Generation and analysis of random point mutations in an antibody CDR2 sequence: many mutated antibodies lose their ability to bind antigen

Computational Modeling and Characterization of Peptides Derived from Nanobody Complementary-Determining Region 2 (CDR2) Targeting Active-State Conformation of the β2-Adrenergic Receptor (β2AR)

This study assessed the suitability of the complementarity-determining region 2 (CDR2) of the nanobody (Nb) as a template for the derivation of nanobody-derived peptides (NDPs) targeting active-state β2-adrenergic receptor (β2AR) conformation. Sequences of conformationally selective Nbs favoring the agonist-occupied β2AR were initially analyzed by the informational spectrum method (ISM). The derived NDPs in complex with β2AR were subjected to protein-peptide docking, molecular dynamics (MD) simulations, and metadynamics-based free-energy binding calculations. Computational analyses identified a 25-amino-acid-long CDR2-NDP of Nb71, designated P4, which exhibited the following binding free-energy for the formation of the β2AR:P4 complex (ΔG = -6.8 ± 0.8 kcal/mol or a Ki = 16.5 μM at 310 K) and mapped the β2AR:P4 amino acid interaction network. In vitro characterization showed that P4 (i) can cross the plasma membrane, (ii) reduces the maximum isoproterenol-induced cAMP level by approximately 40% and the isoproterenol potency by up to 20-fold at micromolar concentration, (iii) has a very low affinity to interact with unstimulated β2AR in the cAMP assay, and (iv) cannot reduce the efficacy and potency of the isoproterenol-mediated β2AR/β-arrestin-2 interaction in the BRET2-based recruitment assay. In summary, the CDR2-NDP, P4, binds preferentially to agonist-activated β2AR and disrupts Gαs-mediated signaling.

Characterization and Modeling of Reversible Antibody Self-Association Provide Insights into Behavior, Prediction, and Correction

Reversible antibody self-association, while having major developability and therapeutic implications, is not fully understood or readily predictable and correctable. For a strongly self-associating humanized mAb variant, resulting in unacceptable viscosity, the monovalent affinity of self-interaction was measured in the low μM range, typical of many specific and biologically relevant protein-protein interactions. A face-to-face interaction model extending across both the heavy-chain (HC) and light-chain (LC) Complementary Determining Regions (CDRs) was apparent from biochemical and mutagenesis approaches as well as computational modeling. Light scattering experiments involving individual mAb, Fc, Fab, and Fab'2 domains revealed that Fabs self-interact to form dimers, while bivalent mAb/Fab'2 forms lead to significant oligomerization. Site-directed mutagenesis of aromatic residues identified by homology model patch analysis and self-docking dramatically affected self-association, demonstrating the utility of these predictive approaches, while revealing a highly specific and tunable nature of self-binding modulated by single point mutations. Mutagenesis at these same key HC/LC CDR positions that affect self-interaction also typically abolished target binding with notable exceptions, clearly demonstrating the difficulties yet possibility of correcting self-association through engineering. Clear correlations were also observed between different methods used to assess self-interaction, such as Dynamic Light Scattering (DLS) and Affinity-Capture Self-Interaction Nanoparticle Spectroscopy (AC-SINS). Our findings advance our understanding of therapeutic protein and antibody self-association and offer insights into its prediction, evaluation and corrective mitigation to aid therapeutic development.

Cognizance of Molecular Methods for the Generation of Mutagenic Phage Display Antibody Libraries for Affinity Maturation

Antibodies leverage on their unique architecture to bind with an array of antigens. The strength of interaction has a direct relation to the affinity of the antibodies towards the antigen. In vivo affinity maturation is performed through multiple rounds of somatic hypermutation and selection in the germinal centre. This unique process involves intricate sequence rearrangements at the gene level via molecular mechanisms. The emergence of in vitro display technologies, mainly phage display and recombinant DNA technology, has helped revolutionize the way antibody improvements are being carried out in the laboratory. The adaptation of molecular approaches in vitro to replicate the in vivo processes has allowed for improvements in the way recombinant antibodies are designed and tuned. Combinatorial libraries, consisting of a myriad of possible antibodies, are capable of replicating the diversity of the natural human antibody repertoire. The isolation of target-specific antibodies with specific affinity characteristics can also be accomplished through modification of stringent protocols. Despite the ability to screen and select for high-affinity binders, some 'fine tuning' may be required to enhance antibody binding in terms of its affinity. This review will provide a brief account of phage display technology used for antibody generation followed by a summary of different combinatorial library characteristics. The review will focus on available strategies, which include molecular approaches, next generation sequencing, and in silico approaches used for antibody affinity maturation in both therapeutic and diagnostic applications.

Antibody Epitope Specificity for dsDNA Phosphate Backbone Is an Intrinsic Property of the Heavy Chain Variable Germline Gene Segment Used

Analysis of protein sequences by the informational spectrum method (ISM) enables characterization of their specificity according to encoded information represented with defined frequency (F). Our previous data showed that F(0.367) is characteristic for variable heavy chain (VH) domains (a combination of variable (V), diversity (D) and joining (J) gene segments) of the anti-phosphocholine (PC) T15 antibodies and mostly dependent on the CDR2 region, a site for PC phosphate group binding. Because the T15 dsDNA-reactive U4 mutant also encodes F(0.367), we hypothesized that the same frequency may also be characteristic for anti-DNA antibodies. Data obtained from an analysis of 60 spontaneously produced anti-DNA antibody VH domain sequences supported our hypothesis only for antibodies, which use V gene segment in germline configuration, such as S57(VH31), MRL-DNA22, and VH11, members of the VH1 (J558) and VH7 (S107) gene families. The important finding is that out of seven V gene segments used by spontaneous anti-DNA antibodies, F(0.367) is only expressed by the germline configuration of these three V gene segments. The data suggest that antibody specificity for the phosphate group moiety delineated as F(0.367) is the intrinsic property of the V germline gene segments used, whereas paratope/epitope interaction with antigens bearing this epitope, such as PC or dsDNA, requires corresponding antibody VH conformation that is susceptible to somatic mutation(s).

Immune Repertoire after Immunization As Seen by Next-Generation Sequencing and Proteomics

The immune system produces a diverse repertoire of immunoglobulins in response to foreign antigens. During B-cell development, VDJ recombination and somatic mutations generate diversity, whereas selection processes remove it. Using both proteomic and NGS approaches, we characterized the immune repertoires in groups of rats after immunization with purified antigens. Proteomics and NGS data on the repertoire are in qualitative agreement, but did show quantitative differences that may relate to differences between the biological niches that were sampled for these approaches. Both methods contributed complementary information in the characterization of the immune repertoire. It was found that the immune repertoires resulting from each antigen had many similarities that allowed samples to cluster together, and that mutated immunoglobulin peptides were shared among animals with a response to the same antigen significantly more than for different antigens. However, the number of shared sequences decreased in a log-linear fashion relative to the number of animals that share them, which may affect future applications. A phylogenetic analysis on the NGS reads showed that reads from different individuals immunized with the same antigen populated distinct branches of the phylogram, an indication that the repertoire had converged. Also, similar mutation patterns were found in branches of the phylogenetic tree that were associated with antigen-specific immunoglobulins through proteomics data. Thus, data from different analysis methods and different experimental platforms show that the immunoglobulin repertoires of immunized animals have overlapping and converging features. With additional research, this may enable interesting applications in biotechnology and clinical diagnostics.

Extreme genetic fragility of the HIV-1 capsid

Genetic robustness, or fragility, is defined as the ability, or lack thereof, of a biological entity to maintain function in the face of mutations. Viruses that replicate via RNA intermediates exhibit high mutation rates, and robustness should be particularly advantageous to them. The capsid (CA) domain of the HIV-1 Gag protein is under strong pressure to conserve functional roles in viral assembly, maturation, uncoating, and nuclear import. However, CA is also under strong immunological pressure to diversify. Therefore, it would be particularly advantageous for CA to evolve genetic robustness. To measure the genetic robustness of HIV-1 CA, we generated a library of single amino acid substitution mutants, encompassing almost half the residues in CA. Strikingly, we found HIV-1 CA to be the most genetically fragile protein that has been analyzed using such an approach, with 70% of mutations yielding replication-defective viruses. Although CA participates in several steps in HIV-1 replication, analysis of conditionally (temperature sensitive) and constitutively non-viable mutants revealed that the biological basis for its genetic fragility was primarily the need to coordinate the accurate and efficient assembly of mature virions. All mutations that exist in naturally occurring HIV-1 subtype B populations at a frequency >3%, and were also present in the mutant library, had fitness levels that were >40% of WT. However, a substantial fraction of mutations with high fitness did not occur in natural populations, suggesting another form of selection pressure limiting variation in vivo. Additionally, known protective CTL epitopes occurred preferentially in domains of the HIV-1 CA that were even more genetically fragile than HIV-1 CA as a whole. The extreme genetic fragility of HIV-1 CA may be one reason why cell-mediated immune responses to Gag correlate with better prognosis in HIV-1 infection, and suggests that CA is a good target for therapy and vaccination strategies.

The HIV-1 capsid protein (CA) is absolutely essential for viral replication and there is, therefore, intense evolutionary pressure for HIV-1 CA to conserve its functions. However, HIV-1 CA is also a key target of the host immune response, which should provide evolutionary pressure to diversify CA sequence. Genetic robustness, or fragility, is defined as the ability, or lack thereof, of a biological entity to preserve function in the face of sequence changes. Thus, it should be advantageous to HIV-1 CA to evolve genetic robustness. Here, we present the results of extensive, random mutagenesis of single amino acids in CA that reveal an extreme genetic fragility. Although CA participates in several steps in HIV-1 replication, the biological basis for its genetic fragility was primarily the need to participate in the efficient and proper assembly of mature virion particles. The extreme genetic fragility of HIV-1 CA may be one reason why immune responses to it correlate with better prognosis in HIV-1 infection, and suggests that CA is a good target for therapy and vaccination strategies.

Phosphocholine-binding antibody activities are hierarchically encoded in the sequence of the heavy-chain variable region: dominance of self-association activity in the T15 idiotype

A methodology based on the representation of each amino acid of a protein sequence by the electron-ion interaction potential and subsequent analysis by signal processing was used to determine the characteristic or common frequency (in Hz) that reflects the biological activity shared among phosphocholine (PC)-binding antibodies. The common frequency for the variable portion of the heavy chain (VH) of the PC-specific antibodies is found to be at f = 0.37 Hz. The VH sequences of the PC-binding antibodies exhibit three subsites for the PC moiety where hypervariable region 2 (CDR2) plays a role in the interaction with the phosphate group. Mutations in this VH region have an impact on the ability of mutant variants to bind PC and its carrier molecule, as well as on the characteristic frequency shift toward f = 0.12 Hz for mutants failing to bind both hapten and carrier. The VH sequence of mutants that retain the ability to bind PC still shows f = 0.37 Hz, suggesting that this frequency determines PC binding. However, this statement was not confirmed as mutation in another PC subsite impairs PC binding but retains both the phosphate-group recognition and the frequency at f = 0.37 Hz. Herein, this finding is discussed to promote the idea that the VH sequence of the PC-binding antibodies encodes the subsite for phosphate-group binding as a dominant functional activity and that only CDR2 of the T15-idiotype antibodies together with FR3 region form an autonomous self-association function represented by the T15VH50-73 peptide with f = 0.37±0.05 Hz. Thus, these data confirmed that T15VH50-73 peptide might be used in superantibody technology.

Generation of recombinant antibodies and means for increasing their affinity

Highly specific interaction with foreign molecules is a unique feature of antibodies. Since 1975, when Keller and Milstein proposed the method of hybridoma technology and prepared mouse monoclonal antibodies, many antibodies specific to various antigens have been obtained. Recent development of methods for preparation of recombinant DNA libraries and in silico bioinformatics approaches for protein structure analysis makes possible antibody preparation using gene engineering approaches. The development of gene engineering methods allowed creating recombinant antibodies and improving characteristics of existing antibodies; this significantly extends the applicability of antibodies. By modifying biochemical and immunochemical properties of antibodies by changing their amino acid sequences it is possible to create antibodies with properties optimal for certain tasks. For example, application of recombinant technologies resulted in antibody preparation of high affinity significantly exceeding the initial affinity of natural antibodies. In this review we summarize information about the structure, modes of preparation, and application of recombinant antibodies and their fragments and also consider the main approaches used to increase antibody affinity.

Correlation of molecular characteristics, isotype, and in vitro functional activity of human antipneumococcal monoclonal antibodies

Structure-function correlations of pneumococcal antibodies are important in predicting how changes in the pneumococcus (Pnc)-specific B-cell repertoire will influence humoral immunity against invasive Pnc disease. Using a unique panel of human hybridomas derived from memory B cells after pneumococcal conjugate vaccination, we analyzed the structure-function relationship of nine monoclonal antibodies (MAbs) reactive to Pnc polysaccharides. The avidities of the antibodies correlated with the avidity of donor immune serum (R, 0.7; P < 0.025), and this relationship was particularly strong for immunoglobulin A clones (R, 1; P < 0.0005), suggesting that the MAbs may represent important clones contributing to serological memory. Common heavy-light chain combinations and amino acid replacement mutations were seen for clones with the same serospecificity from different individuals. The two highest-avidity MAbs used Vh3-48, and two MAbs with the same serospecificity, using the same V gene pairings (Vh3-7 and Vk2A17), had similar avidities, suggesting that canonical V gene use makes an important contribution to avidity. Although all clones had mutation levels consistent with their being derived from memory B cells, low levels of replacement mutation were associated with high avidities. This relationship was strongest for Vh genes (R, 0.8; P < 0.01). Opsonophagocytosis was demonstrated for all clones, and there was a trend toward clones using canonical genes with low levels of mutation having high opsonophagocytic activities (R, 0.5). These data suggest that the use of canonical genes in the Pnc antibody response is associated with highly functional antibodies and that most somatic mutations seen in these genes are not antigen selected.

Analysis of the intraclonal diversity of HLA-A0201-restricted T lymphocyte epitopes in follicular lymphoma idiotype

Lymphoma-derived immunoglobulin idiotype (Id) is a tumour-specific antigen used for antitumour vaccination in follicular lymphoma (FL). However, FL Ids are subject to hypermutation and subclones may escape antitumour cytotoxic T-cell response. To investigate the intraclonal epitope diversity, we sequenced the FL heavy chain gene (consensus Id gene) and subclones of 24 patients. The derived polypeptide sequences were analysed by bioinformatics for human leucocyte antigen (HLA)-A0201-restricted epitope prediction. Epitopes were classified according to BIMAS and SYFPEITHI scores. Surprisingly in these highly mutated polypeptides, the epitopes concentrated in short hotspots in the conserved framework regions (FRs), both in HLA-A0201(+) and HLA-A0201(-) patients. Similar hotspots have been observed by others in chronic lymphocytic leukaemia Ids which in contrast to FL have low mutation frequency. FR3 amino acids 78-88 displayed the best-score epitopes in Ids containing a VH3-family segment, the most represented in FL Ids. Such VH3-FR3(78-88) epitopes were previously demonstrated as immunogenic. Modifications of the epitope pattern in subclones of HLA-A0201(+) patients were generally absent from high-score peptides, including VH3-FR3(78-88) epitopes (83% unmodified). Therefore, no tendency for loss of HLA-A0201-restricted epitopes was evidenced and, given their limited intraclonal diversity, VH3-FR3(78-88) epitopes may provide a useful target for the induction of cytotoxic response in Id-vaccinated FL patients.

Repertoire shift in the humoral response to phosphocholine-keyhole limpet hemocyanin: VH somatic mutation in germinal center B cells impairs T15 Ig function

Phosphocholine (PC) is a naturally occurring Ag common to many pathogenic microorganisms. Early in the primary response to PC conjugated to keyhole limpet hemocyanin (KLH), T15 Id(+) Abs constitute >90% of the serum Ig in BALB/c mice. During the late primary and memory response to PC-protein, a shift in the repertoire occurs and T15 Id(+) Abs lose dominance. In this study, we use immunohistochemistry and single germinal center microdissection to locate T15 Id(+) cells in the spleen in a primary response to PC-KLH. We demonstrate T15 Id(+) B cells and V(H)1-DFL16.1-JH1 and V kappa 22-J kappa 5 rearrangements in germinal centers early in the immune response; thus loss of T15 dominance is not due to lack of T15 cells within germinal centers. One-hundred thirty one V(H)1 and 57 V kappa 22 rearrangements were cloned and sequenced. Thirty four percent of the V(H)1 clones and 37% of the V kappa 22 clones contained somatic mutations indicating participation in the germinal center response. Six variant T15 H clones were expressed with wild-type T15 L chain in vitro. Two of these Abs were defective in secretion providing the first evidence that mutation occurring in vivo can disrupt Ig assembly and secretion. Of the four secretion-competent Abs, two failed to display binding to PC-protein, while the other two displayed altered carrier recognition. These results indicate that somatic mutation of T15 in vivo can result in the loss of binding and secretion, potentially leading to B cell wastage. The failure of T15 to gain affinity enhancing mutations in the face of these detrimental changes may contribute to repertoire shift.

Restoration of Ig secretion: mutation of germline-encoded residues in T15L chains leads to secretion of free light chains and assembled antibody complexes bearing secretion-impaired heavy chains

We previously described T15H chain mutants that were impaired in assembly with L chain and in ability to be secreted from the cell. The unmutated T15L chain is unusual in that it is secretion-impaired in the absence of assembly with H chain. The T15L chain preferentially pairs with T15H in vivo, suggesting that if we introduced mutations that would allow secretion of free T15L chain, they might also lead to the secretion of the complex with the defective H chain. We mutated four positions in the germline T15L that had amino acids infrequently found in other kappa-chains. Mutation to the most frequently occurring amino acid at three of the four positions allowed secretion of free L chain, while the combination of two secretion-restoring mutations was synergistic. Coexpression of secretion-restored mutant L chains with the secretion-defective mutant H chains rescued secretion of the assembled H(2)L(2) complex, suggesting that during somatic hypermutation in vivo, deleterious mutations at the H chain may be compensated by mutations on the L chain. To our knowledge, this is the first example of mutations in IgL chains that are able to restore secretion-defective H chains to secretion competence in mammalian cells.

Human alpha-defensin 1 (HNP-1) inhibits adenoviral infection in vitro

Adenoviral gene transfer is a promising tool for direct treatment of cystic fibrosis by local application of the CFTR-gene via the airway. However, various host defense mechanisms reduce the adenoviral infectivity and hereby the success of adenoviral transduction. Twenty-eight of 62 BALs from various patients exerted strong inhibition of adenoviral infection of 293 cells. This soluble activity could be attributed to larger peptides rather than to small molecules. Beside immunoglobulins, certain epithelial cell-derived anti-microbial polypeptides called defensins might be involved. Therefore, we investigated the inhibitory potential of the defensins HNP-1 and HBD-2 on adenoviral infectivity. 293 cells infected with adenovirus-type 5 were treated with both peptides. Compared to control, HNP-1 reduced adenoviral infection by more than 95% if administered at 50 microg/ml, and the IC50-value was 15 microg/ml. In contrast, HBD-2 was much less efficient and did not block adenoviral infection at doses up to 50 microg/ml. Our data demonstrate that the presence of certain polypeptides in the BAL, i.e. the defensin HNP-1, might be the major obstacle for adenoviral gene transfer, particularly in patients with inflammatory diseases.

Mutation of a single conserved residue in VH complementarity-determining region 2 results in a severe Ig secretion defect

During an immune response, somatic mutations are introduced into the VH and VL regions of Ig chains. The consequences of somatic mutation in highly conserved residues are poorly understood. Ile51 is present in 91% of murine VH complementarity-determining region 2 sequences, and we demonstrate that single Ile51-->Arg or Lys substitutions in the PCG1-1 Ab are sufficient to severely reduce Ig secretion (1-3% of wild-type (WT) levels). Mutant H chains, expressed in the presence of excess L chain, associate with Ig binding protein (BiP) and GRP94 and fail to form HL and H2L assembly intermediates efficiently. The mutations do not irreversibly alter the VH domain as the small amount of mutant H chain, which assembles with L chain as H2L2, is secreted. The secreted mutant Ab binds phosphocholine-protein with avidity identical with that of WT Ab, suggesting that the combining site adopts a WT conformation. A computer-generated model of the PCG1-1 variable region fragment of Ig (Fv) indicates that Ile51 is buried between complementarity-determining region 2 and framework 3 and does not directly contact the L chain. Thus, the Ile51-->Arg or Ile51-->Lys mutations impair association with the PCG1-1 L chain via indirect interactions. These interactions are in part dependent on the nature of the L chain as the PCG1-1 VH single Ile51-->Arg or Ile51-->Lys mutants were partially rescued when expressed with the J558L lambda1 L chain. These results represent the first demonstration that single somatic mutations in V(H) residues can impair Ig secretion and suggest one reason for the conservation of Ile51 in so many Ig VH.

[gamma][delta] cells: a right time and a right place for a conserved third way of protection

The tripartite subdivision of lymphocytes into B cells, alphabeta T cells, and gammadelta cells has been conserved seemingly since the emergence of jawed vertebrates, more than 450 million years ago. Yet, while we understand much about B cells and alphabeta T cells, we lack a compelling explanation for the evolutionary conservation of gammadelta cells. Such an explanation may soon be forthcoming as advances in unraveling the biochemistry of gammadelta cell interactions are reconciled with the abnormal phenotypes of gammadelta-deficient mice and with the striking differences in gammadelta cell activities in different strains and species. In this review, the properties of gammadelta cells form a basis for understanding gammadelta cell interactions with antigens and other cells that in turn form a basis for understanding immunoprotective and regulatory functions of gammadelta cells in vivo. We conclude by considering which gammadelta cell functions may be most critical.

The structural basis of repertoire shift in an immune response to phosphocholine

The immune response to phosphocholine (PC)-protein is characterized by a shift in antibody repertoire as the response progresses. This change in expressed gene combinations is accompanied by a shift in fine specificity toward the carrier, resulting in high affinity to PC-protein. The somatically mutated memory hybridoma, M3C65, possesses high affinity for PC-protein and the phenyl-hapten analogue, p-nitrophenyl phosphocholine (NPPC). Affinity measurements using related PC-phenyl analogues, including peptides of varying lengths, demonstrate that carrier determinants contribute to binding affinity and that somatic mutations alter this recognition. The crystal structure of an M3C65-NPPC complex at 2.35-A resolution allows evaluation of the three light chain mutations that confer high-affinity binding to NPPC. Only one of the mutations involves a contact residue, whereas the other two have indirect effects on the shape of the combining site. Comparison of the M3C65 structure to that of T15, an antibody dominating the primary response, provides clear structural evidence for the role of carrier determinants in promoting repertoire shift. These two antibodies express unrelated variable region heavy and light chain genes and represent a classic example of the effect of repertoire shift on maturation of the immune response.

Variable heavy chain gene analysis of follicular lymphomas: correlation between heavy chain isotype expression and somatic mutation load

The expansion of follicular lymphomas (FLs) resembles, both morphologically and functionally, normal germinal center B-cell growth. The tumor cells proliferate in networks of follicular dendritic cells and are believed to be capable of somatic hypermutation and isotype switching. To investigate the relation between somatic mutation and heavy chain isotype expression, we analyzed the variable heavy (VH) chain genes of 30 FL samples of different isotypes. The VH genes of the FLs were heavily mutated (29.3 mutations on average). In addition, isotype-switched lymphomas contained more somatic mutations than immunoglobulin M–positive lymphomas (33.8 mutations per VH gene versus 23.0, respectively). In all but one of the FLs, the ratios of replacement versus silent mutations in the framework regions were low, independent of the absolute number of somatic mutations and the level of intraclonal variation. Analysis of relapse samples of 4 FLs showed no obvious increase in somatic mutation load in most FLs and a decrease in intraclonal variation in time. In 3 of 4 cases, we obtained evidence for selection of certain subclones, rather than clonal evolution. Our findings question if intraclonal variation is always a reflection of ongoing somatic hypermutation. This may have implications for the concept of antigen-driven lymphomagenesis.

Cutting Edge: Proteasome Involvement in the Degradation of Unassembled Ig Light Chains

Several studies on disposal of nonsecreted Ig L chains have identified the endoplasmic reticulum as the site of degradation. Here, we examine degradation of a nonsecreted Ig L chain, T15L, and an experimentally endoplasmic reticulum-retained secretion-competent L chain, D16L, in the absence of H chains. We demonstrate that 1) degradation is specifically impaired by the proteasome-specific inhibitors carboxybenzyl-leucyl-leucyl-leucine vinyl sulfone (Z-L3VS) and lactacystin, 2) L chain degradation occurs early in the biosynthetic pathway, and 3) degradation does not require vesicular transport. Our findings indicate that previous assertions of L chain disposal within the endoplasmic reticulum must be modified. To our knowledge, we provide the first direct evidence supporting a new paradigm for removal of nonsecreted Ig L chains via dislocation to cytosolic proteasomes.

In vitro scanning saturation mutagenesis of all the specificity determining residues in an antibody binding site

For the first time, each specificity determining residue (SDR) in the binding site of an antibody has been replaced with every other possible single amino acid substitution, and the resulting mutants analyzed for binding affinity and specificity. The studies were conducted on a variant of the 26-10 antidigoxin single chain Fv (scFv) using in vitro scanning saturation mutagenesis, a new process that allows the high throughput production and characterization of antibody mutants [Burks,E.A., Chen,G., Georgiou,G. and Iverson,B.L. (1997) Proc. Natl Acad. Sci. USA, 94, 412-417]. Single amino acid mutants of 26-10 scFv were identified that modulated specificity in dramatic fashion. The overall plasticity of the antibody binding site with respect to amino acid replacement was also evaluated, revealing that 86% of all mutants retained measurable binding activity. Finally, by analyzing the physical properties of amino acid substitutions with respect to their effect on hapten binding, conclusions were drawn regarding the functional role played by the wild-type residue at each SDR position. The reported results highlight the value of in vitro scanning saturation mutagenesis for engineering antibody binding specificity, for evaluating the plasticity of proteins, and for comprehensive structure-function studies and analysis.

Replacements in the exposed loop of the T15 antibody VH CDR2 affect carrier recognition of PC-containing pathogens

A panel of mutant antibodies of the phosphocholine (PC)-binding antibody, T15, was tested for binding to PC-protein, Streptococcus pneumoniae, Trichinella spiralis and Ascaris suum. Relative to wildtype T15, all the mutant antibodies showed differential recognition of the panel of PC-associated antigens. These mutant antibodies contain amino acid replacements in the CDR2 region of the heavy chain variable region, indicating the importance of CDR2 in recognition of carrier determinants. A model of T15 is shown that illustrates the strategic placement of mutations that could allow interaction with determinants associated with PC. A direct implication of this finding is that the T15 antibody combining site accommodates structures larger than phosphocholine and that recognition of associated carrier determinants could be a significant force in shaping the immune response to PC-containing pathogens.

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.

A simple method for determining K(A)s of both low and high affinity IgG antibodies

A rapid and convenient method for measuring affinity constants (K(A)) of IgG antibody-hapten complexes is described. A key advantage of this method is its suitability for quantification of both low and high affinity interactions. A comparison is made of the K(A)s of the low affinity anti-phosphocholine (PC) antibody T15 (K(A) = 2.9 x 10(5) M(-1)) and five heavy chain complementarity determining region 2 (HCDR2) mutant antibodies expressed as IgG2b transfectants. As a demonstration of the general applicability of the technique, colchicine binding to the high affinity monoclonal IgG2a antibody C44 (K(A) = 1.5 x 10(9) M(-1)) is measured also; thus the assay is applicable over a four-log range of affinities. The assay, based upon use of fixed Staphylococcus aureus Cowan I strain cells as an adsorbent for antibody-radiolabeled antigen complexes, is conducted over a range of hapten concentrations at constant antibody concentration. The K(A) is obtained by Scatchard analysis.

Inefficient Assembly and Intracellular Accumulation of Antibodies with Mutations in VH CDR2

We previously described secretion defects in four mutants of the murine anti-phosphocholine Ab, T15. The mutant heavy (H) chains had amino acid replacements in the VH complementarity-determining region 2 (HCDR2) and were expressed at normal intracellular levels. Here, the intracellular fate of the secretion-defective mutant heavy chains was investigated. Metabolic labeling demonstrated that the T15 wild-type Ab was secreted within a 4-h chase. In contrast, the mutant H chains accumulated with intracellular t1/2 values ranging from 10 to 24 h. The mutant H chains were associated with increased levels of the molecular chaperones BiP and GRP94, and remained endoglycosidase H sensitive, suggesting retention in the endoplasmic reticulum. Assembly of the mutant H chains with T15 light (L) chain was arrested at the H2 and H2L intermediate stages of the T15 wild-type pathway (H2 → H2L → H2L2). Even though some assembly with L chain occurred, it was not as a secretion-competent H2L2 Ig moiety. The T15 L chains coexpressed with mutant H chains were degraded efficiently except for a minor L chain population with a long t1/2 that was apparently protected at the H2L stage. To our knowledge, this is the first study demonstrating that intracellular half-lives of Ig H and L chains can be influenced by somatic mutations in HCDR2.

The signature of somatic hypermutation appears to be written into the germline IgV segment repertoire

We present here a unifying hypothesis for the molecular mechanism of somatic hypermutation and somatic gene conversion in IgV genes involving reverse transcription using RNA templates from the V-gene loci to produce cDNA which undergoes homologous recombination with chromosomal V(D)J DNA. Experimental evidence produced over the last 20 years is essentially consistent with this hypothesis. We also review evidence suggesting that somatically generated IgV sequences from B lymphocytes have been fed back to germline DNA over evolutionary time.

Somatic origin of T-cell epitopes within antibody variable regions: significance to monoclonal therapy and genesis of systemic autoimmune disease

During an immune response, specific antibody variable region genes are diversified by a somatic point mutation process that generates de novo "foreign" V-region sequences. This creates an interesting problem in immune regulation because B cells are highly proficient at self-presenting V-region peptides in the context of class II MHC. Though our studies indicate that the corresponding T-cell repertoire attains a state of tolerance to germline-encoded antibody V-region diversity, it is presently unknown whether the same is true of mutationally generated diversity. On the basis of immunoregulatory considerations, we hypothesize that contact exclusion or tolerance normally precludes T cells from helping B cells via self-presented mutant V-region peptides. The lack of recurrent somatic mutations that create known T-cell epitopes in antibody V regions lends some support to this idea. In contrast, our studies of spontaneously autoreactive B cells in systemic autoimmune disease strongly suggest that precursors of such cells are recruited by T-cell help directed to self-presented mutant idiopeptides. Failures in tolerance or contact exclusion mechanisms may be responsible for this apparently abnormal event. In addition to their importance in immune regulation, somatic mutations or other differences from germline-encoded V-region sequence may be largely responsible for undesirable patient responses to therapeutic monoclonal antibodies. These reactions might be averted or diminished by inducing tolerance in the T-cell repertoire with synthetic peptide correlates of non-germline-encoded V-region sequences in humanized antibodies.

Harmful somatic mutations: lessons from the dark side

The ability of somatic mutation to modify the course of an immune response is well documented. However, emphasis has been placed almost exclusively on the ability of somatic mutation to improve the functional characteristics of representative antibodies. The harmful effects of somatic mutation, its dark side, have been far less well characterized. Yet evidence suggests that the number of B cells directed to wastage pathways as a result of harmful somatic mutation probably far exceeds the number of cells whose antibodies have been improved. Here we review our recent findings in understanding the structural and functional consequences of V-region mutation.

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

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

A single VH family and long CDR3s are the targets for hypermutation in bovine immunoglobulin heavy chains

Bovine immunoglobulins are made from genes belonging to a small family of closely related VH genes. In this respect cattle resemble all species of domesticated mammals, which also use one VH family. The family, named BoVH1, is homologous to the mouse Q52 family, and there are no more than 20 genes of this family in the bovine genome. Another feature of bovine heavy chains is the use of long CDR3s, which have an average of 21 codons. It seems that there are several families of long, closely related D genes rich in glycine and tyrosine responsible for this length. Sequences described as targets for mutations in other species can be found in CDR1, CDR2, and the putative D genes. The mutation mechanism starts at some point between late fetal stage and birth and seems to be antigen independent. Diversity seems to be generated by hypermutation, although other mechanisms cannot be discounted at this time. Contrary to humans and mice, which have several VH gene families comprising more than 100 genes, cattle use only a few genes and long CDR3s followed by somatic mutation to generate the necessary diversity to recognize the universe of antigens they will encounter during their life.

Ig V region hypermutation in B cell hybrids mimics in vivo mutation and allows for isolation of clonal variants

In order to investigate the regulation of Ig hypermutation, we have established a cell culture system in which reversion of a V region stop codon in a stably transfected Ig gene permits the quantitation of mutation rates by fluctuation analysis. Transfected heavy chain V regions associated with the mu constant region undergo low rates of mutation in the NSO plasmacytoma cell line and a moderate rate of mutation in the 18.81 pre-B cell line. Most of the hybrids created by fusing these two cell lines resembled the non-permissive NSO cell line, though a few hybrids had constitutive V region mutation rates that were even higher than 18.81 and similar to the high rates of mutation that occur in vivo (Green, N. S., Rabinowitz, J. L., Zhu, M., Kobrin, B. J. and Scharff, M. D. (1995) Proc. Nat. Acad. Sci. (USA) 92, 6304 6308). Characterization of these hybrids now demonstrates that the transfected genes were integrated outside of the Ig locus. Mutation was due to multiple single base pair replacements in the V region and not the C region, was ongoing and often arose in hot spot motifs described by V region hypermutation in vivo. Subcloning of unstable hybrids allowed for the isolation of highly related clones with 44-70-fold different mutation rates. These results suggest that V region hypermutation in this mode in vitro systems is under both positive and negative regulation.

A method of estimating the numbers of human and mouse immunoglobulin V-genes

Mutations in immunoglobulin V-genes can be due to gene multiplication, allelic variations, mutations induced by antigens or somatic mutations, etc., and various combinations of these. Since the number of different mouse lambda light V-gene nucleotide sequences is relatively small, a pairwise comparison between these sequences can provide a rough idea as to the contributions of the above mechanisms to the number of nucleotide differences between sequences. A plot of occurrences against the number of differences suggests that differences between one to five can be attributed to somatic mutations. Six to 12 differences can be allelic. Thirteen to 17 may be due to allelic variations together with somatic mutations. Differences > 17 appear to be derived from gene multiplication. Although these numbers are most likely somewhat different in humans, they can nevertheless provide a rough guide to sort out the effect of gene multiplication. Estimations of human heavy, kappa and lambda light chain immunoglobulin V-genes are in reasonably good agreement with recent experimental studies. For mouse kappa light and heavy chains, our estimations can provide some insight to future analyses by direct sequencing of these gene segments.

Mechanism of antigen-driven somatic hypermutation of rearranged immunoglobulin V(D)J genes in the mouse

Available data relevant to the mechanism of somatic hypermutation have been critically evaluated in the context of alternative models: (i) error-generating reverse transcription (RT) followed by homologous recombination; and (ii) error-prone DNA replication/repair. A set of basic principles concerning somatic hypermutation has also been formulated and a revised and expanded "RT-Mutatorsome" concept (analogous to telomerase) is presented which is consistent with these principles and all data on the distribution of somatic mutations in normal and Ig transgenic mice carrying particular V(D)J and flanking region constructs. It is predicted that in the mouse VH and Vk loci. the J-C intronic Enhancer-Nuclear Matrix Attachment Region (Ei/MAR) contains a unique sequence motif or secondary structure which ensures that only V(D)J sequences mutate whilst other regions of the genome are not mutated.

Immunology and immunity studied with viruses

Immunity to viruses is used to define important biological parameters of immunology. Specificity, tolerance and T and B cell memory were analysed with murine model infections. The key parameters of antigen kinetics, localization and patterns of T and B cell response induction in maintaining memory and in causing deletion of reactive lymphocytes were compared for self and for viral foreign antigens. Evidence is reviewed that suggests that B cells essentially recognize antigen patterns, whereas T cells react against antigens newly brought into lymphoid tissues; antigens outside lymphoid tissues are ignored, and antigens always present in, or spreading too fast throughout, lymphoid tissues exhaust and delete T cell responses. Finally, effector mechanisms of antiviral immunity are summarized, as they vary with different viruses. On this basis immunological T and B cell memory against viruses is reviewed. Memory studies suggest that increased precursor frequencies of B and T cells appear to remain in the host independent of antigen persistence. However, in order to protect against cytopathic viruses, memory B cells have to produce antibody to maintain protective elevated levels of antibody: B cell differentiation into plasma cells is driven by persisting antigen. Similarly, to protect against infection with a non-cytopathic virus, cytotoxic T cells have to recirculate through peripheral organs. Activation and capacity to emigrate into solid tissues as well as cytolytic effector function are also dependent upon, and driven by, persisting antigen. Because no convincing evidence is yet available of the existence of identifiable B or T cells with specialized memory characteristics, the phenotype of protective immunological memory correlates best with antigen-driven activation of low frequency effector T cells and plasma cells.

On immunological memory

Immunological memory is a hallmark of the immune system. Evolution can teach us which effector arms of immunological memory are biologically relevant against which virus. Antibodies appear to be the critical protective mechanism against cytopathic viruses. Since these viruses cause cell damage and disease directly, particularly in the absence of an immune response, mothers protect their offspring during a critical immunoincompetent period (a consequence of MHC- restricted T cell recognition) by passive transfer of neutralizing antibodies. In contrast, CTL appear to be the crucial effector mechanism against noncytopathic viruses. Since MHC polymorphism has made vertical transmission of T cell memory impossible, immunoincompetent offspring are not, and need not be, protected against such noncytopathic viruses. During the primary response and again during secondary infection, the most important function of CTL is to eliminate noncytopathic viruses, which may otherwise cause lethal immunopathology. Increased precursor frequencies of B and T cells appear to remain in the host independent of antigen persistence. However, in order to protect against cytopathic viruses, memory B cells have to produce antibody to maintain protective elevated levels of antibody; B cell differentiation into plasma cells is driven by persisting antigen. Similarly, to protect against infection with a noncytopathic virus, CTL have to recirculate through peripheral organs. Activation and capacity to emigrate into solid tissues as well as cytolytic effector function are also dependent upon, and driven by, persisting antigen. Because no convincing evidence is available yet of the existence of identifiable B or T cells with specialized memory characteristics, the phenotype of immunological memory correlates best with antigen-driven activation of low frequency effector T cells and plasma cells.

Immunology taught by viruses

The survival of viruses depends on the survival of susceptible hosts. The vertebrate immune system and viruses have therefore coevolved complementary facets. Evidence from various balanced virus-host relationships illustrates that immunological specificity and memory may best be defined biologically and that the mature immune system does not discriminate between "self" and "nonself." Rather, B cells distinguish antigen patterns, whereas T cell responses depend on localization, transport, and kinetics of antigen within lymphatic organs.

Development of the mouse B-cell repertoire

The mouse B-cell repertoire early in ontogeny contains B cells with receptor immunoglobulins exhibiting high connectivity, multi/self-reactivity, and generally low affinity. This is due structurally to extensive restriction in the germline components used to generate the B-cell receptor. The selective pressure acting on the nascent repetoire has both negative and positive components as seen in our in vivo models. VH81X-bearing B cells from the VH81X transgenic mice (and probably from normal mice) are subject to self-selective pressure with two components: a positive one favoring a certain (self-reactive) specificity in the CD23-IgM+ population and a negative one preventing the entry of B cells with this specificity into the CD23+IgM+ compartment.

Random mutagenesis of two complementarity determining region amino acids yields an unexpectedly high frequency of antibodies with increased affinity for both cognate antigen and autoantigen

To gain insight into the mechanism and limitations of antibody affinity maturation leading to memory B cell formation, we generated a phage display library of random mutants at heavy chain variable (V) complementarity determining region 2 positions 58 and 59 of an anti-p-azophenylarsonate (Ars) Fab. Single amino acid substitutions at these positions resulting from somatic hypermutation are recurrent products of affinity maturation in vivo. Most of the ex vivo mutants retained specificity for Ars. Among the many mutants displaying high Ars-binding activity, only one contained a position 58 and 59 amino acid combination that has been previously observed among the monoclonal antibodies (mAbs) derived from Ars-immunized mice. Affinity measurements on 14 of the ex vivo mutants with high Ars-binding activity showed that 11 had higher intrinsic affinities for Ars that the wild-type V region. However, nine of these Fabs also bound strongly to denatured DNA, a property neither displayed by the wild-type V region nor observed among the mutants characteristic of in vivo affinity maturation. These data suggest that ex vivo enhancement of mAb affinity via site-directed and random mutagenesis approaches may often lead to a reduction in antibody specificity that could complicate the use of the resulting mAbs for diagnostic and therapeutic applications. Moreover, the data are compatible with a hypothesis proposing that increased specificity for antigen, rather than affinity per se, is the driving force for formation of the memory B cell compartment.

Somatic diversification and selection of immunoglobulin heavy and light chain variable region genes in IgG+ CD5+ chronic lymphocytic leukemia B cells

Chronic lymphocytic leukemia (CLL) is characterized by the clonal expansion of CD5-expressing B lymphocytes. Most studies have found that these leukemic CD5+ B cells, like their normal counterparts, use immunoglobulin (Ig) variable (V) region genes that exhibit minimal, if any, somatic diversity. These and other observations have suggested that CD5+ B cells may be incapable of generating Ig V gene diversity, and therefore may not be able to develop higher affinity binding sites that could be selected by antigen. However, most of the studies of CLL and normal CD5+ B cells have focused on IgM-producing cells. Since somatic mutations are most often seen in B cells that have undergone an isotype class switch, we analyzed the Ig heavy (H) and light (L) chain variable region genes of seven IgG+CD5+ CLL B cells to determine if somatic diversification and antigen selection had occurred. The data derived provide evidence for skewed use, somatic diversification, and antigenic selection of the Ig V region genes. Nonrandom use of both H and L chain V region genes was manifested by an overrepresentation of VH4 and VKI family genes and the underrepresentation of the JH4 gene segment. Furthermore, VH4 gene use was restricted to only two family members (4.21 and 4.18). In four of the seven cases, the VH and VL genes displayed > or = 5% difference from the most homologous known germline counterparts. Polymerase chain reaction and Southern blot analyses performed in two of these patients demonstrated that their unique VH CDR2 and adjacent sequences were not present in their germline DNA. In addition, a significant level of diversity was seen in the rearranged DJH segments and at the VL-JL junctions of every patient that occurred both at the time of recombination and subsequently. The localization of replacement changes to complementarity determining regions of some patients suggested that antigen selection had occurred. Furthermore, the mutations identified in the VH and VL genes of each individual patient were strikingly similar, both in number and location. Collectively, the data indicate that a subset of CD5+ CLL B cells can display Ig V region gene mutations. In addition, they are consistent with the notions that in some cases antigen selection of these mutations may have occurred, and that antigen stimulation may be a promoting factor in the evolution of certain CLL clones.

A human anti-insulin IgG autoantibody apparently arises through clonal selection from an insulin-specific "germ-line" natural antibody template. Analysis by V gene segment reassortment and site-directed mutagenesis

We analyzed the structural correlates underlying the insulin-dependent selection of the specific anti-insulin IgG1 kappa mAb13-producing cell clone, derived from a patient with insulin-dependent diabetes mellitus treated with recombinant human insulin. First, we cloned the germ-line genes that putatively gave rise to the expressed VH and V kappa segments and used them to generate the full (unmutated) "germ-line revertant" of the "wild-type" (somatically mutated) mAb13, using recombinant PCR methods and an in vitro human C gamma 1 and C kappa expression system. The full "germ-line revertant" bound insulin specifically and in a dose-saturable fashion, but with a relative avidity (AVrel) more than three-fold lower than that of its wild-type counterpart (Avrel, 1.69 x 10(-8) vs 4.91 x 10(-9) g/microliters). Second, we established, by reassorting wild-type and germ-line revertant forms of the mAb13 VH and V kappa segments, that the increased Avrel for insulin of mAb13 when compared with its full "germ-line revertant" counterpart was entirely dependent on the mutations in the VH not those in the V kappa chain. Third, we determined, by site-directed mutagenesis experiments, that of the three mutations in the mAb13 VH segment (Ser-->Gly, Ser-->Thr, and Ser-->Arg at positions 31, 56, and 58, respectively), only Arg58 was crucial in increasing the mAb13 Avrel (from 1.44 x 10(-8) to 5.14 x 10(-9) g/microliters) and affinity (Kd, from 189 to 59 nM) for insulin. The affinity enhancement mediated by the VH segment Arg58 residue reflected about a threefold decrease in dissociation rate constant (Koff, from 4.92 x 10(-3) to 1.54 x 10(-3) s-1) but not an increase in association rate constant (Kon, from 2.60 x 10(4) to 2.61 x 10(4) M-1 s-1), and it contrasted with the complete loss of insulin binding resulting from the substitution of the VH segment Asn52 by Lys. The present findings suggest that human insulin, a self Ag, has the potential to recruit a natural autoantibody-producing cell precursor expressing a specific surface receptor for Ag in unmutated configuration, and drive it through affinity maturation. They also show that binding of insulin by such a receptor can be enhanced or completely abrogated by a single amino acid change.

Defective secretion of an immunoglobulin caused by mutations in the heavy chain complementarity determining region 2

We have investigated four secretion-deficient antibodies (Abs) derived from a panel of 46 mutant T15 anti-phosphocholine Abs, all of which have point mutations in the heavy chain (H) complementarity determining region 2 (CDR2). The level of secretion for these four Abs was < 10% of wild type when expressed together with the T15 light chain (L) in either SP2/0 or P3X63Ag8.653 myeloma cells although normal levels of H and L chain mRNA were produced. Moreover, abundant intracellular H and L chain proteins were detected. Three of the four mutants had little or no assembled H and L complexes intracellularly whereas one had a significant amount of intracellular immunoglobulin (Ig) which was shown to be capable of binding Ag. Thus, we demonstrate for the first time that point mutations confined to CDR2 of the H chain variable (V) region can impede Ab assembly and secretion. We then introduced the same CDR2 mutations into a related H chain which is encoded by the same T15 VH gene but different diversity (D) and joining (J) genes. When these H chains were expressed with a non-T15 L chain, the resulting Abs were secreted normally. The results thus suggest that the effects of the CDR2 mutations on Ab secretion are dependent on their interactions with L and/or H chain D-J sequences. These results also reveal a novel mechanism that could contribute to B cell wastage.

IgG+, CD5+ human chronic lymphocytic leukemia B cells. Production of IgG antibodies that exhibit diminished autoreactivity and IgG subclass skewing

Several questions exist regarding CD5+ B cells. These include the ability of these cells, as compared to CD5- B cells, to undergo an Ig isotype class switch, the subclasses utilized, and the effects that switching may have on antigen binding. To address these issues, ten patients with chronic lymphocytic leukemia (CLL) whose CD5+ leukemic B cell clones produced IgG were studied. Monoclonal IgG was collected from PMA-stimulated CLL cells and from heterohybridomas constructed with these cells, and then analyzed for IgG subclass utilization, autoreactivity, and DNA idiotype expression. The monoclonal B cells from 80% of the CLL patients produced IgG1 and those from 20% produced IgG3. None produced IgG2. In contrast to the known autoreactivity of IgM-producing CD5+ CLL cells (> 50% autoreactive), none of these IgG antibodies reacted significantly with the autoantigens tested. However, three did react significantly with autoantigen after artificially increasing antibody valency by crosslinking. Whereas five of the IgG molecules expressed a cross reactive idiotypic (CRI) marker characteristic of non-mutated kappa anti-DNA antibodies, three expressed a CRI displayed primarily on mutated IgG anti-DNA antibodies. Thus, some CD5+ human B cells can undergo an isotype class switch that for these CLL cells is biased against IgG2 and in favor of the IgG1 and IgG3. In their native state the IgG molecules secreted by these isotype-switched CD5+ cells have diminished autoreactivity, as compared to IgM-producing CLL cells. Since some of the IgG antibodies could be made auto- and poly-reactive by increasing antigen-binding valency, while others expressed idiotypic markers of mutated antibodies, certain of these CD5+ B cells probably utilize non-mutated Ig V genes coding for polyreactive antibodies, whereas others may use genes that have undergone somatic mutation and that code for more restricted specificities. Therefore, both valency and VH gene mutation may account for the diminished autoreactivity of these CD5+ B cell-derived IgG antibodies.