The malaria parasite sheddase SUB2 governs host red blood cell membrane sealing at invasion.

Red blood cell (RBC) invasion by malaria merozoites involves formation of a parasitophorous vacuole into which the parasite moves. The vacuole membrane seals and pinches off behind the parasite through an unknown mechanism, enclosing the parasite within the RBC. During invasion, several parasite surface proteins are shed by a membrane-bound protease called SUB2. Here we show that genetic depletion of SUB2 abolishes shedding of a range of parasite proteins, identifying previously unrecognized SUB2 substrates. Interaction of SUB2-null merozoites with RBCs leads to either abortive invasion with rapid RBC lysis, or successful entry but developmental arrest. Selective failure to shed the most abundant SUB2 substrate, MSP1, reduces intracellular replication, whilst conditional ablation of the substrate AMA1 produces host RBC lysis. We conclude that SUB2 activity is critical for host RBC membrane sealing following parasite internalisation and for correct functioning of merozoite surface proteins.

Malaria kills or disables hundreds of millions of people across the world, especially in developing economies. The most severe form of the disease is caused by Plasmodium falciparum, a single-cell parasite which, once inside a human host, forces its way into red blood cells to feed on a protein called haemoglobin. This invasion relies on P. falciparum being engulfed by the membrane of the red blood cell, which then seals off to form a compartment inside the cell where the parasite can feed and multiply. Invasion takes less than 30 seconds, and it involves P. falciparum losing the coat of proteins that covers its surface. An enzyme calls SUB2 cleaves or cuts off these proteins, but exactly why and how the shedding takes place during infection is still unclear. To investigate, Collins, Hackett et al. deactivated the gene which codes for SUB2, and examined how mutant P. falciparum would survive and multiply. Without the enzyme, the parasites failed to shed many of their proteins, including some that were not previously known to be removed by SUB2. The majority of the genetically modified parasites also failed to invade red blood cells. In particular, most of the host cells ruptured, suggesting that the protein coat needs to be discarded for the engulfing process to be completed properly. When the enzyme-free mutants did manage to make their way into a red blood cell, they starved to death because they could not digest haemoglobin. SUB2 and surface coat shedding therefore appears to be essential for the parasite to survive. P. falciparum is fast becoming resistant to the many drugs that exist to fight malaria. New treatments that target SUB2 may therefore help in combatting this deadly disease.

Mycobacterium tuberculosis cords within lymphatic endothelial cells to evade host immunity.

The ability of Mycobacterium tuberculosis to form serpentine cords is intrinsically related to its virulence, but specifically how M. tuberculosis cording contributes to pathogenesis remains obscure. Here, we show that several M. tuberculosis clinical isolates form intracellular cords in primary human lymphatic endothelial cells (hLECs) in vitro and in the lymph nodes of patients with tuberculosis. We identified via RNA-Seq a transcriptional program that activated, in infected-hLECs, cell survival and cytosolic surveillance of pathogens pathways. Consistent with this, cytosolic access was required for intracellular M. tuberculosis cording. Mycobacteria lacking ESX-1 type VII secretion system or phthiocerol dimycocerosates expression, which failed to access the cytosol, were indeed unable to form cords within hLECs. Finally, we show that M. tuberculosis cording is a size-dependent mechanism used by the pathogen to avoid its recognition by cytosolic sensors and evade either resting or IFN-γ-induced hLEC immunity. These results explain the long-standing association between M. tuberculosis cording and virulence and how virulent mycobacteria use intracellular cording as strategy to successfully adapt and persist in the lymphatic tracts.