Hemogenic Endothelial Fate Mapping Reveals Dual Developmental Origin of Mast Cells.

Hematopoiesis occurs in distinct waves. "Definitive" hematopoietic stem cells (HSCs) with the potential for all blood lineages emerge in the aorta-gonado-mesonephros, while "primitive" progenitors, whose potential is thought to be limited to erythrocytes, megakaryocytes, and macrophages, arise earlier in the yolk sac (YS). Here, we questioned whether other YS lineages exist that have not been identified, partially owing to limitations of current lineage tracing models. We established the use of Cdh5-CreERT2 for hematopoietic fate mapping, which revealed the YS origin of mast cells (MCs). YS-derived MCs were replaced by definitive MCs, which maintained themselves independently from the bone marrow in the adult. Replacement occurred with tissue-specific kinetics. MCs in the embryonic skin, but not other organs, remained largely YS derived prenatally and were phenotypically and transcriptomically distinct from definite adult MCs. We conclude that within myeloid lineages, dual hematopoietic origin is shared between macrophages and MCs.

T Cell Zone Resident Macrophages Silently Dispose of Apoptotic Cells in the Lymph Node.

In lymph nodes (LNs), dendritic cells (DCs) are thought to dispose of apoptotic cells, a function pertaining to macrophages in other tissues. We found that a population of CX3CR1+ MERTK+ cells located in the T cell zone of LNs, previously identified as DCs, are efferocytic macrophages. Lineage-tracing experiments and shield chimeras indicated that these T zone macrophages (TZM) are long-lived macrophages seeded in utero and slowly replaced by blood monocytes after birth. Imaging the LNs of mice in which TZM and DCs express different fluorescent proteins revealed that TZM-and not DCs-act as the only professional scavengers, clearing apoptotic cells in the LN T cell zone in a CX3CR1-dependent manner. Furthermore, similar to other macrophages, TZM appear inefficient in priming CD4 T cells. Thus, efferocytosis and T cell activation in the LN are uncoupled processes designated to macrophages and DCs, respectively, with implications to the maintenance of immune homeostasis.

Development of ICT01, a first-in-class, anti-BTN3A antibody for activating Vγ9Vδ2 T cell-mediated antitumor immune response.

Gamma delta T (γδ T) cells are among the most potent cytotoxic lymphocytes. Activating anti­butyrophilin 3A (BTN3A) antibodies prime diverse tumor cell types to be killed by Vγ9Vδ2 T cells, the predominant γδ T cell subset in peripheral circulation, by mechanisms independent of tumor antigen­major histocompatibility complex (MHC) complexes. In this report, we describe the development of a humanized monoclonal antibody, ICT01, with subnanomolar affinity for the three isoforms of BTN3A. We demonstrate that ICT01-activated Vγ9Vδ2 T cells kill multiple tumor cell lines and primary tumor cells, but not normal healthy cells, in an efficient process requiring approximately 20% target occupancy. We show that ICT01 activity is dependent on BTN3A and BTN2A but independent of the phosphoantigen (pAg)­binding B30.2 domain. ICT01 delays the growth of hematologic and solid tumor xenografts and prolongs survival of NOD/SCID/IL2rγnull (NSG) mice adoptively transferred with human Vγ9Vδ2 T cells. In single- and multiple-dose safety studies in cynomolgus macaques that received up to 100 mg/kg once weekly, ICT01 was well tolerated. With respect to pharmacodynamic endpoints, ICT01 selectively activated Vγ9Vδ2 T cells without affecting other BTN3A-expressing lymphocytes such as αß T or B cells. A first-in-human, phase 1/2a, open-label, clinical study of ICT01 was thus initiated in patients with advanced-stage solid tumors (EVICTION: NCT04243499; EudraCT: 2019-003847-31). Preliminary results show that ICT01 was well tolerated and pharmacodynamically active in the first patients. Digital pathology analysis of tumor biopsies of a patient with melanoma suggests that ICT01 may promote immune cell infiltration within the tumor microenvironment.

Imaging the Lymph Node Stroma.

Lymph node (LN) stromal cells are being recognized as key organizers of the immune system. They assemble in complex 3D networks and hence, need to be studied in situ to fully understand their exact functions. Here, we describe two distinct but complementary procedures that allow analyzing LN stromal cells at high resolution by confocal imaging.

Pancreatic Ductal Adenocarcinoma: A Strong Imbalance of Good and Bad Immunological Cops in the Tumor Microenvironment.

Pancreatic ductal adenocarcinoma (PDAC) is one of the most aggressive and lethal cancers with very few available treatments. For many decades, gemcitabine was the only treatment for patients with PDAC. A recent attempt to improve patient survival by combining this chemotherapy with FOLFIRINOX and nab-paclitaxel failed and instead resulted in increased toxicity. Novel therapies are urgently required to improve PDAC patient survival. New treatments in other cancers such as melanoma, non-small-cell lung cancer, and renal cancer have emerged, based on immunotherapy targeting the immune checkpoints cytotoxic T-lymphocyte-associated antigen 4 or programmed death 1 ligand. However, the first clinical trials using such immune checkpoint inhibitors in PDAC have had limited success. Resistance to immunotherapy in PDAC remains unclear but could be due to tissue components (cancer-associated fibroblasts, desmoplasia, hypoxia) and to the imbalance between immunosuppressive and effector immune populations in the tumor microenvironment. In this review, we analyzed the presence of "good and bad immunological cops" in PDAC and discussed the significance of changes in their balance.

Epidermal γδ T cells originate from yolk sac hematopoiesis and clonally self-renew in the adult.

The murine epidermis harbors two immune cell lineages, Langerhans cells (LCs) and γδ T cells known as dendritic epidermal T cells (DETCs). LCs develop from both early yolk sac (YS) progenitors and fetal liver monocytes before locally self-renewing in the adult. For DETCs, the mechanisms of homeostatic maintenance and their hematopoietic origin are largely unknown. Here, we exploited multicolor fate mapping systems to reveal that DETCs slowly turn over at steady state. Like for LCs, homeostatic maintenance of DETCs is achieved by clonal expansion of tissue-resident cells assembled in proliferative units. The same mechanism, albeit accelerated, facilitates DETC replenishment upon injury. Hematopoietic lineage tracing uncovered that DETCs are established independently of definitive hematopoietic stem cells and instead originate from YS hematopoiesis, again reminiscent of LCs. DETCs thus resemble LCs concerning their maintenance, replenishment mechanisms, and hematopoietic development, suggesting that the epidermal microenvironment exerts a lineage-independent influence on the initial seeding and homeostatic maintenance of its resident immune cells.

Multicolor fate mapping of Langerhans cell homeostasis.

Langerhans cells (LCs) constitute a network of immune sentinels in the skin epidermis that is seeded during embryogenesis. Whereas the development of LCs has been extensively studied, much less is known about the homeostatic renewal of adult LCs in "nonmanipulated" animals. Here, we present a new multicolor fluorescent fate mapping system and quantification approach to investigate adult LC homeostasis. This novel approach enables us to propose and provide evidence for a model in which the adult epidermal LC network is not formed by mature coequal LCs endowed with proliferative capabilities, but rather constituted by adjacent proliferative units composed of "dividing" LCs and their terminally differentiated daughter cells. Altogether, our results demonstrate the general utility of our novel fate-mapping system to follow cell population dynamics in vivo and to establish an alternative model for LC homeostasis.