Compartmentalisation by epithelial barriers is critical in the intestine, as they not only protect the host from potential pathogens but also separate the underlying tissue from foreign material ingested by the host. IFNλs protect intestinal epithelial barrier integrity (16, 74, 75). In a mouse model of colitis induced by dextran sulfate sodium (DSS), IFNλs were shown to control the proliferation of intestinal epithelial cells and accelerate intestinal mucosal healing, which reduced epithelial cell damage (15, 22, 76).

The protective role of IFNλ in intestinal epithelia extends to the context of bacterial infection. Treatment with IFNλ1 was shown to increase trans-epithelial electrical resistance (TEER) in an in vitro model of barrier integrity using polarized T84 colonic epithelial cells (16). Invasive enteric pathogens such as S. flexneri and Salmonella enterica serovar Typhimurium disrupt epithelial barriers to aid bacterial dissemination. While TEER dropped upon infection with these bacteria in control conditions, it was maintained following IFNλ1 pre-treatment. In addition, IFNλ treatment prevented S. flexneri and S. Typhimurium transmigration, showing that IFNλ protects intestinal epithelial barriers from infection and damage mediated by invasive bacteria in vitro (16) (Figure 1). The molecular mechanism of action of IFNλs-mediated protection of barriers has not been uncovered, but intracellular tight junction proteins like claudin-1 were shown to be upregulated by IFNλ2 (75). Whether these observations in cell culture extend to in vivo models of bacterial infection remains to be determined.

While type III IFNs appear to protect against enteric bacterial infections, their roles in the airway epithelium are unclear. At steady state, IFNλs modulate immune cell responses to decrease inflammation (24, 25, 77, 78). However, during infection, the activities of type III IFNs on lung epithelial cells were also shown to increase inflammation and compromise barrier function (79–82) (Figure 1). During infection with Klebsiella pneumoniae, IFNλ treatment lowered epithelial barrier integrity in vitro, facilitating neutrophil transmigration and bacterial translocation. Moreover, Ifnlr1^-/- mice were protected from Klebsiella-induced pneumonia (79). Similarly, IFNλ exacerbated lung inflammatory pathology in a mouse model of Bordetella pertussis infection (80). In this study, antibody neutralization of IFNλ, as well as deletion of IFNLR, led to reduced lung inflammatory pathology (80), congruent with a detrimental role of type III IFN in bacteria-infected lungs.

Compromised barrier functions and increased inflammation during viral infection in the lungs can result in complications such as bacterial superinfection (81–84). Infection with Influenza A virus (IAV) or intratracheal instillation of poly (I:C), a synthetic ligand that mimics viral dsRNA, were shown to up-regulate IFNλ2 and IFNλ3. This led to compromised barrier function and promoted superinfection by Staphylococcus aureus (81) and S. pneumoniae (82). Greater barrier damage and higher bacterial burdens were found in mice treated with poly (I:C), a phenotype reversed in mice lacking IFNLR. Moreover, the co-administration of IFNλ and R848 (a TLR7 agonist that mimics viral ssRNA), but not IFNλ alone, increased sensitivity to S. aureus infection. This suggests that IFNλ promotes superinfections in inflamed lungs by driving inflammation and compromising barrier function (81) (Figure 1).

IFNλ was also shown to impair neutrophil recruitment and phagocytosis during recovery from IAV infection (85). This enhanced superinfections with methicillin-resistant S. aureus (MRSA) or S. pneumoniae (82, 83). Consequently, mice lacking IFNLR that were infected with IAV were less susceptible to superinfections (82, 83).

Although type III IFNs can be protective in the intestinal epithelium and healthy lung, it is evident that they can have a detrimental role in inflamed airways. These findings emphasize that biological context is crucial, and further research is required to understand the nuanced effects of IFNs at epithelial barriers.

At steady-state, ISGs are detected in mouse and human intestinal epithelial cells (87–90). The expression of ISGs is dependent on PRR-mediated sensing of the microbiota, particularly in the ileum, and expression of specific ISGs was found to be ablated following antibiotic treatment to eliminate the microbiota (87, 90). Interestingly, homeostatic ISG expression was shown to control enteric viral infections (89), which benefits the host (Figure 1).

Although the pulmonary microbiota is poorly understood (91–93), a homeostatic ISG response has also been found in mouse lungs (89, 94). Emerging evidence demonstrates that IFNλ is also important for regulating the microbiota and influencing infection at this mucosal barrier in humans (Figure 1).

One study investigated the microbiota composition using random sampling of bronchoalveolar lavage fluid (BALF) from lung transplant recipients. Post-transplant lung microbiota were categorised into four ‘pneumotypes’. In balanced pneumotype, BALF samples analysis of differential expression of host genes, revealed that Ifnlr1 was upregulated, relative to the other groups. This balanced pneumotype was associated with the lowest risk of infection and allogenic responses, resulting in a lower clinical risk (95). Moreover, in a study assessing the contribution of nasal commensal bacteria in antiviral defense against IAV infection in the upper respiratory tract, the commensal bacterium Staphylococcus epidermidis was shown to reduce host susceptibility to IAV infection by inducing the expression of IFNλ (96).

Neutrophils are phagocytes that can engulf foreign materials and pathogens. They also potently produce microbicidal reactive oxygen species (ROS) and promote inflammation via degranulation. However, excessive or prolonged neutrophil activation is a hallmark of many inflammatory pathologies. IFNLR is expressed at higher levels in human and mouse neutrophils than other immune or epithelial cells (22, 25, 78, 97), and its expression in neutrophils is further upregulated by bacterial ligands such as LPS (22). While IFNβ elicits pro-inflammatory activities in neutrophils (78), IFNλs cultivate an anti-inflammatory state by inhibiting neutrophil recruitment (22, 25, 85). Crucially, IFNλ treatment of bone marrow derived neutrophils diminished ROS production and degranulation but did not affect protective cytokine production or phagocytosis. This was protective in the intestine in a mouse model of DSS-induced colitis (22).

Neutrophils are efficient phagocytes and are essential to the resolution of infection. Despite their crucial roles, the functions of IFNλ in neutrophils in the context of bacterial infections have seldom been addressed. In models of bacterial superinfection in murine lungs, IFNλ was shown to reduce the phagocytic abilities of neutrophils which impaired bacterial clearance, exposing the host to infection (85). However, during infection with Pseudomonas aeruginosa, an IFNλ2-mediated decrease in neutrophil recruitment was protective and resulted in less epithelial damage (98) (Figure 1). As such, the balance between inhibiting neutrophil-mediated inflammatory damage and promoting bacterial clearance should be carefully considered when evaluating the therapeutic potential of IFNλ.

Macrophages are phagocytes that engulf and destroy bacteria. They can also stimulate the adaptive immune system via the presentation of foreign antigens to T cells. Exposure to IFNλ increases their phagocytic activity and the production of proinflammatory cytokines and chemokines (23). This promotes bacterial clearance during infection; IFNλ1 was shown to enhance S. aureus uptake in macrophages, and increase bacterial killing (99) (Figure 1).

Interestingly, when monocytes are incubated with Granulocyte Macrophage Colony-Stimulating Factor (GM-CSF) or Macrophage Colony-Stimulating Factor (M-CSF) to respectively produce M1- or M2-shifted macrophages, the differentiated macrophages respond differently to IFNλ3. GM-CSF differentiated macrophages demonstrated greater IFNLR1 expression and had increased levels of STAT1 phosphorylation and ISG expression in response to IFNλ3 than M-CSF differentiated macrophages (23). Moreover, GM-CSF differentiated macrophages exhibited a pro-inflammatory profile and were more potent at recruiting leukocytes and NK cells upon IFNλ3 stimulation. Interestingly, when stimulated with type I IFN, these macrophages did not exhibit a pro-inflammatory phenotype and were incapable of recruiting leukocyte and NK cells (23).

IFNλ also alters the function of pDCs (8, 24), T and B lymphocytes (5, 26) and mouse NK cells (28). However, whether these effects have an impact on bacterial infections has yet to be explored. pDCs are antigen presenting cells which secrete cytokines that recruit and activate cells of the adaptive immune system. IFNλ3 was shown to prolong the survival of human pDCs in vitro, as well as increase their immunoreactivity and ISG response (100). IFNλ was also demonstrated to enhance activation of, and antibody production by, human B lymphocytes (101, 102) and skew the T helper 1 (Th1)/Th2 balance to a Th1-pro-inflammatory response (103–105). Finally, in an in vivo model of bacterial-induced inflammation, IFNλ signaling in mouse NK cells was shown to induce IFNγ production, which promoted inflammation. Although enhanced activation of immune cells may promote bacterial clearance, the dangers of prolonged inflammation and tissue destruction must be considered. As such, studying the role of IFNλ in the resolution of inflammatory responses to infection is essential to uncover whether it causes any detrimental effects.