An improved method for purifying human thymic dendritic cells

Human thymus contains IFN-alpha-producing CD11c(-), myeloid CD11c(+), and mature interdigitating dendritic cells

Three distinct dendritic cell (DC) subsets capable of stimulating allogeneic naive T cells were isolated from human thymus. The most abundant subset was represented by plasmacytoid DCs (pDCs), which secreted high amounts of IFN-alpha upon stimulation with inactivated influenza virus and thus likely correspond to the recently identified peripheral blood natural IFN-alpha/beta-producing cells (IPCs). Like those latter cells, thymic pDCs had distinctive phenotypic features (i.e., Lin(-), HLA-DR(int), IL-3R alpha(hi), CD45RA(hi), CD11c(-), CD13(-), and CD33(lo)) and developed into mature DCs upon culture in IL-3 and CD40L. Of the two other DC subsets, one displayed a phenotype of immature myeloid DCs (imDCs) (HLA-DR(int), CD11c(+), CD13(+), CD33(+)), and the other represented HLA-DR(hi) CD11c(+) mature DCs (mDCs). Since they also expressed DC-LAMP, these mDCs appear to correspond to interdigitating dendritic cells (IDCs). Thymic pDCs, but not myeloid imDCs, strongly expressed lymphoid-specific transcripts such as pre-T alpha, lambda-like, and Spi-B, thereby suggesting a possible lymphoid origin. The detection of Spi-B mRNA, not only upon in vitro maturation of pDCs, but also in freshly purified IDCs, suggests that in vivo pDCs may differentiate into IDCs.

Presence of HLA-DR immunopositive cells in human fetal thymus

Several kinds of thymic cells express MHC class II antigens, including human-leukocyte-associated antigen-DR (HLA-DR) during postnatal development. The present study was focused on the detection and analysis of HLA-DR immunoreactivity in human fetal thymuses (6-7th month of gestation). Using monoclonal antibodies, indirect immunoperoxidase staining (IIP), immunogold electron microscopy (IGEM), enzyme-linked immunosorbent assay (ELISA) and flow cytometry, HLA-DR immunopositive (IP) thymic cells were found in samples studied. IIP and IGEM demonstrated the presence of HLA-DR IP stromal cells (SCs): epithelial cells (ECs), dendritic-like cells (DCs) and macrophages (MCs) as well as HLA-DR IP lymphocytes (Lys) in all thymic regions. HLA-DR immunoreactivity was more prominent in the medullary ECs (mECs) than in the cortical ECs (cECs). Strong staining of Hassall's corpuscles and the adjacent mECs was seen. The differences in the intracellular distribution of HLA-DR molecules were detailed by IGEM as a first attempt to analyse HLA-DR IP cells at ultrastructural level. ELISA data and two-colour flow cytometric analysis revealed the presence of HLA-DR IP and HLA-DR/CD3 double IP Lys in accordance with the immunocytochemical assays. The results presented enrich the information about HLA-DR IP components of the thymic microenvironment in developing human thymus and raise the question of their role during prenatal T cell differentiation and selection processes.

Commercial high speed machines open new opportunities in high throughput flow cytometry (HTFC)

Two recent events have opened a new domain of flow cytometry applications which we term high throughput flow cytometry (HTFC). The release of a commercial high speed sorter in 1994 placed HTFC within the reach of anyone who could buy one of the new machines and not just the handful of advanced laboratories worldwide that had custom built their own high speed sorters. The advent in 1999 of HTFC analysis capabilities of 100000 cells/s marks the second stage in this enabling of HTFC. We describe the technical basis of HTFC. The commercial high speed sorters measure cells in dead-times three to six times shorter than conventional machines. They can sort with high yield and high purity at rates from 25000 to 60000 cells/s, depending on their settings, mainly by virtue of their use of high drop creation rates 100000 drops/s or more. Finally, one series can analyse the measured cells at rates exceeding these sort-rates and at least six times faster than conventional sorters could. The performance of the systems made by the three manufacturers can be readily assessed for single laser systems. Comparison becomes difficult for multiple beam machines, due to requirements for multi-beam sampling for each cell and due to the demands of fluorescence compensation between signals from one laser and between signals from two or three lasers. Applications are described in the field of rare cell analysis and isolation as well as from sorting of abundant cell populations.

Analysis of the human neonatal thymus: evidence for a transient thymic involution

The neonatal period is marked by the impairment of the major components of both innate and adaptive immunity. We report a severe depletion of cortical CD4+CD8+ double-positive thymocytes in the human neonatal thymus. This drastic reduction in immature double-positive cells, largely provoked by an increased rate of cell death, could be observed as early as 1 day after birth, delaying the recovery of the normal proportion of this thymocyte subset until the end of the first month of postnatal life. Serum cortisol levels were not increased in newborn donors, indicating that the neonatal thymic involution is a physiological rather than a stress-associated pathological event occurring in the perinatal period. Newborn thymuses also showed increased proportions of both primitive CD34+CD1- precursor cells and mature TCRalphabetahighCD69-CD1-CD45RO+/RAdull and CD45ROdull/RA+ cells, which presumably correspond to recirculating T lymphocytes into the thymus. A notable reinforcement of the subcapsular epithelial cell layer as well as an increase in the intralobular extracellular matrix network accompanied modifications in the thymocyte population. Additionally neonatal thymic dendritic cells were found to be more effective than dendritic cells isolated from children's thymuses at stimulating proliferative responses in allogeneic T cells. All these findings can account for several alterations affecting the peripheral pool of T lymphocytes in the perinatal period.

Potent immunostimulatory dendritic cells can be cultured in bulk from progenitors in normal infant and adult myasthenic human thymus

Low density cells can readily be enriched from thymus tissue both of children undergoing cardiac surgery and of older patients with myasthenia gravis, and can be cryostored in bulk. When fresh or thawed cells are cultured with granulocyte-macrophage colony-stimulating factor and stem cell factor with or without tumour necrosis factor-alpha (TNF-alpha), they generate numerous cells with the characteristic ultrastructural, phenotypic and functional properties of dendritic cells. These proved to be very potent, both as stimulators of primary mixed leucocyte responses and as costimulators in oxidative mitogenesis. Especially after exposure to TNF-alpha, these dendritic cells also processed a natural epitope from a 437-residue polypeptide and presented it efficiently to an autoimmune T-cell clone (of T helper type 0 phenotype). Thus, immunostimulatory dendritic cells can be cultured in relative abundance from progenitors in infant and adult human thymus. Both are convenient sources of potent antigen-presenting cells of identifiable origins, e.g. for use in selecting human T-cell lines.

Transcription of a Broad Range of Self-Antigens in Human Thymus Suggests a Role for Central Mechanisms in Tolerance Toward Peripheral Antigens

The role of the thymus in the induction of tolerance to peripheral antigens is not yet well defined. One impending question involves how the thymus can acquire the diversity of peripheral nonthymic self-Ags for the process of negative selection. To investigate whether peripheral Ags are synthesized in the thymus itself, we have determined the expression of a panel of circulating and cell-bound peripheral Ags, some of which are targets of autoimmune diseases, at the mRNA level in total thymic tissue and in its main cellular fractions. Normalized and calibrated RT-PCR experiments demonstrated the presence of transcripts of nonthymic self-Ags in human thymi from 8 days to 13-yr-old donors. Out of 12 glands, albumin transcripts were found in 12; insulin, glucagon, thyroid peroxidase, and glutamic acid decarboxylase (GAD)-67 in six, thyroglobulin in five, myelin basic protein and retinal S Ag in three, and GAD-65 in one. The levels of peripheral Ag transcripts detected were age-related but also showed marked interindividual differences. Cytokeratin-positive stromal epithelial cells, which are a likely cellular source for these, contained up to 200 transcript copies of the most expressed peripheral Ags per cell. These results implicate the human thymus in the expression of wide representation of peripheral self-Ags and support the view that the thymus is involved in the establishment of tolerance to peripheral Ags. The existence of such central mechanism of tolerance is crucial for the understanding of organ-specific autoimmune diseases.

Characterization of the cytotoxic factor(s) released from thymic dendritic cells upon human immunodeficiency virus type 1 infection

We previously demonstrated that infection of primary human thymic dendritic cells (DCs) with laboratory strains of HIV leads to the release of soluble factor(s) which induced thymocyte killing. In the present paper, we extend the characterization of this process. Our results reveal that primary HIV-1 isolates are similarly able to induce the production of cytotoxic factor(s) from thymic DCs and that the release of such factor(s) is dependent on viral infection. Interestingly, we observed that CD4+ and CD8+ purified thymocyte subsets, and activated PBMCs are susceptible to the cytotoxic activity, whereas freshly isolated resting PBMCs are resistant to this effect. Cycloheximide treatment prevents the killing of thymocytes exposed to HIV-infected DC supernatant, revealing that this form of cell death is an active biological process requiring protein synthesis. Finally, our data suggest that FasL and TNF alpha could both participate in the killing process. These in vitro observations provide a plausible model, whereby HIV-infected DCs can play a role in vivo in the induction of uninfected thymocyte killing.