The relationship between human effector and memory T cells measured by ex vivo and cultured ELISPOT following recent and distal priming

Maintenance of T-cell responses is an essential feature in protection from many infectious diseases that must be harnessed in vaccination. The relationship between effector T-cell responses and more durable and highly proliferative T-cell memory, particularly in humans, is not well understood. In this study, effector T-cell responses were measured by overnight ex vivo interferon-γ (IFN-γ) enzyme-linked immunosorbent spot-forming cell assay (ELISPOT), whereas memory T cells were measured by 10-day culture followed by IFN-γ ELISPOT (cultured ELISPOT). We observed a significant correlation between IFN-γ responses to CD4-stimulatory, but not to CD8-stimulatory, recall antigens measured by these assays, suggesting a divergence in regulation. In vaccine trial participants who received a prime-boost vaccination regimen comprising malaria antigens delivered by poxviruses, there was a correlation between ex vivo and cultured responses on day 7, but not 3 months post-vaccination, with the ratio of cultured : ex vivo response increasing over time. To compare responses revealed by cultured ELISPOT in more detail, tetramers comprising viral recall antigens were used to ascribe effector-memory and central-memory T-cell phenotypes through CCR7 and CD62L costaining. For CD8+ responses the effector phenotype decreased during the initial culture period and memory populations remained high within the resulting 20-fold to 50-fold increased IFN-γ-secreting or tetramer+ population. This was less marked for CD4+ responses, which had higher starting memory phenotype. Depletion of these central-memory T-cell populations generally ablated responses in cultured ELISPOT and reduced ex vivo responses. This study highlights differences between CD4+ and CD8+ effector and memory T cells, and the more complex phenotype of CD4+ T cells.