**2.3 Alkaloids**

The most valuable and essential plant compounds are alkaloids, which are also powerful medicinal agents [64]. The alkaloid family is the largest class of secondary plant chemicals with one or more nitrogen atoms, typically combined as part of a cyclic structure, which consists of around 5500 identified compounds. At least one nitrogen atom with a basic nucleus is present [65]. Other elements found in alkaloids include carbon, hydrogen, oxygen, sulphur, chlorine, bromine, and phosphorus. These organic substances are neutral or only slightly acidic. A few synthetic substances have structure similar to natural alkaloids. Numerous creatures, including fungus, bacteria, mammals, and plants, contain alkaloids [66]. It has numerous medicinal effects, including anti-malarial, anti-asthmatic, anti-cancer, cholinomimetic, vasodilatory, anti-arrhythmic, analgesic, anti-bacterial, and antihyperglycemic effects. Several alkaloids have stimulant and psychoactive effects on the central nervous system (CNS) [67]. Additionally, it has immunomodulatory effects. Immune effector cells trigger the main immunomodulatory pathways of alkaloids to cause autoimmune responses [68]. It interacts with the forskolin proteins, lysosomes, phagocyte vacuoles, and neutrophil, monocyte, and macrophage cytoskeleton filaments. In addition, it triggers the innate immune response by causing macrophages to perform phagocytic functions [69]. Moreover, active antigen-presenting cells (APCs) i.e. macrophages, contribute to the production of adaptive immune responses [70]. Potent alkaloids i.e., berberine (BBR) and colchicine are identified as potential immunomodulatory drugs.

Berberine (BBR), an isoquinoline alkaloid, is present in several Berberis species. Its chemical name is 5,6-dihydro-9,10-dimethoxybenzo[g]-1,3-benzodioxolo[5,6-a] quinolizinium compound. It has anti-diabetic, hepatoprotective, hypolipidemic, cancer-preventive, anti-hypertensive, anti-oxidant, anti-inflammatory, antidepressant, anti-diarrheal, and anti-microbial effects [71–73]. Multiple mechanisms of action are used to create these pharmacological effects. It interacts with multiple cellular kinases and signalling pathways. Some routes of the actions are interlinked with immunomodulatory pathways i.e., activation of nuclear factor erythroid-2 related factor-2 (Nrf2), MAPKs, NF-κB, & AMPK pathways; and expression of sirtuin 1 (SIRT1), & deacetylation of transcription factors of forkhead box O (FOXO) proteins [74]. In studies, BBR also lowers levels of proinflammatory cytokines such IFNγ, IL-17, IL-6, and TNF-α, which results in immunomodulatory effects in non-obese diabetic (NOD) mice [73]. The overexpression of NADPH oxidase 2/4 is also downregulated, which results in antioxidant effects [75]. Furthermore, BBR stimulates Nrf2 activities, which increases the production of antioxidant enzymes such as NADPH quinine oxidoreductase-1 (NQO-1) and heme oxygenase-1 (HO-1). Phosphatidylinositol 3-kinase (PI3K)/protein kinase B (Akt), P38, and AMPK pathways are a few of the different cell signalling pathways that are promoted by Nrf2 [76]. As a result, it can be employed as an immunomodulatory agent to treat autoimmune diseases.

A significant natural alkaloid called colchicine is derived from the Colchicum autumnale plant (Colchicaceae family). Colchicine is a medication that has anti-inflammatory effects and is used to treat various inflammatory diseases including pericarditis, gout, and familial Mediterranean fever. It is known to impede microtubule polymerisation and reduce the growth of several types of cancer cells as its primary mechanism of action. Additionally, it has been discovered to decrease immunological responses such as neutrophil chemotaxis,

lysosome breakdown, and leukocyte adhesiveness [77]. The immune cell signals i.e., nucleotide-binding and oligomerization domain (NOD), leucine-rich repeat proteins (LRR)-containing protein 3 and pyrin domain proteins [NLRP3 sensors] is suppressed by colchicine as well. SARS-CoV-2 proteins are also capable of initiating and energising comparable pathways. The therapy of COVID-19 with colchicine has tragically failed [78]. Moreover, it inhibits the production of leukotriene B4, neutrophil chemotactic factor and IL-1, which cause the immune cell regulatory action [79–81]. According to a literature review, colchicine can restrict procollagen synthesis, increase collagenase activity, and block mast cell release of histamine. It can also reduce the production of TNF-α that is caused by LPS [82–84]. Hence, autoimmune illnesses can be managed using their immunomodulatory effects.

#### **2.4 Glycosides**

The secondary metabolites of plants that contain sugar are called glycosides, a portion to which non-sugar portions are joined. The binding between the sugar and nonsugar moiety leads to hemiacetal formation. It includes the alcoholic or phenolic hydroxyl group of nonsugar and the aldehyde or keto group of the sugar moiety. These agents play numerous beneficial activities in animals and humans; however, many plants accumulate these chemicals in an inactive form which can be activated by the action of enzymes in the body [85]. Glycosides may be classified depending upon the glycone and aglycone moieties, such as glucoside, fructoside, α-glycosides and βglycosides. Amygdalin and scrocaffeside-A are identified as primary bioactive immunomodulating agents. These substances primarily work to stimulate the immunological, cardiac, and central neurological systems. Furthermore, glycosides also show substantial antibacterial effects [86].

The bitter almond, apricot, plum, apple, and peach fruit kernels contain amygdalin, a cyanogenetic glycoside. Leukoderma, colorectal cancer, emphysema, leprosy, bronchitis, and asthma are all conditions frequently treated by amygdalin [87, 88]. By controlling T-cells, amygdalin has been shown to suppress inflammatory reactions and enhance immunomodulatory effects [89]. Additionally, it activates caspase-3, which suppresses Bcl-2-like protein 4 (Bax, proapoptotic protein) and B-cell lymphoma 2 (Bcl-2, an antiapoptotic protein) [88]. It stops the metastases of cancer cells via prevention of β1 and β4 integrins expression leading to suppression of the Aktmediated mammalian target of rapamycin (mTOR) pathway (immunomodulatory mediators). Moreover, it lowers the β-catenin, integrin-linked kinase (ILK), and focal adhesion kinase (FAK) expression in immune cells [90]. Consequently, amygdalin is also an immunomodulatory agent.

Picrorhiza scrophulariiflora roots contain the caffeoyl glycoside known as scrocaffeside-A. It effectively heals leukoderma, inflammatory illnesses, gastrointestinal & urinary disorders, scorpion stings, and snake bites [91, 92]. Additionally, it controls the immunological responses of splenocytes by triggering concanavalin-A and LPS interactions [93]. According to an in vitro investigation, scrocaffeside-A increases CD4/CD8 population and cytokine production in splenocytes, which activates peritoneal macrophages and natural killer cell activity. Furthermore, it has been observed that scrocaffeside-A exposure increases the production of IFN-α, IL-2, IL-4, and IL-12 in cultured splenocytes. It suggests that scrocaffeside-A stimulates the host's immune system [94] and as a result, scrocaffeside-A is regarded as an immunomodulating substance.
