A Novel Synthetic GMCSF and IL7 Fusion Cytokine (GIFT7) Leads to T Cell Neogenesis by Reversing Age-Related Thymic Atrophy and Overcoming PD-1-Assocaited CD8 Exhaustion

Abstract 3289

Functional T cell immunity depends on the diversity of T cell receptor (TCR) repertoire. Numerous cytolytic assaults can perturb effector cell fitness by inhibiting thymic de novo T cell production or predisposing peripheral CD8⁺ T cells to exhaustion. In fact, age-associated thymic atrophy and PD-1 upregulation remain the critical components of immune dysfunction.

Normal thymopoiesis arises from marrow-derived CD3⁻CD4⁻CD8⁻ triple-negative T cell progenitors (TN) which seed the thymic cortex. Thereafter, TN develop into mature single-positive (SP) CD4 or CD8 T cells after expressing both CD4 and CD8 (double-positive-DP) transiently, leading to de novo T cell production. Interleukin-7 (IL7) is a singularly important common γ-chain interleukin involved in this process. In order to pharmacologically enhance thymopoiesis, we report the development of a novel biopharmaceutical based on the fusion of GMCSF and IL7, hereafter GIFT7. Systemic administration of GIFT7 compared to IL7 in young immune competent mice leads to a transient increase in the number of DN1 (5.48±1.01 v.s. 2.74±1.07 × 10⁶, p=0.046) by day 7. Total thymic cellularity significantly increases in GIFT7-treated group by day 14 (237±51.3 v.s. 123±25.6 × 10⁶, p=0.026). GIFT7-mediated hypertrophic effect is significantly more pronounced in aged thymi: 111±21.8, 67.8±13.9, 60.3±4.6 × 10⁶ for 7-dose of (5ug kg⁻¹) GIFT7, IL7, or PBS treatment respectively. We observed a significant increase in DN1 and DN4 subsets with the greatest fold difference in the CD44^int DN1 subset. Histologically, GIFT7 administration leads to significant cortical thymic hyperplasia (cortex: medulla ratio 2.67:1). Functionally, GIFT7 enhances viral-specific CTL responsiveness as modeled by murine cytomegalovirus (MCMV) peptide MHC⁺ tetramer enumeration (10.4±3.8 v.s. 2.9±1.4 × 10⁶ for GIFT7 and untreated respectively, p<0.05).

In the periphery, GIFT7 selectively expand a CD8⁺ subset with a Central Memory (CM) phenotype defined as CD8⁺CD44⁺CD62L⁺CCR7⁺KLRG⁻CD27⁺PD1⁻. This unique expansive effect of GIFT7 is also demonstrated on peripheral blood mononuclear cells (PBMC) derived from non-human primates (NHP), in that GIFT7 stimulation of pre-activated NHP-PBMC leads to a two-fold increase in total cell number after 3 days and >80% of Ki67⁺ expression in both CD4⁺ and CD8⁺ compartments. Interestingly, GIFT7-mediated proliferation in CD8⁺ T cells is accompanied by a 40% decrease in the mean fluorescent intensity (MFI) of PD1 expression.

Our work identifies a CD44^intCD25⁻ subset of DN thymocytes most responsive to IL7R-mediated STAT5 hypersignalling and their function as T cell progenitors in GIFT7-driven thymic reconstitution. This points to the translational potential of GIFT7 or GIFT7-primed T lineage cells as treatment of immune malfunction.