I want to use a P. aeruginosa PA14 non-essential mutant library from Dr. Fred Ausubel’s lab to screen the molecules that activate p38/PMK-1-dependent genes in worms. However, a professor questioned me how I would do this. The following is what he said. Is he right?
“Orthodoxy says that PAMP should be fundamental and essential for the organism which in turn means that they are less likely to show when mutating nonessential genes. For example lipid A is a PAMP but the O antigen on LPS is not. I was just curious if anyone has been successful delete PAMPs in a similar screen. If they have been, it would argue that orthodoxy is wrong, which would be fun.”
From the information that I found, I guess he is right. “Innate immunity does not recognize every possible antigen. Instead, it is designed to recognize molecules shared by groups of related microbes that are essential for the survival of those organisms and are not found associated with mammalian cells.”
These studies argue that C. elegans senses damage from the infection (DAMPs: damage associated molecular patterns) not the infection itself (PAMPs). Growth on pathogens trigger an aversion
response, and RNAi inactivation of core cellular processes (translation, respiration, protein homeostasis, etc.) also triggers this response. Melo and Ruvkin suggested that many of these processes
are the targets of bacterial toxins, thus the animal senses infection through the damage incurred during infection. Such a system might allow the immune system to be more flexible to evolving
threats and make it more difficult for the pathogen to escape the response.
You could follow McEwan et al.'s approach, where they expressed ToxA in non-pathogenic E. coli and observed that worms carrying an irg-1::GFP immune reporter activated the reporter similar
to animals grown on Pseudomonas.
Jordan, thank you very much. I read Debrah’s paper and also a review paper on her paper and the paper from Troemel’s lab. I thought I could use Debrah’s method to express candidate PA14 genes, but there still have some problems. Although these papers suggest it might be infection that triggers immune response, I still want to test if worm might recognize any PAMP. At least I want to try.
Now, I am still not convinced by the information I found that only essential genes trigger innate immune response. Am I wrong?
It is not only the products of essential genes that trigger immune responses. A well-studied example is the activation of the inflammasome by pore-forming toxins. I suggest you read:
Nat Rev Immunol. 2013 Mar;13(3):199-206. doi: 10.1038/nri3398. Epub 2013 Feb 15.
Effector-triggered versus pattern-triggered immunity: how animals sense pathogens.
Stuart LM, Paquette N, Boyer L.
Even in the case of LPS, bacteria have the capacity to alter their lipid A to modulate the host response. See for example:
Front Cell Infect Microbiol. 2013;3:3. doi: 10.3389/fcimb.2013.00003. Epub 2013 Feb 12.
Recognition of lipid A variants by the TLR4-MD-2 receptor complex.
Maeshima N, Fernandez RC.