Role of IL-15 in the Stimulation of Human CD8+ CMV-Reactive Human T Cells in a Hu-PBL Mouse Model Preconditioned with Self-Differentiated Human Dendritic Cells Programmed with Lentiviral Vectors

Abstract 2966

IL-15 is known to be a key factor in the homeostatic stimulation of T cells and maintenance of central memory lymphocytes involved in long-lasting immune responses. In addition, IL-15 may enhance NK cell function to protect against infection and relapse after allo-HCT. Thus, IL-15 ectopic overexpression provided by dendritic cell (DC) immunotherapy after bone marrow transplantation or donor lymphocyte infusion may direct prompt and durable immune responses against leukemia relapse or viral reactivation.

Here, we show that overnight transduction of monocytes with tricistronic lentiviral vectors (LV) co-expressing GM-CSF/IL-4/IL-15 induced their differentiation into highly viable DC that expressed the major DC markers CD209 (75%), CD86 (95%), HLA-DR (90%) for up to 21 days in culture. These so-called SmartDC/IL-15 maintained stable secretion of several inflammatory and Th1 cytokines (TNF α, MCP-1, IL-1 β, IL-8). A single in vitro priming of T cells obtained from CMV seropositive donors with autologous SmartDC/IL-15 loaded with CMV-pp65 peptides led to expansion of higher frequencies pp65 tetramer-positive CTLs as compared with SmartDC that were not engineered to express IL-15 (26.5% vs. 16.5% respectively). Importantly, IL-15 expression in Smart-DCs led to the enrichment of T cells with effector memory (80% vs. 65%) and central memory (8% vs. 6%) as compared with SmartDC controls. SmartDCs/IL-15 presented high viability in vivo (>6 weeks) in immune deficient NOD.Rag1^—/—.IL2r γ^—/— (NRG) mice. We further evaluated the effects of SmartDCs/IL-15 on lymphatic and anti-CMV immune reconstitution in a humanized peripheral blood lymphocyte (Hu-PBL) mouse model. NRG mice preconditioned with unloaded or pp65-loaded SmartDCs/IL-15 were infused 7 days later with autologous luciferase-labeled T cells for optical imaging analysis. At day 21 after fLUC-T cells infusion, mice injected with pp65-loaded SmartDCs/IL-15 showed higher bioluminescence signal from expanded T cells in spleens (3-fold) and injection site (2-fold) as compared with control SmartDCs. Human T cells recovered from spleens of SmartDC-IL15 showed enhanced frequencies of effector memory (58% vs. 30%) and central memory (5.6% vs. 4%) phenotype as compared with SmartDC controls.

Remarkably, bioluminescent T cells co-localized with the site of SmartDC/IL-15 injection in a pp65-dependent manner and enhanced the bio-distribution of human T cells towards other rudimentary lymphatic structures found in NRG mice. Anti-pp65 reactivity in vivo was confirmed by flow cytometry analyses by tetramer staining in peripheral blood. Thus, SmartDCs/IL-15 induced accelerated anti-CMV immune responses and triggered lymphatic regeneration and repopulation in vivo, which may be explored clinically to enhance homeostatic and antigen-dependent immune reconstitution after transplantation.