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Biodistribution studies of adenovirus-based vaccines support their clinical development by evaluating their spread and persistence following in vivo administration. AZD1222 (ChAdox1 nCov-19) is a replication-deficient non-human adenovirus-vectored vaccine for coronavirus disease 2019. In this nonclinical study, the biodistribution of AZD1222 was assessed in mice for 29 days following intramuscular injection. Results show that AZD1222 was safe and well tolerated, with a spread that was largely confined to administration sites and the proximal sciatic nerve, with low levels observed in sites that are involved in rapid clearance of particulates by the reticuloendothelial system. Accordingly, levels of AZD1222 decreased from Day 2 to Day 29, indicating clearance. There were no quantifiable levels of AZD1222 in the blood, brain, spinal cord, and reproductive tissue, suggesting a lack of widespread or long-term distribution of AZD1222 vector DNA throughout the body following its administration.

AZD1222 (ChAdOx1 nCoV-19) is a COVID-19 vaccine that is not yet licensed for use during pregnancy. To support the inclusion of pregnant and breastfeeding people in AZD1222 clinical studies, a non-clinical developmental and reproductive toxicity study was performed to evaluate its effects on fertility and reproductive processes of female CD-1 mice during the embryofetal development phase, and postnatal outcomes during the littering phase. Immunogenicity assessments were also made in dams, fetuses, and pups. There were no vaccine-related unscheduled deaths throughout the study. Furthermore, there were no vaccine-related effects on female reproduction, fetal or pup survival, fetal external, visceral, or skeletal findings, pup physical development, and no abnormal gross pathology findings in pups or dams. Antibody responses raised in dams were maintained throughout gestation and postnatal periods, and seroconversion in fetuses and pups indicate placental and lactational transfer of immunoglobulins. Together with clinical data from non-pregnant people, these results support the inclusion of pregnant and breastfeeding people in AZD1222 clinical studies.

Objectives Humanised mice have emerged as valuable models for pre‐clinical testing of the safety and efficacy of immunotherapies. Given the variety of models available, selection of the most appropriate humanised mouse model is critical in study design. Here, we aimed to develop a model for predicting cytokine release syndrome (CRS) while minimising graft‐versus‐host disease (GvHD). Methods To overcome donor‐induced variation, we directly compared the in vitro and in vivo immune phenotype of immunodeficient NSG mice reconstituted with human bone marrow (BM) CD34+ haematopoietic stem cells (HSCs), peripheral blood mononuclear cells (PBMCs) or spleen mononuclear cells (SPMCs) from the same human donors. SPMC engraftment in NSG‐dKO mice, which lack MHC class I and II, was also evaluated as a strategy to limit GvHD. Another group of mice was engrafted with umbilical cord blood (UCB) CD34+ HSCs. Induction of CRS in vivo was investigated upon administration of the anti‐CD3 monoclonal antibody OKT3. Results PBMC‐ and SPMC‐reconstituted NSG mice showed short‐term survival, with engrafted human T cells exhibiting mostly an effector memory phenotype. Survival in SPMC‐reconstituted NSG‐dKO mice was significantly longer. Conversely, both BM and UCB‐HSC models showed longer survival, without demonstrable GvHD and a more naïve T‐cell phenotype. PBMC‐ and SPMC‐reconstituted mice, but not BM‐HSC or UCB‐HSC mice, experienced severe clinical signs of CRS upon administration of OKT3. Conclusion PBMC‐ and SPMC‐reconstituted NSG mice better predict OKT3‐mediated CRS. The SPMC model allows generation of large experimental groups, and the use of NSG‐dKO mice mitigates the limitation of early GvHD., In this study, we perform a head‐to‐head comparison between different humanised mouse models in terms of survival, human immune engraftment and response to immunotherapy. We characterise the differences in the immune compartment of these models and demonstrate that some, but not all, can mimic the human immune response to immunotherapies.

Significant advances in immunotherapies have resulted in the increasing need of predictive preclinical models to improve immunotherapeutic drug development, treatment combination, and to prevent or minimize toxicity in clinical trials. Immunodeficient mice reconstituted with human immune system (HIS), termed humanized mice or HIS mice, permit detailed analysis of human immune biology, development, and function. Although this model constitutes a great translational model, some aspects need to be improved as the incomplete engraftment of immune cells, graft versus host disease and the lack of human cytokines and growth factors. In this review, we discuss current HIS platforms, their pathology, and recent advances in their development to improve the quality of human immune cell reconstitution. We also highlight new technologies that can be used to better understand these models and how improved characterization is needed for their application in immuno-oncology safety, efficacy, and new modalities therapy development.

In October 2013, the International Life Sciences Institute - Health and Environmental Sciences Institute Immunotoxicology Technical Committee (ILSI-HESI ITC) held a one-day workshop entitled, “Workshop on Cytokine Release: State-of-the-Science, Current Challenges and Future Directions”. The workshop brought together scientists from pharmaceutical, academic, health authority, and contract research organizations to discuss novel approaches and current challenges for the use of in vitro cytokine release assays (CRAs) for the identification of cytokine release syndrome (CRS) potential of novel monoclonal antibody (mAb) therapeutics. Topics presented encompassed a regulatory perspective on cytokine release and assessment, case studies regarding the translatability of preclinical cytokine data to the clinic, and the latest state of the science of CRAs, including comparisons between mAb therapeutics within one platform and across several assay platforms, a novel physiological assay platform, and assay optimization approaches such as determination of FcR expression profiles and use of statistical tests. The data and approaches presented confirmed that multiple CRA platforms are in use for identification of CRS potential and that the choice of a particular CRA platform is highly dependent on the availability of resources for individual laboratories (e.g. positive and negative controls, number of human blood donors), the assay through-put required, and the mechanism-of-action of the therapeutic candidate to be tested. Workshop participants agreed that more data on the predictive performance of CRA platforms is needed, and current efforts to compare in vitro assay results with clinical cytokine assessments were discussed. In summary, many laboratories continue to focus research efforts on the improvement of the translatability of current CRA platforms as well explore novel approaches which may lead to more accurate, and potentially patient-specific, CRS prediction in the future.

Immunomodulatory therapeutics such as monoclonal antibodies (mAb) carry an inherent risk of undesired immune reactions. One such risk is cytokine release syndrome (CRS), a rapid systemic inflammatory response characterized by the secretion of pro-inflammatory cytokines from immune cells. It is crucial for patient safety to correctly identify potential risk of CRS prior to first-in-human dose administration. For this purpose, a variety of in vitro cytokine release assays (CRA) are routinely used as part of the preclinical safety assessment of novel therapeutic mAbs. One of the challenges for the development and comparison of CRA performance is the lack of availability of standard positive and negative control mAbs for use in assay qualification. To address this issue, the National Institute for Biological Standards and Control (NIBSC) developed a reference panel of lyophilised mAbs known to induce CRS in the clinic: human anti-CD52, mouse anti-CD3 and human superagonistic (SA) anti-CD28 mAb manufactured according to the respective published sequences of Campath-1H® (alemtuzumab, IgG1) , Orthoclone OKT-3® (muromonab, IgG2a) and TGN1412 (theralizumab, IgG4), as well as three isotype matched negative controls (human IgG1, mouse IgG2a and human IgG4, respectively). The relative capacity of these control mAbs to stimulate the release of IFN-γ, IL-2, TNF-α and IL-6 in vitro was evaluated in eleven laboratories in an international collaborative study mediated through the HESI Immuno-safety Technical Committee Cytokine Release Assay Working Group. Participants tested the NIBSC mAbs in a variety of CRA platforms established at each institution. This paper presents the results from the centralised cytokine quantification on all the plasma/supernatants corresponding to the stimulation of immune cells in the different CRA platforms by a single concentration of each mAb. Each positive control mAb induced significant cytokine release in most of the tested CRA platforms. There was a high inter-laboratory variability in the levels of cytokines produced, but similar patterns of response were observed across laboratories that replicated the cytokine release patterns previously published for the respective clinical therapeutic mAbs. Therefore, the positive and negative mAbs are suitable as a reference panel for the qualification and validation of CRAs, comparison of different CRA platforms (e.g. solid vs aqueous phase), and intra- and inter-laboratory comparison of CRA performance. Thus, the use of this panel of positive and negative control mAbs will increase the confidence in the robustness of a CRA platform to identify a potential CRS risk for novel immunomodulatory therapeutic candidates.

There is an urgent unmet need for human tissue bioassays to predict cytokine storm responses to biologics. Current bioassays that detect cytokine storm responses in vitro rely on endothelial cells, usually from umbilical veins or cell lines, cocultured with freshly isolated peripheral blood mononuclear cells (PBMCs) from healthy adult volunteers. These assays therefore comprise cells from 2 separate donors and carry the disadvantage of mismatched tissues and lack the advantage of personalized medicine. Current assays also do not fully delineate mild (such as Campath) and severe (such as TGN1412) cytokine storm-inducing drugs. Here, we report a novel bioassay where endothelial cells grown from stem cells in the peripheral blood (blood outgrowth endothelial cells) and PBMCs from the same donor can be used to create an autologous coculture bioassay that responds by releasing a plethora of cytokines to authentic TGN1412 but only modestly to Campath and not to control antibodies such as Herceptin, Avastin, and Arzerra. This assay performed better than the traditional mixed donor assay in terms of cytokine release to TGN1412 and, thus, we suggest provides significant advancement and a definitive system by which biologics can be tested and paves the way for personalized medicine.

Safety pharmacology (SP) is an essential part of the drug development process that aims to identify and predict adverse effects prior to clinical trials. SP studies are described in the International Conference on Harmonisation (ICH) S7A and S7B guidelines. The core battery and supplemental SP studies evaluate effects of a new chemical entity (NCE) at both anticipated therapeutic and supra-therapeutic exposures on major organ systems, including cardiovascular, central nervous, respiratory, renal and gastrointestinal. This review outlines the current practices and emerging concepts in SP studies including frontloading, parallel assessment of core battery studies, use of non-standard species, biomarkers, and combining toxicology and SP assessments. Integration of the newer approaches to routine SP studies may significantly enhance the scope of SP by refining and providing mechanistic insight to potential adverse effects associated with test compounds.

A subset of patients with relapsing-remitting multiple sclerosis (RRMS) on therapy with interferon beta (IFNβ) develop neutralising anti-drug antibodies (ADA) resulting in reduced, or loss of, therapeutic efficacy. The aims were to characterise the relative contributions of anti-IFNβ antibody isotypes to drug neutralising activity, ability of these antibodies to cross-react with endogenous IFNβ, to form immune complexes and activate complement. IFNβ-specific ADA were measured in plasma from RRMS patients treated with IFNβ1a (Rebif®). Neutralisation of endogenous and therapeutic IFNβ by ADA was determined by IFNβ bioassay. IFNβ-ADA profile was predominantly comprised of IgG1 and IgG4 antibody isotypes. The contribution of IgG4-ADA towards neutralising activity was found to be minimal. Neutralising IFNβ-ADA blocks endogenous IFNβ activity. ADA interaction with therapeutic IFNβ results in immune complex formation and complement activation. In summary, IgG1 and IgG4 IFNβ-ADA have the ability to neutralise therapeutic and endogenous protein and to activate complement., Highlights • IgG4 and IgG1 contributes to IFNβ-ADA profile • Neutralising IFNβ-ADA cross reacts and blocks endogenous IFNβ activity. • ADA-IFNβ results in IC formation and complement activation

Aim To determine if cytokine release with a solid phase assay is predictive of adverse responses for a range of therapeutic mAbs. Methods Cytokine ELISAs and a multi-array system were used to compare responses generated by different therapeutic mAbs using a solid phase assay. Flow cytometry was employed to determine the cellular source of those cytokines. Results Only TGN1412 and muromonab-CD3 stimulated CD4+ T-cell mediated cytokine release characterized by significant (all P < 0.0001) IFNγ, TNFα, IL-2, IL-4, IL-5, IL-10, IL-12, IL-13, IL-17 and IL-22 release, comparable with T-cell mitogen. Significantly greater (P < 0.0001) IL-2 release with TGN1412 (2894–6051 pg ml−1) compared with muromonab-CD3 (62–262 pg ml−1) differentiated otherwise comparable cytokine responses. Likewise, TGN1412 stimulated significantly more (P = 0.0001) IL-2 producing CD4+ T-cells than muromonab-CD3 and induced Th1, Th2, Th17 and Th22 subsets that co-release this cytokine. Significant TNFα release was observed with bevacizumab (P = 0.0001), trastuzumab (P = 0.0031) and alemtuzumab (P = 0.0177), but no significant IL-2 release. TGN1412 and muromonab-CD3 caused pro-inflammatory cytokine release despite significantly (both P < 0.0001) increasing numbers of T-cells with a regulatory phenotype. Conclusions The severity of the adverse response to TGN1412 compared with muromonab-CD3 and other therapeutic mAbs correlates with the level of IL-2 release.

The failure of regulatory science to keep pace with and support the development of new biological medicines was very publically highlighted in March 2006 when the first-in-man Phase I clinical trial of the immunomodulatory CD28-specific monoclonal antibody (mAb) TGN1412 ended in disaster when all six volunteers suffered a life-threatening adverse reaction termed a ‘Cytokine Storm’. The poor predictive value of standard pre-clinical safety tests and animal models applied to TGN1412 demonstrated the need for a new generation of immunotoxicity assays and animal models that are both sensitive and predictive of clinical outcome in man. The non-predictive result obtained from pre-clinical safety testing in cynomolgus macaques has now been attributed to a lack of CD28 expression on CD4+ effector memory T-cells that therefore cannot be stimulated by TGN1412. In contrast, high levels of CD28 are expressed on human CD4+ effector memory T-cells, the source of most TGN1412-stimulated pro-inflammatory cytokines. Standard in vitro safety tests with human cells were also non-predictive as they did not replicate in vivo presentation of TGN1412. It was subsequently shown that, if an immobilized therapeutic mAb-based assay or endothelial cell co-culture assay was used to evaluate TGN1412, then these would have predicted a pro-inflammatory response in man. New in vitro assays based on these approaches are now being applied to emerging therapeutics to hopefully prevent a repeat of the TGN1412 incident. It has emerged that the mechanism of pro-inflammatory cytokine release stimulated by TGN1412 is different to that of other therapeutic mAbs, such that standard pro-inflammatory markers such as TNFα and IL-8 are not discriminatory. Rather, IL-2 release and lymphoproliferation are optimal readouts of a TGN1412-like pro-inflammatory response.

The failure of preclinical testing to predict the severity of the cytokine storm experienced by the recipients of the superagonistic anti-CD28 monoclonal antibody (mAb) TGN1412 during its Phase 1 clinical trial prompted the development of new in vitro experimental approaches for mimicking in vivo cytokine release and lymphoproliferation. Peripheral blood mononuclear cells (PBMC) presented to TGN1412 immobilised on plastic has previously been shown to stimulate a pro-inflammatory cytokine response. The aim of the present study was to investigate a 'co-culture' model for the detection of TGN1412-like immunomodulatory activity in which TGN1412 was presented to PBMC in the presence of monolayers of endothelium-derived cells and other cell types, followed by measurement of cytokine levels in the culture supernatants and proliferation of PBMC. Culturing PBMC with TGN1412 over primary human umbilical vein endothelial cells (HUVEC) and HUVEC-derived cell lines retaining classic endothelial markers, but not cell lines of non-endothelial origin, mediated the specific release of IL-6, IL-8 and TNFα, and proliferation of PBMC. Low levels of IL-2 and IFNγ were also detected in supernatants with most donors of PBMC. An anti-CD28 mAb agonist, i.e., not a superagonist like TGN1412, did not stimulate cytokine release or proliferation of PBMC in co-cultures. In conclusion, co-culture experiments for TGN1412-specific cytokine release required cells of endothelial origin. However, the profile of released cytokines in co-cultures did not mirror that in the clinical trial participants or the responses from PBMC exposed to TGN1412 immobilised on plastic, suggesting that TGN1412 stimulation of PBMC can occur through more than one mechanism.

TGN1412 is a "superagonistic" CD28 monoclonal antibody (IgG4) that caused serious adverse events at its first time in human clinical trial. In the present study, different in vitro methods for detecting and quantifying unwanted pro-inflammatory activity of therapeutic monoclonal antibodies (mAbs) such as TGN1412 are described. The antibody of interest is immobilised by wet-coating or air-drying onto polypropylene or polystyrene 96-well plates prior to the addition of human peripheral blood mononuclear cells (PBMCs). The cells are incubated for 16-24h with the immobilised antibody which allows the accumulation of pro-inflammatory cytokines, quantified by enzyme-linked immunoabsorbent assay (ELISA), in response to the antibody. Cytokine responses stimulated by TGN1412 immobilised by air-drying onto polypropylene and polystyrene plates were much larger than responses to TGN1412 wet-coated onto polypropylene and polystyrene plates, respectively. In additional experiments with other mAbs associated with clinical reactions, air-dried mAbs stimulated larger tumour necrosis factor-alpha (TNF) responses than antibodies added in aqueous phase. Also, TGN1412 air-dried onto plastic plates stimulated large proliferative responses of 3-day cultures of lymphocytes. It was concluded that immobilising mAbs by air-drying offers a useful in vitro method for detecting and quantifying pro-inflammatory activities of therapeutic mAbs.

In 2006, a first-in-man phase-I clinical trial of an immunomodulatory mAb, TGN1412, ended in disaster when six healthy recipients suffered a life-threatening systemic inflammatory response, termed a 'Cytokine Storm'. A subsequent investigation concluded that these serious adverse events, not predicted by pre-clinical safety testing, were unforeseen biological effects in man. However, the adverse events had been exacerbated by administration of a near-maximum immuno-stimulatory dose to volunteers, because the calculation of a safe starting dose in man had been based upon results from pre-clinical safety testing in a non-responsive species. In hindsight, many lessons have been learnt from this experience and these have prompted a revision of the European guidelines for first-in-man phase-I clinical trials of biologics. Perhaps the most important lesson is that greater caution needs to be exercised when evaluating new biologics.

Biological medicines are a rapidly growing area of interest to many pharmaceutical companies, large and small. Under a broad definition they include not only modern high-tech products, such as monoclonal antibodies, enzymes and cytokines, but also older well-established products, such as vaccines and blood products. Despite a long history of standardisation and control of biological medicines, and an elaborate system of licensing and regulation, problems still occur because of their complexity. This review includes historical and regulatory background and three examples of problems seen with biotherapeutics: streptokinase, heparin and TGN1412.

The CD28-specific mAb TGN1412 rapidly caused a life-threatening “cytokine storm” in all six healthy volunteers in the Phase I clinical trial of this superagonist, signaling a failure of preclinical safety testing. We report novel in vitro procedures in which TGN1412, immobilized in various ways, is presented to human white blood cells in a manner that stimulates the striking release of cytokines and profound lymphocyte proliferation that occurred in vivo in humans. The novel procedures would have predicted the toxicity of this superagonist and are now being applied to emerging immunotherapeutics and to other therapeutics that have the potential to act upon the immune system. Data from these novel procedures, along with data from in vitro and in vivo studies in nonhuman primates, suggest that the dose of TGN1412 given to human volunteers was close to the maximum immunostimulatory dose and that TGN1412 is not a superagonist in nonhuman primates.

As a result of the CD28 superagonist biotherapeutic monoclonal antibody (TGN 1412) "cytokine storm" incident, cytokine release assays (CRA) have become hazard identification and prospective risk assessment tools for screening novel biotherapeutics directed against targets having a potential risk for eliciting adverse pro-inflammatory clinical infusion reactions. Different laboratories may have different strategies, assay formats, and approaches to the reporting, interpretation, and use of data for either decision making or risk assessment. Additionally, many independent contract research organizations (CROs), academic and government laboratories are involved in some aspect of CRA work. As a result, while some pharmaceutical companies are providing CRA data as part of the regulatory submissions when necessary, technical and regulatory practices are still evolving to provide data predictive of cytokine release in humans and that are relevant to safety. This manuscript provides an overview of different approaches employed by the pharmaceutical industry and CROs, for the use and application of CRA based upon a survey and post survey follow up conducted by ILSI-Health and Environmental Sciences Institute (HESI) Immunotoxicology Committee CRA Working Group. Also discussed is ongoing research in the academic sector, the regulatory environment, current limitations of the assays, and future directions and recommendations for cytokine release assays.

Methods Groups of 8 macaques were immunised 3 times on weeks 0, 4, 8 with 100μg purified plasmid DNA designed to express SIVmac239 derived gag under the control of the CMV immediate early promoter. Three different SIV gag vaccines were compared. The DNA plasmids expressed native SIVmac239gag, ubiquinated SIVmac239 gag or fragmented, ubiquinated SIVmac239 gag derived peptides. On week 19, the groups were boosted with 10e7 infectious units recombinant Ad 5 expressing the same SIV Gag antigens. Cell mediated immunity was assessed after each immunisation and after virus challenge. At week 23, all vaccinated macaques, along with a group of naive challenge controls began 10 weekly, atraumatic challenges via the rectal mucosal with 150TCD50 SIVmac251.

Background Live-attenuated SIV can induce superinfection resistance; however, the mechanism of this effect is not understood and may have implications for HIV vaccine development. To further investigate the role of virus replication in conferring protection we have analysed T cell phenotype and responses in gut tissue following infection of macaques with either SIVmac239Δnef or a doxycycline-dependent replication variant of SIVmac239 Δnef designated SIVrtTA.

Background In the SIV/macaque model of HIV in man, live attenuated virus confers the most potent protection against wild-type virus challenge. We have developed a novel live attenuated (nef deleted) vaccine SIVmac239 (SIVrtTA) that is dependent on doxycycline for replication in vivo. Withdrawal of doxycycline prevents replication of SIVrtTA. We used this virus to vaccinate rhesus macaques to investigate the role of virus replication and persistence in protection against SIVmac239 challenge.

BACKGROUND: Clinical indications for lymphocyte subset enumeration by flow cytometry include monitoring of disease progression and timing of therapeutic intervention in infection with human immunodeficiency virus. Until recently international standardisation has not been possible due to a lack of suitable stable reference material. METHODS: This study consisted of two trials of a stabilised whole blood preparation. Eleven participants were sent two standard protocols for staining plus gating strategy and asked to report absolute counts for lymphocyte subsets. RESULTS: No significant difference was detected between the two methods when results from the two assays and all partners were pooled. Significant differences in results from the different partners were observed. However, representative mean counts were obtained for geometric means, geometric coefficient of variation, and 95% confidence interval for CD3 910 cells/mul, 9%, and 888 to 933, respectively), CD4 (495 cells/mul, 12%, and 483 to 507), and CD8 (408 cells/mul, 13%, and 393 to 422). CONCLUSION: We have introduced a stabilised blood preparation and a well-characterized biological standard. The availability of this reference material greatly simplifies the validation of new techniques for CD4(+) T-cell enumeration and the expansion of external quality assurance programmes for clinical laboratories, including those that operate in resource-restricted environments. (c) 2005 Wiley-Liss, Inc.

Each year the three main political parties meet for an autumn conference to discuss key policies and outline their political agenda for the year ahead. The people that attend conferences include MPs, MEPs and peers, together with cabinet special advisors and other policy experts. These are exactly the people to whom the College needs to talk to ensure that we get the best for patients and surgeons.

The design of novel anti-HIV vaccination and gene therapy strategies will be presented, including pilot experiments in animal models. Our gene therapy approach uses the RNA interference (RNAi) machinery to make human cells resistant to HIV-1. The problem of viral escape and the control of viral escape by means of a combination-RNAi therapy will be discussed. A humanized mouse model has been set up as pre-clinical test system to address the safety and efficacy of lentiviral vector-delivered RNAi cassettes. Our vaccination approach deals with the novel concept of a conditional-live virus that can be turned on and off at will. Live-attenuated virus confers the most potent protection against wild-type virus challenge in the SIV/macaque vaccination model. However, such a vaccine is not pursued because of safety reasons as the vaccine virus persists and may evolve into a pathogenic variant. We therefore designed an SIVmac239Δnef variant that is dependent on doxycycline for replication by replacing the natural Tat/TAR transcription mechanism by the Tet-system for inducible gene expression. Replication of this virus can be switched off after vaccination, which will prevent evolution. The first rhesus macaque test yielded a marked vaccine effect. This new conditional-live virus will be a useful tool to identify the correlates of protection by this vaccine strategy.

Background Vaccination with live attenuated SIV confers potent protection against wild-type SIV challenge in the SIV/ macaque model of HIV. Although safety concerns preclude direct application of this vaccine approach in humans, a clearer understanding of the mechanistic processes at play may lead to better HIV vaccines. A novel live attenuated SIVmac239Δnef vaccine (SIVrtTA), dependent on doxycycline (dox) administration for replication, has been evaluated for its ability to protect in vivo against independent homologous and heterologous pathogenic wild-type (wt) SIV challenges, and the role of vaccine persistence assessed. We recently demonstrated immunophenotyping of SIVrtTA vaccinees prior to wt challenge drives global polarisation of a T cell memory phenotype, particularly in the CD4 population, toward an effector memory response.