Radiation exposure and longitudinal changes in peripheral monocytes over 50 years: the Adult Health Study of atomic‐bomb survivors

Summary Enhanced inflammatory responses have been suggested decades after radiation exposure in atomic‐bomb survivors, but cellular and molecular alterations related to prolonged inflammation remain unclear. This study, utilizing longitudinal haematological data over 50 years for 14 000 persons, investigated whether radiation exposure promoted the relative increase in peripheral myeloid cells, known as an aging‐associated indicator of low‐grade inflammation. Statistical modelling was performed with a linear mixed‐effects model for leucocyte subsets, together with a proportional hazards regression model for all‐cause mortality. We found that age trends in lymphocyte, neutrophil and monocyte percentages or counts differed before versus after age 60 years. Radiation dose was associated with monocyte percentages and counts, but not with the lymphoid‐myeloid cell ratio. Radiation effects on monocytes were stronger after versus before age 60 years. Increases in monocyte percentages and counts were associated with higher risk of all‐cause mortality. Studies of chromosomal aberrations have shown a clonal expansion of haematopoietic stem cells among atomic‐bomb survivors. Therefore, radiation exposure might accelerate aging‐associated clonal haematopoiesis, which could result in a long‐lasting elevation of circulating monocytes.