key: cord-0971926-q7vv128t authors: Hellerstein, Marc title: What are the roles of antibodies versus a durable, high quality T-cell response in protective immunity against SARS-CoV2? date: 2020-08-28 journal: Vaccine X DOI: 10.1016/j.jvacx.2020.100076 sha: 42e7b56b36fda0c74e687586c96028640caf4621 doc_id: 971926 cord_uid: q7vv128t The first SARS-CoV2 vaccine(s) will likely be licensed based on neutralizing antibodies in Phase 2 trials, but there are significant concerns about using antibody response in coronavirus infections as a sole metric of protective immunity. Antibody response is often a poor marker of prior coronavirus infection, particularly in mild infections, and is shorter-lived than virus-reactive T-cells; strong antibody response correlates with more severe clinical disease while T-cell response is correlated with less severe disease; and antibody-dependent enhancement of pathology and clinical severity has been described. Indeed, it is unclear whether antibody production is protective or pathogenic in coronavirus infections. Early data with SARS-CoV2 support these findings. Data from coronavirus infections in animals and humans emphasize the generation of a high-quality T cell response in protective immunity. Yellow Fever and smallpox vaccines are excellent benchmarks for primary immune response to viral vaccination and induce long-lived virus-reactive CD8 T-cells, which are measurable within 1-4 months of vaccination. Progress in laboratory markers for SARS-CoV2 has been made with identification of epitopes on CD4 and CD8 T-cells in convalescent blood. These are much less dominated by spike protein than in previous coronavirus infections. Although most vaccine candidates are focusing on spike protein as antigen, natural infection by SARS-CoV-2 induces broad epitope coverage, cross-reactive with other betacoronviruses. It will be important to understand the relation between breadth, functionality and durability of T-cell responses and resulting protective immunity. It would be a public health and general trust-in-medicine nightmare - including a boost to anti-vaccine forces - if immune protection wears off or new disease patterns develop among the immunized. Data correlating clinical outcomes with laboratory markers of cell-mediated immunity, not only with antibody response, after SARS-CoV2 natural infection and vaccines may prove critically valuable if protective immunity fades or if new patterns of disease emerge. The most definitive solution to the current world-wide public health and economic crisis will be an effective vaccine against SARS-CoV-2 (COVID- 19) . But due to the urgency of this moment, the first SARS-CoV-2 vaccine(s) will likely be licensed based on laboratory evidence of neutralizing antibodies in earlier (Phase 2) trials, prior to Phase 3 efficacy and safety trials. Both humoral immunity and cell mediated immunity, particularly from CD8 Tcells, play key roles in vaccine-induced protective immunity against intracellular infections like viruses (1) . For coronavirus infections including SARS-Cov-2, the literature is striking on this topic and raises important concerns. infection (2) (3) (4) (5) (6) (7) (8) (9) (10) . In severe acute respiratory syndrome (SARS), only 50% of survivors had detectable antibodies at 3 years and none had antibodies or B-cell responses to SARS-CoV-1 at 6 years, while virus-specific T-cells remains at 6 -17 years (2-4, 8, 9) . Antibody response in Middle East respiratory syndrome (MERS) is low or absent in mild disease (5) . Experimental infection with a common cold coronavirus in humans resulted in antibodies that died away within 1 year (6) . Early data in SARS-CoV-2 infection suggest short-lived, less robust or absent antibody response in mild clinical disease, with 40% of asymptomatic patients being seronegative for anti-spike IgG ~12 weeks after virologic diagnosis and a 70% mean reduction from initial IgG levels (7,10). 2. Strong antibody response correlates with more severe clinical disease while Tcell response is correlated with less severe disease. MERS survivors with higher antibody levels had experienced longer ICU stays and required more ventilator support compared to subjects with no detectable antibodies (11), while higher virus-specific Tcell counts were observed with no detectable antibodies in recovered patients who had less severe disease. The authors (11) proposed that T-cells clear virus rapidly, which reduces disease severity, exposure to virus and the strength of antibody response. Higher IgG levels against spike protein during acute infection were observed in SARS patients who subsequently died, associated with worse clinical lung injury and proinflammatory macrophages, compared to SARS patients who went on to recover (12) . In COVID-19 patients, total T cells counts are markedly lower in most patients compared to healthy controls and low CD8 T-cell counts (<165/mcl) are a predictor of higher risk for death (13, 14) . Antibody response is higher in severe disease than in milder disease (15) and an abrupt elimination of virus is not observed after the appearance of antibodies (16). 3. Antibodies can worsen disease (antibody-dependent enhancement) in coronavirus infection in animals and possibly humans. Feline infectious peritonitis is a coronavirus disease. Severity is worsened by vaccination or passive immunization with serum from cats containing high antibody titers prior to viral infection (17) . SARS-CoV-1 virus causes hepatitis in ferrets only in previously vaccinated animals (18) . In macaques, administering immunoglobulin against spike protein worsened subsequent SARS-CoV-1-induced lung damage, induced inflammatory cytokines and reduced wound healing (12) . These findings parallel dengue hemorrhagic fever in humans, where initial infection and antibody response followed by a second infectious episode is required for serious hemorrhagic disease. Subneutralizing antibodies can promote viral entry into cells, including entry into and activation of macrophages. Vaccine trials for feline infectious peritonitis and dengue had to be halted because of disease enhancement. It has to be considered that antibodies alone might worsen coronavirus disease severity. Emerging data in COVID-19 patients support this concern. High serum IgG levels against SARS-CoV-2 are associated with more serious disease (19, 20) . As Cao wrote (19) , "significant antibody production is observed; however, whether this is protective or pathogenic remains to be determined." T-cells might also amplify tissue damage in lung and heart in established coronavirus infection or after vaccination, due to cytokine excess or an eosinophilic proinflammatory Th2 response of CD4 T-cells (21) . Although eosinophilic Th2 lung damage was reported in mice from viral challenge after a SARS-CoV-1 vaccine (22) , data in COVID-19 patients do not show a Th2 cytokine profile (23, 24) . In mouse models of SARS, virus-specific T-cells are necessary and sufficient for protection against disease (25 -27) . Several authors have come to the conclusion that a coronavirus vaccine should optimally induce virus-specific T-cells, not just antibodies. Zhao (11) wrote , "future vaccines against emerging coronaviruses should emphasize the generation of a memory CD8 T cell response for optimal protection" and Liu (12) concluded "in addition to a strong anti-SARS-CoV antibody response, an optimal memory CD8 T cell response will be an important goal in vaccine design". Recent publications (23, 28 -32) identifying epitopes on CD4 and CD8 T-cells against SARS-CoV-2 in convalescent blood represent a key step toward understanding the role of adaptive T-cell responses in COVID-19 protective immunity. Sette and Crotty's laboratories (23) reported that CD4 T-cell responses are less dominated by spike protein epitopes than in previous coronavirus infections. Spike accounted for 27% of total responsive CD4 T-cells, with M and N proteins accounting for 27% and 11%, respectively. In comparison, spike protein accounted for ~2/3 of reactive CD4 T-cells after previous coronavirus infections in humans (23), with one study (33) reporting no M or N CD4 response in recovered SARS-CoV-1 patients. The results were even more striking for CD8 T-cells. Spike-reactive CD8 T-cells comprised only 26% and M 22% of the total CD8 responsive cells, while nsp6, ORF3a, and N comprised ~50%. This is very different from prior coronavirus infections, where spike generally contributed ~50% and N comprised 36%, although Zhao et al (11) showed broad responses to spike, N and M in MERS survivors. Other studies have confirmed the breadth of CD4 and CD8 T-cell responses in COVID-19 convalescent patients. M-and NP-reactive cells (30, 31) or M and N-reactive cells (10) are equal to or more prevalent than spike-reactive CD8 and CD4 T-cells. (14) . These findings suggest that the adaptive CD8 T-cell immune response in general and broad T-cell specificity in particular confer protective rather than pathologic effects. Peng et al. (30) concluded that "the identification of T cell specificity and functionality associated with milder disease highlights the potential importance of including non-spike proteins within future COVID-19 vaccine design." We are fortunate to have in Yellow Fever (YF) and smallpox vaccines excellent benchmarks for primary immune response to viral vaccination. These vaccines induce remarkably effective and long-lived immune protection and share common features for cellular immunity: generation of CD8 T-cells with broad specificity, high magnitude, polyfunctionality, high proliferative potential and long-term persistence (1). This CD8 Tcell response pattern provides us with criteria for evaluating long-term, "high quality" protective immunity in vaccine trials or after natural infection. In this context, recent data on the durability and cross-reactivity of SARS-CoV-2 responsive T-cells in exposed and unexposed subjects may have important clinical implications. LeBert et al. (28) reported remarkable results about T-cells that react to the coronavirus structural protein NP in a Singapore population. Prevalence of reactive T-cells to SARS-CoV-2 NP in was 100% in COVID-19 recovered patients (36/36 patients) and included coverage of multiple regions of the NP protein. Moreover, 23/23 patients studied 17 years after recovery from SARS-CoV-1 infection still had reactive T-cells to SARS-CoV-1 NP. Importantly, these cells also reacted to SARS-CoV-2 NP. Finally, ~50% (19/37) of subjects who were never clinically exposed to SARS-CoV-1 or -2 infection exhibited SARS-CoV-2 NP-reactive T-cells. These results in unexposed humans for NP-reactive T-cells are supported by findings for spike-reactive T-cells (29) . T-cells that react to SARS-CoV-2 spike protein were present in 34% of seronegative, clinically unexposed healthy controls in a German cohort. COVID-19 recovered patients in this population showed 83% prevalence of SARS-CoV-2 spike-reactive T-cells (and higher levels than in unexposed subjects). An intriguing finding was that the COVID-19 patients' T-cells covered epitopes broadly spaced across the spike protein, whereas the epitopes covered in control subjects were mostly against the C-terminal region, which has greater homology to spike proteins in betacoronviruses that cause the common cold. Prevalence of T-cells that are reactive to SARS-CoV-2 proteins has also been reported in unexposed subjects in studies from the The apparent durability of virus-specific T-cells against SARS-CoV-1 (8, 9) after natural infection is also a central feature of highly effective viral vaccines -in particular, YF and smallpox vaccinations (1, 36) . To understand the basis of long-lived, high quality protective immunity following viral vaccination, we (36) T cells interact with humoral immunity in several ways that can influence both protective immunity and tissue pathology. Knowledge is advancing on how this plays out for natural coronavirus infections (Figure 2 ). Protective natural immunity to coronavirus infections, including SARS-CoV-2, provides criteria for vaccine evaluation. In particular, CD8 T-cells with broad specificity (not just to spike protein) and long persistence, more than a robust antibody response alone, may be a signature of successful protective immunity against SARS-CoV-2 and SARS-CoV-1 infections. A key early question for any candidate vaccine for COVID-19 will therefore be whether it induces durable, high quality T-cell protective immunity. ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Insights into human CD8(+) T-cell memory using the yellow fever and smallpox vaccines Lack of peripheral memory B cell responses in recovered patients with severe acute respiratory syndrome: a six-year follow-up study Duration of antibody responses after severe acute respiratory syndrome Disappearance of antibodies to SARS-associated coronavirus after recovery Antibody response and disease severity in healthcare worker MERS survivors The time course of the immune response to experimental coronavirus infection of man Clinical and immunological assessment of asymptomatic SARS-CoV-2 infections Memory T cell responses targeting the SARS coronavirus persist up to 11 years post-infection SARS-CoV-2-specific T cell immunity in cases of COVID-19 and SARS, and uninfected controls Robust T cell immunity in convalescent individuals with asymptomatic or mild COVID-19 (Preprint) bioRxiv 2020 Recovery from the Middle East respiratory syndrome is associated with antibody and T-cell responses Antispike IgG causes severe acute lung injury by skewing macrophage responses during acute SARS-CoV infection IL-6 and CD8+ T cell counts combined are an early predictor of in-hospital mortality of patients with COVID-19 Reduction and Functional Exhaustion of T Cells in Patients With Coronavirus Disease 2019 (COVID-19) Frontiers in Immunology Magnitude and kinetics of anti-SARS CoV-2 antibody responses and their relationship to disease severity Virological assessment of hospitalized patients with COVID-2019 Early death after feline infectious peritonitis virus challenge due to recombinant vaccinia virus immunization Immunization with modified vaccinia virus Ankara-based recombinant vaccine against severe acute respiratory syndrome is associated with enhanced hepatitis in ferrets COVID-19: immunopathology and its implications for therapy Antibody responses to SARS-CoV-2 in patients of novel coronavirus disease 2019 Clinical features of patients infected with 2019 novel coronavirus in Wuhan A double-inactivated severe acute respiratory syndrome coronavirus vaccine provides incomplete protection in mice and induces increased eosinophilic proinflammatory pulmonary response upon challenge Targets of T Cell Responses to SARS-CoV-2 Coronavirus in Humans with COVID-19 Disease and Unexposed Individuals Phenotype and kinetics of SARS-CoV-2-specific T cells in COVID-19 patients with acute respiratory distress syndrome Virus-specific memory CD8 T cells provide substantial protection from lethal severe acute respiratory syndrome coronavirus infection T cell responses are required for protection from clinical disease and for virus clearance in severe acute respiratory syndrome coronavirusinfected mice Understanding the T cell immune response in SARS coronavirus infection SARS-CoV-2-specific T cell immunity in cases of COVID-19 and SARS, and uninfected controls SARS-CoV-2-reactive T cells in patients and healthy donors Broad and strong memory CD4 + and CD8 + T cells induced by SARS-CoV-2 in UK convalescent COVID-19 patients The SARS-CoV-2 T-cell immunity is directed against the spike, membrane, and nucleocapsid protein and associated with COVID 19 severity Single-cell landscape of bronchoalveolar immune cells in patients with COVID-19 T cell responses to whole SARS coronavirus in humans Immunogenicity and safety of a recombinant adenovirus type-5-vectored COVID-19 vaccine in healthy adults aged 18 years or older: a randomised, double-blind, placebo-controlled, phase 2 trial Safety and immunogenicity of the ChAdOx1 nCoV-19 vaccine against SARS-CoV-2: a preliminary report of a phase 1/2, single-blind, randomised controlled trial Origin and differentiation of human memory CD8 T cells after vaccination Measurement of cell proliferation by heavy water labeling Methods for Measuring T-Cell Memory to Vaccination: From Mouse to Man An mRNA Vaccine against SARS Single-shot Ad26 vaccine protects against SARS-CoV-2 in rhesus macaques DNA vaccine protection against SARS-CoV-2 in rhesus macaques Developing Covid-19 Vaccines at Pandemic Speed