Social animals benefit from sharing information that promotes fitness and survival (1-3). In rodents, there is extensive evidence that the detection of chemosensory cues emitted by other individuals is a fundamental feature of social recognition and a prerequisite for a wide range of social behaviors (4 -6).For example, rodents use such chemical signals to distinguish between parasitized individuals, recognize infected conspecifics, and avoid and display aversive responses to infected individuals (7-9). Defining the cellular and molecular mechanisms and neural circuits underlying the detection of infection-and illness-associated states is not only crucial for understanding the sensing of health status in animals but also for advancing our knowledge of disease-related odors and diagnostic olfactory biomarkers in humans (10, 11).The vomeronasal organ (VNO) 4 is an olfactory substructure of the mammalian nose that is essential for social recognition and chemical communication (12). The VNO houses vomeronasal sensory neurons (VSNs), specialized nerve cells that detect a wide range of volatile and non-volatile molecular cues, including pheromones and predator odors (13-15). Among these cues are major histocompatibility complex class I binding peptides (antigens) and other small urinary peptides (16 -18) thus providing initial support for an evolutionary link between recognition mechanisms in immune cells and subsets of VSNs (14, 19). More recently, clear evidence has emerged that the VNO plays an important role in mediating sick conspecific avoidance and thus seems to function as a sensor of infection (20), but the underlying cellular and molecular basis for this function remains unclear. VSNs are characterized by the expression of large families of G protein-coupled receptors, vomeronasal type 1 receptors (Vmn1r) expressed in the apical vomeronasal layer and vomeronasal type 2 receptor (Vmn2r) that function together with non-classical class I major histocompatibility complex receptors of the H2-Mv subtype in the basal layer (21, 22). An important development has been the identification of a third family of G protein-coupled receptors expressed in subsets of VSNs as follows: the formyl peptide receptors (Fprs) (23,24). Fprs are primarily known from the innate immune system where they are involved in chemoattraction of phagocytic immune cells to sites of infection mediating host defense against invading microorganisms (25-28). Fprs detect a wide range of inflammation markers as well as bacterial and mitochondrial peptides (25-28), and consistent with the results from immune cells, some of these ligands have also been shown to activate specific VSNs (23,29). Hence, members of the Fpr family are currently prime candidates for mediating VNO pathogen sensing, although it cannot be ruled out that

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