Marek's disease (MD) is an alpha-herpesvirus-induced lymphoproliferative disease of chickens which results in CD4+ T cell lymphomas in multiple organ systems, as well as peripheral and central nervous system disorders. Genetic studies of MD-resistant and susceptible chicken lines have identified the major histocompatibility complex (MHC) locus as the most important disease resistance locus in chickens; however, the contribution of the T cell receptor (TCR) repertoire to MD resistance mediated by the peptide-MHC-TCR synapse has not yet been characterized, in contrast to the extensive TCR repertoire studies that have been performed in human herpesviral infections. In this study, we identified differences in the TCR Vbeta repertoire of MD-resistant and susceptible chicken lines, and sought to determine the genetic basis of these differences. Additionally, we studied the contribution of thymic tolerance to MD neuropathogenicity in a non-oncogenic MD model, identifying a potential role of adaptive immune dysregulation in the acute disease. Model pairs of genetically MD-resistant and susceptible chickens that are either MHC-matched or MHC congenic were studied in order to characterize the T cell response, particularly the TCR Vbeta repertoire, during Marek's disease virus (MDV) infection. In contrast to previous models of T cell-mediated genetic resistance which suggested that resistant birds might have less-easily activated CD4+ T cells and thus be resistant to transformation, we were unable to find differences in in vitro mitogen response within the CD4+ T cell populations of MHC-matched MD-resistant and susceptible chickens. However, TCR Vbeta repertoire in vivo differed between MD-resistant and susceptible birds, and shifts towards higher TCR Vbeta-1 usage in response to MDV-infection could be identified within the CD8+ T cell populations, most notably within MD-resistant birds, consistent with CTL-mediated resistance. Chickens resistant to MD showed higher usage of Vbeta-1 TCRs, in both the CD8 and CD4 subsets in the MHC-matched model, and in the CD8 subset only in the MHC-congenic model. Using Illumina sequencing and PacBio long-read sequencing, we characterized the TCR beta locus of the MHC-matched lines, and found that the MD-resistant line expressed a greater number of Vbeta-1 TCRs and an increased number of Vbeta-1 CDR1 loops with a Trp-45 residue. TCR Vbeta-1 CDR1 usage in MHC-matched F1 birds was studied with Illumina RNA-seq, and usage of a susceptible line CDR1 variant was disproportionately high, suggesting that selection for resistance in the MHC-matched model has optimized the TCR repertoire away from dominant recognition of one of the MHC molecules. We also studied TCR down-regulation by MDV infection, and found that in vitro down-regulation could be mediated by viral reactivation independently of TCR activation or apoptosis, suggesting a TCR-targeting immune evasion strategy by MDV. Lastly, we studied a potentially immune-mediated phenotype associated with MD, acute transient paralysis, using an MDV virus which lacks the Meq oncogene. We describe a fatal neuropathy of chicks induced by injection of Meq-deleted MDV in ovo during the thymic tolerizing window (prior to 15 days of embryogenesis), which induces severe bursa and thymic atrophy as well as mild lymphocytic peripheral nerve lesions. This establishes that oncogenicity is not absolutely required for the acute neuropathic syndrome, and suggests that vaccine strains may be capable of inducing neuropathic disease in T-cell-immunity-disregulated birds.
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