Characterization of pre-existing and induced SARS-CoV-2-specific CD8+ T cells.

Emerging data indicate that SARS-CoV-2-specific CD8+ T cells targeting different viral proteins are detectable in up to 70% of convalescent individuals1-5. 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.

BAFF- and TACI-Dependent Processing of BAFFR by ADAM Proteases Regulates the Survival of B Cells.

B cell activating factor (BAFF) provides B cells with essential survival signals. It binds to three receptors: BAFFR, TACI, and BCMA that are differentially expressed by B cell subsets. BAFFR is early expressed in circulating B cells and provides key signals for further maturation. Here, we report that highly regulated BAFFR processing events modulate BAFF responses. BAFFR processing is triggered by BAFF binding in B cells co-expressing TACI and it is executed by the metalloproteases ADAM10 and ADAM17. The degree of BAFF oligomerization, the expression of ADAM proteins in different B cell subsets, and the activation status of the cell determine the proteases involved in BAFFR processing. Inhibition of ADAM10 augments BAFF-dependent survival of primary human B cells, whereas inhibition of ADAM17 increases BAFFR expression levels on germinal center B cells. Therefore, BAFF-induced processing of BAFFR regulates BAFF-mediated B cell responses in a TACI-dependent manner.