Retrovirus-Transduced T Cell Blasts Have Not Only Antigen-Presenting Capabilities but Also Suppressor Regulatory T Cell-Inducing Capability.

EBV-associated T/NK-lymphoproliferative diseases (LPD) are relatively rare but their prognosis is very poor. In order to understand the pathogenesis and rationalize immunotherapy, it is important to know whether EBV-infected T/NK cells have antigen-presenting capacities and how they escape the EBV-specific immunity that controls virus-infected B cells and epithelial cells. As a model for T cell antigen presenting cells (T-APC), we used activated T cells expressing pp65, the immunodominant antigen of CMV, after retroviral transduction. Two to three days after activation with CD3/28, T cells were transduced with retrovirus supernatants for 24h to 48 hr on retronectin-coated plates, and then expanded for one to two weeks with IL-2 before use as APCs or target cells. As control APCs, we used LCLs transduced with a retrovirus vector expressing pp65 (pp65-LCL). Pp65-expressing T cells functioned as target cells, since they were readily killed by pp65-specific cytotoxic T cells (CTL), reactivated using Adenovirus-pp65-pulsed dendritic cells. Furthermore, pp65T-APCs could reactivate pp65-specific CD8+ CTL from autologous PBMC. However, the pp65T-APC could not sustain prolonged pp65-specific CTL expansion. The ratio of CD4+: CD8+ T cells increased in responders after the 2^nd stimulation with pp65-T-APC, and after the 3^rd stimulation, the CD8+ CTL had virtually disappeared so that most of live cells were CD4+ T cells. The majority of the CD4+ T cells co-expressed CD25, the glucocoticoid-induced TNF receptor (GITR) and intracellular antigen CD152, all markers of T regulatory cells. To determine if they had regulatory function, the CD4+CD25+ population was isolated and shown to inhibit the proliferation of autologous PBMCs stimulated by pp65-LCL. In contrast, the CD4+CD25− fraction did not inhibit proliferation. Transwell experiments revealed that the inhibition required cell-to-cell contact. CFSE analysis showed that the CD4+CD25+ cells expanded while inhibiting the proliferation of stimulated PBMC. Taken together, these findings indicated that T-APC could reactivate antigen specific CD8+ CTL, but at the same time they activated regulatory CD4+ T cells, which prevented the long-term expansion of the CD8+ CTL. Similar results were found when the LMP2 antigen of EBV, which is expressed in T/NK-LPD, was used as antigen. Thus the immune escape of EBV-associated T/NK-LPD might result in part from their induction of regulatory T cells which inhibit T cell-specific CTL generation and function. T cells in general may evade immune-mediated recognition of their “novel” T cell receptors in the same way.