Perforin and IL-2 Upregulation Define Qualitative Differences among Highly Functional Virus-Specific Human CD8+ T Cells

The prevailing paradigm of T lymphocyte control of viral replication is that the protective capacity of virus-specific CD8(+) T cells is directly proportional to the number of functions they can perform, with IL-2 production capacity considered critical. Having recently defined rapid perforin upregulation as a novel effector function of antigen-specific CD8(+) T cells, here we sought to determine whether new perforin production is a component of polyfunctional CD8(+) T cell responses that contributes to the control of several human viral infections: cytomegalovirus (CMV), Epstein-Barr virus (EBV), influenza (flu), and adenovirus (Ad). We stimulated normal human donor PBMC with synthetic peptides whose amino acid sequences correspond to defined CTL epitopes in the aforementioned viruses, and then used polychromatic flow cytometry to measure the functional capacity and the phenotype of the responding CD8(+) T cells. While EBV and flu-specific CD8(+) T cells rarely upregulate perforin, CMV-specific cells often do and Ad stimulates an exceptionally strong perforin response. The differential propensity of CD8(+) T cells to produce either IL-2 or perforin is in part related to levels of CD28 and the transcription factor T-bet, as CD8(+) T cells that rapidly upregulate perforin harbor high levels of T-bet and those producing IL-2 express high amounts of CD28. Thus, "polyfunctional" profiling of antigen-specific CD8(+) T cells must not be limited to simply the number of functions the cell can perform, or one particular memory phenotype, but should actually define which combinations of memory markers and functions are relevant in each pathogenic context.