Antibody repertoire development in fetal and neonatal piglets. VI. B cell lymphogenesis occurs at multiple sites with differences in the frequency of in-frame rearrangements

Changes in Immune Response during Pig Gestation with a Focus on Cytokines

Pigs have the highest percentage of embryonic death not associated with specific diseases of all livestock species, at 20-45%. During gestation processes, a series of complex alterations can arise, including embryonic migration and elongation, maternal immunological recognition of pregnancy, and embryonic competition for implantation sites and subsequent nutrition requirements and development. Immune cells and cytokines act as mediators between other molecules in highly complex interactions between various cell types. However, other non-immune cells, such as trophoblast cells, are important in immune pregnancy regulation. Numerous studies have shed light on the crucial roles of several cytokines that regulate the inflammatory processes that characterize the interface between the fetus and the mother throughout normal porcine gestation, but most of these reports are limited to the implantational and peri-implantational periods. Increase in some proinflammatory cytokines have been found in other gestational periods, such as placental remodeling. Porcine immune changes during delivery have not been studied as deeply as in other species. This review details some of the immune system cells actively involved in the fetomaternal interface during porcine gestation, as well as the principal cells, cytokines, and molecules, such as antibodies, that play crucial roles in sow pregnancy, both in early and mid-to-late gestation.

The major role of junctional diversity in the horse antibody repertoire

The antibody repertoire (Rep-seq) sequencing revolutionized the diversity of antigen B cell receptor studies, allowing deep and quantitative analysis to decipher the role of adaptive immunity in health and disease. Particularly, horse (Equus caballus) polyclonal antibodies have been produced and used since the century XIX to treat and prophylaxis diphtheria, tuberculosis, tetanus, pneumonia, and, more recently, COVID-19. However, our knowledge about the horse B cell receptors repertories is minimal. We present a deep horse antibody heavy chain repertoire (IGH) characterization of non-infected horses using NGS (Next generation sequencing). This study obtained a mean of 248,169 unique IgM clones and 66,141 unique IgG clones from four domestic adult horses. Rarefaction analysis showed sequence coverage was between 52 % and 82 % in IgM and IgG isotypes. We observed that besides horses antibody can use all functional IGHV genes, around 80 % of their antibodies use only three IGHV gene segments, and around 55 % use only one IGHJ gene segment. This limited VJ diversity seems to be compensated by the junctional diversity of these antibodies. We observed that the junctional diversity in horse antibodies is widespread, present in more than 90 % of horse antibodies. Besides this, the length of this region seems to be higher in horse antibodies than in other species. N1 and N2 nucleotides addition range from 0 to 111 nucleotides. In addition, around 45 % of the antibody clones have more than ten nucleotides in both the N1 and N2 junction regions. This diversity mechanism may be one of the most important in providing variability to the equine antibody repertoire. This study provides new insights regarding horse antibody composition, diversity generation, and particularities compared to other species, such as the frequency and length of N nucleotide addition. This study also points out the urgent need to better characterize TdT in horses and other species to better understand antibody repertoire characteristics.

On the influence of the source of porcine colostrum in the development of early immune ontogeny in piglets

The effects on the ontogeny of serum cytokines and immune cells caused by feeding suckling piglets with sow/gilt colostrum and milk replacer was assessed in the present study. After farrowing, the piglets born were randomized into six groups: GG and SS (n = 10/group): piglets were kept with their dam; GS (n = 10): piglets were changed from gilts to sows; SG (n = 10): piglets were changed from sows to gilts; GMR (n = 6) and SMR (n = 8): piglets from either gilts or sows were isolated from the dams and were bottle-fed ad libitum with commercial formula milk replacer. The piglets remained in the groups during the first 24 h of life and were later returned to their respective mothers. Serum immunoglobulin concentration and lymphocyte proliferation from the blood, spleen, thymus, and mesenteric lymph node of the piglets were assessed at 24 h and at 28 days of age. Serum cytokine concentrations were measured through a cytokine multiplex assay at 24 h. Overall, piglets suckling on sows (SS and GS) had a higher concentration of serum immunoglobulin at 24 h, which was also associated with a rise in plasma cytokine concentration and greater ability of B and T cells from lymphatic organs and blood mononuclear cells to respond to mitogens. We suggest a bias towards Th1-, Th2-, and Th17-cell polarizing and cytokines during the suckling period, which may be influenced by maternal immunological factors in the colostrum, such as dam parity. All findings suggest sow parity having a possible role, which may contribute to exerting a modulating action on immune response development.

The order of immunoglobulin light chain κ and λ usage in primary and secondary lymphoid tissues of germ-free and conventional piglets

In pigs (Sus scrofa), the initial immunoglobulin rearrangement of the κ light chain is replaced by λ before the heavy chains rearrange, and the light chains may rearrange even later. This study investigates whether these developmental differences are reflected in the usage of IGK and IGL genes. We found large differences between peripheral B cells and those developing in the bone marrow, and between B cells in germ-free piglets and conventional pigs. During early B cell development in the bone marrow, more 3' V and 5' J gene segments for both light chains are used. However, in the peripheral naive repertoire, more 5' IGLV and 3' IGLJ genes are used. A similar shift toward the use of more 5' IGKV and 3' IGKJ genes is observed later after antigen exposure in conventional pigs. The expression profile showed that most λ⁺ B cells are generated earlier, while κ⁺ B cells develop from late precursors that already contain the λ rearrangement. The initial λ rearrangement is retained in both λ⁺ and κ⁺ B lymphocytes, and multiple λ transcripts can be found in individual cells. The overall pool of the IGLV repertoire is therefore much larger and more diversified than for IGKV. The κ repertoire is further restricted to the preferential use of only two major IGKV genes, reflecting the limitation for only two consecutive rearrangements. Tracing of silenced λ transcripts in κ⁺ B cells further confirmed the unconventional mechanism of differential rearrangements in pigs. Our results underline the diversity of the immune system among mammals.

Impact of maternally derived immunity on immune responses elicited by piglet early vaccination against the most common pathogens involved in porcine respiratory disease complex

Background

Newborn piglets can trigger an elementary immune response, but the acquirement of specific antibodies and/or cellular immunity against pathogens before they get infected post-natally is paramount to preserve their health. This is especially important for the pathogens involved in porcine respiratory disease complex (PRDC) as they are widespread, fairly resistant at environment, and genetically variable; moreover, some of them can cause intrauterine/early life infections.

Main body

Piglet protection can be achieved by either passive transfer of maternal derived immunity (MDI) and/or actively through vaccination. However, vaccinating piglets in the presence of remaining MDI might interfere with vaccine efficacy. Hence, the purpose of this work is to critically review the putative interference that MDI may exert on vaccine efficacy against PRDC pathogens. This knowledge is crucial to design a proper vaccination schedule.

Conclusion

MDI transferred from sows to offspring could potentially interfere with the development of an active humoral immune response. However, no conclusive interference has been shown regarding performance parameters based on the existing published literature.

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.

Optimal vaccination strategy against Mycoplasma hyopneumoniae, porcine reproductive and respiratory syndrome virus, and porcine circovirus type 2 in case of early M. hyopneumoniae infection

BACKGROUND

The aim of this study was to determine the optimal vaccination strategies for the control of porcine respiratory disease complex (PRDC) caused by Mycoplasma hyopneumoniae, porcine reproductive and respiratory syndrome virus (PRRSV), and porcine circovirus type 2 (PCV2) in case of early mycoplasmal infection.

METHODS

A total of 120 pigs were randomly divided into 6 groups (20 pigs per group). Four separate vaccine regimen groups were selected. Pigs from the four vaccinated groups were challenged with M. hyopneumoniae at 28 days old followed by a challenge of PRRSV or PCV2 at 49 days old.

RESULTS

Regardless of PRRSV or PCV2 vaccination, pigs vaccinated with one of the M. hyopneumoniae vaccines at 7 days old had a significantly better growth performance over the whole length of the study compared to pigs vaccinated with a second M. hyopneumoniae vaccine at 21 days old. Vaccination of pigs with M. hyopneumoniae at 7 days and PRRSV at either 7, 14 or 21 days old resulted in significantly reduced PRRSV viremia and lung lesions compared to vaccination of pigs with M. hyopneumoniae and PRRSV at 21 days old.

CONCLUSIONS

The efficacy of the PRRSV MLV vaccine is influenced by the different timing of M. hyopneumoniae vaccination whereas the efficacy of the PCV2 vaccine is not. This experiment study demonstrated that early vaccination with a M. hyopneumoniae vaccine should be the highest priority in order to control M. hyopneumoniae and PRRSV infection in cases of early M. hyopneumoniae infection.

Porcine Colostrum Protects the IPEC-J2 Cells and Piglet Colon Epithelium against Clostridioides (syn. Clostridium) difficile Toxin-Induced Effects

Clostridioides difficile toxins are one of the main causative agents for the clinical symptoms observed during C. difficile infection in piglets. Porcine milk has been shown to strengthen the epithelial barrier function in the piglet’s intestine and may have the potential to neutralise clostridial toxins. We hypothesised that porcine colostrum exerts protective effects against those toxins in the IPEC-J2 cells and in the colon epithelium of healthy piglets. The IPEC-J2 cells were treated with either the toxins or porcine colostrum or their combination. Analyses included measurement of trans-epithelial electrical resistance (TEER), cell viability using propidium iodide by flow cytometry, gene expression of tight junction (TJ) proteins and immune markers, immunofluorescence (IF) histology of the cytoskeleton and a TJ protein assessment. Colon tissue explants from one- and two-week-old suckling piglets and from five-week-old weaned piglets were treated with C. difficile toxins in Ussing chamber assays to assess the permeability to macromolecules (FITC-dextran, HRP), followed by analysis of gene expression of TJ proteins and immune markers. Toxins decreased viability and integrity of IPEC-J2 cells in a time-dependent manner. Porcine colostrum exerted a protective effect against toxins as indicated by TEER and IF in IPEC-J2 cells. Toxins tended to increase paracellular permeability to macromolecules in colon tissues of two-week-old piglets and downregulated gene expression of occludin in colon tissues of five-week-old piglets (p = 0.05). Porcine milk including colostrum, besides other maternal factors, may be one of the important determinants of early immune programming towards protection from C. difficile infections in the offspring.

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.

Porcine reproductive and respiratory syndrome virus infection of bone marrow: Lesions and pathogenesis

Red bone marrow is physiologically unique in that it is both the major hematopoietic organ and a primary lymphoid organ. Porcine reproductive and respiratory syndrome virus (PRRSV) affects normal bone marrow functions. The cumulative effect of PRRSV infection is acute bone marrow failure, i.e., hypoplasia characterized by the absence of normal myeloid and erythroid precursors and increased red bone marrow M:E ratios. The measurable clinical consequence of PRRSV infection on normal red bone marrow functions is a reduction in the number of cells emigrating to the peripheral blood resulting in leucopenia, anemia, and thrombocytopenia. These observations may be explained by the fact that bone marrow-derived mononuclear cells, i.e., imDCs, mDCs, monocytes, macrophages, and myeloid precursor cells are susceptible to PRRSV. Apoptosis in bone marrow-derived cells occurs both as a direct consequence of infection and indirectly via a bystander effect. Immunologically, PRRSV-susceptible mononuclear cells are the first line of defense against microbial infection and responsible for antigen recognition, processing, and presentation to T and B cells; a critical step in the initiation and development of an effective adaptive immune. Thus, impairment of normal immune function renders the host less able to resist and/or eliminate secondary infectious agents and partially explains the synergy between PRRSV and bacterial and viral co-infections.

Congenital tremor in piglets: Is bovine viral diarrhea virus an etiological cause?

Congenital tremor in pigs involves several etiologies, including pestivirus, which may cause neurological injuries in different animal species. To evaluate whether bovine viral diarrhea virus (BVDV), an important pestivirus, is one of the etiological agents of congenital tremor in swine, gilts and the fetuses were challenged at 45 days of gestation with BVDV-2. Four pregnant gilts were inoculated oronasally, four gilts underwent fetal intrauterine inoculation, and two gilts constituted the control group. Antibody titers were determined by virus neutralization (VN), and viral RNA was detected by RT-PCR. Blood samples were collected from all gilts and piglets born to obtain whole blood and serum for analysis. One third of the neonates were euthanized at three days old, and samples of the encephalon, brain stem and spinal cord were collected for anatomopathological evaluation and viral RNA detection. The piglets that remained alive were clinically evaluated every day, and blood sampling was performed regularly for 35 days. The piglets from gilts in both inoculation treatment groups showed no clinical neurological signs and were born with no viral RNA in their blood and organs. Piglets born from oronasally inoculated gilts did not present antibodies against BVDV-2 at birth, although they were acquired by passive maternal transfer. In contrast, intrauterine-inoculated piglets were born with high antibody titers (80 to 640) against the agent, which remained high until the end of the experimental period. Microscopically, no noticeable changes were observed. Macroscopically, 29.5% of the total piglets euthanized, from both inoculation groups, were born with a low cerebellar:brain ratio. Nevertheless, some piglets had a high cerebellar:brain ratio, indicating the need for standardizing this value. Thus, it was concluded that BVDV is not an etiological agent for congenital swine tremor.

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.

Identification of an Atypical Enzootic Bovine Leukosis in Japan by Using a Novel Classification of Bovine Leukemia Based on Immunophenotypic Analysis

Bovine leukemia is classified into two types: enzootic bovine leukosis (EBL) and sporadic bovine leukosis (SBL). EBL is caused by infection with bovine leukemia virus (BLV), which induces persistent lymphocytosis and B-cell lymphoma in cattle after a long latent period. Although it has been demonstrated that BLV-associated lymphoma occurs predominantly in adult cattle of >3 to 5 years, suspicious cases of EBL onset in juvenile cattle were recently reported in Japan. To investigate the current status of bovine leukemia in Japan, we performed immunophenotypic analysis of samples from 50 cattle that were clinically diagnosed as having bovine leukemia. We classified the samples into five groups on the basis of the analysis and found two different types of EBL: classic EBL (cEBL), which has the familiar phenotype commonly known as EBL, and polyclonal EBL (pEBL), which exhibited neoplastic proliferation of polyclonal B cells. Moreover, there were several atypical EBL cases even in cEBL, including an early onset of EBL in juvenile cattle. A comparison of the cell marker expressions among cEBL, pEBL, and B-cell-type SBL (B-SBL) revealed characteristic patterns in B-cell leukemia, and these patterns could be clearly differentiated from those of healthy phenotypes, whereas it was difficult to discriminate between cEBL, pEBL, and B-SBL only by the expression patterns of cell markers. This study identified novel characteristics of bovine leukemia that should contribute to a better understanding of the mechanism underlying tumor development in BLV infection.

Antibody repertoire development in fetal and neonatal piglets. XXIV. Hypothesis: The ileal Peyer patches (IPP) are the major source of primary, undiversified IgA antibodies in newborn piglets

The ileal Peyers patches (IPP) of newborn germfree (GF) piglets were isolated into blind loops and the piglets colonized with a defined probiotic microflora. After 5 weeks, IgA levels in the intestinal lavage (IL) of loop piglets remained at GF levels and IgM comprised ∼70% while in controls, IgA levels were elevated 5-fold and comprised ∼70% of total Igs. Loop piglets also had reduced serum IgA levels suggesting the source of serum IgA had been interrupted. The isotype profile for loop contents was intermediate between that in the IL of GF and probiotic controls. Surprisingly, colonization alone did not result in repertoire diversification in the IPP. Rather, colonization promoted pronounced proliferation of fully switched IgA(+)IgM(-) B cells in the IPP that supply early, non-diversified "natural" SIgA antibodies to the gut lumen and a primary IgA response in serum.

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.

Postnatal xenogeneic B-cell tolerance in swine following in utero intraportal antigen exposure

BACKGROUND

The objective of this study was to investigate the humoral immune response to xenogeneic antigens administered during the fetal state utilizing a baboon-to-pig model.

METHODS

Nine fetuses from an alpha-1,3-galactosyltransferase gene knockout (GalT-KO) MGH-miniature swine sow underwent transuterine ultrasound-guided intraportal injection of T-cell depleted baboon bone marrow (B-BM) at mid-gestation. Two juvenile GalT-KO swine undergoing direct B-BM intraportal injection were used as controls.

RESULTS

Postnatal humoral tolerance was induced in the long-term surviving piglets as demonstrated by the absence of any antibody response to baboon donor cells. In addition, a second intraportal B-BM administration at 2.5 months post-birth led to no antibody formation despite re-exposure to xenogeneic antigens. This B-cell unresponsiveness was abrogated only when the animal was exposed subcutaneously to third-party xenogeneic and allogeneic antigens, suggesting that the previously achieved humoral non-responsiveness was donor specific. In comparison, the two juvenile GalT-KO control swine demonstrated increasing anti-baboon IgM and IgG levels following intraportal injection.

CONCLUSIONS

In summary, xenogeneic B-cell tolerance was induced through in utero intraportal exposure to donor cells and this tolerance persisted following postnatal rechallenge with donor B-BM, but was lost on exposure to third-party antigen, possibly as a result of cross-reactive antibody formation.

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.

Unusual outcome of in utero infection and subsequent postnatal super-infection with different PCV2b strains

VC2002, isolated from postweaning multisystemic wasting syndrome (PMWS)-affected pig, is a mixture of two porcine circovirus genotype 2b (PCV2b) viruses, K2 and K39. Preliminary experiments disclosed short-term adverse effects of K39, but not K2, on porcine foetuses. These findings led to the hypothesis that infection of immuno-incompetent foetuses with K2 confers a status of immunotolerance, and postnatal super-infection with K39 triggers PMWS. To explore this hypothesis, nine 55-day-old foetuses were inoculated in utero (three with K2-10(4.3)TCID50, three with K39-10(4.3)TCID50 and three with medium), and foeto-pathogenicity examined. At 21 days post-inoculation (dpi), K2 did not induce pathology, whereas pathological effects of K39 were evident. Twenty-four 45-day-old foetuses were subsequently inoculated to examine the long-term effect of K2, including six with K2-high dose-10(4.3)TCID50, six with K2-low dose-10(2.3)TCID50 and 12 mock-inoculated controls. Both doses resulted in five mummified foetuses and one live-born piglet each (69dpi). K2 was recovered from all mummies. K2 and K2-specific antibodies were not detected in serum of the two live-born piglets at birth, indicating full control of K2 infection. The K2-low dose-infected piglet was immunostimulated at day 2, but not the K2-high dose-infected piglet. Both non-stimulated and stimulated K2-infected piglets were super-inoculated with K39 at day 6 or 8 (taken as 0 days post super-inoculation). Low viral replication was observed in the non-stimulated K2-K39 piglet (up to 10(3.3)TCID50/g; identified as K39). In contrast, viral replication was extremely high in the stimulated K2-K39 piglet (up to 10(5.6)TCID50/g) and identified as K2, indicating that K2 infection is controlled during foetal life, but emerges after birth upon immunostimulation. However, none of the piglets showed any signs of PMWS.

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.

Developmental progression of equine immunoglobulin heavy chain variable region diversity

Humoral immunity is a critical component of the immune system that is established during fetal life and expands upon exposure to pathogens. The extensive humoral immune response repertoire is generated in large part via immunoglobulin (Ig) heavy chain variable region diversity. The horse is a useful model to study the development of humoral diversity because the placenta does not transfer maternal antibodies; therefore, Igs detected in the fetus and pre-suckle neonate were generated in utero. The goal of this study was to compare the equine fetal Ig VDJ repertoire to that of neonatal, foal, and adult horse stages of life. We found similar profiles of IGHV, IGHD, and IGHJ gene usage throughout life, including predominant usage of IGHV2S3, IGHD18S1, and IGHJ1S5. CDR3H lengths were also comparable throughout life. Unexpectedly, Ig sequence diversity significantly increased between the fetal and neonatal age, and, as expected, between the foal and adult age.

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.

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.

The expression of CD25, CD11b, SWC1, SWC7, MHC-II, and family of CD45 molecules can be used to characterize different stages of γδ T lymphocytes in pigs

The expression of selected molecules was chosen to study porcine γδ lymphocytes and their CD2/CD8 subsets in different lymphoid organs in vivo and in vitro. Results indicate that many γδ T cells can constitutively express CD25 and MHC-II and that the frequency of γδ T cells positive for CD25, CD11b, SWC1 and SWC7 can be increased by stimulation. A diversified TCRδ repertoire was found inside CD25(+), CD11b(+), SWC1(-) and CD45RA(-) cells. Ontogenetic studies revealed various age and/or colonization dependency for expression of all studied molecules except of SWC7. Findings generally indicate that CD25 represent an activation molecule that probably marks a functionally distinct subsets, expression of CD11b is perhaps connected to early functions of naive γδ T cells in the periphery, SWC1 is lineage specific marker, SWC7 may represent an activation molecule with intrinsic or transient expression, and the expression of CD45RA/RC most likely defines naive and terminally differentiated cells.

Antibody repertoire development in fetal and neonatal piglets. XXIII: fetal piglets infected with a vaccine strain of PRRS Virus display the same immune dysregulation seen in isolator piglets

The Ig levels and antibody repertoire diversification in fetal piglets infected with an attenuated Porcine Reproductive and Respiratory Syndrome Virus (PRRSV) were measured. Serum Ig levels were greatly elevated in PRRSV-infected fetuses; IgG was elevated >50-fold, IgM>5-15-fold and IgA>2-fold compared to control fetuses. Their IgM to IgG to IgA profile was the same as that in isolator piglets infected for the same period with wild-type PRRSV. Fetal animals showed less repertoire diversification than even isolator piglets that were maintained germfree (GF) while the repertoire diversification index (RDI) for PRRSV-infected isolator piglets was 10-fold higher and comparable to littermates infected with swine influenza (S-FLU). However, when expressed as the RDI:Ig ratio, infected fetuses appeared 10-fold less capable of repertoire diversification than uninfected littermates and GF isolator piglets. Compared to S-FLU isolator piglets that resolve the infection, the RDI:Ig of PRRSV-infected isolator piglets was 100-fold lower. Overall, infection of fetuses with an attenuated virus shows the same immune dysregulation seen postnatally in wild type infected isolator piglets, indicating that: (a) attenuation did not alter the ability of the virus to cause dysregulation and (b) the isolator infectious model reflects the fetal disease.

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.

Ileal Peyer's patches are not necessary for systemic B cell development and maintenance and do not contribute significantly to the overall B cell pool in swine

Based on studies of sheep, ileal Peyer's patches (IPP) have been regarded as a type of primary lymphoid tissue similar to the bursa of Fabricius in chicken. Because bursectomy results in B cell deficiency, we wondered whether resection of the IPP of piglets would have a similar effect. Comparison of IPP-resected, surgical shams and untreated germ-free piglets, all of which were later colonized with a defined commensal flora, demonstrated that resection of the IPP did not alter the level and phenotype of B and T cells in lymphoid tissues and the blood 10 wk after surgery. Additionally, colonization of IPP caused a shift from the fetal type of lymphocyte distribution to the adult type that is characterized by prevalence of B cells, with many of them representing IgA(+) switched B cells or displaying a more mature CD2(-)CD21(+) and CD2(-)CD21(-) phenotype. Moreover, colonization leads to appearance of effector CD4(+)CD8(+) αβ T helper and CD2(+)CD8(-) γδ T cells. Comparison of germ-free with colonized pigs and experiments utilizing surgical transposition of jejunal Peyer's patch into terminal ileum or construction of isolated ileal loops indicated that lymphocyte development in IPP is dependent on colonization. Although our studies confirmed higher mitotic and apoptotic rates in IPP, they failed to identify any cell populations that resemble developing B lineage cells in the bone marrow. These results indicate that porcine IPP are not required for systemic B cell generation or maintenance, but they are secondary lymphoid tissue that appears important in immune responses to colonizing bacteria.

Studies on porcine circovirus type 2 vaccination of 5-day-old piglets

Porcine circovirus type 2 (PCV2) vaccines have become widely used since they became available in 2006. It is not uncommon for producers to use PCV2 vaccines in pigs younger than what is approved by manufacturers. The objective of this study was to determine the efficacy of a chimeric and a subunit PCV2 vaccine administered at 5 or 21 days of age. Forty-eight PCV2-naïve piglets were randomly divided into six groups of eight pigs each. Vaccination was done at day 5 or day 21, followed by triple challenge with PCV2, porcine parvovirus (PPV), and porcine reproductive and respiratory syndrome virus (PRRSV) at day 49. Vaccinated pigs seroconverted to PCV2 approximately 14 days postvaccination and had a detectable neutralizing antibody response by 21 days postvaccination regardless of age at vaccination. At day 49, the pigs vaccinated with the chimeric vaccine had significantly higher levels of neutralizing antibodies than the pigs vaccinated with the subunit vaccine. After challenge, vaccinated pigs had significantly decreased levels of PCV2 viremia and a decreased prevalence and severity of microscopic lesions compared to the positive-control group, which had severe lymphoid lesions associated with abundant PCV2 antigen, compatible with PCV-associated disease. The results of this study indicate that, under the conditions of this study, vaccination of PCV2-naïve pigs at day 5 or day 21 resulted in development of a detectable humoral immune response and provided reduction or complete protection against PCV2 viremia and PCV2-associated lesions after triple challenge with PCV2, PPV, and PRRSV.

Generation of antibody- and B cell-deficient pigs by targeted disruption of the J-region gene segment of the heavy chain locus

A poly(A)-trap gene targeting strategy was used to disrupt the single functional heavy chain (HC) joining region (J_H) of swine in primary fibroblasts. Genetically modified piglets were then generated via somatic cell nuclear transfer (SCNT) and bred to yield litters comprising J_H wild-type littermate (+/+), J_H heterozygous knockout (±) and J_H homozygous knockout (−/−) piglets in the expected Mendelian ratio of 1:2:1. There are only two other targeted loci previously published in swine, and this is the first successful poly(A)-trap strategy ever published in a livestock species. In either blood or secondary lymphoid tissues, flow cytometry, RT-PCR and ELISA detected no circulating IgM⁺ B cells, and no transcription or secretion of immunoglobulin (Ig) isotypes, respectively in J_H −/− pigs. Histochemical and immunohistochemical (IHC) studies failed to detect lymph node (LN) follicles or CD79α⁺ B cells, respectively in J_H −/− pigs. T cell receptor (TCR)_β transcription and T cells were detected in J_H −/− pigs. When reared conventionally, J_H −/− pigs succumbed to bacterial infections after weaning. These antibody (Ab)- and B cell-deficient pigs have significant value as models for both veterinary and human research to discriminate cellular and humoral protective immunity to infectious agents. Thus, these pigs may aid in vaccine development for infectious agents such as the pandemic porcine reproductive and respiratory syndrome virus (PRRSV) and H1N1 swine flu. These pigs are also a first significant step towards generating a pig that expresses fully human, antigen-specific polyclonal Ab to target numerous incurable infectious diseases with high unmet clinical need.

Expression of essential B cell genes and immunoglobulin isotypes suggests active development and gene recombination during equine gestation

Many features of the equine immune system develop during fetal life, yet the naïve or immature immune state of the neonate renders the foal uniquely susceptible to particular pathogens. RT-PCR and immunohistochemical experiments investigated the progressive expression of developmental B cell markers and immunoglobulins in lymphoid tissues from equine fetus, pre-suckle neonate, foal, and adult horses. Serum IgM, IgG isotype, and IgA concentrations were also quantified in pre-suckle foals and adult horses. The expression of essential B cell genes suggests active development and gene recombination during equine gestation, including immunoglobulin isotype switching. The corresponding production of IgM and IgG proteins is detectable in a limited scale at birth. Although the equine neonate humoral response seems competent, B cell activation factors derived from antigen presenting cells and T cells may control critical developmental regulation and immunoglobulin production during the initial months of life.

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.

Two Groups of Porcine TCRγδ+Thymocytes Behave and Diverge Differently

Developmental pathways of gammadelta T cells are still unknown, largely because of the absence of recognized lineage-specific surface markers other than the TCR. We have shown that porcine gammadelta thymocytes can be divided into 12 subsets of the following two major groups: 1) CD4(-) gammadelta thymocytes that can be further subdivided according to their CD2/CD8alphaalpha phenotype, and 2) CD4(+) gammadelta thymocytes that are always CD1(+)CD2(+)CD8alphabeta(+) and have no counterpart in the periphery. In this study, we have analyzed gammadelta thymocyte subsets with respect to behavior during cultivation, cell cycle status, and lymphocyte-specific transcripts. The group of CD4(-) gammadelta thymocytes gives rise to all gammadelta T cells found in the periphery. Proliferating CD2(+)CD8(-)CD1(+)CD45RC(-) gammadelta thymocytes are a common precursor of this group. These precursors differentiate into CD2(+)CD8alphaalpha(+), CD2(+)CD8(-), and CD2(-)CD8(-) gammadelta T cell subsets, which subsequently mature by loss of CD1 and by eventual gain of CD45RC expression. In contrast, the group of CD4(+) gammadelta thymocytes represents transient and independent subsets that are never exported from thymus as TCRgammadelta(+) T cells. In accordance with the following findings, we propose that CD4(+)CD8alphabeta(+) gammadelta thymocytes extinguish their TCRgammadelta expression and differentiate along the alphabeta T cell lineage program: 1) CD4(+) gammadelta thymocytes are actively dividing; 2) CD4(+) gammadelta thymocytes do not die, although their numbers decreased with prolonged cultivation; 3) CD4(+) gammadelta thymocytes express transcripts for RAG-1, TdT, and TCRbeta; and 4) CD4(+) gammadelta thymocytes are able to alter their phenotype to TCRalphabeta(+) thymocytes under appropriate culture conditions.

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.

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.

Development of gammadelta thymocyte subsets during prenatal and postnatal ontogeny

In this report, we describe 12 subpopulations of porcine gammadelta thymocytes based on their expression of CD1, CD2, CD4, CD8- isoforms and CD45RC. Our data suggest that gammadelta thymocytes can be divided into two major families: (a) one large family of CD4-gammadelta thymocytes that could be further subdivided according to the CD2/CD8alphaalpha phenotype and (b) a small family of CD4+ gammadelta thymocytes bearing CD8alphabeta and possessing certain unusual features in comparison with other gammadelta thymocytes. Maturation of gammadelta thymocytes within the CD4- family begins with proliferation of the CD2+ CD8- CD1+ CD45RC- gammadelta common precursor. This developmental stage is followed by diversification into the CD2+ CD8alphaalpha+, CD2+ CD8- and CD2- CD8- subsets. Their further maturation is accompanied by a loss of expression of CD1 and by increased expression of CD45RC. Therefore, individual subsets develop from CD1+ CD45RC- through CD1- CD45RC- into CD1- CD45RC+ cells. On the other hand, gammadelta thymocytes within the CD4+ family bear exclusively CD8alphabeta, always express CD1, but may coexpress CD45RC. These cells have no counterpart in the periphery. Our observations suggest that all peripheral CD8+ gammadelta T cells express CD8alphaalpha and that two subsets of these cells differing in major histocompatibility complex II expression, occur. We propose that one subset acquires CD8alphaalpha in the thymus while the second acquires CD8alphaalpha as a result of stimulation in the periphery.

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.

Antibody repertoire development in fetal and neonatal piglets. XI. The thymic B-cell repertoire develops independently from that in blood and mesenteric lymph nodes

The origin and function of thymic B cells is currently unresolved. In the present study we compared V(H) gene repertoire diversification in >3500 cloned VDJs (from 11 animals at three time-points, using three to five animals per time-point) that were expressed with immunoglobulin (Ig)M, IgD, IgG, IgA and IgE in thymus, mesenteric lymph nodes (MLN) and peripheral blood B cells (PBB) of newborn piglets and 5-week-old isolator piglets maintained germfree (GF) or colonized with Escherichia coli. The results showed that the repertoire expressed with IgM, IgD, IgG and IgA in PBB and MLN remained polyclonal, undiversified and unselected in piglets maintained GF for 5 weeks, that age and colonization resulted in significant repertoire diversification of IgG and IgA in the MLN and of IgG in blood, that the thymic B-cell repertoire was polyclonal, unaffected by colonization and showed no clonal selection in any isotype, and that the thymic IgA and IgE repertoires were more diverse at birth than the repertoire of any isotype in MLN or PBB. IgD was seldom recovered from the PBB of newborn piglets or at any time-point in thymus, but was recovered in the MLN of all 11 animals examined. The IgD and IgM repertoires in all tissues remained polyclonal and unselected, although V(H) usage by IgD transcripts did not always parallel that of IgM in the same tissue. Therefore, isotype-switched B cells in the thymic medulla cannot be accounted for by immigration of B cells diversified by colonization of the gut, and thymic B cells undergo switch recombination and repertoire diversification before birth without clonal selection.

Antibody repertoire development in fetal and neonatal pigs. VII. Characterization of the preimmune kappa light chain repertoire

Combinatorial diversity is highly restricted in the preimmune porcine H chain repertoire compared with that in humans and mice. This raised the question of whether similar restriction characterized the preimmune L chain repertoire. In this study we present evidence that >90% of all expressed Vkappa genes in the porcine preimmune repertoire belong to three subfamilies of Vkappa genes that share 87% sequence similarity with human IGKV2. This porcine Vkappa family also shares sequence similarity with some, but not all, Vkappa genes from sheep. Hybridization with sperm DNA and sequence analyses of polynucleotides from overlapping bacterial artificial chromosome clones suggest swine possess approximately 60 IGVK2 genes. The latter method also revealed that certain IGKV2 subfamilies are not expressed in the preimmune repertoire. Six members of an IGVK1 family were also expressed as part of the preimmune repertoire, and these shared 87% sequence similarity with human IGVK1. Five Jkappa segments, complete with recombination signal sequences and separated by approximately 300 nt, were identified approximately 3 kb upstream of a single Ckappa. Surprisingly, Jkappa2 accounted for >90% of all framework region 4 sequences in the preimmune repertoire. These findings show that swine use approximately 10 IGVK2 genes from three of six subfamilies and preferentially one Jkappa segment to generate their preimmune kappa repertoire. These studies, like those of porcine Ig constant regions and MHC genes, also indicate unexpected high sequence similarity with their human counterparts despite differences in phylogeny and the mechanism of repertoire diversification.