How many types of viral vectors?

Viruses have evolved specialized molecular mechanisms to efficiently transport their genomes inside the cells they infect. Viral vectors are engineered viruses to deliver genetic material into cells and have been used for gene therapy and the development of vaccines.

Several types of viruses, including retrovirus, adenovirus, adeno-associated virus (AAV), and herpes simplex virus, have been modified in the laboratory to deliver genetic materials into cells. These vector systems have unique advantages and limitations for specific applications.

RETROVIRUS

Retroviral vectors can permanently integrate into the genome of the infected cell, but require mitotic cell division for transduction. Hence many cells (e.g., neurons) are resistant to retrovirus infection and integration. Lentiviruses are a subclass of retroviruses, but in contrast to other retroviruses, lentiviruses can integrate into the genome of non-dividing cells. Lentiviruses are commonly used for cell therapy. 

ADENOVIRUS

Adenoviruses are double-stranded DNA viruses that replicate in the cell nucleus of vertebrates. Adenoviral vectors can efficiently deliver genes to a wide variety of dividing and non-dividing cell types, but immune elimination of infected cells often limits gene expression in vivo. In addition, adenoviruses have been shown to induce a broad immune response, including cytotoxic T cells. Hence, adenovirus is one of the most explored viral vectors for use in vaccines. 

ADENO-ASSOCIATED VIRUS (AAV)

AAV is a single-stranded DNA virus that can infect many non-dividing and dividing cell types. AAV causes a very mild immune response and currently no evidence shows that it can cause disease. AAV is a very attractive candidate for creating viral vectors for gene therapy, but has a limited DNA capacity- AAV can only deliver smaller inserts of up to 5 kb.

AAV has become the preferred virus for the following reasons: only causes a mild immune respon; infects both dividing and non-dividing cells; persists in cells without directly inserting into the host genome (remains in an extra-chromosomal state).

Image: Adeno-associated virus serotype 2 structure from 1LP3. One fivefold axis shown center. Wikipedia.org

Herpes simplex virus can deliver large amounts of exogenous DNA. However, cytotoxicity and maintenance of transgene expression remain as obstacles. 

Alternatively, chimeric viral-vector systems that combine advantageous properties of two or more viral systems are also being explored. 

References 

en.wikipedia.org

ncbi.nlm.nih.gov

beckman.com

cytivalifesciences.com