Interleukin-2-induced Tumor Necrosis Factor-alpha (TNF-α) Gene Expression in Human Alveolar Macrophages and Blood Monocytes

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Abstract
Recent investigations have demonstrated interleukin-2 receptor (IL-2R) expression on both human alveolar macrophages (AMø) and blood monocytes (PBM), but the function of these receptors has not been fully elucidated. In this study, we demonstrate that human AMø, as well as PBM, can be induced to express biologically active TNF-α after challenge with interleukin-2 (IL-2). Furthermore, we examined the expression of TNF-α at the mRNA level via Northern blot and in situ hybridization analysis. Normal AMø, obtained by bronchoalveolar lavage, and PBM were stimulated with either IL-2 (2,000 U/ml) or lipopolysaccharide (LPS) (10 µg/ml) for 18 h. Specificity was demonstrated by neutralizing TNF-α activity with a polyclonal rabbit anti-human TNF-α antibody. PBM TNF-α biologic activity from 11 subjects challenged with either IL-2 or LPS was 19 ± 6 and 85 ± 15U/ml/106 cells, respectively, which represented 5-fold and 21-fold increases over control values. AMø TNF-α biologic activity from nine subjects was 110 ± 28 (IL-2-mediated) and 304 ± 69 (LPS-mediated) U/ml/106 cells, which represented 2- and 6-fold increases over controls. AMø exhibited statistically greater (p < 0.05) TNF production in response to both IL-2 and LPS as compared to PBM. IL-2 challenge resulted in an induction of TNF-α mRNA accumulation, as demonstrated by Northern blot and in situ hybridization analyses. TNF-α mRNA was quantitated by laser densitometry for Northern blots or by counting the number of silver grains/mononuclear phagocytic cell in the in situ hybridization analysis. As a control, the accumulation of actin mRNA was determined in unstimulated as well as in IL-2- and LPS-challenged macrophages. IL-2 challenge resulted in a 193% increase in TNF-α mRNA by AMø compared to controls. The peak in IL-2-induced TNF-α mRNA was delayed 3 to 6 h in both AMø and PBM as compared to the peak induced by LPS challenge. Actin mRNA accumulation was unaffected by either stimuli. These data indicate that the effects of IL-2 are not restricted to lymphocyte populations but may influence macrophage effector functions. Thus, IL-2 may represent an additional cell-to-cell communication signal between lymphocytes and macrophages which may modulate chronic cell mediated inflammatory processes in the lung.