Immunomodulation and SARS-CoV-2

Could plants with immunomodulation activity be useful in the treatment of SARS-CoV-2 infection?

SARS-CoV-2 belongs to the order Nidovirales, family Coronaviridae, and genus Betacoronavirus. This infectious entity is an enveloped virus of spherical shape (80–160 nm in length) with a single-stranded RNA genome (27–32 kb in length) in the nucleus. Its virion surface comprises mainly proteins such as a spike protein, envelope protein, small membrane protein, and nucleocapsid protein (Bhuiyan et al. 2020; Pal et al. 2020). SARS-CoV-2 has reached all continents, affected more than 430 million people, and caused almost 6 million deaths worldwide (WHO 2020). The human–human transmission of this virus occurs through close contact by coughing, sneezing, and respiratory droplets. It is classified as an asymptomatic infection (patients with high nasopharyngeal viral levels); a mild illness with characteristics such as fever, dry cough, shortness of breath, systemic fatigue, dyspnea, anosmia, ageusia, and reduction in oxygen saturation  <94%; a moderate illness with prostration, severe asthenia, fever  >38 °C, or persistent cough; a severe illness with lung lesion  >50%, respiratory rate  ≥30 breaths/min, oxygen saturation  ≤92% at a rest state, and aerial partial pressure of oxygen (PaO2)/inspired oxygen fraction (FiO2)  ≥300; and a critical illness (Gandhi et al. 2020). Severely infected SARS-CoV-2 patients are associated with increased cytokine levels called a “cytokine storm,” such as interleukins IL-2, IL-4, IL-5, IL-6, and IL-10, tumor necrosis factor-α (TNF-α), ferritin, macrophage inflammatory protein-1a, and D-dimer, lymphopenia (CD4 + and CD8 + T cells), and decreased IFN-γ expression in CD4 + T cells, C-reactive protein, and erythrocyte sedimentation rate. This triggers a hyperinflammatory pathological state, which consequently leads to overwhelming systematic inflammation, exacerbating viral pathogenesis and causing sepsis, acute respiratory distress syndrome, and multi-organ dysfunction or failure (Bhuiyan et al. 2020; Dutta et al. 2020). With this in mind, there is the possibility of finding potential therapeutic targets against some of these symptoms in COVID-19 patients.

Palliative drugs and vaccines against COVID-19 have been developed (Babich et al. 2020; Barbosa and Nunes de Carvalho 2021). The UK National Health Service offers treatments to people with coronavirus (COVID-19) who are at the highest risk of becoming seriously ill. The treatments available are as follows: (1) antiviral synthetic medicines, such as nirmatrelvir and ritonavir (Paxlovid or Bexovid), remdesivir (Veklury), and molnupiravir (Lagevrio); and (2) a neutralizing monoclonal antibody, sotrovimab (Veklury). However, the behavior of SARS-CoV-2 is not fully known, but mutations in new strains have caused new symptoms and unexpected effects. Herbal medicines may provide effective treatments to alleviate this serious health situation, either as a main therapy or as a secondary therapy combined with other medicines (Catanzaro et al. 2018; Babich et al. 2020). Nowadays, immunomodulators based on medicinal plants, such as Echinacea purpurea (L.) Moench, Asteraceae, or Curcuma longa L., Zingiberaceae, have become an alternative medicine for integrative therapies such as complementary medicine, probably because they are mostly considered to have fewer side effects based on popular beliefs (Di Sotto et al. 2020). A good alternative would therefore be to look for new plant species as a therapy against immune system dysfunctions, as their chemical components can act on the immune system, affecting the functions of immune cells directly (adaptive and innate immunity) or indirectly by modulating the function of non-immune cells, reducing inflammation, scavenging free radicals, and influencing the secretion of cytokines or modulating angiogenesis. We cannot rule out the possibility of direct antimicrobial and antiviral action from these chemical compounds (Babich et al. 2020; Behl et al. 2021).

Natural immunomodulators have a wide range of chemical structures and can act directly or indirectly in modulating the immune response. Plants have been demonstrated to be a rich source of immunomodulating agents such as alkaloids, terpenoids, polyphenols (e.g., phenolic acids and flavonoids), and sesquiterpene lactones. They also present immunomodulator agents originating from a plant’s primary metabolism, such as polysaccharides, glycoproteins and lectins, fatty acids, and other organic compounds (e.g., aldehydes, and primary/secondary alcohols) (Alhazmi et al. 2021; Barbosa and Nunes de Carvalho 2021; Behl et al. 2021).

Promising Plants

A total of 116 papers were included for the nine selected plants, summarizing botanical, ethnomedicinal, phytochemical, pharmacological, and clinical applications. Information on the immunomodulatory effects and their anti-inflammatory, antioxidant, cytotoxic, and antiviral activities were extracted from the full-text articles included. These are presented descriptively in table 1S.

Chenopodium quinoa

Chenopodium quinoa Willd., Amaranthaceae (popular name: “quinoa”), is considered a pseudo-cereal and has been cultivated as a food for centuries by native communities from the Andean highland (Peru, Bolivia, Ecuador, Chile, Argentina, and Colombia). Quinoa seed has been consumed in a similar way to rice, in soup, in breakfast cereals, or ground into flour to produce other food alternatives. Interestingly, fermented quinoa seeds are also used in alcoholic beverages for traditional ceremonies. Finally, quinoa leaves are eaten like spinach (Graf et al. 2015).

The plant usually reaches 1–2 m in height, branching with large panicles, and produces large to small flat grains, which are oval and usually pale-yellow in color, but can vary from pink to black, depending on the variety. Its main characteristic, compared with other grains, is its ability to grow in the most adverse climatic conditions (great flexibility) such as extreme temperatures (~4 to 38 °C), frost, very low rainfall, nutrient-poor soils with pH ranging from 6 to 8.5, and high salinity (40 mS/cm) (mainly Peru and Bolivia) (Graf et al. 2015); consequently, its cultivation has recently expanded to other countries, such as Canada, US, Australia, China, India, and England, among others (Pereira et al. 2019). According to the Food and Agriculture Organization (FAO 2022), quinoa is considered to be a promising plant for humanity.

Chemically, quinoa seeds contain carbohydrates (59–74%), constituted mainly by polysaccharides. Experimental studies have extracted homo- and heteropolysaccharide fractions (codified fractions such as CQP, QWP, QAP, and QPS1) with unit sugars such as d-mannose, d-xylose, rhamnose, maltose, arabinose, fructose, and glycose (Yao et al. 2014a; Graf et al. 2015; Hu et al. 2017; Fan et al. 2019); proteins (10–18%) were constituted by globulins (37% constituted by chenopodin, a globulin 11S-type protein), albumins (35%, a 2S-type protein), and traces of prolamins (0.5–7.0%) (Verza et al. 2012; Zevallos et al. 2012; Yao et al. 2014b; Capraro et al. 2020); lipids (4–10%); minerals (3–4%); vitamins (E, B group, and C); and carotenoids (lutein, zeaxanthin, and neochrome) (Graf et al. 2015).

The molecular richness coming from primary metabolism could be responsible for their pharmacological properties in the immune system. Nevertheless, the secondary metabolites identified in C. quinoa could also be responsible for its ascribed immunological properties. Quinoa triterpenoids-saponins with a tetracyclic or pentacyclic core are the most studied metabolites (Lin et al. 2019). El Hazzam et al. (2020) gathered information about quinoa seed saponins, highlighting oleanolic acid derivatives, such as hederagenin acid, spergulagenic acid, serjanic acid, phytolaccagenic acid, gypsogenin acid, and 3β-hydroxy-27-oxo-olean-12-en-28-oic acid. Other molecules, such as tocopherols, tocotrienols, steroids (e.g., ∆7-stigmastenol (51.3%), sitosterol (27.2%), and ∆7-avenasterol (8.7%)), polyphenols (e.g., benzoic acid derivatives such as gallic acid), cinnamic acid derivatives (predominantly ferulic acid and derivatives, as well as vanillic acid), and flavonoids (mainly kaempferol and quercetin and derivatives, as well as acacetin, myricetin, and daidzein) were also the focus in experimental studies (Gómez-Caravaca et al. 2011; Navruz-Varli and Sanlier 2016; Lin et al. 2019; El Hazzam et al. 2020).

The nutritional and pharmacological properties of quinoa have also been reported in research and review papers. Although numerous studies on vegetal species are focused on secondary metabolites, the primary metabolites of the quinoa seed reveal strongly pharmacological properties related to the immune system, anti-inflammatory, and inclusive, cytotoxic properties, which are the mainly target in the polysaccharide and protein fractions of some research (Verza et al. 2012; Zevallos et al. 2012; Yao et al. 2014a; Hu et al. 2017; Fan et al. 2019; Capraro et al. 2020). For instance, an early study by Zevallos et al. (2012) evaluated a protein fraction rich in prolamin on patients with celiac problems (an ex vitro method using an organ culture of a celiac duodenal biopsy sample) and showed not only its safety but also activation of the innate immune response, stimulating T cell lines and the secretion of some cytokines (IFN-γ and IL-15). Two years later, Yao et al. (2014a) observed an increasing production of IL-6, TNF-α, and nitric oxide (NO) in a dose-dependent manner (max. concentration at 200 μg/ml) using water-extractable polysaccharide fractions from quinoa. Likewise, a new quinoa polysaccharide promoted the proliferation of RAW264.7 macrophages, while suppressing NO production in a dose- and time-dependent manner (Hu et al. 2017). This study was corroborated by Fan et al. (2019), wherein quinoa crude polysaccharides (QPS1) successfully improved the levels of IFN-γ, IL-6, IFN-ɑ, IgM, and lysozymes in serum, enhancing the phagocytic function of mononuclear macrophages and reducing the allergic reaction in mice (Fan et al. 2019). Recently, Capraro et al. (2020) described the potential immunomodulation capacity and anti-inflammatory effects of fractions rich in chenopodin, the major protein of quinoa seeds. Chenopodin may exert biological effects on intestinal cell models, activating the canonical nuclear factor kappa B (NF-ƘB) signaling pathway and decreasing IL-8 expression.

Facing these findings, molecules belonging to secondary metabolism were also studied and ascribed immunomodulatory and anti-inflammatory properties (Lin et al. 2019). In quinoa, for example, Verza et al. (2012) observed a significant enhancement of humoral and cellular immune responses to ovalbumin in mice, promoted by two quinoa saponin fractions, a mixture of hederagenin (1), phytolaccagenic acid (2), serjanic acid (3), and oleanolic acid (4) glycosylated saponin derivatives. A decrease in the production of inflammatory mediators such as NO, TNF-α, and IL-6 was observed in lipopolysaccharide-induced RAW264.7 cells using other fractions of quinoa saponin at different concentrations (Yao et al. 2014b). Moreover, Lozano et al. (2013) reported significant anti-inflammatory effects for extracts of saponin and its isolated compounds, such as oleanolic acid, methyl oleanate (5), hederagenin, and phytolaccagenic acid, in ear and paw edema assays. This anti-inflammatory action could be complemented with phytosterols present in quinoa seeds, such as β-sitosterol (Lin et al. 2019). On the other hand, studies with polyphenols, e.g., ferulic acid (6), ferulic acid 4-O-glucoside (7), isoferulic acid (8), kaempferol (9), kaempferol 3-O-glucoside (10), and kaempferol 3,7-di-O-α-l-rhamnoside (11), are occasionally mentioned. The review article by Lin et al. (2019) highlighted some polyphenols in quinoa seeds and leaves that showed several pharmacological properties, including antioxidant and anti-inflammatory activity.

Chuquiraga spinosa

Chuquiraga spinosa Less., Asteraceae (popular name: “huamanpinta”), is an Andean evergreen shrub whose growth is mostly restricted to areas with elevations of  ~2500–4000 m (Peru, Bolivia, and Argentina). Its branched stems are semi-woody at the base and herbaceous in the aerial parts. The shrub can reach up to 1.5 m in height and shows sessile small leaves with axial hooks, and inflorescence like a calix, with numerous flowers of yellow-orange color. Infusions and decoctions of the stems and leaves have been widely used in folk medicine by Andean communities, basically for inflammation and infection of the genitourinary regions, problems related to the reproductive system, and respiratory diseases (Sotelo-Córdova 1998; Casado et al. 2011). Other plants, e.g., Bixa orellana L., Bixaceae, and Plantago major L., Plantaginaceae, are also reported to be added to C. spinosa infusion preparations (Perez-Chauca et al. 2020).

Despite its presence in national and international markets, and in addition to being included in the list of medicinal plants of the Formulario Nacional de Recursos Naturales y Afines (ESSALUD, Lima, Peru), phytochemical and pharmacological studies are scarce. The first studies were done by Casado et al. (2011) and Senatore (1996, 1999), and glycosylated flavonoids (quercetin-3-O-rutinoside, kaempferol-3-O-rutinoside, and kaempferol-3-O-glucoside), in addition to an acetophenone (p-hydroxyacetophenone), were isolated from hydroalcoholic extracts of the aerial parts (Senatore 1999). Years later, other flavonoids (e.g., kaempferol, quercetin, isorhamnetin, and some derivatives) were reported (Landa et al. 2009). Furthermore, seventy compounds, mainly carbonyl compounds, such as p-methoxyacetophenone and p-hydroxyacetophenone, were characterized from the essential oil of C. spinosa leaves, which include sesquiterpenes (β-humulene, ar-curcumene, cuparene, and spathulenol), monoterpenes, and phenylpropanoids (apiol), among others (Senatore 1996).

Only one study related to immunomodulation has been reported for this medicinal plant, in which the anti-inflammatory and antioxidant properties were related to flavonoids, such as 5,6,7-trihydroxy-4′-methoxyflavone (12), 3′,5,6,7-tetrahydroxy-4′-methoxyflavanone (13), 4′,5,7,8-tetrahydroxyflavone (14), and 5,7,8-trihydroxy-4′-methoxyflavone (15) (Ramírez et al. 2014). The administration of a chloroform extract of C. spinosa leaves to rats, at 200 mg/kg (48.23%) and 300 mg/kg (46.76%), resulted in an increase of the phagocytic activity of the macrophages, which was comparable to the positive control isoprinosine (59.9%). The anti-inflammatory activity of hydroalcoholic extracts was evaluated in paw edema induced by carrageenan, with results being comparable to ibuprofen (Ramírez et al. 2014). The results were corroborated by Casado et al. (2011) and Sotelo-Córdova (1998) using hydroalcoholic extracts from aerial parts, with indomethacin as the positive control. Both authors suggested that flavonoids and acetophenones, mainly p-hydroxyacetophenone (16), should be responsible for the anti-inflammatory action. Additionally, the hydroethanolic extract from aerial parts showed potent antioxidant activity and cytotoxicity properties with IC50 values ranging from 5 to 10 μg/ml, suggesting that kaempferol (10), quercetin (17), and rutin (18) glycosides could be the bioactive compounds (Herrera-Calderon et al. 2017).

Croton lechleri

Croton lechleri Müll.Arg., Euphorbiaceae (popular name: “sangre de grado”), is an Amazonian tree that grows up to 20 m in height. Its trunk has a diameter of 30 cm and shows a white or gray bark that exudes clear red and viscous sap when lacerated (Jones 2003; Rossi et al. 2011). This species occurs mainly in the Amazonian basin (Peru and Ecuador). Croton lechleri has been a well-known medicinal plant for centuries, reputed for its red latex-bearing sap. Products based on C. lechleri extracts are widely used by Amazonian communities, being included in the list of medicinal plants of ESSALUD and have been exported in recent decades as raw materials (Lock et al. 2016). Its ethnomedicinal uses include healing wounds, and inflammatory and septic processes (mainly skin conditions). This plant is also used to treat gastrointestinal ulcers, pyorrhea, menstrual cramps, fevers of digestive causes, and bleeding after childbirth; information collected through community use indicates that ca. 8 drops are administrated in almost all folk medicine uses, although this dose could reach 20–30 drops in an infusion combined with other aromatic plants (Jones 2003; Rossi et al. 2011; Lock et al. 2016). The sap obtained from the stem bark of C. lechleri has received great attention from scientists due to its use in folk medicine. Cai et al. (1991) suggested that abundant polyphenols could be participating in the regulation of the immune system. These polyphenols are mainly composed of proanthocyanidins (oligomerics and polymerics of flavan-3-ols), for instance, SP-303 (19), a proanthocyanidin heterogeneous oligomer. These chemical constituents are predominance in aqueous fractions from sap’s bark (Cai et al. 1991). Meanwhile, other molecules, such as catechin/epicatechin (20 and 21) or gallocatechin/epigallocatechin (22 and 23), were identified in low quantities (Cai et al. 1991). Two years later, the same research group identified clerodane-type diterpenes, such as korberin A (24) and B (25), from the bark (Cai et al. 1993). These and other chemical groups, such as flavonols (quercitrin, 26), lignans (3’,4-O-dimethylcedrusin from the sap, 27), and alkaloids such as sinoacutine (28) and glaucine (29) in the leaves, and taspine (30) (>1% dry wt. from sap) were highlighted by Jones (2003) in a review article. Meanwhile, other researchers also found 74 substances in the essential oil of C. lechleri (Rossi et al.