**8. Biopolymers of Algae Are Adjuvants for the Influenza Vaccines**

Biopolymers of algae are currently being also investigated as candidate adjuvants for the next generation of influenza vaccines. The main direction of improvement for anti-influenza vaccines is to increase their safety. That is why, from whole virion vaccines, the transition was made to split vaccines, and from there to subunit vaccines. However, immunogenicity is often reduced with highly purified antigens [103]. In this regard, scientists study the polysaccharides of algae, whose influence on the formation of innate and adaptive immunity has been described in numerous papers in Russia and in other countries [75,87,88,104]. In the analysis of adjuvant technologies for the creation of vaccines, preference is given to modifiers of functions of the receptors of innate immunity and their signaling pathways [105]. The sulphate polysaccharides of brown algae have some excellent properties as adjuvants: almost complete absence of toxicity, safety, and excellent biocompatibility [106].

In the mechanisms of action of polysaccharides, which are important for the manifestation of the adjuvant effect, one should highlight the ability to exhibit the properties of TLR agonists of innate immunity cells, designed to recognize microbial pathogen-associated molecules. TLRs are major targets for the development of new adjuvants, and TLR agonists are the most preferred adjuvants for vaccines. In the investigation of the specific interaction of polysaccharides with human TLRs, it was found that fucoidans from algae *Saccharina japonica*, *Saccharina cichorioides*, and *Fucus evanescens* specifically bind TLR2 and TLR4, causing activation of the nuclear factor NF-κB. Subsequent expression of genes of proinflammatory cytokines and interferon-inducible genes promote the activation of immunocompetent cells, and the development of an adaptive immune response to unrelated antigens of the Th1 type [107]. Experimental data demonstrate the adjuvant properties of polysaccharides in relation to various antigens and vaccine strains of infectious agents, including influenza virus [104,108]. The results of our experimental studies also indicate the adjuvant activity of fucoidan from the brown alga *F. evanescens*, manifested as an increase in the immunogenicity of the inactivated influenza virus A/California/7/09 H1N1pdm09. At the same time, the effect of fucoidan was more pronounced compared with the traditional licensed adjuvant aluminum hydroxide. In addition, with repeated

immunization of animals, fucoidan provided a reduction in antigenic load. The results indicate the promising application of fucoidan as an adjuvant in vaccines of influenza [109]. Of significant interest are the results of a randomized, double-blind, placebo-controlled study with elderly volunteers, focusing on the ability of fucoidan to have an adjuvant effect when administered orally. The volunteers took the fucoidan from *U. pinnatifida* at a dose of 300 mg/day orally for 4 weeks. Subsequent immunization with the trivalent influenza vaccine led to the identification of higher antibody titers against all strains of the virus contained in the vaccine, compared with antibody titers in individuals who received placebo. In the group of volunteers who received fucoidan, after nine weeks, there was a clear tendency to increase the activity of natural killer cells, and the absence of allergic and other undesirable immune reactions [110]. Despite the positive results of testing of sulphated polysaccharides as adjuvants, it should be taken into account that the use of fucoidans as drugs is currently limited, due to difficulties with obtaining structurally characterized and homogeneous samples or oligomeric fractions of fucoidans. In this regard, active work is underway to obtain low molecular weight polysaccharides or fucooligosaccharides (homo- or hetero-oligosaccharides containing from 2 to 10 monosaccharide residues) related to natural fucoidans. A number of studies have indicated the high immunomodulatory activity of low molecular weight, structurally characterized fractions of fucoidan or its oligosaccharides, but studies on their adjuvant properties are rare [109]. Thus, the adjuvant activity of low molecular weight polysaccharides obtained from the brown alga *F. evanescens* was investigated; this was done using enzymes that provided a stable, reproducible structure. The authors considered that this substance can be used as a pharmaceutical substance or adjuvant as a part of vaccine preparations [109]. Therefore, polysaccharides from brown algae can apparently be used as safe and effective adjuvants in the composition of next generation influenza vaccines. Fucoidans may form a new molecular basis for the creation of immune adjuvants, including for influenza vaccines, due to their high biocompatibility, lack of toxicity, and good tolerance by the human body.
