PDCD1 and Sepsis: Clinical studies have confirmed that the expression of PD-1 in immune cells of sepsis patients is upregulated, accompanied by the downregulation of HLA-DR and CD28 and the increase in Treg activation (75), directly correlating the PD-1 pathway with poor prognosis.CTLA-4: Highly homologous to CD28, it competitively binds to CD80/CD86, interrupting the CD28 co-stimulatory signal, and transmitting inhibitory signals to inactivate T cells (76).