Listeria's Vis­it­ing Card

by Elio

Dur­ing in­fec­tion, host and par­a­site carry out re­peated and in­tense con­ver­sa­tions that of­ten rise to the level of shout­ing matches. The lan­guage they use is chem­i­cal, the words and sen­tences elo­quent and force­ful. Eaves­drop­ping on the con­ver­sa­tion be­tween Lis­te­ria mono­cy­to­genes and im­mune cells of its host, Port­noy and col­leagues dis­cov­ered an in­trigu­ing lin­guis­tic use for a novel "sec­ond mes­sen­ger," cyclic-di-AMP (c‑di-AMP).

Lis­te­ria mono­cy­to­genes (Scan­ning EM). Source

It has been known that Lis­te­ria, like many in­tra­cel­lu­lar pathogens, pro­motes the mak­ing of an in­ter­feron (IFN‑β), which in turn sig­nals cells of the im­mune sys­tem, thus ac­ti­vates them. But what sets this in mo­tion? What are the sig­nals emit­ted by Lis­te­ria and how are they "heard" by the host cells? The an­swer is that the bac­te­ria ex­crete c‑di-AMP, which ac­ti­vates the so-called cy­toso­lic sur­veil­lance path­way. One as­pect of this is the mak­ing of in­ter­feron. The im­mune re­sponse is now off and run­ning; mo­bi­lized killer lym­pho­cytes and other host de­fenses ward off the in­vad­ing or­gan­isms.

The bac­te­ria must be alive for this kind of im­mune sys­tem ac­ti­va­tion. How come? It turns out that live Lis­te­ria, like other bac­te­ria, have pow­er­ful, mul­tidrug ef­flux pumps, usu­ally thought to be there to get rid of harm­ful com­pounds such as an­tibi­otics. But these pumps func­tion here to se­crete a con­stituent of the bac­te­ria them­selves: c‑di-AMP. Such se­cre­tion is harm­ful to the bac­te­ria be­cause they now have to cope with the con­se­quences of hav­ing elicited pow­er­ful host de­fenses. Makes you won­der how come it was not se­lected against and what else is go­ing on with this story. Nev­er­the­less, here it is, an­other case where the in­vad­ing agents an­nounce their pres­ence to their own detri­ment. (In the words of Lewis Thomas, bac­te­r­ial lipopolysac­cha­ride is just such a "vis­it­ing card.")

How did the in­ves­ti­ga­tors make this dis­cov­ery? Ob­serv­ing Lis­te­ria mu­tants with mul­tidrug ef­flux pumps with vary­ing lev­els of ac­tiv­ity, they found that the amount of in­ter­feron made cor­re­lates with pump ac­tiv­ity. This sug­gested that in­duc­tion of in­ter­feron pro­duc­tion was due to a sec­retable, rel­a­tively low mol­e­c­u­lar weight com­pound. So, they frac­tion­ated cul­ture su­per­natants and iden­ti­fied the ac­tive com­pound as c‑di-AMP. Store-bought c‑di-AMP works the same way and in a dose-de­pen­dent fash­ion. What is novel and ex­cit­ing here is that this cyclic nu­cleotide works out­side the bac­terium. Such sig­nal­ing mol­e­cules are sup­posed to work within the cell that makes them. For a dis­cus­sion on the se­cre­tion mech­a­nisms of vir­u­lence fac­tors, see a re­view by Vance, Is­berg, and Port­noy.

Cyclic-di-AMP. Source

Like Lis­te­ria, many other Bac­te­ria and Ar­chaea also have pro­teins with a c‑di-adeny­late­cy­clase do­main, the ac­tive do­main thought to make our fa­vorite cyclic din­u­cleotide. In B. sub­tilis, this do­main is part of a pro­tein called DisA (DNA integrity scan­ning pro­tein A). In­ter­est­ingly, DisA is said to have a role in sporu­la­tion by mon­i­tor­ing DNA dam­age at the be­gin­ning of the process. DisA mol­e­cules are seen in in­tra­cy­to­plas­mic foci that move in­side bac­te­r­ial cells, ap­par­ently scan­ning the DNA as they go, to de­tect the pres­ence of le­sions. This seems to­tally un­re­lated to the Lis­te­ria story, but it in­di­cates that c‑di-AMP may play a role else­where. Just what it might be do­ing in Lis­te­ria or other bac­te­ria is a mys­tery. This is surely not the end of the story. c‑di-AMP's cousin, c‑di-GMP, is get­ting a lot of play as a reg­u­la­tor, so we can put our money on the dis­cov­ery of fur­ther roles of cyclic din­u­cleotides.

There may be also a prac­ti­cal as­pect to this dis­cov­ery. If Lis­te­ria has de­vel­oped into such a pow­er­ful im­muno­gen, might one not ex­ploit it for mak­ing bet­ter vac­cines? The Port­noy lab is giv­ing this a good try and we will re­port of any progress they or oth­ers make along these lines.

Ref­er­ence

Wood­ward JJ, Iavarone AT, & Port­noy DA (2010). c‑di-AMP se­creted by in­tra­cel­lu­lar Lis­te­ria mono­cy­to­genes ac­ti­vates a host type I in­ter­feron re­sponse. Sci­ence (New York, N.Y.), 328 (5986), 1703−1705. PMID 20508090