Antibody repertoire development in fetal and neonatal piglets. II. Characterization of heavy chain complementarity-determining region 3 diversity in the developing fetus

A Rapid Method for Direct Quantification of Antibody Binding-Site Concentration in Serum

The quantitation of the available antibody binding-site concentration of polyclonal antibodies in serum is critical in defining the efficacy of vaccines against substances of abuse. We have conceptualized an equilibrium dialysis (ED)-based approach coupled with fluorimetry (ED-fluorimetry) to measure the antibody binding-site concentration to the ligand in an aqueous environment. The measured binding-site concentrations in monoclonal antibody (mAb) and sera samples from TT-6-AmHap-immunized rats by ED-fluorimetry are in agreement with those determined by a more established equilibrium dialysis coupled with ultraperformance liquid chromatography tandem mass spectrometry (ED-UPLC-MS/MS). Importantly, we have shown that the measured antibody binding-site concentrations to the ligand by ED-fluorimetry were not influenced by the sample serum matrix; thus, this method is valid for determining the binding-site concentration of polyclonal antibodies in sera samples. Further, we have demonstrated that under appropriate analytical conditions, this method resolved the total binding-site concentrations on a nanomolar scale with good accuracy and repeatability within the microliter sample volumes. This simple, rapid, and sample preparation-free approach has the potential to reliably perform quantitative antibody binding-site screening in serum and other more complex biological fluids.

Comparative Aspects of Immunoglobulin Gene Rearrangement Arrays in Different Species

Studies in humans and mice indicate the critical role of the surrogate light chain in the selection of the productive immunoglobulin repertoire during B cell development. However, subsequent studies using mutant mice have also demonstrated that alternative pathways are allowed. Our recent investigation has shown that some species, such as pig, physiologically use preferential rearrangement of authentic light chains, and become independent of surrogate light chains. Here we summarize the findings from swine and compare them with results in other species. In both groups, allelic and isotypic exclusions remain intact, so the different processes do not alter the paradigm of B-cell monospecificity. Both groups also retained some other essential processes, such as segregated and sequential rearrangement of heavy and light chain loci, preferential rearrangement of light chain kappa before lambda, and functional κ-deleting element recombination. On the other hand, the respective order of heavy and light chains rearrangement may vary, and rearrangement of the light chain kappa and lambda on different chromosomes may occur independently. Studies have also confirmed that the surrogate light chain is not required for the selection of the productive repertoire of heavy chains and can be substituted by authentic light chains. These findings are important for understanding evolutional approaches, redundancy and efficiency of B-cell generation, dependencies on other regulatory factors, and strategies for constructing therapeutic antibodies in unrelated species. The results may also be important for explaining interspecies differences in the proportional use of light chains and for the understanding of divergences in rearrangement processes. Therefore, the division into two groups may not be definitive and there may be more groups of intermediate species.

Consequences of the different order of immunoglobulin gene rearrangements in swine

Swine use a reverse order of immunoglobulin chain rearrangement compared to humans and mice, and this altered and modified order should have measurable consequences. Here we perform new and defining experiments with developing and mature B cells, characterizing the B cell populations that do not exist in other species. First, we have finally confirmed that light chains κ and λ are rearranged and expressed on the surface before any heavy chain rearrangements using western-blot. And second, we have analyzed a pool of mature B cells on the single-cell level to demonstrate that many κ⁺ mature B cells carry λ transcripts. According to these findings, we believe that there may be more groups of mammals; one of which uses a pre-BCR-driven developmental pathway for B cell generation (like mice and humans), the second group uses a pre-BCR-independent one (like swine), and some may be even intermediate.

High-throughput sequencing of the porcine antibody repertoire with or without PEDV infection: A proof-of-concept study

A host's adaptive immune system can protect against a wide variety of pathogens by producing diverse antibodies. The antibody repertoire is so vast that traditional low-throughput methods cannot fully sequence it. In this study, we developed a high-throughput sequencing (HTS) method for antibody repertoire assessment in swine, and tested it with or without porcine epidemic diarrhea virus (PEDV) infection. We isolated peripheral blood mononuclear cells from normal or PEDV-seropositive pigs and applied multiplex PCR to amplify the porcine B cell receptor heavy chain library, followed by HTS using the Illumina Miseq system to obtain full sequence information. The results from sequence analysis demonstrated that in normal conditions, several V gene segments were preferentially used, with IGHV1-4 and IGHV1S2 being the two most frequent. The IGHV usage in PEDV-seropositive pigs was not exactly the same as that of PEDV-seronegative pigs, with an increased usage of IGHV1-6. Our study provides an effective approach to comprehensively understand the overall porcine antibody repertoire, as well as to monitor broad antibody responses to viral challenge in pigs.

Organogenesis of Ileal Peyer's Patches Is Initiated Prenatally and Accelerated Postnatally With Comprehensive Proliferation of B Cells in Pigs

Morphogenesis and differentiation of organs is required for subsequent functional maturation. The morphological features of Peyer's patches vary among species. In pigs, they develop extensively in the ileum as ileal Peyer's patches (IPPs). However, the role of IPPs in the porcine immune system remains to be elucidated because of a lack of complete understanding of IPP organogenesis. Results of the present study revealed that development of porcine IPPs is initiated prenatally between embryonic days 76 and 91. The process of IPP organogenesis is concomitant with increased transcriptional patterns of CXCL13 and CCL19. IPPs undergo further development postnatally by forming central, marginal, and subepithelial zones. Importantly, a large number of proliferating B cells and apoptotic cells are found in porcine IPPs postnatally, but not prenatally. The expression level of IgM in proliferating B cells depends on the zone in which distinct B cells are separately localized after birth. Specifically, IgM⁺ cells are predominantly found in the central zone, whereas IgM^-/low cells are abundant in the marginal zone. Importantly, the cellular feature of IPPs differs from that of mesenteric lymph nodes (MLNs) where such distinct zones are not formed both prenatally and postnatally. Our findings suggest that IPPs (not MLNs) in postnatal pigs are involved in complementing functions of the primary lymphoid tissue that promotes the differentiation and maturation of B cells.

Sexual Dimorphism in Immune Development and in Response to Nutritional Intervention in Neonatal Piglets

Although sex disparity in immunological function and susceptibility to various inflammatory and infectious disease is recognized in adults, far less is known about the situation in young infants during immune development. We have used an outbred piglet model to explore potential early sex disparity underlying both mucosal immune development and systemic responses to novel antigen. Despite similarities in intestinal barrier function and therefore, presumably, antigen exposure, females had less CD172⁺ (Sirp-α) antigen presenting cells and expression of MHCIIDR at 28 days old compared to males, along with greater regulatory T-cell numbers. This suggests that, during infancy, females may have greater potential for local immune regulation than their male counterparts. However, females also presented with significantly greater systemic antibody responses to injected ovalbumin and dietary soya. Females also synthesized significantly more IgA in mesenteric lymph nodes, whereas males synthesized more in caecal mucosa, suggesting that plasma cells were retained within the MLN in females, but increased numbers of plasma cells circulated through to the mucosal tissue in males. Significant effects of inulin and Bifidobacterium lactis NCC2818 on the developing immune system were also sex-dependent. Our results may start to explain inconsistencies in outcomes of trials of functional foods in infants, as distinction between males and females is seldom made. Since later functionality of the immune system is highly dependent on appropriate development during infancy, stratifying nutritional interventions by sex may present a novel means of optimizing treatments and preventative strategies to reduce the risk of the development of immunological disorders in later life.

T cells in swine completely rearrange immunoglobulin heavy chain genes

Porcine thymus contains three independent populations of cells that have rearranged immunoglobulin heavy chain VDJ_H genes. The first population can be found exclusively in medulla and it consists of existing mature B cells and plasma cells. The second consists of developing B cells characterized by the presence of selected VDJ_H rearrangement, similar to B cell lymphogenesis in the bone marrow. The third population is entirely unaffected by selection mechanism for productive VDJ_H rearrangement and represents T lineage cells that rearrange immunoglobulin genes. Transcription of unselected VDJ_H repertoire is not allowed in T cells. Sequence analysis of unselected VDJ_H repertoire from T cells also revealed important consequences for B cell lymphogenesis and selection of B cell repertoire. As far as we know, this is the first evidence that some species completely rearrange VDJ_H genes in T cells. Our results also support the finding that B cells actively develop in the thymus.

Characterization and comparative analysis of immunoglobulin lambda chain diversity in a neonatal porcine model

To elucidate how antigen exposure and selection shape the porcine antibody repertoires, we investigated the immunoglobulin lambda light chain (IGL) gene repertoires of the binary cross-bred (Yorkshire×Landrace) pig at different developmental stages, pre-suckle neonate (0days), wean piglet (35days) and growing pig (75days) under normal farming conditions. Immunoglobulin lambda light transcript (IGLV-J-C) clones of the peripheral blood mononuclear cells (PBMCs) from these different developmental stages were assessed for IGL combination, junction and sequence diversity. Previous research has revealed that IGLV8 plays a major role in immunity during the early fetus stage and that IGLV3 accounts for 30% of the neonatal IGLV repertoires. Here, we found that the antibody profile exhibited salient features at different stages. The usage of the IGLV3-3 subclass gradually decreased during development, in contrast, the utilization of IGLV8 (IGLV8-10, IGLV8-13 and IGLV8-18), which started in the fetal stage, has increased in the growing stage. Moreover, the junction diversity, as measured by the IGLV hypervariable complementarity determining region 3 (CDR3L) lengths, was constant during the different stages. The complete junction mutation ratio clearly increased in the growing pig compared to that in the younger pig. Our data provide new insights into the postnatal porcine IGLV repertoires maturation which can lay the foundation for porcine antibody gene research.

Antibody Repertoire Development in Swine

We describe the domestication of the species, explore its value to agriculture and bioscience, and compare its immunoglobulin (Ig) genes to those of other vertebrates. For encyclopedic information, we cite earlier reviews and chapters. We provide current gene maps for the heavy and light chain loci and describe their polygeny and polymorphy. B-cell and antibody repertoire development is a major focus, and we present findings that challenge several mouse-centric paradigms. We focus special attention on the role of ileal Peyer's patches, the largest secondary lymphoid tissues in newborn piglets and a feature of all artiodactyls. We believe swine fetal development and early class switch evolved to provide natural secretory IgA antibodies able to prevent translocation of bacteria from the gut while the bacterial PAMPs drive development of adaptive immunity. We discuss the value of using the isolator piglet model to address these issues.

Weaning stress and gastrointestinal barrier development: Implications for lifelong gut health in pigs

The gastrointestinal (GI) barrier serves a critical role in survival and overall health of animals and humans. Several layers of barrier defense mechanisms are provided by the epithelial, immune and enteric nervous systems. Together they act in concert to control normal gut functions (e.g., digestion, absorption, secretion, immunity, etc.) whereas at the same time provide a barrier from the hostile conditions in the luminal environment. Breakdown of these critical GI functions is a central pathophysiological mechanism in the most serious GI disorders in pigs. This review will focus on the development and functional properties of the GI barrier in pigs and how common early life production stressors, such as weaning, can alter immediate and long-term barrier function and disease susceptibility. Specific stress-related pathophysiological mechanisms responsible for driving GI barrier dysfunction induced by weaning and the implications to animal health and performance will be discussed.

Progress in the use of swine in developmental immunology of B and T lymphocytes

The adaptive immune system of higher vertebrates is believed to have evolved to counter the ability of pathogens to avoid expulsion because their high rate of germline mutations. Vertebrates developed this adaptive immune response through the evolution of lymphocytes capable of somatic generation of a diverse repertoire of their antigenic receptors without the need to increase the frequency of germline mutation. The focus of our research and this article is on the ontogenetic development of the lymphocytes, and the repertoires they generate in swine. Several features are discussed including (a) the "closed" porcine placenta means that de novo fetal development can be studied for 114 days without passive influence from the mother, (b) newborn piglets are precocial permitting them to be reared without their mothers in germ-free isolators, (c) swine are members of the γδ-high group of mammals and thus provides a greater opportunity to characterize the role of γδ T cells and (d) because swine have a simplified variable heavy and light chain genome they offer a convenient system to study antibody repertoire development.

Hematopoiesis in the equine fetal liver suggests immune preparedness

We investigated how the equine fetus prepares its pre-immune humoral repertoire for an imminent exposure to pathogens in the neonatal period, particularly how the primary hematopoietic organs are equipped to support B cell hematopoiesis and immunoglobulin (Ig) diversity. We demonstrated that the liver and the bone marrow at approximately 100 days of gestation (DG) are active sites of hematopoiesis based on the expression of signature messenger RNA (mRNA) (c-KIT, CD34, IL7R, CXCL12, IRF8, PU.1, PAX5, NOTCH1, GATA1, CEBPA) and protein markers (CD34, CD19, IgM, CD3, CD4, CD5, CD8, CD11b, CD172A) of hematopoietic development and leukocyte differentiation molecules, respectively. To verify Ig diversity achieved during the production of B cells, V(D)J segments were sequenced in primary lymphoid organs of the equine fetus and adult horse, revealing that similar heavy chain VDJ segments and CDR3 lengths were most frequently used independent of life stage. In contrast, different lambda light chain segments were predominant in equine fetal compared to adult stage, and surprisingly, the fetus had less restricted use of variable gene segments to construct the lambda chain. Fetal Igs also contained elements of sequence diversity, albeit to a smaller degree than that of the adult horse. Our data suggest that the B cells produced in the liver and bone marrow of the equine fetus generate a wide repertoire of pre-immune Igs for protection, and the more diverse use of different lambda variable gene segments in fetal life may provide the neonate an opportunity to respond to a wider range of antigens at birth.

Diversity of immunoglobulin lambda light chain gene usage over developmental stages in the horse

To further studies of neonatal immune responses to pathogens and vaccination, we investigated the dynamics of B lymphocyte development and immunoglobulin (Ig) gene diversity. Previously we demonstrated that equine fetal Ig VDJ sequences exhibit combinatorial and junctional diversity levels comparable to those of adult Ig VDJ sequences. Herein, RACE clones from fetal, neonatal, foal, and adult lymphoid tissue were assessed for Ig lambda light chain combinatorial, junctional, and sequence diversity. Remarkably, more lambda variable genes (IGLV) were used during fetal life than later stages and IGLV gene usage differed significantly with time, in contrast to the Ig heavy chain. Junctional diversity measured by CDR3L length was constant over time. Comparison of Ig lambda transcripts to germline revealed significant increases in nucleotide diversity over time, even during fetal life. These results suggest that the Ig lambda light chain provides an additional dimension of diversity to the equine Ig repertoire.

The comparative profile of lymphoid cells and the T and B cell spectratype of germ-free piglets infected with viruses SIV, PRRSV or PCV2

Lymphocyte subsets isolated from germ-free piglets experimentally infected with swine influenza virus (SIV), porcine reproductive and respiratory syndrome virus (PRRSV) or porcine circovirus type 2 (PCV2) were studied and the profile of these subsets among these three infections was monitored. Germ-free piglets were used since their response could be directly correlated to the viral infection. Because SIV infections are resolved even by colostrum-deprived neonates whereas PRRSV and PCV2 infections are not, SIV was used as a benchmark for an effectively resolved viral infection. PRRSV caused a large increase in the proportion of lymphocytes at the site of infection and rapid differentiation of B cells leading to a high level of Ig-producing cells but a severe reduction in CD2^—CD21⁺ primed B cells. Unlike SIV and PCV2, PRRSV also caused an increase in terminally differentiated subset of CD2⁺CD8α⁺ γδ cells and polyclonal expansion of major Vβ families suggesting that non-specific helper T cells drive swift B cell activation. Distinct from infections with SIV and PRRSV, PCV2 infection led to the: (a) prevalence of MHC-II⁺ T cytotoxic cells, (b) restriction of the T helper compartment in the respiratory tract, (c) generation of a high proportion of FoxP3⁺ T cells in the blood and (d) selective expansion of IgA and IgE suggesting this virus elicits a mucosal immune response. Our findings suggest that PRRSV and PCV2 may negatively modulate the host immune system by different mechanisms which may explain their persistence.

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.

Development of gut immunoglobulin A production in piglet in response to innate and environmental factors

The current review focuses on pre- and post-natal development of intestinal immunoglobulin A (IgA) production in pig. IgA production is influenced by intrinsic genetic factors in the foetus as well as extrinsic environmental factors during the post-natal period. At birth, piglets are exposed to new antigens through maternal colostrums/milk as well as exogenous microbiota. This exposure to new antigens is critical for the proper development of the gut mucosal immune system and is characterized mainly by the establishment of IgA response. A second critical period for neonatal intestinal immune system development occurs at weaning time when the gut environment is exposed to new dietary antigens. Neonate needs to establish oral tolerance and in the absence of protective milk need to fight potential new pathogens. To improve knowledge about the immune response in the neonates, it is important to identify intrinsic and extrinsic factors which influence the intestinal immune system development and to elucidate their mechanism of action.

The enigma of the lower gut-associated lymphoid tissue (GALT)

Artiodactyls possess GALT that appears in fetal life and is located at the extreme end of the ileum. These IPP contain mostly B cells and involute early in postnatal life. Rabbits have a similarly located lymphoid organ, called the sacculus rotundus. Studies in sheep and rabbits have led to the concept that the lower hindgut GALT represents primary lymphoid tissue for B cells and is necessary for normal B cell development, analogous to the bursa of Fabricius. This review traces the history of the observations and theories that have led to the existing concept concerning the role of lower GALT. We then review recent data from piglets with resected IPP that challenges the concept that the IPP is primary B cell lymphoid tissue and that artiodactyls and rabbits are members of the GALT group in the same context as gallinaceous birds. Eliminating the IPP as the primary lymphoid tissue for B cells leads to the hypothesis that the IPP acts as first-responder mucosal lymphoid tissue.

Antibody repertoire development in fetal and neonatal piglets. XII. Three IGLV genes comprise 70% of the pre-immune repertoire and there is little junctional diversity

We characterized 239 lambda rearrangements from fetal and germfree (GF) piglets to: (1) determine if transcripts recovered from the earliest sites of B cell lymphogenesis were unique (2) determine what proportion of the genome is used to form the pre-immune repertoire (3) estimate the degree of somatic hypermutation and junctional diversity during ontogeny and (4) test whether piglets maintained germfree in isolators (GF piglets) have a more diversified repertoire than fetal piglets. We show that all expressed lambda genes belong to the IGLV3 and IGLV8 families and only IGLJ2 and IGLJ3 were expressed and used equally throughout fetal and neonatal life. Only genes of the IGLV8 family were used in yolk sac and fetal liver and in these tissues, IGLV8-10 comprised >50%. However, the IGLV8 genes recovered at these early sites of B cell lymphogenesis were recovered at all stages of development. Thus, no unique lambda rearrangement was recovered at the first sites of B cell development. The frequency of somatic hypermutation (SHM) in fetal piglets was ~5.9 per Kb equivalent, mutation were concentrated in CDR regions and did not increase in GF piglets. The average CDR3 length was 30 nt ± 2.7 and did not change in GF piglets. Similar to the heavy chain pre-immune repertoire in this species, three IGLV genes account for ~70% of the repertoire. Unlike the heavy chain repertoire, junctional diversity was very limited.

Marginal zone B cells: virtues of innate-like antibody-producing lymphocytes

Protective responses to microorganisms involve the nonspecific but rapid defence mechanisms of the innate immune system, followed by the specific but slow defence mechanisms of the adaptive immune system. Located as sentinels at the interface between the circulation and lymphoid tissue, splenic marginal zone B cells rapidly respond to blood-borne antigens by adopting 'crossover' defensive strategies that blur the conventional boundaries of innate and adaptive immunity. This Review discusses how marginal zone B cells function as innate-like lymphocytes that mount rapid antibody responses to both T cell-dependent and T cell-independent antigens. These responses require the integration of activation signals from germline-encoded and somatically recombined receptors for microorganisms with helper signals from effector cells of the innate and adaptive immune systems.

Dietary supplementation with Bifidobacterium lactis NCC2818 from weaning reduces local immunoglobulin production in lymphoid-associated tissues but increases systemic antibodies in healthy neonates

Weaning is associated with a major shift in the microbial community of the intestine, and this instability may make it more acquiescent than the adult microbiota to long-term changes. Modulation achieved through dietary interventions may have potentially beneficial effects on the developing immune system, which is driven primarily by the microbiota. The specific aim of the present study was to determine whether immune development could be modified by dietary supplementation with the human probiotic Bifidobacterium lactis NCC2818 in a tractable model of weaning in infants. Piglets were reared by their mothers before being weaned onto a solid diet supplemented with B. lactis NCC2818, while sibling controls did not receive supplementation. Probiotic supplementation resulted in a reduction in IgA (P<0·0005) and IgM (P<0·009) production by mucosal tissues but had no effect on IgG production (P>0·05). Probiotic-supplemented pigs had more mast cells than unsupplemented littermates (P<0·0001), although numbers in both groups were low. In addition, the supplemented piglets made stronger serum IgG responses to fed and injected antigens (P<0·05). The present findings are consistent with B. lactis NCC2818 reducing intestinal permeability induced by weaning, and suggest that the piglet is a valuable intermediate between rodent models and human infants. The results also strongly suggest that measures of the effect of probiotic supplementation on the immune system need to be interpreted carefully as proxy measures of health benefit. However, they are useful in developing an understanding of the mechanism of action of probiotic strains, an important factor in predicting favourable health outcomes of nutritional intervention.

Antibody repertoire development in fetal and neonatal piglets. XVI. Influenza stimulates adaptive immunity, class switch and diversification of the IgG repertoire encoded by downstream Cγ genes

Infection of germ-free isolator piglets with swine influenza (S-FLU) that generates dsRNA during replication causes elevation of immunoglobulins in serum and bronchoalveolar lavage, a very weak response to trinitrophenyl conjugates but an immune response to S-FLU. The increased immunoglobulin levels result mainly from the polyclonal activation of B cells during the infection, but model antigen exposure may contribute. The 10-fold increase in local and serum IgG accompanies a 10-fold decrease in the transcription of IgG3 in the tracheal-bronchial lymph nodes and in the ileal Peyer's patches. Infection results in class switch recombination to downstream Cγ genes, which diversify their repertoire; both features are diagnostic of adaptive immunity. Meanwhile the repertoires of IgM and IgG3 remain undiversified suggesting that they encode innate, natural antibodies. Whereas IgG3 may play an initial protective role, antibodies encoded by downstream Cγ genes with diversified repertoires are predicted to be most important in long-term protection against S-FLU.

Antibody repertoire development in fetal and neonatal piglets. XXII. λ Rearrangement precedes κ rearrangement during B-cell lymphogenesis in swine

VDJ and VJ rearrangements, expression of RAG-1, Tdt and VpreB, and the presence of signal joint circles (SJC) were used to identify sites of B-cell lymphogenesis. VDJ, VλJλ but not VκJκ rearrangements or SJC were recovered from yolk sac (YS) at 20 days of gestation (DG) along with strong expression of VpreB and RAG-1 but weak Tdt expression. VλJλ rearrangements but not VκJκ rearrangements were recovered from fetal liver at 30-50 DG. SJC were pronounced in bone marrow at 95 DG where VκJκ rearrangements were first recovered. The VλJλ rearrangements recovered at 20-50 DG used some of the same Vλ and Jλ segments seen in older fetuses and adult animals. Hence the textbook paradigm for the order of light-chain rearrangement does not apply to swine. Consistent with weak Tdt expression in early sites of lymphogenesis, N-region additions in VDJ rearrangements were more frequent at 95 DG. Junctional diversity in VλJλ rearrangement was limited at all stages of development. There was little evidence for B-cell lymphogenesis in the ileal Peyer's patches. The widespread recovery of VpreB transcripts in whole, non-lymphoid tissue was unexpected as was its recovery from bone marrow and peripheral blood monocytes. Based on recovery of SJC, B-cell lymphogenesis continues for at least 5 weeks postpartum.

The porcine antibody repertoire: variations on the textbook theme

The genes encoding the heavy and light chains of swine antibodies are organized in the same manner as in other eutherian mammals. There are ∼30 VH genes, two functional DH genes and one functional JH gene, 14-60 Vκ genes, 5 Jκ segments, 12-13 functional Vλ genes, and two functional Jλ genes. The heavy chain constant regions encode the same repertoire of isotypes common to other eutherian mammals. The piglet models offers advantage over rodent models since the fetal repertoire develops without maternal influences and the precocial nature of their multiple offspring allows the experimenter to control the influences of environmental and maternal factors on repertoire development postnatally. B cell lymphogenesis in swine begins in the fetal yolk sac at 20 days of gestation (DG), moves to the fetal liver at 30 DG and eventually to the bone marrow which dominates until birth (114 DG) and to at least 5 weeks postpartum. There is no evidence that the ileal Peyers patches are a site of B cell lymphogenesis or are required for B cell maintenance. Unlike rodents and humans, light chain rearrangement begins first in the lambda locus; kappa rearrangements are not seen until late gestation. Dissimilar to lab rodents and more in the direction of the rabbit, swine utilize a small number of VH genes to form >90% of their pre-immune repertoire. Diversification in response to environmental antigen does not alter this pattern and is achieved by somatic hypermutation (SHM) of the same small number of VH genes. The situation for light chains is less well studied, but certain Vκ and Jκ and Vλ and Jλ are dominant in transcripts and in contrast to rearranged heavy chains, there is little junctional diversity, less SHM, and mutations are not concentrated in CDR regions. The transcribed and secreted pre-immune antibodies of the fetus include mainly IgM, IgA, and IgG3; this last isotype may provide a type of first responder mucosal immunity. Development of functional adaptive immunity is dependent on bacterial MAMPs or MAMPs provided by viral infections, indicating the importance of innate immunity for development of adaptive immunity. The structural analysis of Ig genes of this species indicate that especially the VH and Cγ gene are the result of tandem gene duplication in the context of genomic gene conversion. Since only a few of these duplicated VH genes substantially contribute to the antibody repertoire, polygeny may be a vestige from a time before somatic processes became prominently evolved to generate the antibody repertoire. In swine we believe such duplications within the genome have very limited functional significance and their occurrence is therefore overrated.

Direct detection of antibody concentration and affinity in human serum using microscale thermophoresis

The direct quantification of both the binding affinity and absolute concentration of disease-related biomarkers in biological fluids is particularly beneficial for differential diagnosis and therapy monitoring. Here, we extend microscale thermophoresis to target immunological questions. Optically generated thermal gradients were used to deplete fluorescently marked antigens in 2- and 10-fold-diluted human serum. We devised and validated an autocompetitive strategy to independently fit the concentration and dissociation constant of autoimmune antibodies against the cardiac β1-adrenergic receptor related to dilated cardiomyopathy. As an artificial antigen, the peptide COR1 was designed to mimic the second extracellular receptor loop. Thermophoresis resolved antibody concentrations from 2 to 200 nM and measured the dissociation constant as 75 nM. The approach quantifies antibody binding in its native serum environment within microliter volumes and without any surface attachments. The simplicity of the mix and probe protocol minimizes systematic errors, making thermophoresis a promising detection method for personalized medicine.

Antibody repertoire development in fetal and neonatal piglets XXI. Usage of most VH genes remains constant during fetal and postnatal development

Usage of variable region gene segments during development of the antibody repertoire in mammals is unresolved in part because of the complexity of the locus in mice and humans and the difficulty of distinguishing intrinsic from extrinsic influences in these species. We present the first vertical studies on VH usage that spans the fetal and neonatal period using the piglet model. We tracked VH usage in DNA rearrangements and in VDJ transcripts throughout 75 days of gestation (DG) in outbred fetuses, thereafter in outbred germfree and colonized isolator piglets, isolator piglets infected with swine influenza and in conventionally reared nematode-infected adults. Seven VH genes account for >90% of the pre-immune repertoire which is the same among tissues and in both transcripts and DNA rearrangements. Statistical modeling supports the view that proportional usage of the major genes remains constant during fetal life and that postnatal usage ranking is similar to that during fetal life. Changes in usage ranking are developmental not antigen dependent. In this species exposure to environmental antigens results in diversification of the repertoire by somatic hypermutation of the same small number of VH genes that comprise the pre-immune repertoire, not by using other VH gene available in the germline. Therefore in swine a small number of VH genes shape the antibody repertoire throughout life questioning the need for extensive VH polygeny.

IgA antibody response of swine to foot-and-mouth disease virus infection and vaccination

Foot-and-mouth disease virus (FMDV) continues to be a significant economic problem worldwide. Control of the disease involves the use of killed-virus vaccines, a control measure developed decades ago. After natural infection, the primary site of replication of FMDV is the pharyngeal area, suggesting that a mucosal immune response is the most effective. Humoral immunity to killed-virus vaccination induces antibodies that can prevent the clinical disease but not local infection. Determining whether infection or vaccination stimulates IgA-mediated local immunity depends on the method of analysis. Different assays have been described to analyze the quality of antibody responses of cattle and swine to FMDV, including indirect double-antibody sandwich enzyme-linked immunosorbent assay (IDAS-ELISA) and antibody capture assay-ELISA (ACA-ELISA). We tested these assays on swine and show that vaccinated animals had FMDV-specific IgM and IgG but no IgA in either serum or saliva. After the infection, both assays detected FMDV-specific IgM, IgG, and IgA in serum. Notably, serum IgA was more readily detected using the ACA-ELISA, whereas IgA was not detected in saliva with this assay. FMDV-specific IgA antibodies were detected in saliva samples using the IDAS-ELISA. These data show that parenterally administered, killed-virus vaccine does not induce a mucosal antibody response to FMDV and illuminates limitations and appropriate applications of the two ELISAs used to measure FMDV-specific responses. Further, the presence of the IgA antivirus in serum correlates with the presence of such antibodies in saliva.

The piglet as a model for B cell and immune system development

The ability to identify factors responsible for disease in all species depends on the ability to separate those factors which are environmental from those that are intrinsic. This is particularly important for studies on the development of the adaptive immune response of neonates. Studies on laboratory rodents or primates have been ambiguous because neither the effect of environmental nor maternal factors on the newborn can be controlled in mammals that: (i) transmit potential maternal immunoregulatory factors in utero and (ii) are altricial and cannot be reared after birth without their mothers. Employing the newborn piglet model can address each of these concerns. However, it comes at the price of having first to characterize the immune system of swine and its development. This review focuses on the porcine B cell system, especially on the methods used for its characterization in fetal studies and neonatal piglets. Understanding these procedures is important in the interpretation of the data obtained. Studies on neonatal piglets have (a) provided valuable information on the development of the adaptive immune system, (b) lead to important advances in evolutionary biology, (c) aided our understanding of passive immunity and (d) provided opportunities to use swine to address specific issues in veterinary and biomedical research and immunotherapy. This review summarizes the history of the development of the piglet as a model for antibody repertoire development, thus providing a framework to guide future investigators.

Immunoglobulins, antibody repertoire and B cell development

Swine share with most placental mammals the same five antibody isotypes and same two light chain types. Loci encoding lambda, kappa and Ig heavy chains appear to be organized as they are in other mammals. Swine differ from rodents and primates, but are similar to rabbits in using a single VH family (VH3) to encode their variable heavy chain domain, but not the family used by cattle, another artiodactyl. Distinct from other hoofed mammals and rodents, Ckappa:Clambda usage resembles the 1:1 ratio seen in primates. Since IgG subclasses diversified after speciation, same name subclass homologs do not exist among swine and other mammals unless very closely related. Swine possess six putative IgG subclasses that appear to have diversified by gene duplication and exon shuffle while retaining motifs that can bind to FcgammaRs, FcRn, C1q, protein A and protein G. The epithelial chorial placenta of swine and the precosial nature of their offspring have made piglets excellent models for studies on fetal antibody repertoire development and on the postnatal role of gut colonization, maternal colostrum and neonatal infection on the development of adaptive immunity during the "critical window" of immunological development. This chapter traces the study of the humoral immune system of this species through its various eras of discovery and compiles the results in tables and figures that should be a useful reference for educators and investigators.

The ontogeny of the porcine immune system

Cellular and humoral aspects of the immune response develop sequentially in the fetus. During the ontogeny, the pluripotent stem cells emerge and differentiate into all hematopoietic lineages. Basic questions including the identification of the first lympho-hematopoietic sites, the origin of T and B lymphocytes, the development of different subpopulations of alphabeta T, gammadelta T and B lymphocytes as well as development of innate immunity and the acquisition of full immunological capacities are discussed here for swine and compared with other species. The description of related topics such as fertilization, morphogenesis, maternal-fetal-neonatal physiology and early neonatal development are also discussed.

Isolator and other neonatal piglet models in developmental immunology and identification of virulence factors

The postnatal period is a ‘critical window’, a time when innate and passive immunity protect the newborn mammal while its own adaptive immune system is developing. Neonatal piglets, especially those reared in isolators, provide valuable tools for studying immunological development during this period, since environmental factors that cause ambiguity in studies with conventional animals are controlled by the experimenter. However, these models have limited value unless the swine immune system is first characterized and the necessary immunological reagents developed. Characterization has revealed numerous features of the swine immune system that did not fit mouse paradigms but may be more generally true for most mammals. These include fetal class switch recombination that is uncoupled from somatic hypermutation, the relative importance of the molecular mechanisms used to develop the antibody repertoire, the role of gut lymphoid tissue in that process, and the limited heavy chain repertoire but diverse IgG subclass repertoire. Knowledge gained from studies of adaptive immunity in isolator-reared neonatal pigs suggests that isolator piglets can be valuable in identification of virulence factors that are often masked in studies using conventional animals.

The isolator piglet: a model for studying the development of adaptive immunity

The period from late gestation to weaning in neonatal mammals is a critical window when the adaptive immune system develops and replaces the protection temporarily provided by passive immunity and pre-adaptive antibodies. It is also when oral tolerance to dietary antigen and the distinction between commensal and pathogenic gut bacteria becomes established resulting in immune homeostasis. The reproductive biology of swine provides a unique model for distinguishing the effects of different factors on immune development during this critical period because all extrinsic factors are controlled by the experimenter. This chapter reviews this early stage of development and the usefulness of the piglet model for understanding events during this transitional stage. The review also describes the major features of the porcine immune system and the immune stimulatory and dysregulatory factors that act during this period. The value of the model to medical science in such areas as food allergy, organ transplantation, cystic fibrosis and the production of humanized antibodies for immuno-therapy is discussed.

Antibody repertoire development in fetal and neonatal piglets: XIX. Undiversified B cells with hydrophobic HCDR3s preferentially proliferate in the porcine reproductive and respiratory syndrome

Porcine respiratory and reproductive syndrome virus (PRRSV) causes an extraordinary increase in the proportion of B cells resulting in lymphoid hyperplasia, hypergammaglobulinemia, and autoimmunity in neonatal piglets. Spectratypic analysis of B cells from neonatal isolator piglets show a non-Gaussian pattern with preferential expansion of clones bearing certain H chain third complementary region (HCDR3) lengths. However, only in PRRSV-infected isolator piglets was nearly the identical spectratype observed for all lymphoid tissues. This result suggests dissemination of the same dominant B cell clones throughout the body. B cell expansion in PRRS was not associated with preferential VH gene usage or repertoire diversification and these cells appeared to bear a naive phenotype. The B cell population observed during infection comprised those with hydrophobic HCDR3s, especially sequences encoded by reading frame 3 of DHA that generates the AMVLV motif. Thus, the hydropathicity profile of B cells after infection was skewed to favor those with hydrophobic binding sites, whereas the normally dominant region of the hydropathicity profile containing neutral HCDR3s was absent. We believe that the hypergammaglobulinemia results from the products of these cells. We speculate that PRRSV infection generates a product that engages the BCR of naive B cells, displaying the AMVLV and similar motifs in HCDR3 and resulting in their T-independent proliferation without repertoire diversification.

Antibody repertoire development in fetal and neonatal piglets. XIII. Hybrid VH genes and the preimmune repertoire revisited

The expressed porcine VH genes belong to the VH3 family (clan), four of which, VHA, VHB, VHC, and VHE, alone comprise approximately 80% of the preimmune repertoire. However, so-called "hybrid" VH genes that use CDR1 of one VH gene and the CDR2 of another are frequently encountered. We studied > 3000 cloned VDJs and found that such hybrids can contribute up to 10% of the preimmune repertoire. Based on the 1) recovery of hybrid VH genes from bacterial artificial chromosome clones, 2) frequency of occurrence of certain hybrids in the preimmune repertoire, and 3) failure to recover equal numbers of reciprocal hybrids, we concluded that some chimeric genes are present in the genome and are not PCR artifacts. Two chimeric germline genes (VHZ and VHY), together with VHF and the four genes mentioned above, constitute the major VH genes and these account for > 95% of the preimmune repertoire. Diversification of the preimmune IgG and IgM repertoires after environmental exposure was mainly due to somatic hypermutation of major VH genes with no evidence of gene conversion. Somatic hypermutation was 3- to 10-fold higher in CDRs than in framework regions, most were R mutations and transversions and transitions equally contributed. Data were used to 1) develop an index to quantify the degree of VH repertoire diversification and 2) establish a library of 29 putative porcine VH genes. One-third of these genes are chimeric genes and their sequences suggest that the porcine VH genome developed by duplication and splicing from a small number of prototypic genes.

Antibody Repertoire Development in Fetal and Neonatal Piglets. XVII. IgG Subclass Transcription Revisited with Emphasis on New IgG3

Fetal piglets offer an in vivo model for determining whether Ag-independent IgG subclass transcription proceeds in a manner that differs from subclass transcription in pigs exposed to environmental Ags and TLR ligands. Our data from approximately 12,000 Cgamma clones from > 60 piglets provide the first report on the relative usage of all known porcine Cgamma genes in fetal and young pigs. Studies revealed that among the six Cgamma genes, allelic variants of IgG1 comprised 50-80% of the repertoire, and IgG2 alleles comprised < 10% in nearly all tissues. However, relative transcription of allelic variants of IgG1 randomly deviate from the 1:1 ratio expected in heterozygotes. Most surprising was the finding that IgG3 accounted for half of all Cgamma transcripts in the ileal Peyer's patches (IPPs) and mesenteric lymph nodes but on average only approximately 5% of the clones from the thymus, tonsil, spleen, peripheral blood, and bone marrow of newborns. Lymphoid tissues from late term fetuses revealed a similar expression pattern. Except for IgG3 in the IPPs and mesenteric lymph nodes, no stochastic pattern of Cgamma expression during development was seen in animals from mid-gestation through 5 mo. The age and tissue dependence of IgG3 transcription paralleled the developmental persistence of the IPP, and its near disappearance corresponds to the diversification of the preimmune VDJ repertoire in young piglets. We hypothesize that long-hinged porcine IgG3 may be important in preadaptive responses to T cell-independent Ags similar to those described for its murine namesake.

Development of the neonatal B and T cell repertoire in swine: implications for comparative and veterinary immunology

Birth in all higher vertebrates is at the center of the critical window of development in which newborns transition from dependence on innate immunity to dependence on their own adaptive immunity, with passive maternal immunity bridging this transition. Therefore we have studied immunological development through fetal and early neonatal life. In swine, B cells appear earlier in fetal development than T cells. B cell development begins in the yolk sac at the 20th day of gestation (DG20), progresses to fetal liver at DG30 and after DG45 continues in bone marrow. The first wave of developing T cells is gammadelta cells expressing a monomorphic Vdelta rearrangement. Thereafter, alphabeta T cells predominate and at birth, at least 19 TRBV subgroups are expressed, 17 of which appear highly homologous with those in humans. In contrast to the T cell repertoire and unlike humans and mice, the porcine pre-immune VH (IGHV-D-J) repertoire is highly restricted, depending primarily on CDR3 for diversity. The V-KAPPA (IGKV-J) repertoire and apparently also the V-LAMBDA (IGLV-J) repertoire, are also restricted. Diversification of the pre-immune B cell repertoire of swine and the ability to respond to both T-dependent and T-independent antigen depends on colonization of the gut after birth in which colonizing bacteria stimulate with Toll-like receptor ligands, especially bacterial DNA. This may explain the link between repertoire diversification and the anatomical location of primary lymphoid tissue like the ileal Peyers patches. Improper development of adaptive immunity can be caused by infectious agents like the porcine reproductive and respiratory syndrome virus that causes immune dysregulation resulting in immunological injury and autoimmunity.

Developmental changes in the human heavy chain CDR3

The CDR3 of the Ig H chain (CDR3(H)) is significantly different in fetal and adult repertoires. To understand the mechanisms involved in the developmental changes in the CDR3(H) of Ig H chains, sets of nonproductive V(H)DJ(H) rearrangements obtained from fetal, full-term neonates and adult single B cells were analyzed and compared with the corresponding productive repertoires. Analysis of the nonproductive repertoires was particularly informative in assessing developmental changes in the molecular mechanisms of V(H)DJ(H) recombination because these rearrangements did not encode a protein and therefore their distribution was not affected by selection. Although a number of differences were noted, the major reasons that fetal B cells expressed Ig H chains with shorter CDR3(H) were both diminished TdT activity in the DJ(H) junction and the preferential use of the short J(H) proximal D segment D7-27. The enhanced usage of D7-27 by fetal B cells appeared to relate to its position in the locus rather than its short length. The CDR3(H) progressively acquired a more adult phenotype during ontogeny. In fetal B cells, there was decreased recurrent DJ(H) rearrangements before V(H)-DJ(H) rearrangement and increased usage of junctional microhomologies both of which also converted to the adult pattern during ontogeny. Overall, these results indicate that the decreased length and complexity of the CDR3(H) of fetal B cells primarily reflect limited enzymatic modifications of the joins as well as a tendency to use proximal D and J(H) segments during DJ(H) rearrangements.

Antibody repertoire development in fetal and neonatal piglets. IX. Three pathogen-associated molecular patterns act synergistically to allow germfree piglets to respond to type 2 thymus-independent and thymus-dependent antigens

Newborn piglets maintained germfree (GF) cannot respond to either thymus-dependent (TD) or type 2 thymus-independent Ags (TI-2) unless colonized with bacteria. We show here that pathogen-associated molecular patterns (PAMPs), including muramyl dipeptide (MDP), LPS, and a B-class CpG oligonucleotide (CpG-B), can substitute for gut flora in the induction of neonatal immunoresponsiveness. These PAMPs alone or in combination had little effect on serum IgG and IgA levels, but CpG-B and CpG-B + MDP elevated total IgM levels 3- to 7-fold above that seen in colonized controls after booster immunization. Although only CpG-B could alone stimulate immunoresponsiveness, co-administration of LPS or MDP resulted in a 5-fold increase in the IgG response to both immunogens. Co-administered MDP did not promote secondary IgG responses to either Ag but instead pronounced secondary IgM responses to the epitopes of both immunogens. LPS co-administered with CpG-B may promote class switch recombination or cause differentiation of previously switched cells that become responsive after exposure to CpG-B. Primary and secondary IgG responses equally recognized the epitopes of the TI-2 and TD immunogens, whereas IgM responses favored the TI-2 epitope. Because PAMPs alone can result in Abs to 2,4,6-triitrophenyl and FLU without immunization, it suggests they alone cause differentiation of B cells of the preimmune repertoire. The finding that both bacterial PAMPs and colonization are capable of stimulating Ab responses in both immunized and nonimmunized piglets suggests that PAMPs derived from host flora may play a major role in awakening adaptive immunity in neonates.

Channel catfish immunoglobulins: repertoire and expression

The channel catfish, Ictalurus punctatus, is widely recognized as an important model for studying immune responses in ectothermic vertebrates. It is one of the few fish species for which defined viable in vitro culture systems have been established and is currently the only fish species from which a variety of functionally distinct clonal leukocyte lines are available. Moreover, there is a large basis of biochemical and molecular information on the structure and function of catfish immunoglobulins (Igs). Catfish, as other teleosts, have a tetrameric homolog of IgM as their predominant serum Ig plus a homolog of IgD. They also have genetic elements basically similar to those of mammals, which encode and regulate their expression. The catfish Ig heavy (H) chain locus is a translocon-type locus with three Igdelta genes linked to an Igmu gene or pseudogene. The catfish IgH locus is estimated to contain approximately 200 variable (V) region genes representing 13 families as well as at least three diversity (D) and 11 joining (JH) genes. The catfish has two light (L) chain isotypes, F and G, both encoded by loci organized in multiple cassettes of VL-JL-CL with the VL in the opposite transcriptional orientation. Hence, all requisite components for encoding antibodies are present in the catfish, albeit with certain variations. In the future, whether or not additional unique features of Ig function and expression will be found remains to be determined.

Antibody repertoire development in swine

Swine belong to the Order Artiodactyla and like mice and humans, express IgM, IgD, IgG, IgE and IgA antibodies but a larger number of IgG subclasses. Like rabbits and chickens, expressed V(H) genes belong to the ancestral V(H)3 family and only 5 comprise >80% of the pre-immune repertoire. Since they use primarily two D(H) segments and have a single J(H) like chickens, junctional diversity plays a relatively greater role in repertoire formation than in humans and mice. Proportional light chain usage surprisingly resembles that in humans and is therefore distinctly different from the predominant kappa chain usage (>90%) of lab rodents and predominant lambda chain usage in other ungulates (>90%). The pre-immune V(kappa) repertoire also appears restricted since >95% of V(kappa)J(kappa) rearrangements use only a few members of the IGKV2 family and only J(kappa)2. Two V(lambda) families (IGLV3 and IGLV8) are used in forming the pre-immune repertoire. Antibodies that do not utilize light chains as in camelids, or the lengthy CDR3 regions seen in cattle that use V(H)4 family genes, have not been reported in swine. B cell lymphogenesis first occurs in the yolk sac but early VDJ rearrangements differ from mice and humans in that nearly 100% are in-frame and N-region additions are already present. Swine possess ileal Peyers patches like sheep which may be important for antigen-independent B cell repertoire diversification. The presence of pro B-like cells in interlobular areas of thymus and mature B cells in the thymic medulla that have switched to especially IgA in early gestation, is so far unique among mammals. The offspring of swine are believed to receive no passive immunity in utero and are precosial. Thus, they are a useful model for studies on fetal-neonatal immunological development. The model has already shown that: (a) colonization of the gut is required for responsiveness to TD and TI-2 antigens, (b) responsiveness due to colonization depends on bacterial PAMPs and (c) some viral pathogens can interfere with the establishment of immune homeostasis in neonates. Studies on swine reinforce concerns that caution be used when paradigms arising from studies in one mammal are extrapolated to other mammals, even when similarities are predicted by taxonomy and phylogeny. Swine exemplify a situation in which evolutionary diversification of the immune system is not characteristic of an entire order or even of other related systems in the same species.

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.

Lymphocyte development in fetal piglets: Facts and surprises

The developing porcine fetus offers an excellent opportunity for the study of lymphocyte development. Studies on B cell, alphabeta T cells and gammadelta T cells in the last decade have expanded our knowledge of lymphocyte development in pigs. These studies have revealed several interesting differences between swine, mice and humans. For example, porcine peripheral lymphocytes include CD4+CD8+ alphabeta T cells and an abundance of gammadelta T cells that may even prevail over the alphabeta population. There are numerous CD2- gammadelta T cells in the blood and a large number of CD8alphaalpha-bearing cells that include NK cells, conventional gammadelta and alphabeta T cells. All porcine B lymphocytes are CD25(lo) and sIgM+ B cells may differ in the expression of CD2 antigen. Unlike mice, porcine B cells appear approximately 2 weeks before T cells and progenitors undergo VDJH rearrangement at 20th day of gestation (DG20) in the yolk sac and DG30 in the fetal liver before consummating high level lymphogenesis in the bone marrow after DG45. Early B cells show an unexpectedly high proportion of in-frame rearrangements, undergo switch recombination in thymus on DG60 and use N-region insertion from the time of the earliest VDJ rearrangement. The genomic repertoire of VH, DH and JH genes is small compared to mice and humans and swine appear to depend on junctional diversity for the majority of their repertoire. The limited VH repertoire of swine contrasts sharply with the porcine TCRbeta repertoire, which is extensive, extraordinarily conserved and nearly identical to that in humans. Therefore, swine present an example of two highly related receptor systems that have diverged in the same species.

Developmental progression of immunoglobulin heavy chain diversity in sheep

In order to assess the respective impacts of combinatorial rearrangement, junctional diversification, somatic hypermutation and gene conversion in the generation of immunoglobulin heavy chain variable regions diversity, the sequences of 42 variable regions from late fetal, newborn and young sheep were determined and compared to those of adult animals. At earlier stages of development, the use of germline diversity segments appears restricted, junctional variability is already established, and somatic hypermutations are scarce. The sequence diversity in adults is much higher, which we suggest results from a higher hymermutation activity and possibly from the use of a variety of diversity segments. Altogether, this pattern is very reminiscent of the situation observed in cattle, except for the length of the third complementarity determining regions (CDR3) which are shorter in sheep than in bovine. Unlike the chicken and rabbit systems, it seems that new rearrangements continue to occur in sheep for at least several months after birth.

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.