Polysaccharides, crucial bioactive macromolecules, are polymers consisting of at least twenty monosaccharides linked by glycosidic linkage [1]. Polysaccharides from natural sources have broader bioactivity due to the inherent property of polyvalency which polysaccharide chains have multiple contact sites that can connect to several biological receptors simultaneously [2]. In the last two decades, polysaccharides have attracted increasing attention from glycobiologists for their application in treating various diseases [3]. A wide of polysaccharides derived from animals, plants, and microorganisms exhibit diverse bioactivities, including antiviral, antioxidant, antibacterial, antitumoral, and immunomodulatory activity [4]. The total carbohydrate content in functional plants accounts for more than half of the dry weight of plants, which are significant sources for promoting health and for developing drugs based on carbohydrates [5]. Hence, the correlation between the chemical composition of carbohydrates and their biological activity in natural plants offers significant opportunities for advancements in pharmaceutical research and health-oriented nutrition.

Lepidium meyenii Walp. (maca), native to the Peruvian Central Andes, belongs to the Brassicaceae family and is an important traditional plant in South America [6]. In the past 2000 years, maca has enjoyed the reputation as the “Peruvian ginseng” and the “Peruvian national treasure”, celebrated for its beneficial effects in bolstering overall health, boosting fertility, mitigating osteoporosis, managing diabetes, and alleviating symptoms of menopause [7]. Maca, a functional food known for its beneficial health properties, gained official recognition from the US Food and Drug Administration (FDA) as a health food in 2001 [8]. Countries consuming maca products mainly include the USA, Canada, the UK, Germany, China, Japan, and the Netherlands, with sales of about 100 million yuan in 2022 [9]. Previous studies have proposed that maca produces various bioactive components to withstand severe conditions such as high altitude, coldness, strong UV radiation, low oxygen levels, and capricious climates [10]. The pharmacological investigation has primarily focused on its secondary metabolites, including macamides and macaenes [11]. Recently, our team conducted a comprehensive examination of the secondary metabolites and their biological effects, discovering a unique group of thiohydantoin compounds [12]. These components exhibit moderate neuroprotective activity; however, their low concentrations limit the further exploration of their functional activity. Carbohydrates, a major component in maca, have also attracted significant interest from phytochemists and pharmacologists. The unique chemical structures found in polysaccharides extracted from maca may hold the key to unlocking their functional properties, especially in enhancing bodily strength. Several papers have reported the structure of maca polysaccharides (MPs), which mainly consist of arabinogalactan and heteroglucans [13]. For example, MP-21 was composed of arabinose, galactose, and rhamnose in a molar ratio of 4.84:5.34:1 with an average molecular weight of 368 kDa from maca tuber whereas the primary structure of MP-21 was not fully characterized [14]. Therefore, the detailed characterization of the chemical structure of MPs is urgent for finding more possibilities in the development of healthcare food.

Natural polysaccharides enhance the phagocytic activity of macrophages, modulate the polarization of immune cells, and leverage their low toxicity and high biocompatibility [15]. These properties effectively enhance the response of the immune system to infections and diseases, making natural polysaccharides an ideal choice in the field of immunoregulation. They play a crucial role in maintaining the homeostasis of the innate immune system and modulating immune cell responses [16]. MPs have various bioactivities including immunoregulatory, anti-fatigue, antioxidant, anti-aging, and anti-tumor activities [17]. Among these, the immunomodulatory effect could be crucial in exploring the primary healthcare function of maca in strengthening the body. Several immunostimulatory polysaccharides were isolated from maca, including heteroglucans (MCP-1 and MCP-2) and arabinogalactans (MCP 3) [18,19]. However, the relationship between the chemical structure and immunoregulatory functions of these polysaccharides remains unclear and needs to be further explored. Therefore, continued investigation into the polysaccharides extracted from maca is essential to deepen our understanding of the relationship between their structure and activity.

Here, we aim to investigate the chemical structures of MPs and their immunomodulatory effects to understand the structure-function relationship. Two new polysaccharides were isolated and purified from maca in our study. One of them contains uronic acid, while the other is a neutral polysaccharide. The fine structures were characterized by analyzing molecular weights, monosaccharide compositions, D/L monosaccharide, and glycosidic bond types. The linkage positions and sequences of MCP-1a and MCP-2b were determined using GC–MS, UHPLC-MS/MS, and 1D/2D-NMR technology. Subsequently, the immune activity and potential mechanism of two new polysaccharides were analyzed using RAW 264.7 macrophage cells.