key: cord-1051941-0ave9s2f
authors: Painter, Mark M.; Mathew, Divij; Goel, Rishi R.; Apostolidis, Sokratis A.; Pattekar, Ajinkya; Kuthuru, Oliva; Baxter, Amy E.; Herati, Ramin S.; Oldridge, Derek A.; Gouma, Sigrid; Hicks, Philip; Dysinger, Sarah; Lundgreen, Kendall A.; Kuri-Cervantes, Leticia; Adamski, Sharon; Hicks, Amanda; Korte, Scott; Giles, Josephine R.; Weirick, Madison E.; McAllister, Christopher M.; Dougherty, Jeanette; Long, Sherea; D’Andrea, Kurt; Hamilton, Jacob T.; Betts, Michael R.; Bates, Paul; Hensley, Scott E.; Grifoni, Alba; Weiskopf, Daniela; Sette, Alessandro; Greenplate, Allison R.; Wherry, E. John
title: Rapid induction of antigen-specific CD4+ T cells is associated with coordinated humoral and cellular immune responses to SARS-CoV-2 mRNA vaccination
date: 2021-08-13
journal: Immunity
DOI: 10.1016/j.immuni.2021.08.001
sha: b9391300f04b0da8f970b599fcfd6f7bfccdd751
doc_id: 1051941
cord_uid: 0ave9s2f

SARS-CoV-2 mRNA vaccines have shown remarkable clinical efficacy, but questions remain about the nature and kinetics of T cell priming. We performed longitudinal antigen-specific T cell analyses on healthy SARS-CoV-2 naïve and recovered individuals prior to and following mRNA prime and boost vaccination. Vaccination induced rapid antigen-specific CD4+ T cell responses in naïve subjects after the first dose, whereas CD8+ T cell responses developed gradually and were variable in magnitude. Vaccine-induced Th1 and Tfh cell responses following the first dose correlated with post-boost CD8+ T cell and neutralizing antibody, respectively. Integrated analysis revealed coordinated immune responses with distinct trajectories in SARS-CoV-2 naïve and recovered individuals. Lastly, whereas booster vaccination improved T cell responses in SARS-CoV-2 naïve subjects, the second dose had little effect in SARS-CoV-2 recovered individuals. These findings highlight the role of rapidly primed CD4+ T cells in coordinating responses to the second vaccine dose in SARS-CoV-2 naïve individuals.

The COVID-19 pandemic has had a profound global toll on human life and socioeconomic well-53 being, prompting emergency use authorization of prophylactic mRNA vaccines (Cutler and 54 Summers, 2020) . Recent studies have documented strong memory B cell and antibody 55 responses post-vaccination that neutralize SARS-CoV-2, including variants of concern (VOC) 56 such as B.1.351 (Beta) (Goel et al., 2021; Krammer et al., 2021; Sahin et al., 2020; Widge et al., (Soresina et al., 2020; Wu et al., 2020; Wurm 61 et al., 2020) . Moreover, in patients with hematological malignancy, CD8 + T cells appear to 62 compensate for lack of humoral immunity and were associated with improved outcomes, 63 indicating a role for T cells in protection against SARS-CoV-2 infection (Bange et al., 2021) . The

T cell response to mRNA vaccination is less well-characterized than the humoral response, 65 though initial reports indicate that T cells, particularly CD4 + T cells, are primed by the vaccine 66 (Anderson et al., 2020; Angyal, 2021; Camara et al., 2021; Jackson et al., 2020; Kalimuddin et 67 al., 2021; Lederer et al., 2020; Mazzoni et al., 2021; Prendecki et al., 2021; Sahin et al., 2020;  68 Stamatatos et al., 2021; Tarke et al., 2021b; Woldemeskel et al., 2021) . However, the details of 69 antigen-specific T cell induction following vaccination remain incompletely understood, and 70 questions remain about the trajectory of the adaptive immune response following vaccination.

T cell immunity is functionally heterogeneous, with subsets of both CD4 + and CD8 + T cells 72 contributing to protective immunity and long-term immunological memory. Specifically, CD4 + T 73 follicular helper (Tfh) cells have key roles in the development of memory B cells, plasma cells 74 and antibodies, whereas Th1 cells support and enhance the quality of memory CD8 + T cell 75 responses (Crotty, 2011; Krawczyk et al., 2007; Luckheeram et al., 2012; Williams et al., 2006) .

In addition, the central memory or effector memory differentiation states of CD4 + and CD8 + T 77 cells have implications for durability, recirculation, tissue access and responses upon antigen 78 re-exposure (Kaech et al., 2002; Martin and Badovinac, 2018; Wherry et al., 2003) . In the 79 context of mRNA vaccination, relatively little is known about the nature and differentiation state 80 of antigen-specific CD4 + and CD8 + T cells. For example, it is unclear whether Tfh cells are 81 efficiently primed and whether these cells relate to vaccine-induced antibodies or memory B 82 cells. It is also unclear whether the kinetics of T cell priming differs for CD4 + and CD8 + T cells, 

In this study we sought to address these questions and define the kinetics and differentiation 87 state of vaccine-induced CD4 + and CD8 + T cells following mRNA vaccination. All SARS-CoV-2 88 naïve subjects mounted robust CD4 + T cell responses following the first vaccine dose, and the 89 second dose further boosted both CD4 + and CD8 + T cell responses. In contrast, SARS-CoV-2 90 recovered individuals had maximal CD4 + and CD8 + T cell responses following the first dose of 

We acquired longitudinal peripheral blood samples from a cohort of 36 SARS-CoV-2 naive and 102 11 SARS-CoV-2 recovered individuals who received mRNA vaccines through the University of 103 Pennsylvania Health System (Table S1) . We obtained peripheral blood mononuclear cells 104 (PBMCs) at 4 key timepoints (Fig. 1A) : pre-vaccine baseline (timepoint 1), two weeks post-105 primary vaccination (timepoint 2), the day of the booster vaccination (timepoint 3), and one 106 week post-boost (timepoint 4). PBMCs from each of these timepoints were stimulated with 107 peptide megapools containing SARS-CoV-2 Spike epitopes optimized for presentation by MHC-108 I (CD8-E) or MHC-II (CD4-S) (Grifoni et al., 2020b; Tarke et al., 2021a) . We then assessed 109 peptide-dependent activation induced marker (AIM) expression by flow cytometry compared to 110 unstimulated control samples ( Fig. 1A and Fig. S1A ) (Betts et al., 2003; Reiss et al., 2017) .

AIM + CD4 + T cells were defined by dual-expression of CD200 and CD40L. Although dual 112 expression of IFN-and 41BB was useful to visualize AIM + CD8 + T cell populations (Fig. 1A) , a 113 two-marker strategy alone was sub-optimal for detecting vaccine-elicited responses due to high 114 baseline signals (Fig. S1B) . These responses at baseline likely represent cross-reactive T cells, 115 possibly primed during a prior seasonal coronavirus infection, that can mask low frequencies of 116 vaccine-induced T cells (Grifoni et al., 2020a) . Since we sought to study vaccine responses, 117 AIM + CD8 + T cells were defined by expression of at least four of five markers: CD200, CD40L, 118 41BB, CD107a, and intracellular IFN- (Fig. S1C) . These distinct approaches for defining AIM +

CD4 + and CD8 + T cells provided optimal detection of vaccine-elicited responses relative to 120 background signals in unstimulated controls ( Fig. S2A-C) . Alternative combinations of activation 121 induced markers revealed similar kinetics of antigen-specific T cell responses, though in some 122 cases with higher background (Fig. S2D-E) . We further confirmed that the frequency of AIM + 123 CD4 + and CD8 + T cells correlated strongly with the frequency of activated Ki67 + CD38 + T cells 124 (Fig. S1D) , another method for quantifying antigen-specific responses, after each vaccine dose 125 (Miller et al., 2008; Ndhlovu et al., 2015) . As expected, most SARS-CoV-2 recovered donors had clearly detectable antigen-specific CD4 + 130 and CD8 + T cell populations at baseline (Fig. 1B) . In contrast, pre-vaccination responses to 131 peptide stimulation were undetectable in many SARS-CoV-2 naïve individuals, though some 132 subjects did have low frequencies of pre-vaccination AIM + T cells that may be attributed to 133 J o u r n a l P r e -p r o o f cross-reactive cells from a prior seasonal coronavirus infection (Grifoni et al., 2020a) (Fig. 1B) .

SARS-CoV-2 Spike-specific CD4 + T cells were robustly primed in SARS-CoV-2 naïve and 135 recovered individuals following the first dose of mRNA vaccine, with all participants generating 136 detectable responses to the first dose (Fig. 1B) . SARS-CoV-2 naïve individuals, but not 137 recovered individuals, received an additional boost to antigen-specific CD4 + T cells following the 138 second vaccine dose (Fig. 1B) . Overall, mRNA vaccination induced a universal CD4 + T cell 139 response, as all individuals, regardless of prior infection with SARS-CoV-2, had greater 140 frequencies of AIM + CD4 + T cells post-boost than at pre-vaccine baseline (Fig. S1E) . 150 SARS-CoV-2 naïve individuals (88%) had post-boost CD8 + T cell responses that were 151 detectable above their individual pre-vaccine baseline ( Fig. 1B and Fig. S1E ). Individuals who 152 had previously recovered from SARS-CoV-2 infection experienced no significant increase in the 153 frequency of AIM + CD8 + T cells from either dose of vaccine ( Fig. 1B and Fig. S1E) . A subset of 154 recovered individuals (70%) did appear to have increased AIM + CD8 + T cell frequencies 155 compared to baseline, but as a group this increase did not reach statistical significance (Fig. 156 S1E). In contrast to the modestly weaker induction of antibodies and memory B cells with 157 increasing age observed in this cohort and others (Abu Jabal et al., 2021; Goel et al., 2021; Levi 158 et al., 2021; Prendecki et al., 2021) , T cell responses upon mRNA vaccination were not 159 correlated with age ( Fig. S1F) . Taken together, these data demonstrate robust induction of 160 antigen-specific T cell responses following mRNA vaccination, with more consistent induction of 161 CD4 + T cell responses compared to CD8 + T cell responses.

We next sought to define the differentiation state of vaccine-induced AIM + T cells. We first 166 examined subsets of central and effector memory populations using CD45RA, CD27 and CCR7 167 J o u r n a l P r e -p r o o f (Hamann et al., 1997; Sallusto et al., 1999) . With these markers, we defined central memory 168 (CM), effector memory types 1, 2, and 3 (EM1, EM2, EM3) and terminally differentiated effector 169 memory (EMRA) cells ( Fig. 2A, 2C , and S1A) (Mathew et al., 2020) . Total non-naïve CD4 + T 170 cells were predominantly CM (CD45RA -CD27 + CCR7 + ) in this cohort, and the overall 171 frequencies of these subsets were unchanged by vaccination (Fig. S3A) . The baseline AIM +

CD4 + T cell response in SARS-CoV-2 recovered individuals, presumably generated during prior 173 SARS-CoV-2 infection, was composed mainly of EM1 (CD45RA -CD27 + CCR7 -) and CM cells 174 ( Fig. 2A-B) . The memory T cell subset distribution of these SARS-CoV-2 specific CD4 + T cells 175 did not change substantially following vaccination (Fig. 2B) . In SARS-CoV-2 naïve individuals, 176 the first dose of vaccine primarily induced AIM + CD4 + T cells in the EM1 and CM subsets, 177 similar to the response in recovered donors (Fig. 2B) . Antigen-specific CD4 + EM2 (CD45RA -178 CD27 -CCR7 + ) and EM3 (CD45RA -CD27 -CCR7 -) T cells, which share more effector-like 179 properties (Romero et al., 2007) , were also boosted by the vaccine, but remained minority 180 populations compared to CM and EM1 (Fig. 2B ).

Total non-naïve CD8 + T cells were distributed throughout memory T cell subsets and the 183 frequencies of these subsets were unchanged by vaccination (Fig. S3B) . AIM + CD8 + T cells had 184 a similar subset distribution to AIM + CD4 + T cells. The baseline antigen-specific CD8 + T cell 185 response in recovered subjects was composed of similar proportions of EM1, CM, and 186 terminally-differentiated CD8 + EMRA (CD45RA + CD27 -CCR7 -) T cells ( Fig. 2C-D) . A smaller 187 proportion of AIM + EM2 and EM3 CD8 + T cells was observed at baseline in recovered subjects.

These proportions stayed relatively consistent throughout the course of vaccination in recovered 189 subjects, and there were no statistically significant changes from baseline ( Fig. 2D) . In contrast, 190 in SARS-CoV-2 naive individuals, few AIM + EMRA CD8 + T cells were observed at any time point 191 ( Fig. 2D) . Rather, vaccine-primed AIM + CD8 + T cells in these subjects were largely EM1 with 192 minority populations of CM and EM3 cells (Fig. 2D) . With the exception of the EMRA 193 population, the antigen-specific AIM + CD8 + T cell response in SARS-CoV-2 naïve donors 194 following vaccination resembled that observed in recovered donors (Fig. 2D) . These data 195 indicate that the vaccine-elicited T cell response has a similar memory T cell subset distribution 196 to the response generated following SARS-CoV-2 infection and is comprised of primarily 197 CD45RA -CD27 + memory T cells. 

Tfh in circulation (cTfh) as well as CXCR5 -Th1 (CXCR3 + CCR6 -), Th17 (CXCR3 -CCR6 + ), Th1/17 205 (CXCR3 + CCR6 + ), and CXCR3 -CCR6cells (likely to include Th2) ( Fig. 3A and S1A) (Acosta-206 Rodriguez et al., 2007; Schmitt et al., 2014; Trifari et al., 2009) . Total non-naive CD4 + T cell 207 populations predominantly had Th1 and CXCR3 -CCR6phenotypes (Fig. S3C) . The baseline 208 AIM + CD4 + T cell response in recovered individuals, however, was dominated by cTfh and Th1 209 cells ( Fig. 3A-B) . The first dose of vaccine led to further expansion of AIM + cTfh and Th1 cells in 210 these recovered subjects, and this pattern was largely maintained through the course of 211 vaccination (Fig. 3B) . In SARS-CoV-2 naïve subjects, the first vaccine dose also elicited 212 predominantly antigen-specific Th1 and cTfh cells (Fig. 3B ). This distribution was sustained 213 through booster vaccination in SARS-CoV-2 naïve individuals, with these AIM + subsets being 214 further boosted by the second vaccine dose (Fig. 3B) . The magnitude of AIM + cTfh responses 215 was correlated with activated (Ki67 + CD38 + ) cTfh (CXCR5 + PD-1 + ) analyzed in parallel after each 216 vaccine dose (Fig. S3D) , indicating that the AIM assay accurately captures cTfh known to 217 contain antigen-specific T cells in other settings (Herati et al., 2017) . Thus, the vaccine-elicited 218 AIM + CD4 + T cell response to mRNA vaccination qualitatively resembled the response to natural 219 infection and was characterized by robust induction of antigen-specific cTfh and Th1 cells. (Crotty, 2011; Krawczyk et al., 2007; Luckheeram et al., 2012; Williams et al., 2006) .

Indeed, we observed a strong correlation between the frequency of pre-boost AIM + Th1 cells 229 and the frequency of post-boost AIM + CD8 + T cells in SARS-CoV-2 naïve individuals (Fig. 3C) , (Fig. 3C) . Despite a strong correlation between AIM + Th1 and cTfh (Fig. S3E) , pre-235 J o u r n a l P r e -p r o o f boost Th1 were less well correlated with post-boost neutralizing titers than were cTfh, and pre-236 boost cTfh did not significantly correlate with post-boost CD8 + T cell responses, supporting the 237 distinct associations of these pre-boost immune cell types to post-boost vaccine-elicited immune 238 responses (Fig. S3F) . Notably, the pre-boost Th1:cTfh ratio within the AIM + cells did not 239 correlate with post-boost humoral or CD8 + T cell responses, suggesting that the independent 240 magnitudes of pre-boost AIM + cTfh and Th1, rather than the relative skewing between these 241 responses, contribute to humoral and CD8 + T cell responses to the second dose (Fig. S3G) .

Furthermore, baseline AIM + Th1 and cTfh cells in SARS-CoV-2 naïve subjects did not correlate 243 with post-boost CD8 + T cell or neutralizing responses, respectively, suggesting minimal 244 contribution of pre-existing cross-reactive CD4 + T cells to the immune response to SARS-CoV-2 245 mRNA vaccines (Fig. S3H) . These observations highlight a key functional role for vaccine- (Goel et al., 2021) . Using these data, we integrated 26 antigen-specific features of the immune 258 response to mRNA vaccination into high-dimensional UMAP space (Fig. 4A) . Correlating 259 individual antigen-specific features with the UMAP coordinates revealed that UMAP1 is a 260 measure of the anti-SARS-CoV-2 immune response to vaccination (Fig. 4D) . UMAP1 also 261 revealed a signal of previous SARS-CoV-2 infection, as recovered subjects occupied a location 262 with increased UMAP1 signal at baseline (Fig. 4A-B) . Specifically, UMAP1 captured a 263 coordinated immune response in which antigen-specific CD8 + T cells and CD4 + Th1 and cTfh 264 cells were increased coordinately with antibodies, IgG + memory B cells, RBD-focused humoral 265 responses, and increased neutralizing antibody titers ( Fig. 4D-E) . UMAP2 captures the relative 266 balance between humoral and cellular immune responses, especially CD4 + T cell responses 267 (Fig. 4D) . Total non-naive lymphocyte populations were not altered and did not correlate with 268 the antigen-specific responses, consistent with induction of a targeted vaccine-elicited response 269 J o u r n a l P r e -p r o o f (Fig. S4A) . This UMAP projection revealed trajectory shifts that were notable between naïve 270 and recovered subjects. For example, in SARS-CoV-2 recovered individuals, there was an 271 increase in both UMAP1 and UMAP2 following primary vaccination, but essentially no change 272 following the second vaccine dose, indicating that both the magnitude and relative balance of 273 the antigen-specific response is stabilized after a single dose (Fig. 4A-C) . In SARS-CoV-2 naïve 274 subjects there was a more dynamic trajectory over time with an initial increase in UMAP1 and 275 decrease in UMAP2 signal at timepoints 2 and 3, followed by a coalescence towards increased 276 UMAP1 and UMAP2 after the second vaccine dose (Fig. 4A-C) . (Fig. S4B) . This analysis also highlights pre-boost immune response features like 287 cTfh and Th1 that correlate with post-boost humoral and cellular responses (Fig. S4B) . In In this study, we interrogated the antigen-specific CD4 + and CD8 + T cell responses induced by 292 SARS-CoV-2 mRNA vaccination in a longitudinal cohort of SARS-CoV-2 naïve and recovered 293 individuals. Our data demonstrate robust induction of antigen-specific T cells by mRNA 294 vaccination that may contribute, in addition to previously defined humoral responses, to durable 295 protective immunity. In particular, antigen-specific memory CD4 + and CD8 + T cells are likely to 

Vaccine-induced CD4 + and CD8 + T cells specific for SARS-CoV-2 were qualitatively similar to 307 baseline memory T cell responses generated following natural SARS-CoV-2 infection and 308 mainly mapped to CM and EM1 memory T cell subsets. These two subsets share many 309 functional and memory-like attributes, but differ in CCR7 expression. Since CCR7 promotes 310 homing to secondary lymphoid tissues, EM1 may represent memory T cells that can survey 311 blood and peripheral tissues, whereas CM can home efficiently to lymphoid tissues (Romero et 312 al., 2007) . These memory T cell subsets are longer-lived compared to effector T cells, and 313 access to secondary lymphoid tissues may allow CM cells to contribute to recall responses 314 upon booster vaccination or future infection. Although we await follow-up studies to directly 315 interrogate longevity, the observed induction of memory T cell subsets with capacity for 316 durability by mRNA vaccination supports the hypothesis that vaccine-induced CD4 + and CD8 + T 317 cell responses will be long-lived and capable of contributing to future recall responses.

One key observation was the rapid and universal induction of SARS-CoV-2-specific CD4 + T 326 Polack et al., 2020) , when neutralizing antibody titers are still low in many individuals (Goel et (Zhao et al., 2016) , and the rapid induction of antigen-specific CD4 + T cells after only a single 329 vaccine dose may explain the disconnect between low neutralizing responses and vaccine-330 induced protective immunity following the first dose.

The notion that early CD4 + T cell responses have a functional role in immunity is also supported 

Previous studies have demonstrated that individuals who have recovered from SARS-CoV-2 346 infection achieve maximum antigen-specific humoral immune responses after only a single 347 vaccine dose, raising the question of whether a second vaccine dose is necessary in these 348 individuals (Angyal, 2021; Bradley et al., 2021; Camara et al., 2021; Goel et al., 2021; Mazzoni 349 et al., 2021; Saadat et al., 2021; Samanovic et al., 2021; Stamatatos et al., 2021) . Our current 

 All data reported in this paper will be shared by the lead contact upon request.

 This paper does not report original code 509  Any additional information required to reanalyze the data reported in this paper is 510 available from the lead contact upon request. 

Review Board (IRB# 844642). All participants were otherwise healthy and did not report any 517 history of chronic health conditions. Subjects were identified as SARS-CoV-2 naïve or 518 recovered via combined self-reporting and laboratory evidence of a prior SARS-CoV-2 infection.

All subjects received either Pfizer (BNT162b2) or Moderna (mRNA-1273) mRNA vaccines and 520 were enrolled irrespective of which mRNA vaccine they received. Samples were collected at 4 521 timepoints: pre-vaccine baseline (timepoint 1), two weeks post-primary vaccination (timepoint 522 2), the day of the booster vaccination (timepoint 3), and one week post-boost (timepoint 4).

Each study visit included collection of clinical questionnaire data and 80-100mL of peripheral 524 blood. Full cohort and demographic information is provided in Table S1 . 

well round-bottom plates and rested overnight in a humidifed incubator at 37°C, 5% CO 2 . After 547 16 hours, CD40 blocking antibody (0.5g/mL final concentration) was added to cultures for 15 548 minutes prior to stimulation. Cells were then stimulated for 24 hours with costimulation (anti-549 human CD28/CD49d, BD Biosciences) and peptide megapools (CD4-S for all CD4 + T cell 550 analyses, CD8-E for all CD8 + T cell analyses) at a final concentration of 1 g/mL. Peptide 551 megapools were prepared as previously described (Grifoni et al., 2020b; Tarke et al., 2021a) . Data were acquired on a BD Symphony A5 instrument. Standardized SPHERO rainbow beads 581 (Spherotech) were used to track and adjust photomultiplier tube voltages over time.

Compensation was performed using UltraComp eBeads (Thermo Fisher). Up to 2x10 6 events 583 were acquired per sample. Data were analyzed using FlowJo v10 (BD Bioscience). A full gating 584 strategy for segregation of T cell subsets is shown in Fig. S1A .

The dataset of antibody and memory B cell responses from the same cohort of individuals was 588 published previously (Goel et al., 2021) . 

Impact of age, ethnicity, sex and prior infection status on immunogenicity following a 605 single dose of the BNT162b2 mRNA COVID-19 vaccine: real-world evidence from healthcare 606 workers, Israel

Surface phenotype and antigenic specificity of human 610 interleukin 17-producing T helper memory cells

Origin and differentiation of human 613 memory CD8 T cells after vaccination

Safety and Immunogenicity of 616 SARS-CoV-2 mRNA-1273 Vaccine in Older Adults

627 Lance; de Silva, Thushan I.; Consortium, PITCH (2021). T-Cell and Antibody Responses to First 628

BNT162b2 Vaccine Dose in Previously SARS-CoV-2-Infected and Infection-Naive UK Healthcare 629 Workers: A Multicentre, Prospective, Observational Cohort Study. . (Preprints with The Lancet)

Efficacy and Safety of the mRNA-1273 SARS-CoV-2 632

CD8+ T cells contribute to survival in patients 635 with COVID-19 and hematologic cancer

Sensitive and viable identification of antigen-specific CD8+ T cells by a flow 638 cytometric assay for degranulation

Antibody responses boosted in 640 seropositive healthcare workers after single dose of SARS-CoV-2 mRNA vaccine. medRxiv

Differential effects of the 644 second SARS-CoV-2 mRNA vaccine dose on T cell immunity in naïve and COVID-19 recovered 645 individuals. bioRxiv

Follicular Helper CD4 T Cells (TFH)

The COVID-19 Pandemic and the $16 Trillion Virus

Distinct antibody and memory B cell 651 responses in SARS-CoV-2 naïve and recovered individuals following mRNA vaccination

Targets of T cell responses to SARS-CoV-2 655 coronavirus in humans with COVID-19 disease and unexposed individuals

Targets of T Cell Responses

CoV-2 Coronavirus in Humans with COVID-19 Disease and Unexposed Individuals

Phenotypic and functional separation of memory and effector human CD8+ T cells. 662

Duration of antiviral immunity after smallpox vaccination

Successive annual influenza vaccination induces a 668 recurrent oligoclonotypic memory response in circulating T follicular helper cells

An mRNA Vaccine 672 against SARS-CoV-2 -Preliminary Report

Effector and memory T-cell differentiation: 674 implications for vaccine development

Early T cell and binding antibody responses are associated 677 with Covid-19 RNA vaccine efficacy onset

Antibody 680 Responses in Seropositive Persons after a Single Dose of SARS-CoV-2 mRNA Vaccine. The New 681 England journal of medicine

Memory CD4 T Cells Enhance Primary CD8 T-683 Cell Responses

SARS-CoV-2 mRNA Vaccines Foster 686

Potent Antigen-Specific Germinal Center Responses Associated with Neutralizing Antibody 687 Generation

A 689 cautionary note on recall vaccination in ex-COVID-19 subjects

CD4⁺T cells: differentiation and 691 functions

Defining Memory CD8 T Cell

Deep immune profiling of COVID-19 696 patients reveals distinct immunotypes with therapeutic implications

First dose mRNA vaccination is 699 sufficient to reactivate immunological memory to SARS-CoV-2 in ex COVID-19 subjects. 700 medRxiv

Human effector and memory CD8+ 703 T cell responses to smallpox and yellow fever vaccines

Magnitude and kinetics of CD8+ T cell 706 activation during hyperacute HIV infection impact viral set point

Safety and Efficacy of the BNT162b2 mRNA 709

Covid-19 Vaccine

Effect of previous SARS-CoV-2 infection on humoral and T-712 cell responses to single-dose BNT162b2 vaccine

Comparative analysis of activation induced 715 marker (AIM) assays for sensitive identification of antigen-specific CD4 T cells

Four functionally distinct populations of human effector-719 memory CD8+ T lymphocytes

Single Dose Vaccination in Healthcare Workers Previously Infected with SARS-CoV-2. 722 medRxiv

COVID-19 vaccine BNT162b1 elicits human antibody and 725 T(H)1 T cell responses

Two subsets of memory 727 T lymphocytes with distinct homing potentials and effector functions

Poor antigen-specific responses to 730 the second BNT162b2 mRNA vaccine dose in SARS-CoV-2-experienced individuals. medRxiv

Phenotype and functions of memory Tfh 733 cells in human blood

Two X-linked agammaglobulinemia patients develop 736 pneumonia as COVID-19 manifestation but recover. Pediatric allergy and immunology : official 737 publication of the

mRNA vaccination boosts cross-740 variant neutralizing antibodies elicited by SARS-CoV-2 infection

Comprehensive analysis of T cell immunodominance and 743 immunoprevalence of SARS-CoV-2 epitopes in COVID-19 cases

Negligible impact of SARS-CoV-2 variants on 747 CD4<sup>+</sup> and CD8<sup>+</sup> T cell reactivity in COVID-19 748 exposed donors and vaccinees. bioRxiv

Identification of a human 750 helper T cell population that has abundant production of interleukin 22 and is distinct from 751 T(H)-17, T(H)1 and T(H)2 cells

Lineage relationship and protective immunity of memory CD8 T cell 754 subsets

Durability of Responses after 757 SARS-CoV-2 mRNA-1273 Vaccination

Interleukin-2 signals during priming are 759 required for secondary expansion of CD8+ memory T cells

SARS-CoV-2 mRNA vaccines induce 761 broad CD4+ T cell responses that recognize SARS-CoV-2 variants and HCoV-NL63

Neutralizing antibody responses to SARS-CoV-2 in a COVID-19 recovered patient cohort 765 and their implications. medRxiv

Recovery from 767 COVID-19 in a B-cell-depleted multiple sclerosis patient

Airway Memory CD4(+) T Cells Mediate 771 Protective Immunity against Emerging Respiratory Coronaviruses

SARS-CoV-2 mRNA vaccines have demonstrated remarkable efficacy, but T cell responses to vaccination have not been well-studied. In a longitudinal cohort, Painter, Matthew et al. show that mRNA vaccines activate SARS-CoV-2-specific T cells that could contribute to durable immunity. The findings highlight the central role of T cells in the two-dose vaccine regimen for individuals not previously