Setting: Peripheral blood monocytes (PBM) are the main source of alveolar macrophages, which have an upregulation of inducible nitric oxide synthase (iNOS) in pulmonary tuberculosis (TB). TNF-alpha and IL-1 beta are thought to be involved in the immune response to mycobacterial infection.
Objective: To identify whether iNOS expression and cytokine release of PBM are upregulated and have a connection in TB infection.
Design: The expression of iNOS immunoreactivity on PBM from TB patients and normal subjects was measured by loading with anti-macrophage iNOS polyclonal primary antibody analyzed by flow cytometry. Expression of iNOS mRNA in PBM was detected by RT-PCR. The spontaneous generation of nitrite and cytokines (IL-1 beta and TNF-alpha) by cultured monocytes was also determined.
Results: Compared to normal subjects, iNOS immuno-reactivity, the capacity for spontaneous nitrite generation and the level of TNF-alpha or IL-1 beta secretion of PBM were significantly higher in TB patients. The amount of nitrite, TNF-alpha and IL-1 beta released from PBM of TB patients was inhibited by NG-monomethyl-L-arginine (L-NMMA), a competitive inhibitor of NOS. The level of iNOS immunoreactivity on PBM was highly correlated with nitrite generation both in all the subjects studied and in TB patients alone. Spontaneous TNF-alpha production showed a stronger correlation with nitrite production than with IL-1 beta.
Conclusion: The NO and cytokine synthase activities of monocytes appear to be concomitantly upregulated in response to mycobacterial infection. The enhanced NO generation by monocytes in TB patients may play an autoregulatory role in amplifying the synthesis of pro-inflammatory cytokines.