Figure 3 Working Model for T Cell Responses to SARS-CoV-2: Changes in Peripheral Blood T Cell Frequencies and Phenotype
A decrease in peripheral blood T cells associated with disease severity and inflammation is now well documented in COVID-19. Several studies report increased numbers of activated CD4 and CD8 T cells, which display a trend toward an exhausted phenotype in persistent COVID-19, based on continuous and upregulated expression of inhibitory markers as well as potential reduced polyfunctionality and cytotoxicity. In severe disease, production of specific inflammatory cytokines by CD4 T cells has also been reported. This working model needs to be confirmed and expanded on in future studies to assess virus-specific T cell responses both in peripheral blood and in tissues. In addition, larger and more defined patient cohorts with longitudinal data are required to define the relationship between disease severity and T cell phenotype.
IL, interleukin; IFN, interferon; TNF, tumor necrosis factor; GM-CSF, granulocyte-macrophage colony-stimulating factor; GzmA/B, granzyme A/granzyme B; Prf1, perforin.