1. Introduction
The positive impact of cannabis on human health has been known for many years [
1]. C. sativa plant contains many compounds, but the most well-known are cannabinoids, terpenes, and flavonoids. Cannabinoids act via the endocannabinoid system (ECS), which consists of receptors like cannabinoid 1 (CB1), cannabinoid 2 (CB2), transient receptor potential channels of the vanilloid subtype 1 and 2 (TRPV1, TRPV2), G protein-coupled receptors 18, 55, 119 (GPR18, GPR55, GPR119), endocannabinoids like 2-arachidonoylglycerol and anandamide (2-AG, AEA), and their biosynthetic and degradation enzymes. Homeostasis maintenance is the main function of ECS [
2]. The most studied phytocannabinoids produced by C. sativa plant are cannabidiol (CBD) and Δ9-tetrahydrocannabinol (THC). The effect of cannabis extracts is frequently different from single cannabinoids, with extracts often being more efficient – this is known as an entourage effect. This phenomenon is analogous to the orchestra when the complex interaction between minor cannabinoids, flavonoids, and terpenes are more potent in comparison to each individual compound [
3]. The main cannabinoid receptors, CB1 and CB2, are highly expressed on immune cells, such as macrophages, T lymphocytes, and B cells, as well as in lungs and the bronchial tissue [
4,
5]. Therefore, cannabis extracts, as well as their active ingredients, can modulate the function of immune system and inflammation in general.
Inflammation is a body’s vital defensive mechanism. Epithelial cells and macrophages are often the first line of defence against bacteria, viruses, and toxins [
6]. Bacteria express pathogen-associated molecular patterns (PAMPs) that are recognized by pathogen recognition receptors (PRRs) present on immune and non-immune cells. Lipopolysaccharide (LPS), being a component of gram-negative bacterial cell wall, is an example of PAMPs [
7]. Chronic inflammation plays an important role in the pathogenesis of many chronic conditions, like inflammatory bowel diseases, obesity, diabetes mellitus, atherosclerosis, asthma, Alzheimer’s disease, and many others [
8]. The uncontrolled and continuous release of pro-inflammatory cytokines is associated with organ damage and chronic disease progression [
9]. The most frequently induced cytokines are TNF-α, IL-1β, IL-1, IL-6, IL-8, IL-10, monocyte chemoattractant protein-1 (MCP-1), and IL-8. The pro-inflammatory cytokines (for instance, MCP-1 and IL-8) induce chemotaxis of neutrophils, monocytes, T cells, and dendritic cells to the site of infection and stimulate phagocytosis [
10,
11]. TNF-α and IL-1β are key mediators of the inflammatory response [
12,
13]. IL-6 stimulates inflammation and induces synthesis of acute phase proteins [
14]. The anti-inflammatory cytokines, like IL-10, function to limit a host immune response to pathogens, tissue homeostasis maintenance, the prevention of autoimmune conditions development [
15]. The balance between pro-inflammatory and anti-inflammatory cytokines is crucial for proper tissue homeostasis.
The anti-inflammatory effects of cannabis extracts, as well as its active ingredients, are well documented [
16]. For instance, cannabis users suffering from multiple sclerosis had lower levels of pro-inflammatory cytokines (TNF-α, IL-1, IL-6, IFN-γ), while higher levels of anti-inflammatory cytokine (IL-10) as compared to non-cannabis users [
17]. Cannabis extracts high in CBD and THC suppressed the expression of IL-6, IL-1β, MCP-1, and TNF-α genes induced by TNF-α/IFN-γ in human 3D EpiDermFT tissue [
18]. Cannabis extract high in CBD was demonstrated to be more potent than pure CBD in suppressing inflammation by preventing TNF-α production, reducing pain and paw swelling in the zymosan-induced inflammation mice model [
19]. Another study demonstrated that high CBD cannabis extract was superior to CBD in reducing IL-8 and IL-6 levels in vitro on the A549 alveolar epithelial cell line [
20]. In addition, cannabis extracts high in CBD and THC were shown to be more effective than pure CBD and THC in graft versus host disease in allogeneic bone marrow transplantation model by reducing inflammation and improving survival rate [
21]. Our previous work also demonstrated that cannabis extracts were efficient in reducing inflammation in skin fibroblasts, lung fibroblasts and human 3D tissues [
18,
22,
23].
In this work, we attempted to study the effects of cannabis extracts on the lipopolysaccharides (LPS)-induced cytokine release in human THP-1 leukemia monocytes. We aimed to establish the model of induction of inflammation and hypothesized that cannabinoids will prevent strong induction of inflammation. We further hypothesized that extracts differ from pure cannabinoids in their anti-inflammatory properties. Here, we found that inflammation was induced efficiently at 3 h after application of LPS and was partially prevented by exposure to CBD and various cannabis extracts. We also noted that IL-6, IL-10 and monocyte chemoattractant protein-1 (MCP-1) were more sensitive to pre-treatments with cannabinoids, as compared to IL-1β, IL-6 and TNF-α.
4. Discussion
In this work, we have established an LPS-induced model of induction of inflammation in THP-1 macrophages. We found that pro-inflammatory cytokines are induced the most at 3 h post exposure to LPS. We then demonstrated that pre-treatment with CBD, THC or extracts high in one of these two cannabinoids substantially decrease the induction of cytokines post-LPS. CBD was more efficient than THC, and extracts were more efficient than single cannabinoids. IL-6, IL-8 and MCP-1 were most sensitive to pre-treatments with cannabinoids, while IL-1β, IL-10 and TNF-α were much less responsive.
Several other models of inflammation were used in the past, including U937 [
30], and Mono Mac 6 [
31] cell lines. U937 cells are monocytes derived from tissue origin of human histiocytic lymphoma. These cells can also be transformed into macrophage-like phenotype and be stimulated with LPS for inflammation induction [
32,
33]. The response of U937 cells to LPs was not as efficient as in our work; only 7 out of 34 inflammation-related genes were induced [
34]. Stimulation of Mono Mac 6 cell line, a human monocytic line, with different triggers only upregulated TNF-α, IL-1α/β, and IL-6 [
35]. Another type of cells, that can be used in inflammation is human peripheral blood mononuclear cell (PBMC) derived macrophages. These primary cells are considered superior to cell lines, as they more resemble in vivo settings. The main disadvantages of primary cells are their short lifespan and heterogenic response among donors which arises during cell differentiation from progenitors [
36]. A comparison between the response of THP-1 cells and PBMC derived macrophages to LPS revealed a close correlation in inflammatory gene expression, making a THP-1 cell line a good model for studying LPS-induced changes [
34].
Many studies have demonstrated the effects of single cannabinoids, such as THC and CBD, on inflammation [
37]. Other components of the plant (such as minor cannabinoids, terpenes, terpenoids, flavonoids, and others) may act synergistically with cannabinoids and can be useful from a therapeutic point of view [
19]. The modulating effect of these compounds is known as "an entourage effect"; such modulation is typically positive which means that the medicinal effect of the whole plant extract is more significant than the effect of isolated compounds [
3,
38].
IL-1β is the most prominent pro-inflammatory cytokine which plays a crucial role in inflammasome signaling [
39]. IL-6, a pro-inflammatory cytokine, also plays an important role in the inflammatory response of many chronic inflammatory conditions [
40]. TNF-α cytokine stimulates inflammation and is one of the most studied and most important pro-inflammatory cytokines [
41]. IL-8 and MCP-1 induce chemotaxis of granulocytes to the site of infection and stimulate phagocytosis [
42,
43]. Our cannabis extracts significantly downregulated the levels of several of these cytokines, with IL-6, IL-8 and MCP-1 being affected the most. The results of this study were similar to other studies, where it was shown that cannabinoids and cannabis extracts efficiently inhibit inflammation by suppressing the levels of pro-inflammatory cytokines. For example, cannabis users that suffered from multiple sclerosis had significantly lower levels of many pro-inflammatory cytokines, like TNF-α, IL-1, IL-6, IL-12, IFN-γ, while higher levels of the anti-inflammatory cytokine, IL-10, in comparison to non-cannabis users [
44]. Another study reported that cannabis extract high in CBD was superior to pure CBD in reducing the IL-6 and IL-8 levels effectively in an alveolar epithelial cell line, A549, while cannabis extract high in THC on the same cell line showed only minor anti-inflammatory activity and was more cytotoxic. This study was using comparable concentration of extracts - 5 µg/mL, while in our study, we used 7 µg/mL [
20]. In a 3D EpiDermFT tissue inflammation model, it was shown that the extracts high in CBD and THC were effective in reducing the expression of IL-6, IL-1β, MCP-1, and TNF-α [
18]. Another study demonstrated that cannabis extract high in CBD was more effective than pure CBD in zymosan-induced inflammation mice model. The extract effectively reduced pain and paw swelling, prevented TNF-α production, and overcame the bell-shaped dose response of CBD [
19]. High CBD and high THC extracts were shown to be superior to the pure CBD and THC in reducing inflammation in graft versus host disease in allogeneic bone marrow transplantation model [
21].
The data we obtained on IL-1β levels was quite interesting. This was the only cytokine that decreased in response to CBD more efficiently than in response to the high CBD extract or THC/high THC extracts. It remains to be shown whether this is a specific response to our extracts or the nature of the response of this cytokine to cannabinoids.
It was curious also to note that CBD and THC had no effect on MCP-1 levels while our extracts significantly downregulated high MCP-1 levels post-LPS. Our data and reports by others demonstrated that MCP-1 can indeed be downregulated by cannabis extracts [
45]. MCP-1 was also downregulated in human Epithelial cells (BEAS-2B and NHBE), macrophages (U937), and lung fibroblast cells (HFL-1) exposed to LPS in response to CBD [
46]. In contrast, exposure of THP-1 to LPs with subsequent treatment with increasing concentrations of CBD did not reduce the MCP-1 levels [
47]. Therefore, it appears that decrease in MCP-1 expression in response to pure cannabinoids may be cell-specific.
Another interesting result was the effect of cannabinoids on IL-10 levels. IL-10 is an anti-inflammatory cytokine, which protects cells against profound inflammation. We observed that LPS induced this cytokine, and pure CBD as well as all extracts reduced it; in contrast, THC slightly increased it. Some studies demonstrated that cannabinoids and cannabis extracts enhance the levels of anti-inflammatory cytokines. For example, in the murine model of bone marrow transplantation, it was demonstrated that pure THC and CBD, as well as cannabis extracts high in CBD and THC, reduced inflammation, by reducing IL-17 secretion and enhancing IL-10 secretion [
21]. Another study performed on mouse primary bone marrow-derived macrophages stimulated by LPS showed that flavonoids, luteolin and quercetin, increased the levels of IL-10 secretion [
48]. It was also shown that THC reduced inflammation on endotoxemic mice model by significantly upregulating the plasma level of anti-inflammatory cytokine, IL-10, while suppressing the pro-inflammatory cytokine, MCP-1 [
49]. Along this line, it was reported that CBD and THC suppressed the secretion of IL-17 but elevated the secretion of IL-10 by mouse-derived encephalitogenic T cell line [
50]. CBD was reported to significantly reduce the plasma levels of pro-inflammatory cytokines (IFN-γ, TNF-α), along with the increase in the levels of anti-inflammatory cytokines (IL-10, IL-4) on diabetic mice model [
51].
Since LPS was used to induce the inflammatory response, it was expected that it would also induce anti-inflammatory cytokines to counteract high levels of pro-inflammatory cytokines, which is required for cell survival. For effective clearance of pathogens, the pro-inflammatory response is crucial, while excessive inflammatory response causes tissue damage. Hence, human body always maintains equilibrium by activating counteractive pathways to bring down pro-inflammatory response. Since levels of pro-inflammatory cytokines were elevated by LPS in our study, we observed the corresponding increase in the IL-10 levels in LPS group to counteract the increase in pro-inflammatory cytokines.
We hypothesize that since our extracts and CBD were able to significantly downregulate the levels of several pro-inflammatory cytokines, they reduced the levels of IL-10 possibly to maintain the equilibrium. On the other hand, THC was not able to reduce the levels of pro-inflammatory cytokines as effectively as CBD and hence, did not change the high levels of IL-10 post-LPS. It is also possible that cannabis extracts inhibit all cytokines, regardless of their nature.