Harnessing Mechanistic Knowledge on Beneficial Versus Deleterious IFN-I Effects to Design Innovative Immunotherapies Targeting Cytokine Activity to Specific Cell Types

Abstract

Type I interferons (IFN-I) were identified over 50 years ago as cytokines critical for host defense against viral infections. IFN-I promote anti-viral defense through two main mechanisms. First, IFN-I directly reinforce or induce de novo in potentially all cells the expression of effector molecules of intrinsic anti-viral immunity. Second, IFN-I orchestrate innate and adaptive anti-viral immunity. However, IFN-I responses can be deleterious for the host in a number of circumstances, including secondary bacterial or fungal infections, several autoimmune diseases, and, paradoxically, certain chronic viral infections. We will review the proposed nature of protective versus deleterious IFN-I responses in selected diseases. Emphasis will be put on the potentially deleterious functions of IFN-I in human immunodeficiency virus type 1 (HIV-1) infection, and on the respective roles of IFN-I and IFN-III in promoting resolution of hepatitis C virus (HCV) infection. We will then discuss how the balance between beneficial versus deleterious IFN-I responses is modulated by several key parameters including (i) the subtypes and dose of IFN-I produced, (ii) the cell types affected by IFN-I, and (iii) the source and timing of IFN-I production. Finally, we will speculate how integration of this knowledge combined with advanced biochemical manipulation of the activity of the cytokines should allow designing innovative immunotherapeutic treatments in patients. Specifically, we will discuss how induction or blockade of specific IFN-I responses in targeted cell types could promote the beneficial functions of IFN-I and/or dampen their deleterious effects, in a manner adapted to each disease.

Keywords: bioengineering; chronic viral infections; dendritic cells; immunotherapy; type I interferons.

Figure 1

A simplified model of the potential contributions of selective sensors and cell types to IFN production during viral infections. Different innate immune recognition receptors are involved in sensing various types of viral nucleic acids in distinct categories of cells during viral infections, which may promote different types of anti-viral defenses. For each selected sensor shown, the types of viral nucleic acids recognized and the downstream signaling cascade induced are represented in a simplified, schematic manner. The potential specific role of each cell type in anti-viral defenses is also indicated at the bottom of each panel. (A) Potentially all types of infected cells can detect endogenous viral replication through cytosolic sensors triggering their local production of IFN-β/λ to control viral replication in an autocrine and paracrine fashion in infected tissues. (B) Uninfected XCR1⁺ DCs selectively produce high levels of IFN-λ and IFN-β upon engulfment of materials containing dsRNA and the consecutive triggering of TLR3 in endosomes. The receptor of IFN-λ is mostly expressed by epithelial cells. Hence, XCR1⁺ DCs might be involved in inducing local IFN responses in virally infected epithelial tissues. Since XCR1⁺ DCs are especially efficient at producing IFN-III upon HCV stimulation, they might contribute to local or systemic IFN production during infection with this virus, to promote IFN-λ-mediated protection of hepatocytes. Uninfected XCR1⁺ DCs and other uninfected cells may produce some IFN-β upon engulfment of materials containing ssRNA and the consecutive triggering of TLR8 in endosomes. The contribution of this pathway to anti-viral defense is not well understood yet, in part because mouse TLR8 is deficient for this function. (C) Uninfected pDCs selectively produce high levels of all subsets of IFNs upon engulfment of materials containing ssRNA or CpG DNA and the consecutive triggering of TLR7/9 in endosomes. However, pDCs seem to be activated for this function only in lymphoid tissues. Hence, pDC might contribute to systemic IFN production during blood-borne viral infections or as a failsafe mechanism activated upon abnormal widespread dissemination of a viral infection once it has escaped local confinement at its portal of entry. CM, cell membrane; NM, nuclear membrane.

Figure 2

A simplified and schematic view of the modes of action and specificities of ISG acting as direct anti-viral restriction factors. (A) Different IFN-inducible restriction factors can block viral replication in infected cells in a cell-intrinsic manner at different stages of the viral life cycle. (B) Viral specificity (purple) might be inversely correlated to the breadth of side effects on host cells (orange).

Figure 3

DCs play a central role in IFN-I orchestration of innate and adaptive immune responses. (A) IFN-I exert cell-intrinsic as well as indirect effects on a variety of immune cell populations. DC responses to IFN-I play a major role in promoting protective activation and functional polarization of other innate and adaptive immune cells, not only during viral infections but also in other physiopathological situations including cancer. (B) DC cell-intrinsic responses to IFN-I endow them to deliver appropriate signals for T cell priming and functional polarization. IFN-I can modulate all three types of signals delivered by DC to T cells: MHC-I/antigenic peptide complexes (), co-stimulation (), and cytokines (). This depends both on IFN-I-dependent transcriptional induction in DC of some of the corresponding genes and on IFN-I-dependent metabolic reprogramming of DC. (C) DC cell-intrinsic responses to IFN-I endow them to deliver appropriate signals, in particular IL-15 trans-presentation, for NK cell activation. See main text for further details.

Figure 4

A simplified model of the deleterious role of IFN-I in several autoimmune diseases. When exposed to different kinds of injuries (microbial infection, commensal microbiota dysbiosis, chemical or physical insults), healthy tissues can undergo cell damage and death. These events induce the release of apoptotic bodies encompassing self RNA or DNA. Neutrophil recruitment and activation in inflamed tissues can also constitute a potent source of self nucleic acids, through the release of neutrophil extracellular traps (NET). Self RNA or DNA can associate with cationic peptides (e.g., LL37) as shown in psoriatic patients or with inflammatory molecules (e.g., high mobility group box 1, HMGB1) to generate nanoparticles that are extremely efficient for IFN-I production by pDC and eventually other cell types. pDC can also be efficiently activated for IFN-I production by immune complexes (ICs) generated by the association between self nucleic acids and auto-antibodies as frequently found in the serum of systemic lupus erythematosus patients. IFN-I promote the differentiation and/or the maturation of antigen-presenting cells, in particular different subsets of DC. Activated DC can then present self-antigens for activation of auto-reactive T CD4⁺ cells, including follicular helper lymphocytes, which in turn activate auto-reactive B cells for auto-antibody secretion, leading to a vicious circle of reciprocal activation between innate and auto-reactive adaptive immune cells. iDC, immature DC; mDC, mature DC; Mo-DC, monocyte-derived DC. See main text for further details.

Figure 5

A simplified model of the deleterious role of IFN-I in secondary pulmonary bacterial infections or in fungal infections and of their protective role in multiple sclerosis (MS). IFN-I-dependent signals can block CXCL1 and CXCL2 production, thus inhibiting the recruitment and activation of neutrophils in inflamed tissues, hampering their protective functions against secondary pulmonary bacterial infections or fungal infections. IFN-I-dependent signals can enhance CCL2 production, promoting the recruitment in inflamed tissues of CCR2⁺ monocytes that can potentially differentiate into TipDC or into MDSC. This can contribute to disease either through enforcing T cell activation leading to immunopathology or on the contrary through suppressing anti-microbial immune defenses. IFN-I can either inhibit or promote Th1 responses, which in the latter case occurs at the expense of Th17 responses thus compromising the production of IL-17 and IL-22, which are respectively required for control of microbial replication and for tissue healing. IFN-I-induced IL-10 and IL-27 can directly inhibit Th17. In MS, inhibition of Th17 functions may contribute to the protective effects of IFN-β therapy. MPO, myeloperoxidase; TNF, tumor necrosis factor alpha; NO, nitric oxide. See main text for other abbreviations and further details.

Figure 6

Potential mechanisms through which chronic, low level IFN-I production might promote disease progression in HIV-1 infection. High and sustained expression of ISG in blood and lymphoid organs is a hallmark of progressive infection with immunodeficiency viruses both in human beings and in non-human primates, irrespective of the levels of viral replication. Several mechanisms summarized here have been proposed to explain how chronic, low level IFN-I production might promote disease progression in HIV-1 infection. These mechanisms include direct () and indirect () promotion of the exhaustion of anti-viral CD8 T cell responses, as well as direct () and indirect (-to-, and ) promotion of CD4 T cell depletion with a proposed central role of pDC in this deleterious process. Altogether, these mechanisms may sustain a vicious circle of reciprocal activation between chronic viral replication and deleterious immune responses, driving the progressive depletion of all CD4 T cells ultimately causing the enhanced susceptibility to opportunistic infections characteristic of the acquired immunodeficiency syndrome (AIDS). See main text for further details.

Figure 7

A novel hypothetical model attempting to explain the respective roles of IFN-I and IFN-III in HCV infection. (A) Classification of patients suffering from chronic HCV infection and treated with PEG-IFN-α in non-responders and responders, and identification of IFNL3 (IL28B) gene polymorphisms as the best predictors for treatment response. The side-effects induced by PEG-IFN-α treatment are also listed since they can severely affect the patient’s quality of life and lead to treatment failure due lack of compliance or suicide. Hence, new approaches are needed to promote beneficial over deleterious effects of IFN-I administration in chronic HCV infection. (B) Proposal of a new hypothesis explaining the relationships between endogenous ISG levels in patients prior to treatment, IFNL3 gene polymorphism, endogenous expression of IFN-I, IFN-λ, and IFNλR1, and responsiveness to IFN-I administration. Efficient control of HCV infection may depend on hepatocyte response to IFN-λ rather than IFN-α. Upon HCV infection, the virus induces the expression of host miRNA able to bind the 3′ UTR of IFNL3 mRNA to promote their degradation. The favorable IFNL3 allele associated with responsiveness to PEG-IFN-α treatment may allow endogenous expression of sufficient levels of IFNL3 for efficient induction of cell-intrinsic anti-viral defenses in hepatocytes. This process is, however, hampered by the limited expression of the receptor for this cytokine (IFNλR1) in these patients. PEG-IFN-α treatment might promote resolution of the infection by inducing IFNλR1 in these patients, potentiating their response to their endogenous production of IFNL3. In the patients that do not respond to PEG-IFN-α treatment, endogenous levels of IFNL3 are insufficient for efficient induction of cell-intrinsic anti-viral defenses in hepatocytes, due to the degradation of the corresponding mRNA in infected hepatocytes. In these patient’s hepatocytes, however, IFNλR1 is already expressed to high levels prior to treatment due to their high endogenous IFN-I responses. Administration of exogenous IFN-λ might cure these patients. See main text for further details.

Figure 8

Schematic representation of the ISGF3 and alternative JAK/STAT signaling pathways induced by IFN-I. The receptor for IFN-I, IFNAR, is composed of two chains, IFNAR1 and IFNAR2, which are respectively associated with the JAK family kinases TYK2 and JAK1. IFN-I binding to IFNAR triggers the phosphorylation of TYK2 and JAK1, which in turn phosphorylate a variety of STAT proteins. Activated STATs are able to form complexes, as homo- or hetero-dimers. The heterodimer STAT1-STAT2 binds to a third partner, IFN-regulatory factor 9 (IRF9), in order to form the ISGF3 complex. This complex translocates into the nucleus and binds to specific regulatory sequences, IFN-stimulated response elements (ISRE), to activate the expression of many interferon-stimulated genes (ISGs). In particular, ISGF3 induces most, if not all, of the ISGs encoding effector molecules of cell-intrinsic anti-viral defenses such as OAS or MX1. Alternative JAK/STAT pathways include the formation of STAT1 or STAT4 homodimers, which may drive different functional responses to IFN-I. STAT1 homodimers bind to IFNγ-activated sequences (GAS) in the promoter of certain ISGs, which may promote inflammatory, anti-proliferative, and pro-apoptotic responses. STAT4 homodimers also bind to GAS but promote IFN-γ production and pro-proliferative responses.

Figure 9

Schematic illustration of different mechanisms controlling the diversity of IFN-I effects. Different parameters that contribute to promote and control the diversity of IFN-I responses are depicted on the left side of the figure. References of papers illustrating each mechanism are given on the right side of the figure. (A) Avidity (a combination of affinity and dose). For example, the affinity of IFN-β for IFNAR1 is 100-times higher than that of IFN-α2, and IFNβ is much more potent in inhibiting cellular proliferation or monocyte differentiation into osteoclasts (response A), while both IFN-I subtypes are equipotent in establishing an anti-viral state (response B). The same subset of IFN-I can also exert different biological effects at low versus high doses. For example, low, but not high, doses of IFN-β protect BALB/c mice from progressive cutaneous and fatal visceral disease after Leishmania major infection. (B) Cell type specificity. Mouse DC but not NK cells are strong responders to IFN-I, and cell-intrinsic responses to IFN-I are critical in DC but not in some other cell types for immune defenses against viral infections or tumors. (C) Tissue microenvironment. The response of a given cell type to a given dose of a specific subset of IFN-I can also be modulated by the microenvironment of the cell. For example, in cancer, protective IFN-I effects on infiltrating DC or other immune cells might be dampened by inhibitors of IFNAR signaling locally produced by the tumor, such as ligands of the TAM receptor tyrosine kinases. (D) Timing. Differences in the time and duration of exposure to IFN-I can also determine distinct functional outcomes. For example, during viral infections, early and transient high levels of IFN-I promote protective DC and T cell responses, while delayed, chronic and low level IFN-I production compromises host immune defenses and promotes chronic viral infections. Within a given cell type, the outcome of IFN-I stimulation also depends on time of exposure to these cytokines relative to other modulatory signals (timing relative to other stimuli). For example, in naïve CD8 T cells, TCR signaling prior to IFN-I stimulation leads to increased expression of STAT4 and promotes IFN-γ production and proliferation, while IFN-I stimulation prior to TCR triggering leads to STAT1-dependent anti-proliferative and pro-apoptotic effects.

Figure 10

Strategy for high efficiency cell type-specific targeting of cytokine activity. (A) Most cytokines have evolved to exhibit optimized specific activities. (B) When the antibody moiety of a classical immunocytokine binds its cellular target, the ensuing increase of cytokine-receptor avidity translates into a modest increase of cytokine potency. (C,D) By introducing a mutation that decreases the affinity of the cytokine for its receptor, the activity of the mutated immunocytokine is now focused with a very high efficiency on target cells.