Single-shot Ad26 vaccine protects against SARS-CoV-2 in rhesus macaques.

A safe and effective vaccine for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) may be required to end the coronavirus disease 2019 (COVID-19) pandemic1-8. For global deployment and pandemic control, a vaccine that requires only a single immunization would be optimal. Here we show the immunogenicity and protective efficacy of a single dose of adenovirus serotype 26 (Ad26) vector-based vaccines expressing the SARS-CoV-2 spike (S) protein in non-human primates. Fifty-two rhesus macaques (Macaca mulatta) were immunized with Ad26 vectors that encoded S variants or sham control, and then challenged with SARS-CoV-2 by the intranasal and intratracheal routes9,10. The optimal Ad26 vaccine induced robust neutralizing antibody responses and provided complete or near-complete protection in bronchoalveolar lavage and nasal swabs after SARS-CoV-2 challenge. Titres of vaccine-elicited neutralizing antibodies correlated with protective efficacy, suggesting an immune correlate of protection. These data demonstrate robust single-shot vaccine protection against SARS-CoV-2 in non-human primates. The optimal Ad26 vector-based vaccine for SARS-CoV-2, termed Ad26.COV2.S, is currently being evaluated in clinical trials.

Biophysical studies do not reveal direct interactions between human PF4 and Ad26.COV2.S vaccine.

BACKGROUND: COVID-19 vaccines have been widely used to control the SARS-CoV-2 pandemic. In individuals receiving replication-incompetent, adenovirus vector-based COVID-19 vaccines (eg, ChAdOx1 nCoV-19 [AstraZeneca] or Ad26.COV2.S [Johnson & Johnson/Janssen] vaccines), a very rare but serious adverse reaction has been reported and described as vaccine-induced immune thrombotic thrombocytopenia (VITT). The exact mechanism of VITT following Ad26.COV2.S vaccination is under investigation. Antibodies directed against human platelet factor 4 (PF4) are considered critical in the pathogenesis of VITT, suggesting similarities with heparin-induced thrombocytopenia. It has been postulated that components of these vaccines mimic the role of heparin by binding to PF4, triggering production of these anti-PF4 antibodies. OBJECTIVES: This study aimed to investigate the potential interaction between human PF4 and Ad26.COV2.S vaccine using several biophysical techniques. METHODS: Direct interaction of PF4 with Ad26.COV2.S vaccine was investigated using dynamic light scattering, biolayer interferometry, and surface plasmon resonance. For both biosensing methods, the Ad26.COV2.S vaccine was immobilized to the sensor surface and PF4 was used as analyte. RESULTS: No direct interactions between PF4 and Ad26.COV2.S vaccine could be detected using dynamic light scattering and biolayer interferometry. Surface plasmon resonance technology was shown to be unsuitable to investigate these types of interactions. CONCLUSION: Our findings make it very unlikely that direct binding of PF4 to Ad26.COV2.S vaccine or components thereof is driving the onset of VITT, although the occurrence of such interactions after immunization (potentially facilitated by unknown plasma or cellular factors) cannot be excluded. Further research is warranted to improve the understanding of the full mechanism of this adverse reaction.

Booster vaccination with Ad26.COV2.S or an Omicron-adapted vaccine in pre-immune hamsters protects against Omicron BA.2.

Since the original outbreak of the SARS-CoV-2 virus, several rapidly spreading SARS-CoV-2 variants of concern (VOC) have emerged. Here, we show that a single dose of Ad26.COV2.S (based on the Wuhan-Hu-1 spike variant) protects against the Gamma and Delta variants in naive hamsters, supporting the observed maintained vaccine efficacy in humans against these VOC. Adapted spike-based booster vaccines targeting Omicron variants have now been authorized in the absence of human efficacy data. We evaluated the immunogenicity and efficacy of Ad26.COV2.S.529 (encoding a stabilized Omicron BA.1 spike) in naive mice and in hamsters with pre-existing immunity to the Wuhan-Hu-1 spike. In naive mice, Ad26.COV2.S.529 elicited higher neutralizing antibody titers against SARS-CoV-2 Omicron BA.1 and BA.2, compared with Ad26.COV2.S. However, neutralizing titers against the SARS-CoV-2 B.1 (D614G) and Delta variants were lower after primary vaccination with Ad26.COV2.S.529 compared with Ad26.COV2.S. In contrast, we found comparable Omicron BA.1 and BA.2 neutralizing titers in hamsters with pre-existing Wuhan-Hu-1 spike immunity after vaccination with Ad26.COV2.S, Ad26.COV2.S.529 or a combination of the two vaccines. Moreover, all three vaccine modalities induced equivalent protection against Omicron BA.2 challenge in these animals. Overall, our data suggest that an Omicron BA.1-based booster in rodents does not improve immunogenicity and efficacy against Omicron BA.2 over an Ad26.COV2.S booster in a setting of pre-existing immunity to SARS-CoV-2.

Anti-tumor activity of a T-helper 1 multiantigen vaccine in a murine model of prostate cancer.

Prostate cancer is one of the few malignancies that includes vaccination as a treatment modality. Elements of an effective cancer vaccine should include the ability to elicit a Type I T-cell response and target multiple antigenic proteins expressed early in the disease. Using existing gene datasets encompassing normal prostate tissue and tumors with Gleason Score ≤ 6 and ≥ 8, 10 genes were identified that were upregulated and conserved in prostate cancer regardless of the aggressiveness of disease. These genes encoded proteins also expressed in prostatic intraepithelial neoplasia. Putative Class II epitopes derived from these proteins were predicted by a combination of algorithms and, using human peripheral blood, epitopes which selectively elicited IFN-γ or IL-10 dominant antigen specific cytokine secretion were determined. Th1 selective epitopes were identified for eight antigens. Epitopes from three antigens elicited Th1 dominant immunity in mice; PSMA, HPN, and AMACR. Each single antigen vaccine demonstrated significant anti-tumor activity inhibiting growth of implanted Myc-Cap cells after immunization as compared to control. Immunization with the combination of antigens, however, was superior to each alone in controlling tumor growth. When vaccination occurred simultaneously to tumor implant, multiantigen immunized mice had significantly smaller tumors than controls (p = 0.002) and a significantly improved overall survival (p = 0.0006). This multiantigen vaccine shows anti-tumor activity in a murine model of prostate cancer.

Ad26 vector-based COVID-19 vaccine encoding a prefusion-stabilized SARS-CoV-2 Spike immunogen induces potent humoral and cellular immune responses.

Development of effective preventative interventions against SARS-CoV-2, the etiologic agent of COVID-19 is urgently needed. The viral surface spike (S) protein of SARS-CoV-2 is a key target for prophylactic measures as it is critical for the viral replication cycle and the primary target of neutralizing antibodies. We evaluated design elements previously shown for other coronavirus S protein-based vaccines to be successful, e.g., prefusion-stabilizing substitutions and heterologous signal peptides, for selection of a S-based SARS-CoV-2 vaccine candidate. In vitro characterization demonstrated that the introduction of stabilizing substitutions (i.e., furin cleavage site mutations and two consecutive prolines in the hinge region of S2) increased the ratio of neutralizing versus non-neutralizing antibody binding, suggestive for a prefusion conformation of the S protein. Furthermore, the wild-type signal peptide was best suited for the correct cleavage needed for a natively folded protein. These observations translated into superior immunogenicity in mice where the Ad26 vector encoding for a membrane-bound stabilized S protein with a wild-type signal peptide elicited potent neutralizing humoral immunity and cellular immunity that was polarized towards Th1 IFN-γ. This optimized Ad26 vector-based vaccine for SARS-CoV-2, termed Ad26.COV2.S, is currently being evaluated in a phase I clinical trial (ClinicalTrials.gov Identifier: NCT04436276).

Expression of a natural tumor antigen by thymic epithelial cells impairs the tumor-protective CD4+ T-cell repertoire.

A variety of antigens that display a highly tissue-specific expression pattern have recently found to be also expressed in medullary thymic epithelial cells (mTEC). This unique feature of mTEC plays an important role in preventing hazardous autoimmune responses through thymic tolerization of T-cell subsets directed against autoantigens but could also limit the possibility of exploiting tumor-associated antigens for immune-mediated targeting of cancers. Our present study shows that expression of carcinoembryonic antigen (CEA) in thymic epithelial cells of CEA-transgenic mice results in tolerization of a major fraction of the CD4+ T-cell repertoire against this antigen, thereby markedly limiting the effect of CEA-specific immunization against CEA-overexpressing tumors. The expression of CEA in mTEC of CEA-transgenic mice is mirrored by its expression in human mTEC, arguing that promiscuous gene expression in these thymic stromal cells needs to be considered as a potential hurdle for immunotherapies of cancer that target tissue-specific autoantigens.

Dual effect of Kras(D12) knockdown on tumorigenesis: increased immune-mediated tumor clearance and abrogation of tumor malignancy.

Activating mutations in the human KRAS proto-oncogene are acquired during the earliest stages of colorectal cancer development. If mutant KRAS is to be used as a target for therapy in colorectal cancer, tumor growth should depend on its continued presence. Here, we report that stable knockdown of Kras(D12) in murine C26 colorectal cancer cells by RNA interference resulted in loss of transformed properties in vitro. The incidence of subcutaneous tumor formation was reduced by 60% and the lag time was increased sevenfold. Kras(D12)-knockdown tumors grew noninvasively and did not cause morbidity. Remarkably, some of the Kras(D12)-knockdown tumors regressed spontaneously, which rendered these mice resistant to parental C26 tumor growth. In immune-deficient hosts, the incidence of tumor formation by Kras(D12)-knockdown cells was 100%. None of these tumors regressed spontaneously. We conclude that the reduced incidence of tumor formation by Kras(D12)-knockdown cells is due to tumor cell clearance by the host immune system, but not to an intrinsic inability of these cells to grow out as tumors. Interestingly, Kras(D12) knockdown resulted in increased production of interleukin 18 (Il-18), an immune-stimulatory cytokine that has been implicated in limiting colorectal tumor formation. Thus, mutant Kras(D12) suppresses Il-18 production in colorectal tumor cells, which may contribute to evasion of the local immune system during tumor development.

Immunologic effects of an orally available BRAFV600E inhibitor in BRAF wild-type murine models.

Vemurafenib is an orally available small molecule that targets constitutively activated BRAFV600E, an integral part of the MAPK pathway involved in melanomagenesis. We examined the effects of vemurafenib on cytokine production and antitumor response in a BRAF wild-type (WT) non-tumor-bearing murine model and a BRAF WT murine insulinoma system to determine its effect on immune function during immunotherapy. We demonstrate no significant effect from vemurafenib on CD4+ and CD8+ T-cell cytokine production or on a T-cell-mediated antitumor response. Our data demonstrate that vemurafenib does not significantly affect BRAF WT targets, suggesting that there may be a role for combining vemurafenib treatment with T-cell-directed immunotherapy.

Functional differences between low- and high-affinity CD8(+) T cells in the tumor environment.

Weak T-cell antigen receptor (TCR)-ligand interactions are sufficient to activate naïve CD8(+) T cells, but generally do not result in tumor eradication. How differences in TCR affinity affect the regulation of T-cell function in an immunosuppressive tumor environment has not been investigated. We have examined the functional differences of high- vs. low-affinity CD8(+) T cells and we observed that infiltration, accumulation, survival and cytotoxicity within the tumor are severely impacted by the strength of TCR-ligand interactions. In addition, high-affinity CD8(+) T cells were found to exhibit lower expression of inhibitory molecules including PD-1, LAG-3 and NKG2A, thus being less susceptible to suppressive mechanisms. Interferon γ and autocrine interleukin-2 were both found to influence the level of expression of these molecules. Interestingly, although high-affinity CD8(+) T cells were superior to low-affinity CD8(+) T cells in their ability to effect tumor eradication, they could be further improved by the presence of tumor specific CD4(+) T cells. These findings illustrate the importance of both TCR affinity and tumor-specific CD4 help in tumor immunotherapy.

CD4+ T-cell help in the tumor milieu is required for recruitment and cytolytic function of CD8+ T lymphocytes.

CD4 help for CD8(+) T lymphocytes prevents tolerance and promotes the survival of effector and memory CD8(+) T cells. Here, we describe additional helper functions that require CD4(+) T cells within the tumor environment. CD8(+) T-cell recruitment, proliferation, and effector function within the tumor were greatly enhanced by tumor-specific CD4(+) T cells. Recruitment of CD8(+) T cells was accelerated by IFN-γ-dependent production of chemokines. Production of interleukin-2 by tumor resident CD4(+) T cells enhanced CD8(+) T-cell proliferation and upregulated expression of granzyme B. These results highlight a novel role for tumor-specific CD4(+) T cells in promoting CD8(+) T-cell recruitment and cytolytic function, two previously unappreciated aspects of tumor-specific CD4 help.

Tumor-specific CD4+ T cells render the tumor environment permissive for infiltration by low-avidity CD8+ T cells.

CD4+ T cells enhance tumor destruction by CD8+ T cells. One benefit that underlies CD4+ T cell help is enhanced clonal expansion of newly activated CD8+ cells. In addition, tumor-specific CD4+ help is also associated with the accumulation of greater numbers of CD8+ T cells within the tumor. Whether this too is attributable to the effects of help delivered to the CD8+ cells during priming within secondary lymphoid tissues, or alternatively is due to the action of CD4+ cells within the tumor environment has not been examined. In this study, we have evaluated separately the benefits of CD4+ T cell help accrued during priming of tumor-specific CD8+ T cells with a vaccine, as opposed to the benefits delivered by the presence of cognate CD4+ cells within the tumor. The presence of CD4+ T cell help during priming increased clonal expansion of tumor-specific CD8+ T cells in secondary lymphoid tissue; however, CD8+ T cells that have low avidity for tumor Ag were inefficient in tumor invasion. CD4+ T cells that recognized tumor Ag were required to facilitate accumulation of CD8+ T cells within the tumor and enhance tumor lysis during the acute phase of the response. These experiments highlight the ability of tumor-specific CD4+ T cells to render the tumor microenvironment receptive for CD8+ T cell immunotherapy, by facilitating the accumulation of all activated CD8+ T cells, including low-avidity tumor-specific and noncognate cells.

Balancing between antitumor efficacy and autoimmune pathology in T-cell-mediated targeting of carcinoembryonic antigen.

Carcinoembryonic antigen (CEA) is intensively studied as a potential target for immunotherapy of colorectal cancers. Although overexpressed by tumors, CEA is also expressed in normal tissues, raising questions about the feasibility and safety of CEA-targeted immunotherapy. We investigated these issues in transgenic mice in which the expression of human CEA in normal tissues closely resembles that in man. Our data show that the T-cell response against CEA in these mice is blunted by both thymic and peripheral tolerance. Consequently, effective tumor targeting is only achieved by adoptive transfer of T cells from nontolerant donors in combination with interventions that eliminate peripheral immune regulatory mechanisms. However, such treatments can result in severe intestinal autoimmune pathology associated with weight loss and mortality. Interestingly, preconditioning of recipient mice by depletion of T-regulatory cells results in immune-mediated tumor control in the absence of toxicity. In this setting, CEA-specific T-cell responses are lower than those induced by toxic regimens and accompanied by additional T-cell responses against non-self antigen. These findings illustrate the importance of testing adoptive immunotherapies targeting self antigens such as CEA in preclinical in vivo models and show that the choice of immune intervention regimen critically determines the balance between therapeutic efficacy and toxicity.

Characterization of antigen-specific immune responses induced by canarypox virus vaccines.

Avipoxvirus-based vectors, such as recombinant canarypox virus ALVAC, are studied extensively as delivery vehicles for vaccines against cancer and infectious diseases. Effective use of such vaccines is expected to benefit from proper understanding of the interaction between these viral vectors and the host immune system. We performed preclinical vaccination experiments in a murine tumor model to analyze the immunogenic properties of an ALVAC-based vaccine against carcinoembryonic Ag (ALVAC-CEA), a tumor-associated autoantigen commonly overexpressed in colorectal cancers. The protective CEA-specific immunity induced by this vaccine consisted of CD4(+) T cell responses with a mixed Th1/Th2 cytokine profile that were accompanied by potent humoral responses, but not by CEA-specific CD8(+) CTL immunity. In contrast, protective immunity induced by a CEA-specific DNA vaccine (DNA-CEA) consisted of Th1 and CTL responses. Modification of the ALVAC-CEA vaccine through coinjection of DNA-CEA, admixture with CpG oligodeoxynucleotides, or supplementation with additional transgenes encoding a triad of costimulatory molecules (TRICOM) did not result in induction of CEA-specific CTL responses. Even though these results suggested that ALVAC does not elicit Ag-specific CTLs, immunization with ALVAC vaccines against other Ags efficiently induced CTL responses. Our data show that the capacity of ALVAC vaccines to elicit CTL immunity against transgene-encoded Ags critically depends on the presence of highly immunogenic CTL epitopes in these Ags. This consideration needs to be taken into account with respect to the design and evaluation of vaccination strategies that use ALVAC-based vaccine.