The Interferon-Lambda Family Celebrates 20 Years of Scientific Discovery

The interferon (IFN) λ gene family was discovered and first described 20 years ago in a pair of highly related research reports published in Nature Immunology in 2003 (Kotenko et al., 2003; Sheppard et al., 2003). Both research groups independently identified a cluster of 3 distinct human genes (IFNL1, IFNL2, and IFNL3) on chromosome 19q13.2 that encode the corresponding IFN-λ proteins: IFN-λ1, IFN-λ2, and IFN-λ3. One of these groups proposed a different nomenclature (IL29, IL28A, and IL28B) for these novel genes (Sheppard et al., 2003). However, it is now generally agreed that the IFN-λ nomenclature is the most suitable choice and this nomenclature has gained widespread use by the global scientific community.

In 2013, a fourth IFN-λ gene, IFNL4, was discovered and reported in Nature Genetics by Prokunina-Olsson and coworkers based on a follow-up of a genome-wide association study for spontaneous and treatment (IFN-α/ribavirin)-induced clearance of hepatitis C virus (HCV). This study discovered the previously unannotated gene located between IFNL3 and IFNL2; the protein encoded by this gene was annotated as yet another type III IFN and designated as IFN-λ4 (Prokunina-Olsson et al., 2013).

Furthermore, this study showed that the production of IFN-λ4 protein is fully controlled by a dinucleotide genetic polymorphism IFNL4-ΔG/TT (rs368234815), and IFN-λ4 protein can be expressed only in carriers of the common IFNL4-ΔG allele, which represents 10%–90% of the world, depending on the ancestry, whereas the TT allele introduces a frameshift and abrogates expression of the protein (Prokunina-Olsson et al., 2013). Although the IFN-λ4 protein has only limited sequence homology to the other 3 IFN-λ proteins (∼30% amino acid identity to IFN-λ3), it binds and signals through the same membrane receptor complex as IFN-λ1, 2, and 3 (Hamming et al., 2013; Prokunina-Olsson et al., 2013).

The first special issue of the Journal of Interferon & Cytokine Research (JICR) dedicated to type III IFNs was published in September 2019 (O'Brien et al., 2019), after the “Interferon Lambda – Disease Impact and Therapeutic Potential” satellite meeting held on the NIH campus in Bethesda, Maryland, in October 2018.

The current special issue of JICR highlights a group of presentations that were delivered during the recent “Interferon Lambda 2022” satellite meeting held during September 24–25, 2022, at the Hilton Waikoloa Village in Hawaii. This meeting was co-organized by Ludmila Prokunina-Olsson (National Cancer Institute, USA), Ivan Zanoni (Harvard Medical School, USA), Rune Hartmann (Aarhus University, Denmark), Juan Mendoza (University of Chicago, USA), Thomas O'Brien (National Cancer Institute, USA), and Deanna Santer (University of Manitoba, Canada). The purpose of this meeting was to share recent findings and promote additional research on the IFN-λs (type III IFNs) by enhancing interdisciplinary communication and encouraging new scientific collaborations.

The research report by Bharatiya et al. in this issue of JICR describes a novel inactive protein isoform with a restored reading frame that is expressed from the human IFNL4-TT allele at rs368234815, which is not expected to produce IFN-λ4 due to a frameshift in the first exon (Bharatiya et al., 2023). Surprisingly, using a monoclonal antibody that binds to the C-terminus of the IFN-λ4 protein, the authors found that leukocytes obtained from individuals with the TT/TT genotype can also express proteins that are recognized by this antibody. However, these putative proteins did not induce functional responses such as induction of IFN-stimulated gene expression in IFN-λ receptor-positive target cells. The functional role, if any, of these putative proteins remains to be defined.

The research report by Baker et al. examined the potential association of certain IFNL4 genotypes with the development of pediatric Burkitt Lymphoma in a large cohort of children from East Africa (Baker et al., 2023). As noted by the authors, infection with malaria and Epstein–Barr virus are strong risk factors for Burkitt lymphoma, suggesting a potential role of the IFN-λ4-expressing rs368234815-dG allele (IFNL4-dG). This allele is very common (up to 78% of all individuals) in west sub-Saharan Africa, compared with 35% of Europeans and 5% of individuals from east Asia. The authors found that there was no significant association between the risk of Burkitt lymphoma and 3 coding genetic variants within the IFNL4 gene: rs368234815, rs117648444, and rs142981501.

Paradoxically, the ability to express IFN-λ4 is associated with a weaker antiviral response to HCV infection (O'Brien et al., 2014; Prokunina-Olsson et al., 2013). Direct-acting antiviral (DAA) drugs such as sofosbuvir are now commonly used to treat chronic HCV.

A new study report by O'Brien and coworkers in this issue shows that individuals with a favorable IFNL4 genotype (eg, TT/TT, not producing IFN-λ4) can be successfully cured of HCV infection with a shorter treatment duration (8 weeks) of sofosbuvir versus the standard treatment duration of 12 weeks (O'Brien et al., 2023). These findings indicate that IFNL4 genotyping before initiation of treatment with a DAA drug such as sofosbuvir could be used to reduce the duration of drug treatment and the overall cost of treatment per person.

Several recent studies have shown that individuals who develop critical COVID-19 often possess autoantibodies against type I IFNs, particularly IFN-α (Bastard et al., 2021; Bastard et al., 2020). Circulating anti-IFN-α autoantibodies are associated with weaker antiviral responses against SARS-CoV-2 and a greater risk of developing critical or life-threatening disease. However, it is unclear whether individuals who develop critical COVID-19 also express autoantibodies against type III IFNs.

Vanker and colleagues examined the potential involvement of anti-IFN-λ autoantibodies in a large population of patients with COVID-19 (>1,000 individuals), including ∼500 with severe disease (Vanker et al., 2023). The authors found that the presence of autoantibodies against IFN-λ was not associated with the development of severe COVID-19. In addition, the anti-IFN-λ autoantibodies did not neutralize the bioactivity of IFN-λ. These findings indicate that, unlike anti-IFN-α autoantibodies, the presence of anti-IFN-λ autoantibodies does not correlate with the development of severe COVID-19.

The IFN-λ proteins signal through a heterodimeric membrane receptor complex consisting of the ligand-binding chain, IFN-λR1, and the accessory chain, IL-10R2 (Kotenko et al., 2003; Mendoza et al., 2017; Sheppard et al., 2003). The ability to measure cell surface expression levels of IFN-λR1 by flow cytometry has been limited by the lack of availability of anti-IFN-λR1 monoclonal antibodies (mAbs). The article by de Weerd and collaborators in this issue describes the evaluation of several mouse antihuman IFN-λR1 mAbs for their ability to detect and measure IFN-λR1 expression by flow cytometry (de Weerd et al., 2023).

The authors identified and characterized 1 particular mAb, HLR14, that can be used to detect and quantify IFN-λR1 levels on a variety of IFN-λ receptor-positive target cells. Availability of this anti-IFN-λR1 mAb will help to facilitate additional characterization of IFN-λ receptor expression by various cells and tissues.

Signal transduction induced by binding of IFN-λ to its cognate receptor, IFN-λR1, catalyzes heterodimerization of IFN-λR1 with the IL-10R2 chain. This in turn results in activation of the receptor-associated Janus kinases, JAK1 and TYK2. JAK1 is coupled to the intracellular domain (ICD) of the IFN-λR1 chain and TYK2 is linked to the ICD of the IL-10R2 chain. Several recent studies have indicated that TYK2 is not required for induction of productive signaling by IFN-λ (Fuchs et al., 2016; Schnepf et al., 2021). Mesev and coworkers used a set of synthetic chimeric IFN receptor constructs to further examine the requirement for TYK2 in IFN-λ receptor-mediated signaling (Mesev et al., 2023).

They found that absence of TYK2 markedly reduced the magnitude of signaling through wild-type heterodimeric IFN-λ receptor complexes but did not diminish the magnitude of responses induced by signaling through homodimeric IFN-λ receptors. These findings suggest that noncanonical receptor signaling through homodimeric IFN-λR1 complexes might be able to activate IFN-λ-inducible responses in a TYK2-independent manner. It is possible that IFN-λR1 may be able to heterodimerize with other class-2 accessory chains such as IFN-αR1 or IL-20R2 to mediate productive signaling. However, at present, there is no experimental data to support this possibility. It remains to be determined whether homodimeric clustering of IFN-λR1 chains by IFN-λ contributes significantly or not to IFN-λ-mediated functional responses.

Biliary atresia is a medical condition in infants in which the bile ducts inside and outside the liver become scarred and blocked. These changes can lead to liver damage and cirrhosis. This condition can be modeled in BALB/c but not in C57BL/6 mice by experimental infection with rhesus rotavirus (RRV). A report by Hartman and coworkers in this issue shows that deletion of IFN-λR1 in C57BL/6 mice causes a dramatic change in the phenotype of these mice (Hartman et al., 2023).

Unlike wild-type C57BL/6 mice, C57BL/6 IFN-λR1 KO mice develop liver inflammation and biliary obstruction after experimental infection with RRV. These findings indicate that endogenous IFN-λ is a critical inhibitor of RRV replication in vivo and knockout of the IFN-λ receptor-mediated signaling pathway facilitates enhanced RRV infection. This, in turn, results in liver inflammation and biliary obstruction.

Type III IFNs are best known for their antiviral activities; however, there is increasing evidence to support an important role for type III IFNs in host defense against bacterial infections as well. The review article by Johnson and Carbonetti in this special issue summarizes the emerging role of IFN-λ in the innate immune response to bacterial infections (Johnson and Carbonetti, 2023). The authors discuss examples of various bacterial infections in which a role for IFN-λ has been reported.

In some cases, IFN-λ provides host-protective effects; whereas, in others, IFN-λ appears to inhibit antibacterial immune responses. Some reports have shown that induction of IFN-λ production by viral infection can interfere with antibacterial responses in coinfection studies in mouse models. Similarly, induction of IFN-λ expression by bacterial infection can inhibit host responses to coinfection by certain viruses.

In summary, the articles in this special issue of JICR highlight many interesting new findings regarding the biology of type III IFNs and expand our knowledge of the IFN-λ family. There are several Food and Drug Administration (FDA)-approved IFN drug products, including recombinant human IFN-α, -β, and -γ. In contrast, recombinant human IFN-λ has so far not been approved as a treatment for any clinical indication. However, recent clinical studies showed that treatment with pegylated IFN-λ significantly decreases the risk of developing serious or life-threatening COVID-19 (Feld et al., 2021; Reis et al., 2023). These findings support a potential future role for IFN-λ as a prophylactic and/or therapeutic antiviral agent.