Porcine Reproductive and Respiratory Syndrome Virus Subverts Repertoire Development by Proliferation of Germline-Encoded B Cells of All Isotypes Bearing Hydrophobic Heavy Chain CDR3

Abstract

Isolator piglets infected with porcine reproductive and respiratory syndrome virus (PRRSV), which is related to the lactate dehydrogenase-elevating virus of mice, develop severe hypergammaglobulinemia, lymph node adenopathy, and autoimmune disease. Many of the polyclonally activated B cell clones bear hydrophobic H chain CDR3s (HCDR3s) and are disseminated to most lymphoid tissues. We show in this study that B cells with identical hydrophobic HCDR3s are expressed with all major isotypes in PRRSV-infected piglets (PIPs), explaining why PRRSV-induced hypergammaglobulinemia is seen in all major isotypes. Up to one-third of randomly selected VDJ clones from the respiratory tract of PIPs have hydrophobic HCDR3s exclusively bearing VDJ rearrangements with CDR1, CDR2, and nearly intact DH segments in germline configuration. These HCDR3s are long and DHA and DHB are exclusively used in reading frame 3. A minimal tripeptide motif containing three hydrophobic amino acids (Leu, Val, and Ile) or any two plus alanine is common to this hydrophobic patch. We propose that PRRSV infection causes generalized Ag-independent B cell activation and hypergammaglobulinemia with biased expansion of a subpopulation of the preimmune repertoire with hydrophobic binding sites that normally disappears during Ag-driven repertoire diversification. Elevated Ig levels in PIP cannot be explained as antiviral Abs; some Igs can account for autoantibodies to dsDNA and Golgi, whereas those with hydrophobic binding sites may account for the Ig aggregates seen in PIPs and lactate dehydrogenase-elevating virus-infected mice. This diversion from normal repertoire development may explain the delayed immune response to PRRSV.

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

“…Previously we reported on the humoral parameters of the same animals [5][6][7]28,29]. Based on those studies we were curious about how these infections influenced lymphocyte profiles.…”

“…This study involves monitoring the lymphoid cells profile in the same set of GF piglets that had been previously used in studies on humoral parameters including antibody repertoire development [5][6][7]29]. The clinical response of SIV, PRRSV, and PCV2 infected piglets during the course of these studies (dpi 0-28) is described therein.…”

“…Subjective evaluation revealed that PCV2 infected piglets were similar to the PRRSV infected piglets as regards the size of lymph nodes, albeit TBLN were not as enlarged as with PRRSV but were larger than for SIV. All inoculated piglets in each experiment replicated the virus used for inoculation and were seropositive for respective virus while control piglets remained free of viral infection and remained asymptomatic [5][6][7]29]. Leukograms for individual infections (Additional file 3) show that there was no effect of SIV infection on total number of white blood cells, neutrophils or lymphocytes but the number of monocytes was increased.…”

“…Limitation of vaccines against SIV that uses genetic reassortment is known [4]. Nevertheless, even germ-free (GF) piglets lacking passive antibodies (Abs) can resolve SIV infection within 6-7 days post challenge [5] whereas resolution of PRRSV [6,7] and PCV2 [8] infections is delayed. This delay may result from the ability to block, postpone or dysregulate an effective host immune response allowing the diseases to become pandemic.…”

“…Also, GF piglets are currently the best model for fetal animals [7], which is especially relevant because PRRSV causes considerable fetal mortality [10] and PCV2 displays a high frequency of in utero transmission [3]. While the effect of these three viral infections on humoral response in GF piglets has been previously reported [5][6][7]28,29], the complex information about the phenotype of lymphoid cells and their distribution is missing. Here for the first time we report on the phenotypic profile of T, B and NK cells and their subsets utilizing our recent knowledge about these lymphoid cells in swine [24][25][26][27]30] allowing for comparative analysis of > 20 different lymphoid subpopulations.…”