CD4 and type 1 diabetes mellitus: Trials employing treatment with abatacept, which blocks CD80/CD86-dependent T-cell co-stimulation (the second signal required for T-cell activation), have shown that abatacept slows the decline in beta cell function in new-onset T1D patients in the first 2 years of treatment and impact the CD4 T memory T cell compartment by reducing effector T cells and, specifically, follicular helper T cells [33,34,35].