Virosome

Virosomes are vehicles that have a spherical shape with a phospholipid mono/bilayer membrane. Inside of the virosome, there is a central cavity that holds the therapeutic molecules such as nucleic acids, proteins, and drugs.[2] On the surface of the virosome, there can be different types of glycoproteins. Glycoproteins are a type of protein that have an oligosaccharide chain bonded to amino acid chains. The different types of glycoproteins on the surface of the virosome increases the specificity of the target cells because the surface glycoproteins help with recognition as well as the attachments of the virosomes to their target cells. In the case of the influenza virosome, the glycoproteins are antigen, haemagglutinin, and neuraminidase. Antigens are molecules that triggers an immune response when targeted by a specific antibody that corresponds to the shape of the antigen.[3] Haemagglutinin is a type of antibody that causes red blood cells to congregate together.[4] Neuraminidase are enzymes that break glycosidic breaks glycosidic linkages.[5] The size and surface molecules presented on of the virosome can be modified so that it can target different types of cells.[2]

Virosomes deliver antigens and therapeutic agents to their targeted cells. Virosomes can act as immunopotentiating agents and as agents of targeted drug delivery. Virosomes as immunopotentiating agents activate cell mediated and humoral immune responses. Virosomes are suspended in saline buffers and are administered through respiratory, parenteral, intravenous, oral, intramuscular, and topical routes.[2]

Fig. 3. Components of an influenza virosome

In contrast to liposomes, virosomes contain functional viral envelope glycoproteins: influenza virus hemagglutinin (HA) and neuraminidase (NA) intercalated in the phospholipid bilayer membrane. They have a typical mean diameter of 150 nm. Essentially, virosomes represent reconstituted empty influenza virus envelopes, devoid of the nucleocapsid including the genetic material of the source virus.[6]

The unique properties of virosomes partially relate to the presence of biologically active influenza HA in their membrane. This viral protein not only confers structural stability and homogeneity to virosome-based formulations, but it significantly contributes to the immunological properties of virosomes, which are clearly distinct from other liposomal and proteoliposomal carrier systems. It has been shown that a physical association between the virosome and the antigen of interest is necessary for the full adjuvant effect of virosomes. Such physical association can be achieved by a variety of methods, depending on the properties of the antigen. Antigens can be incorporated into virosomes, adsorbed to the virosome surface, or integrated into the lipid membrane, either via hydrophobic domains or lipid moieties cross-linked to the antigen.

Virosomes therefore represent an innovative, broadly applicable adjuvant and carrier system with prospective applications in areas beyond conventional vaccines. They are one of only three adjuvant systems widely approved by regulatory authorities and the only one that has carrier capabilities.

They are also being considered for HIV-1 vaccine research.[7]

They were used as a drug carrier mechanism for experimental cancer therapies.[8]

The benefits of virosomes are that the specific structure and small size help with the precision of target cells. The phospholipid membrane prevents the virosome from adverse reactions in the body and the membrane allows the virosome to be biocompatible and biodegradable in the body.[2] The challenges of virosomes is the rapid detection and activation of the immune response against the viral glycoproteins, which can result in a decrease of the virosomes. However, glycoproteins can still induce a prophylactic response against the virus, which helps with establishing virosomes as vaccine delivery systems.[2] If the virosome is administered into the bloodstream, the virosome can disintegrate. However, if the virosome can reach the target quick enough, the drug delivery will still happen. There are some challenges with virosomes, but there are ways in which the virosome can still help activate the immune response.  

• "What are virosomes?"
• "Virosome based vaccine"