Oral Presentation Lorne Infection and Immunity 2018

Dissecting the role of TNF signalling in Mycobacterium tuberculosis disease pathogenesis to identify novel therapeutic targets (#28)

Michael Stutz 1 2 , Samar Ojaimi1 Ojaimi 1 2 , Cody Allison 1 2 , Philip Arandjelovic 1 2 , Marc Pellegrini 1 2
  1. Walter and Eliza Hall Institute of Medical Reseach, Parkville, VIC, Australia
  2. Department of Medical Biology,, The University of Melbourne, Victoria , VIC, Australia

TNF is an essential mediator of host protection against Mycobacterium tuberculosis (Mtb), predominantly through its engagement of macrophages – the pathogen’s intracellular replicative niche. TNF promotes cell survival in the presence of cellular inhibitor of apoptosis (cIAP) proteins, while in their absence, programmed cell death via either caspase 8-dependent apoptosis or MLKL-dependent necroptosis is the dominant outcome. However, the aspects of TNF signalling that impact on Mtb disease remain controversial, particularly in relation to cell death pathways, which have been flagged as potential therapeutic targets. We adopted an in vivo genetic approach to dissect the contributions of TNF-mediated death pathways to the control of Mtb infection.

 

Blocking necroptosis by deleting Mlkl had no effect on the survival of macrophages infected with Mtb in vitro, and did not impact on disease outcomes in vivo, with Mlkl-/- mice displaying lung histopathology and Mtb burdens indistinguishable from controls. We therefore examined the contribution of TNF-driven extrinsic apoptosis in Mtb pathogenesis by infecting Caspase 8-/- mice. Inhibiting extrinsic apoptosis resulted in worse lung histopathology and a significant increase in lung bacterial burdens. Surprisingly, Caspase 8-/- mice had a strikingly reduced number of Mtb-specific CD4+ T cells, indicating a potential impairment in the priming of adaptive immunity, which may be partly responsible for the compromised ability of these mice to control infection. Given the apparent host-protective function of apoptosis, we hypothesized that inducing this pathway may promote clearance of Mtb. To examine this, we infected cIAP1-deficient mice, and found a significantly reduced bacterial burden in the lungs of these mice. Importantly, we were able to replicate this phenotype using the clinical stage IAP antagonist compound birinapant.

 

Thus, although necroptosis is redundant in Mtb infection, we have identified TNF-driven apoptosis of infected macrophages as a host-protective process which can be therapeutically targeted to promote clearance of Mtb.