Precision Medicine, CRISPR, and Genome Engineering Moving from Association to Biology and Therapeutics

16

serotypes used in clinical gene therapy applications (AAV5, AAV8, and AAV9)

[ 162 ]. Table 1.1 summarizes some of the important features of each viral vector

discussed previously.

1.6 Ex Vivo CRISPR Therapies

1.6.1 CAR T-Cell Therapy

Adoptive cell immunotherapy, or the transfer of lymphocytes to mediate effector

function, is not a novel concept; in 1992, it was shown that a single infusion for

CMV-specified CD8 CTLs could be used to treat disseminated CMV infection in

post-allogeneic transplant patients [ 163 ]. In 2002, CD4 effector cells were shown to

be efficiently transferred in HIV and elevated CD4 cell counts, and in 2005 it was

shown that vaccine responses could be augmented in patients with myeloma using

autologous T cells [ 164 , 165 ]. It is generally believed that the beginnings of modern

human immunooncology began with the approval of ‘1st Generation’ Sipuleucel-T

in 2010 and Ipilimumab, a CTLA-4 checkpoint inhibitor, in 2011 for treatment of

castrase-resistance prostate cancer. ‘2nd Generation’ agents included programmed

cell death protein 1 (PD1) and PD1 ligand 1 (PD-L1) blocking antibodies as well as

blinatumomab, a bi-specific antibody, an oncolytic GM-CSF-encoding herpes sim-

plex virus known as talimogene laherparepvec or T-vec for metastatic melanoma,

and CAR-T cells in 2014–2015 [ 166 ].

As previously mentioned, one of the most promising emerging uses of retroviral

vectors to treat human disease, specifically cancer, is known as chimeric antigen

receptor or CAR T-cell therapy. It involves the modification of patient T-cells to

target malignant cell populations by employing antigen receptors that bind to cancer

cell-specific antigens. CARs are fusion proteins that incorporate antigen recognition

Table 1.1 A summary of viral vector genome size and notable applications

Viral vector

Approximate genome

size (kilobase pairs) Notable applications

Adenovirus 36 In vitro gene delivery (highly immunogenic)

[ 90 , 126 ]

Vaccination against pathogens and addictive

compounds [ 87 , 113 , 116 ]

Destruction of malignant cells [ 119 , 121 ]

Adeno-associated

virus

4.7 In vitro and in vivo gene delivery [ 140 , 155 ]

Relatively lower immunogenicity [ 145 ]

Broad cell type specificity, depending on

serotype [ 152 ]

Retrovirus (including

lentivirus)

7–12 In vitro and in vivo gene delivery [ 57 , 62 ,

68 , 79 ]

Modification of cells for ex vivo therapy [ 82 ]

J.E. DiCarlo et al.