Characterizing the impact of the mutational landscape of SARS-CoV-2 on epitope presentation and CTL

La pandémie actuelle de COVID-19, causée par le coronavirus 2 du syndrome respiratoire aigu sévère (SRAS-CoV-2), a entraîné plus de 6 millions de décès et près de 680 millions de cas confirmés dans le monde. Depuis l'émergence du virus en décembre 2019, beaucoup d’efforts de recherche mondiaux ont visé à étudier la relation entre le SRAS-CoV-2 et l'immunité adaptative à médiation cellulaire. La caractérisation des réponses immunitaires à base de lymphocytes T CD4+ et CD8+ contre le SRAS-CoV-2 dans le contexte de mutations virales est d'une pertinence immédiate pour l’approfondissement de nos connaissances concernant l'immunité adaptative envers un virus en évolution, ainsi que l'amélioration de vaccins. Dans cette thèse, je passerai en revue les découvertes actuelles concernant la biologie du SRAS-CoV-2 et sa relation avec le système immunitaire adaptatif humain. Je discuterai ensuite les divers mécanismes par lesquels le SRAS-CoV-2, ainsi que d'autres virus, se sont avérés échapper l’immunité adaptative humoral et cellulaire. Enfin, je présenterai mes contributions à la compréhension du paysage mutationnel global du SRAS-CoV-2 et de sa capacité à échapper à la reconnaissance par les lymphocytes T CD8+. Dans ce travail, j'ai observé que le paysage mutationnel global du SRAS-CoV-2 était régi par des biais de mutation au cours de la première année de la pandémie, le plus répandu d’entre eux conduisant à la suppression de la proline. Il a ensuite été prédit que cette élimination globale de la proline conduirait à la perte d’épitopes reconnues par les cellules T CD8+ d'une manière dépendante sur les super-types HLA, avec la perte d'épitopes survenant préférentiellement dans le contexte du super-type HLA-B7. Le modèle développé dans ce travail propose un lien entre les biais mutationnels globaux du SRAS-CoV-2, les allèles HLA et l'évasion des lymphocytes T. Ce travail crée un cadre pour anticiper l'impact des variantes existantes et émergentes du SRAS-CoV- 2 envers la réponse immunitaire à base de lymphocytes T CD8+.The current COVID-19 pandemic, caused by Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2), has led to upwards of 6 million deaths and nearly 680 million confirmed cases worldwide. Since the emergence of the virus in December 2019, astounding global research efforts have been aimed at investigating the relationship between SARS-CoV-2 and cell-mediated adaptive immunity. Characterizing CD4+ and CD8+ T Lymphocyte responses to SARS-CoV-2 in the context of viral mutations is of immediate relevance to understanding the breadth of a population’s adaptive immunity to an evolving virus and is central to the improving existing vaccines. In this thesis, I will review all present findings pertaining to the biology of SARS-CoV-2 and its relationship with the human adaptive immune system. I will then discuss the various mechanisms by which SARS-CoV-2, along with other viruses, have been found to evade the various arms of the adaptive immune system. Finally, I will present my contributions to the understanding of the global mutational landscape of SARS-CoV-2 and its ability to evade recognition by CD8+ T lymphocytes. By investigating over 300,000 SARS-CoV-2 genomic sequences, I observed that the mutational landscape of SARS-CoV-2 was governed by mutation biases during the first year of the pandemic, with the most prevalent bias leading to the removal of proline. The observed global removal of Proline was predicted to lead to the loss of CD8+ T cell epitopes in an HLA-supertype-dependent manner, with the loss of epitopes occurring preferentially in the context of the HLA-B7 supertype. The model developed proposes a link between SARS-CoV-2 global mutational biases, HLA alleles and T cell evasion. This work creates a framework to anticipate the population-specific impact of existing and emerging SARS-CoV-2 variants on CD8+ T cell-based immunity

Transmission of Single HIV-1 Genomes and Dynamics of Early Immune Escape Revealed by Ultra-Deep Sequencing

We used ultra-deep sequencing to obtain tens of thousands of HIV-1 sequences from regions targeted by CD8+ T lymphocytes from longitudinal samples from three acutely infected subjects, and modeled viral evolution during the critical first weeks of infection. Previous studies suggested that a single virus established productive infection, but these conclusions were tempered because of limited sampling; now, we have greatly increased our confidence in this observation through modeling the observed earliest sample diversity based on vastly more extensive sampling. Conventional sequencing of HIV-1 from acute/early infection has shown different patterns of escape at different epitopes; we investigated the earliest escapes in exquisite detail. Over 3–6 weeks, ultradeep sequencing revealed that the virus explored an extraordinary array of potential escape routes in the process of evading the earliest CD8 T-lymphocyte responses – using 454 sequencing, we identified over 50 variant forms of each targeted epitope during early immune escape, while only 2–7 variants were detected in the same samples via conventional sequencing. In contrast to the diversity seen within epitopes, non-epitope regions, including the Envelope V3 region, which was sequenced as a control in each subject, displayed very low levels of variation. In early infection, in the regions sequenced, the consensus forms did not have a fitness advantage large enough to trigger reversion to consensus amino acids in the absence of immune pressure. In one subject, a genetic bottleneck was observed, with extensive diversity at the second time point narrowing to two dominant escape forms by the third time point, all within two months of infection. Traces of immune escape were observed in the earliest samples, suggesting that immune pressure is present and effective earlier than previously reported; quantifying the loss rate of the founder virus suggests a direct role for CD8 T-lymphocyte responses in viral containment after peak viremia. Dramatic shifts in the frequencies of epitope variants during the first weeks of infection revealed a complex interplay between viral fitness and immune escape

HIV-1 Neutralisation and Other Aspects of the Envelope Glycoprotein

The absence of an effective humoral response contributes to the failure of controlling HIV-1 infection. Methods to elicit a potent neutralising antibody response is still underway and this thesis explores three aspects that can affect neutralisation. The pseudovirus-based assay is now recommended as the standard assay for assessing neutralising antibody response. However, recent studies have reported discrepancies between pseduovirus-based assay and conventional PBMCs or other cell-based assays. The first aim is to investigate possible causes behind this difference. The effect of virus producer cell type and virus platform (pseudovirus vs. replication-competent) was examined and both parameters can affect neutralisation. The second aim was to study the the neutralising antibody response in patients with primary HIV-1 infection. A panel of 6 patients were selected from the SPARTAC clinical trial. Pseudoviruses were constructed with the env gene derived from patient samples at week 0 (baseline) and week 52. The evolutionary history and the neutralisation sensitivity of these primary isolates against a panel of broadly neutralising antibodies and heterologous and autologous sera were studied. The week 52 isolates escaped from antisera neutralisation rapidly, most likely at the glycan level. In addition, viral load was found to correlate directly with intra-patient viral diversity and evolutionary divergence over time. Finally, the effect of Nef on HIV-1 neutralisation was investigated. Recent reports suggest that Nef modifies HIV-1 envelope glycoprotein. Hence, neutralising antibody response might also be modified in the absence of Nef. When subjected to neutralisation with a panel of monoclonal antibodies, the Nef-deleted (ΔNef) HIV-1 was more readily neutralised than the wild-type (WT) virus. Immunoprecipitation assays showed that neutralising antibodies can capture ΔNef virus more efficiently than WT virus. However, the enhanced neutralisation of ΔNef virus was neither due to CD4-down regulation nor difference in glycosylation

Expanding the immune self : impact of non-canonical translation on the repertoire of MHC I-associated peptides

Les molécules du complexe majeur d’histocompatibilité de classe I (MHC I) sont des glycoprotéines de surface exprimées par la majorité des cellules nucléés de notre organisme. Ces molécules servent à exposer une vue intégrative de l’état interne de nos cellules (soi immunitaire) via la présentation de courts peptides (MAPs) générés lors de la dégradation des protéines cytosoliques par le protéasome. Le répertoire des MAPs de chaque cellule, véritable carte d’identité peptidique, est constamment passé en revue par nos lymphocytes T CD8+, cellules centrales du système immunitaire, afin de débusquer et d’éliminer toute cellule anormale, e.g., celles présentant des MAPs d’origine virale ou tumorale (TSAs). Au vu du nombre grandissant d’articles démontrant que des ARNs autres que les ARNs messagers peuvent être traduits, nous avons décidé d’évaluer l’impact de ces mécanismes de traduction non-canonique sur le répertoire des MAPs présentés par des cellules B. En développant une approche protéogénomique, i.e., combinant spectrométrie de masse et séquençage d’ARN à haut-débit, nous avons pu démontrer qu’environ 10 % des MAPs présentés par nos cellules B dérivent d’évènements de traduction non-canonique incluant (i) la traduction d’ARNs messagers dans un cadre de lecture alternatif ou (ii) la traduction de régions ou ARNs supposés non-codants. L’analyse subséquente des caractéristiques de ces MAPs dits « cryptiques » suggère que leur biogenèse diffère de celle des MAPs conventionnels, les MAPs cryptiques étant principalement encodés par des ARNs instables produisant de courtes protéines dont la dégradation ne semble pas exiger l’intervention du protéasome. Sachant que la déméthylation globale du génome des cellules cancéreuses permet l’expression d’un plus grand bassin d’ARNs non-codants, nous avons supposé que ces cellules pourraient présenter de nombreux MAPs (et TSAs) cryptiques. En adaptant notre approche protéogénomique, nous avons pu analyser le répertoire des MAPs de cellules cancéreuses, incluant celui de deux lignées tumorales de souris (EL4 et CT26) et sept échantillons primaires humains (quatre leucémies aigues lymphoblastiques B et trois biopsies de cancer du poumon). Cette analyse nous a permis de découvrir qu’environ 90% des TSAs sont des TSAs cryptiques. Ayant observé que la plupart de ces TSAs dérivent de séquences normales dont l’expression est restreinte aux cellules tumorales, comme les retroéléments endogènes, il est plausible que ces TSAs soient partagés par plusieurs patients. Enfin, nos études chez la souris nous ont permis de démontrer qu’au moins deux facteurs influencent positivement le potentiel protectif d’un TSA in vivo : l’expression de cet antigène par les cellules cancéreuses et la fréquence des lymphocytes T capables de le reconnaître. En conclusion, le recours à la protéogénomique pour analyser les MAPs présentés par les cellules normales et cancéreuses nous a permis de démontrer que les MAPs cryptiques contribuent significativement au bassin de peptides constituant le soi immunitaire et qu’ils permettent aux lymphocytes T CD8+ d’effectuer une surveillance immunitaire plus efficace.On their surface, nucleated cells present major histocompatibility complex class I (MHC I) molecules in complex with short peptides, that we will refer to as MHC I-associated peptides (MAPs). These MAPs derive from the degradation of cytosolic proteins by the proteasome and provide an integrative view of the inner state of cells to CD8+ T cells, which can, in turn, eliminate abnormal cells, e.g., those presenting viral MAPs or tumor-specific antigens (TSAs). With the growing body of evidence suggesting that translation does occur outside of protein-coding transcripts, we tried to evaluate the impact of non-canonical translation on the repertoire of MAPs. Combining RNA-sequencing and mass spectrometry to analyze the MAP repertoire of B-lymphoblastoid cell lines, we uncovered that ~ 10 % of the MAP repertoire derives from such non-canonical translation events, including (i) the out-of-frame translation of protein-coding transcripts or (ii) the translation of non-coding regions (UTRs, introns, etc.) or transcripts (antisense, pseudogene, etc.). Interestingly, our data suggest that the biogenesis of cryptic and conventional MAPs differs, as cryptic MAPs derive from unstable transcripts generating short proteins that might be degraded in a proteasome-independent fashion. Because the global DNA hypomethylation observed in cancer cells tend to de-repress non-coding transcripts, we developed another proteogenomic approach to probe the cryptic MAP repertoire of two murine cancer cell lines (EL4 and CT26) and seven humor primary tumor samples (four B-lineage acute lymphoblastic leukemias and three lung tumor biopsies). This second analysis revealed that ~ 90% of TSAs are cryptic TSAs. Interestingly, most of those TSAs derived from cancer-restricted yet non-mutated sequences, such as endogenous retroelements, thereby suggesting that such TSAs could be shared between patients. Lastly, our validation study in mice demonstrated that at least two parameters can influence the in vivo protective effect of TSAs, namely TSA expression in cancer cells and the frequency of TSA-specific T cells. Altogether, our proteogenomic studies on the MAP repertoire of normal and cancer cells demonstrate that cryptic MAPs significantly expand the immune self and, consequently, the scope of CD8+ T cell immunosurveillance

Development of novel adenoviral vectors to overcome challenges observed with HAdV-5 based constructs

Recombinant vectors based on human adenovirus serotype 5 (HAdV-5) have been extensively studied in pre-clinical models and clinical trials over the last two decades. However, the thorough understanding of the HAdV-5 interaction with human subjects has uncovered major concerns about its product applicability. High vector-associated toxicity and widespread pre-existing immunity have been shown to significantly impede the effectiveness of HAdV-5 mediated gene transfer. It is therefore that the in depth knowledge attained working on HAdV-5 is currently being used to develop alternative vectors. Here, we provide a comprehensive overview of data obtained in recent years disqualifying the HAdV-5 vector for systemic gene delivery as well as novel strategies being pursued to overcome the limitations observed with particular emphasis on the ongoing vectorization efforts to obtain vectors based on alternative serotypes

Immunodominance, clonal composition and TCRß repertoire of the bovine CD8⁺ T-cell response to Theileria parva

In view of the evidence that CD8+ T-cells are involved in mediating immunity against Theileria parva, the antigens recognised by these cells are obvious candidates for inclusion in a subunit vaccine. Results from previous studies have inferred that the CD8+ T-cell response to T. parva is focused on a limited number of immunodominant antigens that exhibit polymorphism between different parasite strains. This could pose a major challenge to the design of a broadly effective subunit vaccine. The recent identification of CTL target antigens has provided the opportunity to characterise immunodominance within the T. parva systemThe objective of this study was to quantitatively assess immunodominance in the CD8+ T-cell response to T. parva and to characterise the clonal composition and TCRp repertoires ofthe epitope-specific T-cell populations. The results from four animals presented in this study have demonstrated that the CDS T-cell response restricted by two MHC class I haplotypes is reproducibly dominated by single polymorphic epitopes. Using a suite of molecular tools developed during this study it was determined that the T-cell populations specific for both these epitopes were polyclonal but dominated by a limited number of large clonal expansions and the TCRP repertoires expressed by these populations was diverse.During the course of this work several novel bovine TCRp genes were identified. Further examination ofTCRp cDNA transcripts and the bovine genome assembly substantially expanded the known bovine TCRP repertoire, which is now the largest characterised for any species. Notably several VP subfamilies, especially Vpi and 13 have undergone extensive duplication and contain large numbers of genes. By annotating the available genomic data it has been shown that the bovine TCRB locus has a highly conserved synteny with the human TCRB locus. Furthermore, this annotation has demonstrated that prodigious duplication of a cassette containing a Vpi and Vpi3 gene has contributed to the large membership of these two subfamilies and that there are three D-J-Cp clusters in the bovine TCRB locus rather than the two seen in the other mammalian TCRB loci described