Infection of mice with sporozoites of Plasmodium berghei or Plasmodium yoelii has been used extensively to evaluate liver-stage protection by candidate preerythrocytic malaria vaccines. Unfortunately, repeated success of such vaccines in mice has not translated readily to effective malaria vaccines in humans. Thus, mice may be used better as models to dissect basic parameters required for immunity to Plasmodium-infection than as preclinical vaccine models. In turn, this basic information may aid in the rational design of malaria vaccines. Here, we describe a model of circumsporozoite-specific memory CD8 T cell generation that protects mice against multiple P. berghei sporozoite challenges for at least 19 months. Using this model we defined a threshold frequency of memory CD8 T cells in the blood that predicts long-term sterilizing immunity against liver-stage infection. Importantly, the number of Plasmodium-specific memory CD8 T cells required for immunity greatly exceeds the number required for resistance to other pathogens. In addition, this model allowed us to identify readily individual immunized mice that exceed or fall below the protective threshold before infection, information that should greatly facilitate studies to dissect basic mechanisms of protective CD8 T cell memory against liver-stage Plasmodium infection. Furthermore, the extremely large threshold in memory CD8 T cell frequencies required for long-term protection in mice may have important implications for development of effective malaria vaccines.

Background. Vaccine development in human Plasmodium falciparum malaria has been hampered by the exceptionally high levels of CD8+ T cells required for efficacy. Use of potently immunogenic human adenoviruses as vaccine vectors could overcome this problem, but these are limited by preexisting immunity to human adenoviruses.Methods. From 2007 to 2010, we undertook a phase I dose and route finding study of a new malaria vaccine, a replication-incompetent chimpanzee adenovirus 63 (ChAd63) encoding the preerythrocytic insert multiple epitope thrombospondin-related adhesion protein (ME-TRAP; n = 54 vaccinees) administered alone (n = 28) or with a modified vaccinia virus Ankara (MVA) ME-TRAP booster immunization 8 weeks later (n = 26). We observed an excellent safety profile. High levels of TRAP antigen–specific CD8+ and CD4+ T cells, as detected by interferon γ enzyme-linked immunospot assay and flow cytometry, were induced by intramuscular ChAd63 ME-TRAP immunization at doses of 5 × 1010 viral particles and above. Subsequent administration of MVA ME-TRAP boosted responses to exceptionally high levels, and responses were maintained for up to 30 months postvaccination.Conclusions. The ChAd63 chimpanzee adenovirus vector appears safe and highly immunogenic, providing a viable alternative to human adenoviruses as vaccine vectors for human use.Clinical Trials Registration. NCT00890019.

Identification of protective T cell responses against SARS-CoV-2 requires distinguishing people infected with SARS-CoV-2 from those with cross-reactive immunity to other coronaviruses. Here we show a range of T cell assays that differentially capture immune function to characterise SARS-CoV-2 responses. Strong ex vivo ELISpot and proliferation responses to multiple antigens (including M, NP and ORF3) are found in 168 PCR-confirmed SARS-CoV-2 infected volunteers, but are rare in 119 uninfected volunteers. Highly exposed seronegative healthcare workers with recent COVID-19-compatible illness show T cell response patterns characteristic of infection. By contrast, >90% of convalescent or unexposed people show proliferation and cellular lactate responses to spike subunits S1/S2, indicating pre-existing cross-reactive T cell populations. The detection of T cell responses to SARS-CoV-2 is therefore critically dependent on assay and antigen selection. Memory responses to specific non-spike proteins provide a method to distinguish recent infection from pre-existing immunity in exposed populations.

The extent to which immune responses to natural infection with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and immunization with vaccines protect against variants of concern (VOC) is of increasing importance. Accordingly, here we analyse antibodies and T cells of a recently vaccinated, UK cohort, alongside those recovering from natural infection in early 2020. We show that neutralization of the VOC compared to a reference isolate of the original circulating lineage, B, is reduced: more profoundly against B.1.351 than for B.1.1.7, and in responses to infection or a single dose of vaccine than to a second dose of vaccine. Importantly, high magnitude T cell responses are generated after two vaccine doses, with the majority of the T cell response directed against epitopes that are conserved between the prototype isolate B and the VOC. Vaccination is required to generate high potency immune responses to protect against these and other emergent variants.

BackgroundPrevious research indicates that a combination vaccine targeting different stages of the malaria life cycle is likely to provide the most effective malaria vaccine. This trial was the first to combine two existing vaccination strategies to produce a vaccine that induces immune responses to both the pre-erythrocytic and blood stages of the P. falciparum life cycle.MethodsThis was a Phase I/IIa study of a new combination malaria vaccine FFM ME-TRAP+PEV3A. PEV3A includes peptides from both the pre-erythrocytic circumsporozoite protein and the blood-stage antigen AMA-1. This study was conducted at the Centre for Clinical Vaccinology and Tropical Medicine, University of Oxford, Oxford, UK. The participants were healthy, malaria naïve volunteers, from Oxford. The interventions were vaccination with PEV3A alone, or PEV3A+FFM ME-TRAP. The main outcome measure was protection from malaria in a sporozoite challenge model. Other outcomes included measures of parasite specific immune responses induced by either vaccine; and safety, assessed by collection of adverse event data.ResultsWe observed evidence of blood stage immunity in PEV3A vaccinated volunteers, but no volunteers were completely protected from malaria. PEV3A induced high antibody titres, and antibodies bound parasites in immunofluorescence assays. Moreover, we observed boosting of the vaccine-induced immune response by sporozoite challenge. Immune responses induced by FFM ME-TRAP were unexpectedly low. The vaccines were safe, with comparable side effect profiles to previous trials. Although there was no sterile protection two major observations support an effect of the vaccine-induced response on blood stage parasites: (i) Lower rates of parasite growth were observed in volunteers vaccinated with PEV3A compared to unvaccinated controls (p = 0.012), and this was reflected in the PCR results from PEV3A vaccinated volunteers. These showed early control of parasitaemia by some volunteers in this group. One volunteer, who received PEV3A alone, was diagnosed very late, on day 20 compared to an average of 11.8 days in unvaccinated controls. (ii). Morphologically abnormal parasites were present in the blood of all (n = 24) PEV3A vaccinated volunteers, and in only 2/6 controls (p = 0.001). We describe evidence of vaccine-induced blood stage efficacy for the first time in a sporozoite challenge study.Trial RegistrationClinicalTrials.Gov NCT00408668

Mucosal-associated invariant T (MAIT) cells are innate sensors of viruses and can augment early immune responses and contribute to protection. We hypothesized that MAIT cells may have inherent adjuvant activity in vaccine platforms that use replication-incompetent adenovirus vectors. In mice and humans, ChAdOx1 (chimpanzee adenovirus Ox1) immunization robustly activated MAIT cells. Activation required plasmacytoid dendritic cell (pDC)–derived interferon (IFN)–α and monocyte-derived interleukin-18. IFN-α–induced, monocyte-derived tumor necrosis factor was also identified as a key secondary signal. All three cytokines were required in vitro and in vivo. Activation of MAIT cells positively correlated with vaccine-induced T cell responses in human volunteers and MAIT cell–deficient mice displayed impaired CD8+ T cell responses to multiple vaccine-encoded antigens. Thus, MAIT cells contribute to the immunogenicity of adenovirus vectors, with implications for vaccine design.

SummaryThe induction and maintenance of T cell memory is critical to the success of vaccines. A recently described subset of memory CD8+ T cells defined by intermediate expression of the chemokine receptor CX3CR1 was shown to have self-renewal, proliferative, and tissue-surveillance properties relevant to vaccine-induced memory. We tracked these cells when memory is sustained at high levels: memory inflation induced by cytomegalovirus (CMV) and adenovirus-vectored vaccines. In mice, both CMV and vaccine-induced inflationary T cells showed sustained high levels of CX3R1int cells exhibiting an effector-memory phenotype, characteristic of inflationary pools, in early memory. In humans, CX3CR1int CD8+ T cells were strongly induced following adenovirus-vectored vaccination for hepatitis C virus (HCV) (ChAd3-NSmut) and during natural CMV infection and were associated with a memory phenotype similar to that in mice. These data indicate that CX3CR1int cells form an important component of the memory pool in response to persistent viruses and vaccines in both mice and humans.

SummarySequestration of parasite-infected red blood cells (RBCs) in the microvasculature is an important pathological feature of both bovine babesiosis caused by Babesia bovis and human malaria caused by Plasmodium falciparum. Surprisingly, when compared with malaria, the cellular and molecular mechanisms that underlie this abnormal circulatory behaviour for RBCs infected with B. bovis have been relatively ignored. Here, we present some novel insights into the adhesive and mechanical changes that occur in B. bovis-infected bovine RBCs and compare them with the alterations that occur in human RBCs infected with P. falciparum. After infection with B. bovis, bovine RBCs become rigid and adhere to vascular endothelial cells under conditions of physiologically relevant flow. These alterations are accompanied by the appearance of ridge-like structures on the RBC surface that are analogous, but morphologically and biochemically different, to the knob-like structures on the surface of human RBCs infected with P. falciparum. Importantly, albeit for a limited number of parasite lines examined here, the extent of these cellular and rheological changes appear to be related to parasite virulence. Future investigations to identify the precise molecular composition of ridges and the proteins that mediate adhesion will provide important insight into the pathogenesis of both babesiosis and malaria.