Functional age-dependent changes in bronchoalveolar lavage rat cells.

Alveolar macrophages (AM) are located at the first line of non-specific defense against inhaled antigens in the lower respiratory tract and therefore represent the major effector cell in antimicrobial defense. Since children under 2 years are known to manifest increased susceptibility to lung infections we used a rat model to study functional capacities of the AM during different stages of development We analyzed several steps of the phagocytic process (adherence, chemotaxis and ingestion) as well as two different mechanisms of cytotoxicity [antibody dependent cellular cytotoxicity (ADCC) and cytotoxicity triggered by immune complex (ICC)] and tumor necrosis factor (TNF-alpha) secretion. We used young (4-6 weeks old), intermediate (16-25 weeks old) and adult (36-45 weeks old) rats. Adherence and phagocytic capacities of AM were lower in young rats compared to intermediate and adult animals. Chemotaxis towards the C5a complement component was low in the first two months of life, then it increased in the intermediate group and fell again in adults. Bronchoalveolar lavage (BAL) cells from young rats did not produce detectable TNF-alpha levels even when stimulated with phorbol 12-myristate 13-acetate (PMA). When we studied two different cytotoxic mechanisms we found that ICC markedly declines from youth to adulthood while ADCC showed a steady increase from youth to adulthood. In conclusion, our data show differences that may help to explain in part the enhanced susceptibility to pulmonary infections found in young children.