Creation of a CAR-T cell requires the ability to add genes of our choosing into T cells. As some viruses have evolved the ability to cross the cell membrane into cells and integrate their own genes into our genome, they can be modified and then employed to insert new genes into the selected target cells (Figure 1.3). These viruses are known as retroviruses, of which HIV is perhaps the most famous example. This is a subtype of retrovirus known as a lentivirus; both retroviruses and lentiviruses are used in gene transfer technologies.
Early on in CAR-T cell development, gene transfer was possible but difficult and very inefficient. As TCRs are made up of two separate proteins (α and β; see Figure 1.2b) encoded by two different genes, the process was even more difficult. This problem was made easier by designing a new, single, artificial gene that produced a functional protein that could both recognise antigen and transmit signals to the cell. This was achieved by connecting the previously separate heavy- and light-chain variable genes into one gene connected by a linker. These constructs are now termed first-generation CAR-T cells (Figure 1.4).
