Increasing the biological activity of IL-2 and IL-15 through complexing with anti-IL-2 mAbs and IL-15Rα-Fc chimera

Highlights

• The in vivo biological activity of IL-2 can be increased by anti-IL-2 mAbs.

• The in vivo biological activity of IL-15 can be increased by IL-15Rα-Fc chimera.

• IL-15/IL-15Rα-Fc complexes are more potent than IL-15 both in vitro and in vivo.

• IL-2/anti-IL-2 mAb immunocomplexes are more potent than IL-2 only in vivo.

• IL-2/anti-IL-2 mAb immunocomplexes show selectivity in the stimulation of immune cells.

Abstract

IL-2 and IL-15 are structurally relative cytokines that share two receptor subunits, CD132 (γ_c chain) and CD122 (β chain). However, the expression pattern and physiological role of IL-2 and IL-15 private receptor α chains CD25 and IL-15Rα, respectively, are strikingly different. CD25, together with CD122 and CD132, forms a trimeric high affinity IL-2 receptor that is expressed and functions on cells acquiring an IL-2 signal. Conversely, IL-15Rα is expressed and binds IL-15 with high affinity per se already in the endoplasmic reticulum of the IL-15 producing cells and it presents IL-15 to cells expressing CD122/CD132 dimeric receptor in trans. Thus, while IL-2 is secreted almost exclusively by activated T cells and acts as a free molecule, IL-15 is expressed mostly by myeloid cells and works as a cell surface-associated cytokine. Interestingly, the in vivo biological activity of IL-2 can be dramatically increased through complexing with certain anti-IL-2 mAbs; such IL-2/anti-IL-2 mAbs immunocomplexes selectively stimulate the proliferation of a distinct population of immune cells, depending on the clone of the anti-IL-2 mAb used. IL-2/S4B6 mAb immunocomplexes are highly stimulatory for CD122^high populations (memory CD8⁺ T and NK cells) and intermediately also for CD25^high populations (Treg and activated T cells), while IL-2/JES6-1 mAb immunocomplexes enormously expand only CD25^high cells. Although IL-2 immunocomplexes are much more potent than IL-2 in vivo, they show comparable to slightly lower activity in vitro. The in vivo biological activity of IL-15 can be dramatically increased through complexing with recombinant IL-15Rα-Fc chimera; however, IL-15/IL-15Rα-Fc complexes are significantly more potent than IL-15 both in vivo and in vitro. In this review we summarize and discuss the features and biological relevance of IL-2/anti-IL-2 mAbs and IL-15/IL-15Rα-Fc complexes, and try to foreshadow their potential in immunological research and immunotherapy.

Introduction

IL-2 and IL-15 belong to a family of γ_c cytokines (IL-2, 4, 7, 9, 15, and 21) which are key regulators of lymphocyte homeostasis and function. They have the potential to promote lymphocyte proliferation and survival and thus overall enhance dominantly adaptive immune responses. IL-2 is an autocrine/paracrine soluble factor produced mainly by activated T cells, whereas IL-15 is produced by activated monocytes and dendritic cells (DCs) [1], [2], [3], and to a lesser extent by epithelial cells, stromal cells and fibroblasts [4], [5], [6].

Both cytokines share two receptor subunits, CD122 and CD132, β, and common γ_c chains, respectively_. IL-2 exerts its pleiotropic activities through binding to either a dimeric receptor composed of CD122 and CD132 or a trimeric receptor composed of CD25, CD122 and CD132 [7]. The isolated CD25 binds IL-2 with low affinity (K_a ∼10⁸ M⁻¹) without transducing a signal [8], [9]. A dimer of CD122 and CD132 binds IL-2 with intermediate affinity (K_a ∼10⁹ M⁻¹) and is present on CD122^high populations, namely memory CD8⁺ T cells (CD3⁺CD8⁺CD44^highCD122^high) and NK cells (CD3⁻NK1.1⁺DX5⁺) [10], [11]. A complex of CD25, CD122 and CD132 binds IL-2 with high affinity (K_a ∼10¹¹ M⁻¹) and is present on activated T cells and T regulatory cells (Treg; CD3⁺CD4⁺CD25⁺Foxp3⁺). CD25 is not expressed in naïve T cells but becomes transiently expressed at high levels upon T cell activation, allowing a selective IL-2 driven expansion of activated T cells. On the other hand, CD25 is constitutively expressed at high levels in Treg cells, enabling them to utilize very low background levels of IL-2.

In contrast to IL-2, IL-15 is membrane-associated cytokine acting through cell-cell contact. The dominant mechanism of IL-15 signaling in vivo is trans-presentation of IL-15 by IL-15Rα (CD215) to a heterodimeric CD122/CD132 receptor [12], [13]. Unlike CD25, IL-15Rα binds IL-15 with a very high affinity (K_a ∼10¹¹ M⁻¹) that is not further increased in the presence of the complete IL-15 receptor, i.e., IL-15 has ∼100-fold higher affinity to IL-15Rα than IL-2 to CD25 [14], [15]. IL-15Rα is not de facto a high-affinity receptor subunit, as it associates with IL-15 already in the endoplasmic reticulum and is necessary for its transport to the cell surface and subsequent delivery of the IL-15 signal [16]. IL-15Rα is expressed on T cells, NK cells, NKT cells, B cells, DCs, monocytes and macrophages, and in thymic and BM stromal cell lines [1], [2], [15], [17], [18]. However, IL-15Rα mRNA have also been detected in a number of non-lymphoid tissues such as liver, heart, spleen, lung, skeletal muscle and endothelial cells [15], [17], [19], which suggests that the IL-15/IL-15Rα signaling system operates in multiple tissues/organs throughout the organism.

IL-2 and IL-15 shares many functions in both the adaptive and innate immune system, because both cytokines signal through the same CD122/CD132 receptor heterodimer. Both cytokines promote proliferation and survival of T cells, induce the proliferation and differentiation of NK cells, as well as their cytolytic activity, and are able to facilitate production of immunoglobulins by B cells. However, IL-15 has in vivo functions distinct from IL-2, which is largely due to the unique manner in which it uses its IL-15Rα receptor. As such, IL-15 is important for the development and homeostasis of memory CD8⁺ T cells, iNKT cells and CD8αα intraepithelial lymphocytes [13].

Although IL-15 was initially described as a T-cell proliferative factor similar to IL-2, subsequent studies have shown that IL-2 and IL-15 can have quite opposite effects in vivo on T-lymphocyte growth and survival. IL-2 is involved in limiting T-cell numbers, as a continued exposure to IL-2 causes activation-induced cell death (AICD); additionally, IL-2 is critical to the development and function of Treg cells [20]. Mice deficient in IL-2 or CD25 develop a polyclonal lymphoproliferation with massive lymphadenopathy, splenomegaly and T-cell dependent autoimmunity [21], [22]. In contrast, IL-15 can inhibit IL-2-induced AICD [23]; IL-15-deficient (IL-15 KO) or IL-15Rα-deficient (IL-15Rα KO) mice exhibit lymphopenia rather than lymphoproliferation [24], [25]. IL-2 signaling in continuously dividing T cells leads to down-regulation of the common γ_c receptor and decreased bcl-2 expression, resulting in increased susceptibility to apoptosis; IL-15 signaling has no such effect [26]. IL-15 has been shown to be critical for the maintenance of memory CD8⁺ T cells [25], [27], [28], [29] and NK cells, since IL-15 KO and IL-15Rα KO mice do not have these populations [24], [30].

IL-2 has a potent stimulatory effect on T and NK cells and some other cells of the immune system. More than 20 years ago, IL-2 was approved by the FDA for treatment of metastatic renal cell carcinoma and malignant melanoma [31], [32]. However, although standard high-dose IL-2 therapy results in a modest clinical response rate of 14% [33], the treatment is limited by substantial toxicities [33], [34], vascular leak syndrome (VLS) being the most serious one [35], [36], [37]. Alternatively, low-dose IL-2 treatment has shown activity in renal cell cancer (response rates of 18–23%), without the toxicity of high dose IL-2 [38], [39]. Yet, IL-2 cancer therapy is of continued interest in part because about one-third of the clinical responses are complete, durable remissions.

So far, IL-2 immunotherapy has been shown to be beneficial in a variety of clinical trials [40], [41], [42], [43], [44]. For example, studies to identify antigen-nonspecific strategies for enhancing immune reconstitution in individuals with HIV infection include those using IL-2 [45]. Two recent studies that used IL-2 at much lower doses in different immune-mediated diseases now suggest that IL-2 may have a dominant role in immunosuppression, rather than in immune stimulation as originally thought. Koreth et al. [46] showed that IL-2 therapy reversed graft-versus-host disease in patients who had undergone allogeneic HSCT to treat lymphoma or leukemia. Similarly, Saadoun et al. [47] found that IL-2 treatment had clinical benefit in hepatitis C virus-related vasculitis. Increased numbers of Treg cells were observed after IL-2 treatment in both reports.

Treatment with IL-15 has many of the same effects as with IL-2 in mice, as it induces the proliferation of T cells, with a preference toward memory CD8⁺ T cells and NK cells. IL-15 could have advantage over IL-2 as it does not increase Treg levels as observed for IL-2. IL-15 can boost patients’ immune systems and restore immune responses against cancer and infectious diseases like HIV. Therefore, IL-15 has been studied since 2009 in patients with malignant melanoma and in patients with renal cell carcinoma [48]. Furthermore, IL-15 is currently being investigated in settings of immune deficiency, for the in vitro expansion of T and NK cells and as an adjuvant for vaccine strategies [49].