T-cell immunoglobulin and mucin domain 3 (Tim-3) is widely expressed on many immune cells including T cells and macrophages [[1], [2], [3], [4]]. The establishment of Tim-3 as an exhaustion marker in immune cells of both tumors and infectious diseases makes Tim-3 an attractive target for immunotherapy similar to PD-1 and CTLA-4. Recently, a report showed that increased Tim-3 expression on immune cells in patients with coronavirus disease (COVID-19) is associated with an exhaustion phenotype [5]. Compared with the relatively clear mechanisms by which PD-1 induces tolerance in T cells, very little is known about Tim-3 signaling in immune cells, especially innate immune cells. We previously demonstrated that Tim-3 may regulate the function of macrophage via NF-kB or STAT-1 pathway [6]. However, as Tim-3 does not have an inhibitory motif within its tail, and the mechanism by which Tim-3 mediates inhibitory signaling remains largely unclear [9,10].

Deubiquitinating enzymes (DUBs) are capable of recognizing and cleaving peptide bonds between ubiquitin and the substrate or within the ubiquitin chain and have multiple biological activities [11]. Recently, studies have shown that ubiquitin specific protease family molecule 25 (USP25) is involved in the pathogenesis of various diseases including infectious diseases, neurodegenerative disorders, and cancer [[12], [13], [14], [15]]. Lin et al. reported that compared with wild-type mice, USP25-deficient mice were more susceptible to infection with herpes simplex virus 1 or influenza A virus subtype H5N1 [16]. Studies have shown that USP25 promotes the antiviral immune response by cleaving ubiquitin moieties from critical signaling proteins of the type I IFN signaling pathway such as RIG-I, TRAF3, and TRAF6 [17]. USP25 tends to play an essential role in infectious diseases, but many of the mechanisms involved are not fully understood.

The threaten of viral infectious diseases calls for more detail investigation. Innate immunity is the first line of defence against viral infection. A variety of effector cells in the innate immune system can quickly recognize pathogens and produce type I interferons and other cytokines to inhibit virus replication and transmission [18,19]. Interferon regulatory factor 7 (IRF7), which is a signaling cascade downstream of TRAF3, plays critical roles in antiviral innate immunity by regulating the response of type I interferons, interferon-stimulated genes (ISGs) and other proinflammatory cytokines. The signaling activity of IRF7 is regulated by multiple mechanisms. An investigation of the mechanisms by which IRF7 is regulated under different physiopathological conditions will provide much information concerning antiviral innate immunity.

Here we identified a new mechanism by which Tim-3 promotes viral immune evasion, that is, by suppressing USP25 expression, Tim-3 inhibits the USP25-mediated deubiquitination of TRAF3 and suppresses the TRAF3- IRF7-type I interferon pathway. We thus identified a new signaling pathway of Tim-3 mediated immune tolerance with clinical significance.