The RSVPreF3-AS01 vaccine elicits broad neutralization of contemporary and antigenically distant respiratory syncytial virus strains

The RSVPreF3-AS01 vaccine, containing the respiratory syncytial virus (RSV) prefusion F protein and the AS01 adjuvant, was previously shown to boost neutralization responses against historical RSV strains and to be efficacious in preventing RSV-associated lower respiratory tract diseases in older adults. Although RSV F is highly conserved, variation does exist between strains. Here, we characterized variations in the major viral antigenic sites among contemporary RSV sequences when compared with RSVPreF3 and showed that, in older adults, RSVPreF3-AS01 broadly boosts neutralization responses against currently dominant and antigenically distant RSV strains. RSV-neutralizing responses are thought to play a central role in preventing RSV infection. Therefore, the breadth of RSVPreF3-AS01-elicited neutralization responses may contribute to vaccine efficacy against contemporary RSV strains and those that may emerge in the future.

Risk Factors Associated with Severe RSV Infection in Infants: What Is the Role of Viral Co-Infections?

The respiratory syncytial virus (RSV) represents the leading cause of viral lower respiratory tract infections (LRTI) in children worldwide and is associated with significant morbidity and mortality rates. The clinical picture of an RSV infection differs substantially between patients, and the role of viral co-infections is poorly investigated. During two consecutive winter seasons from October 2018 until February 2020, we prospectively included children up to 2 years old presenting with an acute LRTI, both ambulatory and hospitalized. We collected clinical data and tested nasopharyngeal secretions for a panel of 16 different respiratory viruses with multiplex RT-qPCR. Disease severity was assessed with traditional clinical parameters and scoring systems. A total of 120 patients were included, of which 91.7% were RSV positive; 42.5% of RSV-positive patients had a co-infection with at least one other respiratory virus. We found that patients suffering from a single RSV infection had higher pediatric intensive care unit (PICU) admission rates (OR = 5.9, 95% CI = 1.53 to 22.74), longer duration of hospitalization (IRR = 1.25, 95% CI = 1.03 to 1.52), and a higher Bronchiolitis Risk of Admission Score (BRAS) (IRR = 1.31, 95% CI = 1.02 to 1.70) compared to patients with RSV co-infections. No significant difference was found in saturation on admission, O2 need, or ReSViNET-score. In our cohort, patients with a single RSV infection had increased disease severity compared to patients with RSV co-infections. This suggests that the presence of viral co-infections might influence the course of RSV bronchiolitis, but heterogeneity and small sample size in our study prevents us from drawing strong conclusions. IMPORTANCE RSV is worldwide the leading cause of serious airway infections. Up to 90% of children will be infected by the age of 2. RSV symptoms are mostly mild and typically mimic a common cold in older children and adolescents, but younger children can develop severe lower respiratory tract disease, and currently it is unclear why certain children develop severe disease while others do not. In this study, we found that children with a single RSV infection had a higher disease severity compared to patients with viral co-infections, suggesting that the presence of a viral co-infection could influence the course of an RSV bronchiolitis. As preventive and therapeutic options for RSV-associated disease are currently limited, this finding could potentially guide physicians to decide which patients might benefit from current or future treatment options early in the course of disease, and therefore, warrants further investigation.

Toll-Like Receptors: Are They Taking a Toll on the Heart in Viral Myocarditis?

Myocarditis is an inflammatory disease of the heart with viral infections being the most common aetiology. Its complex biology remains poorly understood and its clinical management is one of the most challenging in the field of cardiology. Toll-like receptors (TLRs), a family of evolutionarily conserved pattern recognition receptors, are increasingly known to be implicated in the pathophysiology of viral myocarditis. Their central role in innate and adaptive immune responses, and in the inflammatory reaction that ensues, indeed makes them prime candidates to profoundly affect every stage of the disease process. This review describes the pathogenesis and pathophysiology of viral myocarditis, and scrutinises the role of TLRs in every phase. We conclude with directions for future research in this field.

A systematic literature review on the effects of exercise on human Toll-like receptor expression

BACKGROUND

Toll-like receptors (TLRs) are a family of transmembrane pattern recognition receptors that are mainly expressed on immune cells. Recognition of various exogenous and endogenous molecular patterns activates the TLR signalling cascade, which orchestrates an inflammatory immune response. Dysfunctional immune responses, including aberrant TLR signalling, are increasingly implicated in the associations between sedentarism, chronic low-grade systemic inflammation and various non-communicable diseases. Conversely, exercise exerts anti-inflammatory effects, which could be conferred through its immunomodulatory properties, potentially affecting TLRs. This study aims to systematically review the effects of exercise on human TLR expression.

METHOD

A systematic literature search of Pubmed, Embase, The Cochrane Library and SPORTDiscus for articles addressing the impact of exercise (as isolated intervention) on TLRs in humans was conducted, ending in February 2020.

RESULTS

A total of 66 articles were included. The publications were categorised according to exercise modality and duration: acute resistance exercise (4 studies), acute aerobic exercise (26 studies), resistance training program (9 studies), aerobic training program (16 studies), combined (i.e. resistance and aerobic) training program (8 studies) and chronic exercise not otherwise classifiable (9 studies). Five articles investigated more than one of the aforementioned exercise categories. Several trends could be discerned with regard to the TLR response in the different exercise categories. Acute resistance exercise seemed to elicit TLR upregulation, whereas acute aerobic exercise had less activating potential with the majority of responses being neutral or, especially in healthy participants, downregulatory. Chronic resistance and combined exercise programs predominantly resulted in unaltered or decreased TLR levels. In the chronic aerobic exercise category, mixed effects were observed, but the majority of measurements demonstrated unchanged TLR expression.

CONCLUSION

Currently published research supports an interplay between exercise and TLR signalling, which seems to depend on the characteristics of the exercise. However, there was large heterogeneity in the study designs and methodologies. Therefore, additional research is required to further corroborate these findings, to define its pathophysiological implications and to elucidate the mechanism(s) linking exercise to TLR signalling.

Respiratory syncytial virus (RSV) entry is inhibited by serine protease inhibitor AEBSF when present during an early stage of infection

Background

Host proteases have been shown to play important roles in many viral activities such as entry, uncoating, viral protein production and disease induction. Therefore, these cellular proteases are putative targets for the development of antivirals that inhibit their activity. Host proteases have been described to play essential roles in Ebola, HCV, HIV and influenza, such that specific protease inhibitors are able to reduce infection. RSV utilizes a host protease in its replication cycle but its potential as antiviral target is unknown. Therefore, we evaluated the effect of protease inhibitors on RSV infection.

Methods

To measure the sensitivity of RSV infection to protease inhibitors, cells were infected with RSV and incubated for 18 h in the presence or absence of the inhibitors. Cells were fixed, stained and studied using fluorescence microscopy.

Results

Several protease inhibitors, representing different classes of proteases (AEBSF, Pepstatin A, E-64, TPCK, PMSF and aprotinin), were tested for inhibitory effects on an RSV A2 infection of HEp-2 cells. Different treatment durations, ranging from 1 h prior to inoculation and continuing for 18 h during the assay, were evaluated. Of all the inhibitors tested, AEBSF and TPCK significantly decreased RSV infection. To ascertain that the observed effect of AEBSF was not a specific feature related to HEp-2 cells, A549 and BEAS-2B cells were also used. Similar to HEp-2, an almost complete block in the number of RSV infected cells after 18 h of incubation was observed and the effect was dose-dependent. To gain insight into the mechanism of this inhibition, AEBSF treatment was applied during different phases of an infection cycle (pre-, peri- and post-inoculation treatment). The results from these experiments indicate that AEBSF is mainly active during the early entry phase of RSV. The inhibitory effect was also observed with other RSV isolates A1998/3–2 and A2000/3–4, suggesting that this is a general feature of RSV.

Conclusion

RSV infection can be inhibited by broad serine protease inhibitors, AEBSF and TPCK. We confirmed that AEBSF inhibition is independent of the cell line used or RSV strain. The time point at which treatment with the inhibitor was most potent, was found to coincide with the expected moment of entry of the virion with the host cell.

Development and Characterization of New Species Cross-Reactive Anti-Sialoadhesin Monoclonal Antibodies

Sialoadhesin (Sn) is a surface receptor expressed on a subset of macrophages in steady state conditions. During inflammation and diseases, Sn is highly upregulated on macrophages and blood monocytes. Therefore, therapies using monoclonal antibodies (mAbs) to target Sn-positive (Sn⁺) cells are a potential strategy for targeted treatment. It has been shown that Sn internalizes after binding with a mAb, though it is not clear whether this is species-specific. In this study, new Sn-specific mAbs were developed and analyzed for cross-reactivity between species. In addition, the newly developed mAbs were compared to mAbs used in previous research for their epitope recognition and other Sn-specific characteristics. Both species-specific and cross-reactive antibodies could be identified. Furthermore, sialic acid-binding of red blood cells (RBC) could be inhibited with mAbs recognizing different epitopes and all mAb showed internalization of Sn. The newly developed mAbs can be used as novel tools for Sn research and further analysis of Sn internalization in different species.

Evaluation of viral peptide targeting to porcine sialoadhesin using a porcine reproductive and respiratory syndrome virus vaccination-challenge model

Targeting antigens to professional antigen presenting cells resident at the sites where effective immune responses are generated is a promising vaccination strategy. As such, targeting sialoadhesin (Sn)-expressing macrophages, abundantly present in spleen and lymph nodes where they appear to be strategically placed for antigen capture and processing, is recently gaining increased attention. Previously, we have shown that humoral immune responses to the model antigen human serum albumin can be enhanced by using a porcine Sn-specific monoclonal antibody to target the model antigen to Sn-expressing macrophages. To date however, no studies have been performed to evaluate whether targeted delivery of a pathogen-derived antigen can enhance the pathogen-specific immune response. Therefore, we selected a linear epitope on glycoprotein 4 of porcine reproductive and respiratory syndrome virus (PRRSV), which is known to be a target of virus-neutralizing antibodies. This paper reports on the targeted delivery of this viral peptide to porcine Sn-expressing macrophages and the evaluation of the subsequent immune response in a vaccination-challenge set-up. Four copies of the selected PRRSV epitope were genetically fused to a previously developed porcine Sn-targeting recombinant antibody or an irrelevant isotype control. Fusion proteins were shown to be efficiently purified from HEK293T cell supernatants and subsequently, only Sn-specific fusion proteins were shown to bind to and to be internalized into Sn-expressing cells. Subsequent immunizations with a single dose of the fusion proteins showed that peptide-specific immune responses and neutralizing antibody responses after PRRSV challenge were enhanced in animals receiving a single 500 μg intramuscular dose of the Sn-targeting fusion protein, although correlations between the two read-outs were hard to effectuate. Furthermore, a minor beneficial effect on viral clearance was observed. Together, these data show that viral peptide targeting to porcine Sn-expressing macrophages can improve the anti-viral immune response, although more research will be needed to further explore vaccination potential.

Development of a recombinant antibody to target peptides and proteins to sialoadhesin-expressing macrophages

Background

Sialoadhesin (Sn)-expressing monocytes/macrophages have been associated with several diseases like inflammatory and autoimmune disorders as well as viral infections, and they also appear to play a role in the initiation of an adaptive immune response. This makes Sn-expressing cells not only attractive targets for cell-directed therapies, but also an appealing target for vaccination. Furthermore, since Sn was shown to be an endocytic receptor, the conjugation of effector molecules to an Sn-specific ligand should allow intracellular delivery of these conjugates. Previously, we developed functional Sn-specific immunoconjugates that were generated via chemical coupling. Although successful, the system requires significant optimization for each immunoconjugate to be made. To generate a more flexible and controlled system, we developed a recombinant antibody vector allowing the creation of genetic antibody fusion constructs. This paper reports on the characterization of the recombinant antibody and the evaluation of its use for Sn-directed targeting.

Results

The variable domains of the porcine Sn-specific monoclonal antibody 41D3 were sequenced and cloned in frame with a mouse IgG1 backbone. Transfection of HEK293T cells with the resulting plasmid led to the secretion of fully assembled IgG into the culture medium. This recombinant antibody rec41D3 was shown to specifically bind to porcine Sn with a comparable affinity as the native monoclonal antibody. In addition, rec41D3 also induced Sn endocytosis in primary macrophages and resided for prolonged times in early/late endosomes. To allow the generation of antibody fusion constructs, a multiple cloning site was introduced at the C-terminus of the heavy chain. Two fusion constructs were generated, one containing a V5 peptide tag and one containing an eGFP molecule. Both constructs were shown to be efficiently produced in HEK293T cells and easily purified using standard protein G chromatography. In addition, both V5 and eGFP were shown to be co-internalized together with rec41D3 into Sn-expressing primary macrophages.

Conclusions

A recombinant antibody allowing targeted delivery of peptides and proteins to Sn-expressing macrophages was developed. Production and purification of antibody fusion constructs was possible without major optimization and with batch to batch consistency, confirming the development of a versatile antibody vector to evaluate Sn-directed targeting strategies in a porcine animal model.

Porcine sialoadhesin: a newly identified xenogeneic innate immune receptor

Extracorporeal porcine liver perfusion is being developed as a bridge to liver allotransplantation for patients with fulminant hepatic failure. This strategy is limited by porcine Kupffer cell destruction of human erythrocytes, mediated by lectin binding of a sialic acid motif in the absence of antibody and complement. Sialoadhesin, a macrophage restricted lectin that binds sialic acid, was originally described as a sheep erythrocyte binding receptor. Given similarities between sialoadhesin and the unidentified macrophage lectin in our model, we hypothesized porcine sialoadhesin contributed to recognition of human erythrocytes. Two additional types of macrophages were identified to bind human erythrocytes-spleen and alveolar. Expression of sialoadhesin was confirmed by immunofluorescence in porcine tissues and by flow cytometry on primary macrophages. A stable transgenic cell line expressing porcine sialoadhesin (pSn CHO) bound human erythrocytes, while a sialoadhesin mutant cell line did not. Porcine macrophage and pSn CHO recognition of human erythrocytes was inhibited approximately 90% by an antiporcine sialoadhesin monoclonal antibody and by human erythrocyte glycoproteins. Furthermore, this binding was substantially reduced by sialidase treatment of erythrocytes. These data support the hypothesis that porcine sialoadhesin is a xenogeneic receptor that mediates porcine macrophage binding of human erythrocytes in a sialic acid-dependent manner.

Single amino acid mutations in the capsid switch the neutralization phenotype of porcine circovirus 2

Porcine circovirus 2 (PCV2) is the causative agent of porcine circovirus-associated diseases in pigs. Previously, it was demonstrated that mAbs 16G12, 38C1, 63H3 and 94H8 directed against the PCV2 capsid protein recognize PCV2 strains Stoon-1010 (PCV2a), 48285 (PCV2b), 1121 (PCV2a), 1147 (PCV2b) and II9F (PCV2b), but only neutralize Stoon-1010 and 48285. This points to the existence of two distinct PCV2 neutralization phenotypes: phenotype α (mAb recognition with neutralization; Stoon-1010 and 48285) and phenotype β (mAb recognition without neutralization; 1121, 1147 and II9F). In the present study, amino acids that are important in determining the neutralization phenotype were identified in the capsid. Mutation of T at position 190 to A in strain 48285 (phenotype α) resulted in a capsid resembling that of strain 1147 (phenotype β) and caused a loss of neutralization (switch from α to β). Mutations of P at position 151 to T and A at position 190 to T in strain II9F (phenotype β) resulted in a capsid resembling that of strain 48285 (phenotype α) and gave a gain of neutralization (switch from β to α). Mutations of T at position 131 to P and of E at position 191 to R in Stoon-1010 (phenotype α) changed the capsid into that of 1121 (phenotype β) and reduced neutralization (switch from α to β). This study demonstrated that single amino acid changes in the capsid result in a phenotypic switch from α to β or β to α.

Interaction of the European genotype porcine reproductive and respiratory syndrome virus (PRRSV) with sialoadhesin (CD169/Siglec-1) inhibits alveolar macrophage phagocytosis

Porcine reproductive and respiratory syndrome virus (PRRSV) is an arterivirus that shows a restricted in vivo tropism for subsets of porcine macrophages, with alveolar macrophages being major target cells. The virus is associated with respiratory problems in pigs of all ages and is commonly isolated on farms with porcine respiratory disease complex (PRDC). Due to virus-induced macrophage death early in infection, PRRSV hampers the innate defence against pathogens in the lungs. In addition, the virus might also directly affect the antimicrobial functions of macrophages. This study examined whether interaction of European genotype PRRSV with primary alveolar macrophages (PAM) affects their phagocytic capacity. Inoculation of macrophages with both subtype I PRRSV (LV) and subtype III PRRSV (Lena) showed that the virus inhibits PAM phagocytosis. Similar results were obtained using inactivated PRRSV (LV), showing that initial interaction of the virion with the cell is sufficient to reduce phagocytosis, and that no productive infection is required. When macrophages were incubated with sialoadhesin- (Sn) or CD163-specific antibodies, two entry mediators of the virus, only Sn-specific antibodies downregulated the phagocytic capacity of PAM, indicating that interaction with Sn, but not CD163, mediates the inhibitory effect of PRRSV on phagocytosis. In conclusion, this study shows that European genotype PRRSV inhibits PAM phagocytosis in vitro, through the interaction with its internalization receptor Sn. If similar events occur in vivo, this interaction may be important in the development of PRDC, as often seen in the field.

Antibody binding to porcine sialoadhesin reduces phagocytic capacity without affecting other macrophage effector functions

Sialoadhesin (Sn) is a macrophage-restricted endocytic receptor involved in cell-cell, cell-matrix and cell-pathogen interactions. Recently, porcine Sn (pSn) was shown to be involved in signaling and lately Sn is gaining interest as a potential target for immunotherapy. However, little is known about the effect of ligand binding to Sn on macrophage effector functions. In this study, we tested the effect of antibody binding to pSn on macrophage viability, phagocytosis of microspheres, uptake and processing of soluble antigens, reactive oxygen/nitrogen species production, MHC I and MHC II cell surface expression and cytokine production. This was done by treatment of porcine primary alveolar macrophages with the pSn-specific mAb 41D3, or an isotype-matched control mAb. No significant effect on most effector functions under study was observed, except for a significant reduction of phagocytosis. Thus, antibody binding to pSn can downregulate phagocytosis, which could have implications on homeostasis, infectious and immune diseases, and immunotherapy.

Characterization of a circulating PRRSV strain by means of random PCR cloning and full genome sequencing

PRRS is a pig disease of major economic importance that causes respiratory and reproductive problems in pigs. Over the last years it has become clear that PRRSV heterogeneity is increasing. Consequently, this has a potential impact on diagnosis and strategies to counter this disease. The use of sequence-independent PCR techniques for the detection and characterization of PRRSV could be useful to bypass problems associated with the heterogeneity of this virus. A random PCR cloning approach was tested for the characterization of PRRSV strain 07V063 of unknown genetic background that circulated on a Belgian farm. By using this approach, 7305 bp of sequence data were obtained, distributed randomly across the genome. Using RT-PCR with strain-specific primers, the full length sequence (15014 nt) was obtained. Phylogenetic relationships using ORF5 and ORF1a (NSP2) sequences showed that 07V063 was classified in type 1 subtype 1 and that 07V063 was genetically different from prototype Lelystad Virus (LV). 07V063 showed 87-93% aa identity with LV ORFs coding for structural proteins. Most variation (compared to LV) was noticed in Nsp2 (81% identity) with a deletion of 28 aa. This deletion was different from other known deletions in this ORF. In conclusion, it is shown that this random PCR cloning approach can be used for the characterization of new PRRSV strains of unknown genetic background.

Porcine sialoadhesin (CD169/Siglec-1) is an endocytic receptor that allows targeted delivery of toxins and antigens to macrophages

Sialoadhesin is exclusively expressed on specific subpopulations of macrophages. Since sialoadhesin-positive macrophages are involved in inflammatory autoimmune diseases, such as multiple sclerosis, and potentially in the generation of immune responses, targeted delivery of drugs, toxins or antigens via sialoadhesin-specific immunoconjugates may prove a useful therapeutic strategy. Originally, sialoadhesin was characterized as a lymphocyte adhesion molecule, though recently its involvement in internalization of sialic acid carrying pathogens was shown, suggesting that sialoadhesin is an endocytic receptor. In this report, we show that porcine sialoadhesin-specific antibodies and F(ab')₂ fragments trigger sialoadhesin internalization, both in primary porcine macrophages and in cells expressing recombinant porcine sialoadhesin. Using chemical inhibitors, double immunofluorescence stainings and dominant-negative constructs, porcine sialoadhesin internalization was shown to be clathrin- and Eps15-dependent and to result in targeting to early endosomes but not lysosomes. Besides characterizing the sialoadhesin endocytosis mechanism, two sialoadhesin-specific immunoconjugates were evaluated. We observed that porcine sialoadhesin-specific immunotoxins efficiently kill sialoadhesin-expressing macrophages. Furthermore, porcine sialoadhesin-specific albumin immunoconjugates were shown to be internalized in macrophages and immunization with these immunoconjugates resulted in a rapid and robust induction of albumin-specific antibodies, this compared to immunization with albumin alone. Together, these data expand sialoadhesin functionality and show that it can function as an endocytic receptor, a feature that cannot only be misused by sialic acid carrying pathogens, but that may also be used for specific targeting of toxins or antigens to sialoadhesin-expressing macrophages.

Susceptible cell lines for the production of porcine reproductive and respiratory syndrome virus by stable transfection of sialoadhesin and CD163

Background

Porcine reproductive and respiratory syndrome virus (PRRSV) causes major economic losses in the pig industry worldwide. In vivo, the virus infects a subpopulation of tissue macrophages. In vitro, PRRSV only replicates in primary pig macrophages and African green monkey kidney derived cells, such as Marc-145. The latter is currently used for vaccine production. However, since virus entry in Marc-145 cells is different compared to entry in primary macrophages, specific epitopes associated with virus entry could potentially alter upon growth on Marc-145 cells. To avoid this, we constructed CHO and PK15 cell lines recombinantly expressing the PRRSV receptors involved in virus entry into macrophages, sialoadhesin (Sn) and CD163 (CHO^Sn-CD163 and PK15^Sn-CD163) and evaluated their potential for production of PRRSV.

Results

Detailed analysis of PRRSV infection revealed that LV and VR-2332 virus particles could attach to and internalize into the CHO^Sn-CD163 and PK15^Sn-CD163 cells. Initially, this occurred less efficiently for macrophage grown virus than for Marc-145 grown virus. Upon internalization, disassembly of the virus particles was observed. The two cell lines could be infected with PRRSV strains LV and VR-2332. However, it was observed that Marc-145 grown virus infected the cells more efficiently than macrophage grown virus. If the cells were treated with neuraminidase to remove cis-acting sialic acids that hinder the interaction of the virus with Sn, the amount of infected cells with macrophage grown virus increased. Comparison of both cell lines showed that the PK15^Sn-CD163 cell line gave in general better results than the CHO^Sn-CD163 cell line. Only 2 out of 5 PRRSV strains replicated well in CHO^Sn-CD163 cells. Furthermore, the virus titer of all 5 PRRSV strains produced after passaging in PK15^Sn-CD163 cells was similar to the virus titer of those strains produced in Marc-145 cells. Analysis of the sequence of the structural proteins of original virus and virus grown for 5 passages on PK15^Sn-CD163 cells showed either no amino acid (aa) changes (VR-2332 and 07V063), one aa (LV), two aa (08V194) or three aa (08V204) changes. None of these changes are situated in known neutralizing epitopes.

Conclusions

A PRRSV susceptible cell line was constructed that can grow virus to similar levels compared to currently available cell lines. Mutations induced by growth on this cell lines were either absent or minimal and located outside known neutralizing epitopes. Together, the results show that this cell line can be used to produce vaccine virus and for PRRSV virus isolation.

Porcine reproductive and respiratory syndrome virus entry into the porcine macrophage

Porcine reproductive and respiratory syndrome virus (PRRSV) emerged in the late 1980s and rapidly became one of the most significant viral pathogens in the swine industry. In vivo, the virus shows a very narrow cell tropism and targets specific subsets of porcine macrophages. The entry of PRRSV into its host cell is the first crucial step in infection and has been the focus of many fundamental studies. This review provides a comprehensive overview of the current knowledge on PRRSV entry into the porcine macrophage, covering virus binding, internalization and genome release, and integrates these findings into a general model of the entry process.

Scavenger receptor CD163, a Jack-of-all-trades and potential target for cell-directed therapy

Scavenger receptor CD163 contains nine scavenger receptor cysteine-rich (SRCR) domains and because of the presence of this ancient and highly conserved protein motif, CD163 belongs to the SRCR superfamily. Expression of CD163 is restricted to cells of the monocyte/macrophage lineage and is tightly regulated, with a general tendency of anti-inflammatory signals to induce CD163 synthesis, while pro-inflammatory signals rather seem to downregulate CD163 expression. The first-identified and most-studied function of CD163 is related to its capacity to bind and internalize haemoglobin-haptoglobin (HbHp) complexes. Later on, its functional repertoire was expanded, with the identification of CD163 as an erythroblast adhesion receptor, a receptor for tumour necrosis factor-like weak inducer of apoptosis (TWEAK), as well as a receptor for distinct pathogens encompassing bacteria and viruses. Interaction of one of these ligands with CD163 might result in receptor-mediated endocytosis, but might as well trigger a signalling cascade leading to the secretion of signalling molecules, which implicates that CD163 also acts as an immunomodulator. Not only the membrane-bound form of CD163 has an immunomodulating capacity, but also soluble CD163, which is generated via ectodomain shedding, is able to exert anti-inflammatory effects. Furthermore, the concentration of this soluble protein is significantly increased under specific pathological conditions, making it a useful marker protein for certain diseases. Finally, its restricted expression pattern and potential to internalize make CD163 an attractive candidate as gateway for cell-directed (immuno)therapy. This review aims to summarize current knowledge on CD163's biology and its different biological functions beyond HbHp scavenging, thereby mainly focussing on the more recently discovered ones. Furthermore, current data supporting the capacity of CD163 to serve as a diagnostic marker in certain diseases and its potential as a target molecule for cell-directed therapy are surveyed.

Detection of truncated circular DNA species in Escherichia coli with a PCV2-containing plasmid with a single PCV2 origin of replication

OBJECTIVE

Porcine circovirus type 2 (PCV2) is a small circular single-stranded DNA virus that causes postweaning multisystemic wasting syndrome in pigs. Cloning of the PCV2 genome in a plasmid allows the construction of infectious clones. Our objective was to clone single PCV2 genomes from an isolate containing a mixture of strains, in a plasmid in order to obtain pure PCV2 strains.

METHODS

PCR amplification of PCV2 genomes and cloning followed by restriction enzyme analysis and sequencing. Transfection and PCV2 titration on PK-15 cells.

RESULTS

Single-copy PCV2 genomes from three Belgian PCV2 strains were cloned. Unexpectedly, agarose gel analysis revealed that additional circular DNA species were generated in Escherichia coli. Restriction enzyme analysis and sequencing showed that the circular DNA species were truncated and derived from the plasmid containing the PCV2 genome. Mutagenesis of the PCV2 replicase gene abolished the formation of these DNA species. The infectious clones were transfected in PK-15 cells and pure PCV2 viral strains were obtained.

CONCLUSION

Infectious clones were obtained that can be used for antigenic mapping and mutagenesis. In addition, our findings suggest that the replicase protein was expressed in E. coli and involved in the generation of the truncated DNA species.

Assessing the functionality of viral entry-associated domains of porcine reproductive and respiratory syndrome virus during inactivation procedures, a potential tool to optimize inactivated vaccines

Porcine reproductive and respiratory syndrome virus (PRRSV) causes severe economic losses in the pig industry worldwide. Currently, vaccines based on inactivated PRRSV provide limited protection of pigs against infection, most likely because viral epitopes associated with the induction of neutralizing antibodies are not or poorly conserved during inactivation. To analyze the effect of inactivation procedures on the interaction of PRRSV with receptors involved in virus entry, a new assay was set up in this study. Viral entry-associated domains are most likely important for the induction of neutralizing antibodies, since neutralizing antibodies block interaction of PRRSV with cellular receptors. To investigate the interaction of PRRSV with the cellular receptors upon different inactivation procedures, attachment to and internalization of inactivated PRRSV into macrophages were monitored. AT-2 could not inactivate PRRSV completely and is therefore not useful for vaccine development. PRRSV inactivated with ultraviolet light, binary ethyleneimine and gamma irradiation, which all mainly have an effect at the genomic level, showed no difference compared to control live virus at all levels of virus entry, whereas PRRSV treated with formaldehyde, glutaraldehyde and pH changes, which all have a modifying effect on proteins, was not able to internalize into macrophages anymore. These results suggest that inactivation with methods with a main effect on the viral genome preserve PRRSV entry-associated domains and are useful for future development of an effective inactivated vaccine against PRRSV. Although PRRSV incubation at 37 degrees C can completely inactivate PRRSV with preservation of entry-associated domains, this method is not recommended for vaccine development, since the mechanism is yet unknown.

Functional impairment of PRRSV-specific peripheral CD3+CD8high cells

The replication of porcine reproductive and respiratory syndrome virus (PRRSV) in lungs and lymphoid tissues of PRRSV-infected pigs is already strongly reduced before the appearance of neutralizing antibodies, indicating that other immune mechanisms are involved in eliminating PRRSV at those sites. This study aimed to determine whether PRRSV Lelystad virus (LV)-specific cytotoxic T-lymphocytes (CTL) can efficiently eliminate PRRSV-infected alveolar macrophages. Therefore, CTL assays were performed with PRRSV-infected alveolar macrophages as target cells and autologous peripheral blood mononuclear cells (PBMC) from PRRSV-infected pigs as a source of PRRSV-specific CTL. PBMC of 3 PRRSV-infected pigs were used either directly in CTL assays, or following restimulation in vitro. CTL assays with pseudorabies virus (PRV) Begonia-infected alveolar macrophages and autologous PBMC, from 2 PRV Begonia-inoculated pigs, were performed for validation of the assays. In freshly isolated PBMC, derived from PRRSV-infected pigs, CTL activity towards PRRSV-infected macrophages was not detected until the end of the experiment (56 days post infection-dpi). Restimulating the PBMC with PRRSV in vitro resulted in proliferation of CD3+CD8high cells starting from 14 dpi. Although CD+CD8high cells are generally considered to be CTL, CTL activity was not detected in PRRSV-restimulated PBMC of the 3 pigs until 49 dpi. A weak PRRSV-specific CTL activity was observed only at 56 dpi in PRRSV-restimulated PBMC of one pig. In contrast, a clear CTL activity was observed in PRV Begonia-restimulated PBMC, derived from PRV Begonia-infected pigs, starting from 21 dpi. This study indicates that PBMC of PRRSV-infected pigs contain proliferating CD3+CD8high cells upon restimulation in vitro, but these PBMC fail to exert CTL activity towards PRRSV-infected alveolar macrophages.

Sialoadhesin and CD163 join forces during entry of the porcine reproductive and respiratory syndrome virus

The porcine reproductive and respiratory syndrome virus (PRRSV) shows a restricted tropism for subsets of porcine macrophages in vivo. To date, two PRRSV receptors have been identified on primary macrophages, heparan sulphate for binding and sialoadhesin for binding and internalization. However, additional factors are needed because the expression of both receptors in non-permissive cells results in virus internalization but not in virus uncoating and productive infection. Recently, CD163 was described as a PRRSV receptor on Marc-145 cells that renders non-permissive cells susceptible to PRRSV. Therefore, the potential role of CD163 in PRRSV entry in macrophages and its potential interplay with sialoadhesin were studied. Incubation of macrophages at 37 degrees C with either sialoadhesin- or CD163-specific antibodies reduced PRRSV infection by up to 75 %, while infection was completely blocked by a combination of both antibodies. When incubated at 4 degrees C, only sialoadhesin- and not CD163-specific antibodies reduced PRRSV infection. In addition, confocal analysis of PRRSV entry in non-permissive cells expressing only sialoadhesin showed PRRSV internalization but no uncoating. In contrast, when both sialoadhesin and CD163 were expressed, PRRSV was uncoated upon internalization, resulting in productive infection. Virus internalization was not observed when only CD163 was expressed; although, cells became productively infected. Thus, sialoadhesin is confirmed as a PRRSV internalization receptor and CD163 is shown to be involved in PRRSV entry, probably during uncoating. Co-expression of recombinant sialoadhesin and CD163 in non-permissive cells increased virus production 10-100 times compared with cells expressing only CD163, sustaining the requirement of both for efficient PRRSV infection.

Genome-wide transcriptional response of primary alveolar macrophages following infection with porcine reproductive and respiratory syndrome virus

Porcine reproductive and respiratory syndrome is a major cause of economic loss for the swine industry worldwide. Porcine reproductive and respiratory syndrome virus (PRRSV) triggers weak and atypical innate immune responses, but key genes and mechanisms by which the virus interferes with the host innate immunity have not yet been elucidated. In this study, genes that control the response of the main target of PRRSV, porcine alveolar macrophages (PAMs), were profiled in vitro with a time-course experiment spanning the first round of virus replication. PAMs were obtained from six piglets and challenged with the Lelystad PRRSV strain, and gene expression was investigated using Affymetrix microarrays and real-time PCR. Of the 1409 differentially expressed transcripts identified by analysis of variance, two, five, 25, 16 and 100 differed from controls by a minimum of 1.5-fold at 1, 3, 6, 9 and 12 h post-infection (p.i.), respectively. A PRRSV infection effect was detectable between 3 and 6 h p.i., and was characterized by a consistent downregulation of gene expression, followed by the start of the host innate immune response at 9 h p.i. The expression of beta interferon 1 (IFN-β), but not of IFN-α, was strongly upregulated, whilst few genes commonly expressed in response to viral infections and/or induced by interferons were found to be differentially expressed. A predominance of anti-apoptotic transcripts (e.g. interleukin-10), a shift towards a T-helper cell type 2 response and a weak upregulation of tumour necrosis factor-α expression were observed within 12 h p.i., reinforcing the hypotheses that PRRSV has developed sophisticated mechanisms to escape the host defence.

Involvement of proteases in porcine reproductive and respiratory syndrome virus uncoating upon internalization in primary macrophages

Porcine reproductive and respiratory syndrome virus (PRRSV) replicates in differentiated macrophages. In macrophages, heparan sulphate glycosaminoglycans mediate the initial PRRSV attachment and the receptor sialoadhesin mediates both PRRSV attachment and internalization into endosomes. Upon a pH drop, PRRSV is uncoated and its genome is released from the endosomes into the cytoplasm, which allows virus replication. However, expression of heparan sulphate and sialoadhesin in non-susceptible cells only allows virus internalization, but no virus uncoating and infection, indicating that other factors are involved. In the present study, it is shown that treatment of macrophages with serum (mainly the alpha-globulin fraction) inhibited PRRSV infection without affecting attachment and internalization. Because alpha-globulins contain several protease inhibitors, macrophages were treated with different protease inhibitors to investigate the involvement of proteases in PRRSV uncoating. Treatment of macrophages with broadly active inhibitors of serine or aspartic proteases, but not cysteine- or metallo-proteases, inhibited PRRSV uncoating and infection. Further investigation using specific inhibitors indicated that the aspartic protease cathepsin E is involved during PRRSV uncoating, but did not allow identification of the serine protease involved. The involvement of cathepsin E during PRRSV uncoating was confirmed by partial co-localization of internalized PRRSV with cathepsin E. Furthermore, cathepsin E expression increased with macrophage cultivation, which was positively correlated with an increased susceptibility to PRRSV infection. Together, these data show that, in macrophages, both the aspartic protease cathepsin E and an unidentified trypsin-like serine protease are involved in uncoating of internalized PRRSV and subsequent infection.

Antigenic differences among porcine circovirus type 2 strains, as demonstrated by the use of monoclonal antibodies

This study examined whether antigenic differences among porcine circovirus type 2 (PCV-2) strains could be detected using monoclonal antibodies (mAbs). A subtractive immunization protocol was used for the genotype 2 post-weaning multisystemic wasting syndrome (PMWS)-associated PCV-2 strain Stoon-1,010. Sixteen stable hybridomas that produced mAbs with an immunoperoxidase monolayer assay (IPMA) titre of 1,000 or more to Stoon-1,010 were obtained. Staining of recombinant PCV-2 virus-like particles demonstrated that all mAbs were directed against the PCV-2 capsid protein. Cross-reactivity of mAbs was tested by IPMA and neutralization assay for genotype 1 strains 48,285, 1,206, VC2,002 and 1,147, and genotype 2 strains 1,121 and 1,103. Eleven mAbs (9C3, 16G12, 21C12, 38C1, 43E10, 55B1, 63H3, 70A7, 94H8, 103H7 and 114C8) recognized all strains in the IPMA and demonstrated neutralization of Stoon-1,010, 48,285, 1,206 and 1,103, but not VC2,002, 1,147 and 1,121. mAbs 31D5, 48B5, 59C6 and 108E8 did not react with genotype 1 strains or had a reduced affinity compared with genotype 2 strains in the IPMA and neutralization assay. mAb 13H4 reacted in the IPMA with PMWS-associated strains Stoon-1,010, 48,285, 1,206 and VC2,002, and the porcine dermatitis and nephropathy syndrome-associated strain 1,147, but not with reproductive failure-associated strains 1,121 and 1,103. mAb 13H4 did not neutralize any of the tested strains. It was concluded that, despite the high amino acid identity of the capsid protein (>or=91 %), antigenic differences at the capsid protein level are present among PCV-2 strains with a different genetic and clinical background.

IFN-alpha treatment enhances porcine Arterivirus infection of monocytes via upregulation of the porcine Arterivirus receptor sialoadhesin

The Arterivirus porcine reproductive and respiratory syndrome virus (PRRSV) has a specific tropism for a subset of differentiated macrophages. Porcine sialoadhesin was identified as a PRRSV internalization receptor that is, similarly to sialoadhesins from other species, only expressed on subsets of macrophages. Sialoadhesin is not expressed or only expressed at low levels on monocytes, which might explain why monocytes are largely refractory to PRRSV infection. Different molecules have been identified that regulate human, mouse, or rat sialoadhesin expression in in vitro cultivated monocytes and macrophages, but the effect of these varies greatly between species. In this study, we observed that interferon-alpha (IFN-alpha) induces sialoadhesin expression on monocytes to levels similar as those on macrophages and that it increases sialoadhesin on macrophages. IFN-alpha-induced sialoadhesin expression was shown to be functional using a red blood cell (RBC) binding assay. Furthermore, a 2 or 3 day IFN-alpha pretreatment of monocytes caused a 20-fold increase in the numbers of PRRSV-infected monocytes and increased production of infectious virus. We conclude that IFN-alpha, although it is a potent antiviral molecule, upregulated sialoadhesin infection on in vitro cultivated monocytes, which results in enhanced PRRSV infection of monocytes.

Porcine arterivirus attachment to the macrophage-specific receptor sialoadhesin is dependent on the sialic acid-binding activity of the N-terminal immunoglobulin domain of sialoadhesin

The sialic acid-binding lectin sialoadhesin (Sn) is a macrophage-restricted receptor for porcine reproductive and respiratory syndrome virus (PRRSV). To investigate the importance of pSn sialic acid-binding activity for PRRSV infection, an R(116)-to-E mutation was introduced in the predicted sialic acid-binding domain of pSn, resulting in a mutant, pSn(RE), that could not bind sialic acids. PSn, but not pSn(RE), allowed PRRSV binding and internalization. These data show that the sialic acid-binding activity of pSn is essential for PRRSV attachment to pSn and thus identifies the variable, N-terminal domain of Sn as a PRRSV binding domain.

Antiviral activity of carbohydrate-binding agents against Nidovirales in cell culture

Coronaviruses are important human and animal pathogens, the relevance of which increased due to the emergence of new human coronaviruses like SARS-CoV, HKU1 and NL63. Together with toroviruses, arteriviruses, and roniviruses the coronaviruses belong to the order Nidovirales. So far antivirals are hardly available to combat infections with viruses of this order. Therefore, various antiviral strategies to counter nidoviral infections are under evaluation. Lectins, which bind to N-linked oligosaccharide elements of enveloped viruses, can be considered as a conceptionally new class of virus inhibitors. These agents were recently evaluated for their antiviral activity towards a variety of enveloped viruses and were shown in most cases to inhibit virus infection at low concentrations. However, limited knowledge is available for their efficacy towards nidoviruses. In this article the application of the plant lectins Hippeastrum hybrid agglutinin (HHA), Galanthus nivalis agglutinin (GNA), Cymbidium sp. agglutinin (CA) and Urtica dioica agglutinin (UDA) as well as non-plant derived pradimicin-A (PRM-A) and cyanovirin-N (CV-N) as potential antiviral agents was evaluated. Three antiviral tests were compared based on different evaluation principles: cell viability (MTT-based colorimetric assay), number of infected cells (immunoperoxidase assay) and amount of viral protein expression (luciferase-based assay). The presence of carbohydrate-binding agents strongly inhibited coronaviruses (transmissible gastroenteritis virus, infectious bronchitis virus, feline coronaviruses serotypes I and II, mouse hepatitis virus), arteriviruses (equine arteritis virus and porcine respiratory and reproductive syndrome virus) and torovirus (equine Berne virus). Remarkably, serotype II feline coronaviruses and arteriviruses were not inhibited by PRM-A, in contrast to the other viruses tested.

Porcine reproductive and respiratory syndrome virus-infected alveolar macrophages contain no detectable levels of viral proteins in their plasma membrane and are protected against antibody-dependent, complement-mediated cell lysis

Porcine reproductive and respiratory syndrome virus (PRRSV) can evade the host immune system, which results in prolonged virus replication for several weeks to several months. To date, the mechanisms of PRRSV immune evasion have not been investigated in detail. One possible immune-evasion strategy is to avoid incorporation of viral proteins into the plasma membrane of infected cells, as this prevents recognition by virus-specific antibodies and consequent cell lysis either by the classical complement pathway or by antibody-dependent, cell-mediated cytotoxicity. In this study, viral proteins were not observed in the plasma membrane of in vitro-infected macrophages by using confocal microscopy or flow cytometry. Subsequently, the sensitivity of PRRSV-infected macrophages towards antibody-dependent, complement-mediated cell lysis (ADCML) was determined by using an ADCML assay. A non-significant percentage of PRRSV-infected cells were killed in the assay, showing that in vitro PRRSV-infected macrophages are protected against ADCML. PRRSV proteins were not detected in the plasma membrane of in vivo-infected alveolar macrophages and ADCML was also not observed. Together, these data indicate that viral proteins are not incorporated into the plasma membrane of PRRSV-infected macrophages, which makes infected cells invisible to PRRSV-specific antibodies. This absence of viral proteins on the cell surface could explain the protection against ADCML observed for in vitro and in vivo PRRSV-infected macrophages, and may play a role in virus persistence.

Porcine circovirus 2 uses heparan sulfate and chondroitin sulfate B glycosaminoglycans as receptors for its attachment to host cells

Monocyte/macrophage lineage cells are target cells in vivo for porcine circovirus 2 (PCV2) replication. The porcine monocytic cell line 3D4/31 supports PCV2 replication in vitro, and attachment and internalization kinetics of PCV2 have been established in these cells. However, PCV2 receptors remain unknown. Glycosaminoglycans (GAG) are used by several viruses as receptors. The present study examined the role of GAG in attachment and infection of PCV2. Heparin, heparan sulfate (HS), chondroitin sulfate B (CS-B), but not CS-A, and keratan sulfate reduced PCV2 infection when these GAG were incubated with PCV2 prior to and during inoculation of 3D4/31 cells. Enzymatic removal of HS and CS-B prior to PCV2 inoculation of 3D4/31 cells significantly reduced PCV2 infection. Similarly, when PCV2 virus-like particles (VLP) were allowed to bind onto 3D4/31 cells in the presence of heparin and CS-B, attachment was strongly reduced. Titration of field isolates and low- and high-passage laboratory strains of PCV2 in the presence of heparin significantly reduced PCV2 titers, showing that the capacity of PCV2 to bind GAG was not acquired during in vitro cultivation but is an intrinsic feature of wild-type virus. When Chinese hamster ovary (CHO) cells were inoculated with PCV2, relative percentages of PCV2-infected cells were 27% +/- 8% for HS-deficient and 12% +/- 10% for GAG-deficient cells compared to wild-type cells (100%). Furthermore, it was shown using heparin-Sepharose chromatography that both PCV2 and PCV2 VLP directly interacted with heparin. Together, these results show that HS and CS-B are attachment receptors for PCV2.

Analysis of porcine reproductive and respiratory syndrome virus attachment and internalization: distinctive roles for heparan sulphate and sialoadhesin

Heparan sulphate and sialoadhesin were previously identified on porcine macrophages as receptors for porcine reproductive and respiratory syndrome virus (PRRSV). In this study, the exact role and cooperation of heparan sulphate and sialoadhesin during PRRSV attachment and internalization was analysed. It was observed that both heparan sulphate and sialoadhesin mediate PRRSV attachment and that only these two receptors are involved in attachment. Analysis of attachment kinetics of PRRSV to macrophages revealed that early attachment is mediated mainly via an interaction with heparan sulphate, followed by a gradual increase in interaction with sialoadhesin. By using wild-type CHO and CHO deficient in heparan sulphate expression, it was shown that heparan sulphate alone is sufficient to mediate PRRSV attachment, but not entry, and that heparan sulphate is not necessary for sialoadhesin to function as a PRRSV internalization receptor, but enhances the interaction of the virus with sialoadhesin.

Effect of virus-specific antibodies on attachment, internalization and infection of porcine reproductive and respiratory syndrome virus in primary macrophages

Porcine reproductive and respiratory syndrome virus (PRRSV) induces respiratory distress in young pigs and reproductive failure in sows. In PRRSV infected pigs, virus persists for several weeks to several months. Although IPMA antibodies are detected from 7 days post inoculation (pi), virus neutralizing (VN) antibodies are commonly detected starting from 3 weeks pi with an SN test on Marc-145 cells. Since infection of Marc-145 cells is quite different compared to infection of macrophages, the in vivo target cell, the role of these VN antibodies in in vivo protection is questionable. In our study, we demonstrated that antibodies from pigs early in infection with PRRSV Lelystad virus (14 days pi) showed no neutralization in the SN test on Marc-145 cells, but partially reduced Lelystad virus infection of porcine alveolar macrophages. At 72 days pi, VN antibodies were detected by the SN test on Marc-145 cells, and these protected macrophages completely against Lelystad virus infection. In contrast, these VN antibodies only partially reduced porcine alveolar macrophage infection of a Belgian PRRSV isolate (homologous virus), and had no effect on infection of porcine alveolar macrophages with the American type VR-2332 strain (heterologous virus). Confocal analysis of Lelystad virus attachment and internalization in macrophages showed that antibodies blocked infection through both a reduction in virus attachment, and a reduction of PRRSV internalization. Western immunoblotting analysis revealed that sera from 14 days pi, which showed no neutralization in the SN test on Marc-145 cells but partially reduced Lelystad virus infection of macrophages, predominantly recognized the Lelystad virus N protein, and reacted faintly with the M envelope protein. Sera from 72 days pi, with VN antibodies that blocked infection of Marc-145 cells and PAM, reacted with the N protein and the two major envelope proteins M and GP5. Using the Belgian PRRSV isolate 94V360 an identical but less intense reactivity profile was obtained. VN sera also recognized the VR-2332 N and M protein, but not the GP5 protein.

Porcine arterivirus infection of alveolar macrophages is mediated by sialic acid on the virus

Recently, we showed that porcine sialoadhesin (pSn) mediates internalization of the arterivirus porcine reproductive and respiratory syndrome virus (PRRSV) in alveolar macrophages (Vanderheijden et al., J. Virol. 77:8207-8215, 2003). In rodents and humans, sialoadhesin, or Siglec-1, has been described as a macrophage-restricted molecule and to specifically bind sialic acid moieties. In the current study, we investigated whether pSn is a sialic acid binding protein and, whether so, whether this property is important for its function as a PRRSV receptor. Using untreated and neuraminidase-treated sheep erythrocytes, we showed that pSn binds sialic acid. Furthermore, pSn-specific monoclonal antibody 41D3, which blocks PRRSV infection, inhibited this interaction. PRRSV attachment to and infection of porcine alveolar macrophages (PAM) were both shown to be dependent on the presence of sialic acid on the virus: neuraminidase treatment of virus but not of PAM blocked infection and reduced attachment. Enzymatic removal of all N-linked glycans on the virus with N-glycosidase F reduced PRRSV infection, while exclusive removal of nonsialylated N-linked glycans of the high-mannose type with endoglycosidase H had no significant effect. Free sialyllactose and sialic acid containing (neo)glycoproteins reduced infection, while lactose and (neo)glycoproteins devoid of sialic acids had no significant effect. Studies with linkage-specific neuraminidases and lectins indicated that alpha2-3- and alpha2-6-linked sialic acids on the virion are important for PRRSV infection of PAM. From these results, we conclude that pSn is a sialic acid binding lectin and that interactions between sialic acid on the PRRS virion and pSn are essential for PRRSV infection of PAM.

Involvement of sialoadhesin in entry of porcine reproductive and respiratory syndrome virus into porcine alveolar macrophages

Porcine reproductive and respiratory syndrome virus (PRRSV) shows a very restricted tropism for cells of the monocyte/macrophage lineage. It enters cells via receptor-mediated endocytosis. A monoclonal antibody (MAb) that is able to block PRRSV infection of porcine alveolar macrophages (PAM) and that recognizes a 210-kDa protein (p210) was described previously (MAb41D3) (X. Duan, H. Nauwynck, H. Favoreel, and M. Pensaert, J. Virol. 72:4520-4523, 1998). In the present study, the p210 protein was purified from PAM by immunoaffinity using MAb41D3 and was subjected to internal peptide sequencing after tryptic digestion. Amino acid sequence identities ranging from 56 to 91% with mouse sialoadhesin, a macrophage-restricted receptor, were obtained with four p210 peptides. Using these peptide data, the full p210 cDNA sequence (5,193 bp) was subsequently determined. It shared 69 and 78% amino acid identity, respectively, with mouse and human sialoadhesins. Swine (PK-15) cells resistant to viral entry were transfected with the cloned p210 cDNA and inoculated with European or American PRRSV strains. Internalized virus particles were detected only in PK-15 cells expressing the recombinant sialoadhesin, demonstrating that this glycoprotein mediated uptake of both types of strains. However, nucleocapsid disintegration, like that observed in infected Marc-145 cells as a result of virus uncoating after fusion of the virus with the endocytic vesicle membrane, was not observed, suggesting a block in the fusion process. The ability of porcine sialoadhesin to mediate endocytosis was demonstrated by specific internalization of MAb41D3 into PAM. Altogether, these results show that sialoadhesin is involved in the entry process of PRRSV in PAM.

Involvement of the matrix protein in attachment of porcine reproductive and respiratory syndrome virus to a heparinlike receptor on porcine alveolar macrophages

The porcine reproductive and respiratory syndrome virus (PRRSV) has a very restricted tropism for well-differentiated cells of the monocyte-macrophage lineage, which is probably determined by specific receptors on these cells. In this study, the importance of heparinlike molecules on porcine alveolar macrophages (PAM) for PRRSV infection was determined. Heparin interacted with the virus and reduced infection of PAM up to 92 or 88% for the American and European types of PRRSV, respectively. Other glycosaminoglycans, similar to heparin, had no significant effect on infection while heparinase treatment of PAM resulted in a significant reduction of the infection. Analysis of infection kinetics showed that PRRSV attachment to heparan sulfate occurs early in infection. A heparin-sensitive binding step was observed which converted completely into a heparin-resistant binding after 120 min at 4 degrees C. Using heparin-affinity chromatography and sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), it was observed that the structural matrix (M) and nucleocapsid (N) proteins attached to heparin. Nonreducing SDS-PAGE revealed that M bound to heparin mainly as a complex with glycoprotein GP(5) and that the N protein bound to heparin as a homodimer. GP(3), which was identified as a minor structural protein of European types of PRRSV, did not bind to heparin. Since the N protein is not exposed on the virion surface, it was concluded that the structural M protein and the M-GP(5) complex contribute to PRRSV attachment on a heparinlike receptor on PAM. This is the first report that identifies a PRRSV ligand for a cell surface heparinlike receptor on PAM.