Blistering at Birth
Mammalian neonates receive Abs from their mothers that can cause autoimmunity if they are specific for self-Ags. Bullous pemphigoid (BP) is a life-threatening autoimmune blistering skin disease mediated by IgG autoantibodies directed against type XVII collagen (COL17). Nishie et al. (p. 4088) developed a novel mouse model of BP induced by maternally transferred Ab specific for the human COL17 Ag. Heterozygous mCol17+/− female mice were grafted with skin from human COL17 transgenic mice to induce anti-human COL17 IgG. These “immunized” female mice were mated with COL17-humanized male mice (mCol17−/−, hCOL17+/+); half of the offspring of this cross expressed only human COL17 in their skin. These neonates demonstrated circulating maternally transferred anti-COL17 Abs for at least 2 wk after birth and showed severe skin fragility and blistering. The skin inflammation observed in these mice was similar to that seen in BP patients and involved the deposition of complement and IgG1 and IgG2c Abs in the epidermal basement membrane. As in humans, IgG Abs directed against the NC16A domain of COL17 played a major role in the disease. Maternal IgG was also transmitted to neonates through milk, but these Abs appeared to play less of a pathogenic role than those transferred during pregnancy. This model may be useful for studies of both BP and maternally transferred autoimmunity.
The World According to STAT4
Most IL-12-mediated functions, including the differentiation of Th1 cells, require the activity of STAT4. However, relatively few of the target genes of STAT4 have been identified. To analyze the genetic program established by STAT4 activity, Good et al. (p. 3839) used an anti-STAT4 Ab to immunoprecipitate chromatin from activated, IL-12-stimulated CD4+ T cells, and the resultant DNA was hybridized to a DNA microarray consisting of genome-wide promoter sequences. Over 1500 genes whose promoters were bound by STAT4 were identified, of which only 72 overlapped with genes previously reported to be induced by IL-12. All STAT4-bound promoters contained a consensus binding motif, and three other common motifs were also identified. Interestingly, there was no correlation between the intensity of STAT4 binding and the level of gene induction. Further classification of STAT4-inducible genes revealed, in addition to those associated with Th1 differentiation, genes involved in TCR and IL-18R signaling, IFN-inducible genes, and genes encoding multiple cytokines and cytokine receptors. This genome-wide analysis of STAT4 activity provides insight into Th1 development and suggests that this transcription factor’s activity is more complex than previously appreciated.
Manufacturing New CD8+ Treg
The effective induction and expansion of human Ag-specific CD8+ regulatory T cells (Treg) would be of use for immunotherapy in autoimmunity and transplantation. Zheng et al. (p. 3742) developed a method to induce these cells ex vivo and in the process identified a novel population of CD8high Treg. Incubation of naive human T cells with allogeneic CD40-activated B cells resulted in the induction of CD8low and CD8high suppressive T cell populations, but only the CD8high cells were found to be potent alloantigen-specific Treg. The CD8high Treg expressed Foxp3, CD25, CD27, CD28, CD62L, and CD45RO but did not express CCR7. These cells could suppress T cell proliferation and mediate relatively low potency cytotoxicity; both functions were cell contact dependent and required Ag-specific stimulation. The suppression of proliferation involved soluble mediators, including IL-10 and TGF-β, as well as the cell surface molecules GITR and CTLA-4, whereas FasL, perforin, TNF-α, and IFN-γ were involved in Treg-mediated cytotoxicity. Suggesting their potential for immunotherapeutic development, CD8high Treg could be effectively expanded in vitro by incubation with CD40-activated B cells through a process requiring IFN-γ, IL-2, IL-4, and CTLA-4. The description of this new Treg subtype may be useful both for the understanding of human immune regulation and for clinical applications.
A New Friend for TLR4
Signaling through many TLRs requires the involvement of accessory molecules, such as CD14 and MD2 for TLR4. A microarray screen analyzing LPS-inducible genes led Carpenter et al. (p. 3989) to identify a novel component of the TLR4 signaling complex, which they named TLR4 interactor with leucine-rich repeats (TRIL). This protein, which was found to be highly expressed in both murine and human brain, contained a signal sequence, 13 leucine-rich repeats, a type III fibronectin domain, and a single transmembrane region. LPS could induce TRIL expression in vitro and in the brain in vivo, and TRIL overexpression could feed back to increase LPS-induced cytokine production. In addition, TRIL was found to interact with both TLR4 and LPS. To determine whether TRIL played a role in the TLR4 signaling pathway, the authors knocked down TRIL expression with small interfering RNA in human astrocytoma and monocytic cell lines, human PBMC, and primary murine mixed glial cells. In all cell types tested, TRIL knockdown inhibited LPS-induced signaling and cytokine/chemokine production. These data identify TRIL as a novel participant in LPS/TLR4 signaling that may have particular relevance to neuroinflammation.
Igκ Gets a Little Loopy
Higher order chromatin structure is modulated during gene regulation by cis-acting elements, often resulting in the looping out of DNA between the regulatory regions. The mouse Igκ locus has a promoter and three downstream enhancers, Ei, E3′, and Ed, that interact during LPS-induced gene transcription through a poorly understood process involving DNA looping and the activation of NF-κB. Using chromatin conformation capture technology combined with chromatin immunoprecipitation, Liu et al. (p. 3819) investigated the establishment of the active state of the Igκ locus. Before LPS stimulation, Ei and E3′ constitutively interacted with Vκ promoters, and upon transcriptional induction the three enhancers formed all possible pairwise interactions with one another. The formation, but not the maintenance, of these complexes required RelA and actin polymerization but did not require new protein synthesis, transcription, or c-Rel. In fact, c-Rel appeared to inhibit LPS-induced Igκ transcription. All of the complexes of enhancers and promoters, which contained different NF-κB species, were associated with RNA polymerase II, which the authors proposed acted as a scaffold for the formation of DNA loops. Taken together, these data define many of the requirements for the establishment of the active state of the Igκ locus and show that different NF-κB family members can have different effects on the activation of a single gene.
Basophils Do It All
The mechanisms by which classically activated (M1) vs alternatively activated (M2) macrophages are generated are of great interest because of the dramatically different effects these cell subsets have on infectious diseases and cancer. SHIP and IL-3 have recently been implicated in inhibiting or enhancing M2 differentiation, respectively, leading Kuroda et al. (p. 3652) to analyze the contribution of cytokines that signal through common β-chain-containing cytokine receptors to M2 development. IL-3 and to a lesser degree GM-CSF were found to promote M2 differentiation, especially in the absence of SHIP. As expected, IL-3-induced M2 differentiation was inhibited in the absence of STAT5, as IL-3 and GM-CSF signal via STAT5. Unexpectedly, M2 differentiation was also reduced in progenitors lacking STAT6, which is only known to be activated by IL-4 and IL-13. Further analysis determined that IL-3 and GM-CSF caused M2 skewing by inducing IL-4 production from basophils, which was suppressed by SHIP via inhibition of the PI3K pathway. Basophils, in addition to their recently described role as APCs, therefore appear to also play an important role in macrophage differentiation.
MOG Attack in Pediatric MS
Compared with adult-onset multiple sclerosis (MS), little is known about the pathogenesis of pediatric-onset MS. There is no strong gender bias when MS develops before puberty, and disease occurs in a variety of ethnic groups, suggesting that immunological mechanisms may differ from those in play in adult-onset MS. To address the involvement of B cell responses in pediatric MS, McLaughlin et al. (p. 4067) analyzed autoantibodies to myelin surface proteins in a large cohort of pediatric and adult MS patients. Abs to native myelin oligodendrocyte glycoprotein (MOG) were detected in a significantly larger subset of pediatric vs adult patients, and these Abs were found to be most common in patients under age 10. Anti-MOG Abs were also significantly associated with an initial diagnosis of acute disseminated encephalomyelitis (ADEM). The specificity of these Abs, almost all of which were of the IgG1 subtype, was verified through affinity purification and binding competition. MOG-specific Abs from pediatric patients bound to myelin, myelinated axons, and glial cells with oligodendrocyte morphology in human brain sections. Abs to other myelin surface proteins were generally not detected in the sera of pediatric MS patients. MS patients with very early-onset disease therefore demonstrate B cell autoimmunity to MOG, an Ag that appears to have less relevance to adult-onset disease.
Resistin is a proinflammatory molecule that was originally identified as an adipocyte-derived hormone. In sepsis, systemic resistin levels correlate with the severity of disease, but the cellular sources of this molecule and the triggers of its production are not known. To better understand the potential role of resistin in sepsis, Johansson et al. (p. 4047) analyzed patients with severe group A streptococcal infections. Patients with streptococcal toxic shock syndrome (STSS) demonstrated prolonged high systemic levels of resistin. In patients with severe group A streptococcal tissue infections such as necrotizing fasciitis, high levels of resistin were observed at the site of infection and correlated with high bacterial loads. In both groups of patients, neutrophils were identified as the main source of resistin and stored preformed resistin in azurophilic granules in preparation for rapid release upon cellular activation. Although streptococcal superantigens did not trigger resistin release, streptococcal cell wall components and the streptococcal M1 protein strongly induced the release of resistin from primary neutrophils. These data identify neutrophils as an important source of resistin and suggest that this molecule may be relevant to the pathogenesis of such dangerous conditions as STSS.
Summaries written by Jennifer Hartt Meyers, Ph.D.