Small-cell lung cancer: anatomy of an immune-cold tumor

Small-cell lung cancer (SCLC) is an aggressive neuroendocrine (NE) tumor and a leading cause of cancer-related morbidity. The introduction of immune checkpoint inhibitors (ICIs) transformed the treatment of many other cancers but has so far failed to benefit all but a minority of SCLC patients who gain a modest increase in overall survival. Although SCLC is often considered to be 'immune-cold', there is no consensus mechanistic view on why most patients fail to respond to ICI therapy. We address this important question by reviewing recent genomic profiling studies that reveal a complex immune landscape. Each molecular subtype is associated with a unique pattern of immune infiltration and a program of cellular plasticity that involves loss of NE traits. This immunobiology presents a rapidly evolving case study in mechanisms of ICI response and resistance. We discuss recent developments, present new hypotheses, and explore future directions for the field.