Naive T cells are maintained by thymic output in early ages but by proliferation without phenotypic change after age twenty

Analysing T-cell receptor excision circle numbers in healthy individuals we find a marked change in the source of naive T cells before and after 20 years of age. The bulk of the naive T cell pool is sustained primarily from thymic output for individuals younger than 20 years of age whereas proliferation within the naive phenotype is dominant for older individuals. Over 90% of phenotypically naive T cells in middle age are not of direct thymic origin. Moreover, this change in source of naive T cells is accompanied either by an increased death rate of T cells from the thymus or reduced thymic export. Modelling of these processes shows that new naive T cells of a thymic origin have a half-life of approximately 50 days before this change occurs, and that either the life-span of recent thymic emigrants (but not necessarily of all naive cells) decreases approximately threefold in middle age, or thymic production drops by this same amount. The decay rate of T-cell receptor excision circle levels for individuals over 20 years of age is consistent with the decay rate of the productive thymus. Our modelling suggests that at age 25, thymic export is responsible for 20% of naive T-cell production and that this percentage decreases with the 15.7 year half-life of the productive thymus so that by age 55 only 5% of naive production arises from thymic export.     

Apoptosis facilitates antigen presentation to T lymphocytes through MHC-I and CD1 in tuberculosis

Protective immunity against Mycobacterium tuberculosis involves major histocompatibility complex class I (MHC-I)- and CD1-restricted CD8 T cells, but the mechanisms underlying antigen delivery to antigen-presenting molecules remain enigmatic. Macrophages, the primary host cells for mycobacteria, are CD1-negative. Here we show that M. tuberculosis phagosomes are secluded from the cytosolic MHC-I processing pathway and that mycobacteria-infected cells lose their antigen-presenting capacity. We also show that mycobacteria induce apoptosis in macrophages, causing the release of apoptotic vesicles that carry mycobacterial antigens to uninfected antigen-presenting cells (APCs). Inhibition of apoptosis reduced transfer of antigens to bystander cells and activation of CD8 T cells. Uninfected dendritic cells, which engulfed extracellular vesicles, were indispensable for subsequent cross-presentation of antigens, through MHC-I and CD1b, to T cells from mycobacteria-sensitized donors. This new 'detour' pathway for presentation of antigens from a phagosome-contained pathogen shows the functional significance of infection-induced apoptosis in the activation of CD8 T cells specific for both protein and glycolipid antigens in tuberculosis.     

TIP,a T-cell factor identified using high-throughput screening increases survival in a graft-versus-host disease model

A coordinated effort combining bioinformatic tools with high-throughput cell-based screening assays was implemented to identify novel factors involved in T-cell biology. We generated a unique library of cDNAs encoding predicted secreted and transmembrane domain-containing proteins generated by analyzing the Human Genome Sciences cDNA database with a combination of two algorithms that predict signal peptides. Supernatants from mammalian cells transiently transfected with this library were incubated with primary T cells and T-cell lines in several high-throughput assays. Here we describe the discovery of a T cell factor, TIP (T cell immunomodulatory protein), which does not show any homology to proteins with known function. Treatment of primary human and murine T cells with TIP in vitro resulted in the secretion of IFN-gamma, TNF-alpha, and IL-10, whereas in vivo TIP had a protective effect in a mouse acute graft-versus-host disease (GVHD) model. Therefore, combining functional genomics with high-throughput cell-based screening is a valuable and efficient approach to identifying immunomodulatory activities for novel proteins.     

The gel-forming behaviour of dextran in the presence of KCl: a quantitative 13C and pulsed field gradient (PFG) NMR study

Although the gel forming ability of certain polysaccharides in the presence of ions is a well-known phenomenon, detailed physicochemical mechanisms of such processes are still unknown. In this investigation high resolution 13C NMR as well as 1H pulsed field gradient (PFG) NMR were used to investigate the mobility of dextran in the sol and in the gel state. Gel-formation of dextran can be easily induced by the addition of large amounts of potassium chloride. No major differences in the T(1) relaxation times of dextran in the sol and in the gel state could be observed. Accordingly, the analysis of the 13C NMR spectroscopic data did not provide any indication of an observable line-broadening upon gel-formation. However, a KCl concentration dependent decrease of signal intensity in comparison to an internal standard was detected. On the other hand, the PFG NMR studies clearly indicated a gradual diminution of the self-diffusion coefficient of the dextran with increasing molecular weight as well as in the presence of potassium chloride. These measurements revealed in agreement with spectroscopic data that at least one potassium ion per monomer subunit (i.e. one glycopyranose residue) is necessary for gel formation.     

Cutting edge: contribution of NK cells to the homing of thymic CD4+NKT cells to the liver

In contrast to peripheral lymphoid organs, in the liver a high proportion of T cells are CD4+NKT cells. We have previously reported that LFA-1 plays a pivotal role in the homing of thymic CD4+NKT cells to the liver. In the present study, we further assessed which cell type participates in the homing of thymic CD4+NKT cells to the liver. The accumulation of donor thymocyte-derived CD4+NKT cells in the liver of SCID mice that had been reconstituted with thymocytes from C57BL/6 mice was severely impaired by in vivo depletion of NK cells, but not Kupffer cells in recipients. These results suggest that NK cells participate in the homing of thymic CD4+NKT cells to the liver. We assume that LFA-1 expressed on NK cells is involved in this mechanism.     

Helicobacter pylori infection in wild-type and cytokine-deficient C57BL/6 and BALB/c mouse mutants

Helicobacter pylori causes gastroduodenal ulcer disease in humans. T lymphocytes and their cytokines are thought to play a substantial role in the control of H. pylori infection. To determine the importance of T helper (Th) cytokines and background genes we investigated the natural course of H. pylori infection in BALB/c and C57BL/6 wild-type or mutant mice deficient for either interleukin (IL)-4 or interferon (IFN)-gamma. H. pylori SPM 326 persisted for at least six months in C57BL/6 but was cleared by BALB/c wild-type mice nine weeks postinfection. H. pylori was recovered more frequently from IFN-gamma(-/-) BALB/c and IFN-gamma( -/-) C57BL/6 mice than from the respective wild-type animals. In contrast, IL-4 deficiency had no detectable effect on H. pylori recovery rates from either strain of mice. Our data suggest a protective role of IFN-gamma by mediating inflammation in murine H. pylori infection. In addition, our data emphasize that background genes which differ between BALB/c and C57BL/6 mice regulate the clearance of H. pylori.     

Different signaling pathways are involved in CCK(B) receptor-mediated MAP kinase activation in COS-7 cells

Recently, the involvement of the MAP kinase ERK in mitogenic signaling of cholecystokininB (CCK(B)) receptors has been shown. However, the intracellular effector systems involved in this signaling pathway are poorly defined. In this study, we used COS-7 cells transiently transfected with the human CCK(B) receptor to investigate cholecystokinin-induced MAP kinase activation. CCK-8 induced activation of ERK2 which is associated with its phosphorylation and localization in the nucleus. The CCK-8-dependent ERK stimulation is sensitive to wortmannin an inhibitor of phosphoinositide 3-kinases (PI3Ks) indicating the involvement of PI3K activity. To identify the PI3K species involved in mitogenic signaling of the CCK(B) receptor several dominant-negative mutants of PI3K regulatory and catalytic subunits were transiently expressed. Surprisingly, different catalytically inactive mutants of the G protein-sensitive PI3Kgamma did not affect ERK stimulation induced by CCK, whereas a dominant-negative mutant of the regulatory p85 subunit induced significant inhibition of CCK-dependent ERK activity. These results indicate an involvement of PI3K class 1A species alpha, beta or/and delta in signal transduction via CCK(B) receptors. In addition, protein kinase C (PKC)-dependent signaling pathways contribute to CCK(B)-mediated MAP kinase signaling as shown by inhibition of CCK-8-induced ERK activation by the PKC inhibitor bisindolylmaleimide.