Melatonin Controlling the Balance between Th17 & Treg Cells

Both Treg dysfunction and hyperactive responder T-cell proliferation contribute to disease. In this article, we will discuss the importance of regulating between Th17 and Treg cells.

Figure 1.  IL‐6  mediates Th17/Treg balance.
IL‐6 induces Th17 differentiation from naïve T cells together with TGF‐β.
On the other hand, IL‐6 inhibits Treg differentiation induced by TGF‐β.

IL-6: Regulator of Treg/Th17 Balance

Inflammatory T cells are thought to be central to the pathology of autoimmune diseases. Especially, the recently identified T‐cell subset, the Th17 cells, shows pro‐inflammatory functions and plays a critical role in various autoimmune disorders.

Recent studies have demonstrated that IL-6 has a very important role in regulating the balance between (see Figure 1)

• Th17 cells (T Helper 17 Cells)
• Is a key player in the pathogenesis of autoimmune diseases and protection against bacterial infections
• Both melatonin and active form of vitamin D (1,25-Dihydroxyvitamin D3) are  inhibitors for Th17 differentiation
• Treg (Regulatory T cells)
• Functions to restrain excessive effector T-cell responses

Dysregulation or overproduction of IL-6 leads to autoimmune diseases such as multiple sclerosis (MS) and rheumatoid arthritis (RA), in which Th17 cells are considered to be the primary cause of pathology. 

IL-6 is also central to the pathogenesis of both haemophagocytic lymphohistiocytosis (HLH) and cytokine release syndrome (CRS) and, several studies had shown a correlation between IL-6 levels and adverse outcomes in patients with COVID-19.^[13]

Given the critical role of IL-6 in altering the balance between Treg and Th17 cells, controlling IL-6 activities (i.e., by Tocilizumab an anti–interleukin-6 receptor monoclonal antibody) is potentially an effective approach in the treatment of various autoimmune and inflammatory diseases. 

Figure 2. Schematic representation of the host immune response
against microbial pathogens (Source: 12).

What's Treg?

Regulatory T cells (Tregs) are involved in shutting down immune responses after they have successfully eliminated invading organisms, and also in preventing autoimmunity:

• Come in many forms
• With the most well-understood being those that express CD4, CD25, and FOXP3 (CD4+CD25+ regulatory T cells). 
• These "Tregs" are different from helper T cells.
• Act to suppress activation of the immune system 
• Thereby maintain immunological homeostasis and tolerance to self-antigens (self Ags)
• Contribute to maintaining self-tolerance by down-regulating immune response to self and non-self Ags in an Ag-nonspecific manner.
• Elimination/reduction of CD4+CD25+ Tregs relieves this general suppression, thereby not only enhancing immune responses to non-self antigens (Ags), but also eliciting autoimmune responses to certain self-Ags. 
• Abnormality of this T cell-mediated mechanism of peripheral tolerance can be a possible cause of various autoimmune diseases
• Defects in FOXP3+ Tregs can elicit type 1 diabetes in most individuals regardless of other genetic or environmental influences, thus pointing to a key role for these cells in maintaining islet-specific tolerance.
• Therapies that increase the number or functional capacity of FOXP3+ Tregs can lead to prevention or cure of disease in preclinical models of autoimmunity, including type 1 diabetes
• Suppress Th2 cell function
• Treg from atopic individuals are defective in suppressing Th2 cell function compared with those from non-atopic individuals.

Figure 3: TCR recognition of MHC complexes. When a TCR binds an antigen-MHC complex displayed by a sick or infected cell, the T cell can induce cell death called apoptosis (top). In order for mature, antigen-recognizing T cells to develop without being self-reactive and causing autoimmunity, T cells must go through both positive and negative selection. In positive selection, T cells in the thymus that bind moderately to MHC complexes receive survival signals (middle). However, T cells whose TCRs bind too strongly to MHC complexes, and will likely be self-reactive, are killed in the process of negative selection (bottom).

Treg Selection/Activation

The process of Treg selection is determined by the affinity of interaction with the self-peptide MHC complex. Selection of a T cell to become a Treg is a “Goldilocks” process - i.e. not too high, not too low, but just right.^[9] A T cell that receives (see Figure 3):

• A very strong signals 
• It will undergo apoptotic death (i.e., negative selection)
• A weak signal 
• It will survive and be selected to become an effector cell (i.e., positive selection)
• An intermediate signal
• It will become a regulatory cell

Due to the stochastic nature of the process of T cell activation, all T cell populations with a given TCR will end up with a mixture of Teff and Treg – the relative proportions determined by the affinities of the T cell for the self-peptide-MHC.

Th17 

T helper 17 cells (Th17) are a subset of pro-inflammatory T helper cells defined by their production of interleukin 17 (IL-17). They are also characterized by:

• Th17s are developmentally distinct from Th1 and Th2 lineages (See Figure 2)
• Providing protection against viral infections, and they are also associated with the development of autoimmune diseases because of the recruitment of cells in the granulocyte lineage, especially neutrophils.
• The signals that cause Th17s to differentiate actually inhibit Treg differentiation.
• IL-6 induces the development of Th17 cells from naïve T cells together with TGF-beta; in contrast, IL-6 inhibits TGF-beta-induced Treg differentiation (See Figure 1). 
• Playing an important role in maintaining mucosal barriers and contributing to pathogen clearance at mucosal surfaces; such protective and non-pathogenic Th17 cells have been termed as Treg17 cells.[2]
• The loss of Th17 cell populations at mucosal surfaces has been linked to chronic inflammation and microbial translocation.
• Th17 cell differentiation
• TGF‐β, IL‐6, IL‐1β, and IL‐23 promote Th17 cell differentiation.
• Both melatonin and active form of vitamin D (1,25-Dihydroxyvitamin D3)  inhibit the differentiation of Th17 cells.
• mTOR signaling, which integrates input from insulin, growth factors, and amino acids, while sensing cellular nutrient and energy levels, also regulates Th17 cell differentiation and IL‐17 gene expression.

Figure 4.  Melatonin had an anti-senescent effect in cADMSCs by inhibiting ERS through activation of rhythmic expression of NRF2, activating the ERAD pathway, and inhibiting the NF-κB pathway (Source: [15]).  NF-κB is a major transcription factor that regulates genes responsible for both the innate and adaptive immune response.  Through a cascade of phosphorylation events, the kinase complex is activated and NF-κB is able to enter the nucleus to upregulate genes involved in T-cell development, maturation, and proliferation.

Melatonin 

Based its wide distribution in cells of mammals, it is not surprising that melatonin affects a variety of molecular pathways including those for sleep, circadian rhythms, sexual behavior, immune function, apoptosis, proliferation, metastasis, and angiogenesis oxidative stress.

Melatonin is reported to exert biorhythm regulation, anti-oxidation,^[16] and anti-senescence effects^[15]  in various animal and cell models.

Melatonin significantly influences T-cell-mediated immune responses, which are crucial to protect mammals against cancers and infections, but are associated with pathogenesis of many autoimmune diseases. 

Through modulation in T-cell responses, melatonin exerts beneficial effects in various inflammatory diseases, such as 

• Type 1 diabetes
• In which effector T cells destroy beta cells while Treg counter this assault
• Systemic lupus erythematosus
• Multiple sclerosis
• Melatonin alleviates the clinical symptoms of EAE in mice 
• EAE in mice is a well‐known animal model for MS because there are numerous pathological and histological similarities shared by EAE and MS. 
• The secretion of melatonin is inversely correlated with clinical relapses of Multiple sclerosis (MS) in humans in an epidemiological study
• COVID-19
• In one study, it confirmed the effectiveness of melatonin in treating mild to moderate outpatients with COVID-19.^[14]  

Mechanistically, melatonin 

• Limits peripheral and central Th1/Th17 responses 
• By inhibiting the differentiation of Th17 cells
• Limits the T effector memory population
• Elevates responses of Treg cells

References

1. Melatonin signaling in T cells: Functions and applications (important)
2. Role of Endogenous Melatonin in the Regulation of Th17/Treg Balance during Pregnancy
3. Immunologic self-tolerance maintained by activated T cells expressing IL-2 receptor alpha-chains (CD25). Breakdown of a single mechanism of self-tolerance causes various autoimmune diseases.
4. The Type of Responder T-Cell Has a Significant Impact in a Human In Vitro Suppression Assay
5. Critical stoichiometric ratio of CD4+ CD25+ FoxP3+ regulatory T cells and CD4+ CD25− responder T cells influence immunosuppression in patients with B-cell acute lymphoblastic leukaemia
6. Study on ratio imbalance of peripheral blood Th17/Treg cells in patients with rheumatoid arthritis
7. Ratio balance of Th17 and Treg cells in peripheral blood of patients with chronic lymphocytic leukemia
8. A bovine whey protein extract can induce the generation of regulatory T cells and shows potential to alleviate asthma symptoms in a murine asthma model
9. Li M (August 2016). "T Cell Receptor Signaling in the Control of Regulatory T Cell Differentiation and Function". Nature Reviews Immunology. 16 (4): 220–233.
10. IL-6: regulator of Treg/Th17 balance
11. Recognition of self-peptide–MHC complexes by autoimmune T-cell receptors
12. Intracellular Pathogens: Host Immunity and Microbial Persistence Strategies
13. Coomes, E. A. & Haghbayan, H. Interleukin-6 in Covid-19: a systematic review and meta-analysis. Rev. Med. Virol. 30, e2141 (2020).
14. A Pilot Study on Controlling Coronavirus Disease 2019 (COVID-19) Inflammation Using Melatonin Supplement
15. Melatonin prevents senescence of canine adipose-derived mesenchymal stem cells through activating NRF2 and inhibiting ER stress
16. Reiter RJ, Tan DX, Rosales-Corral S, Galano A, Zhou XJ, Xu B. Mitochondria: central organelles for melatonin’s antioxidant and anti-aging actions. Molecules. 2018; 23:509.
17. Bruns DR, Drake JC, Biela LM, Peelor FF 3rd, Miller BF, Hamilton KL. Nrf2 signaling and the slowed aging phenotype: evidence from long-lived models. Oxid Med Cell Longev. 2015; 2015:732596.
18. Hiebert P, Wietecha MS, Cangkrama M, Haertel E, Mavrogonatou E, Stumpe M, Steenbock H, Grossi S, Beer HD, Angel P, Brinckmann J, Kletsas D, Dengjel J, Werner S. Nrf2-mediated fibroblast reprogramming drives cellular senescence by targeting the matrisome. Dev Cell. 2018; 46:145–161.
• This suggests that time-controlled activation of NRF2 may be critical for homeostasis in multicellular organism.
19. Antigenic drift: Understanding COVID-19 (good)