Autovaccine

1.- BACTERIAL CAPSULAR EXOPOLYSACCHARIDES

2.- BACTERIAL BIOFILMS: THE ROLE OF EXOPOLYSACCHARIDES IN CHRONIC PROCESSES

3.- LIPOSOMES: ANTIGEN VEHICLES

4.- LATEST GENERATION AUTOVACCINES

In this section of the website, four chapters will be dedicated to explaining the reasons for using exopolysaccharides and liposomes in our autovaccines.

1.- BACTERIAL CAPSULAR EXOPOLYSACCHARIDES

The formulation and elaboration of veterinary autovaccines has been perfected in recent years, thanks to the application of knowledge obtained in human medicine. In this series of four articles, starting today, we will address the basic principles of these developments.

Fig. 1: Schematic representation of the opsonization process (antibody adhesion to the bacterial surface) and posterior phagocytosis of a bacterium.

Phagocytes, neutrophils and macrophages are able to internalize bacteria in an either nonspecific or specific manner. In the former case, the phagocyte simply ingests the bacterium upon "stumbling into it". If the bacterium in question is "naked", i.e., if it lacks enveloping external layers, its surface will be relatively hydrophobic, and phagocytosis is performed without difficulty. Such nonspecific recognition of foreign bacteria is in turn facilitated by the complement system and a series of blood proteins, though in general it constitutes only a primary response mechanism. In order for the immune response against an extracellular bacterium to exert a protective effect, the mechanism must be mediated by specific antibodies (Roit, 1998).

Once the phagocytes have digested a number of bacteria, opsonizing antibody production begins — fundamentally targeted to the antigens of the bacterial wall. The process of phagocytosis is greatly facilitated in this way. The circulating antibodies encounter the individual bacteria and bind to them in the Fac region. Such binding induces a conformational change in the Fc region of the antibody that on one hand activates the blood complement system and on the other facilitates binding to phagocytes. The elimination of opsonized bacteria is both rapid and effective (Fig. 1).

Production of capsular exopolysaccharides

In fact, things are not so easily resolved for the infected host. Indeed, bacteria, in the course of their evolution have adapted to the host immune defense mechanisms, for example by creating a scantly dense outer layer of capsular exopolysaccharides that coat the bacterial wall (Quessy, 1994).

Macrophages are able to recognize a foreign body such as a bacterium only if it has been opsonized beforehand. As can be seen in the schematic representation, we are interested in producing antibodies that bind to the exopolysaccharide.

The capsular exopolysaccharide layer is very thick yet sufficiently permeable to allow the passage of nutrients, antibiotics or even large proteins. Antibodies are therefore able to penetrate without difficulty, reaching and binding to the bacterial wall in their Fab region. However, due to the thickness of the bacterial exopolysaccharide layer, the bound antibodies are effectively masked or hidden, and their Fc region may be unable to establish contact with the corresponding phagocyte receptors (Fig. 2). For this reason, antibodies directed against the bacterial wall are often unable to eliminate exopolysaccharide-producing bacteria, and the infectious process continues despite the existence of prior vaccination (Roit, 1998). In order to effectively phagocytose a bacterium surrounded by an exopolysaccharide capsule, the produced antibodies must be targeted to the exopolysaccharide component rather than to the bacterial wall itself.

Macrophages are able to recognize a foreign body such as a bacterium only if it has been opsonized beforehand. As can be seen in the schematic representation, we are interested in producing antibodies that bind to the exopolysaccharide.

Fig. 3: Schematic tridimensional representation of a protein.

The host response

The host is only able to generate antibodies against antigens unrelated to the host tissues, in order to avoid potentially fatal autoimmune reactions (Roit 1998). It is relatively simple to find antigens foreign to the host tissues in proteins and even other molecules of the bacterial wall. Proteins are composed of up to twenty different amino acids, and the different combinations of the latter allow for infinite different protein amino acid sequences. In turn, these sequences fold in the three dimensions of space so that two given amino acids spaced far apart in the chain sequence may actually be positioned together tridimensionally — creating a so-called epitope that the antibodies are able to recognize (Fig. 3). Even amino acids belonging to different proteins are able to form an epitope as a result of interaction between several proteins (Roit 1998).

However, the capsular exopolysaccharides are only composed of two or three linearly interlinked sugar molecules that form a repetitive structure; although these macromolecules branch at intervals, they do not have a tridimensional configuration but rather a bidimensional structure (Fig. 4). This lack of "variety", and the great spatial homogeneity of these macromolecules cause them to have very little antigenic potential, and antibody production against them is thus very limited (Mond, 1995).

Bacteria with exopolysaccharides

The idea of a capsular exopolysaccharide may seem strange, though we are all familiar with the concept of bacterial serotypes.

Fig. 4: Nearly bidimensional bacterial polysaccharide.

In this sense, the serotypes of bacteria often correspond to different polysaccharides. Thus, in the case of Salmonella we have more than 180 serotypes; in the case of Streptococcus suis there are over 23 (Prieto, 1993), and more than 20 in Staphylococcus aureus (Poutrel, 1993) or in Erysipelothrix rhusiophatiae (Takahashi, 1999), etc. Virtually all bacterial species of interest in veterinary practice produce exopolysaccharides, though in some cases the latter have been little studied to date — such as the Mycoplasmas (Niang, 1998; Neyrolles, 1998). In general, only a few serotypes are responsible for most clinical processes, and these are the serotypes typically included in commercial vaccines (e.g., serotypes 1 and 2 in Aujesky infection, serotypes 2, 4 and 5 in Actinobacillus pleuropneumoniae, etc). However, in many cases the disease is caused by other, different serotypes; in such situations, the administration of an autovaccine may be preferable.

In order to afford effective protection against any infectious process, many more serotypes should be included. In human medicine, vaccines for meningitis caused by Streptococcus pneumoniae contain over 23 different purified capsular serotypes - representing 25% of the global serotypes implicated in the disease (Rubin, 2000). The efficacy of commercial vaccines against Streptococcus pneumoniae has been well demonstrated, after having been used to vaccinate millions of children around the world, including in Spain.