Formulation in DDA-MPLA-TDB Liposome Enhances the Immunogenicity and Protective Efficacy of a DNA Vaccine against Mycobacterium tuberculosis Infection

Despite the vaccine Mycobacterium bovis Bacillus Calmette–Guérin is used worldwide, tuberculosis (TB) remains the first killer among infectious diseases. An effective vaccine is urgently required. DNA vaccine has shown prophylactic as well as therapeutic effects against TB, while its weak immunogenicity hinders the application. As a strong inducer of Th1-biased immune response, DMT, consisting of dimethyldioctadecylammonium (DDA) and two pattern recognition receptor agonists monophosphoryl lipid A and trehalose 6,6′-dibehenate (TDB), was a newly developed liposomal adjuvant. To elucidate the action mechanism of DMT and improve immunological effects induced by DNA vaccine, a new recombinant eukaryotic expression plasmid pCMFO that secretes the fusion of four multistage antigens (Rv2875, Rv3044, Rv2073c, and Rv0577) of Mycobacterium tuberculosis was constructed. pCMFO/DDA and pCMFO/DMT complexes were then prepared and their physicochemical properties were analyzed. The immunogenicity and protection against M. tuberculosis infection in vaccinated C57BL/6 mice were compared. Formulation of DNA and two agonists into the DDA liposome decreased zeta potential but increased the stability of storage, which resulted in a slower and longer-lasting release of DNA from the DNA–DMT complex than the DNA–DDA liposome. Besides Th1-biased responses, pCMFO/DMT vaccinated mice elicited more significantly CFMO-specific IL2⁺ T_CM cell responses in the spleen and provided an enhanced and persistent protection against M. tuberculosis aerosol infection, compared to pCMFO/DDA and pCMFO groups. Therefore, the adjuvant DMT can release DNA and agonists slowly, which might attribute to the improved protection of DMT adjuvanted vaccines. pCMFO/DMT, a very promising TB vaccine, warrants for further preclinical and clinical trials.

Particulate formulations for the delivery of poly(I:C) as vaccine adjuvant

Current research and development of antigens for vaccination often center on purified recombinant proteins, viral subunits, synthetic oligopeptides or oligosaccharides, most of them suffering from being poorly immunogenic and subject to degradation. Hence, they call for efficient delivery systems and potent immunostimulants, jointly denoted as adjuvants. Particulate delivery systems like emulsions, liposomes, nanoparticles and microspheres may provide protection from degradation and facilitate the co-formulation of both the antigen and the immunostimulant. Synthetic double-stranded (ds) RNA, such as polyriboinosinic acid-polyribocytidylic acid, poly(I:C), is a mimic of viral dsRNA and, as such, a promising immunostimulant candidate for vaccines directed against intracellular pathogens. Poly(I:C) signaling is primarily dependent on Toll-like receptor 3 (TLR3), and on melanoma differentiation-associated gene-5 (MDA-5), and strongly drives cell-mediated immunity and a potent type I interferon response. However, stability and toxicity issues so far prevented the clinical application of dsRNAs as they undergo rapid enzymatic degradation and bear the potential to trigger undue immune stimulation as well as autoimmune disorders. This review addresses these concerns and suggests strategies to improve the safety and efficacy of immunostimulatory dsRNA formulations. The focus is on technological means required to lower the necessary dosage of poly(I:C), to target surface-modified microspheres passively or actively to antigen-presenting cells (APCs), to control their interaction with non-professional phagocytes and to modulate the resulting cytokine secretion profile.

A case-study investigating the physicochemical characteristics that dictate the function of a liposomal adjuvant

A range of particulate delivery systems have been considered as vaccine adjuvants. Of these systems, liposomes offer a range of advantages including versatility and flexibility in design format and their ability to incorporate a range of immunomodulators and antigens. Here we briefly outline research, from within our laboratories, which focused on the systematic evaluation of cationic liposomes as vaccines adjuvants. Our aim was to identify physicochemical characteristics that correlate with vaccine efficacy, with particular consideration of the interlink between depot-forming action and immune responses. A variety of parameters were investigated and over a range of studies we have confirmed that cationic liposomes, based on dimethyldioctadecylammonium bromide and trehalose 6,6'-dibehenate formed a depot at the injection site, which stimulates recruitment of antigen presenting cells to the injection site and promotes strong humoral and cell-mediated immune responses. Physicochemical factors which promote a strong vaccine depot include the combination of a high cationic charge and electrostatic binding of the antigen to the liposome system and the use of lipids with high transition temperatures, which form rigid bilayer vesicles. Reduction in vesicle size of cationic vesicles did not promote enhanced drainage from the injection site. However, reducing the cationic nature through substitution of the cationic lipid for a neutral lipid, or by masking of the charge using PEGylation, resulted in a reduced depot formation and reduced Th1-type immune responses, while Th2-type responses were less influenced. These studies confirm that the physicochemical characteristics of particulate-based adjuvants play a key role in the modulation of immune responses.