IQGAP1’s Dual Identity
T cell activation requires cytoskeletal reorganization, including F-actin accumulation and microtubule organizing center (MTOC) polarization. The multimodular scaffold protein IQGAP1 acts in both signal transduction and cytoskeletal regulation in non-T cells. To investigate the role of IQGAP1 in T cell development and activation, Gorman et al. (p. 6135) analyzed T cell responses in the absence of IQGAP1. IQGAP1-deficient mice showed no defects in T cell development or in CD8+ T cell MTOC polarization or cytotoxic activity. Instead, IQGAP1−/− T cells demonstrated significantly increased global tyrosine phosphorylation levels, both basally and following TCR crosslinking, relative to wild-type T cells. IQGAP1-deficient T cells or T cells in which IQGAP1 expression was suppressed also upregulated IL-2 and IFN-γ expression and LCK activation. IQGAP1 also was found to control the extent and velocity of F-actin accumulation at the immunological synapse. Whereas the N-terminal portion of IQGAP1 was responsible for the regulation of TCR signaling leading to IL-2 production, the C-terminal portion was more important for F-actin regulation. IQGAP1 therefore plays a dual regulatory role in T cell activation and acts through distinct mechanisms to negatively regulate T cell signaling and F-actin dynamics.
DCs Endure Exhaustion
Stimulation of dendritic cells (DCs) with LPS has been described to rapidly cause their “exhaustion,” resulting in a shift from a Th1- to a Th2-promoting phenotype. In this issue, Abdi et al. (p. 5981) found that, although LPS stimulation made DCs unresponsive to further effects of LPS, it left them acutely receptive to T cell-mediated stimulation. Bone marrow-derived DCs were generated from RAG-deficient mice and treated with LPS, resulting in “exhausted” DCs, or LPS + IFN-γ, resulting in “hammered” DCs. Although they lost their responsiveness to LPS, both “exhausted” and “hammered” DCs produced large amounts of IL-12 following incubation with activated, but not naive, T cells. CD40L expression on these T cells, combined with their production of one or more additional cytokines, were necessary to overcome DC “exhaustion.” Despite the ability of IL-4 and CD40L to synergistically enhance IL-12 production from “exhausted” DCs, IL-10 production by Th2 cells was found to inhibit IL-12 production. Overall, the resulting DC phenotype varied according to the effector T cell type it encountered and the activation state of the DC. Thus, LPS stimulation does not appear to exhaust or tolerize DCs, nor does it necessarily cause them to promote Th2 responses. Instead, the authors propose that environmental signals like LPS prime DCs to respond to signals from activated T cells and thus affect the outcome of the immune response.
Cycling with Care
To avoid aberrant chromosomal rearrangements, V(D)J recombination must be tightly regulated and carefully coordinated with the cell cycle. IL-7 is important for the proliferation and survival of B and T cell progenitors and, in pro-B cells, is involved in the seemingly contradictory processes of downregulating Rag expression and enhancing Igh locus accessibility. Johnson et al. (p. 6084) reconcile these observations by determining that pro-B cells are heterogeneous and vary their expression of the IL-7R during the course of the cell cycle. Both the IL-7R and activated STAT5 (phospho-STAT5), which acts downstream of IL-7R, were present at low levels during the G0/G1 phases, when V(D)J recombination occurs, and at higher levels during the S/G2/M phases. During the G2 phase of the cell cycle, when pro-B cells were responsive to IL-7, the Rag locus was repositioned to pericentromeric heterochromatin, and Rag expression was downregulated. Levels of phospho-STAT5 inversely correlated with Rag expression, both in bone marrow and the fetal liver, and STAT5 was found to bind to a putative regulatory region ∼6 kB downstream of the Rag locus. Levels of phosho-STAT5 at different phases of the cell cycle varied in response to variations in IL-7 levels. These data help clarify the role of IL-7/STAT5 in the regulation of Rag expression that must occur to prevent recombination during cell division.
BLyS-fully Boosting HIV Vaccines
The elicitation of broadly neutralizing Abs (bNAb) is an important goal of HIV vaccine development that has thus far proven elusive. Dosenovic et al. (p. 6018) hypothesized that B cell clonotypes able to produce bNAb might be selected against in the transitional developmental stages in the periphery and assessed whether treatment with the B cell survival factor BLyS could rescue these clonotypes in vivo. Transient treatment of mice with BLyS significantly increased total numbers of B cells in the spleen and lymph nodes, particularly transitional and marginal zone B cells. Following BLyS treatment, mice were immunized with HIV envelope (Env) protein in adjuvant, and the resulting B cell response was analyzed. No significant differences were observed in Env-specific Ab titers or numbers of IgG-secreting cells or memory B cells. Despite this lack of an effect on the magnitude of the vaccine-elicited B cell response, clear differences were observed in neutralizing Abs following BLyS treatment. Compared with controls, mice that had been treated with BLyS demonstrated neutralizing Ab responses that were increased in both frequency and potency. Pre-treatment with BLyS may therefore represent a promising avenue for future investigation in the development of HIV vaccines.
IL-17 has important roles in host defense but also promotes inflammation in autoimmunity and cardiovascular disease. To investigate the potential involvement of IL-17 in leukocyte influx to inflammatory sites, Griffin et al. (p. 6287) assessed its ability to enhance the effects of the proinflammatory cytokine TNF-α on endothelial cells. IL-17 alone consistently had no significant effects on adhesion molecule expression or leukocyte migration. However, when combined with TNF-α, it synergistically increased the endothelial expression of E-selectin and P-selectin, but not other adhesion molecules, relative to TNF-α alone. Treatment with IL-17 and TNF-α also synergistically enhanced leukocyte rolling and the duration of leukocyte slow-rolling in vivo. Consistent with the expression data, the effects on leukocyte slow-rolling were E-selectin dependent. Compared with TNF-α alone, addition of IL-17 also synergistically augmented leukocyte, particularly neutrophil, transendothelial migration, both in vitro and in vivo. When combined with TNF-α, IL-17 increased the endothelial expression of neutrophil-attracting CXCR2 ligands but did not affect T cell-activating chemokines or CD4+ T cell migration. In contrast, addition of IFN-γ to TNF-α enhanced endothelial expression of integrin ligands and T cell-activating chemokines and promoted the transendothelial migration of effector CD4+ T cells. This study reveals a role for IL-17 in cell trafficking that will be important to the understanding of leukocyte accumulation during inflammation.
B Cells Patrol the Skin
Although B cells are known to enter the skin during chronic inflammation, little is known about the homing, phenotype, or function of skin B cells. Geherin et al. (p. 6027) used a sheep model of granulomatous skin inflammation to study skin B cells through methods, such as lymph cannulation, that would not be possible in mice or humans. A heterogeneous population of B cells was found to recirculate through the skin and exit via the afferent lymph during homeostatic conditions. Chronic inflammation greatly enhanced B cell accumulation and trafficking in the skin. B cells from inflamed skin upregulated CD1, MHC class II, and B7.1/B7.2 relative to cells from lymph nodes and uninflamed skin, suggesting these cells could act as APCs for skin T cells. Compared with lymph nodes, both inflamed and uninflamed skin also had a higher representation of B cells with an IgMhi, B-1–like phenotype. Chronic inflammation increased Ab titers and the presence of Ab-secreting cells, compared with uninflamed skin. B cells from the skin migrated in response to the CCR6 ligand CCL20 but were unresponsive to previously identified skin-homing chemokines for CD4+ T cells. Taken together, these data suggest that B cells continuously recirculate through the skin and may play a previously unappreciated role in skin immunity.
SLC Components at Cross-Purposes
The precursor B cell receptor (preBCR) is expressed on large preB II cells following productive IgH rearrangement and consists of two Igμ H chains together with two surrogate L chains (SLCs). The SLCs consist of VpreB and λ5 proteins, which each contain both Ig domain-like and non-Ig components. To better understand the function of the preBCR, Knoll et al. (p. 6010) generated preB cell lines deficient in SLCs and reconstituted these cells with SLCs containing either wild-type VpreB and λ5 or mutant molecules lacking the non-Ig domains. Compared with controls, cell surface deposition of the preBCR was decreased in the presence of mutant VpreB and strongly increased in the presence of mutant λ5. The data suggested that the non-Ig portion of VpreB was important for optimal surface expression of the preBCR and acted to slow preBCR turnover, whereas the non-Ig portion of λ5 promoted preBCR turnover. Deletion of the non-Ig part of λ5 led to increased phosphorylation of the preBCR signaling molecules Syk, SLP65, and PLC-γ2 and a dramatic increase in calcium flux following IgM crosslinking, relative to controls. These increases resulted in signaling levels comparable to those observed following stimulation of a mature BCR. The components of the preBCR SLCs thus appear to have opposing roles in controlling the cell surface quantity and the activity of preBCR molecules during B cell development.
EVI1 Averts Excess
Ecotropic viral integration site 1 (EVI1) is a zinc finger transcription factor and proto-oncogene that is upregulated by inflammatory stimuli and can mediate both gene activation and repression. In this issue, Xu et al. (p. 6371) analyzed the effects of EVI1 on NF-κB to address its potential role in inflammatory regulation during infection with the human pathogen nontypeable Haemophilus influenzae (NTHi). Both in vitro and in vivo, EVI1 inhibited NF-κB activation and NF-κB–dependent proinflammatory gene transcription in response to NTHi or TNF-α. EVI1 acted by inhibiting the DNA-binding activity of NF-κB without significantly affecting p65 nuclear translocation. NTHi or TNF-α induced a direct interaction between p65 and EVI1, and EVI1 inhibited p65 acetylation at lysine 310, a modification critical for p65 activity. NTHi could also induce the upregulation of EVI1 through a mechanism requiring p65 and IKKβ. These data suggest that NF-κB activation and the subsequent inflammatory response may be controlled through a negative feedback loop involving EVI1. Thus, EVI1 upregulation has the potential to be utilized in therapeutic strategies aimed at limiting excessive inflammation.
Summaries written by Jennifer Hartt Meyers, Ph.D.