How and why do T cells and their derived cytokines affect the injured and healthy brain?

Recent evidence suggests that T cells and their derived cytokines affect the brain in disease and health. In this Opinion article, Kipnis and colleagues describe their effects and possible underlying mechanisms, and propose an evolutionary model to explain why the T cell-derived cytokine interferon-γ has both pro-social and immune effects. The evolution of adaptive immunity provides enhanced defence against specific pathogens, as well as homeostatic immune surveillance of all tissues. Despite being 'immune privileged', the CNS uses the assistance of the immune system in physiological and pathological states. In this Opinion article, we discuss the influence of adaptive immunity on recovery after CNS injury and on cognitive and social brain function. We further extend a hypothesis that the pro-social effects of interferon-regulated genes were initially exploited by pathogens to increase host–host transmission, and that these genes were later recycled by the host to form part of an immune defence programme. In this way, the evolution of adaptive immunity may reflect a host–pathogen 'arms race'.