BAFF production by antigen‐presenting cells provides T cell co‐stimulation

The B cell‐activating factor from the tumor necrosis factor family (BAFF) is an important regulator of B cell immunity. Recently, we demonstrated that recombinant BAFF also provides a co‐stimulatory signal to T cells. Here, we studied expression of BAFF in peripheral blood leukocytes and correlated this expression with BAFF T cell co‐stimulatory function. BAFF is produced by antigen‐presenting cells (APC). Blood dendritic cells (DC) as well as DC differentiated in vitro from monocytes or CD34+ stem cells express BAFF mRNA. Exposure to bacterial products further up‐regulates BAFF production in these cells. A low level of BAFF transcription, up‐regulated upon TCR stimulation, was also detected in T cells. Functionally, blockade of endogenous BAFF produced by APC and, to a lesser extent, by T cells inhibits T cell activation. Altogether, this indicates that BAFF may regulate T cell immunity during APC-T cell interactions and as an autocrine factor once T cells have detached from the AP

Human memory T cells: lessons from stem cell transplantation.

Animal models have revealed the rules for the organization of mature T-cell pools. However, in humans, little is known about memory T cells, which differ in lifespan and in the number of times that the same antigen is encountered. Here, Nathalie Rufer and colleagues discuss their findings in stem-cell-transplanted patients, which provide interesting data on the human T-cell compartment

Kinetics of MHC-antigen complex formation on antigen-presenting cells

With the use of flow cytometry, we recorded changes in intracellular ionized calcium [Ca2+]i of Indo-1 loaded T cells that were triggered by contact with APC. This rapid readout of TCR perturbation enabled us to monitor the formation of stimulatory Ag-MHC complexes on EBV-transformed B cells that were either pulsed with native tetanus toxoid (TT) or with a 12-amino-acid fragment of this protein. Neither unpulsed APC nor Ag-specific APC that were pulsed with native Ag and kept at +4 degrees C were able to induce changes in basal T cell [Ca2+]i in TT-specific T cell clones. After 1 h at 37 degrees C, however, the Ag-pulsed APC were able to induce a three-to-fourfold increase in [Ca2+]i. This length of time appeared to be almost independent of the concentration of Ag with which the APC were pulsed, suggesting that the lag time was due more to intracellular transit than to association of the processed Ag with the MHC molecule. Furthermore, the same lag time and independence of Ag concentration were found when the EBV-transformed B cells were pulsed with a mouse-anti-transferrin receptor mAb and tested for their capacity to trigger a T cell clone specific for processed mouse Ig. This indicates that, in addition to surface Ig, other receptors that are internalized can function in the same fashion in the uptake and processing of a soluble Ag. In contrast to what was found with intact native Ag, no lag time was observed when the APC were pulsed with high concentrations of a 12-amino-acid peptide, containing the amino acid sequence recognized by a TT-specific T cell clone, suggesting that the formation of MHC-peptide complexes occurs instantly. Pulsing with a lower peptide concentration, however, caused the appearance of a time-dependent increase in efficacy of Ag presentation, suggesting a slow accumulation of MHC-peptide complexes on the B cell membrane

Two monoclonal anti-CD3 antibodies can induce different events in human T lymphocyte activation

Two monoclonal antibodies, WT32 and CLB-T3/4.2a, directed against the CD3 complex were used to study the mechanism of activation of human peripheral T lymphocytes. WT32, a mouse monoclonal IgG2a antibody with a low avidity (much less than OKT3) for the CD3 complex, effectively induces mitogenesis of purified T lymphocytes when used in the 1 ng-10 micrograms range in the presence of monocytes or recombinant interleukin 2 (IL2). In contrast, CLB-T3/4.2a, a mouse monoclonal antibody of the same isotype with a high avidity (much greater than OKT3) for the CD3 complex, induces IL2 receptor expression and IL2 responsiveness only at very low concentrations (less than 5 ng/ml), yet in the presence of monocytes this antibody induces proliferation within a similar dose range as WT32. Apparently, in the absence of accessory cells which can cross-link the antibody CD3 complexes, the binding properties (avidity) of an antibody and thereby the number of receptors that are occupied are important parameters for induction of IL2 responsiveness. Furthermore, we show that Ca2+ mobilization only occurs when the cells are stimulated by saturating amounts of antibody, so that, when the conditions are optimal for the induction of IL2 responsiveness, no Ca2+ mobilization will be detecte

Limited telomere shortening in hematopoietic stem cells after transplantation.

The number of cell divisions in hematopoietic stem cells (HSCs) following transplantation of bone marrow or mobilized peripheral blood into myelo-ablated recipients is unknown. This number is expected to depend primarily on the number of transplanted stem cells, assuming that stem cells do not differ in engraftment potential and other functional properties. In a previous study, we found that the telomere length in circulating granulocytes in normal individuals shows a biphasic decline with age, most likely reflecting age-related changes in the turnover of HSCs. In order to study HSCs' proliferation kinetics following stem cells transplantation, we analyzed the telomere length in donor-derived nucleated blood cells in four HLA-matched bone marrow transplant recipients relative to comparable cells from the sibling donors. In each case, the telomeres in granulocytes were shorter in the recipient than in the donor. This difference was established in the first year post transplantation and did not change after that. The telomere length in naïve and memory T cells showed marked differences after transplantation, complicating the interpretation of telomere length data using unseparated nucleated blood cells. Interestingly, the telomere length in naïve T cells that were first observed six months post transplantation was very similar in donor and recipient pairs. Our observations are compatible with a limited number of additional cell divisions in stem cell populations after bone marrow transplantations and support the idea that different populations of stem cells contribute to short-term myeloid and long-term lympho myeloid hematopoiesis

Telomere length dynamics in human lymphocyte subpopulations measured by flow cytometry.

To measure the average length of telomere repeats at chromosome ends in individual cells we developed a flow cytometry method using fluorescence in situ hybridization (flow FISH) with labeled peptide nucleic acid (PNA) probes. Results of flow FISH measurements correlated with results of conventional telomere length measurements by Southern blot analysis (R = 0.9). Consistent differences in telomere length in CD8+ T-cell subsets were identified. Naive and memory CD4+ T lymphocytes in normal adults differed by around 2.5 kb in telomere length, in agreement with known replicative shortening of telomeres in lymphocytes in vivo. T-cell clones grown in vitro showed stabilization of telomere length after an initial decline and rare clones capable of growing beyond 100 population doublings showed variable telomere length. These results show that flow FISH can be used to measure specific nucleotide repeat sequences in single cells and indicate that the very large replicative potential of lymphocytes is only indirectly related to telomere length