In This Issue

doi:10.4049/jimmunol.1290025

A CD26 Twist on Th17

IL-17–producing CD4⁺ Th cells, known as Th17 cells, help fight microbial and fungal infections, but excessive Th17 responses have been linked to autoimmune diseases. Expression of the ectoenzyme dipeptidylpeptidase IV (CD26) has been shown to increase in Th cells under inflammatory conditions, but these studies preceded the characterization of Th17 cells. Bengsch et al. (p. 5438) have found that human Th17 cells can be specifically characterized by high surface expression of CD26 (CD26⁺⁺). PBMC-derived Th1, Th2, and regulatory T cells expressed measurably lower levels of CD26. CD26⁺⁺CD4⁺ T cells expressed the Th17-associated cytokines IL-17, IL-22, GM-CSF, and TNF, as well as the type 17 differentiation markers CD161, CCR6, IL-23R, and RORγt. Coexpression of CD26 with CD161 or CCR6 on Th17 or IL-17–producing CD8⁺ T cells provided a more defined phenotypic characterization of these subsets than revealed by CD161 or CCR6 staining alone. CD26⁺⁺ Th17 cells were identified in inflamed tissues of patients with hepatitis or inflammatory bowel disease. CD26⁺⁺ CD4⁺ T cells prevented CD4⁺ T cell chemotactic responses to CXCL9–12, which could be restored in the presence of a CD26 inhibitor, indicating that CD26 was enzymatically active. These results define CD26 as a phenotypic marker on human Th17 cells and suggest that its enzymatic activity may influence T cell migration in inflamed tissues.

GC-Free Plasma Cells

Long-lived plasma cells reside primarily in the bone marrow (BM) and secrete high affinity Abs for prolonged time periods. Previous studies support a model in which T cell-dependant (TD) Ags induce long-lived plasma cells to differentiate in germinal centers (GCs). Bortnick et al. (p. 5389) now show that long-lived plasma cells can develop in response to a T cell independent (TI) hapten Ag in the absence of GCs. C57BL/6 (B6) or T cell-deficient B6 mice were immunized with the hapten (4-hydroxy-3-nitrophenyl)acetyl (NP) conjugated to LPS. As expected, long-lived NP-specific plasma cells were detected in the BM of B6 mice; however, these cells were also found in the BM of T cell-deficient B6 mice as long as 400 days postimmunization despite the absence of GCs. A TD Ag also induced the development of long-lived plasma cells in mice treated to inhibit GC formation. Together, these results support a model in which either TI or TD Ag can induce the development of long-lived plasma cells in the absence of GCs.

Found: A Fungal Epitope

Fungal infections can cause severe, life-threatening complications, but are difficult to treat with drugs and, to date, cannot be prevented by vaccination. T cell responses contribute to immunity against fungi, yet little is known about the identity of natural fungal T cell epitopes. Bär et al. (p. 5636) identified an MHC class II (MHC II)-bound peptide from Candida albicans that could induce protective peptide-specific Th cell responses. Immunoproteomic analysis of C. albicans-infected dendritic cells identified two potential peptide epitopes bound to the I-A^b MHC II molecule. One of these peptides, pALS, was validated as a T cell epitope as it was recognized by and stimulated IL-17A production in a subset of T cells derived from C. albicans-infected mice. pALS-specific T cell responses were detected in mice infected with different Candida species, as well as in uninfected human PBMCs stimulated with the peptide, indicating that pALS is a highly conserved epitope that can associate with both murine and human MHC II molecules. Mice immunized with pALS combined with curdlan, a fungal β-glucan with adjuvant activity, generated protective IL-17A–secreting T cells upon i.v. C. albicans challenge. These results highlight the immunological properties of a C. albicans peptide that may help with new approaches to antifungal vaccines and treatments.

TLR9’s Toll on Pregnancy

Intrauterine inflammation and infection have been linked to preterm birth, but little is known about the specific mechanism triggering this outcome. Cell-free fetal DNA is found at significantly higher levels in the maternal circulation of women with pregnancy complications, and Scharfe-Nugent et al. (p. 5706) investigated whether TLR9-mediated recognition of fetal DNA contributes to preterm fetal death and resorption. Compared with adult DNA, fetal DNA is hypomethylated, and TLR9 is known to detect hypomethylated microbial DNA. In this study, a B cell line or PBMCs exposed to CpG, a TLR9 ligand, or fetal DNA showed significantly greater NF-κB activation and IL-6 production relative to control cells or adult DNA-treated cells. The authors confirmed previous observations that i.p. injection of CpG or LPS induced fetal resorption in pregnant mice and observed that i.p. injection of fetal DNA caused a significant and rapid increase in fetal resorption. In contrast, TLR9^−/− mice showed no signs of fetal resorption following injection with fetal DNA. Interestingly, pregnant wild-type mice treated with chloroquine, a TLR9 inhibitor, were protected against fetal DNA-induced resorption. These results support a mechanism by which fetal DNA triggers TLR9-mediated inflammatory responses that can contribute to pregnancy complications like preterm birth.

Stem Cell Factor Split Identity

Mast cell (MC) development from bone marrow progenitors is mediated in part through activation of KIT upon binding to its ligand, stem cell factor (SCF). SCF has been observed to trigger MC degranulation and cytokine release, although prolonged SCF exposure has been shown to attenuate IgE-induced anaphylactic responses in mice. Ito et al. (p. 5428) characterized a mechanism by which SCF exposure induces a hyporesponsive state in MCs through its effect on the MC cytoskeleton. In vitro culture of bone marrow-derived MCs (BMMCs) with SCF combined with IL-3 initially promoted significantly more proliferation than treatment with either SCF or IL-3 alone. Prolonged treatment of BMMCs with SCF and IL-3 or SCF alone significantly attenuated their ability to undergo FcεRI-mediated degranulation, but this response could not be attributed to defects in FcεRI or KIT expression, granule formation, calcium mobilization, or protein kinase C signaling. Impaired degranulation was instead attributed to defective cytoskeletal reorganization, which was accounted for in part by decreased expression of the Src kinase Hck. Together, these results confirm that SCF can have dramatically disparate effects on MCs and may provide insight into how chronic allergic responses can be managed or treated.

Traumatic Injury Insults Immunity

Impaired adaptive immune responses have been observed following spinal cord injury (SCI). Oropallo et al. (p. 5257) investigated the effect of SCI on humoral immunity to better understand how SCI contributes to immune suppression. Mice received a crush injury at the third (T3) vertebral body, which is known to significantly impact splenic innervation, or at T9, which mostly preserved splenic innervation. The splenic architecture was not dramatically altered following either type of SCI, but the frequency of B cell progenitors was significantly reduced after acute T3 or T9 SCI. These B cell pools began to recover during chronic SCI, and the frequency of mature B cells in the spleen was not significantly reduced at either time point. Primary responses to either a thymus-independent type 2 Ag or thymus-dependent Ag were significantly reduced in T3 SCI, but not T9 SCI mice compared with uninjured control mice. B cells in T3 SCI mice immunized with thymus-dependent Ag underwent affinity maturation, but the frequency of germinal center B cells and the level of class-switched Ab were significantly lower relative to uninjured mice. Interestingly, secondary responses by pre-existing memory B cells were intact in chronic SCI mice. These results provide insight into the impact of chronic SCI on both primary and memory B cell responses and confirm previous observations of SCI-associated immune suppression.

TRIM38 Nips NAP1

Tripartite motif (TRIM) 38 can negatively regulate TLR-driven production of proinflammatory cytokines, and Zhao et al. (p. 5311) now examine the mechanism by which TRIM38 influences TLR3/4 and retinoic acid-inducible gene-I (RIG-I) signaling. Small-interfering RNA-mediated knockdown of TRIM38 in mouse macrophages stimulated with a TLR3 ligand (polyinosinic-polycytidylic acid [poly(I:C)]) or a TLR4 ligand (LPS) resulted in significantly greater IFN regulatory factor 3 (IRF3) activity and IFN-β production relative to stimulated cells with normal TRIM38 expression. Similarly, TRIM38 overexpression significantly reduced intracellular RIG-I responses to poly(I:C). TRIM38 targeted a protein involved in IRF3 activation, NF-κB-activating kinase-associated protein 1 (NAP1), for proteasomal degradation via K48-linked polyubiquitination. Knockdown of TRIM38 caused NAP1 to accumulate in macrophages. TRIM38 appeared to negatively regulate antiviral responses to vesicular stomatitis virus (VSV) triggered through detection of viral RNA by RIG-I. Relative to VSV-infected control cells, TRIM38 overexpression correlated with increased viral replication, whereas TRIM38 knockdown was associated with reduced replication. TRIM38 appears to negatively regulate IFN-β production by targeting NAP1 for degradation, thus revealing a mechanism by which viruses that promote TRIM38 expression can interfere with innate antiviral responses.

Dueling Adenosine Responses

Engagement of the A2A adenosine receptor (A2AAR) on the surface of lymphocytes by extracellular adenosine triggers potent anti-inflammatory responses. Mills et al. (p. 5713) now show how A2AAR signaling in lymphocytes or cells of the CNS can differentially impact experimental autoimmune encephalomyelitis (EAE). A2AAR^−/− mice developed more severe EAE, accompanied by greater infiltration of lymphocytes and microglia/macrophages into the CNS relative to wild-type (WT) mice. A2AAR^−/− lymphocytes produced more IFN-γ and underwent significantly more proliferation upon ex vivo stimulation relative to WT lymphocytes, but the frequency and suppressive capacity of regulatory T cells (Tregs) from A2AAR^−/− mice were similar to those of WT Tregs. A2AAR was highly expressed in the choroid plexus, a site known to mediate immune cell migration to the CNS. Gamma-irradiated WT mice reconstituted with A2AAR^−/− hematopoietic cells developed more severe EAE than mice reconstituted with WT cells. In contrast, gamma-irradiated A2AAR^−/− mice reconstituted with WT hematopoietic cells were protected from severe EAE. Additionally, A2AAR-expressing T cell-deficient mice treated with an A2AAR antagonist and reconstituted with A2AAR^−/− lymphocytes were protected from EAE. These data indicate that A2AAR expression on nonimmune cells, including CNS cells, potentiates EAE development, whereas lymphocyte expression of A2AAR reduces EAE severity.