IN THIS ISSUE

doi:10.4049/jimmunol.0990094

Hot and Bothered Skin

Inflammatory skin conditions like psoriasis are characterized by the up-regulation of inflammatory molecules induced by activation of innate immune pathways. The fundamental mechanisms controlling these cutaneous responses are not completely understood, but Lee et al. (p. 6839 ) now show that apurinic/apyrimidinic endonuclease 1 (APE1) regulates TLR2-mediated inflammatory responses in the skin. The multifunctional APE1 protein can mediate DNA repair and activate transcription factors, including hypoxia-inducible factor (HIF)-1α and NF-κB. A significant infiltration of APE1-expressing cells was observed in the epidermis of psoriatic but not normal skin. APE1 gene silencing in keratinocytes reduced TLR2-driven activation of HIF-1α and NF-κB and expression of TNF-α, CXCL8 and LL-37. Keratinocyte proliferation was also influenced by APE1 silencing, as measured by reduced expression of cyclin D1/cyclin-dependent kinase 4 and decreased phosphorylation of Akt and ERK1/2. Increased expression and nuclear translocation of APE1 occurred upon TLR2 stimulation of keratinocytes through signaling events driven by the production of reactive oxygen species. Thus, APE1 appears to be a key regulator of inflammatory responses induced by TLR2 activation in keratinocytes, and overexpression of APE1 in psoriatic skin cells may contribute to disease-associated inflammation.

Disturbing Immunity

Natural killer cells are vital contributors to innate immunity and serve as a first line of defense against viral infections. Several subsets of NK cells have been distinguished in humans based on CD56 surface expression that have different cytolytic and cytokine production capacities. Gonzalez et al. (p. 6612 ) have observed that CD56⁻CD16⁺ NK cells expand during chronic hepatitis C virus (HCV) infection but are functionally defective. In chronically infected individuals, CD56⁻ NK cells were impaired compared with CD56^dim or CD56^bright NK cells, as indicated by the reduced cytokine production following exposure to target cells. CD56⁻ NK cells were also less polyfunctional, which was quantified by reduced coexpression of IFN-γ, TNF-α, MIP-1β, and the degranulation marker CD107a. MIP-1β expression was the most distinctive functional feature of CD56⁻ NK cells. Strikingly, the majority of HCV-infected individuals with levels of CD56⁻ NK cells similar to those of healthy controls showed a sustained virological response (SVR) against HCV upon treatment with IFN-α and ribavirin. A much smaller proportion of HCV-infected individuals who had significantly elevated numbers of CD56⁻ NK cells reached a SVR against HCV. This newly defined presence of CD56⁻ NK cells during HCV infection provides insight into the immunological factors contributing to effective treatment of chronic viral infections.

Turkey MHC, Anyone?

The MHC is comprised of polymorphic gene loci involved in vertebrate immunity. Comparative analysis between mammalian and avian MHC sequences has revealed several features restricted to the avian MHC, including a unique gene locus organization and an overall reduction in length and complexity. Now Chaves et al. (p. 6530 ) provide new insight into the avian MHC through sequence analysis of the turkey (Meleagris gallopavo) MHC-B region. Earlier sequence analysis defined the chicken MHC-B and MHC-Y loci and revealed that a limited number of classical MHC molecules were expressed by the chicken MHC-B locus, thus constraining Ag presentation. In the present study, sequence analysis of turkey MHC-B identified 34 highly homologous genes within the locus in almost perfect synteny with chicken MHC-B, including a single MHC class I gene. Turkey MHC-B differed from the chicken sequence in that it included three class II B and three BG loci, whereas chicken MHC-B contained two class II B loci and a single BG locus. The significant similarities between chicken and turkey MHC-B are striking given the estimation that turkey and chicken genomes diverged ∼50 million years ago and suggest that these two avian species could exhibit similar disease susceptibilities.

Keeping a Lid on Myelopoiesis

Immune responses against pathogens can create an acute demand for myeloid cells that cannot be met by bone marrow (BM) hematopoiesis and often leads to extramedullary myelopoiesis (EM) in the spleen. EM must be carefully regulated to avoid excessive inflammation and autoimmunity, and Lee et al. (p. 6377 ) observed that CD4⁺CD25⁺FoxP3⁺ regulatory T cells (Treg) are chief regulators of EM in the spleen. As characterized by increased CD11b⁺GR-1⁺ myelopoietic cell clusters and increased myeloid progenitors in the spleens, Scurfy mice, which are FoxP3⁺ T cell deficient due to a mutation in the FoxP3 gene, had significantly more splenic EM than wild-type mice. Treg depletion before treatment with the staphylococcal enterotoxin B (SEB) superantigen resulted in a significant increase in EM but not in BM myelopoiesis. EM was positively regulated by GM-CSF and IL-3, which were secreted by SEB-activated CD4⁺CD25⁻ T cells. Treg could suppress production of these cytokines and prevent differentiation of naive CD4⁺CD25⁻ T cells into GM-CSF⁺ T cells, and suppression was dependent on cell contact but not TGF-β. These data support a role for FoxP3⁺ Treg in modulating EM in the spleen by suppression of myelopoiesis-promoting T cells, and these observations may be applied to future therapeutic approaches to curb EM in chronic inflammatory syndromes.

A Tale of Two CpGs

Induction of TLR9 signaling by DNA oligonucleotides with CpG motifs activates an array of cell types. Avalos et al. (p. 6262 ) have focused on the mechanisms of autoreactive B cell activation by the CpG class A (CpG-A) molecule, which forms aggregates and induces IFN-α secretion by plasmacytoid dendritic cells and the CpG class B (CpG-B) molecule, a known B cell mitogen. Autoreactive B cells from AM14 mice internalized large CpG-A macromolecules, but a 10-fold greater concentration of CpG-A was required for an equivalent level of B cell activation compared with the nonaggregating CpG-B. B cell activation was enhanced by treatment with immune complexes (ICs) formed with biotin-conjugated CpG-A molecules and anti-biotin mAbs through a TLR9-dependent mechanism. Surprisingly, CpG-A ICs induced significantly less IL-6 production than either uncomplexed CpG-B molecules or CpG-B ICs. In contrast to CpG-B ICs, CpG-A ICs induced more RANTES, suggesting that cytokine production was influenced by both the sequence and size of CpG-containing molecules. Uncomplexed CpG-A molecules or CpG-rich DNA induced B cell activation by cotreatment with DNA-free protein ICs, suggesting that BCR crosslinking enhanced TLR9 engagement by CpG-A. These results clarify the mechanisms underlying the effects of different CpG molecules on B cell activation.

Channeling Calcium to the Synapse

Antigen recognition by T cells initiates immunological synapse (IS) formation with APCs and activates multiple T cell signaling events, including changes in intracellular Ca²⁺ concentrations ([Ca²⁺]_i). Kv1.3 ion channels are expressed by T cells and contribute to Ca²⁺ influx. Nicolaou et al. (p. 6296 ) have shown that Kv1.3 channels can amass in the IS through lateral movement in the plasma membrane and influence the magnitude of the Ca²⁺ response. Lateral movement of Kv1.3 channels in Jurkat T cells was confirmed by crosslinking with a Kv1.3-specific Ab, which prevented channels from relocating to the IS but did not affect other IS-associated molecules. Kv1.3 recruitment involved pre-existing proteins on the cell surface, as inhibition of protein synthesis or endosomal recycling did not prevent Kv1.3 localization to the IS. Compared with noncrosslinked cells, T cells treated with the Kv1.3-crosslinking Ab showed a significant increase in [Ca²⁺]_i following IS induction with anti-CD3/anti-CD28-coated beads, but the kinetics of Ca²⁺ responses were not affected by Kv1.3 crosslinking. These results support a role for Kv1.3 ion channels as regulators of Ca²⁺ responses during T cell activation that is facilitated by their ability to accumulate in the IS through lateral movement.

Rebuilding Antiviral Responses

Cytomegalovirus reactivation complications have been linked to poor T cell responses in recipients of allogeneic hematopoietic stem cell transplants (allo-HSCT), and new therapies are being explored that can restore T cell responses in CMV-positive patients receiving allo-HSCT from CMV-negative donors. To this end, Schub et al. (p. 6819 ) transferred CMV-specific TCRs with different HLA restrictions into naive T cells, and these genetically modified T cells exhibited a variety of anti-CMV effector functions. CMV-specific TCRs were derived from CD8⁺ T cell clones that reacted specifically to the CMV pp65 protein and were retrovirally transduced into naive T cells from CMV-negative donors. Transgenic CMV-TCR CD8⁺ T cells recognized endogenously processed pp65 epitopes and exhibited multiple effector functions, including cytotoxicity and IL-2 and IFN-γ secretion. Ag restimulation induced expansion and enrichment of CMV-TCR CD8⁺ T cells and enhanced Ag-specific effector functions. Memory formation was also indicated, as a subset of CMV-TCR T cells expressed the central memory markers CCR7 and CD62L. This study offers a novel approach to protecting CMV-positive recipients from viral reactivation with transgenic T cells, although further studies are needed to assure that these T cells would not cause any dangerous crossreactivity.

Histones and Hematopoiesis

Hematopoietic stem cells (HSCs) are a pool of adult stem cells that can provide a continuous supply of hematopoietic progenitors able to differentiate into multiple lineages. Little is known about the mechanisms that maintain the HSC pool. Loizou et al. (p. 6422 ) have now defined an essential role for the histone acetyltransferase (HAT) cofactor TRansformation/tRanscription domain-Associated Protein (TRRAP) in this process. TRRAP-conditional knockout (TRRAP-CKO) mice had profound defects in multiple hematopoietic cell lineages in the spleen and bone marrow (BM) and increased cytopenia and anemia-related death. Transfer of BM cells from TRRAP-CKO mice into lethally irradiated TRRAP-CKO mice was associated with increased lethality compared with mice receiving TRRAP-containing BM and was characterized by an inability of these TRRAP-deficient HSCs and hematopoietic progenitors to proliferate in the BM. TRRAP-deficient BM progenitor cells showed a significant increase in p53-independent apoptosis compared with wild-type controls, and c-Myc and N-Myc transcription factor expression was also deregulated in the absence of TRRAP. Thus, increased apoptosis and Myc family member deregulation are likely to be key causes of HSC loss in the BM of TRRAP-CKO mice. These findings affirm a role for histone modifications, especially those influenced by TRRAP, in hematopoiesis and HSC renewal.

Summaries written by Christiana N. Fogg, Ph.D.