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The Journal of Experimental Medicine (2)
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Andris, Fabienne (3)
Urbain, Jacques (2)
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C-Terminal Mutants of Apolipoprotein L-I Efficiently Kill Both Trypanosoma brucei brucei and Trypanosoma brucei rhodesiense
Mansfield, John M.
Apolipoprotein L-I (apoL1) is a human-specific serum protein that kills Trypanosoma brucei through ionic pore formation in endosomal membranes of the parasite. The T. brucei subspecies rhodesiense and gambiense resist this lytic activity and can infect humans, causing sleeping sickness. In the case of T. b. rhodesiense, resistance to lysis involves interaction of the Serum Resistance-Associated (SRA) protein with the C-terminal helix of apoL1. We undertook a mutational and deletional analysis of the C-terminal helix of apoL1 to investigate the linkage between interaction with SRA and lytic potential for different T. brucei subspecies. We confirm that the C-terminal helix is the SRA-interacting domain. Although in E. coli this domain was dispensable for ionic pore-forming activity, its interaction with SRA resulted in inhibition of this activity. Different mutations affecting the C-terminal helix reduced the interaction of apoL1 with SRA. However, mutants in the L370-L392 leucine zipper also lost in vitro trypanolytic activity. Truncating and/or mutating the C-terminal sequence of human apoL1 like that of apoL1-like sequences of Papio anubis resulted in both loss of interaction with SRA and acquired ability to efficiently kill human serum-resistant T. b. rhodesiense parasites, in vitro as well as in transgenic mice. These findings demonstrate that SRA interaction with the C-terminal helix of apoL1 inhibits its pore-forming activity and determines resistance of T. b. rhodesiense to human serum. In addition, they provide a possible explanation for the ability of Papio serum to kill T. b. rhodesiense, and offer a perspective to generate transgenic cattle resistant to both T. b. brucei and T. b. rhodesiense.
The serum protein apolipoprotein L-I (apoL1) is responsible for human innate immunity against Trypanosoma brucei brucei, because this protein kills the parasite by generating ionic pores in the lysosomal membrane. Two T. brucei subspecies (T. b. rhodesiense and T. b. gambiense) can resist apoL1 and therefore, infect humans and cause sleeping sickness. In T. b. rhodesiense, resistance to human serum is linked to interaction of the Serum Resistance-Associated (SRA) protein with the C-terminal region of apoL1. We show that mutations targeted to this region reduced its interaction with SRA while preserving the activity of the ionic pore-forming domain. While some mutants also lost their trypanolytic potential, C-terminal mutants inspired by apoL1-like sequences of Papio anubis conserved this activity, but also acquired the ability to efficiently kill T. b. rhodesiense, both in vitro and in mice. These findings demonstrate that interaction of SRA with the C-terminus of apoL1 inactivate this protein and is responsible for the resistance of T. b. rhodesiense to human serum. Moreover, they suggest that apoL1-like proteins could be responsible for the trypanolytic potential of Papio species. Finally, Papio-like human apoL1 mutants could be used to generate transgenic cattle that would resist both T. b. brucei and T. b. rhodesiense.
Glucocorticoids Attenuate T Cell Receptor Signaling
Van Laethem, François
Smyth, Lesley A.
The Journal of Experimental Medicine
Glucocorticoids (GCs) affect peripheral immune responses by inhibiting T cell immunity at several stages of the activation cascade, causing impaired cytokine production and effector function. The recent demonstration that the thymic epithelium and possibly thymocytes themselves produce steroids suggests that endogenous GCs also play a role in the control of T cell development. As both peripheral responsiveness and thymic differentiation appear to be regulated by the quantity and quality of intracellular signals issued by antigen–major histocompatibility complex-engaged T cell receptor (TCR) complexes, we investigated the effects of GCs on the signaling properties of T cells stimulated by anti-CD3 monoclonal antibodies or agonist peptides. We demonstrate in this work that dexamethasone, a synthetic GC, inhibits the early signaling events initiated upon TCR ligation, such as tyrosine phosphorylation of several TCR-associated substrates including the ζ chain, the ZAP70 kinase, and the transmembrane adapter molecule linker for activation of T cells. Hypophosphorylation was not a consequence of reduced kinase activity of src protein tyrosine kinases, but was correlated with an altered- membrane compartmentalization of these molecules. These observations indicate that in addition to their well-described ability to interfere with the transcription of molecules involved in peripheral responses, GCs inhibit T cell activation by affecting the early phosphorylating events induced after TCR ligation.
T lymphocyte; signal transduction; tyrosine kinases; membrane rafts; glycosphingolipid-enriched microdomains
B Lymphocytes Regulate Dendritic Cell (Dc) Function in Vivo
The Journal of Experimental Medicine
Increasing evidence indicates that dendritic cells (DCs) are the antigen-presenting cells of the primary immune response. However, several reports suggest that B lymphocytes could be required for optimal T cell sensitization. We compared the immune responses of wild-type and B cell-deficient (μMT) mice, induced by antigen emulsified in adjuvant or pulsed on splenic dendritic cells. Our data show that lymph node cells from both control and μMT animals were primed, but each released distinct cytokine profiles. Lymph node T cells from control animals secreted interferon (IFN)-γ, interleukin (IL)-2, and IL-4, whereas those from μMT mice produced IFN-γ and IL-2 but no IL-4. To test whether B cells may influence the T helper cell type 1 (Th1)/Th2 balance by affecting the function of DCs, we immunized mice by transferring antigen-pulsed DCs from wild-type or mutant mice. Injection of control DCs induced the secretion of IL-4, IFN-γ, and IL-2, whereas administration of DCs from μMT animals failed to sensitize cells to produce IL-4. Analysis of IL-12 production revealed that DCs from μMT mice produce higher levels of IL-12p70 than do DCs from wild-type animals. These data suggest that B lymphocytes regulate the capacity of DCs to promote IL-4 secretion, possibly by downregulating their secretion of IL-12, thereby favoring the induction of a nonpolarized immune response.
T helper cell type 1/type 2 balance; primary response; interleukin 4; interleukin 10; dendritic–B cell interaction
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