CD83 is an inducible glycoprotein expressed predominantly by dendritic cells (DC) and B lymphocytes. Expression of membrane CD83 (mCD83) is widely used as a marker of differentiated/activated DC but its function and ligand(s) are presently unknown. We report the existence of a soluble form of CD83 (sCD83). Using both a sCD83-specific ELISA and Western blotting, we could demonstrate the release of sCD83 by mCD83(+) B cell and Hodgkin's disease-derived cell lines, but not mCD83(-) cells. Inhibition of de novo protein synthesis did not affect the release of sCD83 during short-term (2 h) culture of cell lines although mCD83 expression was significantly reduced, suggesting sCD83 is generated by the release of mCD83. Isolated tonsillar B lymphocytes and monocyte-derived DC, which are mCD83(low), released only low levels of sCD83 during culture. However, the differentiation/activation of these populations both up-regulated mCD83 and increased sCD83 release significantly. Analysis of sera from normal donors demonstrated the presence of low levels (121 +/- 3.6 pg/ml) of circulating sCD83. Further studies utilizing purified sCD83 and the analysis of sCD83 levels in disease may provide clues to the function and ligand(s) of CD83.

Dendritic cells (DCs) have a central role in the initiation and regulation of immune responses in both lymphoid and nonlymphoid tissues. They share a number of common features, notably MHC class II expression, in combination with an absence of the lineage-specific markers CD3, CD14, CD16, and CD19 (lin Ϫ ). [1][2][3] There is however considerable intra-and intertissue variation in the phenotype, morphology, function, and tissue localization of different DC populations. [1][2][3][4][5][6][7] After the identification of distinct myeloid DC and lymphoid DC subsets in mice 3 there has been increasing interest in the characterization of human DC subsets. These have been categorized on the basis of differential expression of myeloid and lymphoid lineage-associated markers, as well as their responsiveness to maturation or differentiation stimuli. 8 -13 There is evidence that myeloid DCs and lymphoid DCs direct immunogenic and tolerogenic responses, respectively. 13 It is unclear whether human DC subsets represent distinct cell lineages 10,12 or differing maturation states. 9,14 As a result, the ontogeny and interrelationship of human DC subsets requires further investigation.Tonsils have been used as a readily available source of lymphoid tissue to characterize human DCs. 15 Three tonsil DC subsets have been identified: interdigitating DCs (IDCs), 16 plasmacytoid DCs, 17 and germinal center DCs (GCDCs). 18 These DC subsets were isolated after a period of in vitro culture, which is likely to alter cell phenotype and morphology. 19 They were also positively selected from lin Ϫ cells based on their expression of the CD4, CD11c, or CD40 antigens, which would exclude any DC subsets lacking these antigens and would incorporate all DC subsets expressing those antigens into one population. The observation that both IDCs and GCDCs were heterogeneous with regard to CD11c, CD83, HLA-DR, and CD13 intensity raised the possibility that these tonsil DC populations might contain additional subsets. This is the first study to analyze the composition of tonsil DC subsets within the entire lin Ϫ HLA-DR ϩ DC population isolated using a new method that maintains the cells at 4°C to minimize changes in cellular differentiation/activation induced by the isolation procedure. We report the presence of five distinct DC subsets within human tonsils. The phenotype of each tonsil DC subset was analyzed by threecolor flow cytometry and two-color immunohistochemistry using an extensive panel of antigens relevant to DC lineage, activation state, and function. Materials and Methods Patients and SamplesAfter approval by the Canterbury Ethics Committee and appropriate informed consent, tonsils were obtained from 68 patients undergoing routine tonsillectomies, which were excised in a noninflamed state. They were trans-

Exosomes are lipid-bound nanovesicles formed by inward budding of the endosomal membrane and released following fusion of the endosomal limiting membrane with the plasma membrane. We show here that primary leukocytes do not release exosomes unless subjected to potent activation signals, such as cytokine or mitogen stimulation. In particular, high levels of exosomes were released when murine splenic B cells were stimulated via CD40 and the IL-4 receptor. This property was shared by B cells from different anatomic locations, as newly formed, marginal zone and follicular B cells were capable of secreting exosomes upon CD40/IL-4 triggering. B cell exosomes expressed high levels of MHC class I, MHC class II, and CD45RA (B220), as well as components of the BCR complex, namely, surface Ig, CD19, and the tetraspanins CD9 and CD81. Ig on the plasma membrane of primary B cells was targeted to the exosome pathway, demonstrating a link between the BCR and this exocytic pathway. IgD and IgM were the predominant Ig isotypes associated with CD40/IL-4 elicited exosomes, though other isotypes (IgA, IgG1, IgG2a/2b, and IgG3) were also detected. Together, these results suggest that exosome release is not R constitutive activity of B cells, but may be induced following cell: cell signaling.

Maximal T lymphocyte responses require presentation of antigen by major histocompatibility complex molecules and delivery of one or more co-stimulatory signals. Interaction of the CD28 molecule on T lymphocytes with its ligands on antigen-presenting cells (APC) initiates a critical co-stimulatory pathway inducing T lymphocyte proliferation and cytokine secretion. Dendritic cells (DC) are potent APC for a primary T lymphocyte response and potential CD28/CTLA-4 ligands on DC are, therefore, of particular functional relevance. In these experiments, the expression and function of the CD28/CTLA-4 ligands B7.1 (CD80) and B7.2 (CD86) were examined on human blood DC. Resting DC populations directly isolated by immunodepletion of lineage marker-positive cells lacked cell membrane expression of CD80 and expressed little or no CD86, although CD86, but not CD80 mRNA was detected by reverse transcription-polymerase chain reaction analysis. In contrast, low-density DC isolated after culture in vitro strongly expressed CD86 surface protein, but expressed limited or no CD80, although mRNA for both molecules were detected. Short-term culture of directly isolated DC up-regulated both CD80 and CD86 expression. Analysis of the kinetics of CD28/CTLA-4 ligand induction showed that surface CD86 was present within 8 h, whereas CD80 antigen was first detected after 24 h of culture. The functional importance of CD28/CTLA-4 ligand up-regulation on DC during T lymphocyte interactions was demonstrated by the ability of both CTLA-4Ig and CD86 monoclonal antibodies (mAb), but not CD80 mAb, to block an allogeneic mixed lymphocyte reaction stimulated by DC populations initially negative for CD80 and CD86. These results demonstrate that CD86 is both the earliest and functionally the predominant co-stimulatory CD28/CTLA-4 ligand on DC.