The enteric nervous system (ENS) arises from neural crest cells that migrate, proliferate, and differentiate into enteric neurons and glia within the intestinal wall. Many extracellular matrix (ECM) components are present in the embryonic gut, but their role in regulating ENS development is largely unknown. Here, we identify heparan sulfate proteoglycan proteins, including collagen XVIII (Col18) and agrin, as important regulators of enteric neural crest-derived cell (ENCDC) development. In developing avian hindgut, Col18 is expressed at the ENCDC wavefront, while agrin expression occurs later. Both proteins are normally present around enteric ganglia, but are absent in aganglionic gut. Using chick-mouse intestinal chimeras and enteric neurospheres, we show that vagal- and sacral-derived ENCDCs from both species secrete Col18 and agrin. Whereas glia express Col18 and agrin, enteric neurons only express the latter. Functional studies demonstrate that Col18 is permissive whereas agrin is strongly inhibitory to ENCDC migration, consistent with the timing of their expression during ENS development. We conclude that ENCDCs govern their own migration by actively remodeling their microenvironment through secretion of ECM proteins.

Small cell lung cancer (SCLC) has recently been subcategorized into neuroendocrine (NE)‐high and NE‐low subtypes showing ‘immune desert’ and ‘immune oasis’ phenotypes, respectively. Here, we aimed to characterize the tumor microenvironment according to immune checkpoints and NE subtypes in human SCLC tissue samples at the protein level. In this cross‐sectional study, we included 32 primary tumors and matched lymph node (LN) metastases of resected early‐stage, histologically confirmed SCLC patients, which were previously clustered into NE subtypes using NE‐associated key RNA genes. Immunohistochemistry (IHC) was performed on formalin‐fixed paraffin‐embedded TMAs with antibodies against CD45, CD3, CD8, MHCII, TIM3, immune checkpoint poliovirus receptor (PVR), and indoleamine 2,3‐dioxygenase (IDO). The stroma was significantly more infiltrated by immune cells both in primary tumors and in LN metastases compared to tumor nests. Immune cell (CD45+ cell) density was significantly higher in tumor nests (P = 0.019), with increased CD8+ effector T‐cell infiltration (P = 0.003) in NE‐low vs NE‐high tumors. The expression of IDO was confirmed on stromal and endothelial cells and was positively correlated with higher immune cell density both in primary tumors and in LN metastases, regardless of the NE pattern. Expression of IDO and PVR in tumor nests was significantly higher in NE‐low primary tumors (vs NE‐high, P < 0.05). We also found significantly higher MHC II expression by malignant cells in NE‐low (vs NE‐high, P = 0.004) tumors. TIM3 expression was significantly increased in NE‐low (vs NE‐high, P < 0.05) tumors and in LN metastases (vs primary tumors, P < 0.05). To our knowledge, this is the first human study that demonstrates in situ that NE‐low SCLCs are associated with increased immune cell infiltration compared to NE‐high tumors. PVR, IDO, MHCII, and TIM3 are emerging checkpoints in SCLC, with increased expression in the NE‐low subtype, providing key insight for further prospective studies on potential biomarkers and targets for SCLC immunotherapies.

Background & Aims Neuroinflammation in the gut is associated with many gastrointestinal (GI) diseases, including inflammatory bowel disease. In the brain, neuroinflammatory conditions are associated with blood-brain barrier (BBB) disruption and subsequent neuronal injury. We sought to determine whether the enteric nervous system is similarly protected by a physical barrier and whether that barrier is disrupted in colitis. Methods Confocal and electron microscopy were used to characterize myenteric plexus structure, and FITC-dextran assays were used to assess for presence of a barrier. Colitis was induced with dextran sulfate sodium, with co-administration of liposome-encapsulated clodronate to deplete macrophages. Results We identified a blood-myenteric barrier (BMB) consisting of extracellular matrix proteins (agrin and collagen-4) and glial end-feet, reminiscent of the BBB, surrounded by a collagen-rich periganglionic space. The BMB is impermeable to the passive movement of 4 kDa FITC-dextran particles. A population of macrophages is present within enteric ganglia (intraganglionic macrophages [IGMs]) and exhibits a distinct morphology from muscularis macrophages, with extensive cytoplasmic vacuolization and mitochondrial swelling but without signs of apoptosis. IGMs can penetrate the BMB in physiological conditions and establish direct contact with neurons and glia. Dextran sulfate sodium-induced colitis leads to BMB disruption, loss of its barrier integrity, and increased numbers of IGMs in a macrophage-dependent process. Conclusions In intestinal inflammation, macrophage-mediated degradation of the BMB disrupts its physiological barrier function, eliminates the separation of the intra- and extra-ganglionic compartments, and allows inflammatory stimuli to access the myenteric plexus. This suggests a potential mechanism for the onset of neuroinflammation in colitis and other GI pathologies with acquired enteric neuronal dysfunction.

The enteric nervous system shares embryological, morphological, neurochemical, and functional features with the central nervous system. In addition to neurons and glia, the CNS includes a third component, microglia, which are functionally and immunophenotypically similar to macrophages, but a similar cell type has not previously been identified in enteric ganglia. In this study we identify a population of macrophages in the enteric ganglia, intermingling with the neurons and glia. These intraganglionic macrophages (IMs) are highly ramified and express the hematopoietic marker CD45, major histocompatibility complex (MHC) class II antigen, and chB6, a marker specific for B cells and microglia in avians. These IMs do not express antigens typically associated with T cells or dendritic cells. The CD45 /ChB6 /MHCII signature supports a hematopoietic origin and this was confirmed using intestinal chimeras in GFP-transgenic chick embryos. The presence of green fluorescent protein positive (GFP /CD45 cells in the intestinal graft ENS confirms that IMs residing within enteric ganglia have a hematopoietic origin. IMs are also found in the ganglia of CSF1R chicken and CX3CR1 mice. Based on the expression pattern and location of IMs in avians and rodents, we conclude that they represent a novel non-neural crest-derived microglia-like cell population within the enteric ganglia.

Embryonic tissues contain highly ramified stellate-shaped cells expressing CD45 and MHC II antigens but their origin and immunophenotype are unknown. Using staged avian embryos and cell-type-specific antibodies, we establish a detailed spatiotemporal ontogeny of cells that express CD45, the earliest marker of hematopoietic stem cells in the chick. CD45 immunostaining marks three distinct embryonic cell populations: round, ramified and amoeboid cells. The round and ramified CD45+ cells appear first in yolk-sac blood islands before the onset of circulation. A subpopulation of round cells co-expresses the thrombocyte-specific CD51/CD61 antigen. Amoeboid cells express macrophage-specific antigens and frequently occur in regions of apoptosis. Ramified cells are distributed uniformly in the embryonic mesenchyme, colonize lymphoid and non-lymphoid organs and later express MHC II. To study the origin of CD45+ cells, 2-day-old chick embryos were ablated from the yolk sac before the establishment of circulation and incubated for 2-5 days. Large numbers of CD45+MHC II+ ramified cells differentiated in the yolk sac. Yolk-sac chimeras were generated by grafting embryos into GFP-expressing de-embryonated yolk sacs. GFP/CD45 co-expressing ramified and amoeboid cells colonized all organ primordia in the donor embryo. We also recombined GFP+ yolk sac with the bursa of Fabricius and found ramified GFP+CD45+ cells in the bursa where they differentiated into dendritic cells. Thus, CD45 cells are first present in the yolk sac during primitive hematopoiesis and then migrate from the extra-embryonic yolk sac to give rise to cells throughout all organ primordia, including dendritic cells in the bursa of Fabricius.