Unveiling the Multifaceted Roles of ISG15: From Immunomodulation to Therapeutic Frontiers

Abstract

The Interferon Stimulated Gene 15 (ISG15), a unique Ubiquitin-like (Ubl) modifier exclusive to vertebrates, plays a crucial role in the immune system. Primarily induced by interferon (IFN) type I, ISG15 functions through diverse mechanisms: (i) covalent protein modification (ISGylation); (ii) non-covalent intracellular action; and (iii) exerting extracellular cytokine activity. These various roles highlight its versatility in influencing numerous cellular pathways, encompassing DNA damage response, autophagy, antiviral response, and cancer-related processes, among others. The well-established antiviral effects of ISGylation contrast with its intriguing dual role in cancer, exhibiting both suppressive and promoting effects depending on the tumour type. The multifaceted functions of ISG15 extend beyond intracellular processes to extracellular cytokine signalling, influencing immune response, chemotaxis, and anti-tumour effects. Moreover, ISG15 emerges as a promising adjuvant in vaccine development, enhancing immune responses against viral antigens and demonstrating efficacy in cancer models. As a therapeutic target in cancer treatment, ISG15 exhibits a double-edged nature, promoting or suppressing oncogenesis depending on the tumour context. This review aims to contribute to future studies exploring the role of ISG15 in immune modulation and cancer therapy, potentially paving the way for the development of novel therapeutic interventions, vaccine development, and precision medicine.

Keywords: IFN; ISG15; ISGylation; adjuvant; cytokines; immunity; inflammation; vaccines.

Figure 1

Human ISG15 structure. Left: Ribbon diagram of ISG15 with two Ubl domains linked by a hinge region (PDB entry 1ZM2). The two Ubl domains are conformed by four β-sheets and one α-helix. Right: The same structure rotated 180°. The C-terminal domain is indicated in blue, and the N-terminal domain is depicted in pink. Adapted from Ref. [3].

Figure 2

Induction and functions of ISG15. (A) Different stimuli, such as the presence of type I and III IFNs or genomic damage, induce the expression of ISG15. (B) The immature precursor of ISG15 is processed into its mature form, capable of conjugating with other proteins through a process called ISGylation. (C) ISG15 can be removed from its target by the action of deubiquitinases such as USP18. (D) Unconjugated ISG15 can be released into the extracellular environment acting as immunomodulator. Created with BioRender.com.

Figure 3

ISGylation antiviral activity. Different stimuli, such as microbial infections, IFNs and genomic damage, induce the expression of ISG15. This molecule conjugates with several target proteins (viral and host origin) through a reversible process called ISGylation. An antiviral effect mediated by ISGylation has been reported. ISGylation inhibits RSV and SINV replication in a yet-to-be-clarified manner, and HPV and HIV-1 replication through the respective viral Major capsid protein L1 (L1) and the cellular STING proteins. Additionally, ISGylation can block IAV packaging acting on the viral protein NS1 and viral budding of EBOV VLP modifying Nedd4. Created with BioRender.com.

Figure 4

Schematic representation of ISG15 extracellular signalling functions. (A) Extracellular ISG15 induces NK cell proliferation. (B) Extracellular ISG15 binds to LFA-1 receptor in leukocytes triggering the production of type II IFN and pro-inflammatory cytokines. (C) Type II IFN activates JAK/STAT pathway, inducing STAT1 phosphorylation, which binds to GAS, and production of ISGs. (D) Extracellular ISG15 acts as a chemotactic factor for neutrophils. (E) ISG15 in its extracellular form leads to elevated e-cadherin expression in dendritic cells. GAS: Gamma Activated Sequence; IFN: Interferon; IFNGR: Interferon Gamma Receptor; ISG: Interferon Stimulated Gene; JAK: Janus Kinase; LFA-1: Lymphocyte Function-Associated Antigen 1; STAT: Signal Transducers and Activators of Transcription; NK: Natural killer. Created with BioRender.com.

Figure 5

Immunization regimens in mice using ISG15 as an adjuvant in combination with HIV-1 vaccine candidates. (A) Mice were primed with 50 µg of DNA-ISG15GG (able to perform ISGylation) or DNA-ISG15AA (unable to perform ISGylation) + 50 µg of DNA-gp120, followed by a boost with 1 × 10⁷ PFU/mouse of MVA-B at 28 days post-prime. The administration of DNA-ISG15GG in the prime increased the potency and quality of the HIV-1-specific CD8 T cells compared to DNA-ISG15AA [83]. (B) Mice were primed with 50 µg of DNA-gp120, followed by a boost with 1 × 10⁷ PFU/mouse of MVA-B + MVA-Δ3-ISG15GG or MVA-Δ3-ISG15AA at 28 days post-prime. The immunization with MVA-Δ3-ISG15AA induced higher magnitude and quality of the HIV-1-specific CD8 T cells along with an increased type I IFN response compared to MVA-Δ3-ISG15GG [84]. IFN: Interferon; MVA: Modified Vaccinia virus Ankara; PFU: Plaque Forming Unit. Created with BioRender.com.

Figure 6

ISG15 in tumorigenesis and cancer therapy. The expression of ISG15 is increased in breast, renal, and colorectal cancers [90,91,92]). Immunogenicity studies using the Lm-LLO-ISG15 vaccine candidate in the mouse model against these types of cancer and its effect in the generated immune response are indicated. Created with BioRender.com.