Porcine Macrophage Markers and Populations: An Update.

Besides its importance as a livestock species, pig is increasingly being used as an animal model for biomedical research. Macrophages play critical roles in immunity to pathogens, tissue development, homeostasis and tissue repair. These cells are also primary targets for replication of viruses such as African swine fever virus, classical swine fever virus, and porcine respiratory and reproductive syndrome virus, which can cause huge economic losses to the pig industry. In this article, we review the current status of knowledge on porcine macrophages, starting by reviewing the markers available for their phenotypical characterization and following with the characteristics of the main macrophage populations described in different organs, as well as the effect of polarization conditions on their phenotype and function. We will also review available cell lines suitable for studies on the biology of porcine macrophages and their interaction with pathogens.

CD9 expression in porcine blood CD4+ T cells delineates two subsets with phenotypic characteristics of central and effector memory cells.

In this report, we describe the characterization of a new monoclonal antibody, named 4H5CR4, against porcine CD9. Its use in combination with antibodies to CD4, CD8α, and 2E3 allows to distinguish at least five main CD4+ T cell subsets. Analysis on these subsets of CD45RA, CD27, CD29, CD95, CCR7, and SLA-DR markers depicts a progressive model of CD4+ T cell development. CD4+ 2E3+ CD8α- CD9- cells are the least differentiated population of naïve cells, whereas the CD4+ 2E3- CD8α+CD9+ and CD4+ 2E3- CD8α+ CD9- cells display phenotypic features of central and effector memory T helper cells, respectively. The latter subsets were able to produce IFN-γ after polyclonal activation with PMA/Ionomycin; however, in vitro virus-specific IFN-γ production of PBMCs collected at 38-44 days after pseudorabies virus vaccination was dominated by cells with a CD9+ phenotype. Therefore, CD9 appears to be a useful marker to investigate CD4+ T cell heterogeneity in swine.

CD200R family receptors are expressed on porcine monocytes and modulate the production of IL-8 and TNF-α triggered by TLR4 or TLR7 in these cells.

The inhibitory receptor CD200R1 and its paired activating receptor CD200R1L are involved in the regulation of myeloid cell immune responses. The aim of this study was to analyze their distribution, regulation by cytokines, and function in porcine monocyte subsets. We had previously observed that CD200R1 and CD200R1L genes can generate different protein isoforms through alternative mRNA splicing, therefore in this study, we explored the diversity of transcripts in monocyte subsets, and described several new splicing variants of both CD200R1 and CD200R1L, some of which could be expressed on the porcine monocyte surface. A substantial proportion of CD163-SLAII+ and most CD163+SLAII+ monocytes expressed CD200R1 and CD200R1L receptors, while CD163-SLAII- monocytes did not. CD200R1 and CD200R1L expression was down-regulated in monocytes polarized by IFN-É£, a cytokine that induces classical activation of macrophages, while IL-10 which gives rise to regulatory macrophages, increased the expression of CD200R1. Finally, treatment of monocyte subsets with a monoclonal antibody specific for the inhibitory CD200R1 receptor and its splicing variants enhanced TNFα and IL-8 production, induced by TLR4 or TLR7 stimulation, suggesting a modulatory role for these receptors on porcine monocyte functions.

CD200R1 and CD200R1L expression is regulated during B cell development in swine and modulates the Ig production in response to the TLR7 ligand imiquimoid.

The CD200R family comprises a group of paired receptors that can modulate the activation of immune cells. They are expressed both on myeloid cells and lymphocyte subsets. Here we report that the expression of these receptors on porcine B cells is tightly regulated, being mainly expressed on mature cells. The expression of the inhibitory receptors CD200R1 and/or its splicing variant CD200R1X2, either in combination or not with the activating receptor CD200R1L, is upregulated in sIgM+ effector/memory cells, and tends to decline thereafter as these cells progress to plasmablasts or switch the Ig isotype. sIgM+ naïve and primed cells only express, by contrast, the CD200R1X2 receptor. B-1 like cells also express CD200R1 isoforms, either alone or in combination with CD200R1L. Treatment of peripheral blood mononuclear cells with a monoclonal antibody specific for inhibitory receptors, enhances the IgM and IgG production induced by TLR7 stimulation suggesting a modulatory role of B cell functions of these receptors.

Expression of CLEC4A in porcine tissues and leukocyte populations and characterization of mRNA splice variants.

CLECs are a group of molecules of the superfamily of C-type lectin domain containing receptors. Several receptors of this group have been described in humans and mice, as well as in other species. Many of them are expressed in immune cells, and have been shown to be involved in immune response modulation. Several molecules of this family have been proposed as targets for antigen delivery, which is what drew our interest in studying them in the pig. In this species only CLEC7A (also named Dectin-1) had been studied in some detail, and we have characterized two other members, CLEC12A and CLEC12B. Another molecule of interest was CLEC4A, for which we could not find any expression studies in pigs, although up to 5 different mRNA variants had been predicted by bioinformatics analysis of the genomic sequence in databases. Here we present an analysis of the expression of CLEC4A mRNA isoforms in tissues and blood cell subsets by PCR. Cloning and sequencing of cDNA of the different isoforms allowed us to identify all 5 predicted isoforms, in addition to characterizing 5 additional isoforms expressed in alveolar macrophages, of which the form coding for a canonical CLEC4A molecule was apparently the most abundant isoform. Analysis of the sequences of the 10 cDNA cloned allowed us to determine their exon usage and to identify their potential for protein coding. Only 8 of the characterized variants contain a recognizable coding sequence. The sequences of the coded proteins show both proteins with a putative transmembrane segment, and others without this feature, as well as proteins with the complete CLEC domain or with diverse truncated forms. We also prove that most of the cDNA can drive protein expression upon transfection in the CHO cell line. We discuss the relevance that the diversity of products of the porcine CLEC4A gene may have in the regulation of the expression and function of these molecules.

Characterization of the Porcine CLEC12A and Analysis of Its Expression on Blood Dendritic Cell Subsets.

CLEC12A has been proposed as a suitable target for delivering antigen to dendritic cells (DCs) to enhance vaccine efficacy both in human and mouse. In this study, we have characterized the porcine homolog of CLEC12A (poCLEC12A). Using new monoclonal antibodies (mAb), raised against its ectodomain, poCLEC12A was found to be expressed on alveolar macrophages, blood conventional type 1 and type 2 DCs and plasmacytoid DCs, but not on monocytes, T cells, B cells or NK cells, in contrast to its human and murine homologs. Western blot analysis showed that in alveolar macrophages this receptor is expressed both as a monomer and a dimer. After binding to DCs, anti- poCLEC12A mAb was efficiently internalized. No significant changes were observed in TNFα or IFNα secretion by plasmacytoid DCs stimulated with either CpGs (ODN2216) or polyinosinic-polycytidylic acid (poly I:C), upon incubation with mAb. These results provide the basis for future investigations aimed to assess the ability of anti-poCLEC12A mAbs to improve vaccine efficacy by targeting antigen to DCs.

Porcine CLEC12B is expressed on alveolar macrophages and blood dendritic cells.

CLEC12B is a C-type lectin-like receptor expressed on myeloid cells. In this study, we have characterized the porcine homologue of CLEC12B (poCLEC12B). To this end, we have generated constructs encoding a c-myc tagged version of the whole receptor, or its ectodomain fused to the Fc portion of human IgG1, from a cDNA clone obtained from an alveolar macrophage library, and raised monoclonal antibodies (mAb) against this molecule. Using these mAbs, poCLEC12B was found to be expressed on alveolar macrophages and, at lower levels, on blood conventional type 1 dendritic cells (cDC1) and plasmacytoid DCs. No binding was detected on monocytes, monocyte-derived macrophages or monocyte-derived DCs. Engagement of CLEC12B on alveolar macrophages with mAbs had no apparent effect on cytokine production (TNF-α, IL-8) induced by LPS. These results provide the basis for future investigations aimed to assess the role of poCLEC12B in different microbial infections and to evaluate its potential in vaccination strategies targeting DCs.

Swine T-Cells and Specific Antibodies Evoked by Peptide Dendrimers Displaying Different FMDV T-Cell Epitopes.

Dendrimeric peptide constructs based on a lysine core that comprises both B- and T-cell epitopes of foot-and-mouth disease virus (FMDV) have proven a successful strategy for the development of FMD vaccines. Specifically, B2T dendrimers displaying two copies of the major type O FMDV antigenic B-cell epitope located on the virus capsid [VP1 (140-158)], covalently linked to a heterotypic T-cell epitope from either non-structural protein 3A [3A (21-35)] or 3D [3D (56-70)], named B2T-3A and B2T-3D, respectively, elicit high levels of neutralizing antibodies (nAbs) and IFN-γ-producing cells in pigs. To assess whether the inclusion and orientation of T-3A and T-3D T-cell epitopes in a single molecule could modulate immunogenicity, dendrimers with T epitopes juxtaposed in both possible orientations, i.e., constructs B2TT-3A3D and B2TT-3D3A, were made and tested in pigs. Both dendrimers elicited high nAbs titers that broadly neutralized type O FMDVs, although B2TT-3D3A did not respond to boosting, and induced lower IgGs titers, in particular IgG2, than B2TT-3A3D. Pigs immunized with B2, a control dendrimer displaying two B-cell epitope copies and no T-cell epitope, gave no nABs, confirming T-3A and T-3D as T helper epitopes. The T-3D peptide was found to be an immunodominant, as it produced more IFN-γ expressing cells than T-3A in the in vitro recall assay. Besides, in pigs immunized with the different dendrimeric peptides, CD4+ T-cells were the major subset contributing to IFN-γ expression upon in vitro recall, and depletion of CD4+ cells from PBMCs abolished the production of this cytokine. Most CD4+IFN-γ+ cells showed a memory (CD4+2E3-) and a multifunctional phenotype, as they expressed both IFN-γ and TNF-α, suggesting that the peptides induced a potent Th1 pro-inflammatory response. Furthermore, not only the presence, but also the orientation of T-cell epitopes influenced the T-cell response, as B2TT-3D3A and B2 groups had fewer cells expressing both cytokines. These results help understand how B2T-type dendrimers triggers T-cell populations, highlighting their potential as next-generation FMD vaccines.

Identification of an Immunosuppressive Cell Population during Classical Swine Fever Virus Infection and Its Role in Viral Persistence in the Host.

Classical swine fever virus (CSFV) remains a highly important pathogen, causing major losses in the swine industry. Persistent infection is highly relevant for CSFV maintenance in the field; however, this form of infection is not fully understood. An increase in the granulocyte population has been detected in CSFV persistently infected animals. The aim of this work was to evaluate the possible immunosuppressive role of these cells in CSFV persistent infection. The phenotype of peripheral blood and bone marrow cells from persistently infected and naïve animals was evaluated by flow cytometry, and the capacity of specific cell subsets to reduce the interferon gamma (IFN-γ) response against unspecific and specific antigen was determined using co-culture assays. The frequency of granulocytic cells was increased in cells from CSFV persistently infected pigs and they showed a phenotype similar to immunosuppressive cell populations found in persistent infection in humans. These cells from persistently infected animals were able to reduce the IFN-γ response against unspecific and specific antigen. Our results suggest that immature immunosuppressive cell populations play a role in CSFV persistent infection in swine. The information obtained by studying the role of myeloid derived suppressor cells (MDSC) during CSFV persistent infection may extrapolate to other viral persistent infections in mammals.

Interaction of PRRS virus with bone marrow monocyte subsets.

PRRSV can replicate for months in lymphoid organs leading to persistent host infections. Porcine bone marrow comprises two major monocyte subsets, one of which expresses CD163 and CD169, two receptors involved in the entry of PRRSV in macrophages. In this study, we investigate the permissiveness of these subsets to PRRSV infection. PRRSV replicates efficiently in BM CD163+ monocytes reaching titers similar to those obtained in alveolar macrophages, but with a delayed kinetics. Infection of BM CD163- monocytes was variable and yielded lower titers. This may be related with the capacity of BM CD163- monocytes to differentiate into CD163+ CD169+ cells after culture in presence of M-CSF. Both subsets secreted IL-8 in response to virus but CD163+ cells tended to produce higher amounts. The infection of BM monocytes by PRRSV may contribute to persistence of the virus in this compartment and to hematological disorders found in infected animals such as the reduction in the number of peripheral blood monocytes.

Phenotypic and functional characterization of porcine bone marrow monocyte subsets.

Monocytes comprise several subsets with distinct phenotypes and functional capacities. Based on CD163 expression, two major monocyte subsets can be discriminated in the porcine bone marrow. The CD163+ cells expressed higher levels of SLA-DR, Siglec-1, CD11R1 and CD16 when compared to CD163- monocytes, whereas no remarkable differences were observed in the expression of other markers analyzed. Gene expression analysis showed differential expression of several chemokine receptor and TLR genes. Both subsets phagocytosed microspheres with similar efficiency. However, CD163- cells tended to produce higher levels of ROS in response to PMA, whereas CD163+ cells were more efficient in endocytosing and processing antigens (DQ-OVA). CD163- monocytes produced higher levels of TNF-α and IL-10 than CD163+ cells when stimulated with LPS or Imiquimod. Both subsets produced similar amounts of IL-8 in response to LPS; however, CD163+ cells produced more IL-8 after Imiquimod stimulation. Whether these subsets represent different developmental stages, and how are they related remain to be investigated.

Splenic CD163+ macrophages as targets of porcine reproductive and respiratory virus: Role of Siglecs.

CD169 and CD163 have been involved in the process of PRRS virus attachment and infection in macrophages, although recent studies have challenged the requirement for CD169. In addition to CD169, macrophages express other siglecs, whose role in PRRS virus infection is so far unknown. Splenic CD163+ macrophages express Siglec-3 and Siglec-5 but almost undetectable levels of CD169. Hence, we considered this cell population appropriate for analysing the role of these siglecs in the attachment and internalization of PRRS virus into macrophages. PRRS virus replicated efficiently in these macrophages, yielding even higher titres than in alveolar macrophages. Besides, a recombinant protein consisting in the ectodomain of porcine Siglec-3 fused to the Fc fragment of human IgG1 (Siglec3-Fc) was able to bind PRRS virus, while binding to Siglec-5-Fc was inconsistent. Antibodies to CD169 but not to Siglec-3 or Siglec-5 blocked the binding and infection of PRRS virus on alveolar macrophages. Unexpectedly, our antibody to CD169 also blocked the binding of PRRS virus to splenic CD163+ macrophages, whereas antibodies to Siglec-3 or Siglec-5 had no effect. These results show that very low levels of CD169 expression are enough to support the attachment and internalization of PRRS virus into macrophages, whereas Siglec-3 and Siglec-5 do not seem to contribute to the virus entry in these cells.

Use of anaerobic green fluorescent protein versus green fluorescent protein as reporter in lactic acid bacteria.

Lactic acid bacteria (LAB) are commonly used in the production of fermented and probiotic foods. Development of molecular tools to discriminate the strains of interest from the endogenous microbiota in complex environments like food or gut is of high interest. Green fluorescent protein (GFP)-like chromophores strictly requires molecular oxygen for maturation of fluorescence, which restrict the study of microorganisms in low-oxygen environments. In this work, we have developed a noninvasive cyan-green fluorescent based reporter system for real-time tracking of LAB that is functional under anoxic conditions. The evoglow-Pp1 was cloned downstream from the promoters D-alanyl-D-alanine carboxypeptidase and elongation factor Tu of Lactobacillus reuteri CECT925 using pNZ8048 and downstream of the lactococcal P1 promoter using pT1NX. The classical gfp was also cloned in pT1NX. These recombinant expression vectors were electroporated into Lactococccus, Lactobacillus, and Enterococcus strains with biotechnological and/or probiotic interests to assess and compare their functionality under different conditions of oxygen and pH. The expression was analyzed by imaging and fluorometric methods as well as by flow cytometry. We demonstrate that reporter systems pNZ:TuR-aFP and pT1-aFP are two versatile molecular markers for monitoring LAB in food and fecal environments without the potential problems caused by oxygen and pH limitations, which could be exploited for in vivo studies. Production of the fluorescent protein did not disturb any important physiological properties of the parental strains, such as growth rate, reuterin, or bacteriocin production.

Synthetic RNAs Mimicking Structural Domains in the Foot-and-Mouth Disease Virus Genome Elicit a Broad Innate Immune Response in Porcine Cells Triggered by RIG-I and TLR Activation.

The innate immune system is the first line of defense against viral infections. Exploiting innate responses for antiviral, therapeutic and vaccine adjuvation strategies is being extensively explored. We have previously described, the ability of small in vitro RNA transcripts, mimicking the sequence and structure of different domains in the non-coding regions of the foot-and-mouth disease virus (FMDV) genome (ncRNAs), to trigger a potent and rapid innate immune response. These synthetic non-infectious molecules have proved to have a broad-range antiviral activity and to enhance the immunogenicity of an FMD inactivated vaccine in mice. Here, we have studied the involvement of pattern-recognition receptors (PRRs) in the ncRNA-induced innate response and analyzed the antiviral and cytokine profiles elicited in swine cultured cells, as well as peripheral blood mononuclear cells (PBMCs).

Phenotypic and functional heterogeneity of CD169⁺ and CD163⁺ macrophages from porcine lymph nodes and spleen.

Secondary lymphoid organ macrophages are involved in the establishment of innate and acquired immunity. Here, we have isolated and characterized porcine lymph node and spleen CD169(+) and spleen CD163(+) macrophages. Lymph node and spleen CD169(+) macrophages can be both identified as CD172a(+)SLA-DR(hi)CD80/86(hi)CD14(int)TLR2(+)TLR4(+). On the other side, spleen CD163(+) macrophages are CD172a(+)SLA-DR(int)CD80/86(int)CD14(-)/(lo)TLR2(int)TLR4(int). In addition, these macrophages can be subdivided based on the expression of CD11R1 or CD11R3. Lymph node CD169(+) macrophages phagocytozed polystyrene microspheres more efficiently than spleen CD163(+) and CD169(+) macrophages. All macrophages exhibited low capacity to take up and process the soluble antigen DQ-OVA. Finally, spleen CD163(+) macrophages displayed the highest ability to present lysozyme to CD4(+) T cells in a secondary in vitro response, followed by lymph node and spleen CD169(+) macrophages.

Delivery of antigen to sialoadhesin or CD163 improves the specific immune response in pigs.

Delivery of antigens to antigen presenting cell surface receptors represents a promising strategy to improve immune response to weak immunogenic antigens. We have analyzed the potential of porcine sialoadhesin (Sn) and CD163 as antigen targeting receptors using mouse Igs as surrogate antigens. Sn and CD163 are two endocytic receptors mainly expressed on macrophages located in antigen-sampling zones of secondary lymphoid organs. MAbs to CD163 induced in vitro PBMC proliferation at concentrations 50-80 fold lower than the control mAb when using, as responder cells, cells from pigs immunized with mouse serum IgGs. To evaluate in vivo targeting, pigs were immunized s.c. with anti-Sn, anti-CD163 or control mAbs, and the immune response induced to mouse Ig was analyzed. Two weeks after the first immunization, pigs receiving either anti-Sn or anti-CD163 mAbs started to show higher anti-mouse-IgG serum titres than controls. Boosting 6 weeks later, further increased the anti-IgG titres up to 15-60 fold those of controls. In addition, differences in the relative predominance of IgG1 or IgG2 subclasses in the response depending on Sn or CD163 targeting were observed. Peripheral blood mononuclear cells from pigs immunized with anti-Sn mAb showed a higher proliferative response to mouse IgG than cells from pigs immunized with control mAb. These results show that targeting antigen to Sn or CD163 can enhance the immune response in pigs.

DNA immunization of pigs with foot-and-mouth disease virus minigenes: from partial protection to disease exacerbation.

Despite several attempts to design new vaccines, there are as of yet no available alternatives to the conventional FMDV vaccines. Here, we present the divergent results obtained in pigs after immunization with two experimental DNA vaccines encoding one B and two T cell FMDV epitopes, either expressed alone (pCMV-BTT) or fused to a strong signal peptide (pCMV-spBTT). While all pigs vaccinated with pCMV-spBTT showed both a delay in the disease onset and reduced severity of signs and lesions after FMDV challenge, pigs immunized with pCMV-BTT showed an exacerbation of the disease and most of the pigs remained viremic at 10 days post-infection, the end-point of the experiment, thus opening concerns about FMDV-suboptimal immunization. Interestingly, only one of the four pigs vaccinated with pCMV-spBTT showed neutralizing antibodies before challenge, demonstrating that partial protection against FMDV could be afforded in the absence of preexisting neutralizing antibodies.

Porcine monocyte subsets differ in the expression of CCR2 and in their responsiveness to CCL2.

Monocyte subsets have been shown to differ in the pattern of chemokine receptor expression and their migratory properties, both in human and mouse. Previously we have characterized in the swine several monocyte subpopulations, based on the expression of CD163, Tük4 and SLA-II, which share features with the populations described in human and mouse. Here, we have analysed the expression of different chemokine receptors in the CD163-Tük4+SLA-II- and CD163+Tük4-SLA-II+ populations of porcine monocytes. CD163+Tük4-SLA-II+ monocytes expressed higher CX3CR1 but lower CCR2 and CXCR4 mRNA levels than CD163-Tük4+SLA-II- monocytes. Moreover, porcine CCL2 binding on Tük4+SLA-II- but not on Tük4-SLA-II+ cells was detected by using a CCL2-green fluorescence protein (pCCL2-GFP) fusion protein. Finally, flow cytometric analyses of monocytes recovered after chemotaxis assays show a clear increase in the proportion of Tük4+SLA-II- cells in the fraction migrating toward CCL2, consistent with the polarized CCR2 expression in this monocyte population. The pattern of expression of these chemokine receptors reinforces the similarities of these porcine subsets with their human and mouse counterparts.

Bone marrow mesenchymal stem cells stimulate cardiac stem cell proliferation and differentiation.

RATIONALE: The regenerative potential of the heart is insufficient to fully restore functioning myocardium after injury, motivating the quest for a cell-based replacement strategy. Bone marrow-derived mesenchymal stem cells (MSCs) have the capacity for cardiac repair that appears to exceed their capacity for differentiation into cardiac myocytes. OBJECTIVE: Here, we test the hypothesis that bone marrow derived MSCs stimulate the proliferation and differentiation of endogenous cardiac stem cells (CSCs) as part of their regenerative repertoire. METHODS AND RESULTS: Female Yorkshire pigs (n=31) underwent experimental myocardial infarction (MI), and 3 days later, received transendocardial injections of allogeneic male bone marrow-derived MSCs, MSC concentrated conditioned medium (CCM), or placebo (Plasmalyte). A no-injection control group was also studied. MSCs engrafted and differentiated into cardiomyocytes and vascular structures. In addition, endogenous c-kit(+) CSCs increased 20-fold in MSC-treated animals versus controls (P<0.001), there was a 6-fold increase in GATA-4(+) CSCs in MSC versus control (P<0.001), and mitotic myocytes increased 4-fold (P=0.005). Porcine endomyocardial biopsies were harvested and plated as organotypic cultures in the presence or absence of MSC feeder layers. In vitro, MSCs stimulated c-kit(+) CSCs proliferation into enriched populations of adult cardioblasts that expressed Nkx2-5 and troponin I. CONCLUSIONS: MSCs stimulate host CSCs, a new mechanism of action underlying successful cell-based therapeutics.

Targeting to porcine sialoadhesin receptor improves antigen presentation to T cells.

Antibody-mediated targeting of antigen to specific antigen presenting cells (APC) receptors is an attractive strategy to enhance T cell immune responses to weak immunogenic antigens. Here, we describe the characterization of two monoclonal antibodies (mAb) against different epitopes of porcine sialoadhesin (Sn) and evaluate in vitro the potential of targeting this receptor for delivery of antigens to APC for T cell stimulation. The specificity of these mAb was determined by amino acid sequence analysis of peptides derived from the affinity purified antigen. Porcine Sn is expressed by macrophages present in the border between white and red pulp of the spleen and in the subcapsular sinus of lymph nodes, an appropriate location for trapping blood and lymph-borne antigens. It is also expressed by alveolar macrophages and monocyte-derived dendritic cells (MoDC). Blood monocytes are negative for this molecule, but its expression can be induced by treatment with IFN-alpha. MAb bound to Sn is rapidly endocytosed. MAb to sialoadhesin induced in vitro T cell proliferation at concentrations 100-fold lower than the non-targeting control mAb when using T lymphocytes from pigs immunized with mouse immunoglobulins as responder cells and IFN-alpha treated monocytes or MoDC as APC, suggesting a role of sialoadhesin in antigen uptake and/or delivery into the presentation pathway in APC.