key: cord-312115-foy3dsq4
authors: Sekine, Takuya; Perez-Potti, André; Rivera-Ballesteros, Olga; Strålin, Kristoffer; Gorin, Jean-Baptiste; Olsson, Annika; Llewellyn-Lacey, Sian; Kamal, Habiba; Bogdanovic, Gordana; Muschiol, Sandra; Wullimann, David J.; Kammann, Tobias; Emgård, Johanna; Parrot, Tiphaine; Folkesson, Elin; Rooyackers, Olav; Eriksson, Lars I.; Henter, Jan-Inge; Sönnerborg, Anders; Allander, Tobias; Albert, Jan; Nielsen, Morten; Klingström, Jonas; Gredmark-Russ, Sara; Björkström, Niklas K.; Sandberg, Johan K.; Price, David A.; Ljunggren, Hans-Gustaf; Aleman, Soo; Buggert, Marcus
title: Robust T cell immunity in convalescent individuals with asymptomatic or mild COVID-19
date: 2020-08-14
journal: Cell
DOI: 10.1016/j.cell.2020.08.017
sha: 
doc_id: 312115
cord_uid: foy3dsq4

Summary SARS-CoV-2-specific memory T cells will likely prove critical for long-term immune protection against COVID-19. We here systematically mapped the functional and phenotypic landscape of SARS-CoV-2-specific T cell responses in unexposed individuals, exposed family members, and individuals with acute or convalescent COVID-19. Acute phase SARS-CoV-2-specific T cells displayed a highly activated cytotoxic phenotype that correlated with various clinical markers of disease severity, whereas convalescent phase SARS-CoV-2-specific T cells were polyfunctional and displayed a stem-like memory phenotype. Importantly, SARS-CoV-2-specific T cells were detectable in antibody-seronegative exposed family members and convalescent individuals with a history of asymptomatic and mild COVID-19. Our collective dataset shows that SARS-CoV-2 elicits robust, broad and highly functional memory T cell responses, suggesting that natural exposure or infection may prevent recurrent episodes of severe COVID-19.

The world changed in December 2019 with the emergence of a new zoonotic pathogen, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which causes a variety of clinical syndromes collectively termed coronavirus disease 2019 . At present, there is no vaccine against SARS-CoV-2, and the excessive inflammation associated with severe COVID-19 can lead to respiratory failure, septic shock, and ultimately, death (Guan et al., 2020; Wolfel et al., 2020; Wu and McGoogan, 2020) . The overall mortality rate is 0.5-3.5% (Guan et al., 2020; Wolfel et al., 2020; Wu and McGoogan, 2020) . However, most people seem to be affected less severely and either remain asymptomatic or develop only mild symptoms during COVID-19 (He et al., 2020b; Wei et al., 2020; Yang et al., 2020) . It will therefore be critical in light of the ongoing pandemic to determine if people with milder forms of COVID-19 develop robust immunity against SARS-CoV-2.

Global efforts are currently underway to map the determinants of immune protection against SARS-CoV-2. Recent data have shown that SARS-CoV-2 infection generates near-complete protection against rechallenge in rhesus macaques (Chandrashekar et al., 2020) , and similarly, there is limited evidence of reinfection in humans with previously documented COVID-19 (Kirkcaldy et al., 2020) . Further work is therefore required to define the mechanisms that underlie these observations and evaluate the durability of protective immune responses elicited by primary infection with SARS-CoV-2. Most correlative studies of immune protection against SARS-CoV-2 have focused on the induction of neutralizing antibodies (Hotez et al., 2020; Robbiani et al., 2020; Seydoux et al., 2020; Wang et al., 2020) . However, antibody responses are not detectable in all patients, especially those with less severe forms of COVID-19 (Long et al., 2020; Mallapaty, 2020; Woloshin et al., 2020) . Previous work has also shown that memory B cell responses tend to be short-lived after infection with SARS-CoV-1 (Channappanavar et al., 2014; Tang et al., 2011) . In contrast, memory T cell responses can persist for many years (Le Bert et al., 2020; Tang et al., 2011; Yang et al., 2006) and, in mice, protect against lethal challenge with SARS-CoV-1 (Channappanavar et al., 2014) . Ni et al., 2020) . It has nonetheless remained unclear to what extent various features of the T cell immune response associate with serostatus and the clinical course of COVID-19. To address this knowledge gap, we characterized SARS-CoV-2-specific CD4 + and CD8 + T cells in outcome-defined cohorts of donors (total n = 206) from J o u r n a l P r e -p r o o f Sweden, which has experienced a more open spread of COVID-19 than many other countries in Europe (Habib, 2020) .

Our preliminary analyses showed that the absolute numbers and relative frequencies of CD4 + and CD8 + T cells were unphysiologically low in patients with acute moderate or severe COVID-19 ( Figure 1A and Figure S1A , B). This finding has been reported previously (He et al., 2020a; Liu et al., 2020) . We then used a 29-color flow cytometry panel to assess the phenotypic landscape of these immune perturbations in direct comparisons with healthy blood donors and individuals who had recovered from mild COVID-19 acquired early during the pandemic (February to March 2020).

The patient demographics are described in STAR Methods section. None of the parameters were found to be prognostic for the outcome of the disease severity. The To extend these findings, we concatenated all memory CD4 + T cells and all memory CD8 + T cells from healthy blood donors, convalescent individuals, and patients with acute moderate or severe COVID-19. Phenotypically related cells were identified using the clustering algorithm PhenoGraph, and marker expression patterns were visualized using the dimensionality reduction algorithm Uniform Manifold Approximation and Projection (UMAP). Distinct topographical clusters were apparent in each group ( Figure 1D and Figure S2A , B). In particular, memory CD8 + T cells from patients with acute moderate or severe COVID-19 expressed a distinct cluster of markers associated with activation and the cell cycle, including CD38, HLA-DR, Ki-J o u r n a l P r e -p r o o f 67, and PD-1 ( Figure 1D and Figure S2A ). A similar pattern was observed among memory CD4 + T cells from patients with acute moderate or severe COVID-19 ( Figure   S2B ). These findings were confirmed via manual gating of the flow cytometry data ( Figure 1E ). Correlative analyses further demonstrated that the activated/cycling phenotype was strongly associated with the presence of SARS-CoV-2 IgG levels, as well as various clinical parameters, including age, hemoglobin concentration, platelet count, and plasma levels of alanine aminotransferase, albumin, D-dimer, fibrinogen, and myoglobin ( Figure S2C , D), but less strongly associated with plasma levels of various inflammatory markers, including interleukin (IL)-1β, IL-10, and tumor necrosis factor (TNF) (Table S1 ). Collectively, these data suggest that the combination of activation markers on T cells potentially marks a more robust early SARS-COV-2specific adaptive immune response in COVID-19.

Unphysiologically high expression frequencies of CD38, potentially driven by a highly inflammatory environment, were consistently observed among memory CD8 + T cells from patients with acute moderate or severe COVID-19 ( Figure S3A , B). In line with these data, we found that CD8 + T cells specific for cytomegalovirus (CMV) or Epstein-Barr virus (EBV) more commonly expressed CD38, but not HLA-DR, Ki-67, or PD-1, in patients with acute moderate or severe COVID-19 compared with convalescent individuals and healthy blood donors, indicating limited bystander activation and proliferation during the early phase of infection with SARS-CoV-2 ( Figure 2A , B and Figure S3C ). Of note, actively proliferating CD8 + T cells, defined by the expression of Ki-67, exhibited a predominant CCR7 − CD27 + CD28 + CD45RA − CD127 − phenotype in patients with acute moderate or severe COVID-19 ( Figure   S3D ), as reported previously in the context of vaccination and other viral infections (Buggert et al., 2018; Miller et al., 2008) . On the basis of these findings, we used overlapping peptides spanning the immunogenic domains of the SARS-CoV-2 spike, membrane, and nucleocapsid proteins to stimulate peripheral blood mononuclear cells (PBMCs) from patients with acute moderate or severe COVID-19. A vast majority of responding CD4 + and CD8 + T cells displayed an activated/cycling (CD38 + HLA-DR + Ki67 + PD-1 + ) phenotype ( Figure 2C ). These results were confirmed using an activation-induced marker (AIM) assay to measure the upregulation of CD69 and 4-1BB (CD137), suggesting that most CD38 + PD-1 + CD8 + T cells were specific for SARS-CoV-2 ( Figure 2D ).

In further experiments, we used HLA class I tetramers as probes to detect CD8 + T cells specific for predicted optimal epitopes from SARS-CoV-2 ( Figure S3E ) (Table   S2) . A vast majority of tetramer + CD8 + T cells in the acute phase of infection, but not during convalescence, displayed an activated/cycling phenotype ( Figure 2E ). In general, early SARS-CoV-2-specific CD8 + T cell populations were characterized by the expression of immune activation molecules (CD38, HLA-DR, Ki-67), inhibitory receptors (PD-1, TIM-3), and cytotoxic molecules (granzyme B, perforin), whereas convalescent phase SARS-CoV-2-specific CD8 + T cell populations were skewed toward an early differentiated memory (CCR7 + CD127 + CD45RA −/+ TCF1 + ) phenotype ( Figure 2F ). Importantly, the expression frequencies of CCR7 and CD45RA among SARS-CoV-2-specific CD8 + T cells were positively correlated with the number of symptom-free days after infection (CCR7: r = 0.79, p = 0.001; CD45RA: r = 0.70, p = 0.008), whereas the expression frequency of granzyme B among SARS-CoV-2-specific CD8 + T cells was inversely correlated with the number of symptom-free days after infection (r = 0.70, p = 0.007) ( Figure 2G ). Time from exposure was therefore associated with the emergence of stem-like memory SARS-CoV-2-specific CD8 + T cells.

On the basis of these observations, we quantified functional SARS-CoV-2-specific memory T cell responses across five distinct cohorts, including healthy individuals who donated blood either before or during the pandemic, family members who shared a household with convalescent individuals and were exposed at the time of symptomatic disease, and individuals in the convalescent phase after mild or severe COVID-19. We detected potentially cross-reactive T cell responses directed against either the spike or membrane proteins in a total of 28% of the healthy individuals who donated blood before the pandemic, consistent with previous reports Le Bert et al., 2020) , but nucleocapsid reactivity was notably absent in this cohort ( Figure 3A and Figure S4A , B). The highest response frequencies against any of the three proteins were observed in convalescent individuals who experienced severe COVID-19 (100%). Progressively lower response frequencies were observed in convalescent individuals with a history of mild COVID-19 (87%), exposed family members (67%), and healthy individuals who donated blood during the pandemic (46%) ( Figure 3A ).

To assess the functional capabilities of SARS-CoV-2-specific memory CD4 + and CD8 + T cells in convalescent individuals, we stimulated PBMCs with the overlapping spike, membrane, and nucleocapsid peptide sets and measured a surrogate marker of degranulation (CD107a) along with the production of interferon (IFN)-γ, IL-2, and TNF ( Figure 3B , C). SARS-CoV-2-specific CD4 + T cells predominantly expressed IFN-γ, IL-2, and TNF ( Figure 3B ), whereas SARS-CoV-2-specific CD8 + T cells predominantly expressed IFN-γ and mobilized CD107a ( Figure 3C ). We then used the AIM assay to determine the functional polarization of SARS-CoV-2-specific CD4 + T cells. Interestingly, spike-specific CD4 + T cells were skewed toward a circulating T follicular helper (cTfh) profile, suggesting a key role in the generation of potent antibody responses, whereas membrane-specific and nucleocapsid-specific CD4 + T cells were skewed toward a Th1 or a Th1/Th17 profile ( Figure 3D and Figure S5A , B).

In a final series of experiments, we assessed the recall capabilities of SARS-CoV-2specific CD4 + and CD8 + T cells in convalescent individuals, exposed family members, and healthy blood donors. Proliferative responses were identified by tracking the progressive dilution of a cytoplasmic dye (CellTrace Violet; CTV) after stimulation with the overlapping spike, membrane, and nucleocapsid peptide sets, and functional responses to the same antigens were evaluated 5 days later by measuring the production of IFN-γ (Blom et al., 2013; Buggert et al., 2014) .

Anamnestic responses in the CD4 + and CD8 + T cell compartments, quantified as a function of CTV low IFN-γ + events ( Figure 4A ), were detected in most convalescent individuals (MC = 96%, SC = 100%) and exposed family members (92%) ( Figure 4B , C). SARS-CoV-2-specific CD4 + T cell responses were proportionately larger overall than the corresponding SARS-CoV-2-specific CD8 + T cell responses (EF = 1.8-fold, MC = 1.4-fold, SC = 1.8-fold larger accordingly) ( Figure 4D ). In addition, most IFN-γ + SARS-CoV-2-specific CD4 + T cells produced TNF, and most IFN-γ + SARS-CoV-2specific CD8 + T cells produced granzyme B and perforin ( Figure 4E ).

Serological evaluations revealed a strong positive correlation between IgG responses directed against the spike protein of SARS-CoV-2 and IgG responses directed the nucleocapsid protein of SARS-CoV-2 (r = 0.82, p < 0.001) ( Figure S5C ).

Moreover, SARS-CoV-2-specific CD4 + and CD8 + T cell responses were present in J o u r n a l P r e -p r o o f seronegative individuals, albeit at lower frequencies compared with seropositive individuals (41% versus 99%, respectively) ( Figure 4F ). These discordant responses were nonetheless pronounced in some convalescent individuals with a history of mild COVID-19 (3/31), exposed family members (9/28), and healthy individuals who donated blood during the pandemic (5/31) ( Figure 4F and Figure S5D ), often targeting both the internal (nucleocapsid) and surface antigens (spike and/or membrane) of SARS-CoV-2 ( Figure 4G ). Higher frequencies of T cell responses were also found in exposed seronegative family members compared to unexposed donors ( Figure S5E ). Potent memory T cell responses were therefore elicited in the absence or presence of circulating antibodies, consistent with a non-redundant role as key determinants of immune protection against COVID-19 (Chandrashekar et al., 2020) .

J o u r n a l P r e -p r o o f

We are currently facing the biggest global health emergency in decades, namely the devastating outbreak of COVID-19. In the absence of a protective vaccine, it will be critical to determine if exposed and/or infected people, especially those with asymptomatic or very mild forms of the disease who likely act inadvertently as the major transmitters, develop robust adaptive immunity against SARS-CoV-2 (Long et al., 2020) .

In this study, we used a systematic approach to map cellular and humoral immune responses against SARS-CoV-2 in patients with acute moderate or severe COVID-19, individuals in the convalescent phase after mild or severe COVID-19, exposed family members, and healthy individuals who donated blood before (2019) or during the pandemic (2020). Individuals in the convalescent phase after mild COVID-19 were traced after returning to Sweden from endemic areas (mostly Northern Italy).

These donors exhibited robust memory T cell responses months after infection, even in the absence of detectable circulating antibodies specific for SARS-CoV-2, indicating a previously unanticipated degree of population-level immunity against COVID-19.

We found that T cell activation, characterized by the expression of CD38, was a hallmark of acute COVID-19. Similar findings have been reported previously in the absence of specificity data (Huang et al., 2020; Thevarajan et al., 2020; Wilk et al., 2020) . Many of these T cells also expressed HLA-DR, Ki-67, and PD-1, indicating a combined activation/cycling phenotype correlates with an early strong immune response, including an early SARS-CoV-2-specific IgG response, and to a lesser extent with plasma levels of various inflammatory markers. Our data also showed that many activated/cycling T cells in the acute phase were functionally replete and specific for SARS-CoV-2. Equivalent functional profiles have been observed early after immunization with successful vaccines (Blom et al., 2013; Miller et al., 2008; Precopio et al., 2007) . Accordingly, the expression of multiple inhibitory receptors, including PD-1, likely indicates early activation rather than exhaustion (Zheng et al., 2020a; Zheng et al., 2020b) .

Virus-specific memory T cells have been shown to persist for many years after infection with SARS-CoV-1 (Le Bert et al., 2020; Tang et al., 2011; Yang et al., 2006) . In line with these observations, we found that SARS-CoV-2-specific T cells acquired an early differentiated memory (CCR7 + CD127 + CD45RA −/+ TCF1 + ) J o u r n a l P r e -p r o o f phenotype in the convalescent phase, as reported previously in the context of other viral infections and successful vaccines (Blom et al., 2013; Demkowicz et al., 1996; Fuertes Marraco et al., 2015; Precopio et al., 2007) . This phenotype has been associated with stem-like properties (Betts et al., 2006; Blom et al., 2013; Demkowicz et al., 1996; Fuertes Marraco et al., 2015; Precopio et al., 2007) . Accordingly, we found that SARS-CoV-2-specific T cells generated anamnestic responses to cognate antigens in the convalescent phase, characterized by extensive proliferation and polyfunctionality. Of particular note, we detected similar memory T cell responses directed against the internal (nucleocapsid) and surface proteins (membrane and/or spike) in some individuals lacking detectable circulating antibodies specific for SARS-CoV-2. Indeed, about twice as many healthy individuals who donated blood during the pandemic generated memory T cell responses in the absence of detectable circulating antibody responses, implying that seroprevalence as an indicator may underestimate the extent of population-level immunity against SARS-CoV-2.

Our study was cross-sectional in nature and limited in terms of clinical follow-up and overall donor numbers in each outcome-defined group. It therefore remains to be determined if robust memory T cell responses in the absence of detectable circulating antibodies can protect against severe forms of COVID-19. This scenario has nonetheless been inferred from previous studies of MERS and SARS-CoV-1 (Channappanavar et al., 2014; Li et al., 2008; Zhao et al., 2017; Zhao et al., 2016) , both of which have been shown to induce potent memory T cell responses that persist while antibody responses wane (Alshukairi et al., 2016; Shin et al., 2019; Tang et al., 2011) . Notably, waning antibodies, as distinguished in SARS-CoV-2 infection (Ibarrondo et al., 2020; Long et al., 2020) , is a natural phenomenon following coronavirus infections (Callow et al., 1990) . The fact that memory B cells (Juno et al., 2020) and robust T cell memory is formed after SARS-CoV-2 infection, suggests that potent adaptive immunity is maintained to provide protection against severe re-infection. In line with these observations, none of the convalescent individuals in this study, including those with previous mild disease, have experienced further episodes of COVID-19.

Of note, we detected cross-reactive T cell responses against spike or membrane in 28% of the unexposed healthy blood donors, consistent with a high degree of preexisting immune responses potentially induced by other coronaviruses (Braun et al., 2020; Grifoni et al., 2020; Le Bert et al., 2020) . Data on the cross-reactive responses where based on cryopreserved samples, which could have a negative impact on the J o u r n a l P r e -p r o o f frequency of T cell responders in SARS-CoV-2 unexposed donors (Owen et al., 2007) . Although we detected generally broader and stronger T cell responses in seronegative convalescent and exposed individuals compared to unexposed donors, it remains possible that a fraction of the anamnestic SARS-CoV-2-specific T cell response was initially induced by seasonal coronaviruses . The biological relevance of cross-reactive T cell responses remains unclear. However, it is tempting to speculate that such responses may provide at least partial protection against SARS-CoV-2, and different disease severity, given that pre-existing T cell immunity has been associated with beneficial outcomes after challenge with the pandemic influenza virus strain H1N1 (Sridhar et al., 2013; Wilkinson et al., 2012) .

Collectively, our data provide a functional and phenotypic map of SARS-CoV-2specific T cell immunity across the full spectrum of exposure, infection, and disease.

The observation that many individuals with asymptomatic or mild COVID-19, after SARS-CoV-2 exposure or infection, generated highly durable and functionally replete memory T cell responses, not uncommonly in the absence of detectable humoral responses, further suggests that natural exposure or infection could prevent recurrent episodes of severe COVID-19. 

The authors declare that they have no competing financial interests, patents, patent applications, or material transfer agreements associated with this study. 

Further information and requests for resources and reagents should be directed to and will be fulfilled by the Lead Contact, Marcus Buggert (marcus.buggert@ki.se)

Aliquots of synthesized tetramers and monomers utilized in this study will be made available upon request. There are restrictions to the availability of the monomers due to cost and limited quantity.

The published article includes all data generated during this study. All codes are freely available at source.

Maximal disease severity was assessed using the NIH Ordinal Scale and Sequential

Organ Failure Assessment (SOFA) (Beigel et al., 2020; Singer et al., 2016) . The NIH

Ordinal Scale was defined as follows: (1) Donors were assigned to one of seven groups for the purposes of this study. AS:

patients with acute severe disease requiring hospitalization in the high dependency or intensive care unit, with low-flow oxygen support (>10 L/min), high-flow oxygen support, or invasive mechanical ventilation (n = 17). These patients had a median NIH Ordinal Scale score of 7 (IQR 6-7) and a median SOFA score of 6 (IQR 3-6) at the time of sampling 12-17 days after disease onset (47% were viremic, and 82%

were antibody-seropositive for SARS-CoV-2). AM: patients with acute moderate disease requiring hospitalization and low-flow oxygen support (0-3 L/min; n = 10).

These patients had a median NIH Ordinal Scale score of 5 (IQR 5-5) and a median SOFA score of 1 (IQR 1-1) at the time of sampling 11-14 days after disease onset (40% were viremic, and 50% were antibody-seropositive for SARS-CoV-2). SC:

individuals in the convalescent phase after severe disease (n = 26). Samples were collected 42-58 days after disease onset, corresponding to 3-21 days after resolution of symptoms (100% were antibody-seropositive for SARS-CoV-2). MC:

individuals in the convalescent phase after mild disease (n = 40). Samples were collected 49-64 days after disease onset, corresponding to 25-53 days after resolution of symptoms (85% were antibody-seropositive for SARS-CoV-2). Exp:

family members who shared a household with donors in groups MC or SC (n = 30).

These individuals were exposed at the time of symptomatic disease (21% remained asymptomatic, and 64% were antibody-seropositive for SARS-CoV-2). 2020 BD: Table S3 , and immunological assay breakdowns are summarized in Table S4 .

PBMCs were isolated from venous blood samples via standard density gradient 

Peptides Peptides corresponding to known optimal epitopes derived from CMV (pp65) and

EBV (BZLF1 and EBNA-1), overlapping peptides spanning the immunogenic domains of the SARS-CoV-2 spike (Prot_S), membrane (Prot_M), and nucleocapsid proteins (Prot_N), and optimal peptides for the manufacture of HLA class I tetramers were synthesized at >95% purity. Lyophilized peptides were reconstituted at a stock concentration of 10 mg/mL in DMSO and further diluted to 100 µg/mL in PBS.

Peptides were selected from full-length SARS-CoV-2 sequences spanning 82 different strains from 13 countries (National Center for Biotechnology Information).

The predicted binding affinities of conserved 9mer peptides for HLA-A*0201 and HLA-B*0702 were determined using NetMHCpan version 4. 1 (Reynisson et al., 2020) . Binders were defined by a threshold IC 50 value of 500 nM. Strong binders were defined by a % Rank <0.5, and weak binders were defined by a % Rank <2 (Table S2) . A total of 13 strong binders were identified for tetramer generation (Table   S2 ).

HLA class I tetramers were generated as described previously (Price et al., 2005) . 

PBMCs were labeled with CTV (0.5 µM; Thermo Fisher Scientific), resuspended in complete medium at 1 x 10 7 cells/mL, and cultured at 1 x 10 6 cells/well in 96-well Ubottom plates (Corning) with the relevant peptides (each at 1 µg/mL) in the presence of anti-CD28/CD49d (3 µL/mL; clone L293/L25; BD Biosciences) and IL-2 (10 IU/mL; PeproTech). Negative control wells lacked peptides, and positive control wells included SEB (0.5 µg/mL; Sigma-Aldrich) or plate-bound anti-CD3 (1 µg/mL; clone OKT3; BioLegend). Functional assays were performed as described above after incubation for 5 days at 37°C.

PBMCs were resuspended in complete medium at 1 x 10 7 cells/mL and cultured at 1 J o u r n a l P r e -p r o o f Trucount Absolute counts were obtained using Multitest 6-color TBNK reagent with Trucount tubes (BD Biosciences). Samples were fixed with 2% PFA for 2 hr prior to acquisition. Absolute CD3 + cell counts were calculated using the following formula: (# CD3 + events acquired x total # beads x 1000) / (# beads acquired x volume of whole blood stained in µL). CD4 + and CD8 + cell counts were computed from the respective frequencies relative to CD3 + cells.

Analyses were performed using scikit-learn version 0.22.1 in Python. Data were normalized using sklearn.preprocessing.StandardScaler in the same package to generate z-scores for PCA. 

PBMCs were rested overnight in complete medium and seeded at 2 x 10 5 cells/well J o u r n a l P r e -p r o o f 

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Acute phase SARS-CoV-2-specific T cells display an activated cytotoxic phenotype 2. Broad and polyfunctional SARS-CoV-2-specific T cell responses in convalescent phase 3. Detection of SARS-CoV-2-specific T cell responses also in seronegative individuals eTOC Blurb Buggert and colleagues provide a phenotypic and functional map of SARS-CoV-2-specific T cells across the full spectrum of exposure, infection, and COVID-19 severity. They observe that SARS-CoV-2-specific T cells generate a broad, robust and functionally replete response in convalescent individuals

We express our gratitude to all donors, health care personnel, study coordinators, administrators, and laboratory managers involved in this work. 
