Design of Liposomes based Vaccine Adjuvant System

 

Raditya Weka Nugraheni^1,2*, Helmy Yusuf¹, Dwi Setyawan¹

¹Department of Pharmaceutics, Faculty of Pharmacy, Airlangga University. Jalan Dharmawangsa Dalam, 60286 Surabaya, Indonesia

²Department of Pharmacy, Faculty of Health Sciences, University of Muhammadiyah Malang. Jalan Bendungan Sutami, 65154, Malang, Indonesia

 

ABSTRACT:

Vaccines are still the most effective way of preventing infectious diseases. Through vaccination, the immune system is exposed to the antigen so that its response will increase in subsequent exposure. Liposomes were first reported as a vaccine carrier in 1974 by Allison and Gregoriardis. Liposomes are chosen as carriers in a vaccine delivery system because they are made from natural, biodegradable, nontoxic and non-immunogenic phospholipids. The effectiveness of liposome formulation depends on various physicochemical factors such as vesicle size, surface charge, bilayer composition, coating, route of administration, adjuvant usage, encapsulation efficiency, and ultimately dependent on the lipid composition used. DDA (dietildioctadecylammonium) is one of the major lipid components in the vaccine adjuvant system because of its cationic nature, but DDA has a weakness in terms of its physical stability. The addition of cholesterol in the liposome composition may improve lipid packing thereby reducing or eliminating the phase transition temperature of the liposome system. APC contains many molecules expressed on the cell surface, which can be utilized in facilitating specific liposome targeting.

 

 

 

INTRODUCTION:

Vaccines are still the most effective way of preventing infectious diseases. Through vaccination, the immune system is exposed to the antigen so that its response will increase in subsequent exposure. Vaccination can be done in the form of an attenuated pathogen, inactivation of all organisms, or inactivation of toxins. The vaccine of inactivated infectious agents (live attenuated) have the advantage can provide lifelong immunity and protection in the form of cellular and humoral immune system.

 

 

However, this type of vaccine is not suitable for people who have low immunity and the risk that the vaccine can return to its virulent form. On the other hand, inactivated vaccines and sub-unit vaccines have more advantages in terms of safety; but its lower potency could not provide long-term immunity. Therefore, we need an adjuvant to enhance the immune response. Adjuvant is any component added to the vaccine to enhance the immune response. Adjuvant can form a depot, improve the delivery of antigen, enhancing uptake and presentation on APC (Antigen Presenting Cells), as well as induce cytokine and chemokine^1,2. Adjuvant has the ability to enhance a strong specific immune response against exposed antigens so as to overcome or eliminate infections. At the physical level, adjuvants help slowly release the antigen, or increase the uptake of the antigen given with the adjuvant. With the addition of adjuvants, the following benefits will be obtained: (1) decrease of antigen dose required; (2) longer-lasting immune response in the long run; (3) Induction of mucosal immunity^3,4.

 

Liposomes were first reported as a vaccine carrier in 1974 by Allison and Gregoriardis. Liposomes are chosen as carriers in a vaccine delivery system because they are made from natural, biodegradable, nontoxic and non-immunogenic phospholipids. In addition, there may also be variations in liposome preparation in order to obtain expected characteristics such as particle size, lipid composition variation, and the liposome charge, enabling the design of liposomes as the optimal carrier system of an antigen. Liposomes are also able to prevent the occurrence of antigen degradation and increase the rate of absorption through the biological membrane, thereby increasing bioavailability and therapeutic index. Liposomes can be designed to target a particular location, or trigger the release of antigen to its target. A greater immune response and can be obtained through the use of selective targeting molecules on the surface of liposomes will recognize a specific receptor on immune cells that cause the internalization of liposomes, for example on antigen presenting cells (APC) such as dendritic cells and macrophages. Increased uptake of liposome vaccine in the APC system can be adjusted by adding the number of ligands on the liposome surface so that the dose of antigen can be reduced. The effectiveness of liposome formulation depends on various physicochemical factors such as vesicle size, surface charge, bilayer composition, coating, route of administration, adjuvant usage, encapsulation efficiency, and ultimately dependent on the lipid composition used ⁵.

 

THE MECHANISM OF LIPOSOMES AS VACCINES ADJUVANT:

There are several mechanisms that may explain the ability of liposomes as vaccine adjuvants, including induction of signal 0 associated with the induction of molecules of the co-stimulant signal through cell death, signal 1 (depot effect), and signal 2 (induction of harmful signals or stimulation of cytokines Through cell death or cell damage). In addition, liposomes may also work through passive targeting because of their similarity in shape and size to pathogens. Hence, the liposomes are suitable for the delivery PAMP (pathogen-associated molecular pattern) and will easily be endocytosized by APC⁵. PAMP is recognized by Pattern Recognition Receptor (PRR) such as Toll Like Receptor (TLR) and C-type lectin receptors (CLR) and will eventually activate dendritic cells and subsequent processing and presenting antigens on MHC Class I and Class II^2,6. The mechanism allows the retention of the liposomes effect depot at the injection site then released slowly antigens in the immune cells in the area. This released antigen will travel to the lymph node to activate APC and initiate T cells to activate Th1 cell-mediated immune response. Liposome compositions, such as phospholipid structures, membrane fluidity, and liposome loads play an important role in vaccine retention⁷.

 

THE ROLE OF LIPID BILAYER COMPOSITION:

The lipid bilayer composition determines the physicochemical characteristics of the liposomal system, including the charge and fluidity of the bilayer membrane, and the membrane permeability of molecules inside and outside the system. The fluidity of the bilayer membrane can be characterized by the phase transition temperature (Tm) of the bilayer membrane. Tm values ​​can be adjusted based on the phospholipid composition of the liposome system⁸. DDA (dietildiocta decyl ammonium) is one of the major lipid components in the vaccine adjuvant system, but DDA has a weakness in terms of its physical stability because it is easy to aggregate with the presence of salt, albeit at a small level⁹. Therefore, DDA is combined as a minor lipid component with major lipid components such as soy phosphatidylcholine¹⁰.

 

The addition of cholesterol in the liposome composition may improve lipid packing thereby reducing or eliminating the phase transition temperature of the liposome system with lipid components DDA: Chol: TDB (trehalose-6,6-dibehenate). DDA: TDB has a phase transition temperature above 37°C so the system is a regular solid membrane at body temperature. Decreasing the temperature of the gel-liquid phase transition results in increased fluidity of the bilayer membrane and decreases retention of antigens in vitro. However, high cholesterol levels can trigger the formation of crystal habits. Cholesterol is a membrane stabilizer used for 20-50% lipid phase molar¹¹. Cholesterol levels can also affect the release patterns of the active ingredients that are absorbed in the membrane. Park et al¹² reported that elevated cholesterol levels slowed the release of doxorubicin from the DPPC/DSPC liposome system (75:25).

 

EFFECT OF SURFACE LOAD:

Cationic liposomes are widely used in vaccine delivery studies because of their efficiency as adjuvants to enhance the immune response. Korsholm Smith et al. states that the primary mechanism of cationic liposomes DDA as an adjuvant is to target APC cell membrane so as to increase the uptake and antigen presentation¹². Due to its cationic nature, DDA vesicles will aggregate after administration because they interact with proteins in the anionic space of the interstitium, thus preventing the occurrence of clearances and depot effects¹³. Kaur et al ¹¹ stated in its publication that there was an increase in titre of IgG Anti-Ag85-ESAT-6 in response to the Ag85-ESAT-6 antigen by administering the DDA liposome system: TDB. DDA combined with poly I: C (Polyinosinic-polycytidylic acid) which is a ligand for TLR3 can form cationic liposomes with zeta potential+40mV that have the ability to produce specific humoral immunity and cell-mediated against M. tuberculosis compared with traditional BCG vaccine ¹⁴.

 

TARGETED DELIVERY SYSTEMS: LIPOSOMES WITH OLIGOMANNOSA IDENTIFIER:

APC contains many molecules expressed on the cell surface, which can be utilized in facilitating specific liposome targeting. An example is the delivery of antigens through immune complexes directed at receptors for IgG ie FcγRs in dendritic cells. Another example is the use of receptors belonging to the C-type lectin receptors (CLR) are widely expressed on dendritic cells such as DC-SIGN receptor, DEC-205, LOX-1, Dectin-1, and the mannose receptor CD206. CLR includes a large family of receptors that bind to carbohydrates through mechanisms that depend on the presence of calcium. Ligands of carbohydrates such as mannose can be attached to the liposomes surface so that they will be recognized by APC. The modified antigen with mannose residue will be delivered to APC via a CD206 or CD209-mediated uptake, so that the antigen will be presented effectively on T cells⁶.

 

Ishii et al., reported that Oligo-mannose coated liposome (OML) prepared with DPPC, cholesterol, and Man3-DPPE at a 10: 10: 1 molar ratio can be internalized to peritoneal phagocytic cells more effectively than liposomes without decoration Mannose. Both dendritic cells and peritoneal macrophages can count OML uptake rapidly, but only ^dull population of CD11c⁺CD11b cells (dendritic cells) which memprduksi IL-12 in response to the uptake OML, besides there are up-regulated the expression of CD205 and CCR-7 Is a marker of maturity of dendritic cells in mice that receive OML. This result proves that OML can be used as a carrier for delivery of antigen to dendritic cells and as adjuvant can promote maturation, activation, and dendritic cell traficking so as to enhance Th1 immune response¹⁵.

 

CONCLUSION:

Liposomes can be used as carriers for adjuvant vaccine systems by increasing antigen delivery to APC such as dendritic cells and macrophages. The liposomal delivery capability to APC can be controlled by regulating the lipid bilayer composition so that it will affect membrane fluidity and charge on the surface. The addition of cholesterol to the bilayer membrane will affect the character of membrane fluidity based on changes in transition phase temperature. Cationic liposomes are known to be effective in delivering antigens and facilitating endocytosis by APC. The addition of suitable identification molecules such as oligo-mannoses that interact with CLR on the liposomes surface can increase uptake and its activity as adjuvant.

 

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Received on 13.06.2018                Accepted on 19.09.2018               

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