2.1 Immune-stimulants and immune-modulators
Macrophages can either be resting—which is normal—or be active. Activation can occur as a result of a normal infection process or by isolated bacterial and fungal products such as immune-stimulants or immune-modulators. The response to infection, or to microbial products, is an example of immune mechanisms developed through evolution that quickly register structures unique to individual microbes and which therefore have become reliable alarm signals for infection. Examples of bacterial agents which can activate macrophages directly are lipopolysacharides, peptidoglycans, lipotheoic acid, lipoarabinomannan, lipopeptides, and beta-1,3-glucans (Raa 1996). Examples of stimulating components in fungi or protozoa are beta- 1,3-glucans, mannan and glycosyl phoshatidylinositol. There are also substances which inhibit the reaction of macrophages, e.g. lipophoshoglycan from “Leishmania donovani” (Aderem and Ulevitch 2001).
The natural introduction to the body of microbial macrophage activating substances occurs in the form of infections by bacteria, viruses, fungi or protozoa, and with the natural and unavoidable intake of microbial products from the milieu we inhabit. It is possible that, through evolution, the body has become dependent on a certain intake of microbial products in order to function adequately, or “Give us this day our daily germ” (Rook and Stanford 1998).
For many years, researchers have tried to find substances that can induce the same responses as those occurring during early events in a normal infection process. The idea has been to develop controllable procedures that alert innate defenses to respond quickly and effectively to infections without causing serious inflammation or negative side-effects associated with an infectious disease. This is a promising strategy of disease prophylaxis and in curative treatment of certain immune related diseases. The most promising product candidates are bacterial and fungal components of beta-1,3-glucan nature and fragments of bacterial DNA (Medzhitov 2001).
Beta-1,3/1,6-glucans have a long history in medical practice and are therefore closer to far-reaching pharmaceutical applications. Beta- 1,3/1,6-glucans have a very basic mode of action and may affect many immune related disorders. Further, existing and historical use of beta- 1,3/1,6-glucans have already given a strong foundation of anecdotal information about the most promising indications for further clinical development.
2.2 Immune modulating beta-1,3/1,6-glucans and medical indications
In experimental systems, beta-1,3/1,6-glucans have proven extremely effective as immune stimulants that enhance resistance to infection by viruses, bacteria, fungi and parasites (Seljelid et al. 1987; Kaneko and Chihara 1992; Maeda et al. 1994; Williams et al. 1998). They show no toxic effects, even in concentrations much higher than those normally used in fighting infection. Beta-1,3/1,6-glucans work not only by increasing the ability of the organism to kill microbes, but they also protect against shock produced by bacterial endotoxins (lipopolysacharides). This protection is connected with the fact that beta-1,3/1,6-glucan counteracts endotoxin-induced production of tumor necrosis factor (Seljelid et al. 1997) and lipopolysacharide-induced toxicity (Williams et al. 1995; Vereschagin et al. 1998; Rylander and Holt, 1998; Soltys and Quinn, 1999).
The mesenchymal system is also important in relation to the healing of wounds. Macrophages produce growth factors and thereby regulate healing of wounds, a process that can be accelerated by beta-1,3/1,6-glucan (Portera et al. 1997).
Experimental studies have shown striking anti-tumor effects of macrophage-stimulating beta-1,3/1,6-glucans (Seljelid 1989; Sveinbjørnsson et al. 1998). Beta-1,3/1,6-glucan is used to counteract immune-suppression resulting from radium treatment and chemotherapy and to stimulate the body’s own fight to expel cancer cells (Jong and Birmingham 1993). The majority of these studies have been carried out by injecting beta-1,3/1,6-glucan but also oral administration produces a notable systemic effect. This result was unexpected since beta-glucans are not absorbed from the gut into the blood. It was therefore a noteworthy observation when researchers at Sloan Kettering Institute of Cancer Research (New York) showed that orally administered beta-glucans enhance the anti-tumor effects of injected monoclonal antibodies against various cancer types (Cheung et al. 2002). However, the observation is in line with experiments from Japan and experience with farm animals receiving beta-1,3/1,6-glucan as a disease preventing feed additive. The fact that beta-1,3/1,6-glucans act via the oral route to potentiate injected monoclonal tumor antibodies has indeed opened up a complete new strategy in cancer therapy. The observation adds nicely to the experience from Oriental medicine where oral beta-glucans from mushroom have been in use for many years as stand-alone anti-cancer medicines. It is also a nice confirmation of the results from studies with beta-1,3/1,6-glucans in feed or drinking water for farm animals.
2.3. Use of beta-1,3/1,6-glucans in domestic animals
For more than ten years beta-1,3/1,6-glucan has been used with great success to prevent diseases in aquaculture and traditional domestic animal husbandry, mostly as a feed-additive, but also as an adjuvant in vaccines and for immersion of small fish and cultivated shrimp. The results are widely accepted after worldwide use with many different animal species, both aquatic and terrestrial. Beta-1,3/1,6-glucan contributes to increased disease resistance (reduced mortality) and, possibly as a result of this, to better weight and food uptake (Robertsen et al. 1990).
Beta-1,3/1,6-glucan is used in feed for piglets and calves to reduce infections during weaning, for chickens and broilers against virus and bacterial infections, and for dogs against various immune related disorders. In piglets beta-1,3/1,6-glucan causes reduced level of haptoglobin in the blood (Dritz et al. 1995), a parallel to the CRP-lowering effect of beta-1,3/1,6-glucan as oral supplement to humans, shown in recent a “double blind” study with individuals with mild or moderate hypercholesterolemia (in preparation).
The use of beta-1,3/1,6-glucan in domestic animal husbandry is based on a comprehensive scientific documentation and practical experience which is very relevant also for human medicine. It not only constitutes a reliable safety and efficacy test for beta-1,3/1,6-glucans, but has provided “proof-of-concept” for eventual human use. For instance, animal studies have shown that beta-1,3-glucan administered orally or as a nasal spray, has a systemic effect on immunity and may act as an adjuvant for both injected and mucosal vaccine antigens (Raa et al.2002). This is in accordance with the observations by Cheung et al. (2002) that orally administered beta-glucan enhances the anti-tumor effect of monoclonal antibodies that are injected.
It appears that beta- 1,3-glucan in high molecular weight or micro-particle form is not taken up from the intestine in noticeable amounts and transported around in the blood. Nevertheless the substance produces a systemic effect on animals. This can mean that the substance reacts with glucan-receptors on cellular extensions from macrophages and the dendrittic cells in the underlying lymphoid tissue (Nagler-Anderson 2001).