A huge thanks to co-authors @ashleytseng.bsky.social , @danielhfisch.bsky.social, @StephanieRaglandNotOnBlueSky, fearless leader @jkagan1.bsky.social, and to colleagues who gave advice and strains so I could learn to work with Staph. Feel free to comment or DM with any questions/suggestions!

WTA here, despite being “immunogenic” in its ability to promote immune responses, does not directly activate immune receptors. Rather, it regulates availability of the PAMP DNA, essentially acting as a “meta-PAMP” akin to Legionella meta-effectors that regulate activity of direct-acting effectors.

Collectively these data suggest that in contrast to toxigenic S. aureus infection where NLRP3 is robustly activated, during nontoxigenic infection S. aureus protects itself from being detected by cytosolic DNA sensors by O-acetylating its PGN, thus limiting WTA abundance on its cell surface.

Finally, we tested if differences in bacterial DNA release impact other immune DNA sensors as well, in particular cGAS. Using cells designed and generated by my incredible colleague Stephanie Ragland, we found that OatA and TarMS affect cGAS activation in the same way they affect AIM2 activity.

Our finding that S. aureus activates a DNA sensor suggested OatA and TarMS regulate DNA release within infected cells. We purified and quantified extrabacterial DNA from infected macrophages, and consistent with our hypothesis found oatA mutants released more whereas tarMS mutants released less DNA.

Using an AIM2-mScarlet fluorescent fusion protein and GFP-producing bacteria, incredible PhD student @ashleytseng.bsky.social with a major assist from @danielhfisch.bsky.social found that AIM2 specks (red), representing inflammasomes, form in close proximity to bacteria (green).

To clarify the mechanism by which WTA enhances inflammasome activation we turned to the host, reasoning that identifying the receptor required would lead us to the ligand that WTA controls. We identified DNA sensor AIM2 as the receptor activated during nontoxigenic S. aureus infection.

We then returned to TarM, which glycosylates WTA. Glycosylation of WTA is increasingly appreciated to play important roles in host-microbe interactions. We knocked out glycosyltransferases TarM and TarS and found that both promote inflammasome activation, with TarM having a more potent effect.

Further supporting our hypothesis, chemically blocking WTA synthesis ablated inflammasome activation and eliminated the difference between WT and oatA strains. Thus, WTA is essential for inflammasome activation by nontoxigenic S. aureus, and is required for the immunoevasive function of OatA.

TarM is an enzyme that modifies cell wall component wall teichoic acid (WTA). Strikingly, OatA acetylates the identical PGN site that WTA is attached to. We found that oatA mutants have more WTA than WT bacteria, suggesting the way OatA reduces inflammasome activation is by reducing WTA abundance.

We hypothesized OatA instead restricts the activity of a bacterial factor that promotes inflammasome activation. To identify such a factor, we generated a transposon mutant library in the oatA mutant background and isolated mutants defective for activating inflammasomes; we identified a tarM mutant.

PGN acetylation by OatA blocks bacterial killing by phagosomal lysozyme; this was thought to be how OatA prevents inflammasome activation. But we knocked out lysozyme from iBMDMs and found it was dispensable for inflammasome activity. OatA’s role here is thus distinct from its established function.

Prior studies have shown bacteria lacking PGN O-acetyltransferase OatA elicit greater inflammasome activation than WT bacteria. We rederived an oatA mutant in a nontoxigenic S. aureus background and reproduced the finding that OatA helps S. aureus evade inflammasomes during infection of macrophages.

S. aureus secretes toxins that kill host cells and activate the NLRP3 inflammasome. After Staph spreads through the blood it reduces toxin production and can survive inside host cells, triggering minimal immunity. How does nontoxigenic Staph avoid activating inflammasomes within infected cells?

Thanks so much Brian! I hope you and Irma are both doing well yourselves!!

Notably, WTA is not a PAMP.
Rather, WTA controls the release of bacterial DNA into cells, where it can act as a PAMP for AIM2 and cGAS. How many other meta-PAMPs are out there?
@jordanjastrab.bsky.social @harvardmed.bsky.social @bostonchildrens.bsky.social @harvardmicro.bsky.social

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Thanks Lauren!!