The development of immunological tolerance to orally fed antigens depends on the sampling, processing and transportation events followed in the intestinal epithelium. We present here a description of a ''tolerosome'': a supra‐molecular, exosome‐like structure assembled in and released from the small intestinal epithelial cell. The tolerosome is a ≈ 40 nm large vesicular structure that carries MHC class II (MHC II) with bound antigenic peptides sampled from the gut lumen. Tolerosomes isolated from serum shortly after antigen feeding or from an in vitro pulsed intestinal epithelial cell line are fully capable of inducing antigen specific tolerance in naive recipient animals. Purified tolerosomes represent a structure by which fed antigens can be efficiently presented to the immune system. Removal of the tolerosomes from serum by ultracentrifugation or absorption of MHC II results in abrogated tolerance development.

MEP21 is an avian antigen specifically expressed on the surface of Myb-Ets–transformed multipotent hematopoietic precursors (MEPs) and of normal thrombocytes. Using nanoelectrospray tandem mass spectrometry, we have sequenced and subsequently cloned the MEP21 cDNA and named the gene thrombomucin as it encodes a 571–amino acid protein with an extracellular domain typical of the mucin family of proteoglycans. Thrombomucin is distantly related to CD34, the best characterized and most used human hematopoietic stem cell marker. It is also highly homologous in its transmembrane/intracellular domain to podocalyxinlike protein–1, a rabbit cell surface glycoprotein of kidney podocytes.Single cell analysis of yolk sac cells from 3-d-old chick embryos revealed that thrombomucin is expressed on the surface of both lineage-restricted and multipotent progenitors. In the bone marrow, thrombomucin is also expressed on mono- and multipotent progenitors, showing an overlapping but distinct expression pattern from that of the receptor-type stem cell marker c-kit. These observations strengthen the notion that the Myb-Ets oncoprotein can induce the proliferation of thrombomucin-positive hematopoietic progenitors that have retained the capacity to differentiate along multiple lineages. They also suggest that thrombomucin and CD34 form a family of stem cell–specific proteins with possibly overlapping functions in early hematopoietic progenitors.

Two pools of heparan sulfate proteoglycans have been selectively solubilized from rat liver plasma membranes by successive incubations with heparin and detergent. The two populations of proteoglycans have similar polyanionic properties as indicated by identical elution positions on ion-exchange chromatography on DEAE-Sephacel but differ in buoyant density in CsCI when analyzed by density gradient centrifugation in the presence of 4 M guanidine. The detergent-extracted proteoglycan has a lower buoyant density ('1.40 g/ml) and is, as determined by gel chromatography, slightly. larger than the heparin-released proteoglycan (buoyant density, 21.55 g/ml). Furthermore, in contrast to the heparin-released proteoglycan, the detergent-extracted proteoglycan is able to bind detergent micelles, shows affinity for the hydrophobic gel octyl-Sepharose, and can be inserted into liposomes. We conclude that the detergent-extracted ,heparan sulfate represents a proteoglycan species that has its core protein rooted in the lipid bilayer of the plasma membrane.

The complete nucleotide sequence of a mouse mastocytoma proteoglycan core protein mRNA was determined. The mRNA, estimated to contain 1.1 kb, encodes a protein with an Mr of 16715. A 21-amino acid-residue region of the protein is composed of alternating serine and glycine residues. Southern-blot analysis of mouse genomic DNA with cDNA containing sequences corresponding to the Ser-Gly repeat region revealed more than 15 gene fragments. Hybridization with a probe corresponding to the N-terminal portion of the core protein identified two fragments, and cDNA covering the C-terminal part of the core protein and the 3' untranslated part of the mRNA hybridized to a single fragment. Antibodies against the core protein, obtained after immunization of rabbits with a fusion protein, reacted with both chondroitin sulphate proteoglycans and heparin proteoglycans produced by the tumour. In immunoblotting of a microsomal fraction from the mastocytoma, the antiserum recognized a single protein (Mr 17,000), which probably represents the core protein before glycosylation.