An elegant study from Minervina et al. from N M ORF9b ORF3a = S proteins (17). Hence, the linear and positive-sense genome of SARS-CoV-2 shapes the T cell response. Open in a separate window Figure 1 SARS-CoV-2-specific T cell responses to various proteins at different stages of exposure (A) in infection and vaccination can be quantified by (B) activation induced markers, pMHC tetramer or multimer binding for known immunodominant epitopes, or by cytokine induction (B). Figure produced with Biorender. Crossreactive T Cell Responses by SARS-CoV-2 Infection: Getting Ahead From the Baseline Race SARS-CoV-2 T cells have been quantified by a number of different immune assays based on epitope presentation leading to T cell receptor binding or activation for upregulation of surface markers or cytokine secretion (Figure 1B). The characterization of functional antiviral cytokine Rabbit polyclonal to CD48 and activated T cells elicited by SARS-CoV-2 infection by stimulation has used either HLA optimized peptide megapools (18, 19), selected expected crossreactive peptides (20), comprehensive peptidome with the omission of ORF1 (21), or whole peptidome functional pools (22). Antigen-specific responses once a peptide epitope has been identified (23) can also be quantified by pMHC binding using tetramers or multimers which is useful for downstream cellular characterization (23C28). Furthermore, antigen-specific SR9243 responses have also been identified by mapping of HLA presented peptides during infection to reveal cryptic T cell epitopes within proteins that are boosted by recent infection in patients with COVID-19 (17), which can be ORF independent, and therefore cryptic epitopes can be generated during infection (19). Therefore, the definition of SARS-CoV-2 T cells is assay-dependent and contingent upon the epitopes included; however, consistent trends amongst studies with different approaches have shown that robust T cell responses are generated by SARS-CoV-2 infection (Table 1). Table 1 SARS-CoV-2 CD4+ and CD8+ T cell responses in infection and vaccination. – Express CD134, CD137- Recruited early during acute infection- HLA-DPB1*04/S166?180 is a dominant SARS-CoV-2-specific epitope following infection(18, 21, 22, 28, 29)?70% recovered patients activated cells- Produce IFN, TNF, IL2, GzB- Express CD69, CD137- Recruited later during infection at convalescence- HLA-B*07:02/N105?113 is a dominant SARS-CoV-2-specific epitope following infection (38 larger than B7/N257, A2/S269 and A24/S1208)(18, 21, 22, 26, 29C32)Children- Reduced response magnitude compared with adults- T na?ve in children T SR9243 na?ve in adults(22)- Reduced response magnitude compared with adults(22)Severe- Reduced structural response- Hyperactivated CD25+ CD4+ T cells expressing furin- Increased TFH2, reduced TFH1 and TFH17(33, 34)- Reduced response – Less exhaustion, defined by coexpression of PD-1, CD38 and HLA-DR than mild patients.(33, 35)Vaccination- Induction of strong CD4+ responses across most vaccine platforms- No significant differences found in studies comparing two vaccines in parallel(36C40)- Induction of CD8+ responses across most vaccine platforms (at lower levels than CD4+ T cell fold changes)(36C40)Hybrid immunity- Increased responses compared with infection induced immunity alone.- One dose of mRNA vaccination boosts previous response from natural infection or ChAdOx1 nCov-19 vaccination- Two doses did not boost responses further(41, 42)- mRNA vaccination boosts previous response after one dose- Further boosting is seen after two doses alongside increased memory responses(42)Crossreactivity Infection- Unexposed healthy individuals have crossreactive CD4+ T cells against ORF1a/b nsp7 and nsp13 proteins- Dominant epitopes DPB1*04/S166 and /S186 are conserved in currently known VoCs(20, 28, 43, 44)- Dominant epitope B7/N105 epitope is conserved in currently known VoCs(21, 26, 32, 45)Vaccination- mRNA vaccination induces CD4+ T cells crossreactive to several VoCs- Back boosting of HLA-DPB1*04/S816?830 responses by mRNA vaccination. Found in 20% unexposed positive, 50% patients and 97% vaccinees.- 7% of epitopes affected in VoC, crossreactivity maintained(36, 39, 46, 47)- mRNA vaccine recipients maintain crossreactive CD8+- 3% of epitopes affected in VoC, crossreactivity maintained(36, 39) Open in a separate window SARS-CoV-2 T cell epitope immunodominance hierarchies are evident (19, 20, 22, 26, 43), which may be attributable to peptide homology with CCCoV (48, 49) and the relative expression of structural, nsp, and accessory proteins during virus SARS-CoV-2 replication (21). The magnitude of SARS-CoV-2 T SR9243 cell responses generated during infection can also be dictated by the efficiency of T cell recruitment by crossreactivity and avidity (50) and na?ve precursor frequency as evident by T cell receptor diversity (26). Due to the widespread prevalence of CCCoV exposure, 20C60% unexposed individuals (18, 30) display low magnitude of SARS-CoV-2 crossreactive T cells which may provide a baseline of immunity against SARS-CoV-2 (51). These crossreactive responses are mostly reported in the ORF1a/b non-structural proteins (18), and the magnitude of the response has a significant correlation with reduced duration of infection (20). The majority of pre-existing crossreactive T cell responses map to the ORF1a/b (43, 44), especially nsp7 and nsp13 which have high homology to other CoVs (48) (Table 1). SARS-CoV-2 crossreactive T cell responses have also been.