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Aging and hematopoiesis. VI. Neutrophilia and other leukocyte changes in aged mice.

The concentration of blood leukocytes rose progressively as mice aged. All blood leukocytes increased, although a greater degree of increase was seen in neutrophils and monocytes than in lymphocytes and eosinophils. The total number of nucleated cells per marrow cavity of the humerus was also higher in aged than in young adult mice, the increase primarily reflecting peroxidase-positive cells. Both blood and marrow neutrophils of aged mice responded to perturbations induced by bleeding or by endotoxin injection in a manner qualitatively similar to that seen in young adults. When hematopoietic chimeras were produced by marrow transplantation, blood and marrow neutrophils were characteristic of the age of the recipient, not the cells; i.e., young mice kept a "young" neutrophil pattern and old mice kept an "old" neutrophil pattern when given marrow from either old or young mice. Colonies of granulocytes and/or monocyte-macrophages were grown from young marrow cells placed in plasma clots in Millipore chambers in the peritoneal cavity. The number of colonies was the same in young and in old mice, suggesting that long-range humoral stimulation of cell growth was similar in young and old. Thus, neutrophilia due to increased neutrophil production appears to be a normal part of the aging process in the mouse. The increase in neutrophil production may be due to a changing hematopoietic microenvironment.