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If there is a way for an organism to successfully bypass or overcome the immunological defenses, then some bacterial pathogen has probably "discovered" it. Bacteria evolve very rapidly in relation to their host, so that most of the feasible anti-host strategies are likely to have been tried out and exploited. Consequently, pathogenic bacteria have developed numerous ways to bypass or overcome the immunological defenses of the host, which contributes to the virulence of the microbe and the pathology of the disease.

PATHOGEN STRATEGIES  TO DEFEND AGAINST THE SPECIFIC IMMUNE DEFENSES

Immunological Tolerance to a Bacterial Antigen

Tolerance to an Ag can arise in a number of ways, but three are possibly relevant to bacterial infections.

1. Fetal exposure to Ag. If a fetus is infected at certain stages of immunological development, the microbial Ag may be seen as "self", thus inducing tolerance (failure to undergo an immunological response) to the Ag which may persist even after birth.

2. High persistent doses of circulating Ag. Tolerance to a bacterium or one of its products might arise when large amounts of bacterial antigens are circulating in the blood. The immunological system becomes overwhelmed.

3. Molecular mimicry. If a bacterial Ag is very similar to normal host "antigens", the immune responses to this Ag may be weak giving a degree of tolerance. Resemblance between bacterial Ag and host Ag is referred to as molecular mimicry. In this case the antigenic determinants of the bacterium are so closely related chemically to host tissue components that the immunological cells cannot distinguish between the two and an immunological response cannot be raised. Some bacterial capsules are composed of polysaccharides (hyaluronic acid, sialic acid) so similar to host tissue polysaccharides that they are not immunogenic.

Antigenic Disguises

Some pathogens can hide their unique antigens from opsonizing antibodies or complement. Bacteria may be able to coat themselves with host proteins such as fibrin, fibronectin, or even immunolobulin molecules. In this way they are able to hide their own antigenic surface components from the immunological system.

S. aureus produces cell-bound coagulase and clumping factor that cause fibrin to clot and to deposit on the cell surface. It is possible that this disguises the bacteria immunologically so that they are not readily identified as antigens and targets for an immunological response.

Protein A produced by S. aureus, and the analogous Protein G produced by Streptococcus pyogenes, bind the Fc portion of immunoglobulins, thus coating the bacteria with antibodies and canceling their opsonizing capacity by the disorientation.

The fibronectin coat of Treponema pallidum provides an immunological disguise for the spirochete.

E. coli K1, that causes meningitis in newborns, has a capsule composed predominantly of sialic acid providing an antigenic disguise, as does the hyaluronic acid capsule of Streptococcus pyogenes.

Immunosuppression

In mild cases of leprosy there is frequently an associated immunological suppression that is specific for M. leprae antigens. This is separate from tolerance, since unique antigens (proteins) of M. leprae have been associated as the cause of this immunosuppression. This could be explained by (1) lack of costimulatory signals (interference with cytokine secretion); (2) activation of suppressor T cells; (3) disturbances in T_H1/T_H2 cell activities.

At present, little is known of the mechanisms by which bacterial pathogens inhibit general immune responses. It seems probable that it is due to interference with the functions of B cells, T cells or macrophages. Since many intracellular bacteria infect macrophages, it might be expected that they compromise the role of these cells in an immunological response.

General immunosuppression induced in a host may be of immediate value to an invading pathogen, but it is of no particular significance (to the invader) if it merely promotes infection by unrelated microorganisms. Perhaps this is why it does not seem to be a commonly used strategy of the bacteria.

Persistence of a Pathogen at Bodily Sites Inaccessible to Specific Immune Response

Some pathogens can avoid exposing themselves to immune forces. Intracellular pathogens can evade host immunological responses as long as they stay inside of infected cells and they do not allow microbial Ag to form on the cell surface. This is seen in macrophages infected with Brucella, Listeria or M. leprae. The macrophages support the growth of the bacteria and at the same time give them protection from immune responses. Some intracellular pathogens (Yersinia, Shigella, Listeria, E. coli) may take up residency in cells that are neither phagocytes nor APC's and their antigens are not displayed on the infected cell's surface. They are virtually unseen by cells of the immune system.

Some pathogens persist on the luminal surfaces of the GI tract, oral cavity and the urinary tract, or the lumen of the salivary gland, mammary gland or the kidney tubule. If there is no host cell destruction, the pathogen may avoid inducing an inflammatory response, and there is no way in which sensitized lymphocytes or circulating antibodies can reach the site to eliminate the infection. Secretory IgA could react with surface antigens on bacterial cells, but the complement sequence would be unlikely to be activated and the cells would not be destroyed. Conceivably, IgA antibodies could immobilize bacteria by agglutination of cells or block adherence of bacteria to tissue or cell surfaces, but it is unlikely that IgA would kill bacteria directly or inhibit their growth.

Some examples of bacterial pathogens that grow at tissue sites generally inaccessible to the forces of AMI and CMI are given below.

Streptococcus mutans can initiate dental caries at any time after the eruption of the teeth, regardless of the immune status of the host. Either the host does not undergo an effective immune response or secretory IgA plays little role in preventing colonization and subsequent plaque development.

Vibrio cholerae multiplies in the GI tract where the bacteria elaborate a toxin which causes loss of fluids and diarrhea in the host which is characteristic of the disease cholera. IgA antibodies against cellular antigens of the cholera vibrios are not completely effective in preventing infection by these bacteria as demonstrated by the relative ineffectiveness of the cholera vaccine prepared from phenol-killed vibrios.

The carrier state of typhoid fever results from a persistent infection by the typhoid bacillus, Salmonella typhi. The organism is not eliminated during the initial infection and persists in the host for months, years or a life time. In the carrier state, S. typhi is able to colonize the biliary tract (gall bladder) away from the immune forces, and be shed into urine and feces.

Some bacteria cause persistent infections in the lumen of glands. Brucella abortus persistently infects mammary glands of cows and is shed in the milk. Leptospira multiplies persistently in the lumen of the kidney tubules of rats and is shed in the urine and remains infectious.

Bacteria causing infections of the hair follicles, such as acne, seldom encounter the immunological tissues.

Induction of Ineffective Antibody

Many types of antibody (Ab) are formed against a given Ag, and some bacterial components may display various antigenic determinants. Antibodies tend to range in their capacity to react with Ag (the ability of specific Ab to bind to an Ag is called avidity). If Abs formed against a bacterial Ag are of low avidity, or if they are directed against unimportant antigenic determinants, they may have only weak antibacterial action. Such "ineffective" (non-neutralizing) Abs might even aid a pathogen by combining with a surface Ag and blocking the attachment of any functional Abs that might be present.

In the case of Neisseria gonorrhoeae the presence of antibody to an outer membrane protein called rmp interferes with the serum bactericidal reaction and in some way compromises the surface defenses of the female urogenital tract. Increased susceptibility to reinfection is highly correlated with the presence of circulating rmp antibodies.

Antibodies Absorbed by Soluble Bacterial Antigens

Some bacteria can liberate antigenic surface components in a soluble form into the tissue fluids. These soluble antigens are able to combine with and "neutralize" antibodies before they reach the bacterial cells. For example, small amounts of endotoxin (LPS) may be released into surrounding fluids by Gram-negative bacteria.

Autolysis of Gram-negative or Gram-positive bacteria may release antigenic surface components in a soluble form. Streptococcus pneumoniae and Neisseria meningitidis are known to release capsular polysaccharides during growth in tissues. They are found in the serum of patients with pneumococcal pneumonia and in the cerebrospinal fluid of patients with meningitis. Theoretically, these released surface antigens could "mop up" antibody before it reached the bacterial surface which should be an advantage to the pathogen. These soluble bacterial cell wall components are powerful antigens and complement activators so they contribute in a major way to the pathology observed in meningitis and pneumonia.

Protein A, produced by S. aureus may remain bound to the staphylococcal cell surface or it may be released in a soluble form. Protein A will bind to the Fc region of IgG. On the cell surface, protein A binds IgG in the wrong orientation to exert its antibacterial activity, and soluble protein A agglutinates and partially inactivates IgG.

Local Interference with Antibody Activity

There are probably several ways that pathogens interfere with the antibacterial action of antibody molecules. Some pathogens produce enzymes that destroy antibodies.

Neisseria gonorrhoeae, N. meningitidis, Haemophilus influenzae, Streptococcus pneumoniae and Streptococcus mutans, which can grow on the surfaces of the body, produce IgA proteases that inactivate secretory IgA by cleaving the molecule at the hinge region, detaching the Fc region of the immunoglobulin.

Soluble forms of Protein A produced S. aureus agglutinate immunoglobulin molecules and partially inactivate IgG.

Antigenic Variation

One way bacteria can trick forces of the immunological response is to periodically change antigens, i.e., to undergo antigenic variation. Antigens may vary or change in the host during the course of an infection, or an organism can exist in nature as multiple antigenic type (serotypes or serovars). Antigenic variation is an important mechanism used by pathogenic microorganisms for escaping the neutralizing activities of antibodies.

Many pathogenic bacteria exist in nature as multiple antigenic types or serotypes, meaning that they are variant strains of the same pathogenic species. For example, there are multiple serotypes of Salmonella enterica based on differences in cell wall (O) antigens and/or flagellar (H) antigens. There are 80 different antigenic types of Streptococcus pyogenes based on M-proteins on the cell surface. There are over one hundred strains of Streptococcus pneumoniae depending on their capsular polysaccharide antigens. Based on minor differences in surface structure chemistry there are multiple serotypes of Vibrio cholerae, Staphylococcus aureus, Escherichia coli, Neisseria gonorrhoeae and an assortment of other bacterial pathogens. Antigenic variation is prevalent among pathogenic viruses as well.

If the immunological response is a critical defense against a pathogen, then being able to shed old antigens and present new ones to the immune system might allow infection or continued invasion by the pathogen to occur. Furthermore, the infected host would seem to be the ideal selective environment for the emergence of new antigenic variants of bacteria, providing the organism's other virulence determinants remain intact. Perhaps this explains why many successful bacterial pathogens exist in a great variety of antigenic types.