Ex vivo expansion of human CD8 ⁺ T cells using autologous CD4 ⁺ T cell help

Background: Using in vivo mouse models, the mechanisms of CD4 ⁺ T cell help have been intensively investigated. However, a mechanistic analysis of human CD4 ⁺ T cell help is largely lacking. Our goal was to elucidate the mechanisms of human CD4 ⁺ T cell help of CD8 ⁺ T cell proliferation using a novel in vitro model. Methods/Principal Findings: We developed a genetically engineered novel human cell-based artificial APC, aAPC/mOKT3, which expresses a membranous form of the anti-CD3 monoclonal antibody OKT3 as well as other immune accessory molecules. Without requiring the addition of allogeneic feeder cells, aAPC/mOKT3 enabled the expansion of both peripheral and tumor-infiltrating T cells, regardless of HLA-restriction. Stimulation with aAPC/mOKT3 did not expand Foxp3 ⁺ regulatory T cells, and expanded tumor infiltrating lymphocytes predominantly secreted Th1-type cytokines, interferon-ã and IL-2. In this aAPC-based system, the presence of autologous CD4 ⁺ T cells was associated with significantly improved CD8 ⁺ T cell expansion in vitro. The CD4 ⁺ T cell derived cytokines IL-2 and IL-21 were necessary but not sufficient for this effect. However, CD4 ⁺ T cell help of CD8 ⁺ T cell proliferation was partially recapitulated by both adding IL-2/IL-21 and by upregulation of IL-21 receptor on CD8 ⁺ T cells. Conclusions: We have developed an in vitro model that advances our understanding of the immunobiology of human CD4 ⁺ T cell help of CD8 ⁺ T cells. Our data suggests that human CD4 ⁺ T cell help can be leveraged to expand CD8 ⁺ T cells in vitro.