Pathogens have developed myriad strategies to counteract the host immune response, which include the vaccinia virus-encoded A46 protein that inhibits TLR signaling. TLR signaling is important for an efficacious immune response, but it also plays a role in inflammatory diseases, making it an attractive target for therapeutic modulation. Accordingly, Lysakova-Devine et al. (p. 4261) tested A46 sequence-derived peptides for TLR specific targeting. One of these 11-aa–long peptides, termed VIPER (viral inhibitory peptide of TLR4), inhibited LPS-induced TNF-α production. The inhibitory effects of VIPER required intracellular targeting and were limited to TLR4 signaling. TLR4 signaling pathway inhi-bition occurred through direct interaction with the TLR4-specific adaptor protein TRAM and the TLR2-shared adaptor protein Mal via a critical leucine residue in VIPER. TLR2 signaling, however, was not affected by VIPER-mediated inhibition of Mal, indicating that TLR4 and TLR2 bind Mal through disparate domains. VIPER functioned in both human and murine cells in vitro and inhibited in vivo LPS-induced IL-12/23p40 expression in mice. These data, coupled with evidence that the d-enantiomer of VIPER had near-equivalent potency to the l-enantiomer, make VIPER attractive for therapeutic investigation, as well as providing potential therapeutic target sites on TRAM and Mal.

Increasing expression levels of the Th-inducing POK family transcription factor (ThPOK) induce the commitment of CD4+CD8+ double positive (DP) thymocytes to the CD4+ T cell lineage. To investigate a role for protein stability in controlling ThPOK expression levels, Zhang et al. (p. 3960) compared thymocytes from AND (MHC class II-restricted) and OT-1 (MHC class I-restricted) TCR-transgenic mice. Compared with ThPOK protein levels in OT-1 thymocytes, ThPOK protein levels were elevated in AND thymocytes, which was determined to be due to enhanced protein stability. Further studies revealed that ThPOK associated with the acetyltransferase p300, and when co-expressed with wild-type p300, but not a dominant negative p300 mutant, ThPOK protein levels increased. p300 expression levels were found to be more abundant in CD4+ cells compared with CD8+ cells, and p300-mediated acetylation conferred stability to ThPOK protein by competitively modifying three lysine residues in the middle domain of ThPOK that were also targets for ubiquitin conjugation. Collectively, these data reveal that p300-mediated acetylation of ThPOK in DP thymocytes stabilizes ThPOK expression, thereby enabling ThPOK to direct CD4-restricted/MHC II-restricted developmental progression.

The flavivirus GB virus C (GBV-C) is sexually transmitted and infects B and T lymphocytes. Although phylogenetically related to hepatitis C, GBV-C is not known to cause any disease in humans. Conversely, infection with GBV-C appears to confer a degree of protection against HIV-1 pathogenesis, as HIV-1–positive individuals expressing GBV-C envelope glycoprotein (E2)-specific Abs exhibit prolonged survival, compared with GBV-C naive counterparts. In this issue, Mohr et al. (p. 4496) investigated the possibility that structural mimicry between E2 and HIV particles elicits HIV-1–neutralizing Ab. Sera obtained from naturally infected GBV-C donors and experimentally infected mice and rabbits inhibited cellular infectivity by both CCR5- and CXCR4-tropic HIV-1 human isolates. The binding of isolated E2 Abs to HIV-1 particles blocked HIV-1 attachment, but could not inhibit the entry of already bound virus. However, anti-E2 polyclonal IgG preparations significantly inhibited HIV-1 replication and cell–cell spreading, suggesting an ability to neutralize nonenvelope viral particles as well. E2 Abs were specific for HIV-1 particles, as they did not neutralize mumps or yellow fever viral particles, thereby demonstrating potential relevance to HIV-1 vaccine development.

Activated Ag-specific B cells form germinal centers (GCs), where they undergo division, isotype switching, somatic hypermutation, and differentiation into memory and plasma B cells. Formation and maintenance of the GC reaction requires help from follicular dendritic cells and T follicular helper (TFH) cells and has been shown to require the catalytic activity of the PI3K subunit p100δ. To define which GC-affiliated cell type requires p100δ catalytic activity for GC formation, Rolf et al. (p. 4042) created cell-specific conditional cre-lox mouse models that produced a catalytically deficient form of p100δ. Surprisingly, B cell-intrinsic p100δ activity was not required for GC formation, nor was follicular dendritic cell-intrinsic p100δ activity. Rather, deficiency of p100δ activity in TFH cells correlated with decreased numbers of GCs, impaired affinity maturation, reduced numbers of memory B cells, and a defective B cell memory response. p100δ catalytic deficiency in T cells severely impaired TFH cell formation by disrupting ICOS signaling, and TFH cells expressed reduced levels of effector molecules, including CD40L and IL-21. These findings provide timely mechanistic insights into the role of p100δ in the humoral immune response, as studies testing the therapeutic efficacy of p100δ inhibitors for established inflammatory and autoimmune diseases are under way.

Elevated CO2 levels, or hypercapnia, have been found to decrease mortality associated with adverse innate immune responses that can occur during acute respiratory distress syndrome or endotoxin-induced acute lung injury but to exacerbate infection-induced morbidity. To gain insights into these observations, Cummins et al. (p. 4439) investigated the effect of CO2 levels on the innate mammalian immune system and found that high levels of CO2 reversibly induced nuclear localization of IKKα. Treatment with high levels of CO2 inhibited LPS-induced nuclear translocation of the p65 subunit of NF-κB while inducing the translocation of IKKα into the nucleus. This inhibition occurred independently of IKKβ association with IKKα, indicating that both canonical and noncanonical NF-κB pathways were CO2 sensitive. CO2-induced nuclear localization of IKKα was insensitive to changes in both extracellular and intracellular pH levels and unlinked to cell viability and O2 levels. Hypercapnia induced the downregulation of numerous NF-κB–dependent pro-inflammatory response-associated genes, including CCL2, ICAM-1, and TNF-α, whereas expression of the anti-inflammatory cytokine IL-10 was enhanced. Collectively, these data reveal an innate sensitivity of mammalian cells to CO2 levels that modulates the innate immune system through the NF-κB signaling pathway.

Anti-CD20 mAb-mediated depletion of B cells in patients with multiple sclerosis (MS) leads to reduced relapse rates and fewer brain lesions. Myelin protein-specific autoantibodies often persist after treatment, however, suggesting that the role of B cells in MS initiation and/or progression is not exclusively Ab mediated. To investigate the effects of B cell depletion on disease progression in common marmosets, Kap et al. (p. 3990) used the MS animal model recombinant human myelin oligodendrocyte glycoprotein-induced experimental autoimmune encephalomyelitis (EAE). Beginning 3 wk after glycoprotein injection, marmosets were infused weekly with a marmoset cross-reactive human anti-CD20 Ab. In control (PBS-infused) marmosets, clinical signs of EAE became overt for all animals. In contrast, four of seven Ab-treated animals failed to develop neurological symptoms, and the remaining three exhibited substantially reduced symptoms. Accordingly, postmortem magnetic resonance imaging revealed only small brain lesions in Ab-treated animals compared with control animals. At necropsy, the proliferative capacity and production of IL-7 and proinflammatory cytokines by splenic- and axillary lymph node-derived CD3+ T cells proved to be substantially impaired. These data indicate that B cell-mediated effects upon T cell effector functions play a role in the development of EAE-associated neurological symptoms and brain disease.

Noncoding microRNAs (miRNAs) regulate protein expression levels by inducing the translational repression and/or degradation of target miRNA transcripts. In this issue, Bezman et al. (p. 3835) interrogated the relevance of miRNAs in NK cell homeostasis and effector functions. Independent conditional deletion of the genes for miRNA processing enzymes Dicer and Dgcr8 resulted in the significant global reduction of respective miRNAs in both Dicer- and Dgcr8-deficient NK cells. Compared with heterozygote littermate controls, Dicer- and Dgcr8-deficient mice exhibited reduced overall percentages of NK cells, which appeared to stem from increased apoptosis and decreased turnover rates. Both Dicer- and Dgcr8-deficient mice developed reduced percentages of the most mature (CD27loCD11bhi) population of NK cells, which expressed abnormally low levels of the activating receptor NKG2D and exhibited impaired ITAM-mediated IFN-γ production as well. The cytokine receptor-mediated signaling pathways of these NK cells remained intact, however. Finally, expression of Dicer and Dgcr8 was found to be critical for the survival of Ly49H+ NK cells in response to MCMV infection. Taken together, these data reveal that Dicer and Dgcr8 deficiencies induce identical phenotypes in NK cells, thereby demonstrating a critical role for miRNAs in NK cell activation, survival, and function.

Memory B cells (MBCs) can respond rapidly to T cell independent (TI) Ag. In this issue, Weisel et al. (p. 4011) used virus-like particles (VLPs) to investigate the roles of various cell populations and lymphoid structures for TI-induced MBC activation. To induce a measurable VLP-induced Ab response, MBCs were adoptively transferred into RAG−/− mice. Depletion of macrophages, CD11c+ dendritic cells, or follicular dendritic cells from recipient mice did not negatively affect MBC activation. Maximal activation of transferred MBCs required a delay in VLP injection, suggesting that B cells needed to home to specific niches to become responsive. Accordingly, MBCs transferred into TNF/lymphotoxin-α doubly deficient mice, which have disorganized lymphoid tissue, exhibited defective Ab responses, which were rescued by partial normalization of the lymphoid architecture via reconstitution with normal bone marrow. Splenectomy of reconstituted mice, in contrast, profoundly reduced the MBC response to VLP challenge. VLP activation of MBCs did not require localization to the splenic marginal zones for optimal reactivation, but did require homing to lymphoid follicles. Finally, tracking of fluorescent-labeled VLPs revealed their ability to enter the edges of follicular areas, indicating that MBCs bind and react to TI Ags within a preferred niche.

Summaries written by Meredith G. Safford, Ph.D.