Antibody Repertoire Development in Fetal And Neonatal Piglets. IV. Switch Recombination, Primarily in Fetal Thymus, Occurs Independent of Environmental Antigen and Is Only Weakly Associated with Repertoire Diversification

Abstract

The epitheliochorial placenta of swine is considered a barrier to Ag and selective transport of IgG, so this species should be an excellent model with which to determine whether switch recombination is Ag dependent. Analysis of Ig levels and Ig isotype profiles in >150 normal and virus-infected fetuses from 38–110 days of gestation (DG) suggested that IgG, IgA, and IgM were most likely the result of de novo fetal synthesis. Although transcripts for IgM could be recovered at DG 50 (114 DG is full gestation) in all major fetal lymphoid tissues, those for IgG and IgA first became prominent at 60 DG in thymus, and transcription and spontaneous secretion became especially pronounced in this organ in older fetuses. Data on transcription, secretion, and serum isotype profiles suggest that although all fetal IgA and IgM may result from de novo synthesis, some IgG may result from low-level selective transport. The complementarity-determining region 3 spectratypes of thymic IgA and IgG transcripts at 70 and 90 days, respectively, were as polyclonal as that of IgM, indicating a broad repertoire of switched B cells although the VDJs transcribed with these switched isotypes in normal fetuses were not diversified in comparison to those from animals exposed to environmental Ags such as age-matched, virus-infected fetuses, colonized isolator piglets, and conventional adults. However, VDJs expressed with switched isotypes were more diversified than those expressed with IgM. Thus, switch recombination in fetal life does not appear to be driven by environmental Ag and is only weakly coupled to VDJ diversification. These findings, and the fact that the oligoclonal IgA and IgM repertoires in a noninductive site of the mucosal immune system (parotid gland) become polyclonal in piglets reared germfree, suggest that initial expansion of the switched cells in the B cell compartment of fetal and neonatal piglets is not driven by environmental Ag.

Antibody Repertoire Development in Fetal and Neonatal Piglets. VIII. Colonization Is Required for Newborn Piglets to Make Serum Antibodies to T-Dependent and Type 2 T-Independent Antigens

“…In any case, the immediate availability of natural Abs, without the delay associated with Ag-driven germinal center formation, could allow the piglets to quickly respond to environmental challenges, including the release of numerous Ab specificities such as those to Fl and TNP that they never encountered. Since switch recombination in the fetal piglet is an Ag-independent event (27), the use of natural IgG Abs seems consistent with the overall precosial nature of piglets compared with the altricial offspring of rodents and humans (27,63).…”

“…Serum IgM, IgA, and IgG levels were determined by sandwich ELISA as described previously (6,27,29). Specific Ab responses to TNP and FL were determined by specific Ab immunoassay (SpAbI) using FL-swine albumin or TNP-swine albumin as the solid phase Ag and following the principles previously described for SpAbI (35,36).…”

“…However, the intrinsically developed immune system of the fetal piglet appears precosial by comparison with that of the fetal mouse. In piglets, terminal deoxynucleotide transferase activity is present at the onset of VDJ rearrangement (28), and switch recombination is an in utero event (27,29). The Ab repertoire in all major isotypes is polyclonal at birth (29) and therefore seemingly capable of recognizing many environmental Ags.…”

“…Piglets receive virtually no passive Abs in utero as do rodents (25)(26)(27) and are therefore free at birth from maternal immunological factors or from environmental Ags that may be transported in utero as immune complexes. Thus, studies on newborn piglets come closer to zero with regard to the potential effects of environmental and maternal influences.…”

Antibody Repertoire Development in Fetal and Neonatal Piglets: XIX. Undiversified B Cells with Hydrophobic HCDR3s Preferentially Proliferate in the Porcine Reproductive and Respiratory Syndrome

“…Rearranged VDJs and HCDR3 were recovered from solid tissue preserved at Ϫ70°C and those stored in TRIzol. These were processed as previously described for the preparation of total RNA and subsequently cDNA (12,60). VDJs expressed on IgM and IgG transcripts were recovered as described previously (60,61).…”