Summary of PD-1 and CTLA-4 function. Simplified representation of the function of the PD-1 and CTLA-4 immune checkpoint pathways. APC indicates antigen-presenting cell; TCR, T-cell receptor; MHC, major histocompatibility complex; CD, cluster of differentiation; IL-2, interleukin-2; PD-1, programmed death-1; CTLA-4, cytotoxic T-lymphocyte associated protein 4; ITIM, immunoreceptor tyrosine-based inhibitory motif; and ITSM, immunoreceptor tyrosine-based switch motif. Reprinted from Armand.³⁸