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.

The coronavirus disease 2019 (COVID-19) pandemic poses an enormous challenge to healthcare systems in affected communities. Older patients and those with pre-existing medical conditions have been identified as populations at risk of a severe disease course. It remains unclear at this point to what extent chronic liver diseases should be considered as risk factors, due to a shortage of appropriate studies. However, patients with advanced liver disease and those after liver transplantation represent vulnerable patient cohorts with an increased risk of infection and/or a severe course of COVID-19. In addition, the current pandemic requires unusual allocation of healthcare resources which may negatively impact the care of patients with chronic liver disease that continue to require medical attention. Thus, the challenge hepatologists are facing is to promote telemedicine in the outpatient setting, prioritise outpatient contacts, avoid nosocomial dissemination of the virus to patients and healthcare providers, and at the same time maintain standard care for patients who require immediate medical attention.

Chronic hepatitis C virus (HCV) infection is associated with impaired proliferative, cytokine, and cytotoxic effector functions of HCV-specific CD8⁺T cells that probably contribute significantly to viral persistence. Here, we investigated the potential role of T cells with a CD4⁺CD25⁺regulatory phenotype in suppressing virus-specific CD8⁺T-cell proliferation during chronic HCV infection. In vitro depletion studies and coculture experiments revealed that peptide specific proliferation as well as gamma interferon production of HCV-specific CD8⁺T cells were inhibited by CD4⁺CD25⁺T cells. This inhibition was dose dependent, required direct cell-cell contact, and was independent of interleukin-10 and transforming growth factor beta. Interestingly, the T-cell-mediated suppression in chronically HCV-infected patients was not restricted to HCV-specific CD8⁺T cells but also to influenza virus-specific CD8⁺T cells. Importantly, CD4⁺CD25⁺T cells from persons recovered from HCV infection and from healthy blood donors exhibited significantly less suppressor activity. Thus, the inhibition of virus-specific CD8⁺T-cell proliferation was enhanced in chronically HCV-infected patients. This was associated with a higher frequency of circulating CD4⁺CD25⁺cells observed in this patient group. Taken together, our results suggest that chronic HCV infection leads to the expansion of CD4⁺CD25⁺T cells that are able to suppress CD8⁺T-cell responses to different viral antigens. Our results further suggest that CD4⁺CD25⁺T cells may contribute to viral persistence in chronically HCV-infected patients and may be a target for immunotherapy of chronic hepatitis C.

Marcin Krawczyk, Monika Rau, Jörn M. Schattenberg, Heike Bantel, Anita Pathil, Münevver Demir, Johannes Kluwe, Tobias Boettler, Frank Lammert, Andreas Geier, for the NAFLD Clinical Study Group