Enhancing immunity by engineering DAMPs

Manipulation of innate immunity of living organisms to render them more rapid or sensitive in responding to microbial and environmental challenges is becoming a feasible strategy to combat diseases. Our knowledge of immunity-related molecules (signal molecules, receptors, interacting proteins, transducers, genes and defence proteins) is continually increasing in both animals and plants. In both kingdoms, recognition of invading microbes often activates powerful defences that restrict the growth of pathogens. Immunity is frequently triggered by microbe-associated molecular pattern molecules (MAMPs) [1]. However both plants and animals also recognize pathogens indirectly by the disruption of cellular homeostatic processes caused by infections. For example, in mammals, the presence of pathogens is sensed indirectly by the release of low molecular weight molecules such as oligonucleotides, uric acid, or ATP from damaged host tissues or alternatively, upon tissue injury, by the release of oligosaccharides, such as hyaluronan (HA) fragments from the extracellular matrix [2]. All these endogenous molecules act as “damageassociated molecular patterns” (DAMPs) that activate inflammatory responses and immunity. In particular, HA and derived fragments have been recently recognized to play an important role in the regulation of many immunerelated diseases including cancer [3]. It is expected that strategies will be developed in the near future to control the expression of these molecules and their interactions to strengthen immune responses and improve health in both plants and animals. In a recent paper [4], we have investigated and verified the possibility of engineering in plants the controlled release of DAMPs derived from the extracellular matrix (the plant cell wall), to enhance immune responses and confer resistance against several microbial pathogens. The concept of engineering DAMPs in plants is quite new and may potentially be applied to the animal field. We have exploited the knowledge that oligogalacturonides (OG) with a degree of polymerization between 10 and 15, i.e. oligosaccharides released by the action of microbial polygalacturonases from homogalacturonan, a component of the plant cell wall pectin, are capable of activating a wide range of defence responses. In other words, plants are capable of sensing a breach in the wall through the perception of oligogalacturonides and promptly defend themselves. Activation of immunity mediated by oligogalacturonides occurs in many plants and is effective against many microbes. The cascade linking the perception of oligogalacturonides to the activation of immunity has been partly elucidated [5, 6]. During microbial infections, the generation of elicitor-active oligogalacturonides is promoted by plant-encoded polygalacturonase inhibitor proteins (PGIP), which block the complete hydrolysis of homogalacturonan and favour the accumulation of elicitor-active oligomers vs. the formation of elicitorinactive oligomers and monomers of galacturonic acid [6, 7]. We devised an experimental strategy to test whether transgenic plants engineered to accumulate equimolar levels of polygalacturonase and PGIP generate elicitoractive oligogalacturonides. We constructed a chimeric protein by fusing a polygalacturonase from a fungal pathogen to a plant-derived PGIP and showed that transgenic Arabidopsis thaliana plants expressing the enzyme-inhibitor chimera indeed accumulate elicitoractive oligogalacturonides. The chimera was named the “OG-machine”. Expression of the OG-machine under Editorial