We all use anti-PD1 or anti-CTLA4 to release the brake of immune response. And we all know that CD3/28 activator can always help T cell expansion and activation. So, why don't we use anti-CD3/28 as treatment in cancer setting?
The main reason this is not done in vivo is that polyclonally activating ALL T cells via CD3/CD28 will cause CRS - cytokine release syndrome [also called cytokine storm] which is life-threatening and can be fatal.