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Phospholipases A₂ (PLA₂), particularly gIVaPLA₂, have been proposed to play important roles in asthmatic inflammation. Muñoz et al. (p. 4800 ) found that the secreted enzyme gVPLA₂, independently of the cytosolic enzyme gIVaPLA₂, strongly induced airway hyperreactivity (AHR). gVPLA₂ expression was observed in the lungs of OVA-sensitized allergic mice, and inhalational administration of this enzyme induced dramatic airway constriction. A blocking anti-gVPLA₂ Ab could both prevent AHR and, importantly, reduce established airway inflammation, supporting a direct role for this enzyme in AHR development. Although gIVaPLA₂ has been reported to be important in AHR, nasal administration of gVPLA₂ induced identical airway narrowing in mice lacking gIVaPLA₂ and in wild-type controls. Thus, gVPLA₂ may be a major player in asthma pathogenesis, and it may serve as a useful target of drug development for the prevention and treatment of human asthma.

Infection with influenza A virus can be detected by the host via both TLRs and RIG-1-like receptors (RLRs); however, the distinct contributions of pattern recognition pathways to antiviral defense remain undefined. Using a panel of mice deficient in components of the TLR and RLR pathways, Koyama et al. (p. 4711 ) determined that innate antiviral responses were specifically mediated by the TLR7/MyD88 pathway in plasmacytoid dendritic cells, but by the RLR/IPS-1 pathway in conventional dendritic cells and fibroblasts. The authors found that these pathways were compensatory in the innate response, such that either could effectively mount an antiviral immune response in the absence of the other. However, a different story emerged in analyzing the adaptive recall response to the virus, where the TLR7/MyD88 pathway, but not the RLR/IPS-1 pathway, was found to be required for effective generation of Abs and CD4⁺ T cell responses to the immunodominant viral Ags. Thus, TLR7 recognition of viral RNA mediates the protective adaptive immune response to influenza A, whereas multiple overlapping mechanisms control initial innate immunity.

Ectopic germinal center-like structures (GC-LS) are associated with the production of autoantibodies in the salivary glands (SG) of Sjögren’s syndrome patients. Bombardieri et al. (p. 4929 ) investigated the expression of activation-induced cytidine deaminase (AID), the enzyme required for class switch recombination and somatic hypermutation, in SG from Sjögren’s syndrome patients to determine the role of ectopic lymphoid neogenesis in the pathogenesis of this autoimmune disease and its associated low-grade lymphomas. AID expression was observed in SG solely in GC-LS and surrounding interfollicular large B cells and appeared only in the presence of CD21⁺ follicular dendritic cell networks. These data provide direct evidence that autoantibodies can be locally produced in an autoimmune target organ and represent the first observation of the novel interfollicular B cell subset in ectopic lymphoid tissue. Additionally, investigation of SG MALT lymphomas demonstrated AID expression in reactive B cells in GC-LS but not in malignant marginal zone B cells, supporting the hypothesis that these lymphomas emerge from autoreactive B cells undergoing somatic hypermutation.

Lipopolysaccharide (LPS) invades the inflamed small intestine in necrotizing enterocolitis (NEC), leading Leaphart et al. (p. 4808 ) to hypothesize that the LPS receptor TLR4 might play an important role in NEC pathogenesis. Indeed, TLR4 was up-regulated in the intestines of both mouse and human infants suffering from NEC, and mice deficient in TLR4 signaling were strongly resistant to NEC development. Specifically, compared with wild-type mice, TLR4-deficient mice demonstrated both significantly reduced enterocyte apoptosis and increased effectiveness of mucosal repair mechanisms as measured by enterocyte proliferation and migration. In examining the potential mechanisms of TLR4-mediated inhibition of intestinal repair, the authors discovered that TLR4 could directly interact with focal adhesion kinase (FAK). Accordingly, inhibition of FAK restored the enterocyte migration that was decreased following TLR4 stimulation. This work defines a role for TLR4 in regulating the balance between intestinal injury and repair and establishes this molecule as a potential NEC therapeutic target.

The transcription factor Zbtb7b/cKrox/Thpok supports CD4 lineage choice during T cell development, both by directly repressing CD8 expression and by promoting CD4 expression, but the mechanism by which this putative transcriptional repressor enhances CD4 expression is unknown. Wildt et al. (p. 4405 ) found that Zbtb7b could act by preventing Runx3- and Runx1-mediated transcriptional repression of the CD4 gene. Runx3 and Runx1 are transcriptional repressors that bind to a silencer in the CD4 gene and are specifically expressed during distinct phases of T cell differentiation. The authors excluded a direct interaction between Zbtb7b and Runx proteins, instead finding that inhibition required the Zbtb7b DNA-binding motif and could be inhibited by histone deacetylase inhibitors. These data suggest that Zbtb7b might act by repressing the expression of as-yet-unknown genes whose products then act as Runx cofactors in the inhibition of CD4 expression. This complex regulatory mechanism likely acts both to induce CD4 expression in developing thymocytes and to maintain CD4 expression later in T cell differentiation.

Expression of the MHC class I-like molecule FcRn in the vascular endothelium has been shown to maintain high serum IgG levels by recycling internalized IgG to the cell surface; however, it is not known whether FcRn plays a similar role in IgG homeostasis at extravascular sites. As study of FcRn in mice has been quite limited, Akilesh et al. (p. 4580 ) first conducted a systematic analysis of mouse FcRn expression in different tissues. In several organs, mouse FcRn expression was found to be distinct from that of human FcRn, and expression varied significantly between the vasculature of different organs. Most interestingly, in several tissues mouse FcRn expression was observed in professional APCs, particularly CD11b⁺ macrophages. Experiments using wild-type vs FcRn-knockout bone marrow reconstitution demonstrated that during a humoral immune response, both bone marrow-derived FcRn-expressing cells and FcRn-expressing vascular endothelial cells could extend IgG half-life. The identification of other potential functions of FcRn in APCs awaits future study.

Commitment of T cells to full activation, as measured by proliferation and IL-2 production, is known to require sustained TCR signaling, but the mechanistic basis for this requirement is not fully understood. Using F5 transgenic mice lacking the Src-family kinases Fyn and/or Lck, Filby et al. (p. 4635 ) described a multilayered mechanism controlling CD8⁺ T cell activation. T cells from mice lacking Fyn showed initial stimulation comparable to that of wild-type T cells but displayed dramatically increased proliferation, IL-2 production, and cytotoxic effector function 72 h after stimulation. This amplified activation required the engagement of both TCR and CD8 but necessitated a shorter period of TCR:pMHC contact than in wild-type T cells. It was therefore suggested that Fyn normally acts to temper Lck signaling and thus controls the threshold to full T cell activation. Experiments using mice with dampened Lck expression suggested that Lck could additionally down-regulate its own activity, and the Src kinase Csk was implicated as the key inhibitor of Lck in both pathways. This study sheds light on the signaling requirements for full T cell activation early in an immune response.

Unlike mature B cells, transitional immature B cells are unable to translocate their BCRs into lipid rafts and thus generate sustained BCR signaling. Such developmentally regulated differences in signal transduction have been postulated to regulate the decision between B cell tolerance and activation. To further understand BCR-induced signaling in transitional immature vs mature B cells, Brezski and Monroe (p. 4464 ) assessed the abilities of these cells to undergo actin-dependent processes upon BCR stimulation. The authors observed that, although both transitional immature and mature B cells underwent BCR capping, only mature B cells demonstrated membrane ruffling and cell spreading in response to BCR stimulation. The latter two processes were found to be dependent upon BCR-induced Rac1 activation. Interestingly, increasing the membrane cholesterol of transitional immature B cells to the level seen in mature B cells induced the immature cells to undergo membrane ruffling, cell spreading, and Vav/Rac1/JNK activation at levels comparable to those seen in mature B cells. These data suggest that membrane cholesterol levels directly regulate cell fate decisions in B cell development.

Summaries written by Jennifer Hartt Meyers, Ph.D.