Cytokines and tuberculosis : an investigation of tuberculous lung tissue and a comparison with sarcoidosis
The formation of granulomas at the site of antigen presentation in both tuberculosis and sarcoidosis is an essential component of host immunity for controlling inflammation. Granuloma formation is a complex process that also requires recruitment and activation of lymphocytes and macrophages to the site of infection and arrangement into a granuloma. It is dependant on the activation of especially IFNγ secreting CD4+ T cells, resulting in a Th1 profile. However, it is suggested that a persistently high IFNγ is responsible for the damage caused by granulomatous disease and that moderating cytokines, resulting in a Th0 profile, are necessary to down-regulate the IFNγ response to more appropriate levels later in the disease process, after the antigen has been effectively contained. I propose that: “Cytokine profiles determine clinical and histopathological phenotypes of disease. This thesis tests the hypothesis that it will be reflected by cytokine expression profiles in granulomas in different forms of tuberculosis and in sarcoidosis.” To examine this, biopsy tissue was obtained from patients with pulmonary cavitary tuberculosis, pleural tuberculosis in HIV sero-negative and sero-positive patients, and sarcoidosis. The diagnosis of tuberculosis or sarcoidosis was confirmed, granulomas were characterised as necrotic or non-necrotic, sarcoidosis cases were graded histologically and in situ hybridisation was performed for IL-12-, IFNγ-, TNFα- and IL-4-mRNA. In all patients with pleural tuberculosis, a Th0 profile was noted, while necrotic granulomas were more evident in HIV positive than HIV negative patients. There was a clear association between TNFα and necrosis in tuberculous granulomas that may be ascribed to the increased apoptotic activity of TNFα. An increase in IFNγ correlated with an increase in necrosis, supporting the theory that high IFNγ levels later in disease is detrimental. This effect may be enhanced by a strong presence of TNFα positive cells. An increase in both Th1 and Th2 cytokine mRNA in HIV positive patients supports the theory that an overproduction of cytokines may be a mechanism to compensate for the failure of another immune effector mechanism. Findings in pulmonary tuberculosis were similar to those in pleural tuberculosis. In all sarcoidosis cases the presence of a very strong Th1 and TNFα, but no Th0 response was confirmed. None of the differences in either the histological grading, or the clinical outcome of patients were reflected in the cytokine profile. It is possible that this profile does not reflect the histological grade of disease or that it may reflect various stages of disease. These findings support the theory that a strong Th1 presence later in disease, in conjunction with TNFα may induce fibrosis, as most of these cases showed signs of at least focal fibrosis. Numerous aspects, including a T helper response are involved in granulomatous inflammation. The earlier dogma of good, beneficial (Th1) versus evil, detrimental (Th2), is an oversimplification of a very complex process. It is clear that the effect of a cytokine depends at least partially on the stage of disease. The balance between the various cytokines, and the levels of these cytokines contribute to their role in resolution or disease progression. An early, pure Th1 response may be beneficial if effectively clearing the granuloma-inducing antigen. At this stage, a Th2 presence will be harmful as clearing of the antigen will not be as effective. In chronic disease where failure to remove the antigen results in progression of granulomas with subsequent necrosis and/or fibrosis, a proinflammatory Th1 response may be detrimental and minimising of this effect is needed. An overly strong presence of the various cytokines may also be detrimental, while lower levels will be beneficial.