CD4 and leukemia: Strategies to target CD4 immunity are notably bolstered by evidence in mice and humans that ICOS+ CD4+ T cells are an important correlate of response to CTLA-4 immune checkpoint blockade [68,69] and longitudinal tracking of CAR-T recipients showing that CD4+ responses, particularly TH2 CD4+ responses, underscore functional T cell persistence and durable remission in leukaemia [70].