The innate immune system is responsible for protecting the human body against threats that can cause disease or infection. The system relies on innate immune sensors to detect and transmit signals about these threats. One of the most important innate immune strategies to respond to threats is cell death. New research from St. Jude Children's Research Hospital discovered that NLRC5 plays a previously unknown role as an innate immune sensor, causing cell death. The findings, published in Cell, show how NLRC5 drives PANoptosis, a prominent form of inflammatory cell death. This insight has implications for the development of therapies that target 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 an emergency broadcast system that is activated quickly, while the PANoptosome is more like an emergency response unit that generally integrates more signals and components to respond to the threat. How innate immune sensors work—what prompts them to take action—is a mystery that has puzzled researchers for decades.
Nucleotide-binding oligomerization domain-like receptors (NLRs) are a large family of important molecules involved in inflammatory signaling. They are generally thought to function as innate immune sensors that detect threats. However, the specific role of different NLRs in the field of detection is not yet clear. St. Jude scientists ran a large screen and tested a specific NLR, NLRC5, to see which threats activate it. Through their efforts, they discovered that depletion of nicotinamide adenine dinucleotide (NAD), a molecule essential in energy production, causes NLRC5-mediated cell death by PANoptosis.
“One of the biggest questions in the field of immunology and innate immunity is what the different members of the NLR family sense and what their functions are,” said corresponding author Thirumala-Devi Kanneganti, PhD, St. Jude Department of Immunology vice chair . “NLRC5 has been a puzzling molecule, but now we have the answer: it acts as an innate immune sensor and cell death regulator, stimulating inflammatory cell death, PANoptosis, by forming a complex.”
Identification of the NLRC5 trigger
Scientists in the Kanneganti lab conducted a rigorous study to get to the bottom of what threats are causing NLRC5. This included looking at 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 or mimic an infection or the cause of an injury or disease. other danger signals such as cytokines (immune signaling molecules).
The researchers also looked at heme, the component of hemoglobin that is responsible for transporting oxygen. Infections or diseases can cause red blood cells to rupture in a process called hemolysis. This releases hemoglobin into the bloodstream. When hemoglobin is broken down into its components, free heme is released, 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.
“Of all the combinations we tested, we found that the combination of heme plus PAMPs or cytokines specifically induces NLRC5-dependent inflammatory cell death, PANoptosis,” said co-first author Balamurugan Sundaram, PhD, St. Jude Department of Immunology. “Our results showed for the first time that NLRC5 is central to responses to hemolysis, which can occur during infections, inflammatory diseases and cancer.”
Energy depletion activates the NLRC5 function
After identifying the heme-containing PAMP, DAMP, and cytokine combinations that drive NLRC5-dependent inflammatory cell death, the researchers further investigated how NLRC5 is regulated. They found that NAD levels drive NLRC5 protein expression. When NAD is depleted, it sounds an alarm that a threat exists that the immune system should recognize. The researchers found that NAD depletion is sensed by NLRC5, which causes PANoptosis.
“By supplementing with the NAD precursor, nicotinamide, we reduced NLRC5 protein expression and PANoptosis,” said co-first author Nagakannan Pandian, PhD, St. Jude Department of Immunology. “Therapeutically, nicotinamide has been extensively studied as a dietary supplement, and our findings suggest that it could be useful in the treatment of inflammatory diseases.”
The researchers also found that NLRC5 is in an NLR network with NLRP12, which together with other cell death molecules form a NLRC5-PANoptosome complex that causes inflammatory cell death. The finding builds on previous research from the Kanneganti lab demonstrating 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. The Kanneganti lab's work shows that deleting Nlrc5 can protect against inflammatory cell death due to 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 to countless diseases and conditions,” Kanneganti said. “Aging, infectious diseases, inflammatory diseases – things for which there are no targeted therapies, this could be an option.”
Authors and funding
The study's other authors are Emily Alonzo, Research and Development Department at 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, 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.
