The ontogeny of antigen-specific T cells

Intranasal DNA Vaccine for Protection against Respiratory Infectious Diseases: The Delivery Perspectives

Intranasal delivery of DNA vaccines has become a popular research area recently. It offers some distinguished advantages over parenteral and other routes of vaccine administration. Nasal mucosa as site of vaccine administration can stimulate respiratory mucosal immunity by interacting with the nasopharyngeal-associated lymphoid tissues (NALT). Different kinds of DNA vaccines are investigated to provide protection against respiratory infectious diseases including tuberculosis, coronavirus, influenza and respiratory syncytial virus (RSV) etc. DNA vaccines have several attractive development potential, such as producing cross-protection towards different virus subtypes, enabling the possibility of mass manufacture in a relatively short time and a better safety profile. The biggest obstacle to DNA vaccines is low immunogenicity. One of the approaches to enhance the efficacy of DNA vaccine is to improve DNA delivery efficiency. This review provides insight on the development of intranasal DNA vaccine for respiratory infections, with special attention paid to the strategies to improve the delivery of DNA vaccines using non-viral delivery agents.

Technologies for enhanced efficacy of DNA vaccines

Despite many years of research, human DNA vaccines have yet to fulfill their early promise. Over the past 15 years, multiple generations of DNA vaccines have been developed and tested in preclinical models for prophylactic and therapeutic applications in the areas of infectious disease and cancer, but have failed in the clinic. Thus, while DNA vaccines have achieved successful licensure for veterinary applications, their poor immunogenicity in humans when compared with traditional protein-based vaccines has hindered their progress. Many strategies have been attempted to improve DNA vaccine potency including use of more efficient promoters and codon optimization, addition of traditional or genetic adjuvants, electroporation, intradermal delivery and various prime-boost strategies. This review summarizes these advances in DNA vaccine technologies and attempts to answer the question of when DNA vaccines might eventually be licensed for human use.

DNA vaccines: immunology, application, and optimization*

The development and widespread use of vaccines against infectious agents have been a great triumph of medical science. One reason for the success of currently available vaccines is that they are capable of inducing long-lived antibody responses, which are the principal agents of immune protection against most viruses and bacteria. Despite these successes, vaccination against intracellular organisms that require cell-mediated immunity, such as the agents of tuberculosis, malaria, leishmaniasis, and human immunodeficiency virus infection, are either not available or not uniformly effective. Owing to the substantial morbidity and mortality associated with these diseases worldwide, an understanding of the mechanisms involved in generating long-lived cellular immune responses has tremendous practical importance. For these reasons, a new form of vaccination, using DNA that contains the gene for the antigen of interest, is under intensive investigation, because it can engender both humoral and cellular immune responses. This review focuses on the mechanisms by which DNA vaccines elicit immune responses. In addition, a list of potential applications in a variety of preclinical models is provided.

DNA vaccination and the immune responsiveness of neonates

Neonates often respond poorly to conventional vaccines or microbial infections. Immaturity of the immune system has been considered to play a role in this regard. However, accumulating evidence shows that in certain conditions, neonatal inoculation of antigens leads to protective immunity. In the particular case of DNA vaccines administered to neonates, the rule is immunity rather than tolerance. Exceptions to the rule give opportunities to further understand the neonatal responsiveness and the mechanism of DNA vaccination. Due to the very nature of the vaccine vector, inhibition of neonatal DNA vaccination by maternal antibodies may be limited to the humoral immunity.

Factors Associated with the Development of Neonatal Tolerance After the Administration of a Plasmid DNA Vaccine

A plasmid DNA vaccine encoding the circumsporozoite protein of malaria (pCSP) induces tolerance rather than immunity when administered to newborn mice. We find that this tolerance persists for >1 yr after neonatal pCSP administration and interferes with the induction of protective immunity in animals challenged with live sporozoites. Susceptibility to tolerance induction wanes rapidly with age, disappearing within 1 wk of birth. Higher doses of plasmid are more tolerogenic, and susceptibility to tolerance is not MHC-restricted. CD8+ T cells from tolerant mice suppress the in vitro Ag-specific immune response of cells from adult mice immunized with pCSP. Similarly, CD8+ T cells from tolerant mice transfer nonresponsiveness to naive syngeneic recipients. These findings clarify the cellular basis and factors contributing to the development of DNA vaccine-induced neonatal tolerance.

Spaceflight and development of immune responses

The NIH.R1 Space Shuttle experiment was designed to study the effects of spaceflight on rodent development. Pregnant rats were flown on the Space Shuttle for 11 days, and pregnant control rats were maintained in animal enclosure modules in a ground-based chamber under conditions approximating those in flight. Additional controls were in standard housing. The effects of the flight on immunological parameters of dams, fetuses, and pups were determined. Blastogenesis of spleen cells in response to mitogen was inhibited in flown dams but was not inhibited in cells from their pups. Interferon-gamma production by spleen cells showed a trend toward inhibition in flown dams but not in their pups. The response of bone marrow cells to colony-stimulating factor showed a trend toward inhibition after spaceflight in dams, but the response of fetus and pup liver cells was not inhibited. Total serum IgG was not affected by spaceflight. None of the examined immune parameters that were altered in rat dams after spaceflight was found to be altered in their offspring.

Plasmid DNA: a new era in vaccinology

DNA vaccination is a novel approach for inducing an immune response. Purified plasmid DNA containing an antigen's coding sequences and the necessary regulatory elements to express them is introduced into the tissue via intramuscular injection or particle bombardment. Once the DNA reaches the tissue, the antigen is expressed in enough quantity to induce a potent and specific immune response and to confer protection against further infections. The effectiveness of DNA vaccines against viruses, parasites, and cancer cells has been demonstrated in numerous animal models. This new approach comes as an aid for the prevention of infectious diseases for which the conventional vaccines have failed. Research on DNA vaccines is providing new insights into some of the basic immunological mechanisms of vaccination such as antigen presentation, the role of effector cells, and immunoregulatory factors. In addition, DNA vaccines may enable us to manipulate the immune system in situations where the response to agents is inappropriate or ineffective. The study of the potential deleterious effects of DNA vaccines is furthering our knowledge regarding the relationship between bacterial DNA and the immune system, as well as its potential application for the study of neonatal tolerance and autoimmunity.

DNA vaccines: safety and efficacy issues

DNA technology has been harnessed to produce a variety of plasmid-based vaccines designed to prevent viral, bacterial and parasitic infections. The rapid adoption and implementation of this novel vaccine strategy carries with it important safety and efficacy concerns. This review will focus on whether DNA vaccines (1) are likely to induce systemic or organ-specific autoimmune disease, (2) have the potential to induce tolerance rather than immunity, and (3) are as effective in individuals with depressed immune function as they are in healthy adults.

Induction of neonatal tolerance by plasmid DNA vaccination of mice

Plasmid DNA vaccines capable of preventing viral, bacterial, and parasitic infections are currently under development. Our labs have shown that a plasmid DNA vaccine encoding the circumsporozoite protein of the malaria parasite elicits protective immunity against live sporozoite challenge in adult BALB/c mice. We now find that the same DNA vaccine induces tolerance rather than immunity when administered to 2-5 d-old mice. Neonatally tolerized animals were unable to mount antibody, cytokine or cytotoxic responses when rechallenged with DNA vaccine in vitro or in vivo. Tolerance was specific for immunogenic epitopes expressed by the vaccine-encoded, endogenously produced antigen. Mice challenged with exogenous circumsporozoite protein produced antibodies against a different set of epitopes, and were not tolerized. These findings demonstrate important differences in the nature and specificity of the immune response elicited by DNA vaccines versus conventional protein immunogens.

Ontogeny of B-lymphocyte function. IX. Difference in the time of maturation of the capacity of B lymphocytes from foetal and neonatal mice to produce a heterogeneous antibody response to thymic-dependent and thymic-independent antigens

The ontogeny of the capacity of the B-lymphocyte population to produce a response which is heterogeneous with respect to antibody affinity was studied in a cell transfer system. Lethally irradiated mice were reconstituted with B cells from donors of various ages, together with adult thymus cells when the response to T-dependent antigens was studied. The animals were immunized with one of a variety of antigens one day after cell transfer and the distribution of their splenic plaque-forming cells (PFC) with respect to affinity was assayed, by hapten inhibition of plaque formation, 2 to 3 weeks after immunization. Mice reconstituted with B cells from neonatal donors produced a response of low affinity and restricted heterogeneity. With four different thymic-dependent antigens (DNP-BGG, F-BGG, DNP-KLH and Dan-KLH) the splenic B-cell population acquired the capacity to reconstitute irradiated mice to produce a normal adult-like, highly heterogeneous, high affinity PFC response between 7 and 10 days after birth. The capacity to produce a heterogeneous response to the thymic-dependent protein antigen BGG matured just slightly later between 10 and 14 days of age. The bone marrow matures with regard to the capacity to reconstitute irradiated mice to give a heterogeneous response several days after the spleen, possibly as a consequence of the redistribution of peripheral B cells to the bone marrow. In contrast, maturation of the capacity of the splenic B-cell population to reconstitute irradiated recipients to give a heterogeneous, adult-like PFC response to three 'thymic-independent' antigens (TNP-PA, DNP-Ficoll and TNP-BA) takes place considerably later (between 3 and 4 weeks of age). These results suggest that the population of B-cell precursors which responds to thymic-dependent antigens may represent a different subpopulation of B cells from the population that responds to thymic independent antigens. Furthermore, the results suggest that these B-cell subsets mature at different times, presumably under independent controls.

Evolutionary and developmental aspects of T-cell recognition

Studies relating to the nature of the antigen-specific T-cell receptor are reviewed in the light of present knowledge of phylogenetic and ontogenetic development. It is suggested that this evidence supports the concept that immunoglobulin (Ig) is the T-cell receptor, and that the following conclusions may be tentatively drawn.