Developmental regulation of the human antibody repertoire

Normal IgH Repertoire Diversity in an Infant with ADA Deficiency After Gene Therapy

PURPOSE

Adenosine deaminase (ADA) deficiency causes severe combined immunodeficiency (SCID) through an accumulation of toxic metabolites within lymphocytes. Recently, ADA deficiency has been successfully treated using lentiviral-transduced autologous CD34+ cells carrying the ADA gene. T and B cell function appears to be fully restored, but in many patients' B cell numbers remain low, and assessments of the immunoglobulin heavy (IgHV) repertoire following gene therapy are lacking.

METHODS

We performed deep sequencing of IgHV repertoire in peripheral blood lymphocytes from a child following lentivirus-based gene therapy for ADA deficiency and compared to the IgHV repertoire in healthy infants and adults.

RESULTS

After gene therapy, Ig diversity increased over time as evidenced by V, D, and J gene usage, N-additions, CDR3 length, extent of somatic hypermutation, and Ig class switching. There was the emergence of predominant IgHM, IgHG, and IgHA CDR3 lengths after gene therapy indicating successful oligoclonal expansion in response to antigens. This provides proof of concept for the feasibility and utility of molecular monitoring in following B cell reconstitution following gene therapy for ADA deficiency.

CONCLUSION

Based on deep sequencing, gene therapy resulted in an IgHV repertoire with molecular diversity similar to healthy infants.

The Origin of B-cells: Human Fetal B Cell Development and Implications for the Pathogenesis of Childhood Acute Lymphoblastic Leukemia

Human B-lymphopoiesis is a dynamic life-long process that starts in utero by around six post-conception weeks. A detailed understanding of human fetal B-lymphopoiesis and how it changes in postnatal life is vital for building a complete picture of normal B-lymphoid development through ontogeny, and its relevance in disease. B-cell acute lymphoblastic leukemia (B-ALL) is one of the most common cancers in children, with many of the leukemia-initiating events originating in utero. It is likely that the biology of B-ALL, including leukemia initiation, maintenance and progression depends on the developmental stage and type of B-lymphoid cell in which it originates. This is particularly important for early life leukemias, where specific characteristics of fetal B-cells might be key to determining how the disease behaves, including response to treatment. These cellular, molecular and/or epigenetic features are likely to change with age in a cell intrinsic and/or microenvironment directed manner. Most of our understanding of fetal B-lymphopoiesis has been based on murine data, but many recent studies have focussed on characterizing human fetal B-cell development, including functional and molecular assays at a single cell level. In this mini-review we will give a short overview of the recent advances in the understanding of human fetal B-lymphopoiesis, including its relevance to infant/childhood leukemia, and highlight future questions in the field.

Antibodies against Phosphorylcholine and Malondialdehyde during the First Two Years of Life

Abs against phosphorylcholine (anti-PC) and Abs against malondialdehyde (anti-MDA) may be protective in chronic inflammation, like atherosclerosis and cardiovascular disease. It is not known how they develop early in life. Ab titers were measured using ELISA in healthy women (n = 105; born into life study) and their children. Plasma samples were collected from the mothers before conception and from the children at birth as well as at 1 and 2 y after birth. Extracted Abs were compared using a proteomics de novo sequencing approach. It was observed that children were born with very low levels of IgM anti-PC, whereas IgM anti-MDA was present at birth. Both IgM anti-PC and anti-MDA increased during the first 2 y of life, but IgM anti-PC in contrast to IgM anti-MDA was still significantly lower than in the mothers. IgG anti-PC decreased after 1 y but reached similar levels as mothers' after 2 y, whereas IgG anti-MDA reached similar levels as mothers' already after 1 y. Proteomics peptide sequencing analysis indicated large peptide sequence variation without specific clone expression during the early stage of life compared with the adult stage for which specific peptide sequences dominated. IgM anti-PC levels develop much slower than anti-MDA and are still relatively low at 2 y. We hypothesize that anti-PC is developed by a combination of preprogramming and exposure to the external world, in which infectious agents may play a role. For anti-MDA, preprogramming is likely to play a major role and at an earlier stage than for anti-PC.

Putative role of marginal zone B cells in pathophysiological processes

The maintenance of inner integrity of an organism is founded on the proper performance of two immunity branches, innate and adaptive immune responses. Recently, it became apparent that subset of splenic B cells named marginal zone B cells (MZB cells) exhibits unique developmental and functional features that bridge these two immunity branches. Strategically positioned at the site where blood and lymph are filtered, MZB cells represent a population of sentinels that rapidly proliferate and differentiate into IgM plasmablast cells when encountered with blood-borne, thymus-independent (TI) Ags. Moreover, MZB cells have intrinsic capability to induce potent CD4⁺ helper T cell response and cytokine production upon stimulation with soluble antigens. Due to their ability to overcome a time gap prior the establishment of the full adaptive response towards pathogens, MZB cells connect and direct innate and adaptive immunity. An additional interesting characteristic of MZB cells is capacity to function as regulatory cells in autoimmune processes. MZB cells may also contribute to the control of autoimmunity via the induction of tolerance by apoptotic cells. Importantly, in the clear association with inflammation and autoimmunity, MZB cells may transform into MALT lymphoma, representing a concurrence point for the infection, immunity and malignancy. This paper presents an insight into the complex biology of marginal zone B cells and their role in intertwining and directing innate and adaptive immune processes at the physiological and pathological level.

Heterogeneity of Memory Marginal Zone B Cells

The marginal zone (MZ) is largely composed of a unique subpopulation of B cells, the so-called MZ-B cells. At a molecular level, memory B cells are characterized by the presence of somatically mutated IGV genes. The earliest studies in the rat have documented the presence of hapten-specific MZ-B cells after immunization in the MZ. This work later received experimental support demonstrating that the IGHV-Cµ transcripts expressed by phenotypically defined splenic MZ-B cells (defined as CD90^negIgM^highIgD^low B cells) can carry somatic hypermutation. However, only a minor fraction (< 10%-20%) of these MZ-B cells is mutated and is considered to represent memory B cells. Memory B cells can either be class-switched (IgG, IgA, IgE), or non-class-switched (IgM) B cells. B cells in the MZ are a heterogeneous population of cells and both naïve MZ-B cells; class switched and unswitched memory MZ-B cells are present at this unique site in the spleen. Naïve MZ-B cells carry unmutated Ig genes, produce low-affinity IgM molecules and constitute a first line of defense against invading pathogens. Memory MZ-B cells express high-affinity Ig molecules, directed to (microbial) antigens that have been encountered. In this review, we report on the memory compartment of splenic MZ-B cells in the rat to provide insights into the origin and function of these memory MZ-B cells.

Terminal Deoxynucleotidyl Transferase Is Not Required for Antibody Response to Polysaccharide Vaccines against Streptococcus pneumoniae and Salmonella enterica Serovar Typhi

B cell antigen receptor (BCR) diversity increases by several orders of magnitude due to the action of terminal deoxynucleotidyl transferase (TdT) during V(D)J recombination. Unlike adults, infants have limited BCR diversity, in part due to reduced expression of TdT. Since human infants and young mice respond poorly to polysaccharide vaccines, such as the pneumococcal polysaccharide vaccine Pneumovax23 and Vi polysaccharide (ViPS) of Salmonella enterica serovar Typhi, we tested the contribution of TdT-mediated BCR diversity in response to these vaccines. We found that TdT^+/- and TdT^-/- mice generated comparable antibody responses to Pneumovax23 and survived Streptococcus pneumoniae challenge. Moreover, passive immunization of B cell-deficient mice with serum from Pneumovax23-immunized TdT^+/- or TdT^-/- mice conferred protection. TdT^+/- and TdT^-/- mice generated comparable levels of anti-ViPS antibodies and antibody-dependent, complement-mediated bactericidal activity against S Typhi in vitro To test the protective immunity conferred by ViPS immunization in vivo, TdT^+/- and TdT^-/- mice were challenged with a chimeric Salmonella enterica serovar Typhimurium strain expressing ViPS, since mice are nonpermissive hosts for S Typhi infection. Compared to their unimmunized counterparts, immunized TdT^+/- and TdT^-/- mice challenged with ViPS-expressing S Typhimurium exhibited a significant reduction in the bacterial burden and liver pathology. These data suggest that the impaired antibody response to the Pneumovax23 and ViPS vaccines in the young is not due to limited TdT-mediated BCR diversification.

5' Rapid Amplification of cDNA Ends and Illumina MiSeq Reveals B Cell Receptor Features in Healthy Adults, Adults With Chronic HIV-1 Infection, Cord Blood, and Humanized Mice

Using 5′ rapid amplification of cDNA ends, Illumina MiSeq, and basic flow cytometry, we systematically analyzed the expressed B cell receptor (BCR) repertoire in 14 healthy adult PBMCs, 5 HIV-1+ adult PBMCs, 5 cord blood samples, and 3 HIS-CD4/B mice, examining the full-length variable region of μ, γ, α, κ, and λ chains for V-gene usage, somatic hypermutation (SHM), and CDR3 length. Adding to the known repertoire of healthy adults, Illumina MiSeq consistently detected small fractions of reads with high mutation frequencies including hypermutated μ reads, and reads with long CDR3s. Additionally, the less studied IgA repertoire displayed similar characteristics to that of IgG. Compared to healthy adults, the five HIV-1 chronically infected adults displayed elevated mutation frequencies for all μ, γ, α, κ, and λ chains examined and slightly longer CDR3 lengths for γ, α, and λ. To evaluate the reconstituted human BCR sequences in a humanized mouse model, we analyzed cord blood and HIS-CD4/B mice, which all lacked the typical SHM seen in the adult reference. Furthermore, MiSeq revealed identical unmutated IgM sequences derived from separate cell aliquots, thus for the first time demonstrating rare clonal members of unmutated IgM B cells by sequencing.

Factors Affecting the Immunity to Respiratory Syncytial Virus: From Epigenetics to Microbiome

Respiratory syncytial virus (RSV) is a common pathogen that infects virtually all children by 2 years of age and is the leading cause of hospitalization of infants worldwide. While most children experience mild symptoms, some children progress to severe lower respiratory tract infection. Those children with severe disease have a much higher risk of developing childhood wheezing later in life. Many risk factors are known to result in exacerbated disease, including premature birth and early age of RSV infection, when the immune system is relatively immature. The development of the immune system before and after birth may be altered by several extrinsic and intrinsic factors that could lead to severe disease predisposition in children who do not exhibit any currently known risk factors. Recently, the role of the microbiome and the resulting metabolite profile has been an area of intense study in the development of lung disease, including viral infection and asthma. This review explores both known risk factors that can lead to severe RSV-induced disease as well as emerging topics in the development of immunity to RSV and the long-term consequences of severe infection.

Natural IgM and the Development of B Cell-Mediated Autoimmune Diseases

Most serum immunoglobulin M (IgM) is "natural IgM", which is produced apparently spontaneously by a distinct subset of B cells requiring no exogenous antigenic or microbial stimuli. Natural IgM is an evolutionarily conserved molecule and reacts with a variety of epitopes expressed on both self- and non-self antigens. It has long been understood that secreted (s) IgM contributes to the removal of altered self-antigens, such as apoptotic and dying cells. As we outline in this review, it is thought that this sIgM housekeeping function removes potential triggers of autoresponse induction. However, we recently demonstrated an unexpected and distinct role for sIgM in the control of autoreactive B cells: the regulation of bone marrow B cell development. The absence of sIgM blocked pro- to pre- B-cell transition and greatly altered the BCR repertoire of the developing B cells and the peripheral B-cell pools in genetically engineered mice. This finding strongly suggests that IgM is critical for B-cell central tolerance induction. Given that treatment of sIgM-deficient mice with polyclonal IgM corrected these developmental defects, therapeutic application of IgM could be of clinical relevance in the treatment of some B-cell-mediated autoimmune diseases.

Characterization of clonal immunoglobulin heavy (IGH) V-D-J gene rearrangements and the complementarity-determining region in South Indian patients with precursor B-cell acute lymphoblastic leukemia

Background

This study characterized clonal IG heavy V-D-J (IGH) gene rearrangements in South Indian patients with precursor B-cell acute lymphoblastic leukemia (precursor B-ALL) and identified age-related predominance in VDJ rearrangements.

Methods

IGH rearrangements were studied in 50 precursor B-ALL cases (common ALL=37, pre-B ALL=10, pro-B ALL=3) by polymerase chain reaction (PCR) heteroduplex analysis. Twenty randomly selected clonal IGH rearrangement sequences were analyzed using the IMGT/V-QUEST tool.

Results

Clonal IGH rearrangements were detected in 41 (82%) precursor B-ALL cases. Among the IGHV1-IGHV7 subgroups, IGHV3 was used in 25 (50%) cases. Among the IGHD1-IGHD7 genes, IGHD2 and IGHD3 were used in 8 (40%) and 5 (25%) clones, respectively. Among the IGHJ1-IGHJ6 genes, IGHJ6 and IGHJ4 were used in 9 (45%) and 6 (30%) clones, respectively. In 6 out of 20 (30%) IGH rearranged sequences, CDR3 was in frame whereas 14 (70%) had rearranged sequences and CDR3 was out of frame. A somatic mutation in Vmut/Dmut/Jmut was detected in 14 of 20 IGH sequences. On average, Vmut/Dmut/Jmut were detected in 0.1 nt, 1.1 nt, and 0.2 nt, respectively.

Conclusion

The IGHV3 gene was frequently used whereas lower frequencies of IGHV5 and IGHV6 and a higher frequency of IGHV4 were detected in children compared with young adults. The IGHD2 and IGHD3 genes were over-represented, and the IGHJ6 gene was predominantly used in precursor-B-ALL. However, the IGH gene rearrangements in precursor-B-ALL did not show any significant age-associated genotype pattern attributed to our population.

Decreased IL7Rα and TdT expression underlie the skewed immunoglobulin repertoire of human B-cell precursors from fetal origin

Newborns are unable to mount antibody responses towards certain antigens. This has been related to the restricted repertoire of immunoglobulin (Ig) genes of their B cells. The mechanisms underlying the restricted fetal Ig gene repertoire are currently unresolved. We here addressed this with detailed molecular and cellular analysis of human precursor-B cells from fetal liver, fetal bone marrow (BM), and pediatric BM. In the absence of selection processes, fetal B-cell progenitors more frequently used proximal V, D and J genes in complete IGH gene rearrangements, despite normal Ig locus contraction. Fewer N-nucleotides were added in IGH gene rearrangements in the context of low TdT and XRCC4 expression. Moreover, fetal progenitor-B cells expressed lower levels of IL7Rα than their pediatric counterparts. Analysis of progenitor-B cells from IL7Rα-deficient patients revealed that TdT expression and N-nucleotides additions in Dh-Jh junctions were dependent on functional IL7Rα. Thus, IL7Rα affects TdT expression, and decreased expression of this receptor underlies at least in part the skewed Ig repertoire formation in fetal B-cell precursors. These new insights provide a better understanding of the formation of adaptive immunity in the developing fetus.

Natural antibody repertoires: development and functional role in inhibiting allergic airway disease

In this review we discuss the effects of microbial exposure on the B cell repertoire. Neonatal exposure to conserved bacterial carbohydrates and phospholipids permanently reprograms the natural antibody repertoire directed toward these antigens by clonal expansion, alterations in clonal dominance, and increased serum antibody levels. These epitopes are present not only in bacterial cell walls, but also in common environmental allergens. Neonatal immunization with bacterial polysaccharide vaccines results in attenuated allergic airway responses to fungi-, house dust mite-, and cockroach-associated allergens in mouse models. The similarities between mouse and human natural antibody repertoires suggest that reduced microbial exposure in children may have the opposite effect, providing a potential mechanistic explanation for the hygiene hypothesis. We propose that understanding the effects of childhood infections on the natural antibody repertoire and the mechanisms of antibody-mediated immunoregulation observed in allergy models will lead to the development of prevention/interventional strategies for treatment of allergic asthma.

Sterile DJH rearrangements reveal that distance between gene segments on the human Ig H chain locus influences their ability to rearrange

Rearrangement of the Ig locus occurs in two steps. First, a JH gene is rearranged to a D gene followed by a VH gene rearranging to the DJH rearrangement. By next generation sequencing, we analyzed 9969 unique DJH rearrangements and 5919 unique VHDJH rearrangements obtained from peripheral blood B cells from 110 healthy adult donors. We found that DJH rearrangements and nonproductive VHDJH rearrangements share many features but differ significantly in their use of D genes and propensity for somatic hypermutation. In D to JH gene rearrangements, the D genes proximal to the JH locus are used more frequently than JH locus distal D genes, whereas VH locus proximal D genes were observed more frequently in nonproductive VHDJH rearrangements. We further demonstrate that the distance between VH, D, and JH gene segments influence their ability to rearrange within the human Ig locus.

Expansion of the preimmune antibody repertoire by junctional diversity in Bos taurus

Cattle have a limited range of immunoglobulin genes which are further diversified by antigen independent somatic hypermutation in fetuses. Junctional diversity generated during somatic recombination contributes to antibody diversity but its relative significance has not been comprehensively studied. We have investigated the importance of terminal deoxynucleotidyl transferase (TdT) -mediated junctional diversity to the bovine immunoglobulin repertoire. We also searched for new bovine heavy chain diversity (IGHD) genes as the information of the germline sequences is essential to define the junctional boundaries between gene segments. New heavy chain variable genes (IGHV) were explored to address the gene usage in the fetal recombinations. Our bioinformatics search revealed five new IGHD genes, which included the longest IGHD reported so far, 154 bp. By genomic sequencing we found 26 new IGHV sequences that represent potentially new IGHV genes or allelic variants. Sequence analysis of immunoglobulin heavy chain cDNA libraries of fetal bone marrow, ileum and spleen showed 0 to 36 nontemplated N-nucleotide additions between variable, diversity and joining genes. A maximum of 8 N nucleotides were also identified in the light chains. The junctional base profile was biased towards A and T nucleotide additions (64% in heavy chain VD, 52% in heavy chain DJ and 61% in light chain VJ junctions) in contrast to the high G/C content which is usually observed in mice. Sequence analysis also revealed extensive exonuclease activity, providing additional diversity. B-lymphocyte specific TdT expression was detected in bovine fetal bone marrow by reverse transcription-qPCR and immunofluorescence. These results suggest that TdT-mediated junctional diversity and exonuclease activity contribute significantly to the size of the cattle preimmune antibody repertoire already in the fetal period.

Natural IgM: beneficial autoantibodies for the control of inflammatory and autoimmune disease

Natural IgM are highly represented in the circulation at birth, and these often autoreactive antibodies have been postulated to have innate-like properties and play crucial roles in apoptotic cell clearance, tissue homeostasis, and immune modulation. This review summarizes the known properties of these IgM autoantibodies, and the evidence that these anti-apoptotic cell IgM natural antibodies can regulate inflammatory responses through ancient pathways of the innate immune system that first arose long before the initial emergence of the adaptive immune system. While the regulatory contributions of these natural IgM autoantibodies are certainly not an essential and fundamental component of host defenses, these provide an additional layer to further protect the host. More importantly, these IgM antibody responses are highly inducible and their up-regulation can be a powerful means for the host to survive in a setting of chronic inflammation. The observed beneficial clinical associations for cardiovascular disease and autoimmunity, as well as opportunities for potential therapeutic implications are discussed.

Strategies to stabilize compact folding and minimize aggregation of antibody-based fragments

Monoclonal antibodies (mAbs) have proven to be useful for development of new therapeutic drugs and diagnostic techniques. To overcome the difficulties posed by their complex structure and folding, reduce undesired immunogenicity, and improve pharmacokinetic properties, a plethora of different Ab fragments have been developed. These include recombinant Fab and Fv segments that can display improved properties over those of the original mAbs upon which they are based. Antibody (Ab) fragments such as Fabs, scFvs, diabodies, and nanobodies, all contain the variable Ig domains responsible for binding to specific antigenic epitopes, allowing for specific targeting of pathological cells and/or molecules. These fragments can be easier to produce, purify and refold than a full Ab, and due to their smaller size they can be well absorbed and distributed into target tissues. However, the physicochemical and structural properties of the immunoglobulin (Ig) domain, upon which the folding and conformation of all these Ab fragments is based, can limit the stability of Ab-based drugs. The Ig domain is fairly sensitive to unfolding and aggregation when produced out of the structural context of an intact Ab molecule. When unfolded, Ab fragments may lose their specificity as well as establish non-native interactions leading to protein aggregation. Aggregated antibody fragments display altered pharmacokinetic and immunogenic properties that can augment their toxicity. Therefore, much effort has been placed in understanding the factors impacting the stability of Ig folding at two different levels: 1) intrinsically, by studying the effects of the amino acid sequence on Ig folding; 2) extrinsically, by determining the environmental conditions that may influence the stability of Ig folding. In this review we will describe the structure of the Ig domain, and the factors that impact its stability, to set the context for the different approaches currently used to achieve stable recombinant Ig domains when pursuing the development of Ab fragment-based biotechnologies.

Natural and man-made V-gene repertoires for antibody discovery

Antibodies are the fastest-growing segment of the biologics market. The success of antibody-based drugs resides in their exquisite specificity, high potency, stability, solubility, safety, and relatively inexpensive manufacturing process in comparison with other biologics. We outline here the structural studies and fundamental principles that define how antibodies interact with diverse targets. We also describe the antibody repertoires and affinity maturation mechanisms of humans, mice, and chickens, plus the use of novel single-domain antibodies in camelids and sharks. These species all utilize diverse evolutionary solutions to generate specific and high affinity antibodies and illustrate the plasticity of natural antibody repertoires. In addition, we discuss the multiple variations of man-made antibody repertoires designed and validated in the last two decades, which have served as tools to explore how the size, diversity, and composition of a repertoire impact the antibody discovery process.

Antibody responses to mycobacterial antigens in children with tuberculosis: challenges and potential diagnostic value

The identification of easily detectable biomarkers for active tuberculosis (TB) is a global health priority. Such biomarkers would be of particular value in childhood TB, which poses greater diagnostic challenges than adult TB. Serum antibodies can be detected by simple formats that provide extremely rapid results. However, attempts to develop accurate serodiagnostic tests for TB have been unsuccessful. Whereas antibody responses to mycobacterial antigens in adult TB have been studied extensively and reviewed, the same cannot be said for serologic data in pediatric populations. Here we appraise studies on serological responses in childhood TB and discuss findings and limitations in the context of the developing immune system, the age range, and the spectrum of TB manifestations. We found that the antibody responses to mycobacterial antigens in childhood TB can vary widely, with sensitivities and specificities ranging from 14% to 85% and from 86% to 100%, respectively. We conclude that the limitations in serodiagnostic studies of childhood TB are manifold, thereby restricting the interpretation of currently available data. Concerns about the methodology used in published studies suggest that conclusions about the eventual value of serodiagnosis cannot be made at this time. However, the available data suggest a potential adjunctive value for serology in the diagnosis of childhood TB. Despite the difficulties noted in this field, there is optimism that the application of novel antigens and the integration of those factors which contribute to the serological responses in childhood TB can lead to useful future diagnostics.

Human B-cell ontogeny in humanized NOD/SCID γc(null) mice generates a diverse yet auto/poly- and HIV-1-reactive antibody repertoire

Characterization of the human antibody (Ab) repertoire in mouse models of the human immune system is essential to establish their relevance in translational studies. Single human B-cells were sorted from bone marrow and periphery of humanized NOD/SCID γc^null mice at 8–10 months post-engraftment with human cord blood-derived CD34⁺ stem cells. Human immunoglobulin variable heavy (V_H) and kappa (V_κ) genes were amplified, cognate V_H-V_κ gene-pairs assembled as single-chain variable fragment-Fc antibodies (scFvFcs) and functional studies performed. Although overall distribution of V_H genes approximated the normal human Ab repertoire, analysis of the V_H-third complementarity determining regions (H-CDR3) in the mature B-cell subset demonstrated an increase in length and positive charges suggesting autoimmune characteristics. Additionally, >70% of V_κ sequences utilized V_κ4-1, a germline gene associated with autoimmunity. The mature B-cell subset-derived scFvFcs displayed the highest frequency of autoreactivity and polyspecificity, suggesting defects in checkpoint control mechanisms. Furthermore, these scFvFcs demonstrated binding to recombinant HIV envelope corroborating previous observations of poly/autoreactivity in anti-HIVgp140 antibodies. These data lend support to the hypothesis that anti-HIV BnAbs may be derived from auto/polyspecific Abs that escaped immune elimination and that the hNSG mouse could provide a new experimental platform for studying the origin of anti-HIV neutralizing Ab responses.

Challenges for development of meningococcal vaccines in infants and children

Neisseria meningitidis causes significant disease in the form of severe sepsis syndrome or meningococcal meningitis. Owing to the susceptibility of the immune system in early life, the risk of disease after infection is significantly higher in infants. Thus far, vaccines targeted against meningococcal serogroups have struggled to provide lasting protection in young children. Even conjugate vaccines that are now routinely used in the immunization of infants require multiple dosing and the duration of protection has been shown to wane over time and require repeated booster doses. After briefly summarizing the current epidemiology according to age and serogroup, this article will consider the reasons for poor immunogenicity of vaccines in infants and will discuss the relative efficacy of the different vaccine types in this age group. It will then go on to consider strategies for optimizing the protection of infants against meningococcal disease.

DH and JH usage in murine fetal liver mirrors that of human fetal liver

In mouse and human, the regulated development of antibody repertoire diversity during ontogeny proceeds in parallel with the development of the ability to generate antibodies to an array of specific antigens. Compared to adult, the human fetal antibody repertoire limits N addition and uses specifically positioned VDJ gene segments more frequently, including V6-1 the most D(H)-proximal V(H,) DQ52, the most J(H)-proximal D(H), and J(H)2, which is D(H)-proximal. The murine fetal antibody repertoire also limits the incorporation of N nucleotides and uses its most D(H) proximal V(H), V(H)81X, more frequently. To test whether D(H) and J(H) also follow the pattern observed in human, we used the scheme of Hardy to sort B lineage cells from BALB/c fetal and neonatal liver, RT-PCR cloned and sequenced V(H)7183-containing VDJCμ transcripts, and then assessed V(H)7183-D(H)-J(H) and complementary determining region 3 of the immunoglobulin heavy chain (CDR-H3) content in comparison to the previously studied adult BALB/c mouse repertoire. Due to the deficiency in N nucleotide addition, perinatal CDR-H3s manifested a distinct pattern of amino acid usage and predicted loop structures. As in the case of adult bone marrow, we observed a focusing of CDR-H3 length and CDR-H3 loop hydrophobicity, especially in the transition from the early to late pre-B cell stage, a developmental checkpoint associated with expression of the pre-B cell receptor. However, fetal liver usage of J(H)-proximal D(H)Q52 and D(H)-proximal J(H)2 was markedly greater than that of adult bone marrow. Thus, the early pattern of D(H) and J(H) usage in mouse feta liver mirrors that of human.

The immunogenicity of humanized and fully human antibodies: residual immunogenicity resides in the CDR regions

Monoclonal antibodies represent an attractive therapeutic tool as they are highly specific for their targets, convey effector functions and enjoy robust manufacturing procedures. Humanization of murine monoclonal antibodies has vastly improved their in vivo tolerability. Humanization, the replacement of mouse constant regions and V framework regions for human sequences, results in a significantly less immunogenic product. However, some humanized and even fully human sequence-derived antibody molecules still carry immunological risk. To more fully understand the immunologic potential of humanized and human antibodies, we analyzed CD4(+) helper T cell epitopes in a set of eight humanized antibodies. The antibodies studied represented a number of different VH and VL family members carrying unique CDR regions. In spite of these differences, CD4(+) T cell epitopes were found only in CDR-sequence containing regions. We were able to incorporate up to two amino acid modifications in a single epitope that reduced the immunogenic potential while retaining full biologic function. We propose that immunogenicity will always be present in some antibody molecules due to the nature of the antigen-specific combining sites. A consequence of this result is modifications to reduce immunogenicity will be centered on the affinity-determining regions. Modifications to CDR regions can be designed that reduce the immunogenic potential while maintaining the bioactivity of the antibody molecule.

Terminal deoxynucleotidyl transferase is required for an optimal response to the polysaccharide α-1,3 dextran

An understanding of Ab responses to polysaccharides associated with pathogenic microorganisms is of importance for improving vaccine design, especially in neonates that respond poorly to these types of Ags. In this study, we have investigated the role of the lymphoid-specific enzyme TdT in generating B cell clones responsive to alpha-1,3 dextran (DEX). TdT is a DNA polymerase that plays a major role in generating diversity of lymphocyte AgRs during V(D)J recombination. In this study, we show that the DEX-specific Ab response is lower, and the dominant DEX-specific J558 idiotype (Id) is not detected in TdT(-/-) mice when compared with wild-type (WT) BALB/c mice. Nucleotide sequencing of H chain CDR3s of DEX-specific plasmablasts, sorted postimmunization, showed that TdT(-/-) mice generate a lower frequency of the predominant adult molecularly determined clone J558. Complementation of TdT expression in TdT(-/-) mice by early forced expression of the short splice variant of TdT-restored WT proportions of J558 Id+ clones and also abrogated the development of the minor M104E Id+ clones. J558 Id V(D)J rearrangements are detected as early as 7 d after birth in IgM-negative B cell precursors in the liver and spleen of WT and TdT-transgenic mice but not in TdT(-/-) mice. These data show that TdT is essential for the generation of the predominant higher-affinity DEX-responsive J558 clone.

The human neonatal B cell response to respiratory syncytial virus uses a biased antibody variable gene repertoire that lacks somatic mutations

The human Ab repertoire exhibits restrictions during fetal life characterized by biases of variable gene segment usage and lack of junctional diversity. We tested the hypotheses that Ab repertoire restriction persists in the early postnatal period and contributes to the observed poor quality of specific Ab responses made by neonates to viruses and vaccines. We analyzed the molecular determinants of B cell responses in humans to respiratory syncytial virus (RSV). Analysis of the variable gene segment usage of adult RSV-specific B cells revealed a repertoire profile in these cells similar to that seen in randomly selected B cells, which was V(H)3-dominant. Four gene segments (V(H)3-23, V(H)3-30, V(H)3-33 and V(H)4-04) accounted for almost half of the V(H) genes used. In contrast, very young infant RSV-specific antibodies exhibited a biased repertoire characterized by comparable use of the V(H)1, V(H)3, and V(H)4 families, and less common use of the four immunodominant gene segments. Infants and children older than three months used an antibody repertoire similar to that of adults. Mutational analysis revealed that the antibody variable genes of infants under three months of age also possessed significantly fewer somatic mutations in both framework and complementarity-determining region (CDR) regions than those of adults, even in a child with recurrent RSV infection. These data suggest that neonates use a biased antibody gene repertoire that is less V(H)3-focused and that possesses a dramatically lower frequency of somatic mutations. These biased features of the RSV-specific repertoire likely contribute to the poor functional Ab response in very young infants.

Going adaptive: the saga of antibodies

Because of their extreme importance to human health, we probably know more about the structure and function of antibodies than practically any other molecule. Despite all the knowledge that has been accrued in the understanding of antibodies, modern approaches, especially comparative genomics, continue to yield novel findings regarding their underlying biology and evolution. In this review, we describe recent research that led to these revelations, and discuss the broad evolutionary implications of these findings. We have restricted our discussion to three vignettes. Considerable attention has been paid to the recent discovery that the teleost IgH locus is highly similar in organization to the Tcra-Tcrd locus, implicating an evolutionary common ancestor and parallels between the functions of B and T cells during development. Second, we discuss how a new type of antibody, recently discovered in jawless vertebrates, composed not of immunoglobulins but leucine-rich repeats, sheds new light on the overall forces driving evolution of all adaptive antigen receptors. Lastly, we discuss how accumulation of genomic sequences of various human subpopulations leads to better understanding of the directionality of antibody evolution. There is always more to learn from the unfolding saga of antibodies.

Antistaphylococcal immunoglobulins to prevent staphylococcal infection in very low birth weight infants

BACKGROUND

Nosocomial infection is a major problem affecting the immediate health and long-term outcome of preterm and very low birth weight neonates. More than half of these infections are caused by staphylococci. Various type specific antibodies targeted at different antigenic markers of Staphylococcus have been developed and have shown promise in animal studies.

OBJECTIVES

To evaluate the efficacy and safety of antistaphylococcal immunoglobulins in the prevention of Staphylococcal infection in very low birth weight infants.

SEARCH STRATEGY

Medline, Embase, CINAHL, Cochrane Central Register of Controlled Trials (The Cochrane Library) were searched from their inception until February 2009. In addition, abstracts of major pediatric meetings of last seven years were searched.

SELECTION CRITERIA

Randomized and quasi-randomized studies of antistaphylococcal immunoglobulins for the prevention of staphylococcal infections in preterm or very low birth weight neonates were reviewed by both authors for their eligibility for inclusion. Studies of any dose and/or route were included. Quality of studies was evaluated using criteria of masking of randomization, masking of intervention, completeness of follow-up and masking of outcome assessment by both review authors.

DATA COLLECTION AND ANALYSIS

Data from the primary author were obtained where published data provided inadequate information for the review or where relevant data could not be abstracted. Data were abstracted independently by both review authors. Statistical methods included calculation of relative risk (RR), risk difference (RD), number needed to treat (NNT) and weighted mean difference (WMD) when appropriate. Ninety five percent confidence intervals (CI) was used for these estimates of treatment effects. A fixed effect model was used for meta-analyses.

MAIN RESULTS

Three eligible studies were included (two studies of INH A-21 and one study of Altastaph involving a total of 2,701 neonates). Three reports of Pagibaximab were published as abstracts and will be considered for inclusion when further information is obtained. There were no significant differences noted in the risk of Staphylococcal infection between INH A-21 vs. placebo (typical RR 1.07, 95% CI 0.94, 1.22) or Altastaph vs. placebo (RR 0.86, 95% CI 0.32, 2.28); the risk of other bacterial infection between INH A-21 vs. placebo (typical RR 0.87, 95% CI 0.72, 1.06) or Altastaph vs. placebo (RR 0.93, 95% CI 0.53, 1.64); or the risk of any infection between INH A-21 vs. placebo (RR 1.00, 95% CI 0.91, 1.09) or Altastaph vs. placebo (RR 0.93, 95% CI 0.54, 1.62). There was no significant difference in the incidence of relevant secondary outcomes (chronic lung disease at 28 days, patent ductus arteriosus, necrotizing enterocolitis, intraventricular hemorrhage, retinopathy of prematurity or duration of antibiotic and vancomycin use).

AUTHORS' CONCLUSIONS

Antistaphylococcal immunoglobulins (INH A-21 and Altastaph) are not recommended for prevention of staphylococcal infections in preterm or VLBW neonates. Further research to investigate the efficacy of other products such as Pagibaximab is needed.

Characterization and genetic diversity of the sturgeon Acipenser schrenskii Ig heavy chain

It is still unknown about the VH gene organization and diversity of the immunoglobulin (Ig) heavy chain locus in Amur sturgeon. In this study, Ig heavy chain alleles were cloned by RT-PCR using the specific primers. Sequence analysis showed that Amur sturgeon's VH regions belonged to the same family with higher than 90% identities of their leader peptide (LP). Moreover, a number of conserved motifs in the D segment were identified, and the variability of the CDR3 region was substantial. Further, we speculated that there were at least 12 different JH segments in the locus, contributing to the antibody repertoire of the sturgeon. The genetic diversity of the sturgeon Ig should be associated with the random rearrangement of VH, D and JH segments, action of exonuclease and insertion of N and/or probably P nucleotides at the site of rearrangement.

Large-scale analysis of human heavy chain V(D)J recombination patterns

Background

The processes involved in the somatic assembly of antigen receptor genes are unique to the immune system and are driven largely by random events. Subtle biases, however, may exist and provide clues to the molecular mechanisms involved in their assembly and selection. Large-scale efforts to provide baseline data about the genetic characteristics of immunoglobulin (Ig) genes and the mechanisms involved in their assembly have recently become possible due to the rapid growth of genetic databases.

Results

We gathered and analyzed nearly 6,500 productive human Ig heavy chain genes and compared them with 325 non-productive Ig genes that were originally rearranged out of frame and therefore incapable of being biased by selection. We found evidence for differences in n-nucleotide tract length distributions which have interesting interpretations for the mechanisms involved in n-nucleotide polymerization. Additionally, we found striking statistical evidence for pairing preferences among D and J segments. We present a statistical model to support our hypothesis that these pairing biases are due to multiple sequential D-to-J rearrangements.

Conclusion

We present here the most precise estimates of gene segment usage frequencies currently available along with analyses regarding n-nucleotide distributions and D-J segment pair preferences. Additionally, we provide the first statistical evidence that sequential D-J recombinations occur at the human heavy chain locus during B-cell ontogeny with an approximate frequency of 20%.

Active immunization of premature and low birth-weight infants: a review of immunogenicity, efficacy, and tolerability

Preterm infants are at increased risk of disease and hospitalization from a number of vaccine-preventable diseases. However, these same infants have immunologic immaturities that may impact vaccine responses. Larger premature infants mount immune responses to vaccines similar to those of full-term infants, but very premature infants (<28-32 weeks' gestation at birth) may have specific defects in vaccine responsiveness. Although there are minor differences in immunogenicity, the immune responses to diphtheria, tetanus, pertussis, and polio antigens are similar enough between full-term and premature infants that clinical consequences are unlikely to result. However, the immunogenicity of Haemophilus influenzae type b conjugate vaccines varies widely among studies of premature infants, and may be affected by the choice of conjugate protein, inclusion in a combination vaccine, and by an infant's overall health. Pneumococcal conjugate vaccine is efficacious in larger premature infants, but little information is available about immunogenicity in smaller premature infants. Meningococcal group C conjugate vaccine appears immunogenic in even very premature infants, but the duration of immunity may be limited. Hepatitis B vaccine given at birth appears poorly immunogenic in infants with birth weights <1500-2000 g, with delay in the administration of the first dose yielding improved immunogenicity. Few data on influenza vaccine in premature infants are available, but infants with pulmonary disease may respond less robustly than others. Bacille Calmette Guérin vaccine appears to be most immunogenic if delayed until at least 34-35 weeks' postmenstrual age in very premature infants, although there may be non-specific advantages to its earlier administration. Premature infants may have persistently lower antibody titers than full-term infants, even years after initial immunization. Sick premature infants experience increased episodes of apnea or cardiorespiratory compromise following vaccine administration, necessitating careful monitoring. Specific factors that impair immune response, quality of the immune response, and safety and immunogenicity evaluation of new vaccines in premature infants are topics needing further research. Premature infants are at significant risk for decisions from healthcare providers that delay beginning and completing their vaccine regimens. A major challenge facing those who care for these infants is the provision of timely immunization.

Intraperitoneal inoculation of Sandhoff mouse neonates with an HIV-1 based lentiviral vector exacerbates the attendant neuroinflammation and disease phenotype

We aimed to evaluate the efficacy of VSV-G pseudotyped, defective HIV-1 based lentiviral vectors for the neonatal transfer of therapeutic genes following systemic administration in Sandhoff mouse pups. Despite transgene expression in mouse brains, these animals presented with significant exacerbation and acceleration of the disease neurological phenotype. We observed an increase and acceleration in the presence of MHC-II and CD45+ cells in their brains, along with neuroinflammation, but not in control heterozygous or wild type littermates that also received the same treatment.

Categorical selection of the antibody repertoire in splenic B cells

In the bone marrow, the passage of developing B cells through critical checkpoints of differentiation is associated with a reduction of specific categories of CDR3 of the Ig heavy chain (CDR-H3), particularly those with excessive hydrophobic or charged amino acids and those with a length of eight or fewer residues. To gain insight into the role of CDR-H3 content in the development of B cells in the spleen, we compared the sequences of V(H)7183DJCmu transcripts from sorted transitional T1, marginal zone, and follicular B cell subsets to those expressed by immature IgM(+)IgD(-) and mature IgM(lo)IgD(hi) B cells in the bone marrow. Although differences in V(H) utilization were noted, the T1 CDR-H3 repertoire showed extensive similarity to that of immature bone marrow B cells, and the follicular CDR-H3 repertoire most resembled that of mature bone marrow B cells. Unlike the splenic follicular and bone marrow mature B cell CDR-H3 repertoires, the marginal zone B cell CDR-H3 repertoire retained both short and highly charged amino acid motifs, including those with two arginines. Our findings suggest that antigen binding sites containing specific categories of CDR-H3 sequence content may inhibit, permit, or even facilitate passage of the host B cell through critical checkpoints in peripheral as well as central development.

Autoantibody selection and production in early human life

Natural antibodies are autoreactive/polyreactive antibodies believed to be secreted in the absence of xenoantigens. The origin and functional role of this limited and selective autoimmunity are not clear, nor is the specificity and range of autoantigens that drive the development of B cells producing natural antibodies. In this issue of the JCI, Merbl et al. report that in utero, humans generate natural IgM and IgA antibodies that recognize a uniform set of autoantigens (see the related article beginning on page 712), some of which are associated with autoimmune diseases. The authors postulate that this "autoimmunity" at birth favors the emergence of autoimmune diseases in later life. We present a molecular basis for the limited and common repertoire of antibodies produced by fetal B cells, which may be distinct from the abnormalities in B cell development described in patients with autoimmune diseases.

Similarity and divergence in the development and expression of the mouse and human antibody repertoires

Over the past twenty years diverse groups in Northeast Asia, Western Europe, and North America have competed to map, sequence, and characterize the immunoglobulin loci of mouse and man. Now that this work is near completion, it has become evident that the human and mouse germline repertoires share broad similarities in gene composition, organization, and other general principles. In spite of these similarities, the repertoires expressed by adult mice and humans are distinct and differ from each other in detail. In both species the mechanisms used to create repertoire diversity appear designed to generate a random range of antigen binding sites. However, a detailed analysis reveals significant constraints in the sequence and amino acid composition of the third complementarity region of the H chain (CDR-H3), which lies at the center of the antigen binding site. The mechanisms used to regulate the composition of the repertoire, their significance to the development and maintenance of immune competence, and the contribution of violation of normal repertoire boundaries to the development of diseases of immune function remain foci of ongoing investigation.

Limited VH gene usage in B-cell clones established with nurse-like cells from patients with rheumatoid arthritis

OBJECTIVES

Nurse-like stromal cells (NLC) in synovia and bone marrow of patients with rheumatoid arthritis (RA) can support pseudoemperipolesis, protect from apoptosis and enhance immunoglobulin production of peripheral blood B cells isolated from healthy individuals, suggesting the profound contribution of hyperactivation of B cells in RA. In the course of establishing RA-NLC from RA patients, we observed the growth of B cells in the presence of RA-NLC.

METHODS

We cloned B cells from the synovium or bone marrow of RA patients using the limiting dilution technique. For established clones, nucleotide sequences of immunoglobulin and surface antigens were investigated. To investigate the dependence of these clones on NLC, differences in the proliferation and the amount of immunoglobulin produced in the presence or absence of NLC were compared. Immunocytochemical staining of various cells was performed using the antibody these clones produced.

RESULTS

Nine B-cell clones established from RA patients showed RA-NLC-dependent growth. These B-cell clones expressed CD19, CD20, CD38, CD39 and CD40, suggesting that the cloned cells were mature and activated. All clones secreted immunoglobulins in culture media, which were specific for intracellular components of various cell lines, including RA-NLC. Interestingly, we found limited usage of immunoglobulin heavy-chain variable regions (VH) among B-cell clones from RA patients. These repertoires were reported to be detected preferentially in fetal livers.

CONCLUSION

The present study provides a novel insight into the involvement of RA-NLC in the immunopathogenesis of RA via an autoreactive B cell development and/or activation mechanism.

The pre-immune variable kappa repertoire of swine is selectively generated from certain subfamilies of Vkappa2 and one Jkappa gene

Combinatorial diversity is highly restricted during formation of the pre-immune heavy chain repertoire of swine, raising the question of whether the same is true for the pre-immune light chain repertoire. Before addressing this question, we first used competitive PCR to show that kappa and lambda light chains in swine are equally expressed in mature B cells similar to the situation in humans but alike that in other studied Ungulates. This justified efforts to examine the repertoire of both light chain types. These studies also revealed that lambda is preferentially expressed at sites of B cells lymphogenesis, perhaps because of the use of a surrogate light chain containing lambda5. Data are presented here on >100 VkappaJkappa-containing transcripts and approximately 180 genomic Vkappa genes to show that >90% of the pre-immune repertoire is generated from three subfamilies of IGKV2 genes and one of five Jkappa segments. The kappa locus contains >or=50 IGKV2 genes belonging to at least five subfamilies and an undetermined but perhaps equal number of IGKV1 genes. The porcine IGKV1 and IGKV2 genes share 87% sequence similarity with their human counterparts and Jkappa1 through Jkappa5 share sequence and organizational homology with those in sheep, horse, human and mouse. Swine have a single Ckappa gene. These findings contrast with those from rodents and primates but are reminiscent of those on the pre-immune heavy chain repertoire of swine in that it is generated using a relatively restricted number of gene segments. These restricted pre-immune repertoires may reflect the minimal exposure of the fetus to maternal factors and environmental antigens. The significance for swine immunology of characterizing the pre-immune repertoire is discussed.

Recruitment and selection of marginal zone B cells is independent of exogenous antigens

Marginal zone B (MZ-B) cells of the spleen contribute significantly to the immunity against invasive infections with polysaccharide-encapsulated bacteria. Recent evidence indicates that recruitment and selection of MZ-B cells occurs on the basis of positive selection constraints that likely operate via B cell receptor (BCR) signaling. Previous studies have shown that MZ-B cells carry relatively shorter immunoglobulin (Ig) heavy (H) chain complementarity-determining region 3 (H-CDR3) sequences and express BCR which are thought to be polyreactive. In this scenario, MZ-B cell selection proceeds via engagement of the BCR with exogenous (i.e. microbial gut flora-derived) and/or endogenous (self) antigens. Here, we studied the influence of exogenous antigens on the selection process of MZ-B cells using non-genetically manipulated adult germ-free and conventionally reared infant rats. This study was carried out by H-CDR3 spectratype analysis of V(H)(PC7183)-encoded Ig V(H)DJ(H)-mu transcripts expressed by purified splenic MZ-B cells and other B cell subsets. We show that MZ-B cells in both adult germ-free and conventionally reared infant (14-day-old) rats are H-CDR3-selected cells, providing strong evidence that recruitment and selection of MZ-B cells is driven by self antigens.

Novel Substitution Polymorphisms of Human Immunoglobulin VH Genes in Mexicans

It has been proposed that the defense and recognition functions of the immune system, especially those mediated by antibodies, require a great diversity of receptors. Nonetheless, functional and structural evidence has demonstrated the presence of restrictions, both in the use of the repertoire and in the recognition of antigens. Fifty-one functional genes have been described in the IghV locus; however, there is a variety of evidences indicating that only a small fraction of the immunoglobulin genes plays a central role in determining the fundamental properties of the antibody repertoire of the immune system. On the basis of this functional and structural information, we selected four IghV genes and characterized their polymorphism in a sample of Mexican individuals. We also analyzed the implications for the recognition mechanism of the substitutions found in the sequenced alleles. We found that diversification through allelism varies from segment to segment, both in the amount of alleles encountered and in the nature and distribution of mutations in the codifying zone, which might depend on its importance for the repertoire. Such functional characteristics may be useful in the interpretation of differential gene usage in certain physiological, ontological, and/or pathological conditions.

Comparison of the expressed porcine Vbeta and Jbeta repertoire of thymocytes and peripheral T cells

Transcripts of more than 300 unique T-cell receptor-beta (TCR-beta) V-D-J rearrangements recovered from porcine thymocytes and peripheral T cells were compared. We identified 19 groups (families) of porcine Vbeta genes in seven supergroups and provisionally named 17 groups based on their sequence similarity with recognized human Vbeta gene families. TRBV4S, 5S, 7S and 12S accounted for >80% of all Vbeta usage, and usage of these groups by thymocytes and peripheral T cells was highly correlated. No TRBV group was uniquely expressed in significant numbers in thymocytes, although small numbers of TRBV groups 2S, 9S and 15S were only recovered from T cells. Usage of Jbeta segments from the 5' D-J-C duplicon in thymocytes and peripheral T cells directly correlated with their 5' position in the locus, and Jbeta1.1, 1.2 and 1.3 accounted for >or= 35% of all Jbeta usage in both cell types. This contrasts with the usage of Jbeta2 segments in that Jbeta2.4, 2.5 and 2.7 accounted for approximately 30% of Jbeta usage by T cells and thymocytes. Jbeta2.7 was threefold more frequent among T cells than thymocytes. The Vbeta/Jbeta combination was not random. Jbeta1.1 and 1.2 were used in 29% of rearrangements with high frequency among the major Vbeta groups. Combinations of TRBV4 and V12 with Jbeta2.7 were only found in T cells and accounted for half of all Jbeta2.7 usage. These studies show that unlike porcine heavy chain V(H) genes, the occurrence and relative usage of porcine TCR-Vbeta groups resembles that of humans. Thus, highly related gene systems can individually diverge within a species.

Immunoglobulin gene rearrangement, repertoire diversity, and the allergic response

The immunoglobulin repertoire arises as a consequence of combinatorial diversity, junctional diversity, and the process of somatic point mutation. Each of these processes involves biases that limit and shape the available immunoglobulin repertoire. The expressed repertoire is further shaped by selection, to the extent that biased gene usage can become apparent in many disease states. The study of rearranged immunoglobulin genes therefore may not only provide insights into the molecular processes involved in the generation of antibody diversity but also inform us of pathogenic processes and perhaps identify particular lymphocyte clones as therapeutic targets. Partly as a consequence of the low numbers of circulating IgE-committed B-cells, studies of rearranged IgE genes in allergic individuals have commenced relatively recently. In this review, recent advances in our understanding of the processes of immunoglobulin gene rearrangement and somatic point mutation are described, and biases inherent to these processes are discussed. The evidence that some diseases may be associated with particular gene rearrangements is then considered, with a particular focus on allergic disease. Reviewed data suggest that an important contribution to the IgE response may come from cells that use relatively rare heavy chain V (V(H)) segment genes, which display little somatic point mutation. Some IgE antibodies also seem to display polyreactive binding. In other contexts, these 3 characteristics have been associated with antibodies of the B-1 B-cell subset, and the possibility that B-1 B-cells contribute to the allergic response is therefore considered.

Regulation and chance in the ontogeny of B and T cell antigen receptor repertoires

The adaptive immune system has to economically generate a large array of T and B cell antigen receptors (T cell receptors [TCRs], B cell receptors [BCRs]) that eliminate both longstanding and novel antigens from the host while preventing the production of deleterious (e.g., autoreactive) antigen receptors. Our studies focus on the mechanisms that shape the development of these antigen receptor repertoires during human ontogeny. The key to BCR and TCR diversity is the third complementarity determining region (CDR3) of the variable domain, which in the immunoglobulin heavy chain and TCR beta chain, is created by the junction between the variable, diversity, and joining gene segments. The CDR3 diversity is constrained by overrepresentation of gene segments and lack of N regions during the first trimester of gestation and then increases exponentially during ontogeny until it reaches adult levels months after birth. This process parallels, and may contribute to, the stepwise acquisition of the ability to respond to specific antigens. Recent studies indicate that maturation of the CDR3 repertoire is not accelerated by premature exposition to extrauterine antigen and thus appears to follow a strictly developmentally regulated program whose pacemaker(s) is still unknown.

Antibody production in early life supported by maternal lymphocyte factors

To examine the influence of maternal lymphocyte factors on the immune responses in offspring in early life, antibody production in neonates born to either normal or lymphocyte-deficient mothers was analyzed. Recombination activating gene (Rag)-2(+/-) mouse neonates born to Rag-2(+/+), Rag-2(+/-)or Rag-2(-/-)mothers were injected with goat anti-mouse IgD antiserum, and IgE and IgG(1) production was evaluated. The levels of IgE and IgG(1) were higher in the pups born to Rag-2(+/+)and Rag-2(+/-) dams than to lymphocyte-deficient Rag-2(-/-) dams. The enhanced antibody production in the former compared with the latter neonates was also found following immunization with ovalbumin or TNP-Ficoll. Thus, the presence of maternal lymphocyte factors was suggested in neonates that augmented antigen-specific antibody production in both T cell-dependent and -independent pathways. A reduction in antibody production was observed in normal neonates when they were foster-nursed by Rag-2(-/-) mothers. Thus, the maternal lymphocyte factors enhancing the immune responses in newborns were shown to be present in breast-milk.

Neonatal and early life vaccinology

Preclinical and human vaccine studies indicate that, although neonatal immunisation does not generally lead to rapid and strong antibody responses, it may result in an efficient immunological priming, which can serve as an excellent basis for future responses. The apparent impairment of CD4 and CD8 T-cell function in early life seems to result from suboptimal antigen-presenting cells-T cell interactions, which can be overcome by use of specific adjuvants or delivery systems. Although persistence of maternal antibodies may limit infant antibody responses, induction of T-cell responses largely remain unaffected by these passively transferred antibodies. Thus, neonatal priming and early boosting with vaccine formulations optimised for sufficient early life immunogenicity and maximal safety profiles, could allow better control of the huge infectious disease burden in early life.

Precursor-B-ALL with D(H)-J(H) gene rearrangements have an immature immunogenotype with a high frequency of oligoclonality and hyperdiploidy of chromosome 14

The IGH gene configuration was investigated in 97 childhood precursor-B-ALL patients at initial diagnosis. Rearrangements were found by Southern blotting in all but three patients (97%) and in 30 cases (31%) we observed oligoclonal IGH gene rearrangements. Heteroduplex PCR analysis revealed at least one clonal PCR product in all Southern blot-positive cases. In 89 patients (92%) complete V(D)J rearrangements were found, while incomplete D(H)-J(H) rearrangements occurred in only 21 patients (22%). In 5% of cases the D(H)-J(H) rearrangements were the sole IGH gene rearrangements. Sequence analysis of the 31 identified incomplete rearrangements revealed preferential usage of segments from the D(H)2, D(H)3 and D(H)7 families (78%). While D(H)2 and D(H)3 gene rearrangements occur frequently in normal B cells and B cell precursors, the relatively frequent usage of D(H)7-27 (19%) in precursor-B-ALL patients is suggestive of leukemic transformation during prenatal lymphopoiesis. Among J(H) gene segments in the incomplete D(H)-J(H) rearrangements, the J(H)6 segment was significantly overrepresented (61%). This observation together with the predominant usage of the most upstream D(H) genes indicates that many of the identified clonal D(H)-J(H) gene rearrangements in precursor-B-ALL probably represent secondary recombinations, having deleted pre-existing D(H)-J(H) joinings. The patients with incomplete D(H)-J(H) gene rearrangements were frequently characterized by hyperdiploid karyotype with additional copies of chromosome 14 and/or by IGH oligoclonality. The presence of incomplete D(H)-J(H) joinings was also significantly associated with a less mature immunogenotype: overrepresentation of V(H)6-1 gene segment usage, absence of biallelic TCRD deletions, and low frequency of TCRG gene rearrangements. This immature immunogenotype of precursor-B-ALL with incomplete IGH gene rearrangements was not associated with more aggressive disease.

The diversity of rearranged immunoglobulin heavy chain variable region genes in peripheral blood B cells of preterm infants is restricted by short third complementarity-determining regions but not by limited gene segment usage

The immunoglobulin diversity is restricted in fetal liver B cells. This study examined whether peripheral blood B cells of extremely preterm infants show similar restrictions (overrepresentation of some gene segments, short third complementarity-determining regions [CDR3]). DNA of rearranged immunoglobulin heavy chain genes was amplified by polymerase chain reaction, cloned, and sequenced. A total of 417 sequences were analyzed from 6 preterm infants (25-28 weeks of gestation), 6 term infants, and 6 adults. Gene segments from the entire V(H) and D(H) gene locus were rearranged in preterm infants, even though the D(H)7-27 segment was overrepresented (17% of rearrangements) compared to term infants (7%) and adults (2%). CDR3 was shorter in preterm infants (40 +/- 10 nucleotides) than in term infants (44 +/- 12) and adults (48 +/- 14) (P <.001) due to shorter N regions. Somatic mutations were exclusively found in term neonates and adults (mutational frequency 0.8% and 1.8%). We conclude that preterm infants have no limitations in gene segment usage, whereas the diversity of CDR3 is restricted throughout gestation.

Early expression of iepsilon, CD23 (FcepsilonRII), IL-4Ralpha, and IgE in the human fetus

BACKGROUND

A major predictor of childhood atopy is the concentration of IgE in the cord blood, but whether the source of cord blood IgE is maternal or fetal remains unclear.

OBJECTIVE

We sought to determine the pattern of in situ IgE production during ontogeny.

METHODS

Ninety-seven fetal, 142 natal, and 96 childhood samples were analyzed by using reverse transcription PCR for transcription of VDJCepsilon, Iepsilon, and CD23. Thirty-eight fetal liver samples were analyzed for the IL4RA genotype.

RESULTS

IL-4Ralpha, CD23a, CD23b, and sterile Iepsilon transcripts were present as early as 8 weeks' gestation. VDJCepsilon transcripts were found in second-trimester fetal liver and third-trimester cord blood, although they were rare. VDJCepsilon transcripts were more common in the blood of children 9 months and older. Sequence analysis suggested that fetal VDJCepsilon was the product of selection. All fetal livers actively transcribing Iepsilon, VDJCepsilon, and IL-4Ralpha contained at least one copy of the atopy-associated IL4RA*A1902G polymorphism.

CONCLUSION

The human fetus contains B cells that are primed to undergo IgE class switching from the earliest stages of ontogeny and can produce endogenous IgE by 20 weeks' gestation. However, IgE-producing cells are rare until 9 months after birth.

Vaccination in the neonatal period and early infancy

Immune maturation is responsible for a progressive increase in antibody responses that can be elicited during the first year of life, such that neonatal immunization may currently not be expected to induce strong antibody responses. In contrast, B and T cell priming can be induced very early in life, without interference of maternal immunity. Strong IL-5 and IL-13 responses in young mice, and limited IL-12 and IFN-gamma release capacity by early life APC and T cells both in young mice and infants, could contribute to the severity of infections with intracellular pathogens in early life. It calls for evaluation of novel delivery systems, adjuvants and/or prime-boost immunization strategies capable to meet the challenge of both strong neonatal immunogenicity and acceptable reactogenicity. The extent to which early life murine immunization models may be useful for preclinical evaluation of infant responses is outlined in this review.