CD4 and sarcoidosis: CD28-mediated co-stimulation, essential for cell cycle progression through PI3K/AKT signaling, is counteracted by the immune checkpoint receptor PD-1.129In progressive sarcoidosis, CD4+T cells exhibit high PD-1 expression, which suppresses TCR-driven activation of the PI3K/AKT/mTOR pathway, impairing proliferation and cell cycle progression130and subsequently contributing to T cell exhaustion.