The innate immune system is responsible for protecting the human body from threats that could cause disease or infection. The system relies on innate immune sensors to detect and transmit signals about these threats. One of the key innate immune strategies to respond to threats is through cell death. New research from St. Jude Children’s Research Hospital found that NLRC5 plays a previously unknown role as an innate immune sensor, triggering cell death. The findings, published in Cell, show how NLRC5 drives PANoptosis, a prominent type of inflammatory cell death. This knowledge has implications for the development of therapies targeting NLRC5 for the treatment of infections, inflammatory diseases, and aging.
Depending on the threat, innate immune sensors can assemble complexes such as inflammasomes or PANoptosomes. The inflammasome can be thought of as a rapidly activated emergency transmission system, while the PANoptosome is more of an emergency response unit that typically integrates more signals and components to respond to the threat. How innate immune sensors work—what prompts them to act—has been a mystery that researchers have been unraveling for decades.
Nucleotide-binding oligomerization domain-like receptors (NLRs) are a large family of important molecules involved in inflammatory signaling. They are generally believed to function as innate immune sensors that detect threats. However, the specific roles of various NLRs in sensing are not yet understood. St. Jude scientists conducted a large screen, testing a specific NLR, NLRC5, to see what threats activate it. Through their efforts, they discovered that depletion of nicotinamide adenine dinucleotide (NAD), an essential molecule in energy production, triggers NLRC5-mediated cell death through PANoptosis.
“One of the most important questions in the fields of immunology and innate immunity is what the different members of the NLR family feel and what their functions are,” said corresponding author Thirumala-Devi Kanneganti, PhD, vice chair of the Department of Immunology. from St. Jude. chair. “NLRC5 was an enigmatic molecule, but now we have the answer: it acts as an innate immune sensor and regulator of cell death, driving inflammatory cell death, PANoptosis, through the formation of a complex.”
Identify the NLRC5 trigger
Scientists at the Kanneganti lab conducted a rigorous evaluation to get to the bottom of what threats trigger NLRC5. This included the analysis of pathogens such as bacteria and viruses, as well as pathogen-associated molecular patterns (PAMPs) and damage-associated molecular patterns (DAMPs) that can be released by an infection or the cause of or mimic an injury or disease, as well as other danger signals such as cytokines (immune signaling molecules).
The researchers also looked at heme, the component of hemoglobin responsible for transporting oxygen. Infections or illnesses can cause red blood cells to break down in a process called hemolysis. This releases hemoglobin into the bloodstream. When hemoglobin breaks down into its components, it releases free heme, which is known to cause significant inflammation and organ damage. The researchers tested many different combinations of pathogens, PAMPs, and DAMPs to see if NLRC5 was required for a response.
“Among all the combinations we tested, we identified that the combination of heme plus PAMPs or cytokines specifically induces NLRC5-dependent inflammatory cell death, PANoptosis,” said co-author Balamurugan Sundaram, PhD, St. Jude Department of Immunology. “Our results showed for the first time that NLRC5 is critical for responses to hemolysis, which can occur during infections, inflammatory diseases, and cancers.”
Energy depletion activates NLRC5 function
By identifying the combinations of PAMPs, DAMPs, and heme-containing cytokines that trigger NLRC5-dependent inflammatory cell death, the researchers further investigated how NLRC5 is regulated. They found that NAD levels drive the expression of the NLRC5 protein. If NAD is depleted, an alarm sounds that there is a threat that the immune system should recognize. The researchers discovered that NLRC5 senses NAD depletion, which triggers PANoptosis.
“By supplementing with the NAD precursor nicotinamide, we reduced NLRC5 protein expression and PANoptosis,” said co-author Nagakannan Pandian, PhD, St. Jude Department of Immunology. “Therapeutically, nicotinamide has been widely studied as a nutritional supplement and our findings suggest that it could be useful in the treatment of inflammatory diseases.”
The researchers also discovered that NLRC5 is in an NLR network with NLRP12, which bind with other cell death molecules and form an NLRC5-PANoptosome complex that triggers inflammatory cell death. The finding builds on previous research by the Kanneganti laboratory showing the role of NLRP12 in PANoptosis.
A promising target for therapeutic development
NLRs are associated with diseases related to infections, inflammation, cancers, and aging. This makes them intriguing targets for the development of new therapies. Work from the Kanneganti laboratory shows that deletion of Nlrc5 can provide protection against inflammatory cell death through PANoptosis and prevent disease pathology in hemolytic and inflammatory disease models, making NLRC5 an exciting therapeutic prospect.
“The fundamental knowledge we have gained about how innate immune sensing works can be translated into numerous diseases and conditions,” Kanneganti said. “Aging, infectious diseases, inflammatory disorders… things for which there are no targeted therapies, this could be an option.”
Authors and financing
The other authors of the study are Emily Alonzo, Research and Development Department, Cell Signaling Technology; and Hee Jin Kim, Hadia Abdelaal, Omkar Indari, Roman Sarkar, Rebecca Tweedell, Jonathan Klein, Shondra Pruett-Miller and Peter Vogel, all of St. Jude, and Raghvendra Mall, formerly of St. Jude and now of the Technology Innovation Institute , Abu Dhabi.
The study was supported by grants from the National Institutes of Health (AI101935, AI124346, AI160179, AR056296, and CA253095) and ALSAC, St. Jude’s fundraising and awareness organization.