A Novel Fusion of ALT-803 (Interleukin (IL)-15 Superagonist) with an Antibody Demonstrates Antigen-specific Antitumor Responses

Abstract

Edited by Luke O'Neill IL-15 and its receptor ␣ (IL-15R␣IL-15, a four-helix, common ␥ chain (␥ C ) 2 cytokine, is a critical factor for the development, proliferation, and activation of natural killer (NK) cells and CD8 ϩ T cells (1, 2). IL-15 is co-expressed with its ␣ chain receptor (IL-15R␣) by antigen-presenting cells, and the two proteins form a complex on the cell surface that is transpresented to NK and T cells bearing the IL-2R␤␥ C complex (2). IL-15 binds to IL-15R␣ at high affinity, and IL-15R␣ functions as a chaperone and conformational stabilizer to enhance the interaction between IL-15 and IL-2R␤␥ C (2). We identified a novel IL-15 variant carrying an asparagineto-aspartic acid mutation at amino acid 72 (N72D) that exhibits superior binding to IL-2R␤␥ C on immune cells and increased immunostimulatory activity (3). Our previous studies have demonstrated that this IL-15 variant, when associated with a soluble IL-15R␣ sushi domain fusion to IgG1 Fc (IL15R␣SuFc), could form a heterodimeric complex, IL-15N72D⅐ IL-15R␣SuFc (designated ALT-803), that also exhibits increased binding activity to the IL-2R␤␥ C complex, enhanced capacity to stimulate NK and T cells, and has a longer biological half-life compared with native IL-15 (4). In various animal models, ALT-803 acts as a potent immunostimulant that is capable of simultaneously activating the innate and adaptive arms of the immune system to elicit both rapid and long-lasting protective responses against neoplastic challenges (5). Moreover, ALT-803, in combination with checkpoint blockade or therapeutic antibodies, is effective in reducing the tumor burden and prolonging survival in mouse tumor models (6, 7). To make ALT-803-based molecules more specific and efficient in combating disease, we converted ALT-803 into a targeted immunotherapeutic agent by genetically fusing it with singlechain antibodies (scFv) at the N termini of IL-15N72D and IL-15R␣SuFc proteins. In this study, we used the anti-CD20 scFv as the target recognition domain to demonstrate that ALT-803 is a versatile, functional scaffold for creating diseasetargeted immunostimulatory molecules. This novel single fusion protein approach was also found to improve the antibody-dependent cellular cytotoxicity (ADCC) and apoptotic functions of the anti-CD20 therapeutic antibody rituximab.

The potential and promise of IL-15 in immuno-oncogenic therapies

“…The fusion product of the IL-15 agonist, RLI to Rituximab (anti-CD20 Ab), which targets B cell lymphomas, was found to be more effective in inducing long term survival of lymphoma-bearing SCID mice than either RLI or Rituximab alone [84]. Additionally, a fusion of IL-15 agonist, ALT-803 with single chain antibody chains of Rituximab also induced more robust anti-lymphoma responses in mice and mediated depletion of B cells in cynomolgus monkeys [85,86]. Altogether, these studies demonstrate that fusing IL-15 analogs to various scaffolds/antibodies can retain IL-15 activity as well as mediate more specific cell targeting.…”

Natural killer cells unleashed: Checkpoint receptor blockade and BiKE/TriKE utilization in NK-mediated anti-tumor immunotherapy

“…Furthermore, ALT-803 rescued function of NK cells derived from ovarian cancer patient ascites[131]. ALT-803 also enhanced in vivo CD16-triggered NK cell clearance of B-cell lymphomas when treated with an anti-CD20 monoclonal antibody[132]. In conjunction with the postulated role of γ c cytokine-mediated bypass of some checkpoint inhibition pathways[133], the ability of IL-15 to prime CD16 mediated functions on NK cells makes this cytokine an attractive immunotherapeutic target.…”

Immunobiology of the IL-15/IL-15Rα complex as an antitumor and antiviral agent

“…In this model, ALT-803 not only increased the percentage of CD8 + T cells infiltrating into the microenvironment, but more importantly, established a long-term immune memory against tumor rechallenge. ALT-803 also improved NK cell-mediated granulation, IFN-γ production and antibody-dependent cell-mediated cytotoxicity (ADCC) in combination with anti-CD20 monoclonal antibodies against B cell lymphoma (44, 45). In addition, ALT rescued functionality of NK cells from patients with ovarian cancer and enhanced NK cell cytotoxicity against ovarian cancer cell lines in vitro and in vivo (46).…”

“…Although ALT-803 is a human IL-15 superagonist (human IL-15N72D/human IL-15 Rα-IgG1-Fc), it has been shown to exhibit effectiveness against both human and murine tumor cells (44, 45, 47, 48). Human IL-15 shares 70% amino acid sequence identity with murine IL-15.…”