Porins are the constitutive pore-forming proteins present in the outer membrane of the Gram negative bacteria. They play crucial roles in the nutrient exchange, and in maintaining other functions important for bacterial physiology. Being exposed on the bacterial surface, porins have the potential to modulate the host responses as well. Vibrio cholerae porin OmpU plays crucial roles in the survival of the bacteria in the gut of the host during infection. In our study, we have explored various host-modulatory functions of OmpU. We have observed that V. cholerae OmpU can activate monocytes and macrophages in terms of pro-inflammatory cytokine production. The mechanistic pathway underlying the response varies to some extent in both these cells. In both monocytes and macrophages, NFkB and AP-1 transcription factors are involved in OmpU-induced responses. However, in monocytes MAP kinases are activated in TLR1/2-dependent manner, while in macrophages MAP-kinases are activated in both the TLR-dependent and independent manner. In response to OmpU, the additional pathway that kicks in the macrophages may reflect upon the notion why macrophages are more pro-inflammatory than monocytes. Further, we have also observed that OmpU possesses the ability to attenuate LPS-mediated pro-inflammatory responses by interfering with the LPS-dependent TLR-signaling pathway.
Furthermore, we have observed that OmpU has the ability to translocate from the bacterial outer membrane to the host-cell mitochondria, causing non-apoptotic programmed cell death by inducing mitochondrial permeability transition, and release of apoptosis-inducing factor (AIF), which then translocates to nucleus and chops up the DNA. Though we observed release of cytochrome c, no caspase-activation is observed in the process.
Overall, our findings highlight the fact that a single bacterial ligand can modulate host cells in so many different ways. Also, the immune cells of the same origin can differ in their responses to the same ligand.