Microglia are the resident macrophages of the central nervous system and are associated with the pathogenesis of many neurodegenerative and brain inflammatory diseases; however, the origin of adult microglia remains controversial. We show that postnatal hematopoietic progenitors do not significantly contribute to microglia homeostasis in the adult brain. In contrast to many macrophage populations, we show that microglia develop in mice that lack colony stimulating factor-1 (CSF-1) but are absent in CSF-1 receptor–deficient mice. In vivo lineage tracing studies established that adult microglia derive from primitive myeloid progenitors that arise before embryonic day 8. These results identify microglia as an ontogenically distinct population in the mononuclear phagocyte system and have implications for the use of embryonically derived microglial progenitors for the treatment of various brain disorders.

The clinical successes in immunotherapy have been both astounding and at the same time unsatisfactory. Countless patients with varied tumor types have seen pronounced clinical response with immunotherapeutic intervention; however, many more patients have experienced minimal or no clinical benefit when provided the same treatment. As technology has advanced, so has the understanding of the complexity and diversity of the immune context of the tumor microenvironment and its influence on response to therapy. It has been possible to identify different subclasses of immune environment that have an influence on tumor initiation and response and therapy; by parsing the unique classes and subclasses of tumor immune microenvironment (TIME) that exist within a patient’s tumor, the ability to predict and guide immunotherapeutic responsiveness will improve, and new therapeutic targets will be revealed.

| The COVID-19 pandemic caused by infection with SARS-CoV-2 has led to more than 200,000 deaths worldwide. Several studies have now established that the hyperinflammatory response induced by SARS-CoV-2 is a major cause of disease severity and death in infected patients. Macrophages are a population of innate immune cells that sense and respond to microbial threats by producing inflammatory molecules that eliminate pathogens and promote tissue repair. However, a dysregulated macrophage response can be damaging to the host, as is seen in the macrophage activation syndrome induced by severe infections, including in infections with the related virus SARS-CoV. Here we describe the potentially pathological roles of macrophages during SARS-CoV-2 infection and discuss ongoing and prospective therapeutic strategies to modulate macrophage activation in patients with COVID-19. volume 20 | June 2020 | 361 nature reviews | ImmunOlOgy P r o g r e s s

Monocytes and macrophages are critical effectors and regulators of inflammation and the innate immune response, the immediate, pre-programmed arm of the immune system. Dendritic cells initiate and regulate the highly pathogen-specific adaptive immune responses, and are central to the development of immunologic memory and tolerance. Recent in vivo experimental approaches in the mouse have unveiled new aspects of the developmental and lineage relationships among these cell populations. Despite this, the origin and differentiation cues for many tissue macrophages, monocytes, and dendritic cell subsets in mice, and the corresponding cell populations in humans, remain to be elucidated.White blood cells or leukocytes are a diverse group of cell types that mediate the body's immune response. They circulate through the blood and lymphatic system and are recruited to sites of tissue damage and infection. Leukocyte subsets are distinguished by functional and physical characteristics. They have a common origin in hematopoietic stem cells and develop along distinct differentiation pathways in response to internal and external cues. The mononuclear phagocyte system represents a subgroup of leucocytes originally described as a population of bone marrow-derived myeloid cells that circulate in the blood as monocytes and populate tissues as macrophages in the steady state and during inflammation (1). In different tissues they can show significant heterogeneity with respect to phenotype, homeostatic turnover and function. The discovery of dendritic cells (DCs) as a distinct lineage of mononuclear phagocytes, specialized in antigen presentation to T cells and the initiation and control of immunity (2), revealed additional roles of these cells in shaping the immune response to pathogens, vaccines and tumors, as well as additional heterogeneity. Whereas a detailed map of the relationship between monocytes, DCs and their progenitors begins to emerge, other areas like the origin and renewal of tissue macrophage subsets remain less defined. (Fig. 1A) circulate in the blood, bone marrow, and spleen and do not proliferate in a steady state (3,4). Monocytes represent immune effector cells, equipped with chemokine Monocytes

COVID-19 disease, caused by SARS-CoV-2 infection, has resulted in more than 15.5 million infections and 634,000 deaths worldwide. A recent study of hospitals in New York City, at the initial epicenter of the COVID-19 pandemic in the United States, reported that, during March 2020, 21% of patients hospitalized with confirmed COVID-19 died 1 . These findings are aligned with outcomes observed in the Mount Sinai Health System 2,3 . There are currently no curative or preventive therapies for COVID-19, highlighting the need to enhance current understanding of SARS-CoV-2 pathogenesis for the rational development of therapeutics.Recent studies have suggested that, in addition to direct viral damage, uncontrolled inflammation contributes to disease severity in 5 ). Consistent with this hypothesis, high levels of inflammatory markers, including C-reactive protein (CRP), ferritin and D-dimer, high neutrophil-to-lymphocyte ratio [6][7][8][9] and increased levels of inflammatory cytokines and chemokines 6,8-11 have been observed in patients with severe diseases. Pathogenic inflammation, also referred to as cytokine storm, shares similarities with what was previously seen in patients infected with other severe coronaviruses, including SARS-CoV and Middle East respiratory syndrome coronavirus 12 , and bears similarities to cytokine release syndrome (CRS) observed in patients with cancer treated with chimeric antigen receptor-modified (CAR) T cells 13 . Tocilizumab, an IL-6 receptor inhibitor, is a US Food and Drug Administration (FDA)-approved treatment for CRS in patients receiving CAR T cells 14 . Several single-center studies have used IL-6 inhibitors to treat patients with COVID-19 with some clinical benefits 15 and reported failures 14 . Beyond IL-6, several cytokines have been shown to be elevated in CRS and to contribute to tissue damage. TNF-α is important in nearly all acute inflammatory reactions, acting as an amplifier of inflammation. TNF-α blockade has been used to treat more than ten different autoimmune inflammatory diseases, suggesting that this might be a potential therapeutic approach to reduce organ damage in patients with ). IL-1 is also a highly active pro-inflammatory cytokine, and monotherapy blocking