Anti-CD3 sFv/IL-18 fusion DNA for allergy therapy

Since the late 1970s, the prevalence of allergic disease has increased substantially in industrialized countries where it has been linked to: increased hygiene and cleanliness;¹ increased diesel pollution;² increased use of carpets in which house dust mites grow;³ and decreased early childhood infections.⁴ During the early phases of allergic sensitisation, CD4 T helper cells become polarized to a T helper 2 (Th2) phenotype that is characterized by the cytokines interleukin (IL)-4, IL-5, IL-6, IL-9, IL-1 and IL-13.^5–8 These Th2 cells provide help to B cells for class switching to immunoglobulin E (IgE)^9,10 and contribute to the lung inflammation that causes asthma.¹¹ Treatment of Th2-mediated immune diseases, to date, has focused on anti-inflammatory drugs and allergen immunotherapy with incremental doses of allergen. The latter is not completely understood but is associated with the induction of immunoglobulin G4 (IgG4) antibodies, increased T helper 1 (Th1) cytokines,^12,13 as well as IL-10-producing regulatory T cells.¹⁴ In this issue of Immunology, Kim and colleagues¹⁵ show that Th2 immune responses can be redirected to Th1 by using anti-CD3 single-chain Fv/IL-18 fusion DNA.¹⁵

IL-18 is produced by antigen-presenting cells (APC) and was initially described as an interferon-γ (IFN-γ)-inducing factor that circulated during endotoxaemia in mice infected with Propionibacterium acnes.¹⁶ IL-18 is synthesized as a 24 000-molecular weight (MW) precursor, which is enzymically cleaved to the 18 000-MW mature IL-18 protein^16–18 by the action of IL-1β-converting enzyme (ICE, also called caspase-1).¹⁷

IL-18 synergizes with IL-12 to induce IFN-γ synthesis in T and natural killer (NK) cells; each cytokine alone is much less effective. This synergy is mediated by the induction of IL-18Rα on naïve T cells by IL-12 and by up-regulation of IL-12Rβ2 by IL-18.^19,20 IL-18 is produced by APCs. Upon encounter with an antigen, dendritic cells (DCs) migrate to peripheral lymphoid organs where they present antigen to naïve Th precursors, resulting in the production of IL-12 and IL-18 by the DC, which skews T-cell maturation to a Th1 phenotype.^21–23 IL-12 production by DCs is suppressed by IL-10 and prostaglandin E2, which then skew T-cell maturation towards Th2.²⁴

Evidence that IL-18 controls Th1 cell development and therefore regulates Th2 and IgE responses has been advanced in a significant number of studies.^16,25–30 Like IL-12, IL-18 on its own only induces the production of small amounts of IFN-γ from anti-CD3-stimulated Th1 cells. However, combined with IL-12, IL-18 induces large amounts of IFN-γ.^16,25–30In vivo and in vitro, IL-18, combined with IL-12, inhibits IgE and thus Th2 immune responses by inducing IFN-γ in B cells.²⁰ Yoshimoto and colleagues have also shown that mast cell and basophil progenitors cultured with IL-3 and IL-18 produce increased amounts of IL-4 and histamine.³¹ Furthermore, serum IgE and CD4 T-cell IL-4 and IL-13 were all increased in mice given daily injections of high-dose recombinant IL-18.³² Administration of IL-18 also increased CD40L expression.³³ Thus, IL-18 plus IL-12 promotes Th1 responses, but IL-18 without IL-12 favours Th2 responses.³⁴ Recent data suggest that the source of increased IL-4 in IL-18-treated mice is the NK T cell.³⁵

Kim and colleagues have shown that Th1 immune responses can be stimulated by using anti-CD3 single-chain Fv/IL-18 fusion DNA.¹⁵ By linking IL-18 cDNA with the anti-CD3sFv gene, they effectively targeted IL-18 to the T cells to induce IFN-γ synthesis. We assume that the fusion protein homes in on CD3 but can still bind to the IL-18 receptor; however, this has yet to be formally proven. The authors have previously used a similar approach with IL-2 to prevent dexamethasone-induced T-cell apoptosis.³⁶ In a study described in the current issue of Immunology, Kim and colleagues showed that intramuscular injection of ovalbumin (OVA)-immunized mice with an anti-CD3sFv/IL-18 DNA plasmid construct induced an OVA-specific Th1 response. It is not clear whether they also induced allogeneic Th1 cells, as the anti-CD3 could potentially stimulate all T cells. The Th1 immune response induced by plasmid anti-CD3sFv/IL-18 DNA was characterized by high titres of OVA-specific IgG2a and substantially elevated OVA-specific CD4 T-cell IFN-γ. Of particular interest is the fact that these immune responses persisted over the entire 8-week observation period, whereas mice injected with the plasmid anti-CD3sFv DNA alone were unaffected. Furthermore, a mixture of plasmid anti-CD3sFv DNA and plasmid IL-18 DNA was unable to induce a Th1 response – indicating the requirement for the cytokine to be targeted to the T cell. As well as inducing a vigorous Th1 response, the plasmid anti-CD3sFv/IL-18 DNA also inhibited OVA-specific IL-4 production by CD4 T cells.¹⁵ It would be of particular interest to determine whether this plasmid is capable of suppressing ongoing Th2-dominated responses, as previously shown by Maecker et al., where vaccination with an allergen–IL-18 fusion DNA conferred protection while reversing established airway hyper-reactivity in a murine asthma model.³⁷ In these experiments, the effect of IL-18 was dependent on IFN-γ and CD8 T cells,³⁷ and the authors suggest that allergen linked to IL-18 may be more readily presented to CD8 T cells via the major histocompatibility complex I (MHC I).³⁷ Thus, antigen–cytokine fusions may be targeted to APCs rather than T cells.

The study by Kim and colleagues¹⁵ provides compelling evidence for a novel therapeutic approach to the treatment of allergic disease using plasmid anti-CD3sFv/IL-18 DNA that targets IL-18 to T cells. In addition, such constructs could be used to boost protective Th1 responses and enhance the efficacy of vaccines. The mechanism needs further clarification, and the cell population(s) responsible for suppressing OVA-specific IL-4 and enhancing OVA-specific IFN-γ have yet to be identified. By targeting cytokines to specific compartments of the immune system, the therapeutic potential of cytokines is greatly enhanced and raises the possibility of new and exciting treatments for many diseases.