Evaluation of phagocytosis and arachidonate metabolism by alveolar macrophages and recruited neutrophils from F344xBN rats of different ages.

The incidence of infectious respiratory diseases increases with aging. Resident alveolar macrophages (AMs) and recruited leukocytes (PMNL) mediate cellular defense against bacterial infections in the lung, and phagocytosis and lipid mediator synthesis are important components of their antimicrobial capacity. The objective of this study was to determine if either phagocytic capacity or lipid mediator generation declines with normal aging, in either AMs or PMNL recruited to a site of inflammation. The F344xBN rat hybrid has a lower incidence of pathologies associated with aging, particularly up to 20 months; animals aged 6,12 and 18 months were chosen to evaluate changes associated with normal aging. As previously reported for peripheral blood leukocytes, phagocytosis by recruited PMNL declined with aging: recruited PMNL from 18 months rats showed a significantly decreased capacity to phagocytose live Klebsiella pneumoniae bacteria, compared to PMNL from 6 months rats. Surprisingly, however, the phagocytic capacity of AMs increased with aging: the phagocytic index of AMs from 18 months rats was more than three times that of AMs from 6 months rats. The capacity of AMs and recruited PMNL to release arachidonic acid or synthesize leukotrienes or prostaglandins did not change with aging. This study demonstrates that, although phagocytosis by recruited PMNL declines with aging, other aspects of immune function do not decline, and may actually increase, with normal aging. These results suggest that impaired phagocytosis by recruited PMNL may be an important component of the increased susceptibility to infectious respiratory diseases during normal aging.