Ex Vivo Expansion of Human CD8+ T Cells Using Autologous CD4+ T Cell Help

Abstract

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. 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-γ 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. 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.