NINJ1 identified as culprit in TB-infected cell rupture

Lytic cell death — where infected immune cells burst open — is a major factor in tissue damage and spread of tuberculosis, but the exact events that make infected cells rupture have been unclear. In work led by Trude Helen Flo, researchers searched for the missing link between infection and the final breakdown of the cell membrane. They focused on Ninjurin-1 (NINJ1), a protein already known to mediate plasma membrane rupture (PMR) in other settings. Using human macrophages infected with Mycobacterium tuberculosis (Mtb), the team examined how infection changed NINJ1 behavior and how blocking NINJ1 affected cell lysis. The group observed that infection triggered NINJ1 to form oligomers — clumps of the protein that are associated with membrane rupture. When NINJ1 activity was reduced by genetic methods or by pharmacological inhibition, release of lactate dehydrogenase (LDH), a marker of membrane breakdown, dropped substantially. These observations pointed to NINJ1 as a central executioner of macrophage lysis during Mtb infection rather than a bystander effect.

The researchers tested whether known cell death pathways explained plasma membrane rupture in Mtb-infected macrophages. They found that individual or combined inhibition of pyroptosis, apoptosis, necroptosis, or ferroptosis failed to prevent PMR, indicating that none of these classical programs alone explained the membrane rupture seen. Instead, attention turned to the Mtb ESX-1 secretion system and its effector EsxA. The team showed that the ESX-1–secreted effector EsxA directly induced NINJ1 oligomerization, implicating pathogen-driven membrane damage as the upstream trigger for NINJ1 activation. NINJ1 activation did not depend on calcium or on high-MW PEG, although polyethylene glycol (PEG) reduced LDH release, suggesting that swelling-related membrane changes contribute to rupture. Cytokine measurements revealed that most inflammatory release was NINJ1-independent, but CXCL-10 levels were reduced in NINJ1-deficient macrophages, linking NINJ1 to at least one specific immune signal.

These findings establish NINJ1 as a key executioner of lytic cell death during Mtb infection and shift the focus from canonical death pathways to a membrane-focused mechanism triggered by a bacterial effector. The direct activation of NINJ1 by EsxA suggests a model in which pathogen-induced membrane damage leads to NINJ1 oligomerization and subsequent plasma membrane rupture, allowing cellular contents to spill into the tissue. That cytokine release was mostly unaffected by loss of NINJ1 — except for CXCL-10 — indicates that blocking NINJ1 might prevent damaging cell rupture while leaving much of the immune signaling intact. This distinction is important for thinking about interventions: targeting NINJ1 or its activation by EsxA could potentially limit tissue damage and bacterial spread without completely suppressing immune responses. At minimum, the work by Trude Helen Flo and colleagues provides a clearer molecular picture of how tuberculosis causes infected immune cells to lyse.