Evaluation of Human Circulating T Follicular Helper Cells in Influenza- and SARS-CoV-2-Specific B Cell Immunity.

Generation of effective immune protection against viral infection and vaccination depends greatly on a successful engagement and stimulation of adaptive immune B cells and a specialized CD4 T cell subset called T follicular helper cells (T cells). Since T cells primarily reside in lymphoid tissues, they can be challenging to study in human settings. However, a counterpart of these cells, circulating T (cT) cells, can be detected in peripheral blood. Assessment of cT cells serves as an informative marker of humoral responses following viral infection and vaccination and can be predictive of antibody titers. Here, we describe a comprehensive flow cytometry detection method for dissecting cT subsets and activation, together with the assessment of antibody-secreting cells (ASCs), from a small volume of human whole blood. This approach allows the investigation of cellular events that underpin successful immune responses following influenza and SARS-CoV-2 infection/vaccination in humans and is applicable to other viral disease settings.