In This Issue

Degenerate Abs

Although V(D)J recombination introduces an impressive degree of diversity into the Ab repertoire, this diversity is insufficient to address the infinite range of Ags that could arise during an infection. To evaluate the mechanisms by which a single Ab might be able to recognize multiple Ags upon initial encounter (i.e., without the benefit of affinity maturation), Khan and Salunke (p. 1819) compared the structures of a single germline mAb unliganded or bound to four different Ags. The Ab, BBE6.12H3, is specific for the well-characterized hapten NP and has also been found to bind to a number of independent peptide Ags. Structural analysis of BBE6.12H3 Fab in the absence of Ag revealed two conformations of the CDRH3 region. When the Fab was bound to an Ag, the paratope assumed one of four distinct conformations, allowing specific interaction with each of the four different peptides. The CDRH3 region showed the most structural variability, followed by the CDRH2 region. The peptides demonstrated extremely variable topologies but bound the Ab in the same orientation via specific sets of interactions. Three residues of the Ab formed a minimal conserved recognition motif for these very different Ags. Thus, it is possible for the same germline Ab to adopt different conformations to recognize multiple structurally independent Ags and thereby further expand the Ab repertoire.

Tregs Decide To Divide or Die

Based on their low proliferative responses in vitro, regulatory T cells (Tregs) have been described as anergic. However, Tregs are able to proliferate at a high level in vivo. To address this discrepancy, Chen et al. (p. 1698) simultaneously monitored Treg proliferation and death in vitro. Compared with conventional T cells, Tregs were found to be hyperproliferative but also much more susceptible to cell death following TCR stimulation. Most Tregs died before the first cell division, whereas the small subset that survived proliferated at a high level. Freshly isolated Tregs also demonstrated higher levels of both proliferation and apoptosis than conventional T cells. Treg proliferation was increased by IL-2 and was less dependent than that of conventional T cells on CD28 costimulation. Analysis of signaling events revealed differences between Tregs and conventional T cells in several pathways, and TCR-induced signaling was impaired at several levels in Tregs. The subset of Tregs that was death resistant and hyperproliferative was found to express ICOS, whereas ICOS⁻ Tregs were prone to death and had a suppressive capacity that was inferior to that of ICOS⁺ cells. Thus, murine Tregs are not anergic and can be divided into functional subsets based on ICOS expression.

New Tools To Target Inflammation

Matrix metalloproteinase (MMP)-9 is a type IV collagenase that plays a role in the pathogenesis of chronic inflammatory diseases and cancer. Its expression can be induced by a process involving MMP-1– and MMP-3–dependent TNF-α release, induction of cyclooxygenase-2 (Cox-2), and PGE₂ engagement of the EP4 receptor. To better understand the mechanism of MMP-9 induction, Khan et al. (p. 1970) assessed the potential coupling of MMP-induced Cox-2 expression to changes in the expression of PGE synthases (PGES) in macrophages. Induction of microsomal PGES-1 (mPGES-1), the major inducible form of PGES, was required for MMP-9 expression. MMP-1 and MMP-3 were able to induce mPGES-1 expression through a mechanism requiring TNF-α and involving MAPK^erk1/2 activation, which led to induction of the transcription factor early growth response protein 1 (Egr-1). Utilization of a host of pharmacological and genetic methods identified a positive feedback loop that served to drive mPGES-1 production and, consequently, PGE₂ synthesis. In this pathway, PGE₂ binding to the prostanoid receptor EP4 upregulated mPGES-1 via MAPK^erk1/2 and Egr-1. Finally, an EP4 antagonist inhibited MMP-9 expression and PGE₂ synthesis by inhibiting the expression of both Cox-2 and mPGES-1. These data suggest that EP4 antagonists could serve as useful therapeutic agents to target MMP-9, Cox-2, and mPGES-1 in the treatment of inflammatory and neoplastic disease.

License To Present

Human γδ T cells express a limited TCR repertoire and display characteristics of both innate and adaptive immunity. Although the idea has been somewhat controversial, studies have suggested that peripheral blood γδ T cells can act as APCs. In this issue, Himoudi et al. (p. 1708) confirm the APC function of human γδ T cells and define requirements for the licensing of these cells to present Ag. Vγ9Vδ2 T cells efficiently cross-presented an intracellular tumor-associated Ag following coculture with opsonized (Ab-coated), but not nonopsonized, tumor cells. Coculture with a similar tumor cell line that was opsonized but resistant to γδ T cell-mediated killing did not fully license γδ T cells for potent APC function. However, these unlicensed γδ T cells still upregulated MHC class II and costimulatory molecules, suggesting that the licensing process is independent of this upregulation. This finding is reminiscent of the independence of dendritic cell licensing and maturation. Licensing of γδ T cells was abrogated by blockade of the Fc receptor CD16, implying that licensing requires an interaction of CD16 with the Fc regions of opsonizing Abs on the target cell. Taken together, these data demonstrate the professional APC function of γδ T cells and propose a mechanism by which licensing of these cells to present Ag is regulated.

Pathogenic IL-21

The pleiotropic cytokine IL-21 can both promote and inhibit immune responses and has been extensively studied in lymphocyte differentiation and autoimmunity. However, very little is known about the role of IL-21 in viral pathogenesis, particularly in acute respiratory viral infections. Spolski et al. (p. 1924) therefore assessed the involvement of IL-21 in infection with pneumonia virus of mice (PVM), which resembles human respiratory syncytial virus infection. IL-21 expression was upregulated in the lung, spleen, and lymph nodes during PVM infection, particularly in CD4⁺ T cells expressing markers of T follicular helper cells. Following PVM infection, the lungs of mice lacking the IL-21R demonstrated lower inflammation than wild-type lungs and reduced infiltration of neutrophils, corresponding with downregulation of CXCL1. The lungs of Il21r^−/− mice also contained lower numbers of CD4⁺ T, CD8⁺ T, γδ T, and NK cells. At the height of disease, infected Il21r^−/− mice produced normal levels of Th17 cytokines but significantly less IL-6 than wild-type mice. Analysis of lung-draining lymph nodes revealed similar inflammatory cytokine responses in Il21r^−/− mice and controls, suggesting that the reduced lung inflammation did not indicate a systemic impairment. Interestingly, PVM-infected Il21r^−/− mice had significantly improved survival, compared with infected wild-type mice. IL-21 blockade using an IL-21R/Fc fusion protein could also increase survival in PVM-infected wild-type mice. Taken together, these data indicate that IL-21 promotes a PVM-induced pathogenic inflammatory response, and blockade of IL-21 signaling could be therapeutically useful in respiratory virus infection.

Th Preferences of Preservatives

Following activation by cell stress, the transcription factor NF erythroid 2-related factor 2 (Nrf2) stimulates the transcription of genes containing antioxidant response elements. Nrf2 has been shown to exert anti-inflammatory effects and protect against autoimmunity, but its role in T cell differentiation and function is not well understood. In this issue, Rockwell et al. (p. 1630) used the synthetic antioxidants tert-butylhydroquinone (tBHQ) and butylated hydroxyanisole (BHA), which activate Nrf2, to investigate Nrf2 activity in T cells. tBHQ and BHA are widely used as food preservatives, and their potential effects on the immune system are therefore particularly relevant. tBHQ or BHA treatment of murine splenocytes stimulated with anti-CD3 and anti-CD28 led to a marked reduction of IFN-γ production. In purified T cells, tBHQ treatment upregulated Nrf2 and increased expression of several of its known target genes. Nrf2 activation in T cells by tBHQ led to a decrease in IFN-γ secretion and a corresponding increase in the Th2 cytokines IL-4, IL-5, and IL-13. Nrf2 activation also suppressed T-bet binding to DNA while potentiating GATA-3 DNA binding. All of these effects were reversed in Nrf2-deficient mice. These data suggest that Nrf2 activation by common food preservatives promotes Th2 differentiation while inhibiting Th1 differentiation. Future research will be necessary to address this possibility that food additives may skew the immune response.

Summaries written by Jennifer Hartt Meyers, Ph.D.