PDCD1 and small cell lung carcinoma: An in vitro experimental study using small cell lung cancer (SCLC) stem cells found that the interaction between T cells and CD44+/CD90+ CSCs resulted in an upregulation in ICPs, such as PD-1, CTLA-4, lymphocyte-activation gene 3 (LAG-3), and T cell immunoglobulin and mucin-domain containing-3 (TIM-3) [50].