CXCR4 and CXCL12 signaling regulates the development of extrinsic innervation to the colorectum.

The gastrointestinal tract is innervated by an intrinsic neuronal network, known as the enteric nervous system (ENS), and by extrinsic axons arising from peripheral ganglia. The nerve of Remak (NoR) is an avian-specific sacral neural crest-derived ganglionated structure that extends from the cloaca to the proximal midgut and, similar to the pelvic plexus, provides extrinsic innervation to the distal intestine. The molecular mechanisms controlling extrinsic nerve fiber growth into the gut is unknown. In vertebrates, CXCR4, a cell-surface receptor for the CXCL12 chemokine, regulates migration of neural crest cells and axon pathfinding. We have employed chimeric tissue recombinations and organ culture assays to study the role of CXCR4 and CXCL12 molecules in the development of colorectal innervation. CXCR4 is specifically expressed in nerve fibers arising from the NoR and pelvic plexus, while CXCL12 is localized to the hindgut mesenchyme and enteric ganglia. Overexpression of CXCL12 results in significantly enhanced axonal projections to the gut from the NoR, while CXCR4 inhibition disrupts nerve fiber extension, supporting a previously unreported role for CXCR4 and CXCL12 signaling in extrinsic innervation of the colorectum.

Characterization and functional properties of a novel monoclonal antibody which identifies a B cell subpopulation in bursa of Fabricius.

The bursa of Fabricius (BF) plays a central role in the development of B lymphocytes in birds. During embryonic development the BF primordium is colonized by myeloid and lymphoid prebursal stem cells to form the follicle buds, which ultimately develop into lymphoid follicles with a central medullary and an outer cortical region. Lympho-myeloid differentiation within the medulla is fundamental to normal B cell development. In contrast, the complexity of the cellular composition of the follicular cortex and its role in B cell differentiation has only recently begun to be studied. As an effort to characterize the different bursal cells we have produced a large panel of monoclonal antibodies (mAbs) by immunizing mice with a BF cell suspension of guinea fowl (Numida meleagris). One of these antibodies (clone: 7H3) was found to recognize a 80 kDa cell surface antigen expressed first in the yolk sac blood island of 2-day-old guinea fowl and chicken embryos, and later detected in the embryonic circulation and primary lymphoid organs. Double immunofluorescence revealed that chB6+ (Bu-1+) B cells of embryonic BF co-express the 7H3 antigen. 7H3 immunoreactivity of the bursal follicles gradually diminished after hatching and only a subpopulation of cortical B cells expressed the 7H3 antigen. In addition, in post-hatched birds 7H3 mAb recognizes all T lymphocytes of the thymus, peripheral lymphoid organs and blood. Embryonic BF injected with the 7H3 mAb showed a near complete block of lymphoid follicle formation In conclusion, 7H3 mAb labels a new differentiation antigen specific for avian hematopoietic cells, which migrate through the embryonic mesenchyme, colonize the developing BF lymphoid follicles, and differentiate into a subpopulation of cortical B cells. The staining pattern of the 7H3 mAb and the correlation of expression with cell migration suggest that the antigen will serve as valuable immunological marker for studying the ontogeny of avian B cells.

In and Out of the Bursa-The Role of CXCR4 in Chicken B Cell Development.

In contrast to mammals, early B cell differentiation and diversification of the antibody repertoire in chickens do not take place in the bone marrow but in a specialized gut associated lymphoid tissue (GALT), the bursa of Fabricius. During embryonic development, B cell precursors migrate to the bursa anlage, where they proliferate and diversify their B cell receptor repertoire. Around hatch these diversified B cells start to emigrate from the bursa of Fabricius and populate peripheral lymphoid organs, but very little is known how the migratory processes are regulated. As CXCL12 (syn. SDF-1) and CXCR4 were shown to be essential for the control of B cell migration during the development of lymphoid tissues in mammals, we analyzed expression and function of this chemokine/chemokine-receptor pair in the chicken bursa. We found a strong variation of mRNA abundance of CXCL12 and CXCR4 in different stages of bursa development, with high abundance of CXCL12 mRNA in the bursa anlage at embryonic day 10 (ED10). In situ hybridization demonstrated disseminated CXCL12 expression in the early bursa anlage, which condensed in the developing follicles and was mainly restricted to the follicle cortex post-hatch. Flow cytometric analysis detected CXCR4 protein already on early B cell stages, increasing during bursal development. Post-hatch, a subpopulation with the hallmarks of emigrating B cells became detectable, which had lower CXCR4 expression, suggesting that downregulation of CXCR4 is necessary to leave the CXCL12-high bursal environment. In vivo blockade of CXCR4 using AMD3100 at the time of B cell precursor immigration strongly inhibited follicle development, demonstrating that CXCL12 attracts pre-bursal B cells into the bursal anlage. Altogether, we show that CXCL12 and its receptor CXCR4 are important for both populating the bursa with B cells and emigration of mature B cells into the periphery post hatch, and that CXCR4 function in primary B cell organs is conserved between mammals and birds.

Ontogeny of ramified CD45 cells in chicken embryo and their contribution to bursal secretory dendritic cells.

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.

Dual secretion locations on type II cells in the avian lung suggest local as well as general roles of surfactant.

Transmission electron microscopy indicates that the avian lung surfactant may be secreted in two directions: a) into air passages of parabronchus, atrium and infundibulum where it forms a trilaminar substance serving the respiratory role and b) to the basolateral surface-intercellular space-of type II pneumocytes, contributing to the innate and adoptive immune responses of lung. Basolateral secretion may be confirmed by the presence of trilaminal substance in the intercellular space of type II pneumocytes. Fusion of surfactant containing vesicles with the lateral plasma membrane may result in membrane fusion of neighboring cells and subsequently formation of multinucleated giant cell. The indistinct and in some places discontinuous basal lamina in the parabronchial atrium and infundibulum permits the hydrophilic surfactant proteins to spread into the interstitium of air-blood capillary region. The hydrophilic surfactant proteins may activate lung interstitial macrophages to migrate into the air passages where they appear as "free avian respiratory macrophages." Therefore, in the interstitium the hydrophilic surfactant proteins are essential soluble components of innate immunity. J. Morphol. 277:1062-1071, 2016. © 2016 Wiley Periodicals, Inc.

Sonic hedgehog controls enteric nervous system development by patterning the extracellular matrix.

The enteric nervous system (ENS) develops from neural crest cells that migrate along the intestine, differentiate into neurons and glia, and pattern into two plexuses within the gut wall. Inductive interactions between epithelium and mesenchyme regulate gut development, but the influence of these interactions on ENS development is unknown. Epithelial-mesenchymal recombinations were constructed using avian hindgut mesenchyme and non-intestinal epithelium from the bursa of Fabricius. These recombinations led to abnormally large and ectopically positioned ganglia. We hypothesized that sonic hedgehog (Shh), a secreted intestinal epithelial protein not expressed in the bursa, mediates this effect. Inhibition of Shh signaling, by addition of cyclopamine or a function-blocking antibody, resulted in large, ectopic ganglia adjacent to the epithelium. Shh overexpression, achieved in ovo using Shh-encoding retrovirus and in organ culture using recombinant protein, led to intestinal aganglionosis. Shh strongly induced the expression of versican and collagen type IX, whereas cyclopamine reduced expression of these chondroitin sulfate proteoglycans that are known to be inhibitory to neural crest cell migration. Shh also inhibited enteric neural crest-derived cell (ENCC) proliferation, promoted neuronal differentiation, and reduced expression of Gdnf, a key regulator of ENS formation. Ptc1 and Ptc2 were not expressed by ENCCs, and migration of isolated ENCCs was not inhibited by Shh protein. These results suggest that epithelial-derived Shh acts indirectly on the developing ENS by regulating the composition of the intestinal microenvironment.