MYD88: AN ADAPTOR OF THE INNATE IMMUNE RESPONSE

G. Barassi¹, L. Tauro^2*, T. Ferrulli² and M. Fortunato²

¹  Physiotherapy, Rehabilitation and Reeducation Training Center (CeFiRR), School of Medicine and Health Sciences,
University “Gabriele d’Annunzio” of Chieti-Pescara, Chieti, Italy;
²  Miulli General Hospital, Unit of Clinical Pathology and Microbiology, Acquaviva delle Fonti (Bari), Italy.

*Correspondence to:
Lucio Tauro,
Miulli General Hospital,
Unit of Clinical Pathology and Microbiology,
70021 Acquaviva delle Fonti (Bari), Italy.
e-mail: l.tauro@miulli.it

Received: 25 February, 2025 Accepted: 23 May, 2025

ISSN 2279-5855 print ISSN 2974-6345 online. Copyright © by BIOLIFE 2025 This publication and/or article is for individual use only and may not be further reproduced without written permission from the copyright holder. Unauthorized reproduction may result in financial and other penalties. Disclosure: All authors report no conflicts of interest relevant to this article.

ABSTRACT

Myeloid Differentiation Primary Response 88 (MyD88) is a key adaptor protein of the innate immune system that acts as a bridge between other proteins, connecting components of a signalling pathway or cellular process that would otherwise not interact directly. MyD88 is an intracellular adaptor of the Toll-like receptors (TLRs) and IL-1 receptor (IL-1R) signaling pathways. In the central nervous system (CNS), MyD88 helps regulate the response of microglia against infections. Microglia express TLRs that recognize pathogen-associated molecular patterns (PAMPs) and danger-associated molecular patterns (DAMPs).   Toll-like IL-1 receptor (TIR) domain of the TLR recognizes MyD88, triggering a series of reactions that lead to the formation of NF-κB and the production of cytokines (including IL-1b, TNF, and IL-6) and pro-inflammatory chemokines. Activation of these kinases induces AP-1 and the inflamasome, indirectly stimulating the generation of IL-1b and IL-18. The participation of MyD88 in the inflammatory process can exacerbate both infectious diseases and neurodegenerative disorders.

KEYWORDS: Myeloid Differentiation Primary Response 88, MyD88, adaptor protein, Toll-like receptor, IL-1 receptor

INTRODUCTION

Myeloid differentiation primary response 88 (MyD88) is a key intracellular adaptor of the Toll-like receptors (TLRs) and IL-1 receptor (IL-1R) signaling pathways (1). The relationship between MyD88 and microglia is very important for regulating the innate immune response in the central nervous system (CNS) (2).

MyD88 is expressed in immune cells such as macrophages, dendritic cells, neutrophils, epithelial cells, and B lymphocytes. TLRs are part of the innate immune system and are important as “sensors” for the presence of microorganisms, to recognize microbial signaling molecules. For example, TLR4 recognizes bacterial lipopolysaccharide (LPS), TLR3 and TLR7 recognize viral RNA, TLR9 recognizes bacterial DNA, and TLR2 recognizes peptide-binding molecules (3). After TLR activation, the receptor’s Toll-like IL-1 receptor (TIR) domain binds to the TIR domain of MyD88, which acts as a molecular bridge (4).

MyD88 transmits activation signals from TLRs and IL-1 receptors to the cell nucleus, activating the inflammatory response. The TIR domain is a cytoplasmic protein domain found in TLRs, and in the IL-1 and IL-18 receptors. After ligand-receptor binding, the receptor dimerizes, the cytoplasmic TIR domains approach each other, and a TIR-TIR bond is formed between the receptor and the adaptor (5). The TIR domain is essential for signal transduction. MyD88 is the primary adaptor for IL-1, IL-18, and all TLRs except TLR3 (6). When a TLR recognizes a Pathogen-Associated Molecular Pattern (PAMP), which can be a microorganism or a foreign molecule, MyD88 is recruited, activating IL-1R–associated kinase (IRAK) 4 and leading to the formation of IRAK1, Tumor necrosis factor receptor (TNFR)-associated factor 6 (TRAF) activation, NF-κB activation, and mitogen-activated protein kinase (MAPK) activation, resulting in the transcription of pro-inflammatory genes (7,8) (Table I).

Table I. MyD88-dependent signaling pathway in microglia.

DISCUSSION

MyD88 is a key adaptor in the signaling pathway of many TLRs and the IL-1 receptor (9). Activation of TLRs and IL-1R leads to the activation of NF-κB and MAPK, resulting in the production of inflammatory cytokines and chemokines (10). The role of MyD88 in neuroinflammation can be both protective and pathological (11). It is protective because it allows for the rapid recognition of pathogens such as viruses, bacteria, etc., and pathological because excessive activation of MyD88 can cause inflammation due to the release of pro-inflammatory cytokines such as IL-1 and TNF (12).  These effects can exacerbate the progression of neurodegenerative diseases such as Alzheimer’s disease (AD), Parkinson’s disease (PD), and multiple sclerosis (MS), by amplifying oxidative stress and the toxic effects of microglial cells (13). Therefore, MyD88 is important for immune defense and brain recovery, but if its activation is continuous (chronic), it can become pathogenic (14). Inactivation of MyD88 in knockout animal models leads to inefficient microglia with reduced neuroinflammation, but also with impaired immune defense (15).

TLRs, with the exception of TLR3, use MyD88 as their primary adaptor. Brain-based microglia immune cells express TLRs that recognize PAMPs and danger-associated molecular patterns (DAMPs). When these receptors are activated, they are able to recruit the adaptor MyD88, and PAMPs and DAMPs bind to TLRs on the membrane of microglia cells. The TIR domain of the TLR recognizes MyD88, which recruits the kinases IRAK4 and IRAK1 (16).  A large oligomeric intracellular multiprotein complex called the Myddosome is subsequently formed (17).  IRAK phosphorylates and activates TRAF6, an E3 ubiquitin ligase, activating signaling pathways (18). NF-κB translocates to the nucleus and induces pro-inflammatory cytokine genes such as TNF, IL-1β, and IL-6.

The kinases p38, extracellular signal-regulated kinase (ERK), and c-Jun N-terminal kinase (JNK) activate factors such as AP-1, and the inflammasome, which is indirectly stimulated, participates in the maturation of IL-1β and IL-18 (19). Once activated, microglia produce pro-inflammatory cytokines and chemokines, causing increased expression of major histocompatibility complex class II (MHC-II) surface molecules and CD86, which has the ability to present antigen (9).

Viruses are recognized by endosomal TLR7, TLR8, and TLR9, while bacteria use TLR4 for LPS, TLR2 for peptidoglycan, TLR5 for flagellin, and TLR9 for bacterial DNA (20). Beta-glucans and mannans from fungi are recognized by TLR2 and TLR4 in association with Dectin-1 (21). After activation of the TLR or IL-1R, MyD88 is recruited via the TIR, resulting in the formation of the Myddosome signaling complex (22). MyD88 subsequently recruits IRAKs, which leads to the activation of IRAK4 and the phosphorylation of IRAK1/2 (23). After this, TRAF6 and TAK1 are activated, leading to the MAPK cascade (p38, JNK, ERK), the IKK complex, and NF-κB, which translocates to the nucleus, where genes for pro-inflammatory cytokines are activated and cytokines such as IL-1, TNF, and IL-6 are produced (24). AP-1 activated by MAPK leads to further inflammatory gene transcription (25). In viral inflammation, TLR3 activation does not use MyD88, but TRIF, which leads to the production of type I interferon (IFN) (26) (Fig. 1).

Fig. 1. Activation of Toll-like receptors (TLRs) by microorganisms leads to a biochemical cascade which ultimately results in the release of inflammatory cytokines.

Viral activation of MyD88 can result in positive, and therefore, protective clinical effects due to the production of IFNs and cytokines that limit viral replication. Activation also promotes the development of the immune adaptive response (27). Furthermore, in early infections, such as with RNA viruses such as influenza, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), hepatitis C virus (HCV), etc., MyD88 is important for reducing viral load in the initial stages (28). Hyperstimulation of MyD88 can result in a pro-inflammatory cytokine storm with tissue damage and acute respiratory distress syndrome (ARDS) in severe viral infections (29). In chronic infection, MyD88 contributes to the pathogenesis of viral hepatitis or persistent chronic infections, such as those caused by hepatitis B virus (HBV), HCV, or human immunodeficiency virus (HIV) (30).  In autoimmune diseases, activation of TLR/MyD88 by viral nucleic acids can promote lupus, rheumatoid arthritis, and Sjögren’s disease (31).  In virally induced oncogenesis, such as Epstein-Barr virus (EBV)-positive lymphomas, HCV/HBV-induced liver cancer, and so on, chronic MyD88 stimulation can promote cell proliferation and survival (32).

Respiratory infections may cause fever, cough, and severe inflammation, due to activation of MyD88 leading to NF-κB with the release of inflammatory cytokines and chemokines (33). Autosomal recessive MyD88 deficiency is rare but can lead to severe pyogenic bacterial infections (34). However, compared to viruses, it may cause a less severe cytokine storm and therefore, less damage (35).

Role of MyD88 in the central nervous system

Microglia express TLRs such as TLR4, which recognize pathogenic PAMPs such as LPS, lipoproteins, and viral RNA/DNA. Except for TLR3, all TLRs signal via MyD88. Activation of IRAK, TRAF6, and TAK1 leads to NF-κB and MAPK (AP-1) transcription and signal transduction. In the CNS, MyD88-mediated production of inflammatory cytokines leads to neuroinflammation which can be helpful to fight infections. However, if the response is excessive and chronic, it becomes harmful, as occurs in neurodegenerative diseases. In fact, MyD88 is implicated in AD, MS, PD, and amyotrophic lateral sclerosis (ALS) (13). In these diseases and after trauma and cerebral ischemia, MyD88 can exacerbate the damage through inflammation, but also promote debris removal and repair.

CONCLUSIONS

MyD88 plays an essential physiological role in the innate immune response and is linked with adaptive immunity. MyD88 functions physiologically in the production of pro-inflammatory cytokines, such as IL-1, TNF, and IL-6. It can also activate various immune cells such as macrophages, neutrophils, and dendritic cells. At the molecular level, MyD88 can stimulate the expression of MHC-II costimulatory molecules for antigen presentation. In the absence of MyD88, the innate immune response can be compromised. MyD88 activation leads to immune response polarization (Th1/Th17) and B cell activation. In macrophages and microglia, MyD88 is important for activating the inflammatory response.

MyD88 activation is essential for innate immunity and the body’s immediate defenses. Chronic MyD88 activation can lead to a cytokine storm with clinical consequences from hyperinflammation, or autoimmunity.

An excessive MyD88 response can be harmful in the CNS and mediate neurodegeneration, which has been seen in diseases such as MS, AD, PD, and ALS.

Conflict of interest

The authors declare that they have no conflict of interest.

REFERENCES

1. Casanova JL, Abel L, Quintana-Murci L. Human TLRs and IL-1Rs in Host Defense: Natural Insights from Evolutionary, Epidemiological, and Clinical Genetics. Annual Review of Immunology. 2011;29(1):447-491. doi:https://doi.org/10.1146/annurev-immunol-030409-101335
2. Murugaiyan G, Fujiwara M, Garo LP. Remyelination-Promoting Inflammation: Novel Role for MyD88 Signaling in Microglia/Macrophages. Trends in Neurosciences. 2020;43(7):455-457. doi:https://doi.org/10.1016/j.tins.2020.04.005
3. Li D, Wu M. Pattern Recognition Receptors in Health and Diseases. Signal Transduction and Targeted Therapy. 2021;6(1):1-24. doi:https://doi.org/10.1038/s41392-021-00687-0
4. Brown V, Brown Rachel A, Ozinsky A, Hesselberth Jay R, Fields S. Binding specificity of Toll‐like receptor cytoplasmic domains. European Journal of Immunology. 2006;36(3):742-753. doi:https://doi.org/10.1002/eji.200535158
5. Zhou J, Xiao Y, Ren Y, Ge J, Wang X. Structural basis of the IL-1 receptor TIR domain-mediated IL-1 signaling. iScience. 2022;25(7):104508. doi:https://doi.org/10.1016/j.isci.2022.104508
6. Broad A, Kirby JA, Jones DEJ. Toll-like receptor interactions: tolerance of MyD88-dependent cytokines but enhancement of MyD88-independent interferon-β production. Immunology. 2007;120(1):103-111. doi:https://doi.org/10.1111/j.1365-2567.2006.02485.x
7. Pereira M, Gazzinelli RT. Regulation of innate immune signaling by IRAK proteins. Frontiers in Immunology. 2023;14. doi:https://doi.org/10.3389/fimmu.2023.1133354
8. Huang X, Yang Y. Targeting the TLR9–MyD88 pathway in the regulation of adaptive immune responses. Expert Opinion on Therapeutic Targets. 2010;14(8):787-796. doi:https://doi.org/10.1517/14728222.2010.501333
9. Takeda K, Akira S. TLR signaling pathways. Seminars in Immunology. 2004;16(1):3-9. doi:https://doi.org/10.1016/j.smim.2003.10.003
10. Jeyaseelan S, Young SK, Fessler MB, et al. Toll/IL-1 Receptor Domain-Containing Adaptor Inducing IFN-β (TRIF)-Mediated Signaling Contributes to Innate Immune Responses in the Lung during Escherichia coli Pneumonia. The Journal of Immunology. 2007;178(5):3153-3160. doi:https://doi.org/10.4049/jimmunol.178.5.3153
11. Wang Y, Dilinuer Sadike, Huang B, et al. Regulatory T cells alleviate myelin loss and cognitive dysfunction by regulating neuroinflammation and microglial pyroptosis via TLR4/MyD88/NF-κB pathway in LPC-induced demyelination. Journal of Neuroinflammation. 2023;20(1). doi:https://doi.org/10.1186/s12974-023-02721-0
12. Croker BA, Lawson BR, Rutschmann S, et al. Inflammation and autoimmunity caused by a SHP1 mutation depend on IL-1, MyD88, and a microbial trigger. Proceedings of the National Academy of Sciences. 2008;105(39):15028-15033. doi:https://doi.org/10.1073/pnas.0806619105
13. Xiang W, Chao ZY, Feng DY. Role of Toll-like receptor/MYD88 signaling in neurodegenerative diseases. Reviews in the Neurosciences. 2015;26(4). doi:https://doi.org/10.1515/revneuro-2014-0067
14. Akeret K, Buzzi RM, Thomson BR, et al. MyD88-TLR4-dependent choroid plexus activation precedes perilesional inflammation and secondary brain edema in a mouse model of intracerebral hemorrhage. Journal of Neuroinflammation. 2022;19(1). doi:https://doi.org/10.1186/s12974-022-02641-5
15. Dutta D, Jana M, Majumder M, Mondal S, Roy A, Pahan K. Selective targeting of the TLR2/MyD88/NF-κB pathway reduces α-synuclein spreading in vitro and in vivo. Nature Communications. 2021;12(1):5382. doi:https://doi.org/10.1038/s41467-021-25767-1
16. Yamamoto M, Sato S, Hemmi H, et al. Role of adaptor TRIF in the MyD88-independent toll-like receptor signaling pathway. Science (New York, NY). 2003;301(5633):640-643. doi:https://doi.org/10.1126/science.1087262
17. Balka KR, Nardo D. Understanding early TLR signaling through the Myddosome. Journal of Leukocyte Biology. 2018;105(2):339-351. doi:https://doi.org/10.1002/jlb.mr0318-096r
18. Strickson S, Emmerich CH, Eddy, et al. Roles of the TRAF6 and Pellino E3 ligases in MyD88 and RANKL signaling. Proceedings of the National Academy of Sciences of the United States of America. 2017;114(17). doi:https://doi.org/10.1073/pnas.1702367114
19. Leaf IA, Nakagawa S, Johnson BG, et al. Pericyte MyD88 and IRAK4 control inflammatory and fibrotic responses to tissue injury. Journal of Clinical Investigation. 2016;127(1):321-334. doi:https://doi.org/10.1172/jci87532
20. Martinez J, Huang X, Yang Y. Toll-like receptor 8-mediated activation of murine plasmacytoid dendritic cells by vaccinia viral DNA. Proceedings of the National Academy of Sciences. 2010;107(14):6442-6447. doi:https://doi.org/10.1073/pnas.0913291107
21. Levitz SM. Interactions of Toll-like receptors with fungi. Microbes and Infection. 2004;6(15):1351-1355. doi:https://doi.org/10.1016/j.micinf.2004.08.014
22. George J, Motshwene PG, Wang H, et al. Two Human MYD88 Variants, S34Y and R98C, Interfere with MyD88-IRAK4-Myddosome Assembly. Journal of Biological Chemistry. 2011;286(2):1341-1353. doi:https://doi.org/10.1074/jbc.m110.159996
23. Dubois S, Feigenbaum L, Waldmann TA, Müller JR. NK cells prevent T cell lymphoma development in T cell receptor-transgenic mice. Cellular Immunology. 2020;352:104081. doi:https://doi.org/10.1016/j.cellimm.2020.104081
24. McDermott EP, O’Neill LAJ. Ras Participates in the Activation of p38 MAPK by Interleukin-1 by Associating with IRAK, IRAK2, TRAF6, and TAK-1. Journal of Biological Chemistry. 2002;277(10):7808-7815. doi:https://doi.org/10.1074/jbc.m108133200
25. Peng J, Yuan Q, Lin B, et al. SARM inhibits both TRIF- and MyD88-mediated AP-1 activation. European Journal of Immunology. 2010;40(6):1738-1747. doi:https://doi.org/10.1002/eji.200940034
26. Fitzpatrick JM, Minogue E, Curham L, et al. MyD88-dependent and -independent signalling via TLR3 and TLR4 are differentially modulated by Δ9-tetrahydrocannabinol and cannabidiol in human macrophages. Journal of Neuroimmunology. 2020;343:577217. doi:https://doi.org/10.1016/j.jneuroim.2020.577217
27. Ji J, Rao Y, Wan Q, Liao Z, Su J. Teleost-Specific TLR19 Localizes to Endosome, Recognizes dsRNA, Recruits TRIF, Triggers both IFN and NF-κB Pathways, and Protects Cells from Grass Carp Reovirus Infection. The Journal of Immunology. 2018;200(2):573-585. doi:https://doi.org/10.4049/jimmunol.1701149
28. Honda K, Yanai H, Negishi H, et al. IRF-7 is the master regulator of type-I interferon-dependent immune responses. Nature. 2005;434(7034):772-777. doi:https://doi.org/10.1038/nature03464
29. Fu Y, Xu B, Huang S, et al. Baicalin prevents LPS-induced activation of TLR4/NF-κB p65 pathway and inflammation in mice via inhibiting the expression of CD14. Acta Pharmacologica Sinica. 2020;42(1):88-96. doi:https://doi.org/10.1038/s41401-020-0411-9
30. Berod L, Stüve P, Swallow M, et al. MyD88 signalling in myeloid cells is sufficient to prevent chronic mycobacterial infection. European Journal of Immunology. 2014;44(5):1399-1409. doi:https://doi.org/10.1002/eji.201344039
31. Ciesielska A, Matyjek M, Kwiatkowska K. TLR4 and CD14 trafficking and its influence on LPS-induced pro-inflammatory signaling. Cellular and Molecular Life Sciences. 2020;78(4):1233-1261. doi:https://doi.org/10.1007/s00018-020-03656-y
32. Salcedo R, Christophe Cataisson, Hasan U, Yuspa SH, Giorgio Trinchieri. MyD88 and its divergent toll in carcinogenesis. Trends in immunology. 2013;34(8):379-389. doi:https://doi.org/10.1016/j.it.2013.03.008
33. Conti P, Ronconi G, Caraffa A, et al. Induction of pro-inflammatory cytokines (IL-1 and IL-6) and lung inflammation by Coronavirus-19 (COVI-19 or SARS-CoV-2): anti-inflammatory strategies. Journal of Biological Regulators and Homeostatic Agents. 2020;34(2):327-331. doi:https://doi.org/10.23812/CONTI-E
34. von Bernuth H, Picard C, Jin Z, et al. Pyogenic Bacterial Infections in Humans with MyD88 Deficiency. Science (New York, NY). 2008;321(5889):691-696. doi:https://doi.org/10.1126/science.1158298
35. García-García A, Pérez R, Flores C, et al. Humans with inherited MyD88 and IRAK-4 deficiencies are predisposed to hypoxemic COVID-19 pneumonia. The Journal of Experimental Medicine. 2023;220(5). doi:https://doi.org/10.1084/jem.20220170