Abstract Inflammatory Ly6C+ monocytes are a subset of highly plastic myeloid cells that are rapidly mobilized during multiple types of microbial pathogen infections (bacteria, virus, fungi, parasites) and differentiate into robust microbicidal effector cells that can secrete TNF and produce nitric oxide and reactive oxygen species directly contributing to pathogen killing and elimination. We and others have also reported that Ly6C+ monocytes accumulate and form clusters at foci of microbial pathogen infections. In recent work using mice infected with the intracellular bacterium Listeria monocytogenes and intravital multi-photon imaging, we also revealed that Ly6C+ monocytes arrest at infection foci, exhibit a highly activated phenotype and deliver local effector functions. While it is well established that these cells egress from the bone marrow to the blood via CCR2 in response to the rapid release of CCL2 that occurs upon microbial product sensing, how they reach infected tissues, home to foci of infections and form stable clusters is still largely unknown. Many chemotactic and adhesion mechanisms have been ruled out to regulate these processes, suggesting either other mechanisms or redundancy. Here we formally test the hypothesis that Ly6C+ monocytes need to home to foci of infection and form stable clusters to deliver local and powerful effector functions to help clear pathogens. This exploratory proposal will define the chemotactic cues and receptors involved, the initiating and orchestrating target cells that orchestrate Ly6C+ monocytes homing to infection foci and the molecules involved in monocyte arrest and formation of stable clusters. We will use combinations of genetic gain and loss of function experiments in vivo, bacterial, parasitic and viral infection models, cutting-edge intravital microscopy imaging and in vitro functional chemotactic assays.

Public Health relevant statement This project has the potential to improve antimicrobial immunity. Specifically, the proposal focuses on populations of white blood cells (leukocytes), namely monocytes, that are mobilized from the bone marrow to infected tissues during microbial pathogen infections (bacteria, virus, fungi, parasites). These cells can quickly express multiple functions that are important to protect the host against infectious microbes during initial infection but also during a second infection of vaccinated hosts. We studied how these cells protect vaccinated host by very precisely delivering their microbicidal killing functions at sites of microbial pathogen growth, resulting in rapid pathogen control and elimination from the host. We propose here to understand how these cells rapidly reach infectious foci and deliver killing effector activities leading to the efficient elimination of invading microbial pathogens in vivo with the ultimate goal of designing novel immune therapies.