Antigenic activation of the BCR initiates changes in cell morphology, and the actin cytoskeleton is deeply involved in the ensuing steps of B cell spreading across the surface of APCs, enhanced Ag receptor (AgR) mobility, and the formation of AgR microclusters. The severing of existing F-actin filaments, which generates barbed ends and actin monomers that reassemble into a new, branched actin network, drives the cytoskeleton reorganization process. The aim of the study by Freeman et al. (p. 5887) was to identify mediators of the initial cytoskeletal breakdown. The authors found that dephosphorylation of cofilin, an F-actin–severing protein, resulted in increased F-actin breakdown. The Rap 1 GTPase was required to induce cofilin activation and consequent F-actin severing within the cells. By developing a novel surrogate APC system, the authors were able to show that the Rap/cofilin pathway affected Ag gathering, BCR microcluster formation, and BCR signaling. Finally, real-time imaging of nascent BCR microclusters indicated that actin severing induced by the Rap/cofilin pathway resulted in enhanced mobility of the microclusters within the plasma membrane. These data add another player to the molecular mechanisms regulating early events in AgR signaling and further highlight the crucial role for the actin cytoskeleton in BCR function.

Ag binding by high-affinity TCRs is a well-studied crucial step in the generation of potent immune responses following natural infection or vaccine administration. In contrast, the role played by high-affinity TCRs in autoimmunity has not been characterized. Alli et al. (p. 5521) show that T cells bearing a very high affinity TCR for a self-antigen are protective in the model of myelin oligodendrocyte glycoprotein (MOG)-induced experimental autoimmune encephalomyelitis (EAE). The authors used retroviral TCR transgenesis to generate two murine strains, 1MOG9 and G107S, with MOG35–55-specific high-affinity TCRs differing by a single amino acid. The substitution increased G107S TCR sensitivity to the cognate Ag and led to lineage infidelity with the generation of CD8+ and CD4CD8 T cells capable of killing cells presenting the Ag. Before or after EAE induction, T cells isolated from G107S mice (CD4+, CD8+, and CD4CD8) produced increased amounts of IL-10 and IL-4 and less IFN-γ and IL-17, compared with T cells from 1MOG9 mice (CD4+), and, when transferred into Rag1−/− and mixed chimeras, were only weakly pathogenic. Severity of EAE in G107S mice was greatly diminished compared with the fatal disease in 1MOG9 mice. These results suggest that, in autoimmune settings, high-affinity cross-reactive TCR interactions may modulate disease pathogenicity, as was previously proposed for low-affinity Ag binding.

During the process of T cell development, survival mechanisms are in place to prevent premature apoptosis of CD4+CD8+ double positive (DP) thymocytes that would limit TCRα-chain rearrangement and successful positive selection. RORγt and T cell factor (TCF)-1 are transcription factors playing critical roles in the development of DP thymocytes, as they are required for upregulation of the antiapoptotic gene Bcl-xL. The article by Wang et al. (p. 5964) clarifies the relationship between the two factors by demonstrating that they affect the same survival pathway and that TCF-1 acts upstream of, and is required for, RORγt expression. To reach these conclusions, the authors used mice lacking TCF-1 or RORγt and transgenic mice expressing a constitutively active β-catenin, a coactivator required for optimal stimulation of Bcl-xL expression. Gain- and loss-of-function experiments showed that RORγt forced expression was able to rescue DP TCF-1−/− thymocytes from apoptosis, but ectopic expression of β-catenin/TCF-1 failed to rescue RORγt−/− thymocytes. In addition, β-catenin/TCF-1 was found to bind directly to the RORγt promoter. Thus, this report identifies one of the gene regulatory network pathways that control thymocyte development.

The developmental program of natural Ab-producing B-1 cells is not completely defined. The report by Montecino-Rodriguez and Dorshkind (p. 5712) extends their previous finding that B-1 cells represent a distinct lineage emerging in the mouse embryo from committed progenitors before the development of B-2 progenitors. In this study, the authors found that T1 and T2/T3 cells constituted the predominant B cell population in neonatal murine spleens, and the majority of these transitional cells were identified as B-1 precursors. Kinetic analysis showed that these cells retained a robust B-1 potential up to 12 d after birth, whereas splenic transitional B-2 cells gradually emerged and increased in number with time. By 6 wk after birth, follicular and marginal zone B cells were the dominant B cell populations. The two lineages also exhibited different requirements for classical and alternative NF-κB signaling. Specifically, analysis of knockout mice indicated that the emergence and maturation of the B-1 lineage was not dependent on activation of the alternative pathway that is essential for B-2 cell development, nor was it affected by deficiencies in either Btk- or NF-κB1 signaling. This report further differentiates the two B cell lineages by demonstrating that their transitional precursors arise in subsequent waves within the spleen.

Several new innate immune populations have been recently identified, mostly associated with mucosal lymphoid organs and involved in resistance to helminth infections. They are characterized phenotypically by the lack of expression of lineage and progenitor markers and functionally by the ability to produce the Th2 cytokines IL-5 and IL-13 upon IL-33 stimulation. In this study, Brickshawana et al. (p. 5795) describe the discovery of a type 2 innate cell population in the mouse bone marrow (BM). The cells were named LSKCD25+, as they were negative for lineage markers and cKit but expressed Sca1 and CD25. Interestingly, they also expressed ST2, which is the receptor for IL-33. LSKCD25+ cells were identified and purified from BM of wild-type mice or mice deficient in T cells, B cells, or mast cells. The lymphocyte-like cells were kept in culture with IL-2 or IL-7 and stimulated with IL-13. Upon stimulation, LSKCD25+ cells produced Th2 cytokines and underwent morphological changes. Similarly, in IL-33–treated mice, the number and frequency of LSKCD25+ cells increased, compared with control mice, and the expression of CD25, ST2, and α4β7 was upregulated. The authors speculate that this novel cell type may represent the BM precursors of other type 2 innate populations; alternatively, they may be a specialized BM cell type with a function yet to be discovered.

B7-H1, also known as programmed death ligand 1, has been implicated as a negative regulator of antitumor immunity, and therapeutic applications for B7-H1 blockade have been suggested. In this issue, Pulko et al. (p. 5606) investigate the function of B7-H1 in activated T cells. The authors found that B7-H1 expression peaked during the late phase of an Ag-specific T cell response, and interference with B7-H1 signaling on T cells at this stage resulted in a decreased number of CD8+ T effector cells. Experiments using OT-I transgenic mice, carrying an OVA-specific TCR but lacking B7-H1 expression on CD8+ T cells, demonstrated that, initially, CD8+ T cells expanded normally following Ag stimulation but had lower survival rates than CD8+ T cells in wild-type mice, possibly due to increased sensitivity to CTL lysis. Indeed, B7-H1–deficient CD8+ T cells had reduced expression of antiapoptotic Bcl-xL and were prone to proapoptotic signals. Morever, irradiated mice receiving transferred B7-H1–deficient CD8+ T cells did not completely suppress the growth of B16-OVA tumors, whereas control mice injected with wild-type CD8+ T cells rejected the tumors. These results identify a previously unappreciated role of T cell-associated B7-H1. The potential involvement of the protein in the survival of activated T cells should be considered in the optimization of anti–B7-H1 therapeutic strategies for cancer treatment.

Multiple mechanisms are responsible for Ag uptake by dendritic cells (DCs), including the actin-driven processes phagocytosis and macropinocytosis, as well as receptor-mediated endocytosis. Hematopoietic lineage cell-specific protein 1 (HS1) is required in T cells for actin remodeling and signaling events at immune synapses. Huang et al. (p. 5952) examined the involvement of HS1 in DC function. Bone marrow-derived DCs from HS1-deficient mice had impaired presentation of exogenous Ags on both MHC class I and class II, although they presented peptides efficiently. HS1 was not required for the macropinocytosis of Ags, such as FITC-dextran or Lucifer yellow, or the phagocytosis of FITC-zymosan. However, HS1 dependency was observed in the receptor-mediated uptake of proteins such as transferrin, OVA, and GRP94, which are endocytosed by DCs via the transferrin, mannose, and scavenger receptors, respectively. In contrast, HS1 was dispensable for Ag presentation to T cells when OVA was endocytosed via non-receptor–mediated mechanisms. Additional experiments showed that HS1 associated with the actin cytoskeleton to promote early events during the formation of endocytic plasma membrane invaginations. This study provides novel insights into the mechanisms leading to efficient Ag presentation by DCs.

Asthma is characterized by chronic lung inflammation and airway remodeling that consists of thickening and restructuring of the airway tissues. Tatler et al. (p. 6094) investigated the mechanism of TGF-β activation in human airway smooth muscle (HASM) cells and the cytokine’s role in asthmatic airway remodeling. The experiments demonstrated that contraction agonists, such as lysophosphatidic acid and methacholine, induced TGF-β activation through cytoskeleton and talin interaction with the β5 cytoplasmic domain of the αvβ5 integrin. HASM cells from asthmatic subjects activated more TGF-β than cells from nonasthmatic controls in response to stimulation. These results were confirmed in two in vivo models, OVA-induced asthma and Aspergillus fumigatus-induced allergic airway remodeling. Interestingly, OVA-exposed mice treated with an anti-αvβ5 Ab or mice genetically lacking β5 and exposed to the fungus presented significant reduction in the thickness of the ASM layer, compared with control mice, although they appeared to have increased inflammatory responses. The data support the hypothesis that in chronic asthma, there is no association between inflammation and airway remodeling and suggest a mechanistic explanation for airway remodeling caused by recurrent bronchoconstriction.

Summaries written by Bernardetta Nardelli, Ph.D.