When the checkpoint and partner protein bind to each other, they send a signal that causes the immune system to perform a specific action such as proliferation or destruction of target cells. Activation of immune checkpoint signaling pathways can lead to either suppression or stimulation of the immune system, depending on the signaling pathway.
Co-stimulatory receptors include CD28 and ICOS (inducible T cell co-stimulator), 4-1BB, OX40, CD27, CD30, CD40, and GITR (glucocorticoid inducible TNF receptor-related protein). Co-inhibitory receptors include CTLA4, Programmed Death-1 (PD-1), BTLA (B and T lymphocyte attenuator), lymphocyte activation gene-3 (LAG-3), and TIM3 (T cell immunoglobulin and mucin domain-containing protein 3) on T cells (Ref.1).
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Trends Immunol. 2013 Nov;34(11):556-63. doi: 10.1016/j.it.2013.07.003. Review.