High Proportions of CD4+ T Cells among Residual Bone Marrow T Cells in Childhood Acute Lymphoblastic Leukemia Are Associated with Favorable Early Responses

Residual nonmalignant T cells in the bone marrow of patients with acute leukemias may be involved in active immune responses to leukemic cells. Here, we investigated the phenotypic signature of T cells present at diagnosis in 39 pediatric patients with acute lymphoblastic leukemia (ALL) treated within standardized ALL-BFM study protocols. Previously described age associations of lymphocyte subpopulations in the peripheral blood of healthy children were reproduced in leukemic bone marrow. Analysis of individual lymphocyte parameters and risk-associated variables using univariate linear regression models revealed a correlation of higher CD4/CD8 ratios at diagnosis with a favorable bone marrow response on day 15. Separate analysis of CD4⁺ cells with the CD4⁺CD25(hi)FoxP3⁺ T(reg) cell phenotype showed that the association was caused by non-T(reg) CD4⁺ cells. The association of higher CD4/CD8 ratios with a favorable bone marrow response on day 15 of treatment persisted in a cohort extended to 69 patients. We conclude that CD4⁺ non-T(reg) cells in leukemic bone marrow at diagnosis may have a role in early response to treatment. Prospective analysis of the CD4/CD8 ratio in a large cohort of pediatric patients is now needed. Moreover, future experiments will establish the functional role of the individual T cell subsets in immune control in pediatric ALL.