Porcine Reproductive and Respiratory Syndrome Virus Modulates the Switch of Macrophage Polarization from M1 to M2 by Upregulating MoDC-Released sCD83

Porcine reproductive and respiratory syndrome virus (PRRSV), the most economically important infectious disease of pigs, elicits poor innate and adaptive immune responses. Soluble CD83 (sCD83), a secretion from various immune cell populations, especially MoDCs, is involved in negatively regulating the immune response. We speculate sCD83 may be a critical factor in the process of PRRSV-coordinated macrophage polarization. In this study, we found that PAMs co-cultured with PRRSV-infected MoDCs inhibited the M1 macrophage while enhancing the M2 macrophage. This was accompanied by a decrease in the pro-inflammatory cytokine TNF-α and iNOS and an increase in the anti-inflammatory cytokine IL-10 and Arg1. Meanwhile, sCD83 incubation causes the same specific effects lead to a switch in macrophage from M1 to M2. Neutralization of sCD83 removes the inhibitory effects of PRRSV on PAMs. Using reverse genetics, we generated recombinant PRRSVs with mutations in N protein, nsp1α, and nsp10 (knockout sCD83-concerned key amino acid site). Four mutant viruses lost the suppression of M1 macrophage markers, in contrast to the restriction of the upregulation of M2 macrophage markers. These findings suggest that PRRSV modulates the switch of macrophage polarization from M1 to M2 by upregulating the MoDC-induced secretion of CD83, providing new insights into the mechanism by which PRRSV regulates host immunity.


Introduction
Porcine reproductive and respiratory syndrome (PRRS), resulting from the porcine reproductive and respiratory syndrome virus (PRRSV), is one of the most important swine diseases threatening pigs' health at all ages and causing substantial economic losses worldwide [1]. PRRSV is a positive-strand RNA virus belonging to the Arteriviridae family, whose genome has nine open reading frames and encodes seven structural proteins and 16 non-structural proteins [2,3]. During PRRSV infection, every viral protein could be associated with the immunomodulation capability of the virus. A typical characteristic of PRRSV infection in pigs is evading host defenses through the suppression of innate immunity [3,4]. The absence of a classical innate antiviral immune response occurs as a result of multiple strategies, such as influencing the production of inflammatory and immunomodulatory cytokines, suppressing or enhancing interferon production, enhancing immunosuppressive cytokine expression, or disturbing the development of immune cells including monocytes, macrophages, and T/B cells [1,[4][5][6][7][8][9][10][11][12].
PRRSV infection modulates the ability of MoDCs to stimulate the proliferation of T cells via the induction of sCD83 production [13][14][15]. These functions result from the amino acids R43 and K44 in the N protein, the P192-196 and G214-216 domains in nsp10 and the L5, D6, G45, G48, L61, P62, R63, F65, and P66 domains in nsp1α. The N protein of the In this study, we investigated the possible correlation among PRRSV, M1/M2 polarization, and sCD83 in PAMs. We found that infection with the wild-type PRRSV BB0907 strain targets the expression of sCD83, thus regulating macrophage polarization from the M1 phenotype to M2 phenotype bias. These results might provide new theories and insights into the immune mechanism underlying immune suppression by PRRSV infection, with implications for the development of vaccine optimization strategies.

Cells and Viruses
Marc-145 cells, derived from African green monkey kidney cells, are highly permissive to PRRSV infection. HEK 293T cells (a human embryonic kidney cell line) and MARC-145 cells were obtained from the ATCC and held in our laboratory. The cells were incubated in Dulbecco's modified essential medium (DMEM; Gibco, Carlsbad, CA, USA) and supplemented with 10% heat-inactivated fetal bovine serum (FBS; Gibco) containing 100 U/mL penicillin and 100 µg/mL streptomycin at 37 • C with 5% CO 2 . Primary alveolar macrophages (PAMs) were collected via bronchoalveolar lavage from six-week-old PRRSVnegative pigs, and all processes of the animal were authorized and supervised by the rules approved by the State Council of the People's Republic of China for experimental animal care and use. PAMs were cultured in an RPMI1640 medium supplemented with 10% fetal bovine serum (FBS).
HP-PRRSV isolate BB0907 (GenBank accession number HQ315835) was passaged 10 times through Marc-145 cells and is designated as BB in this study. The virus was isolated in 2009 in Guangxi Province, China, and has been maintained as a stock in our laboratory. Classical PRRSV (C-PRRSV) strain S1 (GenBank accession number AF090173) was isolated from pigs with clinical signs of PRRS in Jiangsu Province in 1997. The low pathogenicity PRRSV (LP-PRRSV) isolate NT0801 (GenBank accession number HQ315836) was isolated in Jiangsu Province, China, in 2008. Recombinant viruses [rN-43-2A (mutant sites at residues R43 and K44 of N protein), rnsp10-2m (mutations at rP192-5A and rG214-3A of nsp10), rnsp1α-3m (includes mutations in rL5-2A, rG45A/G48A, and rL61-6A), and rN/nsp10 (includes mutations in R43 and K44 of N protein and rP192-5A and rG214-3A of nsp10)] were rescued from infectious clone pCMV-BB0907 (constructed in our laboratory) and were propagated in Marc-145 cells upon recovery. The viral titers were determined using the Reed-Muench method and expressed as tissue culture infective dose 50% (TCID 50 ).

Measurements of TNF-α, IL-10 and sCD83
PAM supernatants were collected for the measurement of TNF-α, IL-10, and sCD83. TNF-α and IL-10 were measured by a radioimmunoassay using a Porcine IL-10 ELISA Kit and Porcine TNF-α ELISA kit (Beyotime Biotechnology, Shanghai, China). sCD83 were measured by radioimmunoassay using a Human sCD83 ELISA Kit (Abcam, Cambridge, UK) according to the manufacturer's instructions. The optical density (OD) was measured with a microplate reader at 450 nm, and the secreted protein concentrations were calculated according to the standard curve.

RT-qPCR
Quantitative RT-PCR was performed to measure iNOS and Arg1. Total RNA from PAMs was extracted using a Qiagen RNeasy kit (Qiagen, Hilden, Germany). cDNA was synthesized using an RT-PCR kit (TaKaRa, Kusatsu, Japan) according to the manufacturer's instructions. Quantitative real-time PCR was performed using SYBR Premix Ex TaqTM II (Tli RNaseH Plus) (TaKaRa). Relative quantification of target gene expression was calculated using the 2 −∆∆Cq method. Data are presented as fold changes in gene expression, normalized to GAPDH and relative to the mock-infected control. Each reaction was performed in triplicate, and the data are expressed as means ± standard errors of the means (SEM). Primer sequences are shown in Table 1.

Construction of Infectious PRRSV cDNA Clones
The full-length PRRSV genome was amplified using the five primer pairs listed in Table 2. A recombinant plasmid (pCMV-BB) containing a full-length cDNA copy of the virus was constructed, as shown in Figure 1 and designated as pCMV-BB [13,14,45,46]. It was used as the backbone to construct N protein nsp1α and nsp10 mutants in a PRRSV BB0907 background. Mutagenesis of the N protein was performed on the D fragment of pCMV-BB. Mutagenesis of the nsp1α protein was performed on the A fragment of pCMV-BB. Mutagenesis of the nsp10 protein was performed on the C fragment of pCMV-BB. Fragments containing mutations in N protein, nsp1α, and nsp10 were obtained by sitedirected PCR mutagenesis using pBB/wt as the template. The PCR products were inserted into a pEASY-Blunt Simple vector (designated pEASY-Am, pEASY-Cm, pEASY-Dm). The plasmids were then digested with PacI/XhoI, Afl II/Asc I, or Asc I/Spe I, and fragments containing the mutation were ligated into the same restriction enzyme-digested pBB/wt to obtain the full-length mutant cDNA clones pBB/N-R43/K44A, pBB/nsp10 (P192-5A/G212-5A), pBB/nsp1α-3m, and pBB/N/Nsp10, respectively ( Figure 1). MARC-145 cells were transfected with each plasmid to generate the mutant viruses rBB/wt, rN-43-2A, rnsp10-2m, rnsp1α-3m, and rN/Nsp10. All mutant PRRSV strains were repaired in MARC-145 cells. Briefly, plasmids containing the mutations were modified to restore the N, nsp1α, or nsp10 wild-type genotypes. Fragments containing the repaired mutations were ligated into digested pBB/wt. All recombinant viruses and amino acid substitution primers are listed in Table 3. MARC-145 cells were transfected with the resulting full-length cDNA clones to generate repaired PRRSV. Transfections were conducted with Lipofectamine 3000 reagent according to the manufacturer's protocol. The supernatants were harvested and serially passaged four times in MARC-145 cells until about 80% of cells exhibited cytopathic effects (CPEs). Passage 2 to passage 5 virus stocks were prepared in the same manner. All full-length mutant clones were verified by nucleotide sequencing (data not shown). The plasmids were isolated using the QIAprep spin miniprep kit (Qiagen).   were transfected with each plasmid to generate the mutant viruses rBB/wt, rN-43-2 rnsp10-2m, rnsp1α-3m, and rN/Nsp10. All mutant PRRSV strains were repaired MARC-145 cells. Briefly, plasmids containing the mutations were modified to restore t N, nsp1α, or nsp10 wild-type genotypes. Fragments containing the repaired mutatio were ligated into digested pBB/wt. All recombinant viruses and amino acid substituti primers are listed in Table 3. MARC-145 cells were transfected with the resulting fu length cDNA clones to generate repaired PRRSV. Transfections were conducted w Lipofectamine 3000 reagent according to the manufacturer's protocol. The supernata were harvested and serially passaged four times in MARC-145 cells until about 80% cells exhibited cytopathic effects (CPEs). Passage 2 to passage 5 virus stocks were prepar in the same manner. All full-length mutant clones were verified by nucleotide sequenci (data not shown). The plasmids were isolated using the QIAprep spin miniprep kit (Q agen).

One-Step Viral Growth Curves
MARC-145 cells, seeded in 24-well plates, were inoculated with 10 6 TCID 50 of the virus. At 6, 12, 24, 36, 48, and 72 h post-infection, 100 µL of the infected cell supernatant was removed and the same volume of fresh medium was added back to each well. All samples were stored at −70 • C until virus titration. Virus titers were determined as TCID 50 .

Statistical Analysis
Data were expressed as the mean ± SEM of at least three independent experiments for each cellular experimental group. The data were evaluated by one-way analysis of variance (ANOVA), followed by post-hoc Tukey's test using GraphPad Prism version 5.0 (GraphPad Software, San Diego, CA, USA). p values of less than 0.05 (* p < 0.05, ** p < 0.01, *** p < 0.001) were considered statistically significant. Table 3. Primers used for construction of the CD83 promoter, N protein, nsp1α, and nsp10 mutant.
The genomic positions for the primers are based on GenBank accession number XM_001928655.

Alterations in M1 and M2 Macrophage Types after PRRSV Infection In Vitro
MoDCs were incubated with strains of different virulence levels (HP-PRRSV BB0907, classical PRRSV [C-PRRSV] S1, and low pathogenicity PRRSV [LP-PRRSV] NT0801) for 36 h, and cell supernatant was co-cultured with PAMs. After 24 h, PAMs were polarized to M1 and M2 macrophages. Production of the cytokines TNF-α and IL-10, reflective of the pro-inflammatory and anti-inflammatory function of M1 and M2 macrophages, respectively, was assessed using ELISA (Figure 2A,B). The mRNA and protein expression levels of iNOS (a marker of M1) and Arg1 (a marker of M2) were detected by RT-qPCR ( Figure 2C,D) and Western blot ( Figure 2E). As expected, TNF-α release and iNOS expression of M1 macrophages from PRRSV-infected groups were significantly lower than that of the control group. However, changes in the mRNA level of Arg1 were less marked, while Arg1 protein expression and IL-10 release of M2 macrophages from PRRSV-infected cells were obviously higher. Taken together, these data indicate that PRRSV infection induces a switch in alveolar macrophage polarization from M1 to M2. Because the HP-PRRSV infection has caused immense economic damage in China during the pandemic, the highly virulent HP-PRRSV BB0907 strain was selected for subsequent experiments.

PRRSV Regulates Macrophage Polarization in a Dose-Dependent Manner
Firstly, PRRSV titers in infected MoDCs peaked at 36 hpi ( Figure 3A); accordingly, we choose PRRSV to infect MoDCs for 36 h in subsequent trials. To optimize viral infection, MoDCs were incubated with PRRSV at an MOI of 0, 0.5, 1, or 2 for 36 h, Samples were collected and added to PAMs. Cells treated with LPS were used as a positive control. As shown in Figure 3B,C,E, the level of TNF-α release, iNOS mRNA and protein expression were significantly lower in cells infected with PRRSV at an MOI of 2 than in those infected at an MOI of 0.5. Otherwise, the levels of IL-10 release and Arg1 protein expression were prominently higher at an MOI of 2 than those infected at an MOI of 0.5 ( Figure 3B,D). Curiously, Viruses 2023, 15, 773 7 of 17 the increase of the Arg1 mRNA level was almost negligible in PRRSV infection from MOI 0.5 to 2 ( Figure 3F). These results indicate that PRRSV decreases TNF-α release and iNOS expression and increases IL-10 release and Arg1 expression in a dose-dependent manner. levels of iNOS (a marker of M1) and Arg1 (a marker of M2) were detected by RT-q ( Figure 2C,D) and Western blot ( Figure 2E). As expected, TNF-α release and iNOS ex sion of M1 macrophages from PRRSV-infected groups were significantly lower than of the control group. However, changes in the mRNA level of Arg1 were less mar while Arg1 protein expression and IL-10 release of M2 macrophages from PRRSV-infe cells were obviously higher. Taken together, these data indicate that PRRSV infectio duces a switch in alveolar macrophage polarization from M1 to M2. Because the PRRSV infection has caused immense economic damage in China during the pande the highly virulent HP-PRRSV BB0907 strain was selected for subsequent experimen  infected at an MOI of 0.5. Otherwise, the levels of IL-10 release and Arg1 protein ex sion were prominently higher at an MOI of 2 than those infected at an MOI of 0.5 (F 3B,D). Curiously, the increase of the Arg1 mRNA level was almost negligible in PR infection from MOI 0.5 to 2 ( Figure 3F). These results indicate that PRRSV decreases α release and iNOS expression and increases IL-10 release and Arg1 expression in a dependent manner.

sCD83 Modulates a Switch in Macrophage Polarization from M1 to M2
To further investigate the effects of sCD83 on the phenotypic switch in macrophage polarization, PAMs, in the presence of GST-sCD83 protein at concentrations of 0.1, 0.5, and 1 µg/mL, were measured by Western blotting, ELISA, and RT-qPCR. GST-Cap (5 µg/mL) treatment was used as a control group. As shown in Figure 4A,C,E, levels of TNF-α release and iNOS expression were reduced, but the levels of IL-10 release and Arg1 expression were enhanced in cells treated with GST-sCD83 in a dose-dependent manner ( Figure 4B,E). Simultaneously, the sCD83-induced mRNA level of Arg1 hardly budged ( Figure 4D). The results of the sCD83 treatment corresponded well with the effects of PRRSV infection. µg/mL) treatment was used as a control group. As shown in Figure 4A,C,E, levels of T α release and iNOS expression were reduced, but the levels of IL-10 release and Arg1 pression were enhanced in cells treated with GST-sCD83 in a dose-dependent man ( Figure 4B,E). Simultaneously, the sCD83-induced mRNA level of Arg1 hardly bud ( Figure 4D). The results of the sCD83 treatment corresponded well with the effect PRRSV infection.  and Arg1 (D) mRNA expression were analyzed by RT-qPCR. mRNA levels were calculated relative to known amounts of a template and normalized to GAPDH expression. (E) Cell lysates were examined by Western blotting with anti-iNOS and anti-Arg1 antibodies. PBS treatment was used as a negative control and endogenous GAPDH expression was used as an internal control. Results are representative of three independent experiments. Data are represented as means ± SEM. ***, p < 0.001; **, p < 0.01; *, p < 0.05; ns, not significant.

Anti-CD83 Restores a Switch in Alveolar Macrophage Polarization from M1 to M2 in PRRSV Infection
To test if macrophage polarization responds to sCD83 level in PRRSV infection, MoDCs were incubated with rabbit anti-CD83 or rabbit IgG at 10 µg/mL for 1 h and then infected with PRRSV at an MOI of 1. After 36 h, cells were co-cultured with PAMs. The level of sCD83 in MoDCs supernatant was tested by ELISA ( Figure 5A). Figure 5B,D,F show that the levels of TNF-α release and iNOS expression were obviously higher in the anti-CD83treated group. Similarly, cells pre-treated with anti-CD83 led to lower levels of IL-10 release and Arg1 expression than other groups ( Figure 5C,E,F). This result demonstrates that immunodepletion of soluble CD83 largely restores the macrophage polarization from M1 to M2 resulting from PRRSV infection.
MoDCs were incubated with rabbit anti-CD83 or rabbit IgG at 10 µg/mL for 1 h and then infected with PRRSV at an MOI of 1. After 36 h, cells were co-cultured with PAMs. The level of sCD83 in MoDCs supernatant was tested by ELISA ( Figure 5A). Figure 5B,D,F show that the levels of TNF-α release and iNOS expression were obviously higher in the anti-CD83-treated group. Similarly, cells pre-treated with anti-CD83 led to lower levels of IL-10 release and Arg1 expression than other groups ( Figure 5C,E,F). This result demonstrates that immunodepletion of soluble CD83 largely restores the macrophage polarization from M1 to M2 resulting from PRRSV infection. Figure 5. Anti-CD83 antibody blocks the ability of PRRSV to induce a switch in macrophage polarization from M1 to M2. MoDCs were pre-treated with rabbit anti-CD83 antibody to remove sCD83, or with isotype (rabbit IgG) antibody as a negative control in the cell culture medium. MoDCs were then infected with PRRSV at an MOI of 1 for 36 h, and supernatants from these cultures were added to PAMs. Cell supernatants were studied for sCD83 (A), TNF-α (B), and IL-10 (C) secretion. Cell lysates were analyzed for mRNA and protein levels of iNOS (D), Arg1 (E), and N proteins (F) using RT-qPCR and Western blotting (F), respectively. GAPDH was used as a loading control. Data are representative of at least three independent experiments. ***, p < 0.001; **, p < 0.01; *, p < 0.05 compared with the mock treatment group.

Figure 5.
Anti-CD83 antibody blocks the ability of PRRSV to induce a switch in macrophage polarization from M1 to M2. MoDCs were pre-treated with rabbit anti-CD83 antibody to remove sCD83, or with isotype (rabbit IgG) antibody as a negative control in the cell culture medium. MoDCs were then infected with PRRSV at an MOI of 1 for 36 h, and supernatants from these cultures were added to PAMs. Cell supernatants were studied for sCD83 (A), TNF-α (B), and IL-10 (C) secretion. Cell lysates were analyzed for mRNA and protein levels of iNOS (D), Arg1 (E), and N proteins (F) using RT-qPCR and Western blotting (F), respectively. GAPDH was used as a loading control. Data are representative of at least three independent experiments. ***, p < 0.001; **, p < 0.01; *, p < 0.05 compared with the mock treatment group.

Construction and Identification of Mutant Recombinant PRRSV
To confirm the identity of the amino acid residues responsible for CD83 promoter activation, viruses containing mutations in N, nsp10, and nsp1α were constructed using infectious PRRSV cDNA clones (Figure 1). Four viruses were successfully rescued. These were rN-43-2A (mutant sites at residues R43 and K44 of N protein), rnsp10-2m (mutations at rP192-5A and rG214-3A of nsp10), rnsp1α-3m (mutations in rL5-2A, rG45A/G48A, and rL61-6A) and rN/nsp10 (mutations in R43 and K44 of N protein and rP192-5A and rG214-3A of nsp10). The viruses were functional, as judged by the presence of a cytopathic effect 4 days post-transfection. In addition, rN-43-2A, rnsp10-2m, and rN/nsp10 exhibited growth kinetics similar to those of the parental wild-type rBB/wt virus ( Figure 6), but rnsp1α-3m grew more slowly than wild-type rBB/wt infection, resulting in an approximately 10-fold difference in titer.

Effect of Mutant Viruses on Macrophage Polarization from M1 to M2 by sCD83
To determine whether mutant viruses affect macrophage polarization, MoDCs were infected with the four mutant viruses at an MOI of 1. Twenty-four hours after infection, cells were co-cultured with PAMs and analyzed by ELISA, RT-qPCR, and Western blot. As shown in Figure 7A, the mutants decreased their ability to secrete sCD83 compared with the parental virus rBB/wt. Levels of TNF-α release and iNOS protein expression were higher in untreated cells infected by rN-43-2A, rnsp10-2m, rnsp1α-3m, and rN/nsp10 than cells infected by wild-type rBB/wt ( Figure 7B,F). In contrast, the four mutants significantly decreased in levels of IL-10 release and Arg1 protein expression relative to those in the rBB/wt group ( Figure 7C,F). Similarly, the mRNA level of iNOS is consistent with the result of the Western blot, but the Arg1 mRNA level barely changed in each mutant viral group ( Figure 7D,E).

Effect of Mutant Viruses on Macrophage Polarization from M1 to M2 by sCD83
To determine whether mutant viruses affect macrophage polarization, MoDCs were infected with the four mutant viruses at an MOI of 1. Twenty-four hours after infection, cells were co-cultured with PAMs and analyzed by ELISA, RT-qPCR, and Western blot. As shown in Figure 7A, the mutants decreased their ability to secrete sCD83 compared with the parental virus rBB/wt. Levels of TNF-α release and iNOS protein expression were higher in untreated cells infected by rN-43-2A, rnsp10-2m, rnsp1α-3m, and rN/nsp10 than cells infected by wild-type rBB/wt ( Figure 7B,F). In contrast, the four mutants significantly decreased in levels of IL-10 release and Arg1 protein expression relative to those in the rBB/wt group ( Figure 7C,F). Similarly, the mRNA level of iNOS is consistent with the result of the Western blot, but the Arg1 mRNA level barely changed in each mutant viral group ( Figure 7D,E).

Discussion
Macrophages were identified as phagocytic cells, having a well-defined role in the host's response including eliminating pathogens, inflammation, metabolic diseases, autoimmune diseases, and acute lung injury [22][23][24][25][26][27]29,30]. Macrophages commonly change into two forms: the classically activated (M1) macrophages (inflammatory monocytes) or the alternatively activated (M2) macrophages (anti-inflammatory monocytes). In different pathophysiological conditions and surrounding microenvironments, macrophages phenotypically switch between M1 and M2, i.e., macrophage polarization [23,24]. The Th1 cytokine IFN-γ and the ligand of TLR4, lipopolysaccharide (LPS), polarize monocytes toward classically activated (M1) macrophages and possess antiviral activity [24]. M2 statuses represent cell activation statuses polarized by interleukin (IL)-4/IL-13, which are secreted by Th2 cells and related to immune tolerance [23]. Consequentially, M1 phenotype macrophages express higher levels of pro-inflammatory mediators including granulocyte-macrophage colony-stimulating factor (GM-CSF), TNF-α, IL-1, IL-6, and IL-12 and preferentially express inducible nitricoxide synthase (iNOS; NOS2) [32]. Conversely, M2 macrophages produce abundant anti-inflammatory cytokines, such as IL-10, and exhibit a high expression of arginase 1 (Arg-1) [33]. Cumulative research has shown that macrophage polarization is required for many crucial processes of viral infection. COVID-19 could induce STAT-3 expression and then contribute to the development of M2-like macrophages, which could regulate viral replication [47]. HIV pronounced and prolonged the shift from the M1 to M2 macrophages program and elevated the level of IL-10, thus contributing to the failure of all immunological functions and clinical collapse [48]. The New World arenavirus Junin (JUNV) P strain induced higher levels of IRF-1, SOCS1, and SOCS3, triggering macrophage activation and eliciting a more anti-inflammatory M2 response, which may allow JUNV to evade immune detection [49]. HCV core protein interacts with TLR2/STATs signaling, leading to dysfunction of monocyte polarization toward both M1 and M2 macrophages, while intervening on macrophage-induced autologous and allogeneic CD4+ T cell activation [35]. Indeed, several recent studies have demonstrated that macrophages are polarized to M1/M2 type within the innate immune system, which is associated with viral infections and antiviral states.
Porcine reproductive and respiratory syndrome virus (PRRSV), a virulent pathogen of swine, suppresses the innate immune response and induces persistent infection [5,50,51]. More detailed reviews of host interactions with PRRSV conclude that most PRRSV strains delayed and impaired the adaptive immune response, inhibiting the release of pro-inflammatory cytokines, promoting anti-inflammatory cytokines, and modulating the activity and function of immune cells, including dendritic cells, macrophages, natural killer cells, T lymphocytes, and B lymphocytes [1,3,5,13,14]. PRRSV infection upregulates the expression of negative immune regulators including NF-k B inhibitors (NFKBIA, NFKBID, NFKBIZ, and TNFAIP3) and T-cell exhaustion markers (programmed death ligand-1 [PD-L1], PD-L2, interleukin-10 [IL-10], IDO1, and transforming growth factor b2 [TGFB2]) in PAMs to modulate the host's immune response [7]. PRRSV affects the phagocytic capacity of PAM, resulting from interactions with its entry mediators Sn and/or CD163 mediates the inhibitory effect of PRRSV on PAM phagocytosis [52]. PRRSV infection skews macrophage polarization toward the M2 phenotype, and then boosts IFN-and IL-12 secretion and TNF-a expression, which is connected with the transformation of A1 (g6Ld10T) and A2 (lipo-M5Nt) antigens [18].
CD83, a glycoprotein member of the Ig superfamily of receptors, usually came in one of two forms: membrane-bound CD83 (mCD83) and soluble CD83 (sCD83) [36]. mCD83 is expressed in a variety of cells, including mature dendritic cells (DCs), activated T cells and T regulatory cells, B cells, and macrophages [53]. Soluble CD83 (sCD83), as an immunosuppressive molecule, is cleaved from membrane-bound CD83 (mCD83) [15,38]. Studies have indicated that sCD83 is involved in negatively regulating the immune response. sCD83 binds directly to MD-2, the TLR4 co-receptor, leading to an induction of the anti-inflammatory mediators IDO and IL-10, inhibiting T cell proliferation, blocking IL-2 secretion, and rendering the downstream functions of T cells [54]. sCD83 modulates monocyte-DC differentiation and negatively influences immune regulation for DCmediated T cell immune responses [55]. In donor corneal allografts, sCD83 acts on responder cells, including Mϕ and DCs, induces tolerogenic APCs (macrophage/monocyte/DCs), decreases allogeneic T cell proliferation, hampers IL-6 and TNF-α secretion, and produces immunoregulatory mediators (IL-10, Ido1, TGF-β, and IL-27), thus leading to improved corneal graft acceptance [56]. sCD83 might play an important role in regulating the immune tolerance in the maternal-fetal interface of pregnant animals by inducing Foxp3þ Treg cell generation, promoting IL-10 and IL-4 expressions, inhibiting the expression of TNF-a and IL-6, and shifting the Th1/Th2 cytokine balance to Th2 dominance [15].
As previously noted, in our research we have found PRRSV stimulates the secretion of CD83 and increases CD83 mRNA levels, leading to a decreased expression of the MHCpeptide complex components TAP1 and ERp57 in MoDCs and inhibiting the ability of MoDCs to stimulate the proliferation of T cells [13]. These effects are derived from key amino acid sites (R43 and K44 in the N protein; L5A, D6A, G45A, G48A, L61A, P62A, R63A, F65A, and P66A in the ZF domain of Nsp1α; and the P192-5 and G214-3 domains in nsp10) in the PRRSV-induced release of sCD83 [13,14]. We speculated that PRRSV might influence the immune response through the microenvironment by regulating macrophages through sCD83 expression. To this end, how PRRSV infection causes the imbalance of macrophage polarization in severe respiratory infections in young pigs and reproductive disorders, and whether this is related to PRRSV-induced sCD83, remains unknown.
Accordingly, our data suggested that PRRSV inhibits the allostimulatory capacity of M1 macrophages but increases the allostimulatory capacity of M2 macrophages. In PAMs, PRRSV infection decreased TNF-α production but increased IL-10 release, as well as induced a high protein level of Arg1 but a low protein level of iNOS. Since sCD83 has an inhibitory effect on immune capacity, including inhibition of DC-mediated T cell stimulation and DC maturation, which can disturb the process of antigen presentation in innate immune cells (such as monocytes, macrophages, and DCs) and modulate the differentiation of immune cells [13][14][15]42]. We further confirmed the function of sCD83 in macrophage polarization. PAMs were exposed to sCD83 (0.5 ug/mL) and our data showed that sCD83 induced a switch in macrophage polarization from M1 to M2 in vitro. Our previous studies have shown that PRRSV infection induces CD83 expression in porcine MoDCs through the NF-κB and Sp1 signaling pathways. Key amino acid sites of viral N, nsp10, and nsp1 can stimulate sCD83 release and bring about immunosuppression in immune cells [13,14]. We further investigated whether PRRSV-induced switches in macrophage polarization from M1 to M2 is mediated through the secretion of sCD83. MoDCs were treated with anti-CD83 (0.5 ug/mL) in advance and then infected with PRRSV. The experimental results show that anti-sCD83 preconditioning restricted the conversion from M1 to M2. Similarly, the iNOS expression and TNF-α secretion were restored markedly, but Arg1 expression and IL-10 secretion was apparently diminished compared to the anti-IgG pre-treated group. Then, four mutant viruses with mutations of the sCD83-relative essential site in the N protein, nsp10, and nsp1 were rescued using cDNA infectious clones. The analysis indicated that all mutant viruses (rN-43-2A, rnsp10-2m, rnsp1α-3m, and rN/nsp10) specifically interfere with functions related to macrophage polarization to different degrees.
In conclusion, this study revealed that monocyte polarization toward both M1 and M2 macrophages is modulated by PRRSV infection through the excretion of sCD83, leading to the dysfunction of M1 macrophages and promotion of M1 macrophages. In PAMs, PRRSV regulates both pro-inflammatory (TNF-α) and anti-inflammatory (IL-10) processes, which contribute to the immune response. We propose that PRRSV-induced sCD83 might offer novel insights into the molecular mechanism of immune suppression by PRRSV.

Conclusions
Infection of MoDCs by PRRSV increases the release of soluble CD83 in particular. sCD83 strongly decreases the TNF-α release and iNOS expression of M1 macrophages but enhances IL-10 release and Arg1 expression of M2 macrophages. Viruses containing mutations in R43 and K44 of the N protein, the domains L5, D6, G45, G48, L61, P62, R63, F65, and P66 of Nsp1α, and the P192-196 and G214-216 domains of nsp10 do not affect sCD83 expression or impact the polarization of the macrophages (Figure 8). Informed Consent Statement: Not applicable.
Data Availability Statement: All data generated or analyzed during this study are included in this manuscript.