Pulmonary DNA vaccination: concepts, possibilities and perspectives

Abstract

Mucosal immunity establishes the first line of defence against pathogens entering the body via mucosal surfaces. Besides eliciting both local and systemic immunity, mucosal vaccination strategies that are non-invasive in nature may increase patient compliance and reduce the need for vaccine application by trained personnel. A relatively new concept is mucosal immunization using DNA vaccines. The advantages of DNA vaccines, such as the opportunity to combine the genetic information of various antigen epitopes and stimulatory cytokines, the enhanced stability and ease of production make this class of vaccines attractive and suitable for mucosal application. In contrast to the area of intranasal vaccination, only a few recent studies have focused on pulmonary immunization and the involvement of the pulmonary immune system in eliciting protective immune responses against inhaled pathogens. This review focuses on DNA vaccine delivery to the lung as a promising approach to prevent pulmonary-associated diseases caused by inhaled pathogens. Attractive immunological features of the lung as a site for immunization, the mechanisms of action of DNA vaccines and the pulmonary application of such vaccines using novel delivery systems will be discussed. We also examine pulmonary diseases prone to prevention or therapeutical intervention by application of DNA vaccines.

Fig. 1

The proximal airways represent a pseudostratified columnar epithelium, composed of ciliated cells, basal cells and mucous secreting goblet cells. Particles and pathogens are trapped in the mucus. The mucus–particulate complexes are propelled by the beating cilia to the glottis where they are cleared from the airways by swallowing.

Fig. 2

Maturation steps of DCs and the different models for the origin of the functionally distinct DC subsets, DC1 and DC2. Functionally distinct DCs can originate from a single lineage, which has different activation states that are dependent entirely on the local environment signals (the functional plasticity model) or from completely separate developmental lineages where the signals effect lineage segregation earlier in the developmental pathway and the DCs precursors are already separated and functionally committed (the specialized lineage model). Although distinct in the immunological outcome by skewing Th1 and Th2 responses, both subsets become good antigen-presenting cells upon maturation and increase surface expression of co-stimulatory molecules (modified from [35]).

Fig. 3

Schematic representation of the epithelium present in the mucosal-associated lymphoid tissue (MALT) and the common mucosal immune system (CMIS). Particulate antigens are endocytosed by the M cells that transfer the antigens to the underlying lymphoid tissue where they are processed by macrophages and DCs. Antigen-activated lymphocytes will migrate via the regional lymph nodes, thoracic duct and the blood stream to the lamina propria of other mucosal effector sites and induce IgA production at the mucosal surfaces.

Fig. 4

Schematic representation of a DNA vaccine. Plasmid constructs intended for immunization are bacterially derived and contain the genetic sequence of a desired antigen. The vector requires a strong bacterial origin of replication, an antibiotic selection marker, a strong eukaryotic promoter (usually a strong viral promoter like pCMV) and a transcription terminator.

Fig. 5

CD8+ cytolytic T lymphocytes (CTL) activation mechanisms after pulmonary DNA immunization. Type II alveolar cells were found to express co-stimulatory molecules and were able to deliver co-stimulatory signals to T cells, therefore could lead to CTL activation (a). The other CTL activation mechanisms involve direct transduction of professional APCs after DNA immunization (b) and cross-priming, where the respiratory epithelium is transfected, produces the antigen and then transfer it to professional APCs, which are directly responsible for activation of CTL responses (c).

Fig. 6

Schematic illustration of the internalization and intracellular trafficking of plasmid DNA delivered in a non-viral carrier system.

Fig. 7

Molecular structures of chitosan (a), branched PEI (b) and Poly (d,l-lactide-co-glycolide) (PLGA) polymers (c).