The pUL56 of pseudorabies virus variant induces downregulation of swine leukocyte antigen class I molecules through the lysosome pathway

The Deletion of US3 Gene of Pseudorabies Virus (PRV) ΔgE/TK Strain Induces Increased Immunogenicity in Mice

Re-emerging pseudorabies (PR) caused by pseudorabies virus (PRV) variant has been prevailing among immunized herds in China since 2011, indicating that commercially available PR vaccine strains couldn’t provide complete protection against novel, epidemic PRV variant. Before this study, a gE/TK-gene-deleted virus (PRV ΔgE/TK) was constructed from PRV QYY2012 variant through homologous recombination and Cre/LoxP system. Here, PRV ΔgE/TK/US3 strain was generated by deleting US3 gene based on PRV ΔgE/TK strain using the same method. The growth characteristics of PRV ΔgE/TK/US3 were analogous to that of PRV ΔgE/TK. Moreover, the deletion of US3 gene could promote apoptosis, upregulate the level of swine leukocyte antigen class I molecule (SLA-I) in vitro, and relieve inflammatory response in inoculated BALB/c mice. Subsequently, the safety and immunogenicity of PRV ΔgE/TK/US3 was evaluated as a vaccine candidate in mice. The results revealed that PRV ΔgE/TK/US3 was safe for mice, and mice vaccinated with PRV ΔgE/TK/US3 could induce a higher level of PRV-specific neutralizing antibodies and cytokines, including IFN-γ, IL-2 and IL-4, also higher level of CD8+ CD69+ Tissue-Resident Memory T cells (TRM). The results show that the deletion of US3 gene of PRV ΔgE/TK strain could induce increased immunogenicity, indicating that the PRV ΔgE/TK/US3 strain is a promising vaccine candidate for preventing and controlling of the epidemic PR in China.

Host BAG3 Is Degraded by Pseudorabies Virus pUL56 C-Terminal 181L-185L and Plays a Negative Regulation Role during Viral Lytic Infection

Bcl2-associated athanogene (BAG) 3, which is a chaperone-mediated selective autophagy protein, plays a pivotal role in modulating the life cycle of a wide variety of viruses. Both positive and negative modulations of viruses by BAG3 were reported. However, the effects of BAG3 on pseudorabies virus (PRV) remain unknown. To investigate whether BAG3 could modulate the PRV life cycle during a lytic infection, we first identified PRV protein UL56 (pUL56) as a novel BAG3 interactor by co-immunoprecipitation and co-localization analyses. The overexpression of pUL56 induced a significant degradation of BAG3 at protein level via the lysosome pathway. The C-terminal mutations of ¹⁸¹L/A, ¹⁸⁵L/A, or ¹⁸¹L/A-¹⁸⁵L/A in pUL56 resulted in a deficiency in pUL56-induced BAG3 degradation. In addition, the pUL56 C-terminal mutants that lost Golgi retention abrogated pUL56-induced BAG3 degradation, which indicates a Golgi retention-dependent manner. Strikingly, BAG3 was not observed to be degraded in either wild-type or UL56-deleted PRV infected cells as compared to mock infected ones, whereas the additional two adjacent BAG3 cleaved products were found in the infected cells in a species-specific manner. Overexpression of BAG3 significantly suppressed PRV proliferation, while knockdown of BAG3 resulted in increased viral yields in HEK293T cells. Thus, these data indicated a negative regulation role of BAG3 during PRV lytic infection. Collectively, our findings revealed a novel molecular mechanism on host protein degradation induced by PRV pUL56. Moreover, we identified BAG3 as a host restricted protein during PRV lytic infection in cells.

Functional analysis of prv-miR-LLT11a encoded by pseudorabies virus

Viral-encoded microRNAs (miRNAs) have vital roles in the regulation of virus replications and host immune responses. The results of previous studies have indicated that miRNA clusters are involved in the replication and virulence of the pseudorabies virus (PRV), which may potentially lead to immune escape or facilitation of PRV replication. This study's previous research revealed that prv-miR-LLT11a was differentially expressed during PRV infection. The present study's results have demonstrated that prv-miR-LLT11a could significantly inhibit PRV replication. It was further determined that SLA-1 was the target gene of prv-miR-LLT11a, and simultaneously, that overexpression of prv-miR-LLT11a could downregulate the mRNA and protein levels of SLA-1 in a dose-independent manner. Furthermore, the present study also observed that prv-miR-LLT11a can downregulate TAP1 expression. Our findings provide a better understanding of the molecular mechanism involved in the effects of prv-miR-LLT11a on SLA-1 and TAP1 as well as its involvement in immune system evasion of PRV.

Glycoproteins C and D of PRV Strain HB1201 Contribute Individually to the Escape From Bartha-K61 Vaccine-Induced Immunity

The newly emerged pseudorabies virus (PRV) novel variants can escape from the immunity induced by the classical vaccine Bartha-K61. Here we investigated the underlying mechanisms by constructing chimeric mutants between epidemic strain HB1201 and the Bartha-K61 vaccine. Our analyses focused on three viral envelope glycoproteins, namely gB, gC, and gD, as they exhibit remarkable genetic variations and are also involved in induction of protective immunity. The corresponding genes were swapped reciprocally either individually or in combination by using CRISPR/Cas9 technology and homologous recombination. The rescued chimeric viruses exhibited differential sensitivity to neutralizing antibodies in vitro, and gC was found to be the major contributor to inefficient neutralization against HB1201 by anti-Bartha-K61 serum. When tested in the 4-week-piglet model, substitution with HB1201 gC enabled Bartha-K61 to induce a protective immunity against HB1201 at a high challenge dose of 10⁷ TCID₅₀. Interestingly, despite a relatively lower cross-neutralization ability, the gD exchange also enabled Bartha-K61 to protect piglets from lethal challenge. In both cases, clinical signs and microscopic lesions were eased, and so was the viral tissue load with the exception of brain. A better protection could be achieved when both gC and gD were swapped in terms of reducing viral load in brain and virus-induced microscopic lesions. Thus, our studies not only revealed individual roles of gC and gD variations in the immune escape and also suggested a synergistic effect of both proteins on induction of protective immunity. These findings have important implications in novel vaccine development for PRV control in China.

A GFP-tagged version of the pseudorabies virus protein UL56 localizes to the Golgi and trans-Golgi network through a predicted C-terminal leucine-rich helix in transfected cells

BACKGROUND

Pseudorabies virus (PRV) protein UL56 (pUL56) has been implicated in viral dissemination and virulence in vivo. However, the properties of PRV pUL56 remain largely unknown. In the present study, we aim to investigate the subcellular localization of pUL56 and the underlying molecular basis in transfected cells.

METHODS

Constructs of N-terminal green fluorescent protein (GFP) fused pUL56 and its truncations were employed for investigating subcellular localization and further identifying amino acids crucial for pUL56 localization in transfected Vero cells. Finally, the identified amino acids were replaced with alanine for confirming if these mutations could impair the specific localization of pUL56.

RESULTS

The pUL56 predominantly localized at the Golgi and trans-Golgi network (TGN) through its predicted C-terminal transmembrane helix in transfected Vero cells. A Golgi-associated protein Rab6a, independent of interaction with pUL56, was significantly downregulated by pUL56. Further, we found three truncated pUL56 C-terminal fragments (174-184, 175-185 and 191-195) could restrict GFP in the perinuclear region, respectively. Within these truncations, the ¹⁷⁴proline (P), ¹⁸¹leucine (L), ¹⁸⁵L and ¹⁹¹L were essential for maintaining perinuclear accumulation, thus suggesting an important role of leucine. Alanine (A) mutagenesis assays were employed to generate a series of pUL56 C-terminal mutants on the basis of leucine. Finally, a pUL56 mutant M10 (¹⁷⁴P/A-¹⁷⁷L/A-¹⁸¹L/A-¹⁸⁵L/A-¹⁹¹L/A-¹⁹⁴L/A-¹⁹⁵I/A-^196-197L/A-²⁰⁰L/A) lost Golgi-TGN localization. Thus, our data revealed that the leucine-rich transmembrane helix was responsible for pUL56 Golgi-TGN localization and retention, probably through specific intracellular membrane insertion.

CONCLUSION

Our data indicated that the C-terminal transmembrane helix was responsible for the Golgi-TGN localization of pUL56. In addition, the leucines within C-terminal transmembrane helix were essential for maintaining pUL56 Golgi-TGN retention in cells. Further, the pUL56 can induce downregulation of Golgi-associated protein Rab6a.