at relapse. Furthermore cells carrying the mutation are detectable at presentation in a minor clone suggesting that p53 mutations in ALL may be a mechanism contributing to disease relapse.