Human CD4+ T Cells Help CD8+ T Cells Proliferate Ex Vivo by Secreting Both IL-2/IL-21 and Upregulating IL-21R
Authors: Butler Marcus O. , Imataki Osamu , Yamashita Yoshihiro , Tanaka Makito , Ansén Sascha , Berezovskaya Alla , Milstein Matthew I. , Metzler Genita , Mooney Mary M. , Murray Andrew P. , Mano Hiroyuki , Nadler Lee M. , Hirano Naoto 
Published: October 2019

Abstract
Abstract 4284
While adoptive T cell therapy is a promising treatment modality for cancer, the optimal approach to generate T cell grafts ex vivo is currently unknown. CD4+ T cells help generate effective immune responses by sustaining CD8+ T cell proliferation, preventing exhaustion, and establishing long-lived functional memory. Incorporation of CD4+ T cell help to expand CD8+ T cells may provide a novel strategy to generate CTL grafts for adoptive therapy.
In mouse models, common γ-chain receptor cytokines and CD40/CD40L can mediate CD4+ T cell help. However, CD4+ T cell help in humans has yet to be fully defined. We therefore developed an in vitro model for human CD4+ T cell help, which utilizes a novel artificial APC, aAPC/mOKT3. K562-based aAPC/mOKT3 expresses a membranous form of anti-CD3 mAb, CD54, CD58, CD80, and CD83 and stimulates CD3+ T cells regardless of HLA haplotype or antigen specificity. Using aAPC/mOKT3, we stimulated CD8+ T cells in the presence or absence of CD4+ T cells and found that CD8+ T cells expanded better when coincubated with CD4+ T cells, suggesting the presence of CD4+ T cell help. Coculture experiments using transwell plates suggested that the observed CD4+ T cell help of CD8+ T cell expansion involved both soluble factors and cell-cell contact.
To identify molecules mediating the observed CD4+ T cell help, supernatants of CD4+/CD8+ T cell mixed and separate cultures were measured for a panel of soluble factors. IL-2 and IL-21 were detected at lower levels in mixed cultures, consistent with more consumption or less production of these cytokines. Blockade of either IL-2 or IL-21 in CD4+/CD8+ T cell mixed cultures resulted in a reduction of CD8+ T cell expansion, indicating that, for both cytokines, more consumption rather than less production occurred and that IL-2 and IL-21 may serve as mediators of CD4+ T cell help. However, the addition of IL-21 to CD8+ T cells stimulated with aAPC/mOKT3 in the presence of IL-2 did not improve CD8+ T cell expansion, suggesting that IL-2 plus IL-21 cannot solely replace CD4+ T cell help. We found that the presence of CD4+ T cells upregulated the expression of IL-21R on CD8+ T cells. When we introduced IL-21R on CD8+ T cells and stimulated with aAPC/mOKT3 in the presence of IL-2 and IL-21, CD8+ T cell proliferation was restored. These results suggest that CD4+ T cells help CD8+ T cells proliferate ex vivo by secreting both IL-2/IL-21 and upregulating IL-21R.
When peripheral CD3+ T cells from normal donors were stimulated with aAPC/mOKT3, the number of both CD4+ and CD8+ T cells increased. However, in contrast to other pan T cell expansion systems, aAPC/mOKT3 preferentially expanded CD8+ T cells. No obvious skewing in the Vβ usage of both CD4+ and CD8+ T cell populations was revealed by TCR Vβ repertoire analysis, supporting “unbiased” T cell expansion by aAPC/mOKT3. Moreover, HLA-restricted antigen-specific CD8+ CTL with high functional avidity could be generated from CD3+ T cells initially expanded for 4 weeks using aAPC/mOKT3.
Using aAPC/mOKT3, tumor-infiltrating lymphocytes (TIL) were successfully expanded without adding soluble mAb or allogeneic feeder cells. As in peripheral T cell cultures, CD8+ T cells predominantly expanded in all cultures, including those that initially contained a minimal percentage of CD8+ T cells. Importantly, Foxp3+ Treg cells did not proliferate. Expanded T cells highly expressed CD27 and CD28, which are associated with T cell survival and persistence in vivo. They also secreted high levels of IFN-γ and IL-2, lower amounts of IL-4, and no IL-10. These results demonstrate that the aAPC/mOKT3-based system can expand functional CD8+ TIL in the presence of autologous CD4+ T cells.
In conclusion, we have determined that CD4+ T cell-dependent CD8+ T cell expansion required both soluble factors secreted by and cell contact with CD4+ T cells. Among the soluble factors secreted by CD4+ T cells, IL-2 and IL-21 were necessary. Furthermore, upregulation of IL-21R on CD8+ T cells by CD4+ T cells was critical for an optimized response to IL-21. Thus, in humans, CD4+ T cells help CD8+ T cells proliferate by secreting IL-2/IL-21 and upregulating IL-21R. Our aAPC enabled expansion of CD8+ TIL in the presence of CD4+ T cell help without using soluble mAb or allogeneic feeder cells. Taken together, these results demonstrate the indispensable role of CD4+ T cell help on expanding CD8+ T cells and suggest a novel strategy to generate anti-tumor T cells ex vivo for adoptive therapy.

Disclosures:
No relevant conflicts of interest to declare.