Cold agglutinin activity is common among human monoclonal IgM Rh system antibodies using the V4-34 heavy chain variable gene segment

B cells in the balance: Offsetting self-reactivity avoidance with protection against foreign

Antibodies are theoretically limitless in their diversity and specificity to foreign antigens; however they are constrained by the need to avoid binding to self. Germinal centers (GC) allow diversification and maturation of the antibody response towards the foreign antigen. While self-tolerance mechanisms controlling self-reactivity during B cell maturation are well recognized, the mechanisms by which GCs balance self-tolerance and foreign binding especially in the face of cross-reactivity between self and foreign, remain much less well defined. In this review we explore the extent to which GC self-tolerance restricts affinity maturation. We present studies suggesting that the outcome is situationally dependent, affected by affinity and avidity to self-antigen, and the extent to which self-binding and foreign-binding are interdependent. While auto-reactive GC B cells can mutate away from self while maturing towards the foreign antigen, if no mutational trajectories allow for self-reactive redemption, self-tolerance prevails and GC responses to the foreign pathogen are restricted, except when self-tolerance checkpoints are relaxed. Finally, we consider whether polyreactivity is subject to the same level of restriction in GC responses, especially if polyreactivity is linked to an increase in foreign protection, as occurs in certain broadly neutralizing antibodies. Overall, the outcomes for GC B cells that bind self-antigen can range from redemption, transient relaxation in self-tolerance or restriction of the antibody response to the foreign pathogen.

Primera serie de casos reportada en Colombia de la enfermedad por aglutininas frías primaria y secundaria

Introducción: la enfermedad por aglutininas frías (EAF) es un trastorno hematológico primario o secundario, caracterizado por la anemia hemolítica autoinmune causada por los anticuerpos IgM a bajas temperaturas. Clínicamente, presenta parestesias y acrocianosis inducidos por frío y fiebre, aunque también puede ser asintomática y solo identificarse por alteraciones en el hemograma. Objetivo: describir las manifestaciones clínicas y de laboratorio, las causas primarias y secundarias de la EAF y compararlas con series de casos descritos en la literatura. Materiales y métodos: análisis retrospectivo de datos clínicos de pacientes del Hospital Universitario San Vicente Fundación de Medellín con resultados positivos para aglutininas frías. Dichos análisis se realizaron en el laboratorio de hematología de la Universidad de Antioquia, consideramos como positivo título ≥ 1: 64 o con la prueba de Coombs directa y positiva para anticuerpos fríos. Resultados: se incluyen los títulos de crioaglutininas de 23 casos con EAF: 6 formas primarias, 4 asociadas con los linfoma no Hodgkin (LNH), 8 secundarias a enfermedades infecciosas y autoinmunes y, 5 asociados con enfermedades misceláneas. Discusión y conclusiones: esta es la primera serie de casos en Colombia de EAF. La edad y género fueron similares a los datos reportados en la literatura. Observamos un mayor número de pacientes que presentaban anemia hemolítica y con síntomas asociados al frío. La relación hemoglobina hematocrito fue 1:2. Dentro de las causas secundarias destacamos las vasculitis, el lupus y la malaria. De las causas primarias las más frecuentes fueron los LNH, específicamente, el linfoplasmocítico. El tratamiento más utilizado para pacientes con EAF primaria incluyo rituximab.

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.

The VP8* domain of neonatal rotavirus strain G10P[11] binds to type II precursor glycans

Naturally occurring bovine-human reassortant rotaviruses with a P[11] VP4 genotype exhibit a tropism for neonates. Interaction of the VP8* domain of the spike protein VP4 with sialic acid was thought to be the key mediator for rotavirus infectivity. However, recent studies have indicated a role for nonsialylated glycoconjugates, including histo-blood group antigens (HBGAs), in the infectivity of human rotaviruses. We sought to determine if the bovine rotavirus-derived VP8* of a reassortant neonatal G10P[11] virus interacts with hitherto uncharacterized glycans. In an array screen of >600 glycans, VP8* P[11] showed specific binding to glycans with the Galβ1-4GlcNAc motif, which forms the core structure of type II glycans and is the precursor of H type II HBGA. The specificity of glycan binding was confirmed through hemagglutination assays; GST-VP8* P[11] hemagglutinates type O, A, and B red blood cells as well as pooled umbilical cord blood erythrocytes. Further, G10P[11] infectivity was significantly enhanced by the expression of H type II HBGA in CHO cells. The bovine-origin VP4 was confirmed to be essential for this increased infectivity, using laboratory-derived reassortant viruses generated from sialic acid binding rotavirus SA11-4F and a bovine G10P[11] rotavirus, B223. The binding to a core glycan unit has not been reported for any rotavirus VP4. Core glycan synthesis is constitutive in most cell types, and modification of these glycans is thought to be developmentally regulated. These studies provide the first molecular basis for understanding neonatal rotavirus infections, indicating that glycan modification during neonatal development may mediate the age-restricted infectivity of neonatal viruses.

Immunoglobulin M red blood cell alloantibodies are frequently adsorbed by rabbit erythrocyte stroma

BACKGROUND

Rabbit erythrocyte stroma (RESt, Immucor) adsorption is often used to remove cold autoantibodies from patient samples to facilitate detection of underlying alloantibodies. However, reports in the literature show that adsorption of clinically significant alloantibodies can occur. A 2006 study by Storry and colleagues suggested that immunoglobulin (Ig)M antibodies are adsorbed by RESt regardless of antigen specificity. In our study, we further investigated the adsorption of IgM red blood cell alloantibodies by RESt.

STUDY DESIGN AND METHODS

A total of 12 sera containing monoclonal IgM antibodies of various specificities (anti- D, -C, -c, -E, -e, -K, -Jk(b), and -S) and titers, which were all shown to exhibit only IgM reactivity after dithiothreitol treatment, and two sera with polyclonal IgG (anti-Fy(a) and -K) were all adsorbed by RESt. Titers of unadsorbed, once-adsorbed, and twice-adsorbed IgM and IgG antibodies were determined in parallel.

RESULTS

Ten of the 12 monoclonal IgM samples showed significant (more than fourfold) reduction in titer after RESt adsorptions. Both of the polyclonal IgG samples tested showed insignificant (fourfold or less) reduction in titer.

CONCLUSIONS

RESt is known to effectively remove IgM cold autoantibodies. Our results show that monoclonal IgM alloantibodies are also frequently adsorbed by RESt with significant reduction in titer. Adsorption is variable and some IgM alloantibodies are not adsorbed. Further studies may elucidate the effect of RESt adsorption on IgG alloantibodies. Caution is needed when RESt is employed to remove interferences by cold autoantibodies in pretransfusion testing, and the risk of missed IgM alloantibodies must be considered.

Anti-D and anti-i activities are inseparable in V4-34-encoded monoclonal anti-D: the same framework 1 residues are required for both reactivities

BACKGROUND

The heavy-chain V4-34 germline gene segment is mandatory for pathologic cold-reacting autoantibodies with anti-I/i specificity (cold agglutinins) and is also preferentially used by monoclonal immunoglobulin M alloantibodies against D and other Rh antigens. The use of the V4-34 segment by monoclonal anti-D has previously been shown to also confer anti-I/i reactivity (cold agglutinin activity), which has implications for the use of such antibodies for Rh blood typing. V4-34 framework 1 (FR1) sequence is believed to be critical for cold agglutinin activity of cold agglutinins.

STUDY DESIGN AND METHODS

The aim of this investigation was to use site-directed mutagenesis of a recombinant V4-34-encoded anti-D to determine the contribution of V4-34 FR1 sequence to anti-D activity and whether mutational modifications in the FR1 region could separately alter anti-D and anti-i activities.

RESULTS

The results show that amino acid changes in V4-34 FR1 at W7, A23, and Y25 have a profound effect on anti-D activity as well as on anti-i activity. It was not possible to substantially reduce or remove anti-i activity without reducing anti-D activity to a comparable extent.

CONCLUSIONS

The same nonpolar hydrophobic amino acids in FR1 are critical for maintaining both anti-D and anti-i activity. It is proposed that these residues influence the conformation of the antigen-binding site.

Primary chronic cold agglutinin disease: an update on pathogenesis, clinical features and therapy

Chronic cold agglutinin disease (CAD) is a subgroup of autoimmune hemolytic anemia. Primary CAD has traditionally been defined by the absence of any underlying or associated disease. The results of therapy with corticosteroids, alkylating agents and interferon-a have been poor. Cold reactive immunoglobulins against erythrocyte surface antigens are essential to pathogenesis of CAD. These cold agglutinins are monoclonal, usually IgMκ auto antibodies with heavy chain variable regions encoded by the V_H4-34 gene segment. By flowcytometric and immunohistochemical assessments, a monoclonal CD20⁺κ⁺B-lymphocyte population has been demonstrated in the bone marrow of 90% of the patients, and lymphoplasmacytic lymphoma is a frequent finding. Novel attempts at treatment for primary CAD have mostly been directed against the clonal B-lymphocytes. Phase 2 studies have shown that therapy with the chimeric anti-CD20 antibody rituximab produced partial response rates of more than 50% and occasional complete responses. Median response duration, however, was only 11 months. In this review, we discuss the clinical and pathogenetic features of primary CAD, emphasizing the more recent data on its close association with clonal lymphoproliferative bone marrow disorders and implications for therapy. We also review the management and outline some perspectives on new therapy modalities.

Autoimmune Pathogenesis and Autoimmune Hemolytic Anemia

Autoimmune hemolytic anemia (AIHA) is an autoimmune disorder in which autoantibodies are directed against an individual's own red blood cells (RBCs), leading to enhanced clearance through Fc receptor (FcR)-mediated phagocytosis. Although there is a large literature relating to clinical aspects of AIHA, relatively little work addresses how IgG autoantibodies are actually produced against RBC autoantigens. This review will first discuss the current understanding of autoimmunity in general and then focus on the knowledge of the immunopathogenic mechanisms responsible for autoantibody production in AIHA. Both human and animal studies will be discussed. Understanding theses mechanism is vital for developing antigen-specific immunotherapies to treat the disease.

Molecular studies of anti-HLA-A2 using light-chain shuffling: a structural model for HLA antibody binding

Human leukocyte antigen (HLA) A2 is one of the most immunodominant HLA antigens. Through a process of light-chain variable domain (VL) shuffling, we analyzed the VL domains' role in anti-HLA-A2/A28-binding site diversity. This was achieved by combining a VH3-30-encoded HLA-A2/A28-specific heavy-chain variable domain with 10(4) non-immune VL domains. Twelve HLA-A2/A28-specific antibodies were subsequently identified. VL gene analysis demonstrated an absence of Vlambda domains and that all have VkappaI-encoded light chains. The affinities correlated with the VkappaI gene present, with the seven highest affinity antibodies using Vkappa domains encoded by the O18 gene segment. A 300-fold difference in affinity was observed between the 12 antibodies, and homology modeling demonstrated a correlation between electrostatic surface potential of the antigen-binding site and affinity for HLA. Overlap between the T-cell receptor-binding site and that of the antibodies was indicated by inhibition of cytotoxic T-lymphocyte killing of peptide-pulsed target cells. A model of antibody binding to HLA-A2 suggested contact with both alpha helices of the HLA molecule, such that the antigen-binding site spans the peptide-binding groove. These data increase the understanding of antibody recognition of HLA and may facilitate the production of clonotypic antibodies with peptide-specific binding.

Restriction in the repertoire of the immunoglobulin light chain subgroup in pathological cold agglutinins with anti-Pr specificity

BACKGROUND AND OBJECTIVES

In cold agglutinin disease, monoclonal red blood cell autoantibodies, termed cold agglutinins, induce haemolysis in patients exposed to the cold. Commonly, these autoantibodies are directed against the developmentally regulated I/i blood groups. A second blood group system, the Pr system (located on glycophorins), is involved less frequently. Anti-Pr cold agglutinins recognize either alpha 2,3- or alpha 2,6-linked N-acetylneuraminic acid as the immunodominant group. Cold agglutinins of anti-I/i specificity show a remarkable restriction in their genomic repertoire of the immunoglobulin heavy and light-chain immunoglobulin-variable domain (i.e. exclusive use of VH4-34 in heavy chains). For anti-Pr cold agglutinins, preliminary data on the repertoire of the light-chain variable domain indicate a preference for the subgroup Vkappa IV. To elucidate restrictions in the light-chain variable-domain subgroup repertoire of anti-Pr cold agglutinins systematically, and to discuss these results in the context of their anti-Pr(1-3) subclassification and immunodominant sialic acid, light chains in 13 anti-Pr cold agglutinins were investigated.

MATERIALS AND METHODS

The anti-Pr light chains were isolated using temperature-dependent absorption/elution techniques. Subsequently, they were subjected to N-terminal Edman degradation, and the light chain Vkappa subgroup was affiliated using the Kabat database.

RESULTS

Five of 13 (38%) light chains belonged to Vkappa IV, five of 13 (38%) to Vkappa I and three of 13 (23%) to Vkappa III. Anti-Pr with Vkappa IV subgroup light chains exclusively recognized alpha 2,3-linked N-acetylneuraminic acid.

CONCLUSIONS

Including data from the literature, the repertoire of the light-chain variable domain in pathological anti-Pr cold agglutinins exhibits a clear bias towards the use of the single germline gene-derived subgroup, Vkappa IV (eight of 17 or 47%). The association of Vkappa IV subgroup light chain-containing anti-Pr cold agglutinins with binding to alpha 2,3-, but not alpha 2,6-linked N-acetyneuraminic acid raises speculations about a possible role of subgroup-derived determinants in anti-Pr binding.

The RHCE allele ceRT: D epitope 6 expression does not require D-specific amino acids

BACKGROUND

False-positive D typing in patients may lead to anti-D immunization caused by D+ transfusions or by omission of anti-D prophylaxis. Known causes of such errors are RhCE variants carrying RhD-specific amino acids and cold agglutinin activity of some frequently used monoclonal anti-D.

STUDY DESIGN AND METHODS

The molecular basis of eight samples referred because of "false-positive" reactions with some commercial monoclonal anti-D was investigated by PCR and nucleotide sequencing from genomic DNA. PCR with sequence-specific priming was developed to specifically detect the underlying aberrant RHCE allele. The D epitope profile of the allele was determined by serology.

RESULTS

The aberrant reactivity of the samples was caused by the RHCE allele RHCE(R154T) that occurred in a cde haplotype. The phenotype dubbed ceRT expressed the important D epitope 6, which is the target epitope of most monoclonal anti-D used in routine typing.

DISCUSSION

The characterization of ceRT demonstrated a previously unknown mechanism of antigen D expression that does not require any D-specific amino acid. At least for some D epitopes, D-like structures may be mimicked by RhCE proteins carrying amino acid substitutions not representative for RhD.

Differential signaling via surface IgM is associated with VH gene mutational status and CD38 expression in chronic lymphocytic leukemia

The mutational status of tumor immunoglobulin V(H) genes is providing a powerful prognostic marker for chronic lymphocytic leukemia (CLL), with patients having tumors expressing unmutated V(H) genes being in a less favorable subset. However, the biologic differences correlating with V(H) gene status that could determine the clinical course of the disease are unknown. Here we show that differing responses to IgM ligation are closely associated with V(H) gene status. Specifically, 80% of cases with unmutated V(H) genes showed increased global tyrosine phosphorylation following IgM ligation, whereas only 20% of samples with mutated V(H) genes responded (P =.0002). There was also an association between response to IgM ligation and expression of CD38 (P =.015). The Syk kinase, critical for transducing B-cell receptor (BCR)- derived signals, was constitutively present in all CLL samples, and there was a perfect association between global phosphorylation and induction of phosphorylation/activation of Syk. Nonresponsiveness to anti-IgM could be circumvented by ligation of IgD (10 of 15 samples tested) or the BCR-associated molecule CD79alpha (12 of 15 samples tested). These results suggest that multiple mechanisms underlie nonresponsiveness to anti-IgM in CLL and that retained responsiveness to anti-IgM contributes to the poor prognosis associated with the unmutated subset of CLL. The prognostic power of the in vitro response to IgM ligation remains to be determined in a large series, but the simple technology involved may present an alternative or additional test for predicting clinical course.

Expression and functional analysis of recombinant scFv and diabody fragments with specificity for human RhD

In an attempt to generate recombinant anti-D reagents for possible diagnostic and therapeutic use we cloned the genes encoding the variable (V) domains of a human anti-D antibody secreted by the lymphoblastoid cell line BTSN4. A single-chain Fv (scFv) fragment was constructed using a 21 amino acid linker to join the genes encoding the variable domains of the BTSN4 heavy (VH) and light chains (VL). A diabody construct was also generated by reducing the length of the scFv linker from 21 to 10 residues. The scFv and diabody constructs were cloned into the pFLAG-CTS vector, expressed in E. coli host cells and the recombinant proteins were affinity-isolated from bacterial culture medium. Analysis of the recombinant proteins indicated that they retained the D antigen binding specificity of the parental BTSN4 IgG. Furthermore, both fragments mediated agglutination of papain-treated D positive erythrocytes in the absence of a cross-linking second antibody. While the agglutinating property of BTSN4 diabody was readily explained by the non-covalent association of this protein as a bivalent dimer, oligomeric forms of BTSN4 scFv were not detected when the protein was analysed by size exclusion chromatography. Thus, the agglutinating property of the scFv is not the result of the formation of non-covalently associated multimeric forms of the antibody fragment.

Synthesis of Rh Fv phage-antibodies using VH and VL germline genes

Antibodies to the D antigen of the Rh system use a restricted set of immunoglobulin V and J gene segments, especially VH DP50 and DP63, JH6, Vlambda DPL16 and Jlambda 2/3. These gene segments may confer a natural affinity on the antibodies for the D antigen and this hypothesis has been tested by constructing two single-chain Fv phage-antibody libraries based on the germline gene segments DP50 and DP63; structural variability was obtained by insertion of 11 amino acids in random sequence in the VHCDR3. 10 anti-D antibodies were selected from these libraries, each with a unique VHCDR3. In contrast, selections with the CcEe antigens produced antibodies reacting with the Rh polypeptide molecules but without strict blood group specificity. One of these latter DP50-based antibodies was converted into 12 different antibodies with specificity for E by replacing the original germline light chain with chains from a rearranged L chain library. The CDR1 and CDR2 sequences of the DP50-based antibodies were common to both anti-D and anti-E molecules; differentiation between D and E specificity was dependent on VHCDR3 sequences and their correct pairing with an appropriate L chain.

Human monoclonal antibodies encoded by the V4-34 gene segment show cold agglutinin activity and variable multireactivity which correlates with the predicted charge of the heavy-chain variable region

We have characterized the reactivities of a panel of V4-34-encoded human IgM monoclonal antibodies (mAb) which bind the erythrocyte Rh D antigen, derived from an immunized individual. These were compared with the specificities of V4-34-encoded autoantibodies with I/i reactivity produced from patients with cold agglutinin disease (CAD), and other V4-34-encoded autoantibodies. The antibodies were evaluated for cold agglutinin activity using haemagglutination tests, immunofluorescence microscopy for reactivity with tissue components, and in solid phase radiobinding assays with purified antigens. We found that (i) cold agglutinin activity was a property of all the V4-34-encoded mAb (ii) the cold agglutinins from CAD patients were generally monospecific for I/i whereas most of the anti-D and the other V4-34-encoded mAb displayed multireactive properties, frequently binding to strongly acidic antigens (iii) computation of the net charge of the heavy-chain V regions showed that the multireactive mAb were generally more positively charged than the monospecific cold agglutinins, which could contribute to their multireactive phenotype. The involvement of charge interactions was further indicated by the effects of pH and ionic strength on the immunofluorescence staining patterns.