Ever since the first experiments in variolation in 1721, and Jenner's vaccination methods in 1796, methods and compositions for disease prevention utilizing immunization have been extensively investigated. Many methods rely upon the use of active immunization, in which an antigen (or mixtures of antigens), such as a modified infectious agent or toxin is administered, resulting in active immunity. This active immunity is characterized by the production of antibodies directed against the administered antigen(s), and in some cases, induction of cellular responses mediated by lymphocytes and macrophages.
Traditionally, vaccines used for active immunization have consisted of live attenuated bacteria (e.g., Bacillus Calmette-Guerin) or viruses (e.g., measles virus), killed microorganisms (e.g., Vibrio cholerae), inactivated bacterial products (e.g., tetanus toxoid), or specific single components of bacteria (e.g., Haemophilus influenzae polysaccharide). Although active immunization with live organisms is generally superior to immunization with killed vaccines in producing long-lived immune responses, care must be taken to properly store and administer these vaccines, as serious failures of measles and smallpox immunizations have resulted from improper refrigeration of the vaccine preparations. In addition, pregnant women and individuals with compromised immune systems should, in general, not receive live vaccines, as the organisms may cause serious disease upon vaccination. For example, live vaccines have caused serious and fatal disease in patients receiving corticosteroids, alkylating drugs, radiation, other immunosuppressive treatments, as well as individuals with known or suspected congenital or acquired defects in cell-mediated immunity (e.g., severe combined immunodeficiency disease, leukemia, lymphoma, Hodgkin's disease, and acquired immunodeficiency syndrome [AIDS]). Live vaccines may even cause mild, or rarely, severe disease in immunocompetent hosts. In addition, live vaccines may also contain undesirable components. For example, epidemic hepatitis has resulted from the use of vaccinia and yellow fever vaccines containing human serum.
Passive immunization using preformed immunoreactive serum or cells is sometimes utilized, especially when active immunization is not available or not advisable. In particular, passive immunization finds use in individuals who cannot produce antibodies or other immune system deficiencies, as well as in individuals who are at risk of developing disease before active immunization would be successful in stimulating a sufficient antibody response. Passive immunization is also used in conjunction with vaccine administration in the management of certain diseases (e.g., rabies vaccination and prophylaxis following an animal bite), management of individuals who have been exposed to certain toxins or venoms, and as an immunosuppressant. However, passive immunization does not produce long-term immunity and is sometimes associated with severe reactions due to the presence of foreign proteins in the vaccine preparation (e.g., anaphylaxis resulting from a reaction against human or horse [or other non-human animal] proteins present in the vaccine preparation).
More recently, vaccines comprising recombinant DNA or RNA segments have been developed. However, use of these recombinant vaccines has resulted in problems associated with the expression of the desired antigen(s) in another organism (e.g., an E. coli or yeast host). For example, in addition to the desired antigen, other components, such as other antigens (e.g., protein and other components) from the expression host, preservatives, etc may be present in the preparation. In addition, adjuvants are sometimes required in order to provide efficacious vaccination with these vaccines. However as with passive immunization, undesirable reactions sometimes occur in vaccinated individuals due to the presence of these undesirable components.
Various adenovirus-based gene delivery systems have likewise been investigated for vaccine use. Human adenoviruses are double-stranded DNA viruses which enter cells by receptor-mediated endocytosis. These viruses have been viewed as being particularly well suited for gene transfer because they are easy to grow and manipulate and they exhibit a broad host range in vivo and in vitro. Adenovirus is easily produced at high titers and is stable so that it can be purified and stored. Even in the replication-competent form, adenoviruses generally cause only low level morbidity and are not associated with human malignancies. Various references provide reviews of adenovirus-based gene delivery systems (See, e.g., Haj-Ahmad and Graham, J. Virol., 57:267-274 [1986]; Bett et al., J. Virol., 67:5911-5921 [1993]; Mittereder et al., Human Gene Ther., 5:717-729 [1994]; Seth et al., J. Virol., 68:933-940 [1994]; Barr et al., Gene Ther., 1:51-58 [1994]; Berkner, BioTechn., 6:616-629 [1988]; and Rich et al., Human Gene Ther., 4:461-476 [1993]). However, despite these advantages, adenovirus vector systems still have several drawbacks which limit their effectiveness in gene delivery, such as cytotoxicity. Adenovirus vectors also express viral proteins that may elicit a strong non-specific immune response in the host. This non-specific immune reaction may increase toxicity or preclude subsequent treatments because of humoral and/or T cell responses against the adenoviral particles. Thus, problems remain even with the newer technologies for vaccine administration.
As briefly mentioned above, the major focus in the past has been on the development of antibody responses to vaccination. However, cell-mediated responses are of great importance in some situations. Indeed, cell-mediated immunity is of greater importance than the antibody-mediated response in the response to intracellular parasites (e.g., viruses and obligately intracellular bacteria). T-cells (T lymphocytes) play the primary roles in cell-mediated immunity, although there is communication via cytokines and other signalling compounds between these cells as the antibody-producing B-cells.
Cytotoxic T-lymphocytes (CTLs) play an important role in immune responses directed against intracellular pathogens such as viruses and tumor-specific antigens produced by cancerous cells. In particular, CTLs mediate cytotoxicity of virally infected cells by recognizing viral determinants in conjunction with Class I MHC molecules displayed by the infected cells. Cytoplasmic expression of proteins is a prerequisite for Class I MHC processing and presentation of antigenic peptides to CTLs. However, conventional immunization techniques, such as those using killed or attenuated viruses, often fail to elicit an appropriate CTL response which is effective against an intracellular infection. Thus, there remains a need for the development of vaccines that stimulate appropriate responses (i.e., cell-mediated as well as antibody-mediated immune responses), in order to prevent disease. Indeed, despite advances in vaccine technology, there remains a need for vaccines that are efficacious, yet avoid the problems associated with current vaccine preparations.