Apoptosis is a major cause of so-called ‘Caseous Necrosis’ in mycobacterial granulomas in HIV-infected patients

Abstract

Background: Classical mycobacterial granulomas comprise circumscribed collections of epithelioid histiocytes with scattered Langhans’ type giant cells surrounded by a peripheral cuff of lymphocytes, plasma cells and fibroblasts. These granulomas may display central so-called ‘caseous necrosis’ that is conventionally described as a form of coagulative necrosis.

Aim: To demonstrate that so-called caseous necrosis is the result of apoptosis and investigate the association of B and T cells, and macrophages with the granulomas and their relationship to some apoptosis-related proteins.

Methods: Cervical lymph node biopsies from 55 cases of human-immunodeficiency virus- (HIV) infected Thai patients previously described as showing caseating granulomas, confluent caseating granulomas, sarcoid-like granulomas, foamy macrophage response, pseudo-inflammatory tumour response or non-specific lymphoid hyperplasia were examined histologically and for apoptosis by immunostaining for Caspase 3 and TUNEL. In addition, stains for CD20, CD3, CD4, CD8, CD68, Bax, Fas and Bcl-2 proteins were studied. Classic tuberculoid caseating granulomas in cervical lymph node and lungs from non-HIV-infected patients were also stained with Caspase 3.

Results: All areas of caseous necrosisfrequently displayed extensive apoptosis that readily accounted for the so-called ‘necrosis’. Small foci of apoptosis were present in the other reaction patterns and fibrotic granulomas often showed residual apoptosis. The extent of apoptosis was inversely related to the numbers of identifiable acid-fast bacilli and all epithelioid macrophages revealed strong immunoexpression of the pro-apoptotic proteins Bax and Fas, whereas, the anti-apoptotic protein Bcl-2 was not present. Apoptosis occurred in CD68+ macrophages and CD3+ CD8+ T cells and all nodes were deficient of CD4+ cells. CD8+ T cells were intimately related to the apoptotic foci suggesting a role in the process, particularly in the absence of CD4+ cells. In non-HIV-infected cases, similar extensive apoptosis was confirmed with Caspase 3.

Conclusions: So-called ‘caseous necrosis’ is demonstrated for the first time to be the result of apoptosis. In the absence of CD4+ cells our findings negate many of the postulated mechanisms of apoptosis in the murine model and have implications for the treatment of mycobacterial infections.