Development of a Nonhuman Primate Model for Analysis of the Adoptive Transfer of Antigen-Specific T Cell Clones.

In principle, the adoptive transfer of T cell clones specific for antigens expressed by pathogens or malignant cells could be therapeutically effective and allow precise control of the specificity, function, and magnitude of T cell immunity. However, the infusion of large numbers of cultured T cells or T cell clones in clinical trials has frequently failed to eradicate tumors or provide long-term control of infection. This may be due in part to the acquisition of an effector phenotype by the T cells during in vitro culture, which reduces their ability to survive in vivo and establish an immune response of sufficient magnitude for sustained efficacy. Several approaches including the administration of cytokines such as IL15, or lymphodepletion prior to cell transfer might promote the establishment of T cell memory after T cell transfer. To facilitate the rational development of clinical trials of T cell therapy, we have employed a nonhuman primate model of adoptive T cell transfer in which culture conditions and cell doses identical to those in human studies are utilized, and designed strategies to permit rigorous analysis of the persistence, function, phenotype, and migration of transferred cells. CD8⁺ CTL specific for macaque CMV were detected using an overlapping peptide panel and cytokine flow cytometry, isolated as individual T cell clones by limiting dilution, and propagated to large numbers in vitro. The T cell clones were transduced to express an intracellular truncated CD19 (ΔCD19) surface marker to allow tracking and functional assessment of T cells in vivo, and enriched by immunomagnetic selection to high purity (>98%) prior to transfer. The persistence of transferred ΔCD19⁺ T cells in the blood and their migration to the bone marrow and lymph nodes was determined by flow cytometry after staining with anti CD19, CD8, and CD3 antibodies. The infusion of ΔCD19⁺CD8⁺ CTL (3 x 10⁸/kg) was safe and the cells remained detectable in vivo for >5 months. ΔCD19⁺CD8⁺ T cells were easily detected in the blood 1 day after transfer at a level of 2.7% of CD8⁺ T cells and gradually declined over 56 days to a stable population of 0.15–0.2% of CD8⁺ T cells. At the time of transfer the ΔCD19⁺CD8⁺ T cells had an effector phenotype (CD62L⁻ CD127⁻), but gradually converted to a CD62L⁺CD127⁺ memory phenotype in vivo. The infused T cells were found at high levels in lymph node and bone marrow at day 14 after transfer (1.4% and 2.5%, respectively) and the cells at these sites were predominantly CD62L⁺. The ΔCD19⁺CD62L⁺ T cells lacked direct lytic function and expressed low levels of granzyme B, consistent with memory T cells. Sorting of these cells from post-transfer PBMC showed that in vitro activation restored lytic activity. The transferred ΔCD19⁺CD62L⁺ T cells in post-infusion PBMC produced IFNγ and TNFα comparable to endogenous CMV-specific CD8⁺ CTL. These results demonstrate that a subset (5–10%) of transferred CD8⁺ CTL clones can persist long-term as functional memory T cells. The macaque CD8⁺ T cell clones are responsive to IL15 in vitro and a safe regimen for administering IL15 to macaques that boosts endogenous T cells has been identified. Studies are now in progress to determine if IL15 can enhance the efficiency with which effector and memory CD8⁺ T cell responses can be augmented after adoptive transfer of T cell clones.