SARS-CoV-2 spike mRNA vaccines^1–3 mediate protection from severe disease as early as ten days after prime vaccination³, when neutralizing antibodies are hardly detectable^4–6. Vaccine-induced CD8⁺ T cells may therefore be the main mediators of protection at this early stage^7,8. The details of their induction, comparison to natural infection, and association with other arms of vaccine-induced immunity remain, however, incompletely understood. Here we show on a single-epitope level that a stable and fully functional CD8⁺ T cell response is vigorously mobilized one week after prime vaccination with bnt162b2, when circulating CD4⁺ T cells and neutralizing antibodies are still weakly detectable. Boost vaccination induced a robust expansion that generated highly differentiated effector CD8⁺ T cells; however, neither the functional capacity nor the memory precursor T cell pool was affected. Compared with natural infection, vaccine-induced early memory T cells exhibited similar functional capacities but a different subset distribution. Our results indicate that CD8⁺ T cells are important effector cells, are expanded in the early protection window after prime vaccination, precede maturation of other effector arms of vaccine-induced immunity and are stably maintained after boost vaccination.

Emerging data indicate that SARS-CoV-2-specific CD8⁺ T cells targeting different viral proteins are detectable in up to 70% of convalescent individuals^1,2,3,4,5. However, very little information is currently available about the abundance, phenotype, functional capacity and fate of pre-existing and induced SARS-CoV-2-specific CD8⁺ T cell responses during the natural course of SARS-CoV-2 infection. Here, we define a set of optimal and dominant SARS-CoV-2-specific CD8⁺ T cell epitopes. We also perform a high-resolution ex vivo analysis of pre-existing and induced SARS-CoV-2-specific CD8⁺ T cells, applying peptide-loaded major histocompatibility complex class I (pMHCI) tetramer technology. We observe rapid induction, prolonged contraction and emergence of heterogeneous and functionally competent cross-reactive and induced memory CD8⁺ T cell responses in cross-sectionally analyzed individuals with mild disease following SARS-CoV-2 infection and three individuals longitudinally assessed for their T cells pre- and post-SARS-CoV-2 infection. SARS-CoV-2-specific memory CD8⁺ T cells exhibited functional characteristics comparable to influenza-specific CD8⁺ T cells and were detectable in SARS-CoV-2 convalescent individuals who were seronegative for anti-SARS-CoV-2 antibodies targeting spike (S) and nucleoprotein (N). These results define cross-reactive and induced SARS-CoV-2-specific CD8⁺ T cell responses as potentially important determinants of immune protection in mild SARS-CoV-2 infection.

Exhausted CD8+ T cell responses during chronic viral infections are defined by a complex expression pattern of inhibitory receptors. However, very little information is currently available about the coexpression patterns of these receptors on human virus-specific CD8+ T cells and their correlation with antiviral functions, T cell differentiation and antigen recognition. We addressed these important aspects in a cohort of 38 chronically HCV infected patients and found a coexpression of inhibitory receptors such as 2B4, CD160 and KLRG1 in association with PD-1 in about half of the HCV-specific CD8+ T cell responses. Importantly, this exhaustive phenotype was associated with low and intermediate levels of CD127 expression, an impaired proliferative capacity, an intermediate T cell differentiation stage and absence of sequence variations within the corresponding epitopes, indicating ongoing antigen triggering. In contrast, a low expression of inhibitory receptors by the remaining HCV-specific CD8+ T cells occurred in concert with a CD127hi phenotype, an early T cell differentiation stage and presence of viral sequence variations within the corresponding epitopes. In sum, these results suggest that T cell exhaustion contributes to the failure of about half of HCV-specific CD8+ T cell responses and that it is determined by a complex interplay of immunological (e.g. T cell differentiation) and virological (e.g. ongoing antigen triggering) factors. About 170 million people are infected with hepatitis C virus (HCV), which may cause severe liver disease and liver cancer. Upon acute infection, only about 30% of patients are able to eliminate the virus spontaneously while about 70% of patients develop chronic infection. It is known that a successful immune response against HCV depends on virus-specific CD8+ T cells. However, during chronic infection, these cells are impaired in their antiviral function. In this study, we found that the exhaustion is characterized by the expression of multiple inhibitory receptors, such as PD-1, 2B4, CD160 and KLRG1. Of note, the coexpression of these receptors depends on the ongoing recognition of the viral antigen and the maturation stage of the T cell. The remaining virus-specific T cell responses that are not exhausted do not recognize the virus present in the patients any more due to viral mutations, indicating viral escape. Thus, they fail to exert antiviral activity, although they share characteristics of fully functional memory T cells. In sum, we have found that T cell exhaustion contributes to the failure of about half of HCV-specific CD8+ T cell responses and that it is determined by a complex interplay of immunological and virological factors. These findings will be important to consider in the design of new antiviral vaccination strategies.

Hepatocellular carcinoma (HCC) is the fifth most common malignancy worldwide with a poor prognosis and limited therapeutic options. To aid the development of novel immunological interventions, we studied the breadth, frequency, and tumor-infiltration of naturally occurring CD8⁺ T-cell responses targeting several tumor-associated antigens (TAA). We used overlapping peptides spanning the entire alpha-fetoprotein (AFP), glypican-3 (GPC-3), melanoma-associated gene-A1 (MAGE-A1) and New York-esophageal squamous cell carcinoma-1 (NY-ESO-1) proteins and major-histocompatibility-complex-class-I-tetramers specific for epitopes of MAGE-A1 and NY-ESO-1 to analyze TAA-specific CD8⁺ T-cell responses in a large cohort of HCC patients. After nonspecific expansion in vitro, we detected interferon-γ (IFN-γ)-producing CD8⁺ T cells specific for all four TAA in the periphery as well as in liver and tumor tissue. These CD8⁺ T-cell responses displayed clear immunodominance patterns within each TAA, but no consistent hierarchy was observed between different TAA. Importantly, the response breadth was highest in early-stage HCC and associated with patient survival. After antigen-specific expansion, TAA-specific CD8⁺ T cells were detectable by tetramer staining but impaired in their ability to produce IFN-γ. Furthermore, regulatory T cells (T_reg) were increased in HCC lesions. Depletion of T_reg from cultures improved TAA-specific CD8⁺ T-cell proliferation but did not restore IFN-γ-production. Conclusion: Naturally occurring TAA-specific CD8⁺ T-cell responses are present in patients with HCC and therefore constitute part of the normal T-cell repertoire. Moreover, the presence of these responses correlates with patient survival. However, the observation of impaired IFN-γ production suggests that the efficacy of such responses is functionally limited. These findings support the development of strategies that aim to enhance the total TAA-specific CD8⁺ T-cell response by therapeutic boosting and/or specificity diversification. However, further research will be required to help unlock the full potential of TAA-specific CD8⁺ T-cell responses. (HEPATOLOGY 2014;59:1415-1426)