Apparent killing of Mycobacterium tuberculosis by cytokine-activated human monocytes can be an artefact of a cytotoxic effect on the monocytes.

A protocol using combined exposure to interferon-gamma (IFN-gamma), calcitriol and tumour necrosis factor-alpha (TNF-alpha) has been reported to activate human monocytes in vitro to kill Mycobacterium tuberculosis. We have attempted to repeat the findings in two laboratories, with negative results; treated cells were no different from untreated cells in this respect. However, the treated cells were more sensitive to a toxic effect of the bacteria. We suggest that the reported dramatic mycobacterial killing may have been an illusory consequence of the toxicity leading to cell lysis and loss of the liberated bacteria from the assay.