Marine algal polysaccharides: a new option for immune stimulation

Nowadays, all animal production is concerned by vaccination. This is an essential technique for the protection of the livestock health which, however, entails significant costs for stock breeders. Maximizing the efficiency and profitability of prophylactic vaccination strategies is therefore a major stake. To achieve this, new avenues are constantly explored. One of them concerns the use of new molecules extracted from seaweeds to help optimize the stimulation of the natural defences of the body and its response to vaccination strategies. 

The body’s response to the aggression of a pathogen is based on two types of immunity. They are the innate immune response and the adaptive response.

The innate response is the first line of defence against pathogens. It is activated immediately and acts very quickly. This immune response can be found in all animals. It will be the same whenever the body encounters that pathogen. However, the body does not retain a memory of the infectious agent. The mechanism of action of this type of immunity consists in recognizing the molecular patterns shared by numerous pathogens, which are essentially represented by membrane fractions (glycocalyx). 

The various elements that contribute to the innate immune response are the following:

The elements associated with the innate immune response can act on the pathogen directly or indirectly, by producing effector cells (cytokines etc). The latter will subsequently trigger the adaptive immunity by activating the T and B cells.  

Unlike the innate response, the acquired or adaptive response occurs in vertebrates only. During the first encounter with a given pathogen (primary infection), it acts as the body’s second line of defence. Its activation takes some time - known as latency. However, this response system memorizes the pathogens it encounters and when the body is again exposed to them the latency is much shorter and the immune system reacts to the aggression almost immediately. Adaptive immunity is specific: it recognises the molecular patterns of the already encountered pathogens. 

The various elements that contribute to the adaptive immune response are the following:

In recent years more and more publications have brought to the forefront the relevance of seaweeds in numerous biological applications, particularly to immune mechanisms, taking special interest in some of their components, namely the sulfated polysaccharides. These are complex carbohydrates which do not occur in terrestrial plants. They are supposed to influence the immune system by a vast number of still poorly understood pathways.

Polysaccharides represent a structurally diverse class of macromolecules which are relatively widespread in nature. There are simple and complex forms of polysachharides. Unlike proteins and nucleic acids, polysaccharides contain repetitive structural features which are chains of monosaccharide residues joined together by glycosidic bonds. 

Thus, they form polymer (-type) structures represented in the form of chains that may be homogenous (homopolysaccharides) or not (heteropolysaccharides). The simple forms are the homopolysaccharides composed of a single type of sugar, linked in an essentially linear manner (starch, glycogen, cellulose for example). They are essentially structural compounds or mechanisms of energy storage in an easily releasable form. Their structure may become more complex owing to their capacity of establishing links at various levels of each elementary unit, allowing thus the development of branching structures in the three dimensions. These are the branched heteropolysaccharides. 

The nucleotides in nucleic acids and the amino acids in proteins can interconnect in only one way, while the monosaccharide units in oligosaccharides and polysaccharides can interconnect at several points to form a wide variety of linear or branched structures (Sharon and Lis 1993). For instance, the number of possible permutations for four different sugar monomers can attain up to 35,560 unique tetrasaccharides, while four amino acids can form only 24 different permutations (Hodgson 1991). 

This explains the fact that, among macromolecules, polysaccharides provide the highest capacity for carrying biological information, as they have the greatest potential for structural variability. In addition, one of the particularities that numerous marine polysaccharides possess is their polyanionic character, which confers them a high chemical reactivity.

Of these anionic polysaccharides, the majority of those which occur macroalgae are sulfated polysaccharides: galactan (agar, carraghenans), ulvans, fucans.

The ulvans, for example, the water-soluble polysaccharides found in green seaweed of the order Ulvales (Ulva and Enteromorpha), have sulfate, rhamnose, xylose and  iduronic and glucuronic acids as their main constituents (Lahaye and Ray 1996) (Percival and McDowell 1967).

Ulvan structure shows great complexity and variability as evidenced by the numerous oligosaccharide repeating structural patterns identified (Lahaye and Robic 2007). The main repeating disaccharide units reported are of ulvanobiouronic acid 3-sulfate type, containing either glucuronic or iduronic acid. In addition, a few repeating patterns can be found that contain sulfated xylose replacing uronic acid or glucuronic acid on the O-2 binding/link of the rhamnose-3-sulfate units (Lahaye and Ray 1996) (Lahaye et al. 1997).    

This huge variability in the polysaccharide structure provides the flexibility required for exact regulatory mechanisms in different cell-cell interactions in higher organisms.

Sulfation in particular seems to be conducive to various biological activities noted in polysaccharides extracted from marine macroalgae. 

Sulfated polysaccharides, which are widespread in macroalgae, have been shown to possess anti-infectious (Cumashi et al. 2007) (Witvrouw and De Clercq 1997) (anti-viral, anti-bacterial, anti-tumoral), antioxidant (Wang et al. 2010) (de Souza et al. 2007) and anti-thrombotic (Mao et al. 2006) activities, as well as immune-modulating (Leiro et al. 2007) activities that might find relevance in stimulating the immune response or in controlling the activity of immune cells in order to mitigate negative effects such as inflammation (Chen et al. 2008) One of the pathways of marine sulfated polysaccharides, which has been emphasized recently, is their role in the activation of TLR.

Indeed, more and more studies are demonstrating that marine algal polysaccharides can influence the innate immune response by binding to recognition receptors called Pattern Recognition Receptorsc(PRR), such as the mannose receptors or TLRs of phagocytic cells, including and especially macrophages (Chen et al. 2008). TLRs are transmembrane proteins which detect invading pathogens by binding to ancestral molecules of microbial origin called Pathogen-Associated Molecular Patterns (PAMPs).

The PAMPs contact at TRL level triggers a cascade of responses resulting in the expression of inflammatory response genes. In mammals, these recently identified receptors have been numbered from 1 to 11 (TLR1-TLR11). On contact with their respective PAMPs, TLR specifically activate a signaling pathway leading to the activation of NF-kB (Nuclear Factor-kappa B) and AP1 (ActivatorProtein 1) transcription factors regulating the expression of inflammatory cytokines such as TNFα, IL-1 or IL-6.  

It therefore now appears that TLR play a key role in the adaptive immune response, but the signals produced by their activation lead to the activation of numerous other cells and functions of the immune system, which makes them essential elements of both the innate immune mechanisms and of adaptive immunity. 

The activity of some sulfated algal polysaccharides as TLR activating agents might be the result of a certain structural similarity between these marine polysaccharides and bacterial lipopolysaccharides (LPS). Bacterial LPSs are indeed a type of structure occurring at the surface of their external membrane and recognized as bacteria-specific recognition elements. In particular, bacterial LPS in mammals are shown to be specifically recognised by TLR4. 

In conclusion, seaweeds appear to contain sugars in the form of polysaccharides, some of which - sulfated polysaccharides - are complex polyanionic structures which possess various biological properties. A vast number of studies have already evidenced the effects of some of these sulfated polysaccharides, particularly the fucoidans, the carraghenans and the ulvans, on certain mechanisms of inflammatory response and on immunity.

The identification and selection of these polysaccharides extracted from suitable macroalgae makes it possible to envisage the use of these molecules as agents for the stimulation of the various mechanisms associated with the body defence and, in particular, of the innate immunity mechanisms.

Within the framework of the potential applications in the fields of animal breeding and animal health two non-exclusive strategies can be proposed: 

Regular sequential intakes for a general stimulation of the body’s state of defence: 

Targeted intakes within the framework of a vaccination programme: