Torpedoing myasthenia gravis
Myasthenia gravis is an autoimmune disease characterized by muscular weakness due to the presence of Abs against the muscle nicotinic acetylcholine receptor (AChR). Studies in the animal model, experimental myasthenia gravis, indicate that IL-4 may have a protective function in the disease. On p. 604 , Ostlie et al. present a long term study of IL-4 deficiency on Torpedo AChR-induced experimental myasthenia gravis. They show that IL-4−/− B6 mice developed chronic self-reactive Ab and long lasting muscle weakness after a single immunization with Torpedo AchR. In contrast, more than one immunization was usually required to develop symptoms of experimental myasthenia gravis in IL-4+/+ mice, and the symptoms were of limited duration. In addition, anti-self AChR CD4+ T cells were present in the early phase of the response in IL-4−/− mice, whereas such cells were not detected in IL-4+/+ mice until late in the response, when they were recovering from the symptoms. Therefore, IL-4 appears to play a suppressive role in both the xenogenic response to the Torpedo AChR as well as the CD4+ T cell response to self-AchR. The authors postulate that IL-4 dependent regulatory Th3 cells may be involved in controlling experimental myasthenia gravis in IL-4+/+ mice.
Signaling changes in shape
Activation of the immune system is mediated through a complex array of signaling pathways, and elucidation of these pathways is essential to understanding how the immune system operates. The cell surface receptor CD40 plays an important role in B cell activation and maturation during the humoral immune response. Lee et al. (p. 19 ) identified the p190 guanine nucleotide exchange factor (p190RhoGEF) as a protein induced by CD40 stimulation of primary B cells and the B lymphoma cell line WEHI-231. They found that overexpression of p190RhoGEF mimicked some of the effects of CD40 stimulation in WEHI-231 cells, such as changes in cell size and shape, and that inhibition of the protein blocked the cytoskeletal effects of CD40. In addition, p190RhoGEF overexpression resulted in increased NF-κβ activity. This study identifies p190RhoGEF as a mediator of CD40 signaling.
The great calcium wave
Not long ago, the ability to observe space-time changes in concentration of a molecule or ion within a small cell would have been considered the stuff of science fiction. Kindzelskii and Petty (p. 64 ) have accomplished this feat by using a CCD camera with high-speed rates of acquisition and short exposure times. They have used this technology to map the spatio-temporal dynamics of Ca2+ signaling routes in neutrophils. Their results show propogation of local Ca2+ waves near the plasma membrane during temporal calcium spikes. These waves occured in polarized cells, were initiated near the leading edge of lamellopodium, were counter-clockwise (relative to the frame of reference) in orientation and terminated when they returned to the initiation site. When cells were treated with the chemotactic peptide fMLP, an additional travelling wave arose and moved in a clockwise orientation. Localized Ca2+ waves were also seen to split off from the wave at the plasma membrane and encircle phagosomes in a counter-clockwise direction, suggesting that a line of communication exists between the cell surface and phagosomes. Application of this technology to other cells and molecules holds promise for our understanding of intracellular trafficking.
Death by estrogen
Researchers have long suspected that estrogen plays a role in many autoimmune diseases, since women are affected much more frequently than men. Monocytes and macrophages are important cells in the inflammatory responses that often characterize autoimmune diseases; it is also known that defects in apoptosis are involved in the development of autoimmune diseases. In the study on p. 628 , Mor et al. investigated the link between estrogen, monocytes/macrophages and the Fas/Fas ligand system (the receptor/ligand pair most commonly associated with apoptosis). Estrogen treatment increased Fas ligand expression and induced apoptosis in monocytes, but not in monocyte-differentiated macrophages. Two estrogen recognizing elements were detected in the Fas ligand promoter: the estrogen regulatory element, which binds the estrogen receptor and drives gene expression, and an AP-1 motif. The authors also showed that estrogen receptors α and β are differentially expressed in monocytes and macrophages. Therefore, any evaluation of estrogen action must take into account its influence on the Fas ligand promoter, as well as the status of cell differentiation and the receptor isoform expressed in the cell of study.
V.I.P.s
Vasoactive intestinal peptide is a neuropeptide that is produced by both lymphoid and neural cells. It has a wide range of immunological functions that are mediated through several different receptors on various immunocompetent cells, including acting as a potent anti-inflammatory agent. Voice et al. (p. 308 ) present data showing that vasoactive intestinal peptide is required for induction of a Th2 phenotype in mice. Using transgenic mice constitutively expressing VPAC2, the form of the receptor that is the dominant transducer of vasoactive intestinal peptide on activated Th cells, and VPAC2 knockout mice, they examined the influence of vasoactive intestinal peptide on T cell phenotype. They found an elevated number of IL-4 secreting CD4+ T cells following stimulation of T cells from the VPAC2 transgenic mice. By eliminating vasoactive intestinal peptide from cell cultures, the secretion of IL-4 and IL-10 decreased, while the secretion of IFN-γ increased. T lymphocyte derived vasoactive intestinal peptide thus appears to be a major contributor to the skewing of Th cell responses.
“Bottoms-up” T cell signaling
Appropriate TCR signaling is vital to the maintenance of proper T cell function. Agonist and partial agonist ligands elicit different cellular responses when signaling through the TCR. In an effort to understand the molecular basis of these differences, Xi and Kersh (p. 315 ) examined the promoter of the early growth response gene-1 of T cells during stimulation of the TCR with either agonist or partial agonist ligands. Both stimuli induced transcription of the early growth response gene-1, though the response to the partial agonist was 10-fold lower than to the agonist. Analysis of ternary complexes formed on the promoter revealed that both stimuli resulted in similar binding of the complexes. However, the partial agonist induced a much shorter lifespan of the ternary complex. This was due to the shorter duration of phosphorylation of one of the ternary complex components, Elk-1, by the Erk family of MAP kinases, a finding that correlates with the previously observed transient nature of Erk activation by partial agonists. These results link the observed differences in cellular responses to agonist versus partial agonist stimulation with changes in gene transcription.
First line of defense
In the battle to prevent infection, epithelial cells function as a physical barrier as well as mediating defense responses such as production of antimicrobial peptides. LPS, a glycolipid of the outer membrane of Gram-negative bacteria, is a potent activator of host defense responses that targets predominantly macrophages. A study by Liu et al. (p. 575 ) showed that in human epidermal keratinocyte cultures, induction of the antimicrobial peptide human β-defensin-2 was greatly enhanced by cross-talk between keratinocytes and LPS-stimulated monocyte-derived cells. The increase in human β-defensin-2 was relatively selective, with most other genes being repressed, and was mediated by IL-1 secreted by the monocyte-derived cells. It thus appears that both IL-1 and human β-defensin-2 have important roles in the host defense reaction of the epidermis, expanding our understanding of how infections may be prevented following skin injury.
Regulating recombination
Is T cell lineage determination simply dependent on expression of appropriate TCRs or is it dependent on other developmental programs? Understanding how TCR gene recombination is regulated in the context of the developing T cell is important in addressing this question. V(D)J recombination occurs only between pairs of gene segments that are flanked by a 12- and 23-bp spacer (12/23 rule). However, V to DJ rearrangement at the TCRβ locus is subject to additional restrictions beyond the 12/23 rule. On p. 5 , Tillman et al. examine the mechanistic basis of the “beyond 12/23” restrictions on V(D)J recombination. They use extrachromosomal substrates to show that the preference of Vβ for Dβ rather than Jβ occurs at the level of DNA cleavage, requires no lymphoid specific components other than recombinase activating gene (RAG) proteins, and appears to depend on the relative strengths of the Dβ as compared to the Jβ recombination signal sequences (RSSs), rather than a specific requirement for Vβ/Dβ RSS synapsis. They establish that the precise sequence of RAG recognition sites flanking TCRβ gene segments is the major factor in restricting V to DJβ rearrangement.