Epitope Hide and Seek
Autoantibodies in Goodpasture (GP) disease and alloantibodies in Alport posttransplant nephritis (APTN) recognize the noncollagenous (NC1) domains of α3α4α5(IV) collagen in the glomerular basement membrane (GBM) and induce severe glomerulonephritis. Although the APTN alloantibodies recognize native α3α4α5NC1 collagen hexamers, the epitopes recognized by the GP autoantibodies become cryptic upon hexamer assembly. To determine the pathologic importance of this difference in epitope recognition, Luo et al. (p. 3520) immunized mice with anti-GBM Abs isolated from patients with APTN and GP. APTN alloantibodies, but not GP autoantibodies, bound to mouse GBM in vivo and caused crescentic glomerulonephritis in susceptible mice. Both types of Abs, however, bound to mouse NC1 domains in vitro, suggesting that the epitopes recognized by GP autoantibodies were masked in vivo. Because these GP autoantibodies cause nephritis in humans and squirrel monkeys but not in mice, the authors investigated the possibility that epitopes were masked by species-specific structural differences in the GBM. Indeed, whereas APTN alloantibodies bound to native α3α4α5NC1 hexamers from all species tested, GP autoantibodies bound to native hexamers from squirrel monkey GBM but not mouse GBM. The basis for this difference was traced to intermolecular cross-linking present in mouse but not squirrel monkey NC1 hexamers. These data define a structural basis for variations in self-Ag recognition that can trigger or prevent autoimmunity.
Glutathione Gains Control
Glutathione is critical to the maintenance of dendritic cell (DC) redox homeostasis, and its depletion has been implicated in myriad diseases. D’Angelo et al. (p. 3217) investigated the effects of glutathione modulation on DC activity by analyzing the function of the cystine/glutamate antiporter, which regulates glutathione synthesis. Production and activity of the antiporter increased with DC maturation, and the antiporter was found to control the intracellular concentration of reduced glutathione in DCs. Activity of the cystine/glutamate antiporter was important for the differentiation of monocyte-derived DCs but did not affect their maturation or uptake of Ag. Interestingly, inhibition of antiporter activity disrupted the presentation of exogenous Ag, both by inhibiting direct presentation via MHC class I and by inhibiting cross-presentation via MHC class II. The important role of the cystine/glutamate antiporter in DC differentiation and function helps delineate the mechanisms by which glutathione depletion may affect DC activity in disease states.
The proinflammatory cytokine IL-1β is involved in a wide range of diseases, and its expression is known to be tightly regulated. In addition to regulation at the levels of transcription, mRNA stability, and proteolytic processing, Radwan et al. (p. 3544) have identified a mechanism by which IL-1β production is regulated at the translational level. In LPS-treated macrophages lacking tyrosine kinase 2 (Tyk2), IL-1β protein levels were significantly increased both intracellularly and extracellularly compared with wild-type (WT) macrophages. These increases were not caused by alterations in IL-1β mRNA induction or stability and required type I IFNR and Stat1. Correspondingly, exogenous treatment of macrophages with IFN-β inhibited IL-1β production. The mechanism of enhanced IL-1β production in Tyk2−/− macrophages involved increased translation without alterations in IL-1β protein stability. Association of IL-1β mRNA with polysomes was increased in Tyk2−/− macrophages compared with WT cells, thus providing direct evidence for enhanced IL-1β translation in the absence of Tyk2. Finally, peritoneal, but not systemic, IL-1β was increased in Tyk2−/− versus WT mice in vivo following LPS challenge. This demonstration of translational regulation of IL-1β via type I IFN signaling adds another layer of complexity to the control of IL-1β production.
Epigenetic modifications, such as histone acetylation and deacetylation, are important in the regulation of T cell development and function. However, it is not clear to what extent individual members of the large histone deacetylase (HDAC) family are involved in these processes. To identify a role for HDAC1 in T cells, Grausenburger et al. (p. 3489) generated conditional knockout mice with T cell-specific deletion of HDAC1. These HdacΔ mice demonstrated no defects in the composition of their thymic or peripheral T cell pools, suggesting that HDAC1 was dispensable for T cell development and differentiation. However, in a model of allergic airway inflammation, HdacΔ mice developed more severe disease than wild-type controls. This enhanced disease involved increases in eosinophilic infiltration, mucus hypersecretion, airway resistance, and, importantly, the production of Th2 cytokines by lung CD4+ T cells. In vitro-differentiated HdacΔ Th2 cells also produced higher levels of Th2 cytokines than controls, and Th1-differentiated HdacΔ cells produced higher levels of IFN-γ. HDAC1 was recruited to multiple sites in the Il4 gene locus in CD4+ T cells, indicating that HDAC1 directly controlled Th2 cytokine production. Taken together, these data suggest that HDAC1 attenuates cytokine production in effector T cells and, thereby, controls airway inflammation.
Nibbling on Tumors
The use of dendritic cell (DC)-based tumor vaccines is a promising immunotherapeutic approach, and the ability to load DCs with apoptotic autologous tumor cells would obviate the need for tumor Ag identification. However, DCs generally acquire tolerogenic properties upon uptake of apoptotic cells, which is counterproductive to tumor therapy. In this issue, Reboulet et al. (p. 3337) identified a subset of DCs, dubbed merocytic DCs (mcDCs), that did not induce tolerance after interacting with apoptotic tumor cells. mcDCs were phenotypically CD11c+ but did not express CD11b, CD8α, or plasmacytoid DC Ag-1. The mcDCs acquired small particles from apoptotic cells and stored them in nonacidic intracellular compartments, resulting in a prolonged capacity to present Ag compared with other DC populations. mcDCs efficiently primed both CD4+ and CD8+ T cells and cross-primed CD8+ T cells through a mechanism involving the production of IFN-α. In tumor therapy models, mcDCs exposed to apoptotic tumor cells induced tumor suppression more effectively than other DC subsets by activating CD8+ T cells within the tolerogenic tumor environment. In addition to adding yet another cell subset to the DC family, this study identifies a cell type with distinct Ag processing/presentation properties that could be exploited for the development of DC-based tumor vaccines.
Salmonella enterica serovar Typhimurium is an important human pathogen and causes a typhoid-like disease in mice. Richer et al. (p. 3593) sought to identify determinants of host resistance to this pathogen through the use of random mutagenesis of 129S1 mice. The challenge of mutagenized mice with Salmonella Typhimurium led to the identification of a missense mutation in the ubiquitin-specific peptidase 18 (Usp18) gene that caused enhanced susceptibility to lethal bacterial infection. The phenotype caused by this mutation was designated immunity to Typhimurium, locus 9 (Ity9). Salmonella-infected Usp18Ity9/Usp18Ity9 mice demonstrated an increased bacterial burden in the spleen and liver, yet had impaired production of IFN-γ compared with mice bearing wild-type Usp18 genes. In response to infection or LPS treatment, the Usp18Ity9/Usp18Ity9 mice also had an enhanced inflammatory response, including increased production of IL-6 and type I IFN. The authors proposed that this increased inflammation synergized with the increased bacterial load to cause lethality. Finally, increased STAT1 activation and decreased STAT4 activation levels in Usp18Ity9/Usp18Ity9 mice suggested a mechanistic basis for the inverse correlation of IL-6 and IFN-γ levels. This study highlights the interplay between type I and type II IFN in bacterial infection and the importance of USP18 in protection against Salmonella.
SHP-1 Steers Effector Efforts
Upon encountering Ag, naive CD8+ T cells expand into short-lived effector cell (SLEC) and long-lived memory precursor effector cell (MPEC) populations for which differentiation is regulated at multiple levels. The tyrosine phosphatase Src homology region 2 domain-containing phosphatase-1 (SHP-1) modulates the T cell activation threshold, but in vivo analysis of SHP-1 has been complicated by the severe phenotype of mice globally deficient in this molecule. To clarify the functions of SHP-1, Fowler et al. (p. 3256) developed TCR-transgenic mice with conditional deletion of SHP-1 in mature single-positive T cells. Adoptive transfer of T cells from these mice or controls into wild-type hosts demonstrated that SHP-1, in a cell-intrinsic manner, inhibited the peak magnitude of the CD8+ T cell response to viral infection. SHP-1 was found to negatively regulate the generation and survival of SLECs without affecting MPEC generation or the subsequent development of T cell memory. Upon secondary infection, the generation of CD8+ SLECs was even more sensitive to SHP-1 deficiency than during the primary infection, whereas MPEC generation was again unaffected. Thus, SHP-1 activity is vital to limiting the expansion of effector T cells during acute viral infection, and this knowledge may be useful in future clinical applications.
Organs under Construction
Mouse lymph node (LN) and Peyer’s patch (PP) organogenesis is initiated by lymphoid tissue inducer (LTi) cells, which induce stromal organizer cell differentiation. Distinct mechanisms may drive the development of LNs and PPs, and the specific growth factors involved in these processes are not clear. Chappaz et al. (p. 3514) found that IL-7 and Kit signaling differentially regulated LN and PP development. Kit ligand (Kitl) and IL-7, whose receptors (Kit and IL-7R) are both expressed by LTi cells, synergistically induced LTi cell generation and maintenance in vitro. Mice deficient in both Kit and IL-7 had severely reduced numbers of LTi cells in LN anlagen and no detectable organizer cells, and adult mice lacked most peripheral LNs. Comparison of these doubly deficient mice with singly deficient Il7−/− and KitW/Wv mice indicated that both IL-7 and Kit were required for LN development. In contrast, PP anlagen development, including the generation of intestinal LTi cells and organizer cells, required Kit signaling but not IL-7. Corresponding with their relative requirements in PPs and LNs, Kitl was expressed at a much higher level, and Il7 at a much lower level, in PP organizer cells compared with LN organizer cells. These data expand our understanding of early lymphoid organogenesis by identifying growth factors that control LTi cell numbers in an organ-specific manner.
Summaries written by Jennifer Hartt Meyers, Ph.D.