The role of IFN gamma in inflammation, cancer and autoimmune disease

Interferons (IFN) are a family of cytokines which, upon secretion, play a central role in mediating the innate and adaptive immune response to viral and bacterial challenges. Isaacs and Lindenmann first discovered a molecule they called interferon in the 1950s, which is now referred to as IFN gamma (Isaacs and Lindenmann, 1957). Following several more studies there are now two IFN family members identified; Type I IFN, which include IFNa and IFNb, and Type II IFN which is comprised of IFN gamma (van de Broek et al, 1995), both of which have distinct physiological roles, bind to different receptors, and are structural diversity; however, both Type I and Type II IFNs activate the anti-viral response. Moreover, Type I IFNs are secreted following the activation of intracellular and extracellular anti-viral sensors, mainly by macrophages and dendritic cells (Sadler and Williams, 2008); whereas Type II IFN is primarily secreted by T lymphocytes and natural killer (NK) cells in response to cytokine activation, specifically IL-12 or IL-18 (Schroder et al, 2004), with stimulation from both IFN subtypes leading to downstream cytosolic signalling and subsequent upregulation of gene expression.

IFN gamma binds to the extracellular domain of the IFN gamma R1, leading to the engagement of the IFN gamma R2 which induces signalling intracellularly. The tyrosine kinases Janus Kinase 1 (JAK1) and JAK2 are phosphorylated at the membrane upon association with activated IFN gamma R2, facilitating the binding of signal transducer and activator of transcription 1 (STAT1). Phosphorylation of STAT1 precedes its nuclear translocation and binding to the gamma-activated sequence (GAS), which are short DNA elements in the nucleus which bridge STAT binding and the activation of the transcription factor interferon response factor I (IFN1) (Varinou et al, 2003; Decker et al, 1997; Coccia et al, 1995). A large range of genes have been described to be transcribed in an IFN gamma-dependent manner, including many involved in haematopoiesis, inflammation, cell proliferation, cell differentiation and programmed cell death (Boehm et al, 1997). The IFN gamma pathway is negatively regulated by suppressor of cytokine singalling molecule 1 (SOCS1) (Alexander et al, 1999), which inhibit JAK2 and STAT1 interactions, and also by the dephosphorylation of STAT1 by the phosphatase TCP45 (Kramer et al, 2009). Notable, an alternative, or non-canonical, IFN gamma signaling pathway has been described, implicating n interaction between JAK and MyD88 adaptor-like molecule (Mal), commonly associated with TLR signaling (Ní Cheallaigh et al, 2016).

Genetic deficiency in the IL-12/IL-23/IFN gamma pathway leads to a robust susceptibility to mycobacterial infections (Filipe-Santos et al, 2006). The primary sources of IFN gamma are NK, NKT cells, macrophages and dendritic cells, which mediate the innate immune response, and both CD4+ and CD8+ T cells, which facilitate adaptive immunity. Importantly, both Type I and Type II IFNs were demonstrated to be central mediators of vaccine-induced responses, specifically CD4+ T cell (Th1) responses (Weir et al, 2008; Tudor et al, 2001). In addition to their immunomodulatory role, IFN gamma has a specific role in the maturation of naïve T cells into Th1 effector T cells (Schoenborn and Wilson, 2007).

Although IFN gamma can mediate clearance of pathogenic insults, chronic exposure to IFN gamma has been identified to be involved in several non-infectious pathologies, such as autoimmune diseases, for example rheumatoid arthritis (Nielan et al, 2004), and systemic lupus erythematous (Lu et al, 2016). In the case of autoimmune disease, IFN gamma effectively ‘primes’ and sensitizes cells to secondary ligands, such as TLR agonists and TNFa (Borges da Silva et al, 2015). Interestingly, crosstalk between NF-KB and IRF pathways has been reported (Iwanazsko and Kimmel, 2015), with the discovery of IFN gamma mediated IRF1 transcription enhancing the activation of NF-kappaB in response to human immunodeficiency virus (HIV) (Sgarbanti et al, 2008).

IFN gamma in immunosurveillance and cancer

The term immunosurveillance describes the translocation of several immune cells, including T cells, NK cells, NKT cells, and macrophages into the tumour in an effort to attack the foreign cancer cells, eliciting a robust secretion of cytotoxic factors, such as IFN gamma, TNFα, FasL and TNF-related apoptosis inducing ligand (TRAIL) (Dunn et al, 2004). Although tumour immunology is a complex and constantly expanding field, current research demonstrates that cells have developed a way to evade immunosurveillance by secreting immunosuppressive molecules and reducing the cytotoxic T lymphocyte response. One mechanism cancer cells employ in order to evade the immune response is to downregulate IFN gamma-expressing cells and to limit IFN gamma secretion. IFN gamma negatively regulates tumourigenesis by suppressing tumour development by negatively regulating cell proliferation (Kominsky et al, 1998), or promoting apoptosis (Kim et al, 2002). Moreover, IFN gamma-induced IRF1 activation has been shown to synergise with NF-kappaB to upregulate the MHC-I expression on cytotoxic T cells in neuroblastoma, eliciting a protective response (Lorenzi et al, 2012). However, conversely IFN gamma has also been shown to induce tumour survival in several studies. IFN gamma is demonstrated to promote proliferation in a human melanoma cell line in vitro (Garbe et al 1990), and IFN gamma-producing macrophages have been identified in melanoma tumours (Zaidi et al, 2011).In addition to the cell-specific polarized effects of IFN gamma as a therapeutic, the potentially fatal side effect of immunochterapy termed Cytokine Release Syndrome (CRS) has been described, with IFN gamma and IL-6 rescuing mice from anti-CD3 induced CRS (Matthys et al, 1993). However, in clinical trials using anti-CD19 Chimeric Antigen Receptor (CAR) T cells to treat Acute Lymphoblastic Laukaemia, elevated levels of IFN gamma are indicative of later development of fatal CRS (Teachey et al, 2016).

IFN gamma as a therapeutic

Overall, although there has been some success for IFNa in cancer treatment (Parker et al, 2016; Eto et al, 2015), IFN gamma has been shown to facilitate immunomodulation, and promote both pro-tumourigenic and anti-cancer properties, mainly though cell proliferation and apoptotic pathways. These opposing responses may be dose-dependent based on the intensity of the signal, and cancer type specific, dependent on a range of interacting factors in the tumour microenvironment. However, the therapeutic potential of IFN gamma may prove a worthy success in a subset of cancers.

Figure 1: IFN gamma signalling through the JAK/STAT pathway. Extracellular secreted IFN gamma binds directly to the extracellular domain of the membrane-bound IFN gammaR1, which in turn transduces signalling to the IFN gammaR2 subunit. The carboxy termini of both IFN gammaR1 and IFN gammaR2 associate with JAK1 and JAK2 which are in turn phosphorylated. Janus kinase 1 (JAK1) and JAK2 subsequently phosphorylate STAT1 which induces its nuclear translocation, association with gamma-activated sequence (GAS) which mediate IRF1 activation and transcription of multiple genes governing cell proliferation, differentiation, heamatopoiesis and inflammation. Suppressor of Cytokines 1 (SOCS1) negatively regulates this pathway by inhibiting JAK/STAT1 interactions.