New research decodes the crucial elements of a marsupial’s immune system, and in the process identifies a host of previously unknown proteins with potent antibacterial activity that could have applications in human medicine. The advance, by Princeton researchers Jongbeom Park, Ricardo Mallarino, Mohamed Donia and colleagues, appeared April 16, 2025 in the journal Science Advances.

From the moment of birth, babies come under assault from microbial pathogens. We humans, like many other mammals, are born with a sophisticated and elaborate immune system that levies two layers of defenses to protect us from the onslaught. The first layer of defense, called the innate immune system, consists of specialized immune cells that patrol tissues to intercept and destroy microbes. When these cells run into trouble, they can also recruit a second layer of defenses that can be tailored to address novel threats and recalled when those threats reappear. Our immune systems are ready to engage both layers of defenses from the time of birth, but this is not the case for all mammals.

“[Marsupial] offspring are born very early with almost no immune protection, yet they manage to survive in pouches full of microbes,” said Ricardo Mallarino, an Assistant Professor in the Department of Molecular Biology at Princeton. 

Marsupials—a class of mammal that includes familiar animals such as kangaroos and opossums—are born in a highly immature state and develop for a long time inside the mother’s pouch. There, they are protected from many worldly threats, but not from microbes. Despite this, marsupials are typically born with only innate immune defenses, and don’t develop their second layer of defenses until later in life. Yet, their young survive bacterial assaults. This is partly thanks to the mother, who secretes antimicrobial proteins into her pouch. But what other defenses do marsupial neonates have, and how do they work?

For a long time, this has been a complete mystery. Scientists were limited to studying wild marsupials and had only limited information about marsupial immune systems. Mallarino, who studies developmental processes unique to marsupials using small, nocturnal marsupials called sugar gliders, had solutions to both these problems. Wanting to refine a suite of molecular tools to study sugar glider immunity, he struck up a collaboration with fellow Princeton Department of Molecular Biology faculty member, Mohamed Donia, who studies bacterial communities and their relationship with their hosts. The pair enlisted graduate student Jongbeom Park to investigate. 

Park and colleagues first wanted to get a better idea of what types of immune cells might be present in the animals shortly after birth. They found that the most abundant cells were neutrophils, a type of innate immune cell that can kill invading bacteria by engulfing them or by secreting lethal chemicals and proteins. Among the most prominently expressed genes in sugar glider neutrophils were several that encode small, secreted proteins called cathelicidins.

“Cathelicidins kill bacteria primarily by making pores in bacterial membranes. In addition to that, they can play various immunomodulatory roles,” said Park.

Whereas humans have just one cathelicidin gene, sugar glider neonates’ relative plethora of cathelicidin genes may serve to bolster their immune defenses. To explore what sugar glider cathelicidins contribute to antimicrobial defense, Park and colleagues synthesized several of them and then exposed several types of pathogenic bacteria to them. They found that sugar glider cathelicidins exhibit potent anti-microbial activity.

“Some of the cathelicidins we identified in sugar gliders showed stronger antimicrobial activity against specific pathogens than commonly used antibiotics such as kanamycin and ampicillin.” said Mallarino.

Park and colleagues’ studies of sugar glider cathelicidin genes suggests the expression of certain pairs of cathelicidins can be turned on and off simultaneously in those cells. The team also identified a cellular factor that helps control cathelicidin expression. These findings should help the researchers identify related genes in other marsupial genomes that could have similar or even completely new immune functions.

“We aim to connect these findings to the ecology of each species by investigating whether different peptides have evolved in response to the specific environmental challenges that marsupial species face in the wild,” said Mallarino. 

“By studying sugar glider cells, which frequently express a combination of multiple cathelicidins, we may gain insights on the therapeutic potential of using multiple cathelicidins,” said Park.

“Even beyond marsupials, our evolutionary analysis identified several mammalian species that have similarly expanded their cathelicidin repertoire and where these potent antimicrobial peptides likely play an important defensive role,” said Donia. “The methods and approaches we developed in this study will be instrumental in further exploring this area of research, by both our team and the scientific community at large.” 

Studying how other species defend themselves could also help us humans develop new defenses against ever-growing microbial threats.

Citation: Jongbeom Park, Wenfan Ke, Aellah Kaage, Charles Y. Feigin, Aaron H. Griffing, Yuri Pritykin, Mohamed S. Donia, Ricardo Mallarino. Cathelicidin antimicrobial peptides mediate immune protection in marsupial neonates. 2025. Science Advances. DOI: DOI: 10.1126/sciadv.ads6359.

Funding: This project was supported by funding from NIH (R35GM133758 and F32 GM139240-01); a Princeton University Dean for Research Innovation Award; the Searle Scholars Program; the Sloan Foundation and the Vallee Scholars Program; the Gordon and Betty Moore Foundation (Grants 7620 and 9199); NIH/NIAID (DP2AI171161); and the Ludwig Institute for Cancer Research.