To do that, we used the widely described PLC inhibitors D609 and U73122 [23–25]. Thus, we pre-incubated both Mtb isolates with PLC inhibitors (U73122 and D609) separately or combined (Additional file 1: Figure S1), and analysed the ability of the bacilli to cause necrosis and the effect on PGE2 production. The treatment of Mtb isolates with PLC inhibitors severely reduced necrosis of 97-1505-infected

cells, whereas it did not affect the necrosis of PLC-deficient 97-1200-infected cells. Moreover, treatment with PLC inhibitors had no effect on apoptosis induced by both isolates (Figure 5A, B and Additional file 2: Figure S2A). Likewise, PGE2 production by Mtb 97-1505-infected alveolar macrophages presented levels similar to those produced by 97-1200-infected cells, and PLC inhibition did not affect the PGE2 production in cells infected by 97-1200 (Figure 5C Napabucasin and Additional file 2: Figure S2B). Finally, Epigenetics inhibitor to address the role of PGE2 in cell death, celecoxib, a COX-2 inhibitor, was added to the culture, which increased necrosis rate in cells infected with both isolates. On the other hand, addition of PGE2 prevented cell necrosis during infection with the isolate 97-1505 (Figure 5D and Additional file 2: Figure S2C). Taken together, these data reinforce that infection with Mtb harbouring PLCs induces host-cell necrosis, which

may be related to the subversion of PGE2 synthesis. Figure 5 PLC-expressing Mycobacterium tuberculosis induces alveolar macrophage necrosis through the regulation of PGE 2 synthesis. Alveolar macrophages were infected in vitro for 24 h with Mtb isolates 97-1200 or 97-1505 treated or not with the PLC inhibitors D609 (50 μM) and U73122 (10 μM). (A, B) ELISA assay of apoptosis and necrosis. (C) PGE2 production was assessed in supernatants by ELISA. (D) Celecoxib or PGE2 were added to the GW-572016 cell line culture of alveolar macrophages infected or not with 97-1200 2-hydroxyphytanoyl-CoA lyase or 97-1505 and necrosis was assessed by ELISA. # P < 0.0001 for uninfected cells vs. infected cells (97-1505 or 97-1200); ***P < 0.0001; **P < 0.001 (one-way ANOVA). Data are representative

of three (A, B) and two (C, D) independent experiments (error bars, s.e.m.). Discussion The central finding of this study was that PLC-expressing Mycobacterium tuberculosis is more virulent than Mtb lacking these enzymes, through inducing necrosis of alveolar macrophages, which is associated to subversion of PGE2 production. This is the first study to demonstrate such a role for mycobacterial PLCs using clinical isolates, which actually cause tuberculosis, instead of models of recombinant expression of these enzymes in non-pathogenic mycobacteria. We showed that PLC-expressing Mtb (isolate 97-1505) induced high rates of alveolar macrophage death, especially through necrosis, whereas the PLC-deficient Mtb (isolate 97-1200), despite its ability to cause cell death, did not induce necrosis as efficiently.

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