TRIMming Type I Interferon-Mediated Innate Immune Response in Antiviral and Antitumor Defense

The tripartite motif (TRIM) family comprises at least 80 members in humans, with most having ubiquitin or SUMO E3 ligase activity conferred by their N-terminal RING domain. TRIMs regulate a wide range of processes in ubiquitination- or sumoylation-dependent manners in most cases, and fewer as adaptors. Their roles in the regulation of viral infections, autophagy, cell cycle progression, DNA damage and other stress responses, and carcinogenesis are being increasingly appreciated, and their E3 ligase activities are attractive targets for developing specific immunotherapeutic strategies for immune diseases and cancers. Given their importance in antiviral immune response, viruses have evolved sophisticated immune escape strategies to subvert TRIM-mediated mechanisms. In this review, we focus on their regulation of IFN-I-mediated innate immune response, which plays key roles in antiviral and antitumor defense.


Introduction
In mammalians, interferons (IFNs) include three types, type I, II, and III. Type I IFNs (IFN-Is) include the majority of 26 isoforms of IFNα that are encoded by 13 genes, and one IFNβ that is encoded by the single gene IFNB, as well as other minor subtypes, including IFNε, IFNκ, IFNω, IFNδ, IFNτ, and IFNζ. IFNαs are mainly secreted by plasmacytoid dendritic cells (pDCs) and IFNβ is mainly secreted by fibroblasts. All IFN-Is signal through the integral membrane IFNAR1 and -2 heterodimer, and play crucial roles in the first line of innate immune response and subsequent adaptive immune response in response to viral or bacterial infections [1]. Importantly, recent studies have shown that IFN-Is play a dual role in chronic viral infections. At the early stage of infection, they have potent antiviral activity. However, at late stages, a low level of prolonged IFN-I signaling, exemplified by chronic infection of viruses such as HIV and HCV [2][3][4], triggers long-term chronic immune activation that proceeds to T cell exhaustion and inflammaging/immunosenescence in both direct and indirect manners [5,6] and therefore serves as a bridge that links innate and adaptive immune responses [4,5,[7][8][9][10]. For example, the engagement of TLR7 in HIV-infected CD4 + T cells induces anergy/unresponsiveness, accounting for the impaired T cell function by chronic HIV infection [11]. A prolonged IFN-I response also facilitates the establishment of TME (tumor microenvironment) [4,5,7,[12][13][14]. IFN-Is also play crucial roles in cellular development and homeostasis [5,6,[15][16][17]. Aberrant production of IFN-Is is associated with many types of diseases, including autoimmune disorders and cancers [6,[18][19][20]. Therefore, it is of fundamental importance to understand the precise mechanisms of how IFN-Is are regulated in different biological contexts [21,22].
Ubiquitination is a pervasive theme equally important to phosphorylation of proteins in myriad processes. Ubiquitin (Ub) is a 76-amino acid protein that is ubiquitously dis- Figure 1. Toll-like receptor (TLR)signaling pathways. The TLR family has 13 members, among which, endosomal TLR3, -7, -9, and murine TLR8, and endocytic TLR4 are able to trigger signaling for IFN-I production. TLRs that cannot trigger the activation of Interferon regulatory factors (IRFs) (The middle part in the gray frame) do not contribute to IFN-I production.
All the TLRs have a TIR domain in the cytoplasm, which recruits adaptor proteins also with a TIR domain at their C-terminus. TRIF and MyD88, two adaptor proteins, bridge all TLRs to downstream signaling molecules, leading to the activation of NFκB, IRFs, and AP1. IRF1, -3, -5, -7, and -8 are the transcription factors for both IFNα and IFNβ transcription in different cell contexts, but a full IFNβ transcription requires the enhanceosome complex that contains NFκB, IRF3, -7, ATF-2/c-Jun, and HMGIY (high mobility group I(Y)). ISRE: Interferon-stimulated response element. ISRE3/7: ISRE that binds to IRF3//7. ISRE7: ISRE that binds to IRF7.

The TRIM Family
The tripartite motif (TRIM) family of proteins is large and includes at least 80 members in humans, with most having E3 ligase activity for target-specific ubiquitination, and plays crucial roles in innate immunity, transcription, autophagy, and carcinogenesis [49,50]. The N-terminal TRIM motif includes the conserved RBCC domain that comprises of three subdomains: 1 RING domain that confers with E3 ligase activity (8 human TRIMs do not have the RING domain), 0~2 B-box ZNF domains (B1+B2 or B2 alone), and 0~1 coil-coil region that is associated with B-boxes. According to the diversity of the C-terminuses and genomic organization, TRIM proteins are grouped into Group1 and Group 2.  The N-terminal TRIM motif includes the conserved RBCC domain that comprises of three subdomains: 1 RING domain that confers with E3 ligase activity (8 TRIMs in humans do not have the RING domain), 0~2 B-box ZNF domains (B1+B2 or B2 alone), and 0~1 coil-coil region that is associated with B-boxes. According to the diversity of the C-terminuses and genomic organization, TRIM proteins are grouped into Group1 and Group 2. Members in Group 1 possess a variety of C-terminal domains (COS, FN3, ACID, PRY, PHD-BROMO, FIL, NHL, MATH, ARF, and TM) and exist in both vertebrate and invertebrates, and those in Group 2 possess a C-terminal SPRY domain, and they are absent in invertebrates. Many TRIMs are inducible by IFN-Is, and play crucial roles in IFN-I-mediated innate immune regulation, with the involvement of ubiquitination and sumoylation in most cases [50,52-58]. These TRIMs can target most if not all components of the PRR and Jak-STAT1 IFN-I pathways, including different ligands (PAMPs and DAMPs); the receptors such as TLRs, cGAS, and DDX41; the adaptors MyD88, TRIF, STING, and TRAF6 and -3; the kinases IKKs and TAK1; and the transcription factors IRF3 and -7 and NFκB (Table 1). TRIM genes evolve parallelly with the immune system, further supporting their roles as regulators of immune responses [51,53]. * NA: not assayed. Question marks (?) refer to "very likely but not experimentally revealed".

TRIMs in Regulating PRR Signaling Pathways to IFN-I Production
Upon binding to PAMPs or DAMPs, PRRs trigger signals that transmit via unique adaptors to the Ub E3 ligase TRAF6 or -3 and then orchestrate to activate the kinase cascades IKKs, IRAKs, and MAPKs for the activation of the transcription factors NFκB, IRFs, and AP1. Ubiquitination regulates the cellular trafficking, stability, complex assembly, and activity of different components in PRR signaling cascades ( Figure 3). Viral PAMPs and other viral components can be targeted by the host Ub system, including a subset of TRIMs, for ubiquitination-mediated degradation in most cases, and thus the IFN-I response is blocked at the very beginning to suppress viral replication, with some examples listed in Table 1 [54,58]. Of note, TRIM5α (also TRIM22) targets HIV1 Gag and plays a unique role in restricting HIV1 infection (and other retroviruses), implicating a potential clinical application [61,62]. In fewer cases, TRIMs can promote viral entry and replication by targeting viral proteins. For example, TRIM7 targets Zika virus envelope protein E for K63-linked ubiquitination that enhances viral attachment to the cell surface and promotes viral entry [66]. VP35, the Ebola virus polymerase co-factor, has IFN-I inhibitory activity. TRIM6 promotes VP35 polyubiquitination to enhance viral infection [64].
Cytoplasmic ssRNA and dsRNA are mainly recognized by RIG-I and MDA5 respectively. RIG-I activation is promoted by K63-linked ubiquitination mediated by TRIM4 [60] or TRIM25 [121,122], and MDA5 activation is promoted by K63-linked ubiquitination mediated by TRIM65 [164]. However, the biological role of TRIM25 in the activation of RIG-I has been challenged by the in vivo evidence that the deletion of the gene encoding TRIM25 did not impair, but the deletion of that of RIPLET/RNF135 completely abrogated, RIG-Imediated IFN-I response [165]. TRIM38 stabilizes RIG-I and MDA5 by sumoylation [146]. In contrast, TRIM40 promotes RIG-I and MDA5 proteasomal degradation through both K27-and K48-mediated ubiquitination forms [153]. The membrane-anchored TRIM13 and TRIM59 also promote RIG-I activity but negatively regulate MDA5-mediated IFN-I responses, through unclear mechanisms that may involve additional cofactors [79].
Cytosolic DNA, which is derived from self DNA or from invading pathogens, is mainly sensed by the cGAS-STING pathway and additionally by DDX41, DAI, and MRE11, amongst others [22]. The DNA fragment bound to cGAS is processed by cGAS synthetase activity to generate the second messenger cyclic GMP-AMP (cGAMP) that then binds to STING. cGAS stability and synthetase activity are balanced by ubiquitination, which is mediated by an increasing pool of E3 ligases including TRAF6, RNF185, and several TRIMs [166]. K48-linked ubiquitination of cGAS at K414 is targeted for degradation by p62mediated selective autophagy [167]. TRIM14, a mitochondrial protein that lacks the RING domain, recruits USP14 to cleave K48 chains on cGAS to stabilize cGAS [85]. TRIM38 also stabilizes cGAS, however, through promoting cGAS sumoylation at early but K48 ubiquitination at the late stage of signaling at the same site K479 (K464 in mice) [147]. TRIM41 and TRIM56 both promote cGAS activation through mono-ubiquitination [155,159]. TRIM21 (known as Ro52), an autoantigen in patients with SLE (systemic lupus erythematosus), targets DDX41 for degradation [97].

TRIMs in Regulating the Adaptors
TLRs transmit signals via the adaptors MyD88 or TRIF. MyD88 is required for all TLRs except for TLR3, whereas TRIF is involved in both TLR3 and TLR4 pathways to the activation of IRFs. A subset of TRIMs regulate TRIF activity and stability (Table 1, Figure 3), but to our knowledge, so far, no report shows that MyD88 is targeted by any TRIM, although other E3 ligases target MyD88 for activation or degradation [168]. TRIM56 and TRIM62 promote TRIF activation [86,161], but TRIM32 and TRIM38 target TRIF for degradation. Mechanistically, TRIM56 physically interacts with TRIF upon viral infection, and its E3 ligase activity is not required for activation of TRIF [161]. TRIM32 also interacts with TRIF but targets TRIF to TAX1BP1-mediated selective autophagy for degradation [142]; whereas TRIM38 targets TRIF K228 for K48 ubiquitination, leading to proteosome-mediated degradation [150,151]. TRIM8 promotes K6-and K33-linked ubiquitination of TRIF and therefore disrupts TRIF-TBK1 interaction to specifically inhibit TLR3/4-mediated immune response [68].

TRIMs in Regulating TRAF3/6
TRAF3 is critical for activation of IRFs downstream of PRRs leading to IFN-I production [171,172], whereas TRAF6 is generally required for NFκB and AP1 activation in these settings. Both TRAF3 and TRAF6 are members of the TRAF E3 ligase family, and they can catalyze self-ubiquitination.
TRAF3 activity and stability are regulated by ubiquitination. Its activation requires K63 ubiquitin conjugation, which is mediated by TRIM24, TRIM35, TRAF3 itself, TRAF6, and other E3 ligases in distinct contexts [117,144,173,174]. TRIM23 also interacts with TRAF3 in coimmunoprecipitation assay, but the function of TRIM23 in this setting is unknown [114].

TRIMs in Regulating the Kinase Cascades for Activation of IRFs and NFκB
For IFN-I transcription, the IKK family members TBK1, IKKε, and IKKα (IKKβ for IRF5) are responsible for the activation of IRFs, with the involvement of other kinases including IRAKs, RIP1, and the MAPK family member TAK1. The IKK family members IKKα, IKKβ, and NEMO (IKKi), however, are for NFκB activation. A pool of kinaseinteracting partners and adaptors are also involved, such as TANK, NAP1, TAB1/2, FADD, and TRADD. Ubiquitination is involved in the modulation of the kinase cascades.

TRIMs in Regulating the Transcription Factors IRFs and NF-κB for IFN-I Production
Accumulating evidence has shown that ubiquitination and ubiquitination-like modifications directly target the IFN-I transcription factors IRFs and NFκB for their activation in antiviral and antitumor immunity.
As a multifaceted regulator in IFN-I-mediated defense, TRIM21/Ro52 directly targets a subset of IRFs for degradation, and in turn, it is directly induced by IRF1 and -2 [177]. TRIM21/Ro52 acts as a double edge sword to regulate IRF3: TRIM21/Ro52 directly targets phosphorylated IRF3 (p-IRF3) for proteasomal degradation, which is enhanced by tyrosine phosphorylation of TRIM21 at Y393 [104,105]. TRIM21/Ro52 also interacts with multiple autophagy components, including the selective autophagy receptor p62, ULK1, and Beclin1, and serves as a platform to facilitate the assembly of the autophagy apparatus, which directs IRF3 for lysosomal degradation [106]. On the other hand, TRIM21 interferes with the interaction between p-IRF3 and the prolyl isomerase Pin1 independently of its E3 ligase activity to protect p-IRF3 from Pin1-mediated ubiquitination and degradation [103]. Furthermore, Pin1 itself interacts with sumoylated TRIM19/PML isoform IV (TRIM19IV), which results in its recruitment to PML-NBS, preventing p-IRF3 from degradation [91]. TRIM21 also promotes proteasomal degradation of the phosphorylated forms of IRF7 as well as V1 and V5 isoforms of IRF5 [107,108] but promotes IRF8 activation [109]. In addition to its direct targeting p-IRF7 for degradation, as mentioned above, TRIM21 interacts with FADD downstream of RIG-I signaling, promoting ubiquitination-mediated IRF7 degradation [99].

TRIMs in Regulating the Jak-STAT IFN-I Signaling
The level of initial IFN-Is produced downstream of PRR pathways upon viral infection is relatively low due to the low level of endogenous IRF7 protein; these priming IFN-Is then secret to outside of the cell in autocrine and paracrine manners, and bind to IFN-I receptor (IFNAR) on other cells, consequently triggering the Jak-STAT IFN-I pathway, which serves as the second phase of antiviral response by inducing the expression of more IRF7, which in turn participates in IFN-I production downstream of PRR signaling, therefore amplifying the IFN-I production in a positive regulatory circuit (Figure 4) [42]. . TRIM regulation of the Jak-STAT IFN-I signaling circuit. pathogen-associated molecular patterns (PAMPs) from pathogens activate PRR pathways, leading to the phosphorylation of constitutive high level of IRF3 and low level of IRF7, which induce a low level of IFN-Is. IFN-Is secrete from the cell and bind to IFN-I receptors (IFNAR1 and -2) on the cell membrane, followed by the recruitment and activation of Jak1 and Tyk2, leading to the phosphorylation and activation of STAT1 and -2. Phosphorylated STAT1 and -2 bind to IRF9 to form the ISGF3 (IFN-stimulated gene factor) complex, which functions as the transcriptional activator of more than 300 IFN-inducible genes (ISGs), including IRF7 itself. The induced IRF7 proteins then return to be activated by PRR pathways, and therefore constitute a positive regulatory circuit between IRF7 and IFN-Is, ensuring a potent production of IFN-Is to fight the invading pathogen. TRIM6 promotes free K48-linked Ub chains that serve as a platform to facilitate IKKε dimerization and activation. TRIM8 regulates the Jak-STAT IFN-I signaling at multiple points. The nuclear TRIM19 potentiates the transcription and activation of STAT1 and -2, and nuclear TRIM24 inhibits RARα-mediated STAT1 promoter activation. ISRE: Interferon-stimulated response element; VRE: Virus-responsive element.
Jak-STAT pathways are well known to be negatively regulated by two families: SOCS (Suppressor of cytokine signaling) and PIAS (Protein inhibitor of activated STAT). TRIM8 can shuttle between the cytoplasm and the nucleus [74], and has multiple functions to promote IFN-I signaling. TRIM8 promotes proteasomal degradation of SOCS1 and PIAS3 presumedly in the cytoplasm, and also nuclear TRIM8 promotes PIAS3 nucleus-cytoplasm translocation to inhibit PIAS3 activity [72][73][74]. SOCS1 not only inhibits Jak1 activity by directly binding to phosphorylated Jak1 in the IFN-I Jak-STAT signaling but also acts as a ubiquitin E3 ligase that targets phosphorylated IRF3 and IRF7 (both also targeted by SOCS3 that recruits the Cul-RBX2 E3 complex) for proteasomal degradation in the nucleus [179]. As such, PIAS3 acts as a SUMO E3 ligase that inhibits IRF1 transcriptional activity through sumoylation in addition to its ability to inhibit STAT3 [180]. TRIM14 negatively regulates IFN-I signaling in mouse macrophage in response to Mycobacterium tuberculosis infection by serving as a scaffold that bridges TBK1-STAT3 interaction promoting STAT3 S727 phosphorylation, consequently inducing SOCS3 expression that inhibits IFN-I signaling by targeting phosphorylated IRF3 and IRF7 as well as TBK1 for proteasomal degradation [83]. The nuclear protein TRIM19/PML promotes ISGF3-mediated gene expression by facilitating STAT1 gene transcription and STAT2 protein stabilization, as well as the accumulation of both activated STAT1 and -2 to chromosome [90].
IKKε is not only responsible for the activation of IRF7 and -3 but also plays a role in balancing IFN-I and IFN-II Jak-STAT signaling pathways in immune responses [181]. TRIM6 catalyzes free chains of K48, which promotes IKKε oligomerization and activation to facilitate STAT1 S708 phosphorylation and IFN-I signaling [65]. However, TRIM24 can inhibit retinoic acid-induced STAT1 transcription by interacting with the transcription factor RARα on the STAT1 gene promoter [118].

IFN-Is establish an antiviral state in both virus-infected cells and uninfected bystander cells, by inducing the expression of over 300 ISGs (IFN-stimulated genes) [6]
. Many components of the PRR signaling pathways, such as RIG-I, cGAS, STING, IRF1, and IRF7 belong to ISGs. In addition to these components, many other ISGs, including some TRIMs themselves, are also directly regulated by TRIMs. For example, TRIM11 promotes TRIM5 turnover dependently on its RING domain [78]. Ubiquitination-like modifications, such as sumoylation and ISGylation, are involved in IFN-I-mediated defense mechanisms [30,34,[182][183][184][185]. TRIM19/PML mediates global sumoylation [92], and TRIM25 functions as an ISG15 E3 ligase that mediates ISGylation [124]. Further, TRIM25 has been recently reported to be required for the stability of several ISG products [186]. The zinc-finger antiviral protein ZAP, as an ISG, is activated by TRIM25-mediated ubiquitination to inhibit viral genome translation [125]. The tumor suppressor p53 is also an ISG inducible by IFN-Is [187]. TRIM24 promotes p53 ubiquitination and degradation and, in turn, is inducible by p53 [119]. ATM phosphorylates TRIM24 at S768 and promotes its degradation, stabilizing p53 [188]. Numerous TRIMs, in addition to TRIM24, regulate p53 activity and stability in direct or indirect manners [189].

Viral Strategies to Subvert TRIM-Mediated IFN-I Regulatory Mechanisms
Given the overwhelming evidence that TRIMs play versatile roles in antiviral defense, viruses have developed sophisticated strategies to counteract these TRIM-mediated mechanisms. We list some strategies invoked by herpesviruses and HPV in their lytic and latent infections.
Sumoylation promotes stability of IRF3, RIG-I, MDA5, cGAS, and STING. The SUMO protease SENP2 can reverse the sumoylation process, leading to their degradation [146,147,202,203]. LMP1 was reported to inhibit SENP2 activity in EBV latency [204]. Thus, LMP1 regulation of SENP2-mediated desumoylation may represent one of many strategies of EBV to balance IFN-I response mediated by these PRR pathways to benefit its infection and latency establishment. We have shown that LMP1 exploits the host ubiquitin system to regulate IRF7 activity, including the induction of linear ubiquitination mediated by LUBAC to repress IRF7 activity and to promote NFκB activation in EBV latency [205][206][207].
TRIM19/PML is another important target attacked by various viruses for immune escape [208,209]. As a ubiquitin E3 ligase, HSV1 ICP0 preferentially targets the SUMOmodified TRIM19/PML isoforms, leading to their degradation [210], among its many roles in HSV1 lytic and latent infection [211]. EBV EBNA1 and KSHV LANA2 disrupt PML bodies [212,213]. HCMV IE1 binds to PML in a complex with ISGF3 to impede IFN-I signaling [90]. In contrast, HPV requires PML nuclear bodies to establish infection [214]. Other examples for viral targeting PML include that EMCV 3C protease promotes PML degradation [215] and that the rabies virus phosphoprotein P and its truncated mutants cause the redistribution of PML-NBs into the cytoplasm [216]. Interestingly, TRIM19/PML is a cellular ROS (reactive oxygen species) sensor that possesses basal antioxidant properties but also drives ROS-mediated p53 activation; its depletion promotes ROS accumulation and NRF2-mediated antioxidant defense. Thus, TRIM19 plays a role in ROS-mediated p53 responses, including senescence, cell survival/proliferation, and metabolism [95]. The production of ROS and RNS (reactive nitrogen species) is one of cancer hallmarks and the most common complication of persistent viral infection [217][218][219][220][221]. ROS/RNS also cause endogenous DNA damage that can activate the cGAS-STING pathway, which plays critical roles in natural anti-tumor or pro-tumor immunity as well as in ageing [46,47,[222][223][224]. Moreover, we and others have shown that, in chronic viral infections, ROS/RNS couple with p62-mediated selective autophagy machinery [225,226], which serves as an alternative DNA damage repair strategy in these settings that have impaired traditional DNA repair machinery.
HPV E6 and E7 are two oncogenic proteins critical for HPV-mediated oncogenic transformation. E6 can hijack the host ubiquitin E3 ligase E6AP to inactivate the tumor suppressor p53 by promoting its ubiquitination, whereas E7 hijacks the host cullin 2-Zer1 ubiquitin ligase complex for ubiquitination-mediated degradation of the tumor suppressor Rb. Interestingly, E6 exploits the host deubiquitinase USP15 to target TRIM25 for ubiquitination-mediated degradation, to escape RIG-I-mediated immune response [227].
With a high throughput screen strategy, E6 and E7 were found to interact with a pool of UPS-related proteins, including TRIM32 and -72 for E7 [228]. LUBAC-mediated linear ubiquitination of TRIM25 also promotes its degradation [229]. We have shown that EBV LMP1 exploits LUBAC-mediated linear ubiquitination, which inhibits LMP1-stimulated IRF7 activity in EBV latency [207]. This finding implies that targeting TRIM25 by LUBAC may represent another strategy for EBV to escape IFN-I immune response in its latency.
Many other viruses are also known to subvert TRIM-mediated regulation of IFN-I network. For example, the matrix protein of Nipah virus targets TRIM6 for degradation to inhibit IKKε-mediated IFN-I antiviral response [230]. Identification of virus-specific products in this process is necessary for improving our understanding of the host-virus interactions.

Perspectives
TRIMs have diverse roles in host antiviral and antitumor defense systems, with or without the involvement of ubiquitination and alike modifications. Note that the roles of a given TRIM protein in different PRR signal pathways are probably distinct or even opposite, depending on the stimuli and cell contexts. Even more, the same TRIM has opposite outcomes on the same target through different mechanisms. For example, TRIM21 directly targets phosphorylated IRF3 for proteasomal degradation and also protect p-IRF3 from Pin1-mediated ubiquitination and degradation independently of its E3 ligase activity. In the meantime, TRIM21 indirectly regulates IRF3 activity by targeting the signal intermediators, such as DDX41, MAVS, FADD, TAK1, and the autophagy machinery, upstream of PRR pathways. The ultimate outcomes are manifested by the converge of these regulation events.
Besides those TRIM members listed in Table 1, other members in this family may have potential roles in IFN-I regulation. It is important to identify the unique role and targets of each TRIM member, which can be accelerated with different high throughput screening strategies, such as targeted proteolysis, combined with the CRISPR-mediated knockout or knockin techniques [231]. It is also important to understand how TRIMs mediate the crosstalk between immunity and cancer via the regulation of IFN-I network. Of note, selective autophagy, especially that mediated by p62, is emerging to play a crucial role in this process. As shown in Table 1, selective autophagy targets key components of PRR pathways for degradation and is required for TBK1 activation [116]. A subset of TRIMs, including those IFN-I regulators TRIM5α, -11, -14, -20, -21, -23, -28, -31, -32, -38, -39, -59, and -65 listed in Table 1, amongst others, regulate autophagy [49,56,232]. Since the cGAS-STING pathway plays a crucial role in oncogenesis, by sensing self DNA fragments derived from oxidative, infection, and other stresses, it is plausible that these TRIMs connect the cGAS-STING-mediated innate immunity and carcinogenesis via the autophagy machinery, which plays fundamental roles in antiviral and antitumor defense and is a potential target for cancer immunotherapy [233][234][235][236][237][238][239][240].
The regulation of the IFN-I network by TRIMs mostly requires their E3 ligase activity that possesses high substrate specificity, or their activity is regulated by specific enzymes, providing an applausive opportunity for drug targeting for clinical applications. However, there are no drugs directly targeting the E3 ligase activity of TRIMs so far, although some drugs targeting proteasomes or the autophagy machinery have been developed for this purpose. More efforts are desired towards this objective.  [CrossRef] 39. Zheng, Y.; Gao, C. Chapter Four-Fine-tuning of antiviral innate immunity by ubiquitination. Adv. Immunol. 2020, 145, 95-128.