Ultra‐sensitive class I tetramer analysis reveals previously undetectable populations of antiviral CD8+ T cells

A major breakthrough in cellular immunology has been the development of HLA class I tetramers to analyze CD8+ T cell responses. However, in many situations, including persistent virusinfection, specific T cell responses are rarely detected using this technology. This raises the question of whether such responses are ‘deleted’ (or ‘exhausted’) or present below the conventional detection limit for class I tetramer staining. In particular, persistent hepatitis C virus (HCV) infection is characterized by very weak or apparently absent specific CD8+ T cell responses, even though they are readily detectable in acute disease. Therefore, we assessed the use of anti‐PE‐labeled magnetic beads to enrich tetramer‐positive HCV‐specific T cells and identify previously undetectable populations. Using the enrichment technique, HCV‐specific T cells could be detected in the majority of infected individuals, whereas these responses were not detected using conventional tetramer staining (8/15 vs. 1/15; p=0.01). Magnetic enrichment could reliably detect very rare HCV‐specific responses at frequencies of >0.0011% of CD8+ T cells (∼1/million PBMC), and phenotypic analysis of these rare populations was possible. Therefore, this direct ex vivo technique revealed the persistence of very low frequencies of virus‐specific CD8+ T cells during chronic virus infection and is readily transferable to the study of other viral, self‐ or tumor‐specific T cells.