A Systematic Review of the Anti-Inflammatory and Immunomodulatory Properties of 16 Essential Oils of Herbs

Background Inflammation is a host defense mechanism in the body after it is infected and damaged. If inflammation is not treated in time, then it may cause a variety of diseases, such as cancer and autoimmune diseases. Herbal essential oils are natural extracts that can suppress inflammation effectively and are expected to be used in therapeutic drugs for anti-inflammatory diseases in the future. Aim of the review. We review the anti-inflammatory and immunomodulatory effects of essential oils derived from 16 herbs. Materials and methods. We searched the literature of the fields of anti-inflammatory and immunomodulatory herbal essential oil activity published in English within the past five years via databases (PubMed, EMBASE, Scopus, and The Web of Science). Results A total of 1932 papers were found by searching, and 132 papers were screened after removing duplicates and reading article titles. Fifteen articles met the requirements to be included in this review. Among those selected, 11 articles reported in vivo research results, and 10 articles showed research results. Conclusion Essential oils extracted from herbs can reduce inflammation by regulating the release of inflammatory cytokines involved in multiple signalling pathways. Herbal essential oils are expected to be developed as anti-inflammatory drugs.


Introduction
Inflammation is a defense mechanism against infection and tissue damage [1]. Inflammation and proinflammatory cytokines, reactive oxygen species (ROS), lipid extraction medium: arachidonic acid (AA), hydrolases, transcription factors, etc., are closely related [2]. When not stopped in time, inflammation can cause diabetes and cancer [3], arthritis, Alzheimer's disease, atherosclerosis, cardiovascular disease, eye diseases, and autoimmune diseases, including inflammatory bowel disease [4]. Some clinical and physiopathological data also showed that inflammation could also affect children with inflammatory bowel disease, uveitis, and juvenile idiopathic arthritis by slowing their growth in height and weight [5]. Inflammation seriously affects people's lives.
Inflammation can usually be classified into two categories according to its course: acute inflammation and chronic inflammation. e immune system plays a key role in determining whether inflammation is acute or chronic. e activation of inflammation is closely related to immune cells and biological molecules. In particular, innate lymphoid cells (LCs), with multiple immune functions, play important roles in inflammatory diseases [6]. Infiltration of macrophages and neutrophils is a feature of acute inflammation, and infiltration of T lymphocytes and plasma cells is a feature of chronic inflammation [7]. ese cells play important roles in the inflammatory response.
Steroidal and nonsteroidal anti-inflammatory agents are commonly used in the clinical treatment of inflammatory diseases. Long-term use of these drugs can cause serious adverse reactions, such as gastrointestinal tract, cardiovascular, and liver abnormalities [8]. Finding a safe and effective drug to control inflammation has always been a challenge. In recent years, Chinese herbal oils have been rated as the safest, most promising anti-inflammatory drug candidate [2]. body's immune state to attenuate inflammation [9][10][11]. Volatile Chinese medicine oils are mixtures of multiple compounds [12]. e main components are monoterpene and sesquiterpene, among others, with anti-inflammatory, antibacterial, acaricidal, antiviral, sedative, antianxiety, and antidepression properties [13]. e anti-inflammatory effect of volatile oils of traditional Chinese medicine is mainly realized through the regulation of cyclooxygenase (COX) and induction of nitric oxide synthase (iNOS) and a variety of cytokines, which play roles in the inflammation process [14]. In recent years, essential oils of traditional Chinese medicine have been described as potential sources for the development of new drugs.

Materials and Methods
e review was conducted following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement [15].

Search Strategy.
Every publication in English that was reviewed for this study was extracted from the PubMed, Scopus, EMBASE, and Web of Science databases restricted to the Medical Subjects Headings Index (MeSH/DeCS) to April 2020. e search was based on different combinations of the following keywords: "essential oil", "oils, essential," "essential oils," "volatile oils," "inflammation," "inflammations," "innate inflammatory response," "inflammatory response innate," and "innate inflammatory responses." Furthermore, we reviewed the references in the selected articles for additional reports not included in the original article search.

Study Selection.
e two authors independently extracted and proofread the titles and abstracts of each article. e inclusion criteria were the effects of volatile oils and the active components of traditional Chinese medicine on inflammatory diseases, including in vivo and in vitro models and the possible mechanisms of action. e authors excluded articles based on the following criteria: review articles, meta-analyses, abstracts, editorials/letters, conference proceedings, case reports, studies in humans, and articles published more than 5 years ago. e differences of opinion between the two authors were independently decided by the third author.

Data Extraction.
One author summarized the data from the article, and the other author examined it. Table 1 summarizes the following information from the in vitro experiments: the source of the essential oil, species, inflammation type, test index, cell line, the proposed mechanism (s) of biochemical effects, and conclusion. Table 2 summarizes the following information from the in vivo experiments: the source of the essential oil, species, inflammation type, test index, experimental animal species, the proposed mechanism (s) of biochemical results, and conclusion.

Methodological Quality Assessment.
Preclinical in vivo studies of bias risk and quality were based on an optimized checklist [15,31].
is survey mainly involved the randomization of animal feeding and distribution, blind administration, blind results, and other factors used to evaluate the included methodology.

Data
Analysis. Due to the heterogeneity of this study, pooled statistics and meta-analysis were not used. e data analysis is presented in narrative form.

Results and Discussion
3.1. Search Results. Figure 1 presents a flow chart of the search; 1154 articles were identified (PubMed: 115, EMBASE: 567, Scopus: 518, and Web of Science: 732) after duplicate articles had been deleted. After reading the title, we removed articles that were unrelated to essential oils and inflammation and 132 were thus retained. Finally, 15 articles were selected after full-text browsing, among which 12 articles included in vivo experiments, 9 articles included in vitro experiments, and 7 articles included in vivo and in vitro experiments.

Study Characteristics and Description.
ere were 12 in vivo and 9 in vitro experiments. e influence of essential oil and its components on the inflammatory disease model was investigated in vitro using an inflammatory disease model constructed with a variety of cell lines. Using in vivo experiments, 12 articles studied the effects of traditional Chinese medicine essential oils and their active components on a variety of inflammatory disease models based on mice, rats, or rabbits. e oils showed a significant inhibitory effect on inflammation. Part of the chemical structure of traditional Chinese medicine essential oils is shown in Figure 2. Of the studies examined, 9 articles were from China, 2 were from India, 1 was from Iran, and 1 was from South Korea.
In vitro experiments were performed in human aortic endothelial cells (HAECs), a human keratinocyte cell line (HaCaT), the THP-1 cell line, mouse primary splenocytes and peritoneal macrophages, RAW264.7 cells, and ANA-1 cells. Many in vitro experimental methods were reported in the articles, including immunohistochemical analysis, apoptosis assay, and histological assessment, and proinflammatory cytokines were quantitatively measured by ELISAs, with western blotting and RT-PCR used to detect the expression of various proteins and mRNA, respectively.
In vivo models of inflammation included dextran sulfate sodium (DSS) induced intestinal inflammation, TPA-induced mouse skin inflammation, carrageenan-induced paw oedema, xylene-induced ear oedema model, 2, 4-dinitrofluorobenzene (DNFB)-induced allergic contact dermatitis (ACD), and ischaemic renal rat models. A number of experimental methods were used in the in vivo experiments, including the acetic acid-induced writhing test, oxytocininduced dysmenorrhea in mice, formalin test, complete Freund's adjuvant-induced overt pain test, carrageenaninduced mechanical hyperalgesia test, evaluation of cytokine Essential oil from atractylodes macrocephala Compositae ANA-1 cells.

LPS-induced
NO --is essential oil reduces NO levels after stimulation and suppresses inflammation [23] Evidence-Based Complementary and Alternative Medicine 3  Evidence-Based Complementary and Alternative Medicine 5   Evidence-Based Complementary and Alternative Medicine is a complex biological response to harmful stimuli [32]. Lipopolysaccharide (LPS) is a component of the cell wall of Gram-negative bacteria that causes inflammation by activating MAPK, nuclear factor-kB (NF-kB), and activator protein-1 (AP-1) signalling pathways [18]. In the inflammatory response of cells, COX-2 inhibition exerts a strong anti-inflammatory effect [33]. COX-2 overexpression can also be used as an inflammatory model. e essential oil from Gynura procumbens (GPEO) and its three active components, a-pinene [34], 3-carene, and limonene, [35] can inhibit the inflammatory cell infiltration induced caused by noxious stimuli [36]. ey have a significant pharmacological effect on the migration of RAW264.7 macrophages induced by LPS and inhibit COX-2 overexpression [37]. Because of its good permeability and anti-inflammatory properties, GPEO is a great treatment for relieving skin redness or treating poisonous insect bites. e essential oil from Artemisia argyi (AAEO) [38] can inhibit the release of inflammatory mediators (NO and PGE2) and the expression of cytokines (IL-6, IFN-β, IL-10, and MCP-1). AAEO inhibited the expression of iNOS and COX-2 at the transcription level without affecting its activity [39]. Moreover, AAEO also inhibited the phosphorylation of STAT1 (Tyr701) and STAT3 (Tyr705) and downregulated JAK/STATsignalling and ROS scavenging. ROS production plays a key role in the activation of JAK/STATs in macrophages. According to a previous report, essential oils scavenging ROS have antioxidant activity; therefore, the inhibitory effect of AAEO on JAK/ STATmay be due to its antioxidant activity, especially its effect on ROS production [14,40].
In intestinal inflammation diseases, the levels TNF-α, IL-1β, IL-6, and IL-12 are significantly higher than those in noninflamed intestines [41]. Intestinal inflammation is often accompanied by mucosal immune system disorders. Researchers found that Maqian fruit essential oil (MQEO) reduced LPS-stimulated expression of TLR4 in THP-1 cells and effectively suppressed the production of IL-1β and TNF-α in a dose-dependent manner, and in the past, researchers found that curcumin and ellagic acid [42], which are naturally occurring plant phenols, exhibit anti-inflammatory activity by preventing IκB degradation [42]. Linalool is the main component of the essential oil from the blossoms of Citrus aurantium L. var. amara Engl (CAVAO) [43]. It can suppress inflammatory symptoms by suppressing NF-κB P65 translocation. It blocks the phosphorylation of IKK and IκB [44,45]. e anti-inflammatory effect of MQEO may also be related to the TLR4-mediated NF-κB signalling pathway. ese two kinds of traditional Chinese medicine essential oils have very good prospects in intestinal inflammatory diseases.
Essential oil extracted from fructus Alpinia zerumbet (EOFAZ) [46] changes the expression of ICAM-1 and VCAM-1 and promotes the adhesion of inflammatory cells. Target genes [47] for NF-κB include ICAM-1 and VCAM-1. Atherosclerosis is characterized by chronic systemic inflammation and the formation of large and medium-sized atherosclerotic plaques. e amelioration of inflammation plays an important role in the treatment of this disease [48,49]. Endothelial cells can promote the adhesion of inflammatory cells by changing the expression of ICAM-1 and VCAM-1 [50], which is believed to play an important role in the occurrence of inflammation and the pathogenesis of atherosclerosis [51]. EOFAZ may serve as a new drug source for the treatment of atherosclerosis.
Mast cells play important roles in allergic inflammation [52]. e essential oil from Zanthoxylum coreanum Nakai (ZCO) has been shown to effectively inhibit mast cell degranulation and reduce the level of IL-4, a key inflammatory factor in allergy symptoms [53].
irtyseven active ingredients identified from ZCO, mainly ßocimene and (-)-α-pinene, can alleviate allergic inflammation by suppressing the MAPK and NF-κB signalling pathways. ZCO has good transdermal absorbability [54] and will likely be used to treat allergic inflammation in the future.
e skin is the largest sense organ in humans and plays an important role in immune defense [55]. An inadequate immune defense can cause some types of dermatitis. Skin inflammation is closely related to the levels of cytokines and reactive oxygen species [56]. e essential oil from Curcuma longa (EOCl) [57] significantly reduced the levels of TNF-α, IL-6, and IL-1 in inflammatory models. e main components of EOCl are terpinolene and phellandrene. e essential oil from Citrus limetta Risso peels (Clp-EO) is also a candidate for treating skin inflammation [58]. Its main ingredient is limonene, which is not cytotoxic. It can inhibit the levels of TNF-α, IL-6, and IL-1β in inflammation models and can reduce the levels of ROS under oxidative stress conditions. ese two kinds of Chinese herbal essential oils can reduce the levels of various oxidative stress markers. ey play antioxidant roles, including a therapeutic role in skin inflammation. e essential oils extracted from crude A. macrocephala (CA) and bran-processed A. macrocephala (BA) are CAEOs and BAEOs [59]. e main components of these two essential oils are Atractylone [60]. Atractylone effectively inhibited the level of NO in an allergic rhinitis (AR) model [61]. Atractylone can reduce AR biomarkers and exert a certain anti-inflammatory effect. CAEOs and BAEOs not only exert anti-inflammatory effects but also inhibit gastric cancer, intestinal cancer, and liver cancer cells [62]. ese two essential oils have the potential to treat AR. Essential oils can be inhaled into the nasal cavity, which is the best route of administration [63]. ey will be the preferred raw materials for the treatment of AR in the future.
Acorus gramineus (AG) and Evodia ruticarpa (ER) steam-distilled essential oils (SDEOs) are rich in phytochemicals such as total flavonoids, polyphenols, and saponins. e balance between T helper 1 ( 1) lymphocytes and T helper 2 ( 2) lymphocytes is important for human health [64]. ese two essential oils can regulate the 1 and 2 immune balance and inhibit the TNF-α/IL-10 cytokine ratio during the inflammatory response of macrophages, showing a certain anti-inflammatory ability [65]. 8 Evidence-Based Complementary and Alternative Medicine

Anti-Inflammatory Effects of Traditional Chinese
Medicine Essential Oils In Vivo. For in vivo tests of essential oils of traditional Chinese medicine with respect to anti-inflammatory diseases, most articles reported the use of chemical reagents to induce inflammation. Essential oils of Chinese medicine have inhibitory effects on inflammation induced by chemical agents. ey play inhibitory roles in inflammatory diseases such as skin inflammation and intestinal inflammation, 12-O-tetradecanoylphorbol-13-acetate (TPA) [66] can be used to induce skin inflammation in vivo [67]. After 4 hours of skin exposure to TPA, COX-2 expression in the skin increased significantly, and ear thickness increased significantly. Histological analysis of mouse ear slices can be used to observe the occurrence of inflammation. Skin oedema after successful modelling is obvious, showing many infiltrating neutrophils. is ear oedema model is an ideal in vivo model for screening anti-inflammatory drugs. e essential oils from Gynura procumbens, Citrus limetta Risso, Curcuma longa, and Artemisia argyi use the TPA-induced inflammation model in mice, and all four essential oils can reduce the degree of ear swelling in mice [21]. e overproduction of oxidative markers can be regulated by reducing the expression level of COX-2 protein-improved levels of proinflammatory cytokines (TNF-α, IL-6, IL-1). Immunohistochemical methods have shown that Chinese medicine essential oil can restore histopathological damage to a great extent. It plays a significant role in inhibiting inflammation.
In the essential oil extracted from Cinnamomum cassia Presl, a variety of substances, such as benzenepropanal, (E)cinnamaldehyde and turmerone, were detected [26]. ey can reduce the inflammatory response after different kinds of stimuli. After formalin and CFA were injected into the foot of the mice [68], the mice that took oral oil significantly improved their paw licking behavior. At the same time, the levels of TNF-α, NO, IL-1β, and PGE 2 in paw oedema tissues induced by carrageenan were reduced in a dosedependent manner [68]. EO showed an inhibitory effect on acetic acid-induced abdominal contraction. e expression of COX-2 and iNOS in the skin tissues of the feet of mice with essential oils decreased. is shows that this essential oil has a certain anti-inflammatory effect.
Ischemia reperfusion (I/R) injury is a major problem that occurs after a kidney transplant, and it can lead to inflammation [69]. Lavender oil has a certain anti-inflammatory effect on a rat kidney I/R injury model [29]. Oral essential oil can improve renal function by reducing the level of oxidative stress in the pathological cascade caused by renal I/R. Lavender oil can effectively resist renal I/R injury by targeting oxidative stress and apoptosis.
In in vivo models of intestinal inflammation, DSS is generally used to stimulate the intestinal epithelial cells of mice [70], causing mice to produce various symptoms of intestinal inflammation, such as diarrhoea and granulocyte infiltration. Maqian essential oil can reduce MPO and MMP-9 levels in inflammatory tissues, and its anti-inflammatory activity may be related to the NF-κB signalling pathway.
Acetic acid [71] is a common chemical that causes acute colitis. Injection of acetic acid into the colon cavity can cause acute inflammation of colon tissue. TNF-a is closely related to ulcerative colitis. Anise essential oil reduces inflammation by regulating TNF-a levels and inhibiting acetic acid-induced myelin peroxidase activity in tissues [71].
In the LPS-induced in vivo inflammation model, both Angelica essential oil significantly affected the expression of cytokines, inflammatory mediators, and inflammationrelated enzymes in the inflammation rat model. is occurs mainly by regulating the expression of the endothelial adhesion molecules ICAM-1 and VCAM-1, thereby preventing the inflammation induced by LPS.
For establishing skin inflammation models, a variety of chemicals can be used to cause skin inflammation, such as xylene-induced ear oedema, carrageenan-induced paw oedema, DNFB-induced [72] allergic contact dermatitis (ACD), DNCB-induced ear swelling, and AD-like skin lesion models. Essential oils can be used to evaluate the antiinflammatory effects of anti-inflammatory drugs in vivo. Ginger essential oil can protect against xylene-, carrageenanand DNFB-induced cutaneous inflammation. e essential oil from Zanthoxylum coreanum Nakai inhibited DNCBinduced ear swelling and AD-like skin lesions. Figure 3 introduces the methodological features of this review article. e pedigree, frequency of treatment, administration route, and dose of the oils given to experimental animals were clearly described in all the articles. e main objectives and findings of the studies were clearly expressed. However, not all reports reported blind-assessment procedures. Only a few articles reported blind breeding and blind allocation of experimental animals. Figure 4 and Figure 5 describe the year and country of publication of each article reviewed. Traditional Chinese medicine, as a medicinal material with Chinese herbs, is widely studied not only in China but also in India, South Korea, and Iran. is breadth of study reflects the contribution of traditional Chinese medicine to the development of new drugs around the world. Overall, China has contributed much of value to the world, and traditional Chinese medicine is one of these contributions. Traditional Chinese medicine has been widely used around the world, promoting the development of new drugs based on plant species. As identified by the year of publication, the number of publications has increased gradually in recent years, indicating that the use of traditional Chinese medicine essential oils as anti-inflammatory agents has attracted increasing attention from researchers around the world.

Methodological Quality/Risk of Bias.
Evidence-Based Complementary and Alternative Medicine Has the calculation of the sample size been reported?
Has the protocol for animal experiments been approved by the "Animal Care Committee"?
Has the hypothesis/purpose/target of the study been cleared?
Are the main findings of the study clearly described?
Is the distribution report adequate?
Did the investigator turn a blind eye to the treatment used?
Are the animals randomly raised in the experiment?
Are the people who evaluate the results blind?
Are there adequate reports on the dosage and route of administration?
Is the frequency of treatment adequately described?
Is the age/strain of the animal clearly reported?

Conclusion
is review shows that Chinese herbal oils have great potential as naturally extracted compounds in the treatment of inflammatory diseases. e essential oils of traditional Chinese medicine have shown good antiinflammatory effects in various inflammatory disease models. e essential oils of traditional Chinese medicine can regulate the levels of various cytokines and inhibit multiple signalling pathways that trigger responses to inflammatory diseases, as shown in Figure 6. Most of the articles reviewed in this article were of medium quality, and there are still areas for improvement in some areas. However, overall, the essential oils of traditional Chinese medicine will likely play important roles in the development of new anti-inflammatory drugs in the future.
In 15 articles, we found that essential oils of traditional Chinese medicine have excellent performance in inhibiting signal pathways such as NF-κB, MAPK, and AKT. Numerous antigens will be recognized by the corresponding T cell receptors, B cell receptors, and Toll-like receptors, cytokines such as TNF and IL will be recognized by TNF receptors and a variety of IL receptors, which will trigger the three inflammatory signal pathways mentioned above. In addition, oxidative stress can also trigger inflammation signalling pathways. ese three common inflammatory signal pathways are inhibited, expounding the anti-inflammatory mechanism of traditional Chinese medicine essential oils.

Conflicts of Interest
e authors declare that they have no conflicts of interest.