Hydrophobic IgG-Containing Immune Complexes in the Plasma of Autoimmune MRL/lpr Mice, Lactate Dehydrogenase-Elevating Virus-Infected Mice, and Pigs: Association with Transforming Growth Factor-βand pH-Dependent Amplification

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.

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.

Mice expressing HLA-DQ6alpha8beta transgenes develop polychondritis spontaneously

Relapsing polychondritis (RP) is a human autoimmune disease of unknown etiology in which cartilaginous sites are destroyed by cyclic inflammatory episodes beginning, most commonly, during the fourth or fifth decade of life. We have previously described collagen-induced polychondritis that closely mirrors RP occurring in young (6–8 weeks old) HLA-DQ6αβ8αβ transgenic Aβ0 mice, following immunization with heterologous type II collagen (CII).

We present evidence here that transgenic strains expressing the DQ6α8β transgene develop spontaneous polychondritis (SP) at the mouse equivalent of human middle age (4.5–6 months and 40–50 years old, respectively) and display polyarthritis, auricular chondritis and nasal chondritis – three of the most common sites affected in RP. Auricular chondritis in SP, like RP but unlike CII-induced polychondritis, exhibited a relapsing/remitting phenotype, requiring several inflammatory cycles before the cartilage is destroyed. Elevated serum levels of total IgG corresponded with the onset of disease in SP, as in RP and CII-induced polychondritis. No CII-specific immune response was detected in SP, however – more closely mirroring RP, in which as few as 30% of RP patients have been reported to have CII-specific IgG. CII-induced polychondritis displays a strong CII-specific immune response. SP also demonstrated a strong female preponderance, as some workers have reported in RP but has not observed in CII-induced polychondritis. These characteristics of SP allow for the examination of the immunopathogenesis of polychondritis in the absence of an overwhelming CII-specific immune response and the strong adjuvant-induced immunostimulatory influence in CII-induced polychondritis.

This spontaneous model of polychondritis provides a new and unique tool to investigate both the initiatory events as well as the immunopathogenic mechanisms occurring at cartilaginous sites during the cyclic inflammatory assaults of polychondritis.

Polyclonal Hypergammaglobulinemia and Formation of Hydrophobic Immune Complexes in Porcine Reproductive and Respiratory Syndrome Virus-Infected and Uninfected Pigs

Infection of young conventional, domestic pigs with porcine reproductive and respiratory syndrome virus (PRRSV) strains VR2332 and JA142 resulted in a rapid, progressive increase in serum IgG reaching maximum levels of 20-30 mg/mL at about 3 weeks post infection (p.i.), which were maintained until at least 63 days p.i., whereas the level of serum IgG remained at 4-6 mg/mL in sham infected pigs. In most of the VR2332 and JA142-infected pigs hypergammaglobulimenia was associated with the formation of hydrophobic, 150-300-kDa IgG-containing immune complexes that bound in the presence of 0.1% Tween 20 to ELISA plates that were not coated with any antigen. The ELISA plate-binding activity remained low in most infected pigs, but reached high levels in some JA142-infected pigs. Binding of the immune complexes was also observed, but at a lower level, to uncoated ELISA plates in the peptide ELISA for anti-PRRSV Abs. The immune complexes bound to uncoated ELISA plates with a much lower affinity than Abs to plates coated with peptides containing the appropriate epitopes. The immune complexes also bound to HerdChek ELISA plates, but because of low binding affinity for these plates, the bound complexes were removed by the repeated washes with Tween 20 solution. Overall the PRRSV-induced hypergammaglobulinemia and generation of ELISA plate-binding immune complexes resembled those observed in mice infected with the closely related lactate dehydrogenase-elevating virus (LDV) and thus, like the latter, seem a result of a polyclonal activation of B cells. We also found that sera of a group of older sows possessed high levels of IgG as well as of ELISA plate-binding immune complexes, in spite of being PRRSV infection negative by all criteria presently available. On the other hand, sera from wild hogs contained no ELISA plate-binding IgG in spite of possessing high total serum IgG levels.

The primary GP5 neutralization epitope of North American isolates of porcine reproductive and respiratory syndrome virus

I have used indirect ELISA with overlapping synthetic peptides representing the GP5 ectodomain to study the generation and specificity of peptide-binding Abs in pigs that were infected in utero with porcine reproductive and respiratory syndrome virus (PRRSV) strain VR2332 and in North American field sera submitted for PRRSV infection diagnosis. Peptide-binding Abs appeared in sera of the VR2332-infected pigs within about 30 days post-farrowing (dpf), reaching maximum titers 100-200 dpf and then decreasing slowly to about half of maximum titer by about 400 dpf. The formation of peptide-binding Abs and of virus neutralizing Abs correlated and their initial appearance coincided with disappearance of virus from the circulation. The Abs were specific for VR2332-specific peptides. In contrast, anti-N-protein Abs as measured by HerdCheck ELISA appeared within 7 dpf, reached maximum levels at about 100 dpf and had decreased below detectable levels by about 200 dpf. Twenty-seven field serum samples with virus neutralizing activity all possessed high levels of peptide binding Abs, but the Abs bound about equally to VR2332 and strain Lelystad virus (LV)-specific peptides. The indirect ELISA results using various large peptides and competition ELISA using small peptides (8 or 9 amino acids long) confirmed that the epitope recognized by the Abs is located in the GP5 ectodomain sequence 37SHLQLIYNL of VR2332. Use of mutated peptides in the competition ELISA showed that 42I to T and 38HL to TY substitutions blocked Ab recognition, whereas deletion of 41L had no effect. In addition, 26 serum samples submitted by two farms for diagnostic tests were found to possess low levels of Abs that bound to GP5 ectodomain peptides, even though the sera were sero-negative in the HerdChek ELISA and lacked neutralizing activity. Competition ELISA showed that the Abs recognized one or more epitopes located downstream of the PRRSV neutralization epitope. An epitope(s) located in the same area was recognized by Abs generated in mice by immunization with a GP5 ectodomain peptide conjugated to BSA. These Abs also lacked neutralizing activity.

GP5 ectodomain epitope of porcine reproductive and respiratory syndrome virus, strain Lelystad virus

Sera from pigs infected with the European porcine reproductive and respiratory syndrome virus (PRRSV), strain Lelystad virus (LV), were screened by indirect ELISA for antibodies that bound to a series of overlapping synthetic peptides covering amino acids (AA) 19-60 of the primary envelope glycoprotein (GP)5. Antibodies were detected that bound to an epitope(s) located in an ectodomain segment composed of AA 38-54. The antibodies strongly cross-reacted with peptides specific for the North American PRRSV VR2332. Antibodies with the same specificity were present in sera of pigs infected in the US with a European-like PRRSV. Competitive ELISA using overlapping 8-10-AA-long peptides confirmed that the epitope recognized by the antibodies from LV-infected pigs is located in the same segment as the neutralization epitopes previously identified for PRRSV VR2332 and the closely related arterivirus, lactate dehydrogenase-elevating virus (LDV). No antibodies were detected that bound to synthetic peptides representing further upstream GP5 segments that have been reported to carry neutralization or non-neutralization epitopes of some PRRSV isolates.