Naturally occurring CD4+CD25+ regulatory T cells (nTreg) can modulate T cell responses to allergens. Due to conflicting reports, it is unclear whether nTreg require TCR-mediated Ag recognition to mediate their suppressive functions. Joetham et al. (p. 1821) analyzed the ability of OVA-specific nTreg to suppress lung allergic responses and definitively showed that suppression was independent of Ag recognition. These OVA-specific CD4+CD25+ T cells, when transferred to mice that were sensitized and challenged with an unrelated ragweed allergen, suppressed airway hyperreactivity, eosinophilia, and Th2 cytokine production while increasing the production of IL-10, IFN-γ, and TGF-β. Blocking the OVA-specific TCR did not inhibit Treg-mediated suppression of either OVA- or ragweed Ag-specific immune responses in the airway. These nTreg also suppressed activation and accumulation of CD8+ T cells in the airways through a mechanism that involved interactions between MHC I on Treg and CD8 on host cells. In agreement with this observation, these nTreg colocalized with host CD8+ T cells in the airway lumen and lung parenchyma. These data both advance our understanding of nTreg-mediated suppression and suggest a mechanism by which allergic diseases could be modulated.

Vaccinia virus (VV) has been used as an attenuated smallpox vaccine, but its ability to evade both innate and adaptive immunity urges caution in further therapeutic development. Although VV is known to disrupt MHC class II Ag presentation, how this occurs during the course of viral infection is not clear. Wang et al. (p. 1846) found that, at late stages of VV infection (8–10 h), VV dramatically reduced the expression of the invariant chain (Ii) in B cells without significantly affecting the expression of MHC class II. This reduction in Ii involved the inhibition of protein synthesis and could be shown to contribute to the VV-induced decrease in MHC II Ag presentation. VV infection also affected Ii degradation and altered the pattern of lysosomal protease expression in these cells. Specifically, cathepsin L was up-regulated, whereas cathepsin S and other host proteases were down-regulated. Treatment of B cells with the translation inhibitor cycloheximide (CHX) also induced Ii down-regulation and inhibition of MHC II Ag presentation but did not affect protease expression. Thus, VV infection disrupts MHC II Ag presentation late in the viral life cycle through effects on Ii expression and processing. These results may have important implications for the understanding of poxvirus pathogenesis and for future vaccine applications.

Malaria is a major public health threat that kills nearly one million African children annually. The immune response to the malaria-causing parasite Plasmodium falciparum (Pf) is incompletely understood but appears to involve defects in the generation of Pf-specific B cell immunity. A population of hyporesponsive “exhausted memory B cells” expressing the putative inhibitory receptor FCRL4 has recently been identified and is preferentially expanded in HIV-infected individuals with high viremia. To address whether similarly defective B cells might be expanded in malaria, Weiss et al. (p. 2176) analyzed B cell populations in Pf-exposed individuals in Mali. Atypical memory B cells resembling those expanded in HIV-infected individuals were found to be over-represented in the Malian individuals compared with healthy U.S. blood donors. These atypical cells were functionally hyporesponsive, appeared to have undergone isotype switching and somatic hypermutation, and expressed a pattern of inhibitory and homing receptors similar to that of exhausted memory B cells in HIV-viremic individuals. The presence of this atypical memory B cell population in Pf-exposed individuals may explain difficulties in developing malaria vaccines whose success relies on the generation of humoral immunity.

The nucleotide-binding domain, leucine-rich repeat-containing (NLR) family member NLRP3 is an inflammasome component that serves as a critical mediator of inflammation. In vitro data have suggested that this protein may also play a role in the form of monocyte necrosis termed pyronecrosis. To further delineate the in vivo roles of the inflammasome components NLRP3 and apoptotic speck protein containing a caspase recruitment domain (ASC) in inflammation, Willingham et al. (p. 2008) analyzed pulmonary infection with Klebsiella pneumoniae. In vitro, NLRP3 and ASC were found to be necessary for K. pneumoniae-induced IL-1β release as well as caspase-1-independent macrophage cell death and associated high-mobility group box 1 (HMGB1) release. In K. pneumoniae-infected mouse macrophages, NLRP3 and ASC also regulated the expression of multiple proinflammatory cytokines and chemokines. In vivo infection with K. pneumoniae demonstrated protective roles for NLRP3 and ASC, as mice deficient in either molecule showed significantly increased mortality compared with controls. K. pneumoniae-infected NLRP3−/− mice also demonstrated reductions in airway inflammation, IL-1β production, HMGB1 release, and necrotic cell death. These data provide in vivo evidence that NLRP3 and ASC have inflammasome-independent activities and show the dramatic effects of modulating NLR activity in a model of bacterial pneumonia.

Interleukin-4 is involved in respiratory syncytial virus (RSV) pathogenesis, but its cellular source during infection is unknown. To clarify the role of IL-4 in RSV infection, Moore et al. (p. 2016) analyzed two mouse models: BALB/c mice, which exhibit a dominant Th1 response to RSV infection, and STAT1−/− BALB/c mice, which produce an enhanced Th2 response following infection. Although IL-4 did not affect viral load or illness severity in either model, it influenced histopathological characteristics of RSV infection in the lung and regulated levels of IFN-γ and IL-13. A search for the cellular source of IL-4 determined that RSV infection induced IL-4 expression in CD3CD49b+ cells. These cells were clearly identified as basophils and they, rather than T cells, were shown to be the primary source of IL-4 during infection. Following RSV infection, the STAT1−/− mice exhibited significantly more IL-4-producing cells in the lung than did BALB/c mice, suggesting that STAT1 negatively regulated the accumulation of basophils and their induction of IL-4 expression. This identification of a role for basophils in RSV infection provides important insight into the pathogenesis of RSV-mediated disease.

The positive selection of invariant (i)NKT cells requires Ag presentation by CD1d on double-positive thymocytes. iNKT cell maturation requires CD1d-mediated Ag presentation on professional APCs in the periphery, where it is unclear whether the CD1d endocytosis necessary for Ag loading is mediated via a sorting motif in its cytoplasmic tail or via association with invariant chain (Ii). Sillé et al. (p. 1780) generated a knock-in mouse replacing wild-type CD1d with a CD1d-enhanced yellow fluorescent protein (EYFP) fusion protein, which allowed investigation of the CD1d sorting motif in iNKT cell development and function. Comparison of these mice with those expressing wild-type CD1d revealed iNKT cell development with reduced efficiency, most likely due to suboptimal presentation of endogenous lipids rather than to changes in CD1d cell surface expression. In mice expressing CD1d-EYFP, thymic selection of iNKT cells was defective, their maturation was delayed, and mature cells showed defects in effector function. Analysis of CD1d endocytosis in these mice revealed that endosomal sorting in APCs required both the sorting motif in the CD1d tail and interaction with Ii to effectively stimulate iNKT cell activation. Taken together, these data significantly advance our understanding of the requirements for iNKT cell development and acquisition of effector function.

Resistance to leishmaniasis in both humans and mice is associated with a strong Th1 response. The PI3K signaling pathway can either promote or inhibit L. major infection in mice depending on the experimental system. To address the involvement of PI3K activity in L. major infection, Liu et al. (p. 1921) inactivated the p110δ catalytic subunit of PI3K, which was predicted to inhibit both inflammatory cytokine signaling and regulatory T cell (Treg) activity. Surprisingly, despite impairments in the CD4+ T cell response and IFN-γ production, mice with inactive p110δ (p110δD910A) on either the Th1-prone C57BL/6 background or the Th2-prone BALB/c background were highly resistant to L. major infection. Macrophages in wild-type and p110δD910A mice were equivalently susceptible to parasite infection, indicating that the resistance of p110δD910A mice was not caused by enhanced macrophage responsiveness. Instead, resistance was found to be T cell-mediated and dependent on IFN-γ production. Adoptive transfer experiments suggested that impairments in the expansion and recruitment of Treg caused by the inactivation of p110δ were responsible for the enhanced resistance to L. major infection. Thus, the removal of Treg-mediated suppression of Th1 responses allowed the weakened Th1 response to control parasite growth. These data suggest possibilities for the treatment of cutaneous leishmaniasis if similar involvement of p110δ in Treg modulation is found to exist in humans.

The intracellular receptor TLR9 mediates host defense but can also drive autoimmunity if inappropriately activated. The mechanism by which TLR9 is redistributed to the late endosomes/lysosomes following activation is not fully understood, nor are the mechanisms governing TLR9 down-regulation. Yao et al. (p. 1751) asked whether Rab7b, a small GTPase that localizes to late endosomes/lysosomes and promotes degradation of the cell surface receptor TLR4, could also regulate TLR9 trafficking or signaling. TLR9 and Rab7b were found to reciprocally regulate one another’s activity in macrophages. That is, TLR9 signaling through ERK and p38 MAPK inhibited Rab7b expression, whereas Rab7b down-regulated TLR9-mediated production of TNF-α, IL-6, and type I IFNs. Rab7b inhibited TLR9-induced NF-κB activity as well as activation of ERK, JNK, and p38, potentially by acting upstream of MyD88. Although no direct binding was observed between Rab7b and any part of the TLR9 signaling pathway, TLR9 colocalized with Rab7b following activation, suggesting that Rab7b might be involved in TLR9 intracellular trafficking. Finally, Rab7b overexpression promoted the lysosomal degradation of TLR9. These data advance our knowledge of the regulation of TLR9 signaling, which could have useful applications in infectious disease and autoimmune regulation.

Summaries written by Jennifer Hartt Meyers, Ph.D.