A pathogenic microorganism is any microorganism capable of producing disease, and pathogenicity refers to the ability of the microorganism to gain entrance to a host and produce physiological or anatomical changes. To initiate disease, not only must pathogenic microorganisms enter the body in adequate numbers, but many of them must enter through a certain route called the "portal of entry." This mode of entry differs for each individual microorganism depending on its ability to attack certain cells and tissues. For example, the alimentary tract is the portal of entry for the typhoid and cholera organisms. Tuberculosis, diphtheria, and pneumococcus microorganisms enter through the respiratory tract and may set up infections in the bronchi and lungs. The gonococci, herpes virus, and some other sexually transmitted microorganisms generally enter through the urogenital tract from which they can easily attack the genital organs. Still other organisms enter through abrasions or openings in the skin and set up local infections, or spread through the body in the circulatory system.
Protective immunity is the ability of a host to prevent or overcome invasion by these pathogenic micro-organisms. Immunity may be acquired as a result of the host's recovery from a pathogenic infection, or may be prophylactically induced by administration of a vaccine or toxoid.
Vaccines are suspensions of killed, or living but attenuated microorganisms, or their antigenic portions. Toxoids are detoxified but still antigenically active poisons produced by certain bacteria. Injection of these immunogens (vaccine or toxoid) stimulates an immune response against the specific antigen. Upon subsequent exposure to this same or antigenically related pathogen, the host's immunity assists in defense against the pathogen.
The protection against infection afforded by vaccination operates primarily through the humoral arm of the immune response, with the participation of all major classes of immunoglobulin: IgM, IgA and IgG. Circulating neutralizing IgM, IgA and IgG antibodies interrupt or retard the extracellular dissemination of the pathogen. In addition, the roll of cell-mediated immunity appears to have an effect on the type and intensity of the immune response. Local immunity on the other hand, is restricted to the area of pathogen entry and is mediated by local production and release of IgA antibodies. The secretory IgA antibodies function by neutralizing the pathogen before it makes contact with its target cells, thus preventing implantation and formation of infection. IgG and IgM antibodies also appear in secretions but only in low concentrations.
In developing and administering vaccines or toxoids, the pathogenesis of each particular disease must be considered, since the portal of entry and location of the pathogenic microorganism in the host determine what class of antibody will provide a protective role. In viral respiratory infections, for example, where the focus of the infection is in the ciliated epithelium of the respiratory tract, virus is released mainly on the mucosa surface and does not penetrate into the underlying tissue. As a result, antibodies in the secretions play a major role in the host's defense whereas circulating antibodies participate only insofar as they filter through into the secretions.
A major goal in the prevention of infectious diseases has been the development and use of vaccines or toxoids for immunization. Inactivated and attenuated strains of bacteria and viruses are now widely used for immunization against many diseases including typhoid fever, poliomyelitis, influenza, rabies, measles, and hepatitis type B in humans. (Manual of Clinical Laboratory Immunology, 3rd Ed., Rose, Friedman & Fahey, ASM, D.C., 1986). For those bacterial infections where virulence is due in major part to exotoxins such as diphtheria and tetanus, detoxified but antigenically active toxoids can be used to neutralize the toxin. In addition, recent advances in molecular biology and peptide synthesis have allowed production of purified viral proteins or synthetic peptides for use in immunoprophylaxis. (Fundamental Virology, Fields & Knipe, Raven Press, NY, 1986).
In most cases, inactivated virus vaccines are prepared from virus that is grown in eggs (influenza type A and B), monkey kidney cell culture (polio virus, types 1, 2 and 3) or human diploid fibroblast cell culture (polio virus, rabies), and then inactivated with formalin. These vaccines offer the advantage of immunization with little or no risk of infection but are considerably less effective than the actual virus infection.
Failure to successfully inactivate vaccine virus has occasionally had serious consequences, as with the formalin-inactivated measles virus vaccine. Initially this vaccine prevented measles, but after several years the vaccinees lost their immunity. When subsequently infected with measles virus, the vaccinees developed an atypical illness with accentuated systemic symptoms and pneumonia. It is believed that the formalin used to inactivate the virus destroyed the antigenicity of the measles F protein, but did not affect the H-protein. The vaccinees developed an unbalanced response that included H-protein immunity but not F-protein immunity. It is also known that inactivated influenza virus vaccine appears to lose its effectiveness after several years, although the basis for this is not known.
Attenuated vaccines contain viable but weakened organisms. They work by producing a mild infection which is usually of little danger to the host. The major advantage of live virus vaccines is their activation of all phases of the immune system; systemic and local, immunoglobulin and cell-mediated. Furthermore, immunity induced by live virus vaccines is generally more durable, more effective and more cross-reactive than that induced by inactivated vaccine. In addition many live virus vaccines are easy to administer and are relatively inexpensive.
The disadvantages of live attenuated virus vaccines include the potential for contamination with live adventitious agents such as other viruses, and the fact that some live virus vaccines, such as the measles virus, rubella virus and yellow fever virus vaccines, retain a low level of residual virulence which may cause mild symptoms of the disease against which the vaccine is directed. More serious problems include those in which rare members of the population are particularly vulnerable to the vaccine strain of the virus, e.g. poliovaccine virus, resulting in paralysis, or where infection by live vaccine virus occurs in immuno deficient individuals. And finally, stability is a serious problem with labile vaccine viruses, and the need for storage and transport of some vaccines at low temperature (measles vaccine) has limited their usefulness in some tropical areas where this maintenance is difficult.
Immunity to infection by pathogenic microorganisms depends on the development of an immune response stimulated by antigens associated with each organism. In some instances, as with viruses, antigens present on the surface of the virus play an important role. In other cases, as with diphtheria or tetanus, immunity is a function of specific antibody to the antigenic toxins produced. Therefore, a successful strategy of immunoprophylaxis against pathogenic diseases requires the generation of an immune response to these "protective antigens", i.e. those antigens that stimulate immunity against the intact pathogenic microorganism.
In recent years, the protective surface antigens important in immunity have been identified for a wide range of viruses (Fundamental Virology, Fields & Knipe, Raven Press, N.Y. 1986). With knowledge of the structure or conformation of a particular protective antigen, a synthetic or biosynthetic molecule can be prepared that has the same structure and is capable of provoking antibodies reactive with the intact pathogenic organism.
Two separate approaches have been explored for production of these protective antigens. One involves the production of synthetic peptides which represent immunologically important areas on the surface of the pathogen. The other is recombinant DNA technology which involves splicing a DNA sequence that codes for the pathogenic antigen into a prokaryotic or eukaryotic cell. Subsequently, the cloned pathogen DNA can be expressed in the host cell as pathogen antigen. For example, foot-and-mouth disease virus (FMDV), VPl capsid protein, hepatitis B virus (HBV) surface antigen, influenza A virus hemagglutinin, rabies virus surface glycoprotein and the gD protein of herpes virus have been expressed in bacteria.
In addition to the potential hazards of vaccines discussed above, there are many other problems associated with current immunization techniques. Some pathogenic microorganisms themselves possess attributes that create uncertainties for developing satisfactory vaccines. For example, many viruses only produce local infections that tend to shield them from the full play of the host's defense system. Also, the inability to grow hepatitis B virus in tissue culture or in animals other than chimpanzees, has prevented the development of a conventional live vaccine. (Nature, 302:490-495 (1983)).
The use of protective antigen vaccines is an attractive alternative because it enables circumvention of some of the concerns discussed above that are associated with intact viral immunogens. However, synthesis of relevant protective antigens in bacteria or yeast directed by expression vectors containing cloned DNA copies of the appropriate pathogen genes may not always provide abundant amounts of antigen for use in a vaccine. Many viral antigens (e.g. hepatitis-B virus surface antigen, vesicular stomatitis virus G protein and rabies virus glycoprotein) when expressed in E. coli, are either unstable or lethal to the cell. And attempts to directly express the influenza viral surface glycoprotein hemagglutinin (HA) in large quantities using bacterial promoters have failed. (Proc. Nat. Acad. Sci., 78:5376).