[frontierkodiak]

The article states “The first base edit disabled the T-cells targeting mechanism so they would not assault Alyssa's body”. Not an expert but I presume the ‘targeting mechanism’ is the T-cell receptor, whose activation is necessary (but not sufficient) to trigger immune response.

[hooloovoo_zoo]

Maybe, but the second transplant also needs to not assault Alyssa.

[Narretz]

The second transplant is a stem cell (bone marrow) transplant, i.e. it replaces the cells that produce the t-cells.

This is how leukemia therapy is normally finished after chemotherapy / radiation therapy kills the cancer cells.