ABSTRACT Aging is associated with a loss of immune function (immunosenescence) and chronic low-grade inflammation (inflammaging). However, the mechanistic details of our aging immunity are largely enigmatic. Metabolism and immunity have co-evolved, and metabolic pathways are increasingly appreciated as key regulators of our immune system. Mitochondria, being the most important metabolic organelle, have also gained much attention as regulatory hubs of various immune functions. Owing to their bacterial ancestry, mitochondria possess their own genome. While mtDNA itself can trigger immune responses and directly entrap pathogens, it is not known to encode for immune factors. Currently, our immunity is known to be nuclear-encoded. We have recently identified a novel gene encoded within the mitochondrial DNA and named it MOTS-c (Mitochondrial ORF within the Twelve S rRNA). MOTS-c is an age-dependent peptide that regulates metabolic homeostasis and significantly improves aging metabolism and physical fitness in mice. Here, we describe MOTS-c as the first-in-class mitochondrial-encoded immune factor that acts as an antimicrobial peptide (AMP). MOTS-c, consistent with other AMPs, is expressed by various cells including monocytes and macrophages. The identification of MOTS-c was strongly influenced by prior work from the laboratory of Sidney Pestka (aka “father of interferon”), whereby the great majority of mRNAs induced by interferon were from the mitochondrial 12S rRNA in monocyte-like cells (no genes were identified at that time). Indeed, we now demonstrate that MOTS-c peptide expression is induced by interferon gamma. AMPs also regulate immune cell functions, including monocytes/macrophages. This is consistent with our preliminary data whereby MOTS-c moves to the nucleus to program monocyte differentiation to generate unique macrophages that are characterized by increased expression of interferon-stimulated genes (ISGs) and antigen presentation genes. Such “MOTS-c-programmed” macrophages had increased bactericidal capacity. This observation builds on our recent report on MOTS-c as the first-in-class mitochondrial-encoded factor that translocates to the nucleus and directly regulates stress-adaptive nuclear gene expression. Here, we propose to test the hypothesis that MOTS-c is an age-dependent and IFN-inducible mitochondrial- encoded AMP, a first-in-class, that programs monocytes to differentiate into unique IFN-poised macrophages with enhanced bactericidal capacity. We propose three aims to test this hypothesis: (1) Determine whether MOTS-c- programmed macrophages are epigenetically “IFN-poised” for enhanced antibacterial responses, (2) Test whether metabolic rewiring enhances bactericidal capacity of MOTS-c-programmed macrophages, and (3) Determine the functional effect of MOTS-c on monocytes and BMDMs during aging in mice. If successful, we predict that our study will have broad and lasting impact including (i) the first identification of a mitochon...