Non-HLDA8 animal homologue section anti-leukocyte mAbs tested for reactivity with equine leukocytes

Single-cell resolution landscape of equine peripheral blood mononuclear cells reveals diverse cell types including T-bet+ B cells

BACKGROUND

Traditional laboratory model organisms represent a small fraction of the diversity of multicellular life, and findings in any given experimental model often do not translate to other species. Immunology research in non-traditional model organisms can be advantageous or even necessary, such as when studying host-pathogen interactions. However, such research presents multiple challenges, many stemming from an incomplete understanding of potentially species-specific immune cell types, frequencies, and phenotypes. Identifying and characterizing immune cells in such organisms is frequently limited by the availability of species-reactive immunophenotyping reagents for flow cytometry, and insufficient prior knowledge of cell type-defining markers.

RESULTS

Here, we demonstrate the utility of single-cell RNA sequencing (scRNA-Seq) to characterize immune cells for which traditional experimental tools are limited. Specifically, we used scRNA-Seq to comprehensively define the cellular diversity of equine peripheral blood mononuclear cells (PBMC) from healthy horses across different breeds, ages, and sexes. We identified 30 cell type clusters partitioned into five major populations: monocytes/dendritic cells, B cells, CD3+PRF1+ lymphocytes, CD3+PRF1- lymphocytes, and basophils. Comparative analyses revealed many cell populations analogous to human PBMC, including transcriptionally heterogeneous monocytes and distinct dendritic cell subsets (cDC1, cDC2, plasmacytoid DC). Remarkably, we found that a majority of the equine peripheral B cell compartment is comprised of T-bet+ B cells, an immune cell subpopulation typically associated with chronic infection and inflammation in human and mouse.

CONCLUSIONS

Taken together, our results demonstrate the potential of scRNA-Seq for cellular analyses in non-traditional model organisms and form the basis for an immune cell atlas of horse peripheral blood.

Equine bone marrow-derived mesenchymal stem cells: optimization of cell density in primary culture

Background

The primary cell seeding density of bone marrow-derived mononuclear cells (BM-MNCs) affects several cellular behaviors, including attachment to the culture dish, proliferation, and differentiation.

Methods

The aim of this study was to determine the best density of equine BM-MNCs in primary culture (P0) for obtaining the maximum bone marrow-derived mesenchymal stem cell (BM-MSC) yields at the end of P0. Bone marrow samples of two healthy mares were aspirated. The MNCs were isolated and cultured at different densities (1×10⁵, 2×10⁵, 4×10⁵, 8×10⁵, and 1×10⁶ cells/cm²). Within the 7^th and 14^th days after seeding, the colonies containing more than 15 cells were counted and the percentage of confluency and the number of cells were calculated on day 21.

Results

The lowest density of MNCs was associated with the least number of colonies, number of adherent cells, and confluency percentage, whereas the highest density was associated with the maximum number of colonies and confluency percentage (P<0.05). However, the maximum number of cells at the end of P0 was associated with the intermediate (4×10⁵ cells/cm²) and the highest concentration (P<0.05).

Conclusions

The maximum number of MSCs at the end of P0 was obtained at the densities of 1×10⁶ and, especially, at 4×10⁵ cells/cm².

Donor-Matched Comparison of Chondrogenic Potential of Equine Bone Marrow- and Synovial Fluid-Derived Mesenchymal Stem Cells: Implications for Cartilage Tissue Regeneration

Mesenchymal stem cells (MSCs) have been demonstrated to be useful for cartilage tissue regeneration. Bone marrow (BM) and synovial fluid (SF) are promising sources for MSCs to be used in cartilage regeneration. In order to improve the clinical outcomes, it is recommended that prior to clinical use, the cellular properties and, specifically, their chondrogenic potential must be investigated. The purpose of this study is to compare and better understand the in vitro chondrogenic potential of equine bone marrow-derived mesenchymal stem cells (BMMSCs) and synovial fluid-derived mesenchymal stem cells (SFMSCs) populated from the same equine donor. BM- and SF-derived MSCs cultures were generated from five equine donors, and the MSCs were evaluated in vitro for their morphology, proliferation, trilineage differentiation, and immunophenotyping. Differences in their chondrogenic potentials were further evaluated quantitatively using glycosaminoglycan (GAG) content and via immunofluorescence of chondrogenic differentiation protein markers, SRY-type HMG box9, Aggrecan, and collagen II. The BMMSCs and SFMSCs were similar in cellular morphology, viability, and immunophenotype, but, varied in their chondrogenic potential, and expression of the key chondrogenic proteins. The SFMSCs exhibited a significant increase in GAG content compared to the BMMSCs (P < 0.0001) in three donors, suggesting increased levels of chondrogenesis. The expression of the key chondrogenic proteins correlated positively with the GAG content, suggesting that the differentiation process is dependent on the expression of the target proteins in these three donors. Our findings suggest that even though SFMSCs were hypothesized to be more chondrogenic relative to BMMSCs, there was considerable donor-to-donor variation in the primary cultures of MSCs which can significantly affect their downstream application.

Novel monoclonal antibody against alphaX subunit from horse CD11c/CD18 integrin

The αX I-domain of the horse integrin CD11c was successfully expressed in Escherichia coli, purified, biochemically characterized and used as immunogen to generate murine monoclonal antibodies against horse CD11c, which are not yet commercially available. One monoclonal antibody mAb-1C4 against the αX I-domain, is an IgG2a able to interact with the recombinant I-domain, showing an EC50=2.4ng according to ELISA assays. By western blot with horse PBMCs lysates the mAb-1C4 recognized a protein of 150kDa which corresponds well with the CD11c molecule. Using immunohistochemistry in horse lymph node tissue sections, mAb-1C4 marked cells in situ, some with apparent dendritic morphology. Thus the mAb generated to a recombinant epitope from horse CD11c identified the molecule in intact cells within horse lymphoid tissue. By the labelling intensity, the histological location (paracortical and interfollicular areas) and the apparent morphology of the marked cells, we can say that these are putative horse dendritic cells (DCs). The development of a mAb to horse CD11c provides a new tool to better study the horse DC biology and opens other biotechnological avenues, such as DC targeting-based vaccines.

Isolation and characterization of equine nasal mucosal CD172a + cells

The nasal mucosa surface is continuously confronted with a broad variety of environmental antigens, ranging from harmless agents to potentially harmful pathogens. This area is under rigorous control of professional antigen presenting cells (APCs), such as dendritic cells (DCs) and macrophages. Mucosal APCs play a crucial role in inducing primary immune responses and the establishment of an immunological memory. In the present study, a detailed characterization of CD172a(+) cells, containing the APCs residing in the equine nasal mucosa was performed for the first time. CD172a(+) cells were isolated from collagenase-treated equine nasal mucosa fragments by MACS. Expression of surface markers was determined by flow cytometry and functional analysis was done by measuring the uptake of FITC conjugated ovalbumin (FITC-OVA). Cell surface phenotype of the isolated cells was as follows: 90% CD172a(+), 30% CD1c(+), 46% CD83(+), 42% CD206(+) and 28% MHC II(+). This clearly differs from the phenotype of blood-derived monocytes: 96% CD172a(+), 4% CD1c(+), 11% CD83(+), 9% CD206(+), 72% MHC II(+) and blood monocyte derived DCs: 99% CD172a(+), 13% CD1c(+), 30% CD83(+), 51% CD206(+) and 93% MHC II(+). The CD172a(+) nasal mucosal cells were functionally able to endocytose FITC-OVA but to a lesser degree than monocyte-derived DCs. Together, these results demonstrate that the isolated CD172a(+) nasal mucosal cells resemble immature DCs in the nasal area.

Generation and characterization of monoclonal antibodies to equine CD16

The low-affinity Fc receptor CD16 plays a central role in the inflammatory and innate immune responses of many species, but has not yet been investigated in the horse. Using the predicted extracellular region of equine CD16 expressed as a recombinant fusion protein with equine IL-4 (rIL-4/CD16), we generated a panel of mouse monoclonal antibodies (mAbs) that recognize equine CD16. Nine mAbs were chosen for characterization based upon recognition of CD16, but not IL-4, in ELISA. All nine mAbs recognized full-length, cell-surface CD16 expressed as a GFP fusion protein by CHO cells, but not the closely related Fc receptor CD32 expressed in the same system. In flow cytometric analysis with equine peripheral leukocytes, the mAbs labeled cells in the granulocyte, monocyte, and lymphocyte populations in a pattern consistent with other species. Monocytes that were strongly labeled with CD16 mAb 9G5 were also positive for the LPS receptor CD14. Cytospins made with peripheral leukocytes were immunohistochemically labeled and showed mAb recognition of primarily mononuclear cells. ELISA revealed that the nine mAbs can be grouped into three patterns of epitope recognition. These new antibodies will serve as useful tools in the investigation of equine immune responses and inflammatory processes.

Immunophenotype and gene expression profiles of cell surface markers of mesenchymal stem cells derived from equine bone marrow and adipose tissue

Bone marrow and adipose tissue are the two main sources of mesenchymal stem cell (MSC). The aim of this work was to analyse the immunophenotype of 7 surface markers and the expression of a panel of 13 genes coding for cell surface markers in equine bone marrow and adipose tissue-derived MSCs obtained from 9 horses at third passage. The tri-lineage differentiation was confirmed by specific staining. Equine MSCs from both sources were positive for the MSC markers CD29 and CD90, while were negative for CD44, CD73, CD105, CD45 and CD34. The gene expression of these molecules was also evaluated by reverse transcriptase real-time quantitative PCR along with the expression of 5 other MSC markers. Both populations of cells expressed CD13, CD29, CD44, CD49d, CD73, CD90, CD105, CD106, CD146 and CD166 transcripts. Significant differences in gene expression levels between BM- and AT-MSCs were observed for CD44, CD90, CD29 and CD34. Both cell types were negative for CD45 and CD31. The surface antigens tested revealed a similar phenotypic profile between horse and human MSCs, although specific differences in some surface antigens were noticed.

Temporal analysis of equine bone marrow aspirate during establishment of putative mesenchymal progenitor cell populations

Mesenchymal progenitor cells (MPCs) are often characterized using surface markers after expansion and treatment in culture. There are no studies directly comparing gene and protein markers in undifferentiated samples during the very early phases of culture. The goal of this study was to evaluate temporal gene and protein expression changes during establishment of equine MPC cultures. Bone marrow aspirate was obtained from 35 horses and processed by density gradient centrifugation. In freshly isolated bone marrow, mononuclear cells had variable expression of CD44, CD11a/CD18, CD90, and CD45RB cell surface molecules. After 2 h of culture, bone marrow mononuclear cells had a phenotype of CD44(hi), CD29(hi), CD90(lo), CD11a/CD18(hi), and CD45RB(lo). Isolated mononuclear cells were analyzed by flow cytometry and RT-qPCR at 2, 7, 14, 21, and 30 days of culture. At all culture time points, gene expression was in agreement with cell surface protein expression. In established cultures of MPCs, cells remained robustly positive for CD44 and CD29. The proportion of positive cells and the mean fluorescence intensity of positive cells increased in CD90 expression as MPC cultures became more homogeneous. Inversely, the population of cells in culture decreased expression of CD11a/CD18 and CD45RB molecules over time. The decreased expression of the latter molecules makes these useful negative markers of established MPC cultures under normal expansion conditions. The results of this study demonstrate numerous dynamic changes in cell surface molecule expression during early establishment of MPC populations, which may aid to improve MPC isolation methods for research or therapeutic applications.

Complex interactions between the major and minor envelope proteins of equine arteritis virus determine its tropism for equine CD3+ T lymphocytes and CD14+ monocytes

Extensive cell culture passage of the virulent Bucyrus (VB) strain of equine arteritis virus (EAV) to produce the modified live virus (MLV) vaccine strain has altered its tropism for equine CD3(+) T lymphocytes and CD14(+) monocytes. The VB strain primarily infects CD14(+) monocytes and a small subpopulation of CD3(+) T lymphocytes (predominantly CD4(+) T lymphocytes), as determined by dual-color flow cytometry. In contrast, the MLV vaccine strain has a significantly reduced ability to infect CD14(+) monocytes and has lost its capability to infect CD3(+) T lymphocytes. Using a panel of five recombinant chimeric viruses, we demonstrated that interactions among the GP2, GP3, GP4, GP5, and M envelope proteins play a major role in determining the CD14(+) monocyte tropism while the tropism for CD3(+) T lymphocytes is determined by the GP2, GP4, GP5, and M envelope proteins but not the GP3 protein. The data clearly suggest that there are intricate interactions among these envelope proteins that affect the binding of EAV to different cell receptors on CD3(+) T lymphocytes and CD14(+) monocytes. This study shows, for the first time, that CD3(+) T lymphocytes may play an important role in the pathogenesis of equine viral arteritis when horses are infected with the virulent strains of EAV.

Rational design of synthetic peptides to generate antibodies that recognize in situ CD11c(+) putative dendritic cells in horse lymph nodes

A three-dimensional model of the alphaX I-domain of the horse integrin CD11c from dendritic cells provided information for selecting two segments of the primary structure for peptide synthesis. Peptide 1 contains 20 amino acids and peptide 2 has 17 amino acid residues. The first spans from position Thr229 to Arg248 of an alpha-helix segment of the structure, whereas peptide 2 goes from Asp158 to Phe174 and corresponds to an exposed segment of the loop considered to be the metal ion-dependent adhesion site. Murine polyclonal antisera against both peptides were generated and assayed in peripheral blood cell suspensions and in cryosections of horse lymph nodes. Only the serum against peptide 2 was capable of identifying cells in suspension and in situ by immunohistochemistry, some with evident dendritic morphology. Using this approach, an immunogenic epitope exposed in CD11c was identified in cells from horse lymph node in situ.

Characterization of an equine macrophage cell line: application to studies of EIAV infection

EIAV is a monocyte/macrophage tropic virus. To date, even though EIAV has been under investigation for numerous years, very few details have been elucidated about EIAV/macrophage interactions. This is largely due to the absence of an equine macrophage cell line that would support viral replication. Herein we describe the spontaneous immortalization and generation of a clonal equine macrophage-like (EML) cell line with the functional and immunophenotype characteristics of differentiated equine monocyte derived macrophage(s) (eMDM(s)). These cells possess strong non-specific esterase (NSE) activity, are able to phagocytose fluorescent bioparticles, and produce nitrites in response to LPS. The EML-3C cell line expresses the EIAV receptor for cellular entry (ELR1) and supports replication of the virulent EIAV(PV) biological clone. Thus, EML-3C cells provide a useful cell line possessing equine macrophage related properties for the growth and study of EIAV infection as well as of other equine macrophage tropic viruses.

Clinical application of dendritic cells and interleukin-2 and tools to study activated T cells in horses--first results and implications for quality control

Dendritic cells (DCs) are antigen-presenting cells, which are well known for their capacity to stimulate immunity. The ex vivo generation of myeloid DC from monocytes has facilitated the development of DC-vaccination protocols which have been extensively evaluated in tumour immunology and are regarded by some as a gold mine for clinical research. However, there is a considerable amount of work required to overcome the potential risks associated with such therapy. It is therefore mandatory to characterize the system to be applied and to study the reactions, particularly at the level of T cell responses. The first objective of the current study was to test if tumour lysates loaded autologous DC or recombinant human IL-2 are well tolerated in horses and performed an exploratory phase I study on equine sarcoids and squamous cell carcinomas. We consequently intended to establish a robust protocol for the magnetic separation of monocytes such as in use in human clinical studies. Finally we intended to address the limits in the reagents to study equine T cell based immune reactions, and analysed markers for CD25 and FoxP3. The data showed that local application of DC or IL-2 did not cause side effects. Additionally our data show that a polyclonal approach to detect antigens such as CD25 might be successful, where mAbs are not available. Our data also demonstrate that the mAb FJK16s, which has been used successfully in rodents, humans, and dogs, can also be applied in horses. We finally wish to share our concerns regarding quality control for clinical studies and encourage multi-central studies such as in human medicine to ensure that progress along established standards is made for the benefit of veterinary medicine.

Further characterization of cross-reactive anti-human leukocyte mAbs by use of equine leukocyte cell lines EqT8888 and eCAS

The equine leukocyte cell lines, namely eCAS (bone marrow derived) and EqT8888 (lymphoma derived) were used for further analysis, using 40 commercially available mAbs that showed reactivity with equine PBMC. Most mAbs that were detected in these previous studies to react with PBMC, however, did not react with either of these cell lines. Fifteen mAbs were positive on at least one of the cell lines and indicate opportunities to set up further tests using these cells. Notably, two mAbs directed against human CD34 were detected to react with eCAS.

Further analysis of anti-human leukocyte mAbs with reactivity to equine leukocytes by two-colour flow cytometry and immunohistochemistry

We have reported on the reactivity of anti-human CD molecules with equine leukocytes by single-colour flow cytometry (this issue). The objectives of this additional study were to test for the reliability of the results obtained, and to obtain further information on the positive populations of lymphocytes. Two-colour flow cytometry and immunohistochemistry were performed, using many of the positive mAbs and a few questionable ones from the first part of the study. All mAbs analysed by two-colour flow cytometry could be confirmed to their previous designation as "positive" or "questionable". Most of the mAbs tested were effective in immunohistochemistry, supporting previous results. Examples of positive results will be presented and limitations of the study will be discussed briefly.

Screening of anti-human leukocyte monoclonal antibodies for reactivity with equine leukocytes

Three hundred and seventy-nine monoclonal antibodies (mAbs) against various human CD molecules supplied to the HLDA8 animal homologues section (including four isotype controls) were analysed for cross-reactivity with equine leukocytes. First, flow cytometric identification of positively reacting mAbs was performed in one laboratory. Thereafter, a second round of flow cytometric evaluation was performed, involving three laboratories participating in the study. The first test-round indicated 17 mAbs as potentially positive. After the second round of flow cytometric analysis, 14 mAbs remained (directed against CD2, CD11a, CD18, CD44, CD45, CD49d, CD91, CD163 and CD172) where cross-reactivity was anticipated based on similarities between the human and equine staining pattern. Additionally, there was 1 mAb with weak likely positive reactivity, 12 mAbs with positive staining, which likely do not reflect valuable data, 5 mAbs with clear alternate expression pattern from that expected from humans, 5 mAbs with a questionable staining pattern itself, i.e. that was variable between the three labs, 32 mAbs with weak-positive expression and alternate staining pattern, and 279 negative mAbs (including the four isotype controls) were detected. In 31 cases, more appropriate target cells, such as thymocytes or stem cells, were not available for the screening. The results underline the value of this "cross-reactivity" approach for equine immunology. However, as only a few mAbs against leukocyte surface antigens reacted positively (approximately 4% of the mAbs submitted), the analysis of further anti-human mAbs and directed efforts to develop species-specific anti-CD mAb are still required.