Abstract Out of hospital cardiac arrest (OHCA) produces an early systemic inflammatory response associated with significant neurological injury and mortality. Unlike sepsis, OHCA offers an opportunity to intervene at the earliest stages of the immune response. However, the immunology of cardiac arrest is understudied. To address this, we started the multi-center Immunology of Cardiac Arrest Network (I-CAN), a collaboration applying single-cell approaches to a unique biobank of cryopreserved, viable peripheral blood mononuclear cells (PBMC) from OHCA patients. Our premise is that cardiac arrest triggers endogenous, compensatory mechanisms that promote the resolution of inflammation, limit injury and improve survival. Our preliminary studies identified that monocyte and NK cell states expressing immune checkpoints were expanded in patients with poor neurological outcomes. Interactome analysis identified cytokines (IFNγ, IL-10) that mediate cross-talk between Nectin-2+ monocytes and Tim-3+ TIGIT+ NK cells. Subsequent ex vivo studies on PBMC from OHCA patients demonstrated that Nectin-2 is a brake on production of IFNγ by NK cells. IFNγ-deficient mice in experimental cardiac arrest and resuscitation had reduced neurological injury and mortality. These findings suggest our hypothesis that Nectin-2 monocytes are a protective response to ameliorate inflammation and neurological injury after OHCA. Here, we propose to define the mechanisms that resolve inflammation after cardiac arrest. In Aim 1, we perform deep immunophenotyping of OHCA patients at single-cell resolution. In Aim 2, we define the immune checkpoint profile of OHCA and its association with neurological outcomes. We then define the function of immune checkpoints in OHCA, with a focus on Nectin-2+ monocytes. In Aim 3, we test targeting of immune checkpoint receptors as a therapeutic strategy in a mouse model of cardiac arrest and resuscitation. Together, these aims define a new therapeutic approach: augmentation of endogenous, protective mechanisms to reduce inflammation after cardiac arrest. Further, our multi-center and multi- disciplinary team in the Immunology of Cardiac Arrest Network (I-CAN) establishes the resource of a two-site biorepository of clinical cardiac arrest with deep immunophenotyping of the subjects.