high cytotoxic potency is linked to CD8+ effector memory T (TEM) and CD45RA+ TEM (TEMRA) cells, and to CD4+ TEM and central memory T (TCM)-cells due to their large number of preformed perforin and granzyme B-filled vesicles and their great proliferative capacity. Another notable T-cell subset in the context of immunotherapy are CD4+ regulatory T (TREG) cells. Their presence in the tumor microenvironment is implicated in the suppression of naturally occurring anti-tumor T-cell responses

The absence of a need for co-signaling through CD28 for these BiTEs might also be explained by the observation that among all T cell subtypes, CD8+ effector memory cells CD45RO+ (TEM) and CD8+ effector memory CD45RA+ (TEMRA) contribute the most to BiTE activity, whereas naïve T cells do not contribute at all to the killing efficiency.50 It is believed that memory T cells do not require CD28 costimulation for expansion during secondary responses, which could explain the efficiency of BiTEs. However, this dogma has recently been challenged.5