ReviewPlant-derived medicines: A novel class of immunological adjuvants
Introduction
The use of plant-derived medicines in the treatment and prevention of disease has been documented over five millennia. Plants were extensively utilised by ancient civilisations of Mesopotamia, Egypt, Greece, India and China for inflammation, cancer and for the maintenance of health [1], [2], [3]. In particular, popular plant-derived medicines such as Ginseng, Echinacea and garlic have persisted over time and remarkably, continue to be used today for indications similar to those described historically [4], [5]. In the early nineteenth century, the advent of modern medicine saw a rapid decline in botanical medicinal use. Vaccination, the discovery of antibiotics and improvements in medical technology all contributed to this demise. Today, plant-derived medicines are classified as complementary and alternative medicines (CAM) and are regulated by the Therapeutic Goods Administration (TGA) in Australia and by the Dietary Health and Supplement Education Act (DHSEA) under the Federal Drug Administration (FDA) in the USA [6], [7], [8].
It has been estimated by the World Health Organisation (WHO) that up to 80% of the world's population, mostly in developing countries, rely on plant medicines for primary health care [9], [10], [11], [12]. Countries such as Ethiopia, India and Rwanda have reported that 90%, 70% and 70% of the population, respectively, use plant-derived medicines routinely for health [13]. In Western countries, botanical medicinal consumption is on the increase owing to a variety of reasons including an inability of Western medicine to effectively treat certain diseases (e.g. AIDS, malaria, and cancer), a preference for natural alternatives, perceived efficacy and safety of natural products and a willingness to self-medicate [14], [15], [16]. The magnitude of botanical medicinal acceptance in Western society is realised by global market evaluations of at least US $15 billion per year with the WHO recently estimating the market at close to US $60 billion or 20% of the total drug market [17], [18]. In Australia, AU $1 billion is spent annually on CAM therapies comprising AU $200 million on plant-derived medicines [8].
Plant-derived medicines have contributed significantly to Western healthcare. Almost 30% of all FDA-approved pharmaceuticals currently available have a botanical origin [12], [19]. Drugs such as Digoxin, Taxol, Morphine, Salbutamol, Vincristine and Vinblastine are examples of common ‘Western’ prescription medicines based on plant compounds (Table 1; [20], [21]). Moreover, the pain-relieving properties of aspirin, a medicinal compound isolated from the bark of the willow tree and one of the most widely used over-the-counter medications today, was first described by Hippocrates [22]. Although plant medicines are used increasingly in Western society, there is still reluctance by the medical establishment to accept that these may have potential benefits to human health. Only the rigorous scientific investigation of plant-derived medicines will validate or refute any perceived or actual biological properties. Consumer use of plant medicines for health has reached unparalleled levels in Western society and has prompted the necessity for such investigations.
A major research interest has focussed on the immunomodulatory properties of plant-derived medicines. This review will discuss the evidence for plant-derived medicines to modulate immune responses particularly antigen-specific, adaptive immunity.
Section snippets
Immunomodulation by plant-derived medicines
The immune system is a complex defence network that protects the host from disease. Generation of an effective immune response typically involves the critical steps of antigen presentation, activation of T- and/or B-lymphocytes and the resultant secretion of immune effector molecules such as antibodies and cytokines. Indeed, the principles of vaccination rely on the ability to modulate these immune parameters to confer life-long protection from disease. Vaccines induce protective immune
The use of plant-derived medicines in healthcare
The historical record dictates that plants have been integrally associated with the practice of medicine. Over the past twenty years, the widespread use of these natural alternatives has provided the impetus for rigorous scientific investigation to validate the perceived benefits. Many plant-derived medicines are said to provide a ‘tonic’ effect that assists the body in the maintenance of health. From this, it may be reasonable to presume some immune basis for this effect, since improved
Conclusion
Evidence from the scientific literature supports the use of plant-derived medicines to stimulate immune function. Characterisation of these preparations in terms of biological activity and bioactive components will promote the utility of such preparations in the future. While the potential for use of plant-derived medicines should not be underestimated, the cellular and molecular mechanisms of action need to be clearly defined. One major limitation appears to be the lack of congruence in
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