Durable reprogramming of neutralising antibody responses following breakthrough Omicron infection (preprint)

SARS-CoV-2 breakthrough infection of vaccinated individuals is increasingly common with the circulation of highly immune evasive and transmissible Omicron variants. Here, we report the dynamics and durability of recalled spike-specific humoral immunity following BA.1 or BA.2 breakthrough infection, with longitudinal sampling up to 8 months post-infection. Both BA.1 and BA.2 infection robustly boosted neutralisation activity against the infecting strain while expanding breadth against other Omicron strains. Cross-reactive memory B cells against both ancestral and Omicron spike were predominantly expanded by infection, with limited recruitment of de novo Omicron-specific B cells or antibodies. Modelling of neutralisation titres predicts that protection from symptomatic reinfection against antigenically similar strains will be remarkably durable, but is undermined by novel emerging strains with further neutralisation escape.

Rapid recall and de novo T cell responses during SARS-CoV-2 breakthrough infection (preprint)

While the protective role of neutralising antibodies against COVID-19 is well-established, questions remain about the relative importance of cellular immunity. Using 6 pMHC-multimers in a cohort with early and frequent sampling we define the phenotype and kinetics of recalled and primary T cell responses following Delta or Omicron breakthrough infection. Recall of spike-specific CD4+ T cells was rapid, with cellular proliferation and extensive activation evident as early as 1 day post-symptom onset. Similarly, spike-specific CD8+ T cells were rapidly activated but showed variable levels of expansion. Strikingly, high levels of SARS-CoV-2-specific CD8+ T cell activation at baseline and peak were strongly correlated with reduced peak SARS-CoV-2 RNA levels in nasal swabs and accelerated clearance of virus. Our study demonstrates rapid and extensive recall of memory T cell populations occurs early after breakthrough infection and suggests that CD8+ T cells contribute to the control of viral replication in breakthrough SARS-CoV-2 infections.

Dynamics of immune recall following SARS-CoV-2 vaccination or breakthrough infection (preprint)

Vaccination against SARS-CoV-2 results in protection from acquisition of infection as well as improved clinical outcomes even if infection occurs, likely reflecting a combination of residual vaccine-elicited immunity and the recall of immunological memory. Here, we define the early kinetics of spike-specific humoral and T cell immunity after vaccination of seropositive individuals, and after breakthrough infection in vaccinated individuals. Intensive and early longitudinal sampling reveals the timing and magnitude of recall, with the phenotypic activation of B cells preceding an increase in neutralizing antibody titres. In breakthrough infections, the delayed kinetics of humoral immune recall provides a mechanism for the lack of early control of viral replication but likely underpins accelerated viral clearance and the protective effects of vaccination against severe COVID-19.

Tracking the kinetics and phenotype of spike epitope-specific CD4 T cell immunity in the context of SARS-CoV-2 infection and vaccination (preprint)

CD4+ T cells play a critical role in the immune response to viral infection. SARS-CoV-2 infection and vaccination elicit strong CD4+ T cell responses to the viral spike protein, including circulating T follicular helper (cTFH) cells that correlate with the development of neutralising antibodies. Here we use a novel HLA-DRB1*15:01/S751 tetramer to precisely track spike-specific CD4+ T cells following recovery from mild/moderate COVID-19, or after vaccination with spike-encoding vaccines. SARS-CoV-2 infection induces robust S751-specific responses with both CXCR5- and cTFH phenotypes that are maintained for at least 12 months in a stable, CXCR3-biased, central memory pool. Vaccination of immunologically naïve subjects similarly drives expansion of S751-specific T cells with a highly restricted TCR repertoire comprised of both public and private clonotypes. Vaccination of convalescent individuals drives recall of CD4+ T cell clones established during infection, which are shared between the CXCR5- and cTFH compartments. This recall response is evident 5 days after antigen exposure and includes a population of spike-specific cTFH that persist in the periphery after losing expression of PD-1. Overall this study demonstrates the generation of a stable pool of cTFH and memory CD4+ T cells that can be recalled upon spike antigen re-exposure, which may play an important role in long-term protection against SARS-CoV-2 infection.

High expression of CD38 and MHC class II on CD8+ T cells during severe influenza disease reflects bystander activation and trogocytosis (preprint)

Although co-expression of CD38 and HLA-DR on CD8 + T cells reflects activation during influenza, SARS-CoV-2, Dengue, Ebola and HIV-1 viral infections, high and prolonged CD38 + HLA-DR + expression can be associated with severe and fatal disease outcomes. As the expression of CD38 + HLA-DR + is poorly understood, we used mouse models of influenza A/H7N9, A/H3N2 and A/H1N1 infection to investigate the mechanisms underpinning CD38 + MHC-II + phenotype on CD8 + T-cells. Our analysis of influenza-specific immunodominant D b NP 366 +CD8 + T-cell responses showed that CD38 + MHC-II + co-expression was detected on both virus-specific and bystander CD8 + T-cells, with increased numbers of both CD38 + MHC-II + CD8 + T-cell populations observed in the respiratory tract during severe infection. To understand the mechanisms underlying CD38 and MHC-II expression, we also used adoptively-transferred transgenic OT-I CD8 + T-cells recognising the ovalbumin-derived K b SIINFEKL epitope and A/H1N1-SIINKEKL infection. Strikingly, we found that OT-I cells adoptively-transferred into MHC-II −/− mice did not display MHC-II after influenza virus infection, suggesting that MHC-II was acquired via trogocytosis in wild-type mice. Additionally, detection of CD19 on CD38 + MHC II + OT-I cells further supports that MHC-II was acquired by trogocytosis, at least partially, sourced from B-cells. Our results also revealed that co-expression of CD38 + MHC II + on CD8 + T-cells was needed for the optimal recall ability following secondary viral challenge. Overall, our study provides evidence that both virus-specific and bystander CD38 + MHC-II + CD8 + T-cells are recruited to the site of infection during severe disease, and that MHC-II expression occurs via trogocytosis from antigen-presenting cells. Our findings also highlight the importance of the CD38 + MHC II + phenotype for CD8 + T-cell memory establishment and recall. Summary Co-expression of CD38 and MHC-II on CD8 + T cells is recognized as a classical hallmark of activation during viral infections. High and prolonged CD38 + HLA-DR + expression, however, can be associated with severe disease outcomes and the mechanisms are unclear. Using our established influenza wild-type and transgenic mouse models, we determined how disease severity affected the activation of influenza-specific CD38 + MHC-II + CD8 + T cell responses in vivo and the antigenic determinants that drive their activation and expansion. Overall, our study provides evidence that both virus-specific and bystander CD38 + MHC-II + CD8 + T-cells are recruited to the site of infection during severe disease, and that MHC-II expression occurs, at least in part, via trogocytosis from antigen-presenting cells. Our findings also highlight the importance of the CD38 + MHC II + phenotype for CD8 + T-cell memory establishment and recall.

Suboptimal SARS-CoV-2-specific CD8+ T-cell response associated with the prominent HLA-A*02:01 phenotype (preprint)

An improved understanding of human T-cell-mediated immunity in COVID-19 is important if we are to optimize therapeutic and vaccine strategies. Experience with influenza shows that infection primes CD8+ T-cell memory to shared peptides presented by common HLA types like HLA-A2. Following re-infection, cross-reactive CD8+ T-cells enhance recovery and diminish clinical severity. Stimulating peripheral blood mononuclear cells from COVID-19 convalescent patients with overlapping peptides from SARS-CoV-2 Spike, Nucleocapsid and Membrane proteins led to the clonal expansion of SARS-CoV-2-specific CD8+ and CD4+ T-cells in vitro, with CD4+ sets being typically robust. For CD8+ T-cells taken directly ex vivo, we identified two HLA-A*02:01-restricted SARS-CoV-2 epitopes, A2/S269-277 and A2/Orf1ab3183-3191. Using peptide-HLA tetramer enrichment, direct ex vivo assessment of the A2/S269+CD8+ and A2/Orf1ab3183+CD8+ populations indicated that the more prominent A2/S269+CD8+ set was detected at comparable frequency ([~]1.3x10-5) in acute and convalescent HLA-A*02:01+ patients. But, while the numbers were higher than those found in uninfected HLA-A*02:01+ donors ([~]2.5x10-6), they were low when compared with frequencies for influenza-specific (A2/M158) and EBV-specific (A2/BMLF1280) ([~]1.38x10-4) populations. Phenotypic analysis ex vivo of A2/S269+CD8+ T-cells from COVID-19 convalescents showed that A2/S269+CD8+ T-cells were predominantly negative for the CD38, HLA-DR, PD-1 and CD71 activation markers, although the majority of total CD8+ T-cells were granzyme and/or perforin-positive. Furthermore, the bias towards naive, stem cell memory and central memory A2/S269+CD8+ T-cells rather than effector memory populations suggests that SARS-CoV2 infection may be compromising CD8+ T-cell activation. Priming with an appropriate vaccine may thus have great value for optimizing protective CD8+ T-cell immunity in COVID-19.

Distinct systems serology features in children, elderly and COVID patients (preprint)

SARS-CoV-2, the pandemic coronavirus that causes COVID-19, has infected millions worldwide, causing unparalleled social and economic disruptions. COVID-19 results in higher pathogenicity and mortality in the elderly compared to children. Examining baseline SARS-CoV-2 cross-reactive coronavirus immunological responses, induced by circulating human coronaviruses, is critical to understand such divergent clinical outcomes. The cross-reactivity of coronavirus antibody responses of healthy children (n=89), adults (n=98), elderly (n=57), and COVID-19 patients (n=19) were analysed by systems serology. While moderate levels of cross-reactive SARS-CoV-2 IgG, IgM, and IgA were detected in healthy individuals, we identified serological signatures associated with SARS-CoV-2 antigen-specific Fc{gamma} receptor binding, which accurately distinguished COVID-19 patients from healthy individuals and suggested that SARS-CoV-2 induces qualitative changes to antibody Fc upon infection, enhancing Fc{gamma} receptor engagement. Vastly different serological signatures were observed between healthy children and elderly, with markedly higher cross-reactive SARS-CoV-2 IgA and IgG observed in elderly, whereas children displayed elevated SARS-CoV-2 IgM, including receptor binding domain-specific IgM with higher avidity. These results suggest that less-experienced humoral immunity associated with higher IgM, as observed in children, may have the potential to induce more potent antibodies upon SARS-CoV-2 infection. These key insights will inform COVID-19 vaccination strategies, improved serological diagnostics and therapeutics.

Breadth of concomitant immune responses underpinning viral clearance and patient recovery in a non-severe case of COVID-19 (preprint)

We report the kinetics of the immune response in relation to clinical and virological features of a patient with mild-to-moderate coronavirus disease-19 (COVID-19) requiring hospitalisation. Increased antibody-secreting cells, follicular T-helper cells, activated CD4+ and CD8+ T-cells and IgM/IgG SARS-CoV-2-binding antibodies were detected in blood, prior to symptomatic recovery. These immunological changes persisted for at least 7 days following full resolution of symptoms, indicating substantial anti-viral immunity in this non-severe COVID-19.

Breadth of concomitant immune responses underpinning viral clearance and patient recovery in a non-severe case of COVID-19 (preprint)

We report the kinetics of the immune response in relation to clinical and virological features of a patient with mild-to-moderate coronavirus disease-19 (COVID-19) requiring hospitalisation. Increased antibody-secreting cells, follicular T-helper cells, activated CD4+ and CD8+ T-cells and IgM/IgG SARS-CoV-2-binding antibodies were detected in blood, prior to symptomatic recovery. These immunological changes persisted for at least 7 days following full resolution of symptoms, indicating substantial anti-viral immunity in this non-severe COVID-19.Authors Irani Thevarajan and Thi HO Nguyen contributed equally to this work.