One of the major obstacles for generating safe and more effective vaccines that stimulate long lasting protective immunity is the lack of predictive immune correlates with protection. T cell-mediated responses against pathogens can result in potent and long-term protection against diseases, which can be critical to improve the outcome of vaccination. T cells not only mediate effector mechanisms such as cytotoxicity, but they also modulate immunity through helper mechanisms that adequately polarise the immune response towards the elimination of a pathogen. The tumour necrosis factor (TNF) and receptor (TNFR) superfamilies are fundamental in the co-stimulation of many types of immune reaction promoted by T cell help [1
], like antibody secreting plasma cell activation or memory cell differentiation [2
]. A number of costimulatory interactions in the TNF/TNFR superfamily, such as those between OX40L/OX40 or CD70/CD27, are implicated in controlling lymphocyte activation, proliferation and survival during viral immunity [3
OX40 is predominantly expressed on activated T cells, in which its engagement by OX40L, its ligand typically expressed on conventional dendritic cells, expands effector T cells and promotes their survival and memory cell generation. OX40L ligation on OX40 signals through the NF-κB pathway as an independent signalling unit and it may also activate the phosphatyidl-inositol-3 kinase (PI3K) pathways when acting in concert with the T cell receptor [4
]. One important role of this costimulatory pathway is the generation of CD4+
T cell responses including memory cell generation [5
]. Collectively, OX40 is known to control the number of effector T cells that form during primary or secondary immune responses as well as the frequency of memory cells that are generated, reviewed in [4
]. While directly affecting T cell expansion and survival, OX40 signalling has also been shown to indirectly affect cytokine production, thereby skewing the T response depending on the context. Moreover, OX40 signalling has also been found to be critical for CD8 T cell activation in several contexts including viral infections and immunopathology. OX40L is typically present on the surface of antigen-presenting cells as a 34 kDa glycosylated type II trans-membrane protein trimer, like other members of this family [7
]. The extracellular C-terminal domain of OX40L is a structurally compact TNF homology domain (THD) that organises into a characteristic “jelly roll” beta-sandwich structure and assembles into distinctive open trimers that bind to its trimeric OX40 [9
]. The receptor monomers bind the outside of the cork-shaped ligand at the interface between two monomers which constitute the basic signalling unit in this and most other TNF/TNFR interactions. Further receptor clustering at the cell membrane is also essential for efficient signalling [10
Likewise, the interaction between CD70 and its receptor CD27 requires a trimeric CD70 complex interacting with three CD27 molecules on the cell membrane. The stimulation of CD27, a TNF superfamily receptor constitutively expressed on a large range of T-cells, NK-cells, and B-cells, through its ligand CD70 [11
] activates the canonical and the alternative NF-κB pathways [12
]. CD27 engagement on T cells is essential for memory cell differentiation while its engagement on B cell promotes plasma cell formation and enhances IgG production [13
]. CD27 activation appears thus critical for the development of long-lasting adaptive immunity.
Manipulation of these costimulatory pathways to promote antiviral or antitumoural immunity as well as for the control of autoimmune diseases or transplant rejection has been proposed as therapeutic strategy and is addressed by either activation or blockade of the corresponding signalling elements. Particularly agonists of OX40 and CD27, mainly monoclonal antibodies, have been developed and are currently being tested in preclinical models and clinical trials to enhance the efficacy of antitumour immunotherapies [14
]. Due to their critical role for the development of primary as well as memory responses against viral infections [16
], their use as adjuvants in conventional vaccination has also been proposed [18
] and both secreted OX40L- and CD70-based constructs have been shown to enhance immune responses to an HIV-1 DNA vaccine in a murine model [19
In ruminants, vaccination can be used to prevent infections by a set of parasitic, viral, and bacterial pathogens [20
] that collectively produce severe economic and social burden worldwide [22
]. These include important animal diseases such as bluetongue, peste des petits ruminants or foot and mouth disease, as well as zoonotic diseases like rift valley fever [23
]. Improved vaccine designs for these diseases are being developed and these include the development of specific immunomodulators as coadjuvants. As costimulatory signalling pathways in the TNF/TNFR superfamilies in sheep are still uncharacterised, its understanding is of critical relevance to develop their use as immunity modulators.
In this report, we studied the potential orthologues of OX40L-OX40 and CD70-CD27 molecules in sheep, providing evidence for the first time of their interaction and activity using cloned and expressed recombinant proteins. Moreover, we generated recombinant non-replicative human adenoviruses expressing immunostimulatory ligands for both receptors. Non-replicative human adenoviral vectors have been successfully used as efficient in vivo protein delivery systems [27
] and their use is particularly suited for veterinary medicine, as the host lacks pre-existing immunity to this human vector. Additionally, these vectors are safe, present low toxicity, transduce a broad spectrum of host species, and are relatively easy to manipulate and distribute [30
]. This work identifies the capacity of soluble ovine OX40L and CD70 forms to activate cells expressing their cognate receptor and stimulate ovine T cell responses, which points to their potential use as adjuvant for vaccination strategies in ruminants
2. Materials and Methods
2.1. Cells and Viruses
HEK293 cells (ATCC CRL-1573) and its derivative as well as Vero cells (ATCC CCL-81) were grown in Dulbecco’ minimal essential medium (DMEM, Gibco, Dublin, Ireland), supplemented with 10% Foetal Bovine Serum (FBS) (Sigma-Aldrich, Saint Louis, MO, USA), 2 mM L-glutamine, 1% 100× non-essential amino-acids (AANE), 1 mM sodium pyruvate and 100 U/mL Penicillin/100 μg/mL Streptomycin (all from Thermofisher Scientific, Waltham, MA, USA). The HEK293 derived reporter cell line HEK293/pr(IFNβ)-GFP that contains the enhanced Green Fluorescent Protein (eGFP) coding gene under the control of the human IFNβ promoter were kindly provided by Dr. R.E. Randall (St. Andrews University, Scotland, UK). The Trichoplusia ni-derived insect cell line Hi5 (Thermofisher Scientific, Waltham, MA, USA) was grown in TC-100 medium (Sigma-Aldrich, Saint Louis, MO, USA) supplemented with 10% FCS for adherent cell culture and in ExpressFive serum free medium (Gibco, Dublin, Ireland) for suspension culture and protein expression experiments.
Stocks of recombinant baculoviruses vBAC-Oa
OX40L and vBAC-Oa
CD70 were amplified twice on Hi5 cells to generate working stocks for protein expression procedures and stored at 4 °C until used. The recombinant adenoviruses Ad5-Oa
OX40L and Ad5-Oa
CD70 were amplified by sequential rounds of growth on HEK293 cells and final stocks were purified and titrated using standard protocols as described before [31
] and stored at −70 °C until used.
2.2. Cloning and Generation of Recombinant Viruses
The putative Ovis aries
), OX40L (TNFSF4), CD27
) and CD70 (TNFSF7)
were identified using blastp and/or tblastn searches using previously annotated orthologues from other mammalian species as baits and retrieved from different databases. The accession numbers for these molecules are OX40 (Uniprot W5P810), OX40L (Uniprot W5PZ67), CD27 isoform X1 (Genbank XP_004006990), and CD70 (Uniprot W5P639). For sequence alignments shown in Supplementary Figure S1
, orthologues from additional mammalian species were retrieved (accession numbers on figure legend) and aligned online using Clustal omega software (https://www.ebi.ac.uk/Tools/msa/clustalo/
The DNA sequences encoding full length Oa
OX40 and Oa
CD27 were optimised for expression in mammalian cells and synthesised in vitro (GenScript, Piscataway, NJ, USA). For transient expression assays, these were subcloned into plasmid pcDNA3.1V5HisA (Invitrogen, Waltham, MA, USA). Similarly, all DNA sequences for the recombinant Oa
OX40L and Oa
CD70 constructs were assembled in silico and optimised coding sequences designed and synthesised in vitro as above. The details of the relevant elements found in these constructs are shown in Supplementary Figure S1
and correspond to an N- terminal signal peptide derived from insulin, followed by the region encoding Fc (residues P245-K473) form the immunoglobulin gamma-1 chain from Ovis aries
(Accession number CAA49451), an isoleucine trimerization domain [32
] and the predicted extracellular domain of the corresponding TNFSF protein. These constructs were then subcloned into either pSIREN_EF1 plasmid for the generation of recombinant adenoviruses or pOET3 plasmid (Oxford Expression Technologies, Oxford, UK) for the generation of recombinant baculoviruses. The cloning strategies and coding sequences of all plasmids employed is available upon request.
Recombinant baculoviruses were obtained using the flashBAC GOLD system (Oxford Expression Technologies, Oxford UK) following the manufacturer’s instructions. Briefly, the plasmids pOET3-OaOX40L or pOET3-OaCD70 were cotransfected with a linearised DNA corresponding to a modified baculovirus genome optimised for expression of secreted proteins in Hi5 insect cells and recombinant baculoviruses recovered in a single step from the supernatants and stored at 4 °C.
The procedure to obtain recombinant adenoviruses has been described in detail before [31
]. Briefly, the donor plasmids pSIREN-Oa
OX40L or pSIREN-Oa
CD70 were used to transfer the gene of interest to a commercial Acceptor Vector pLP-Adeno-X-PRLS (Clontech, Mountain View, CA, USA) by Cre-loxP mediated recombination. The resulting adenoviral vector DNAs were linearised and transfected into HEK293 cells that provide in trans
the adenoviral replicative capacity and allow the recovery of the replication defective adenoviruses in the supernatants of the transfected cells.
2.3. Protein Detection by Western Blot
Vero cells were seeded in M-24 well plates and infected with Ad5-OaOX40L, Ad5-OaCD70, and Ad5-DsRed (empty recombinant adenovirus used as negative control) at a multiplicity of infection (moi) of 1. Twenty-four hours post-infection the supernatants and cells were harvested separately. They were resuspended in Laemmli buffer and analysed by Western blot with a polyclonal serum. Briefly, equivalent amounts of whole cell extracts (or supernatants) were electrophoresed on SDS-10% polyacrylamide gels and transferred to nitrocellulose membranes. The membranes were incubated with the anti-sheep-Fc (Bethyl, Montgomery, TX, USA) or anti-tubulin mouse IgG1 monoclonal antibody (Sigma-Aldrich, Saint Louis, MO, USA) and then with a peroxidase-labelled anti-mouse serum (GE Life Sciences, Marlborough, MA, USA). Protein bands were detected with the ECL system (GE Life Sciences, Marlborough, MA, USA) according to the manufacturer’s recommendations.
2.4. Purification of Recombinant Ovine CD70 (rOaCD70) and OX40L (rOaOX40L) Proteins
For the purification of recombinant protein constructs, Hi5 cells were grown to high densities in 200 mL batches of suspension cultures in serum free medium. The cells were infected at a multiplicity of infection of approximately 10 pfu/cell. At 72 h post infection, supernatants containing recombinant proteins were harvested by sequential centrifugation at 500 and 3000 g for 15 min to remove cells and cellular debris. The clarified media were concentrated using a Minimate Tangential Flow Filtration System (PALL) and diafiltered into 20 mM phosphate buffer pH 7.0. The recombinant proteins were then subjected to immunoaffinty chromatography using prepacked GE healthcare HiTrap Protein G-sepharose columns (Thermofisher Scientific, Waltham, MA, USA) following the manufacturer’s instructions and dialysed into 0.2 M Hepes, 1.5 M NaCl, pH 7.4 buffer containing 0.01% sodium azide. The purified proteins were analysed by Coomassie blue-stained SDS-PAGE and quantified using a standard BCA assay and stored at −70 °C for use.
2.5. Immunofluorescence Microscopy
Semiconfluent HEK293 or HEK293/pr(IFNβ)-GFP cell monolayers seeded in coverslips were transfected using Mirus Trans
iT-LT1 transfection reagent (Mirus Bio LLC, Madison, WI, USA) following the manufacturer’s instructions. At the corresponding time, 10 coverslips per sample were fixed using a 4% paraformaldehyde solution, washed three times with PBS, and blocked with Dako antibody diluent (Dako, #S3022) for 1 h at room temperature (RT). Proteins were detected by indirect immunofluorescence using primary antibody [mouse anti-V5 (Sigma-Aldrich, Saint Louis, MO, USA) diluted in Dako antibody diluent and incubated overnight at 4 °C, and anti-mouse Alexa-647 or anti-sheep-Fc-Alexa 488 (Thermofisher Scientific, Waltham, MA, USA) secondary antibodies for 1 h at RT. Nuclei were counterstained using 4′,6-diamidino-2-phenylindole DAPI (Sigma-Aldrich, Saint Louis, MO, USA). Cells were washed and coverslips mounted using Prolong Gold antifade reagent (Invitrogen, Waltham, MA, USA). Images were captured using an Olympus CKX41 fluorescence microscope for GFP induction in HEK293/pr(IFNβ)-GFP or a LSM 880 confocal microscope for colocalisation analysis. Image analysis was performed with the ImageJ software (http://rsbweb.nih.gov/ij/
US National Institutes of Health). For rOa
OX40L or rOa
CD70 pixel colocalisation analysis with Oa
OX40 or Oa
CD27 signal, respectively, the percentage of overlapping positive signal per cell was determined as follows: confocal z-stack images (step 0.3–0.5 μm) of transfected cells were captured for each channel, cell mask obtained [33
], positive signal for each channel was determined with the ImageJ threshold tool, and percentage of overlapping pixels in cell masks for each z-plane determined with the ImageJ image calculator tool (“AND” operation).
2.6. PBMC Isolation and T Cell Enrichment
Sheep PBMC were obtained from healthy donor ewes housed at the “Departamento de Reproducción Animal at the INIA”
(Madrid) by standard density gradient centrifugation methods [29
]. For proliferation assays, T cell enrichment was performed with nylon-wool columns [34
]. Briefly, PBMC were incubated for 45 min on nylon wool column, and the initial column effluent, enriched in T cells, was used in the subsequent experiments. T cell enrichment was verified with anti-CD3 staining (clone CD3-12) and flow cytometry. Enriched T cell fractions typically contained >80% CD3+
cells. Enriched T cell fractions and PBMC were cultured in RPMI (Lonza, Basel, Switzerland) supplemented with 10% FBS (Sigma-Aldrich, Saint Louis, MO, USA), 2 mM L-glutamine, 10 mM HEPES, 1% 100× non-essential amino-acids, 1 mM sodium pyruvate, 100 U/mL penicillin/100 μg/mL streptomycin, and 50 nM 2-mercaptoethanol (all from Thermofisher Scientific, Waltham, MA, USA) [36
2.7. CFSE Proliferation Assays
To assess the functionality of the soluble forms produced after Ad5-OaOX40L or Ad5-OaCD70 cell infection, enriched sheep T cells derived from n = 5 animals were independently labelled with CellTrace CFSE (Thermofisher Scientific, Waltham, MA, USA) as described by the manufacturer’s instruction. Enriched T cells (2 × 106 per well) were then cultured in 24-well plates for 4 days in presence of supernatants from Ad5-DsRed-, Ad5-OaOX40L-, or Ad5-OaCD70-infected Vero cells (20% (v/v) supernatant). Cells were then acquired on a FACSCalibur flow cytometer (Becton Dickinson) and data analysed with FlowJo software (Tree Star Inc.).
2.8. Intracellular Cytokine Staining and Flow Cytometry
To induce IFN-γ production, PBMC were stimulated overnight with 1.25 μg/mL concanavalin-A (ConA) (Sigma-Aldrich, Saint Louis, MO, USA) and supernatant obtained from Ad5-DsRed-, Ad5-Oa
OX40L-, or Ad5-Oa
CD70-infected Vero cells. Brefeldin-A (5 μg/mL) (Biolegend, San Diego, CA, USA) was added in the last 4 h of incubation. Cells were then labelled as described in [29
] with anti-sheep-CD4-FITC (clone 44.38), anti-sheep-CD8-PE (clone 38.65), and anti-bovine-IFN-γ-Alexa 647 (clone CC302) antibodies (all from Bio-Rad, Madrid, Spain). Samples were acquired on a FACSCalibur flow cytometer (Becton Dickinson) and data analysed with FlowJo software (Tree Star Inc.). Percentage of IFN-γ+
cells within the CD4+
T cell compartment were measured. Isotype and fluorescence minus one-channel controls were used for gating strategy [37
To assess CD27 expression in sheep PBMC subpopulations, PBMC obtained from healthy donor ewes (n
= 15) were co-stained with cross-reactive anti-human/mouse/rat CD27 antibody (clone LG-3A10 from Biolegend) and either anti-ovine CD4, -ovine CD8, -ovine CD335 (all three from Bio-Rad), or -bovine B cell marker (Kingfisher Biotech) as detailed elsewhere [35
]. Gating for CD27+
events was set using antibody isotype and fluorescence minus-one channel controls. Data were acquired on a FACSCalibur flow cytometer (Becton Dickinson) and analysed with FlowJo software (Tree Star Inc.).
2.9. Statistical Analyses
Data handling and statistical analyses was performed using Prism 6.0 software (GraphPad Software Inc. San Diego, CA, USA). Statistical tests used to compare data are indicated in the figure legend.
2.10. Ethics Statement
This study was carried out in strict accordance with the recommendations in the guidelines of the Code for Methods and Welfare Considerations in Behavioural Research with Animals (Directive 86/609EC; RD1201/2005) and all efforts were made to minimise suffering. Experiments were approved by the Committee on the Ethics of Animal Experiments (CEEA) (Permit number: 10/142792.9/12) of the Spanish Instituto Nacional de Investigación y Tecnología Agraría y Alimentaria (INIA) and the Comisión de ética estatal de bienestar animal (Permit numbers: CBS2012/06 and PROEX 228/14).
In this report, we identified and characterised for the first time the ovine costimulatory molecules of the TNFR superfamily OX40 and CD27 as well as their cognate ligands, OX40L and CD70. These receptors were found to contain the characteristic elements of TNFRs including the extracellular cysteine rich ligand binding domains followed by a predicted transmembrane region and a cytoplasmic tail which, like their human or murine counterparts lack death domains and therefore signal through direct binding to TNF receptor associated factor (TRAF) binding proteins [12
]. Transient expression assays allowed us to determine that the ovine Oa
OX40 or Oa
CD27 proteins could be found at the plasma membrane on transfected cells and colocalised with exogenously added purified rOa
OX40L or rOa
CD70 molecules, respectively. Moreover, addition of their predicted ligands induced NF-κB mediated signalling in a specific and dose dependent manner. Altogether, these results suggest that the ovine ligands bind specifically to their ovine receptors on the cell surface, inducing NF-κB signalling. This shows that the costimulatory signalling axes represented by OX40L-OX40 and CD70-CD27 are conserved and functional in the ovine species and that the recombinant ligands obtained here can effectively elicit biological activity.
With the aim of developing tools for the immunomodulation of these costimulatory pathways, we generated biologically active agonist recombinant forms of the ovine ligands. The oligomerisation of TNF receptor agonists, whether they are modified antibodies or recombinant versions of their ligands has been shown to be essential for their activity [10
]. As described above, members of the TNF ligand superfamily usually signal in a trimeric form that binds in a 1:1 stoichiometry to their respective TNF receptors, although higher order interactions are frequently required for their biological function, as was initially shown for the apoptosis inducing TNF superfamily Fas ligand (FasL) [42
]. To obtain such multimeric forms of the ovine OX40L and CD70 molecules we used a previously employed strategy [43
] in which the predicted extracellular TNF homology domain from human OX40L is fused to an isoleucine zipper trimerisation motif [32
] to stabilise trimerisation of the THD followed by a human IgG1 derived Fc domain, which in our case was replaced with an Fc domain from the ovine species. In the case of the human recombinant protein, this was shown to adopt a functional hexameric structure formed by two OX40L trimers and three disulphide bonded Fc dimers. Stabilising isoleucine zipper trimerisation motifs have been tested before in other chimeric TNFLSF members such as the human CD40L [45
] and TRAIL [46
] proteins, generating higher biological activity molecules. This suggested that such a strategy would have a broad applicability to members of the family from other species, including the ovine CD70 tested in this report. While we did not assess the oligomeric nature of the purified rOa
OX40L and rOa
CD70 proteins, non-reducing SDS-PAGE indicated the presence of disulphide bonded oligomers as in the case above. This suggests that the ovine proteins may adopt also this hexameric structure, or at least a trimeric structure. The oligomeric state of rOa
OX40L and rOa
CD70 was competent to trigger signalling in cells expressing the cognate receptor. Additionally, these recombinant proteins may be used to monitor expression of OX40 and CD27 on the surface of ovine cells.
As a further step for the use of the secreted rOa
OX40L and rOa
CD70 as immunostimulators, we developed recombinant human adenovirus 5 based vectors that express them. Cell culture assays showed that infection with either recombinant virus produced secreted and bioactive proteins equivalent to the purified proteins described before. Human adenovirus 5 has been extensively used in vivo as a sustained gene delivery method in vivo [27
]. While its major drawback for clinical use in humans is the widespread pre-existing immunity in the population [48
], this is not the case for sheep, as this host is unlikely to have been exposed to the recombinant vector [30
]. For example, inoculation of adenoviral vectors expressing bovine IFN type III has been shown to protect cattle from a subsequent FMDV challenge [53
]. While the use of the recombinant cytokines interleukin 1 beta (IL1β) and TNFα as effective adjuvants in vaccination of ruminant species has already been proposed [54
], their use has so far not been widely adopted. Possibly the use of specifically targeted costimulatory molecules such as the Oa
OX40L and Oa
CD70 constructs described here might provide improved vaccine enhancement in vivo. It is worth noting that a recombinant adenovirus expressing trimeric TNFSF ligands 4-1BBL or BAFF was found to effectively enhance anti Gag response in a murine HIV vaccine model [56
], showing that adenoviral delivery of soluble TNF ligands can promote adjuvancy.
An important role of the OX40/OX40L costimulatory pathway is the generation of CD4+
T cell responses including memory cell generation [5
]. OX40 is known to control the number of effector T cells that form during primary or secondary immune responses as well as the frequency of memory cells that are generated (reviewed in [4
]). These reported activities of OX40 are relevant to vaccination protocols and activation of this ligand has therefore been proposed as a potent adjuvant strategy. A good example of OX40 role was demonstrated in vaccinia virus infection in mice, where OX40 was found to be critical for the development of CD8+
T cell responses against dominant and subdominant epitopes as well as for the generation of memory cells [57
]. Relevantly, activation of OX40 signalling by an agonist antibody during vaccination with an attenuated poxviral vector provided enhanced effector and memory cell numbers and allowed complete protection against a lethal respiratory virus infection [58
]. In this report, we show a consistent costimulatory activity on sheep PBMC, with activation of CD4+
cells by the newly developed ovine agonist.
A similar case can be made for the CD70-CD27 signalling axis that is known to enhance primary anti-viral CD8+
T cell responses and generation of memory T cells in mice and probably humans (reviewed in [11
]). Recently, the generation of EBV specific T cell responses was shown to be dependent on this signalling pathway in a patient with EBV-driven recurrent lymphoproliferative disorder who had a CD70 deficiency [59
] and stimulation of CD27 using agonist antibodies has been shown to enhance therapeutic vaccination in cancer immunotherapy [60
]. In the ovine model used here, we were able to show expression of the CD27 molecule on both CD4+
T cells as well as on a subset of NK cells and a fraction of B-cells, which suggests that its role in the development of the immune response may be equivalent to that described in mice and humans. In support of this notion and as in the case of the ovine OX40, the purified ovine ligand was found to be able to induce IFNγ secretion from activated ovine CD4+
cells, confirming their nature as costimulatory molecules.
While CD70-CD27 signalling, like OX40-OX40 signalling, share overall costimulatory activities, differences in their activation dynamics as well as expression patterns on cellular subsets suggest a set of non-overlapping and non-redundant roles in immune response activation that warrant their individual and combined studies in the context of vaccination processes. In particular, future research will need to address sheep disease models where T cell response is thought to play a critical role in protection, as is the case of the Bluetongue virus [28
] or peste des petits ruminants virus [61