Co-stimulation blockade can be used to modulate the immune response for induction of organ transplantation tolerance, treatment of autoimmune disease as well as cancer treatment. Cytotoxic T-Lymphocyte Antigen-4 (CTLA-4), also known as CD152, is an important co-stimulatory molecule which serves as a negative regulator for T cell proliferation and differentiation. CTLA-4/CD28-CD80/CD86 pathway is a critical co-stimulatory pathway for adaptive immune response. T cell activation through the T cell receptor and CD28 leads to increased expression of CTLA-4, an inhibitory receptor for CD80 and CD86. MGH MHC-defined miniature swine provide a unique large animal model useful for preclinical studies of transplantation tolerance and immune regulation. In this study, we have expressed the codon-optimized soluble porcine CTLA-4 in the yeast Pichia pastoris system. The secreted porcine CTLA-4 was captured using Ni-Sepharose 6 fast flow resin and further purified using strong anion exchange resin Poros 50HQ. Glycosylation analysis using PNGase F demonstrated the N-linked glycosylation on Pichia pastoris expressed soluble porcine CTLA-4. To improve the expression level and facilitate the downstream purification we mutated the two potential N-linked glycosylation sites with non-polarized alanines by site-directed mutagenesis. Removal of the two N-glycosylation sites significantly improved the production level from ~2 mg/L to ~8 mg/L. Biotinylated glycosylated and non-N-glycosylated soluble porcine CTLA-4 both bind to a porcine CD80-expressing B-cell lymphoma cell line (KD = 13 nM) and competitively inhibit the binding of an anti-CD80 monoclonal antibody. The availability of soluble porcine CTLA-4, especially the non-N-glycosylated CTLA-4, will provide a very valuable tool for assessing co-stimulatory blockade treatment for translational studies in the clinically relevant porcine model.

Regulatory T cells (Treg) constitute a major inhibitory cell population which suppresses immune responses. Thus, Treg have proven to be key players in the induction of transplantation tolerance, protection from autoimmune disease and prevention of the development of effective anti-tumor immune reactions. Treg express high levels of the high affinity interleukin-2 receptor (IL-2R) consisting of IL-2Rα (CD25) together with IL-2Rβ (CD122) and the common γ-chain (CD132). An effective reagent capable of depleting Treg in vivo would facilitate better cancer treatment and allow mechanistic studies of the role of Treg in transplantation tolerance and the development of autoimmune disease. In this study, we have developed a novel bivalent human IL-2 fusion toxin along with an Ontak®-like monovalent human IL-2 fusion toxin and compared the functional ability of these reagents in vitro. Here we show that genetically linking two human IL-2 domains in tandem, thereby generating a bivalent fusion toxin, results in significantly improved capacity in targeting human CD25+ cells in vitro. Binding analysis by flow cytometry showed that the bivalent human IL-2 fusion toxin has notably increased affinity for human CD25+ cells. In vitro functional analysis demonstrated that the bivalent isoform has an increased potency of approximately 2 logs in inhibiting cellular proliferation and protein synthesis in human CD25+ cells compared to the monovalent human IL-2 fusion toxin. Additionally, we performed two inhibition assays in order to verify that the fusion toxins target the cells specifically through binding of the human IL-2 domain of the fusion toxin to the human IL-2 receptor on the cell surface. These results demonstrated that 1) both monovalent and bivalent human IL-2 fusion toxins are capable of blocking the binding of biotinylated human IL-2 to human CD25 by flow cytometry; and 2) human IL-2 blocked the fusion toxins from inhibiting protein synthesis and cellular proliferation in vitro, thus confirming that the human IL-2 fusion toxins target the cells specifically through binding to the human IL-2 receptor. We believe that the bivalent human IL-2 fusion toxin will be a more potent, and therefore, more optimal agent than the current clinically-used monovalent fusion toxin (denileukin diftitox, Ontak®) for in vivo depletion of Treg.