Multicellular homeostasis is maintained in part by a collection of well-orchestrated, programmed cell death mechanisms. These programs are generally categorized into either “tolerogenic” or “immunogenic” modes. The tolerogenic form (also known as immunologically silent apoptosis) involves well-established intrinsic or extrinsic signaling cascades which lead to caspase activation and asymmetric lipid remodeling of the cell membrane. A crucial phosphatidylserine translocation event, from the inner to outer membrane surface, provides the necessary “eat me” signal whereby dying cells are phagocytized prior to loss of membrane integrity. This form of cell death does not promote inflammation and is generally associated with the efficient removal of aged, damaged, redundant, or unnecessary immune cells or other cell types during the developmental and tissue remodeling processes.
The inflammatory embodiment, known as immunogenic cell death, can be further segregated into subroutines such as necroptosis, pyroptosis, ferroptosis, NETosis, or a distinct form of apoptosis. Regardless of specific subroutine, DAMPs are exposed and/or released into the extracellular environment which perpetuate immune cell infiltration and subsequent localized inflammation. ICD is an important first line of defense for viral and bacterial attack and for the acquired development of tumor-specific cytotoxic T-cells. ICD can also be subverted to produce both acute and chronic pathologies.
Extracellular adenosine triphosphate (eATP) and high mobility group box B1 (HMGB1) protein are two DAMPs regarded as important for establishing the immunogenic cell death phenotype. Although principally known as a universal energy transporting molecule, the extracellular form of ATP is known to participate in a diverse array of cell to cell signaling events. eATP mediates these effects through P2 purinergic receptors expressed on a wide variety of cell types. In the case of ICD, eATP from the dying cell provides the initial chemoattractant gradient for dendritic cells (antigen-processing cells), then sustains the inflammatory event by paracrine release from responding immune cells.
HMGB1 is a major, chromatin-associated, non-histone protein which is compartmentalized in the nucleus. During an ICD event, HMGB1 translocates to the cytoplasm and eventually is released into the extracellular environment by either currently uncharacterized channel activation events or through passive release after a post-mortem loss of membrane integrity. Extracellular HMGB1 exists in various redox states that promote dendritic cell maturation and overall exacerbation of the inflammatory event.
Although each biomarker alone has predictive value for assessing the extent of an immunogenic cell death event, the combined absence or presence of eATP and HMGB1 provides a statistically stronger index for the phenotype than a single parameter measure. Therefore, the capability to rapidly and easily measure both biomarkers is key to discovery and development activities associated with the modulation of ICD.