PD-1 and CTLA-4. Shown is a simplified representation of the function of the PD-1 and CTLA-4 immune checkpoint pathways. APC, antigen-presenting cell; CD, cluster of differentiation; IL-2, interleukin-2; ITIM, immunoreceptor tyrosine-based inhibitory motif; ITSM, immunoreceptor tyrosine-based switch motif; TCR, T-cell receptor.