IL-2 in vivo activities and antitumor efficacy enhanced by an anti-IL-2 mAb

Spatial and functional targeting of intratumoral Tregs reverses CD8+ T cell exhaustion and promotes cancer immunotherapy

Intratumoral Tregs are key mediators of cancer immunotherapy resistance, including anti-programmed cell death (ligand) 1 [anti-PD-(L)1] immune checkpoint blockade (ICB). The mechanisms driving Treg infiltration into the tumor microenvironment (TME) and the consequence on CD8+ T cell exhaustion remain elusive. Here, we report that heat shock protein gp96 (also known as GRP94) was indispensable for Treg tumor infiltration, primarily through the roles of gp96 in chaperoning integrins. Among various gp96-dependent integrins, we found that only LFA-1 (αL integrin), and not αV, CD103 (αE), or β7 integrin, was required for Treg tumor homing. Loss of Treg infiltration into the TME by genetic deletion of gp96/LFA-1 potently induced rejection of tumors in multiple ICB-resistant murine cancer models in a CD8+ T cell-dependent manner, without loss of self-tolerance. Moreover, gp96 deletion impeded Treg activation primarily by suppressing IL-2/STAT5 signaling, which also contributed to tumor regression. By competing for intratumoral IL-2, Tregs prevented the activation of CD8+ tumor-infiltrating lymphocytes, drove thymocyte selection-associated high mobility group box protein (TOX) induction, and induced bona fide CD8+ T cell exhaustion. By contrast, Treg ablation led to striking CD8+ T cell activation without TOX induction, demonstrating clear uncoupling of the 2 processes. Our study reveals that the gp96/LFA-1 axis plays a fundamental role in Treg biology and suggests that Treg-specific gp96/LFA-1 targeting represents a valuable strategy for cancer immunotherapy without inflicting autoinflammatory conditions.

An engineered immunocytokine with collagen affinity improves the tumor bioavailability, tolerability, and therapeutic efficacy of IL-2

The clinical utility of human interleukin-2 (hIL-2) is limited by its short serum half-life, preferential activation of regulatory T (TReg) over immune effector cells, and dose-limiting toxicities. We previously engineered F10 immunocytokine (IC), an intramolecularly assembled cytokine/antibody fusion protein that linked hIL-2 to an anti-IL-2 antibody (denoted F10) that extended IL-2 half-life and augmented the immune effector to TReg ratio. Here, we leveraged molecular engineering to improve the anti-tumor therapeutic efficacy and tolerability of F10 IC by developing an iteration, denoted F10 IC-CBD (collagen binding domain), designed for intratumoral administration and in situ retention based on collagen affinity. F10 IC-CBD retained IL-2 bioactivity exclusively in the tumor and eliminated IL-2-associated toxicities. Furthermore, F10 IC exhibited potent single-agent therapeutic efficacy and synergy with systemic immune checkpoint blockade and elicited an abscopal response in mouse tumors models. This engineered fusion protein presents a prototype for the design of intratumoral therapies.

Robust IL-2-dependent antitumor immunotherapy requires targeting the high-affinity IL-2R on tumor-specific CD8+ T cells

BACKGROUND

Development of interleukin (IL)-2-dependent antitumor responses focus on targeting the intermediate affinity IL-2R to stimulate memory-phenotypic CD8+ T and natural killer (NK) cells while minimizing regulatory T cell (Treg) expansion. However, this approach may not effectively engage tumor-specific T effector cells. Since tumor-antigen specific T cells upregulate the high-affinity IL-2R, we tested an IL-2 biologic, mouse IL-2/CD25, with selectivity toward the high-affinity IL-2R to support antitumor responses to tumors that vary in their immunogenicity.

METHODS

Mice were first implanted with either CT26, MC38, B16.F10, or 4T1 and after a tumor mass developed, they were treated with high-dose (HD) mouse (m)IL-2/CD25 alone or in combination with anti-programmed cell death protein-1 (PD-1) checkpoint blockade. Tumor growth was monitored and in parallel the immune signature in the tumor microenvironment (TME) was determined by a combination of multiparameter flow cytometry, functional assays, and enumeration of tumor-reactive T cells.

RESULTS

We show that HD mIL-2/CD25, which preferentially stimulates the high-affinity IL-2R, but not IL-2/anti-IL-2 complexes with preferential activity toward the intermediate-affinity IL-2R, supports vigorous antitumor responses to immunogenic tumors as a monotherapy that were enhanced when combined with anti-PD-1. Treatment of CT26-bearing mice with HD mIL-2/CD25 led to a high CD8+:Treg ratio in the TME, increased frequency and function of tumor-specific CD8+ T effector cells with a less exhausted phenotype, and antitumor memory responses.

CONCLUSIONS

Targeting the high-affinity IL-2R on tumor-specific T cells with HD mIL-2/CD25 alone or with PD-1 blockade supports antitumor responses, where the resulting memory response may afford long-term protection against tumor re-emergence.

Engineered cytokine/antibody fusion proteins improve delivery of IL-2 to pro-inflammatory cells and promote antitumor activity

Progress in cytokine engineering is driving therapeutic translation by overcoming the inherent limitations of these proteins as drugs. The interleukin-2 (IL-2) cytokine harbors great promise as an immune stimulant for cancer treatment. However, the cytokine's concurrent activation of both pro-inflammatory immune effector cells and anti-inflammatory regulatory T cells, its toxicity at high doses, and its short serum half-life have limited clinical application. One promising approach to improve the selectivity, safety, and longevity of IL-2 is complexation with anti-IL-2 antibodies that bias the cytokine towards the activation of immune effector cells (i.e., effector T cells and natural killer cells). Although this strategy shows therapeutic potential in preclinical cancer models, clinical translation of a cytokine/antibody complex is complicated by challenges in formulating a multi-protein drug and concerns about complex stability. Here, we introduce a versatile approach to designing intramolecularly assembled single-agent fusion proteins (immunocytokines, ICs) comprising IL-2 and a biasing anti-IL-2 antibody that directs the cytokine's activities towards immune effector cells. We establish the optimal IC construction and further engineer the cytokine/antibody affinity to improve immune biasing function. We demonstrate that our IC preferentially activates and expands immune effector cells, leading to superior antitumor activity compared to natural IL-2 without inducing toxicities associated with IL-2 administration. Collectively, this work presents a roadmap for the design and translation of immunomodulatory cytokine/antibody fusion proteins.

Advancing Biologic Therapy for Refractory Autoimmune Hepatitis

Biologic agents may satisfy an unmet clinical need for treatment of refractory autoimmune hepatitis. The goals of this review are to present the types and results of biologic therapy for refractory autoimmune hepatitis, indicate opportunities to improve and expand biologic treatment, and encourage comparative clinical trials. English abstracts were identified in PubMed by multiple search terms. Full-length articles were selected for review, and secondary and tertiary bibliographies were developed. Rituximab (monoclonal antibodies against CD20 on B cells), infliximab (monoclonal antibodies against tumor necrosis factor-alpha), low-dose recombinant interleukin 2 (regulatory T cell promoter), and belimumab (monoclonal antibodies against B cell activating factor) have induced laboratory improvement in small cohorts with refractory autoimmune hepatitis. Ianalumab (monoclonal antibodies against the receptor for B cell activating factor) is in clinical trial. These agents target critical pathogenic pathways, but they may also have serious side effects. Blockade of the B cell activating factor or its receptors may disrupt pivotal B and T cell responses, and recombinant interleukin 2 complexed with certain interleukin 2 antibodies may selectively expand the regulatory T cell population. A proliferation-inducing ligand that enhances T cell proliferation and survival is an unevaluated, potentially pivotal, therapeutic target. Fully human antibodies, expanded target options, improved targeting precision, more effective delivery systems, and biosimilar agents promise to improve efficacy, safety, and accessibility. In conclusion, biologic agents target key pathogenic pathways in autoimmune hepatitis, and early experiences in refractory disease encourage clarification of the preferred target, rigorous clinical trial, and comparative evaluations.

Cancer immunotherapy with T-cell targeting cytokines: IL-2 and IL-7

Clinical trials have demonstrated that an increased number of effector cells, especially tumor-specific T cells, is positively linked with patients’ prognosis. Although the discovery of checkpoint inhibitors (CPIs) has led to encouraging progress in cancer immunotherapy, the lack of either T cells or targets for CPIs is a limitation for patients with poor prognosis. Since interleukin (IL)-2 and IL-7 are cytokines that target many aspects of T-cell responses, they have been used to treat cancers. In this review, we focus on the basic biology of how these cytokines regulate T-cell response and on the clinical trials using the cytokines against cancer. Further, we introduce several recent studies that aim to improve cytokines’ biological activities and find the strategy for combination with other therapeutics.

Treatment of mice with S4B6 IL-2 complex prevents lethal toxoplasmosis via IL-12- and IL-18-dependent interferon-gamma production by non-CD4 immune cells

Toxoplasmic encephalitis is an AIDS-defining condition. The decline of IFN-γ-producing CD4⁺ T cells in AIDS is a major contributing factor in reactivation of quiescent Toxoplasma gondii to an actively replicating stage of infection. Hence, it is important to characterize CD4-independent mechanisms that constrain acute T. gondii infection. We investigated the in vivo regulation of IFN-γ production by CD8⁺ T cells, DN T cells and NK cells in response to acute T. gondii infection. Our data show that processing of IFN-γ by these non-CD4 cells is dependent on both IL-12 and IL-18 and the secretion of bioactive IL-18 in response to T. gondii requires the sensing of viable parasites by multiple redundant inflammasome sensors in multiple hematopoietic cell types. Importantly, our results show that expansion of CD8⁺ T cells, DN T cells and NK cell by S4B6 IL-2 complex pre-treatment increases survival rates of mice infected with T. gondii and this is dependent on IL-12, IL-18 and IFN-γ. Increased survival is accompanied by reduced pathology but is independent of expansion of T_Reg cells or parasite burden. This provides evidence for a protective role of IL2C-mediated expansion of non-CD4 cells and may represent a promising lead to adjunct therapy for acute toxoplasmosis.

The challenges and molecular approaches surrounding interleukin-2-based therapeutics in cancer

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IL2-based cancer therapies are limited by their toxicity and pleiotropy.

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Current engineering approaches target IL2 half-life and cell/receptor specificity.

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IL2 may enhance the efficacy of checkpoint inhibitors and CAR-T-based therapies.

Interleukin-2 has had a long history as a promising cancer therapeutic, being capable of eliciting complete and durable remissions in patients with metastatic renal cell carcinoma and metastatic melanoma. Despite high toxicity and efficacy limited to only certain patient subpopulations and cancer types, the prospective use of novel, engineered IL2 formats in combination with the presently expanding repertoire of immuno-oncological targets remains very encouraging. This is possible due to the significant research efforts in the IL2 field that have yielded critical structural and biological insights that have made IL2 more effective and more broadly applicable in the clinic. In this review, we discuss some of the molecular approaches that have been used to further improve IL2 therapy for cancer.

Combination treatment with hypofractionated radiotherapy plus IL-2/anti-IL-2 complexes and its theranostic evaluation

Background

Immunogenic radiotherapy (RT) can act synergistically with immune checkpoint blockers (ICBs). However, alternatives are needed for non-responding patients and those with pre-existing or ICB-induced autoimmune symptoms. Combination of RT with IL-2 could be an alternative. But IL-2 has a short half-life, and, by binding to its high-affinity receptor, it strongly stimulates immunosuppressive CD4+ Tregs and seems to promote potentially life-threatening vascular leakage. IL-2/anti-IL-2 complexes (IL-2c), which bind to the low-affinity receptor, have been reported to circumvent these disadvantages but they have not yet been thoroughly tested in conjunction with radiotherapy.

Methods

We evaluated, in three mouse models, the antitumoral effects induced by hypofractionated RT (hRT) plus IL-2c. We also used non-invasive imaging with a newly developed PET tracer based on therapeutically active IL-2c and a PD-L1 PET tracer for the theranostic evaluation of the treatment and its side effects.

Results

Treatment of mice bearing established B16 melanomas with hRT + IL-2c was superior to hRT + uncomplexed IL-2 or hRT alone; IL-2c alone was not effective. hRT + IL-2c was also synergistic in mice bearing C51 colon carcinomas or 4T1 mammary carcinomas. The better antitumor response correlated with increased tumor-specific CD8+ T cells and NK cells, but not CD4+ Tregs, in the irradiated tumor and in lymphoid organs. With the new PET tracer, we visualized the whole-body distribution of IL-2c and the bound receptors in naïve mice and tumor-bearing mice. Surprisingly, the tumor uptake was non-specific and only moderate. This prompted experiments demonstrating that specific IL-2c binding in the tumor is limited by IL-2 secreted by tumor-resident effector cells and that extratumorally expanded T and NK cells can infiltrate the irradiated tumor, which suggests that systemic immune activation considerably contributed to the reduction of tumor growth. Lastly, we show that a side effect of IL-2c treatment – a quite dramatic non-specific expansion of CD8+ T and NK cells – is only transient, and we visualized the associated splenomegaly as well as side effects on liver and lung by contrast-enhanced CT and PD-L1 PET.

Conclusions

Our results show that the combination of immunogenic RT with IL-2c that are directed towards the low-affinity IL-2 receptor can be synergistic and more effective than the combination with uncomplexed IL-2. In addition, our theranostic evaluation provided insights into the mechanism of action and the side effects of IL-2c treatment.

Electronic supplementary material

The online version of this article (10.1186/s40425-019-0537-9) contains supplementary material, which is available to authorized users.

Combining computational and experimental biology to develop therapeutically valuable IL2 muteins

High-dose IL2, first approved in 1992, has been used in the treatment of advanced renal cell carcinoma and melanoma. In these indications, IL2 induces long lasting objective responses in 5% to 20% of patients. However, toxicity and the unexpected expansion of regulatory T cells (Tregs) have limited its practical use and therapeutic impact, respectively. At the Center of Molecular Immunology in Havana, Cuba, a project was launched in 2005 to rationally design IL2 muteins that could be deployed in the therapy of cancer. The basic goal was to uncouple the pleiotropic effect of IL2 on different immune T cells, to obtain a mutein with a therapeutic index that was better than that achieved with wild type (wt) IL2. Using a combination of computational and experimental biology approaches, we predicted and developed two novel IL2 muteins with therapeutic potential. The first, designated no-alpha mutein, is an agonist of IL2R signaling with a reduced ability to expand Treg in vivo. In mice, the no-alpha mutein IL2 has higher antitumor activity and lower toxicity than wt IL2. It represents a potential best-in-class drug that has begun phase I/II clinical trials in solid tumors. The second, designated no-gamma mutein, is an antagonist of IL2R signaling, with some preferential affinity for Tregs. This mutein has antitumor activity in mice that likely derives from its ability to reduce Treg accumulation in vivo. It represents a first-in-class drug that offers a novel strategy to inhibit Treg activity in vivo.

Interleukin-15 Complex Treatment Protects Mice from Cerebral Malaria by Inducing Interleukin-10-Producing Natural Killer Cells

Cerebral malaria is a deadly complication of Plasmodium infection and involves blood brain barrier (BBB) disruption following infiltration of white blood cells. During experimental cerebral malaria (ECM), mice inoculated with Plasmodium berghei ANKA-infected red blood cells develop a fatal CM-like disease caused by CD8⁺ T cell-mediated pathology. We found that treatment with interleukin-15 complex (IL-15C) prevented ECM, whereas IL-2C treatment had no effect. IL-15C-expanded natural killer (NK) cells were necessary and sufficient for protection against ECM. IL-15C treatment also decreased CD8⁺ T cell activation in the brain and prevented BBB breakdown without influencing parasite load. IL-15C induced NK cells to express IL-10, which was required for IL-15C-mediated protection against ECM. Finally, we show that ALT-803, a modified human IL-15C, mediates similar induction of IL-10 in NK cells and protection against ECM. These data identify a regulatory role for cytokine-stimulated NK cells in the prevention of a pathogenic immune response.

Biology of IL-2 and its therapeutic modulation: Mechanisms and strategies

Cytokines signal through specific cell surface receptors to broadly regulate immune development, differentiation, proliferation, and survival, thereby influencing cellular fate and function. Accordingly, cytokines are potential therapeutic targets for modulation of immune responses. Indeed, over the last several decades, an increasing number of cytokine-based clinical trials have been performed, collectively using either recombinant forms of cytokines or blocking agents that modulate the actions of cytokines. The pleiotropic actions of cytokines, including their abilities to mediate both inflammatory and protective immune responses, indicate that using or targeting cytokines can have desired but also potentially undesirable effects. Here, we focus on the immunomodulatory cytokine, IL-2. We review the underlying basic science related to IL-2 and its biologic actions, discuss the current state of IL-2-based immunotherapy, and focus on emerging concepts of modulating the activities of this cytokine, including the generation of novel partial cytokine agonists as new potential therapeutics.

TCR Signaling and CD28/CTLA-4 Signaling Cooperatively Modulate T Regulatory Cell Homeostasis

Foxp3⁺ T regulatory cells (Tregs), conventional CD4⁺Foxp3^- T cells, and CD8⁺ T cells represent heterogeneous populations composed of naive phenotype (NP, CD44^low) and memory phenotype (MP, CD44^high) subpopulations. NP and MP subsets differ in their activation state, contribution to immune function, and capacity to proliferate in vivo. To further understand the factors that contribute to the differential homeostasis of NP/MP subsets, we examined the differential effects of CD28 and CTLA-4 interaction with CD80/CD86, as well as MHC class II-TCR interaction within mouse Treg pools and CD4⁺ and CD8⁺ T cell pools. Blockade of CD80/CD86 with CTLA-4-Ig markedly reduced the cycling and absolute numbers of MP Tregs and MP CD4⁺ T cells, with minimal effect on the NP T cell subpopulations. Blockade of MHC class II-TCR interaction led to selective expansion of MP Tregs and MP CD4⁺ and CD8⁺ T cells that was reversed upon cotreatment with CTLA-4-Ig. Treatment with anti-CTLA-4 mAb altered MP Treg and MP CD4⁺ and CD8⁺ T cell homeostasis in a manner similar to that observed with anti-MHC class II. We postulate a complex pathway in which CD28 is the primary driver of Treg proliferation and CTLA-4 functions as the main brake but is likely dependent on TCR signals and CD80/CD86. These findings have important implications for the use of biologic agents targeting such pathways to modulate autoimmune and neoplastic disease.

A Mouse Model of Latent Tuberculosis Infection to Study Intervention Strategies to Prevent Reactivation

Infection with Mycobacterium tuberculosis (Mtb) is the leading cause of death in human immunodeficiency virus (HIV)⁺ individuals, particularly in Sub-Saharan Africa. Management of this deadly co-infection is a significant global health challenge that is exacerbated by the lack of efficient vaccines against both Mtb and HIV, as well as the lack of reliable and robust animal models for Mtb/HIV co-infection. Here we describe a tractable and reproducible mouse model to study the reactivation dynamics of latent Mtb infection following the loss of CD4⁺ T cells as it occurs in HIV-co-infected individuals. Whereas intradermally (i.d.) infected C57BL/6 mice contained Mtb within the local draining lymph nodes, depletion of CD4⁺ cells led to progressive systemic spread of the bacteria and induction of lung pathology. To interrogate whether reactivation of Mtb after CD4⁺ T cell depletion can be reversed, we employed interleukin (IL)-2/anti-IL-2 complex-mediated cell boost approaches. Although populations of non-CD4 lymphocytes, such as CD8⁺ memory T cells, natural killer (NK) cells and double-negative (DN) T cells significantly expanded after IL-2/anti-IL-2 complex treatment, progressive development of bacteremia and pathologic lung alterations could not be prevented. These data suggest that the failure to reverse Mtb reactivation is likely not due to anergy of the expanded cell subsets and rather indicates a limited potential for IL-2-complex-based therapies in the management of Mtb/HIV co-infection.

IL-2/anti-IL-2 mAb immunocomplexes: A renascence of IL-2 in cancer immunotherapy?

The in vivo biological activity of IL-2 can be dramatically increased by complexing with anti-IL-2 mAb. Moreover, IL-2/anti-IL-2 mAb immunocomplexes selectively stimulate different subsets of immune cells, depending on the clone of anti-IL-2 mAb that is used. Thus, IL-2/S4B6 mAb complexes strongly stimulate CD122^high populations, namely NK and memory CD8⁺ T cells. They also intermediately stimulate T_reg cells. Conversely, IL-2/JES6.1 mAb immunocomplexes have no stimulatory activity for CD122^high populations. However, they potently and highly selectively stimulate CD25⁺ cells (i.e., T_reg and activated T cells). IL-2/S4B6 mAb immunocomplexes have also been shown to possess antitumor activity in various mouse tumor models.

Enhancing Dendritic Cell-based Immunotherapy with IL-2/Monoclonal Antibody Complexes for Control of Established Tumors

U.S. Food and Drug Administration-approved high-dose IL-2 therapy and dendritic cell (DC) immunization offer time-tested treatments for malignancy, but with defined issues of short in vivo t1/2, toxicity, and modest clinical benefit. Complexes of IL-2 with specific mAbs (IL-2c) exhibit improved stability in vivo with reduced toxicity and are capable of stimulating NK cell and memory phenotype CD8 T cell proliferation. In this study, we demonstrate that IL-2c treatment in tumor-bearing mice can enhance NK cell and tumor-specific CD8 T cell numbers. Importantly, DC immunization coupled with stabilized IL-2c infusion drastically improves the tumor-specific effector CD8 T cell response. DC + IL-2c treatment enhances number, 41BB and GITR expression, granzyme B production, CTL/regulatory T cell ratio, and per-cell killing capacity of CD8 T cells without increasing inhibitory molecule expression. Notably, IL-2c treatment of anti-CD3-stimulated human CD8 T cells resulted in higher number and granzyme B production, supporting the translational potential of this immunotherapy strategy for human malignancy. DC + IL-2c treatment enhances both endogenous NK cell and tumor Ag-specific CD8 T cell immunity to provide a marked reduction in tumor burden in multiple models of pre-existing malignancy in B6 and BALB/c mice. Depletion studies reveal contributions from both tumor-specific CD8 T cells and NK cells in control of tumor burden after DC + IL-2c treatment. Together, these data suggest that combination therapy with DC and IL-2c may be a potent treatment for malignancy.

Activated regulatory T cells suppress effector NK cell responses by an IL-2-mediated mechanism during an acute retroviral infection

Background

It is well established that effector T cell responses are crucial for the control of most virus infections, but they are often tightly controlled by regulatory T cells (Treg) to minimize immunopathology. NK cells also contribute to virus control but it is not known if their antiviral effect is influenced by virus-induced Tregs as well. We therefore analyzed whether antiretroviral NK cell functions are inhibited by Tregs during an acute Friend retrovirus infection of mice.

Results

Selective depletion of Tregs by using the transgenic DEREG mouse model resulted in improved NK cell proliferation, maturation and effector cell differentiation. Suppression of NK cell functions depended on IL-2 consumption by Tregs, which could be overcome by specific NK cell stimulation with an IL-2/anti-IL-2 mAb complex.

Conclusions

The current study demonstrates that virus-induced Tregs indeed inhibit antiviral NK cell responses and describes a targeted immunotherapy that can abrogate the suppression of NK cells by Tregs.

Electronic supplementary material

The online version of this article (doi:10.1186/s12977-015-0191-3) contains supplementary material, which is available to authorized users.

Combination therapy with dendritic cell vaccine and IL-2 encapsulating polymeric micelles enhances intra-tumoral accumulation of antigen-specific CTLs

Dendritic cell (DC) vaccine is a promising immunotherapy for cancer due to its ability to induce antigen-specific CTLs efficiently. However, a number of clinical studies have implied insufficient therapeutic benefits with the use of MHC class 1 restricted peptide-pulsed DC vaccine. To enhance the clinical efficacy, we examined combination therapy of DC vaccine pulsed with OVA peptide and intravenous low dose unmodified IL-2 (IL-2 solution) administration against EG7 tumor-bearing mice. Unexpectedly, no additional effects of IL-2 solution were observed on CTL induction and the therapeutic effects of DC vaccine, possibly because of the short half-life of IL-2 in plasma. Therefore, we generated IL-2-encapsulating polymeric micelles (IL-2 micelle), which showed prolonged IL-2 retention in the circulation after intravenous administration compared with IL-2 solution. When mice were treated with OVA peptide-pulsed DCs in combination with IL-2 micelle, OVA-specific CTLs were efficiently induced in the spleen in comparison with DC vaccine combined with IL-2 solution or DC vaccine alone. In addition, combination therapy of DC vaccine and IL-2 micelle against EG7 tumor-bearing mice induced the efficient accumulation of antigen-specific CTLs into the tumor and marked anti-tumor effects. Thus, the administration of IL-2 micelle can significantly enhance DC vaccine efficacy against tumors.

Challenges and developing solutions for increasing the benefits of IL-2 treatment in tumor therapy

Interleukin-2 (IL-2) is a cytokine with pleiotropic effects on the immune system. Systemic IL-2 treatment has produced durable responses in melanoma and renal cancer patients, but unfortunately this is effective only in a fraction of patients. Moreover, IL-2 treatment also engenders serious side effects, which limit its clinical utility. It is now appreciated that IL-2 not only stimulates NK and effector T cells but also has a critical role in the generation and maintenance of regulatory T cells, which act to dampen immune responses. Thus, successful immunotherapy of cancers using IL-2 has to address two fundamentally important issues: (1) how to limit side effects yet be active where it is needed, and (2) how to preferentially activate effector T cells while limiting the stimulation of Tregs. Strategies are now being developed to address these critical obstacles that may lead to a renaissance of IL-2 therapy.

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

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.

Mathematical Models of the Impact of IL2 Modulation Therapies on T Cell Dynamics

Several reports in the literature have drawn a complex picture of the effect of treatments aiming to modulate IL2 activity in vivo. They seem to promote either immunity or tolerance, probably depending on the specific context, dose, and timing of their application. Such complexity might derive from the pleiotropic role of IL2 in T cell dynamics. To theoretically address the latter possibility, our group has developed several mathematical models for Helper, Regulatory, and Memory T cell population dynamics, which account for most well-known facts concerning their relationship with IL2. We have simulated the effect of several types of therapies, including the injection of: IL2; antibodies anti-IL2; IL2/anti-IL2 immune-complexes; and mutant variants of IL2. We studied the qualitative and quantitative conditions of dose and timing for these treatments which allow them to potentiate either immunity or tolerance. Our results provide reasonable explanations for the existent pre-clinical and clinical data, predict some novel treatments, and further provide interesting practical guidelines to optimize the future application of these types of treatments.

Fine epitope specificity of antibodies against interleukin-2 explains their paradoxical immunomodulatory effects

The functional dichotomy of antibodies against interleukin-2 (IL-2) is thought to depend upon recognition of different cytokine epitopes. Beyond functional studies, the only molecular evidence obtained so far located the epitopes recognized by the immunoenhancing antibodies S4B6 and JES6-5H4 within the predicted interface of IL-2 with the α receptor subunit, explaining the preferential stimulation of effector cells displaying only β and γ receptor chains. A consistent functional map of the epitope bound by the immunoregulatory antibody JES6-1A12 has now been delineated by screening the interactions of phage-displayed antigen variants (with single and multiple mutations) and antigen mimotopes. The target determinant resides in a region between the predicted interfaces with α and β/γ receptor subunits, supporting the dual inhibitory role of the antibody on both interactions. Binding by JES6-1A12 would thus convert complexed IL-2 into a very weak agonist, reinforcing the advantage of T regulatory cells (displaying the high affinity αβγ heterotrimeric receptor) to capture the cytokine by competition and expand over effector cells, ultimately resulting in the observed strong tolerogenic effect of this antibody. Detailed knowledge of the epitopes recognized by anti-IL-2 antibodies with either immunoenhancing or immunoregulatory properties completes the molecular scenario underlying their use to boost or inhibit immune responses in multiple experimental systems. The expanded functional mapping platform now available could be exploited to study other interactions involving related molecular pairs with the final goal of optimizing cytokine and anti-cytokine therapies.

G-CSF/anti-G-CSF antibody complexes drive the potent recovery and expansion of CD11b+Gr-1+ myeloid cells without compromising CD8+ T cell immune responses

BACKGROUND

Administration of recombinant G-CSF following cytoreductive therapy enhances the recovery of myeloid cells, minimizing the risk of opportunistic infection. Free G-CSF, however, is expensive, exhibits a short half-life, and has poor biological activity in vivo.

METHODS

We evaluated whether the biological activity of G-CSF could be improved by pre-association with anti-G-CSF mAb prior to injection into mice.

RESULTS

We find that the efficacy of G-CSF therapy can be enhanced more than 100-fold by pre-association of G-CSF with an anti-G-CSF monoclonal antibody (mAb). Compared with G-CSF alone, administration of G-CSF/anti-G-CSF mAb complexes induced the potent expansion of CD11b+Gr-1+ myeloid cells in mice with or without concomitant cytoreductive treatment including radiation or chemotherapy. Despite driving the dramatic expansion of myeloid cells, in vivo antigen-specific CD8+ T cell immune responses were not compromised. Furthermore, injection of G-CSF/anti-G-CSF mAb complexes heightened protective immunity to bacterial infection. As a measure of clinical value, we also found that antibody complexes improved G-CSF biological activity much more significantly than pegylation.

CONCLUSIONS

Our findings provide the first evidence that antibody cytokine complexes can effectively expand myeloid cells, and furthermore, that G-CSF/anti-G-CSF mAb complexes may provide an improved method for the administration of recombinant G-CSF.

Chimera of IL-2 linked to light chain of anti-IL-2 mAb mimics IL-2/anti-IL-2 mAb complexes both structurally and functionally

IL-2/anti-IL-2 mAb immunocomplexes were described to have dramatically higher activity than free IL-2 in vivo. We designed protein chimera consisting of IL-2 linked to light chain of anti-IL-2 mAb S4B6 through flexible oligopeptide spacer (Gly(4)Ser)(3). This protein chimera mimics the structure of IL-2/S4B6 mAb immunocomplexes but eliminates general disadvantages of immunocomplexes like possible excess of either IL-2 or anti-IL-2 mAb and their dissociation to antibody and IL-2 at low concentrations. This novel kind of protein chimera is characterized by an intramolecular interaction between IL-2 and binding site of S4B6 mAb similarly as in IL-2/S4B6 mAb immunocomplexes. Our protein chimera has biological activity comparable to IL-2/S4B6 mAb immunocomplexes in vitro, as shown by stimulation of proliferation of purified and activated OT-I CD8(+) T cells. The protein chimera exerts higher stimulatory activity to drive expansion of purified CFSE-labeled OT-I CD8(+) T cells activated by an injection of a low dose of SIINFEKL peptide than IL-2/S4B6 mAb immunocomplexes in vivo.

Application of IL-2 therapy to target T regulatory cell function

Interleukin-2 (IL-2) was originally discovered as a growth factor for activated T cells in vitro. IL-2 promotes CD8(+) T cell growth and differentiation in vivo, but has little effect on CD4(+) T cell function. Regulatory T cells (Treg cells) express all three chains (CD25, CD122, and CD132) of the IL-2 receptor complex and are dependent on IL-2 for survival and function. Exogenous IL-2 can augment Treg cell numbers in vivo and may have therapeutic value in the treatment of autoimmune and inflammatory diseases. Complexes of IL-2 with different IL-2 antibodies can target delivery to cells expressing all three receptor chains (Treg cells and activated T effector cells) or to cells expressing just CD122 and CD132 (NK cells and memory phenotype CD8(+) T cells).

Modeling the role of IL2 in the interplay between CD4+ helper and regulatory T cells: studying the impact of IL2 modulation therapies

Several reports in the literature have drawn a complex picture of the effect of treatments aiming to modulate IL2 activity in vivo. They seem to promote indistinctly immunity or tolerance, probably depending on the specific context, dose and timing of their application. Such complexity might derives from the dual role of IL2 on T-cell dynamics. To theoretically address the latter possibility, we develop a mathematical model for helper, regulatory and memory T-cells dynamics, which account for most well-known facts relative to their relationship with IL2. We simulate the effect of three types of therapies: IL2 injections, IL2 depletion using anti-IL2 antibodies and IL2/anti-IL2 immune complexes injection. We focus in the qualitative and quantitative conditions of dose and timing for these treatments which allow them to potentate either immunity or tolerance. Our results provide reasonable explanations for the existent pre-clinical and clinical data and further provide interesting practical guidelines to optimize the future application of these types of treatments. Particularly, our results predict that: (i) Immune complexes IL2/anti-IL2 mAbs, using mAbs which block the interaction of IL2 and CD25 (the alpha chain of IL2 receptor), is the best option to potentate immunity alone or in combination with vaccines. These complexes are optimal when a 1:2 molar ratio of mAb:IL2 is used and the mAbs have the largest possible affinity; (ii) Immune complexes IL2/anti-IL2 mAbs, using mAbs which block the interaction of IL2 and CD122 (the beta chain of IL2 receptor), are the best option to reinforce preexistent natural tolerance, for instance to prevent allograft rejection. These complexes are optimal when a 1:2 molar ratio of mAb:IL2 is used and the mAbs have intermediate affinities; (iii) mAbs anti-IL2 can be successfully used alone to treat an ongoing autoimmune disorder, promoting the re-induction of tolerance. The best strategy in this therapy is to start treatment with an initially high dose of the mAbs (one capable to induce some immune suppression) and then scales down slowly the dose of mAb in subsequent applications.

Mast cell interleukin-2 production contributes to suppression of chronic allergic dermatitis

The incidence of chronic allergic dermatitis is rapidly increasing. Regulatory control of this disease has not been adequately explored. Here we report that mast cell-derived interleukin-2 (IL-2) contributes to the suppression of chronic allergic dermatitis. Mice deficient in IL-2 production, or deficient in mast cells (Kit(W-sh/W-sh)), showed exacerbated dermatitis upon repeated oxazolone challenge when compared to their wild-type counterparts. Adoptive transfer of wild-type, but not Il2(-/-), mast cells into Kit(W-sh/W-sh) mice dampened the inflammatory response. During the course of disease, mast cell expansion occurred at the site of inflammation and also in the spleen, where production of IL-2 by mast cells was markedly enhanced. In the absence of mast cell IL-2 production, the ratio of activated to regulatory T cells at the site of inflammation was increased. Thus, MC-derived IL-2 contributes to the maintenance of suppression in chronic allergic skin inflammation.

In vivo expansion of regulatory T cells with IL-2/IL-2 mAb complexes prevents anti-factor VIII immune responses in hemophilia A mice treated with factor VIII plasmid-mediated gene therapy

Generation of transgene-specific immune responses can constitute a major complication following gene therapy treatment. An in vivo approach to inducing selective expansion of Regulatory T (Treg) cells by injecting interleukin-2 (IL-2) mixed with a specific IL-2 monoclonal antibody (JES6-1) was adopted to modulate anti-factor VIII (anti-FVIII) immune responses. Three consecutive IL-2 complexes treatments combined with FVIII plasmid injection prevented anti-FVIII formation and achieved persistent, therapeutic-level of FVIII expression in hemophilia A (HemA) mice. The IL-2 complexes treatment expanded CD4(+)CD25(+)Foxp3(+) Treg cells five- to sevenfold on peak day, and they gradually returned to normal levels within 7-14 days without changing other lymphocyte populations. The transiently expanded Treg cells are highly activated and display suppressive function in vitro. Adoptive transfer of the expanded Treg cells protected recipient mice from generation of high-titer antibodies following FVIII plasmid challenge. Repeated plasmid transfer is applicable in tolerized mice without eliciting immune responses. Mice treated with IL-2 complexes mounted immune responses against both T-dependent and T-independent neoantigens, indicating that IL-2 complexes did not hamper the immune system for long. These results demonstrate the important role of Treg cells in suppressing anti-FVIII immune responses and the potential of developing Treg cell expansion therapies that induce long-term tolerance to FVIII.

Induction and function of virus-specific CD4+ T cell responses

CD4+ T cells - often referred to as T-helper cells - play a central role in immune defense and pathogenesis. Virus infections and vaccines stimulate and expand populations of antigen-specific CD4+ T cells in mice and in man. These virus-specific CD4+ T cells are extremely important in antiviral protection: deficiencies in CD4+ T cells are associated with virus reactivation, generalized susceptibility to opportunistic infections, and poor vaccine efficacy. As described below, CD4+ T cells influence effector and memory CD8+ T cell responses, humoral immunity, and the antimicrobial activity of macrophages and are involved in recruiting cells to sites of infection. This review summarizes a few key points about the dynamics of the CD4+ T cell response to virus infection, the positive role of pro-inflammatory cytokines in the differentiation of virus-specific CD4+ T cells, and new areas of investigation to improve vaccines against virus infection.

Constitutive, but not challenge-induced, interleukin-10 production is robust in acute pre-pubescent protein and energy deficits: new support for the tolerance hypothesis of malnutrition-associated immune depression based on cytokine production in vivo

The tolerance model of acute (i.e., wasting) pre-pubescent protein and energy deficits proposes that the immune depression characteristic of these pathologies reflects an intact anti-inflammatory form of immune competence that reduces the risk of autoimmune reactions to catabolically released self antigens. A cornerstone of this proposition is the finding that constitutive (first-tier) interleukin(IL)-10 production is sustained even into the advanced stages of acute malnutrition. The IL-10 response to inflammatory challenge constitutes a second tier of anti-inflammatory regulation and was the focus of this investigation. Weanling mice consumed a complete diet ad libitum, a low-protein diet ad libitum (mimicking incipient kwashiorkor), or the complete diet in restricted daily quantities (mimicking marasmus), and their second-tier IL-10 production was determined both in vitro and in vivo using lipopolysaccharide (LPS) and anti-CD3 as stimulants of innate and adaptive defences, respectively. Both early (3 days) and advanced (14 days) stages of wasting pathology were examined and three main outcomes emerged. First, classic in vitro systems are unreliable for discerning cytokine production in vivo. Secondly, in diverse forms of acute malnutrition declining challenge-induced IL-10 production may provide an early sign that anti-inflammatory control over immune competence is failing. Thirdly, and most fundamentally, the investigation provides new support for the tolerance model of malnutrition-associated inflammatory immune depression.

IL-2 mediates CD4+ T cell help in the breakdown of memory-like CD8+ T cell tolerance under lymphopenic conditions

Background

Lymphopenia results in the proliferation and differentiation of naïve T cells into memory-like cells in the apparent absence of antigenic stimulation. This response, at least in part due to a greater availability of cytokines, is thought to promote anti-self responses. Although potentially autoreactive memory-like CD8⁺ T cells generated in a lymphopenic environment are subject to the mechanisms of peripheral tolerance, they can induce autoimmunity in the presence of antigen-specific memory-like CD4⁺ T helper cells.

Methodology/Principal Findings

Here, we studied the mechanisms underlying CD4 help under lymphopenic conditions in transgenic mice expressing a model antigen in the beta cells of the pancreas. Surprisingly, we found that the self-reactivity mediated by the cooperation of memory-like CD8⁺ and CD4⁺ T cells was not abrogated by CD40L blockade. In contrast, treatment with anti-IL-2 antibodies inhibited the onset of autoimmunity. IL-2 neutralization prevented the CD4-mediated differentiation of memory-like CD8⁺ T cells into pathogenic effectors in response to self-antigen cross-presentation. Furthermore, in the absence of helper cells, induction of IL-2 signaling by an IL-2 immune complex was sufficient to promote memory-like CD8⁺ T cell self-reactivity.

Conclusions/Significance

IL-2 mediates the cooperation of memory-like CD4⁺ and CD8⁺ T cells in the breakdown of cross-tolerance, resulting in effector cytotoxic T lymphocyte differentiation and the induction of autoimmune disease.

Expansion of regulatory T cells via IL-2/anti-IL-2 mAb complexes suppresses experimental myasthenia

Human autoimmune diseases are often characterized by a relative deficiency in CD4(+)CD25(+) regulatory T cells (Treg). We therefore hypothesized that expansion of Treg can ameliorate autoimmune pathology. We tested this hypothesis in an experimental model for autoimmune myasthenia gravis (MG), a B-cell-mediated disease characterized by auto-Ab directed against the acetylcholine receptor within neuromuscular junctions. We showed that injection of immune complexes composed of the cytokine IL-2 and anti-IL-2 mAb (JES6-1A12) induced an effective and sustained expansion of Treg, via peripheral proliferation of CD4(+)CD25(+)Foxp3(+) cells and peripheral conversion of CD4(+)CD25(-)Foxp3(-) cells. The expanded Treg potently suppressed autoreactive T- and B-cell responses to acetylcholine receptor and attenuated the muscular weakness that is characteristic of MG. Thus, IL-2/anti-IL-2 mAb complexes can expand functional Treg in vivo, providing a potential clinical application of this modality for treatment of MG and other autoimmune disorders.

Modeling the role of IL-2 in the interplay between CD4+ helper and regulatory T cells: assessing general dynamical properties

Mathematical models accounting for well-known evidences relating to the dynamics of interleukin 2, helper and regulatory T cells are presented. These models extend an existent model (the so-called cross-regulation model of immunity), by assuming IL-2 as the growth factor produced by helper cells, but used by both helper and regulatory cells to proliferate and survive. Two model variants, motivated by current literature, are explored. The first variant assumes that regulatory cells suppress helper cells by limiting IL-2 production and consuming the available IL-2; i.e. they just trigger competition for IL-2. The second model variant adds to the latter competitive mechanism the direct inhibition of helper cells activation by regulatory cells. The extended models retain key dynamical features of the cross-regulation model. But such reasonable behavior depends on parameter constraints, which happen to be realistic and lead to interesting biological discussions. Furthermore, the introduction of IL-2 in these models breaks the local/specific character of interactions, providing new properties to them. In the extended models, but not in the cross-regulation model, the response triggered by an antigen affects the response to other antigens in the same lymph node. The first model variant predicts an unrealistic coupling of the immune reactions to all the antigens in the lymph node. In contrast, the second model variant allows the coexistent of concomitant tolerant and immune responses to different antigens. The IL-2 derived from an ongoing immune reaction reinforces tolerance to other antigens in the same lymph node. Overall the models introduced here are useful extensions of the cross-regulation formalism. In particular, they might allow future studies of the effect of different IL-2 modulation therapies on CD4+ T cell dynamics.

Elevated blood interleukin-10 levels and undiminished systemic interleukin-10 production rate prevail throughout acute protein-energy malnutrition in the weanling mouse

The objectives were to determine if blood IL-10 levels rise during the early stages of acute (wasting) pre-pubescent malnutrition in metabolically distinct murine models known to depress inflammatory immune competence and whether systemic IL-10 production is affected in these pathologies. Weanling C57BL/6J mice were assigned to dietary protocols that elicited wasting pathologies mimicking the human diseases of marasmus (restricted-intake group) or incipient kwashiorkor (low-protein group). An age-matched control group also was included. Serum IL-10 bioactivities were assessed in the early (day 3) and advanced (day 14) stages of weight loss, and net systemic IL-10 production was assessed at the same stages of pathology by in vivo cytokine capture. Blood IL-10 levels were elevated in both malnourished groups relative to controls at days 3 and 14 (range of P values: 0.03-0.0001). Further, despite a limited supply of energy and nitrogenous substrates, the systemic IL-10 production rate was at least sustained in the malnourished groups and, in fact, was elevated in the marasmic group (P=0.05) throughout the progression of weight loss. IL-10 emerges as an anti-inflammatory mediator positioned to participate in initiating and upholding the depressed immune competence that accompanies acute pre-pubescent deficits of protein and energy.

Cutting edge: IL-2 immune complexes as a therapy for persistent virus infection

There is an urgent need to develop novel therapies for controlling recurrent virus infections in immune suppressed patients. Disease associated with persistent gamma-herpesvirus infection (EBV, HHV-8) is a significant problem in AIDS patients and transplant recipients, and clinical management of these conditions is difficult. Disease occurs because of a failure in immune surveillance to control the persistent infection, which arises in AIDS patients principally because of an erosion of the CD4(+) T cell compartment. Immune surveillance failure followed by gamma-herpesvirus recrudescence can be modeled using murine gamma-herpesvirus in CD4 T cell-depleted mice. We show that enhancement of IL-2 signaling using IL-2/anti-IL-2 immune complexes substantially improves immune surveillance in the context of suppressed immunity and enhances control of the infection. This effect was not due solely to increased numbers of virus-specific CD8 T cells but rather to enhanced cytotoxicity mediated by the perforin-granzyme pathway.

Suppression of murine allergic airway disease by IL-2:anti-IL-2 monoclonal antibody-induced regulatory T cells

Regulatory T cells (Treg) play a decisive role in many diseases including asthma and allergen-induced lung inflammation. However, little progress has been made developing new therapeutic strategies for pulmonary disorders. In the current study we demonstrate that cytokine:antibody complexes of IL-2 and anti-IL-2 mAb reduce the severity of allergen-induced inflammation in the lung by expanding Tregs in vivo. Unlike rIL-2 or anti-IL-2 mAb treatment alone, IL-2:anti-IL-2 complexes dampened airway inflammation and eosinophilia while suppressing IL-5 and eotaxin-1 production. Mucus production, airway hyperresponsiveness to methacholine, and parenchymal tissue inflammation were also dramatically reduced following IL-2:anti-IL-2 treatment. The suppression in allergic airway disease was associated with a marked expansion of Tregs (IL-10(+)CD4(+)CD25(+) and Foxp3(+)CD4(+)CD25(+)) in the tissues, with a corresponding decrease in effector T cell responses. The ability of IL-2:anti-IL-2 complexes to suppress airway inflammation was dependent on Treg-derived IL-10, as IL-10(+/+), but not IL-10(-/-) Tregs, were capable of mediating the suppression. Furthermore, a therapeutic protocol using a model of established airway allergy highlighted the ability of IL-2:anti-IL-2 complexes to expand Tregs and prevent successive airway inflammation and airway hyperresponsiveness. This study suggests that endogenous Treg therapy may be a useful tool to combat the rising incidence of allergic airway disease.

Endoplasmic reticulum stress regulator XBP-1 contributes to effector CD8+ T cell differentiation during acute infection

The transcription factor X-box-binding protein-1 (XBP-1) plays an essential role in activating the unfolded protein response in the endoplasmic reticulum (ER). Transcribed XBP-1 mRNA is converted to its active form by unconventional cytoplasmic splicing mediated by inositol-requiring enzyme-1 (IRE-1) upon ER stress. We report activation of the IRE-1/XBP-1 pathway in effector CD8(+) T cells during the response to acute infection. Transcription of unspliced XBP-1 mRNA is up-regulated by IL-2 signals, while its splicing is induced after TCR ligation. Splicing of XBP-1 mRNA was evident during the expansion of Ag-specific CD8(+) T cells in response to viral or bacterial infection. An XBP-1 splicing reporter revealed that splicing activity was enriched in terminal effector cells expressing high levels of killer cell lectin-like receptor G1 (KLRG1). Overexpression of the spliced form of XBP-1 in CD8(+) T cells enhanced KLRG1 expression during infection, whereas XBP-1(-/-) CD8(+) T cells or cells expressing a dominant-negative form of XBP-1 showed a decreased proportion of KLRG1(high) effector cells. These results suggest that, in the response to pathogen, activation of ER stress sensors and XBP-1 splicing contribute to the differentiation of end-stage effector CD8(+) T cells.

Adjuvants targeting innate and adaptive immunity synergize to enhance tumor immunotherapy

Because of mechanisms of self-tolerance, many tumor-specific CD8 T cells exhibit low avidity for tumor antigens and would benefit from strategies that enhance their numbers and effector function. Here we demonstrate that the combined use of two different types of immune adjuvants, one that directly targets the CD8 cell, IL-2/anti-IL-2 mAb complexes, and one that targets the innate immune system, poly(I:C), can achieve this goal. Provision of IL-2/mAb complexes was found to enhance the activation and effector function of low-avidity tumor-specific T cells, yet this was insufficient to achieve tumor eradication. The addition of poly(I:C) further increased the accumulation of granzyme B-expressing effectors within the tumor and resulted in tumor eradication. This strategy presents many of the benefits of whole-body irradiation, including the provision of high levels of homeostatic cytokines, enhanced expansion of effector cells relative to regulatory T cells, and provision of inflammatory cytokines, and is therefore likely to serve as a strategy for both tumor vaccines and adoptive immunotherapy of cancer.

IL-7 and IL-15 differentially regulate CD8+ T-cell subsets during contraction of the immune response

Although it is known that interleukin-7 (IL-7) and IL-15 influence the survival and turnover of CD8+ T cells, less is known about how these cytokines affect different subsets during the course of the immune response. We find that IL-7 and IL-15 differentially regulate CD8+ T-cell subsets defined by KLRG1 and CD127 expression during the contraction phase of the immune response. The provision of IL-15, or the related cytokine IL-2, during contraction led to the preferential accumulation of KLRG1(hi)CD127(lo) CD8+ T cells, whereas provision of IL-7 instead favored the accumulation of KLRG1(lo)CD127(hi) cells. While IL-7 and IL-15 both induced proliferation of KLRG1(lo) cells, KLRG1(hi) cells exhibited an extraordinarily high level of resistance to cytokine-driven proliferation in vivo despite their dramatic accumulation upon IL-15 administration. These results suggest that IL-15 and IL-2 greatly improve the survival of KLRG1(hi) CD8+ T cells, which are usually destined to perish during contraction, without inducing proliferation. As the availability of IL-15 and IL-2 is enhanced during periods of extended inflammation, our results suggest a mechanism in which a population of cytokine-dependent KLRG1(hi) CD8+ T cells is temporarily retained for improved immunity. Consideration of these findings may aid in the development of immunotherapeutic strategies against infectious disease and cancer.

Interleukin-15/interleukin-15R alpha complexes promote destruction of established tumors by reviving tumor-resident CD8+ T cells

Tumors often escape immune-mediated destruction by suppressing lymphocyte infiltration or effector function. New approaches are needed that overcome this suppression and thereby augment the tumoricidal capacity of tumor-reactive lymphocytes. The cytokine interleukin-15 (IL-15) promotes proliferation and effector capacity of CD8(+) T cells, natural killer (NK) cells, and NKT cells; however, it has a short half-life and high doses are needed to achieve functional responses in vivo. The biological activity of IL-15 can be dramatically increased by complexing this cytokine to its soluble receptor, IL-15R alpha. Here, we report that in vivo delivery of IL-15/IL-15R alpha complexes triggers rapid and significant regression of established solid tumors in two murine models. Despite a marked expansion of IL-2/IL-15R beta(+) cells in lymphoid organs and peripheral blood following treatment with IL-15/IL-15R alpha complexes, the destruction of solid tumors was orchestrated by tumor-resident rather than newly infiltrating CD8(+) T cells. Our data provide novel insights into the use of IL-15/IL-15R alpha complexes to relieve tumor-resident T cells from functional suppression by the tumor microenvironment and have significant implications for cancer immunotherapy and treatment of chronic infections.