4-1BB (CD137) or CD40 Signaling Fails To Improve the Expansion of Antigen Specific T Cells Demonstrated with Engagement of TCR, CD28 and CD83 Ligand.
Authors: Hirano Naoto , Butler Marcus O. , Nadler Lee M. 
Published: October 2019

Abstract
Following the engagement of the T cell receptor by HLA class I and antigenic peptide, naïve CD8+ T cells are primed to receive one or more costimulatory signals. Some of these signals, which are upregulated on and delivered by mature dendritic cells, include members of the immunoglobulin superfamily such as CD80 and CD83. Using a K562 derived artificial antigen presenting cell (aAPC) that expresses HLA-A2, CD80, and CD83, we have shown that the coengagement of CD83 ligand:CD83 and CD28:CD80 induces prolonged and preferential expansion of antigen specific CD8+ T cells. Furthermore, we have found that CD28:CD80 signaling is required for the induction of CD83 ligand expression on peripheral T cells. In order to identify additional immunoaccessory molecules that can augment this response, we have developed a system to efficiently transfer any chosen molecule into aAPC. This provides an excellent platform for studying a potentially immunogenic molecule given the relative lack of immunoaccessory molecules expressed by K562 (i.e. no expression of CD40, CD40 ligand, CD83, CD86, 4-1BB, 4-1BB ligand, OX40, OX40 ligand, HLA class I, or HLA class II). Following the transduction of a candidate molecule under study, the stimulatory capacity of a supertransduced aAPC can be compared to parental aAPC. Attractive candidates include members of the TNF superfamily since they have been shown to deliver important costimulatory signals to T cells. It has been suggested that 4-1BB signaling supports the survival of newly generated effector CD8+ T cells and that CD40 signaling confers “CD4+ T cell-like” help directly to CD8+ T cells. However, the impact of each of these molecules on the stimulation and expansion of antigen specific T cells has not been exhaustively studied. In this report, we transfected aAPC with either 4-1BB ligand or CD40 ligand, allowing us to compare the stimulatory capacity of aAPC/CD40 ligand, aAPC/4-1BB ligand and parental aAPC. We stimulated HLA-A2 positive CD8+ T cells from healthy donors three times at weekly intervals with A2-restricted MART1 peptide pulsed onto either irradiated aAPC/CD40 ligand, aAPC/4-1BB ligand or parental aAPC. Between the stimulations, cells were treated with IL2 and IL15 every three days. When MART1 peptide pulsed aAPC/CD40 ligand were used as stimulators, the total number of CD8+ T cells and number of MART1 specific CD8+ T cells was slightly smaller. IFN-γ ELISPOT analysis revealed that functional avidity of T cell receptors on MART1 specific CD8+ T cells was similar whether they were stimulated by aAPC/CD40 ligand or parental aAPC. These results indicate that CD40 ligand, at least in the human setting, does not directly provide “CD4+ T cell-like” help to antigen-specific CD8+ T cells. In contrast, stimulation with peptide pulsed aAPC/4-1BB ligand did generate a larger total number of CD8+ T cells. Surprisingly, however, most of these T cells were not antigen specific. In fact, significantly fewer MART1 specific CD8+ T cells were generated by aAPC/4-1BB ligand compared to aAPC alone. These results suggest that, unlike CD80 and CD83, 4-1BB ligand delivers a costimulatory signal resulting in the non-specific expansion of CD8+ T cells. This work demonstrates the versatility of our system to dissect the function of particular immunoaccessory molecules and determine the optimal conditions in the stimulation and expansion of antigen-specific human CD8+ T cells ex vivo.