The Split Personality of pDCs
Plasmacytoid dendritic cells (pDCs) regulate antiviral immunity and are implicated in the pathogenesis of autoimmunity. These cells have been found to accumulate in breast cancer and to express the cytotoxic molecule granzyme B, leading Matsui et al. (p. 6815) to assess whether pDCs might display tumoricidal activity. Indeed, activated human pDCs were able to kill tumor cells in vitro in a cell contact-dependent manner. Variations in conjugate formation between pDCs and target cells revealed that human pDCs could be divided into two subsets based on CD2 expression. CD2high and CD2low pDCs demonstrated similar morphologies but distinct patterns of gene and protein expression. CD2high pDCs specifically expressed lysozyme and formed clusters with tumor target cells, while CD2low pDCs expressed molecules implicated in cell cycle regulation. Both subsets expressed IFN-α, granzyme B, and TRAIL upon activation and were able to trigger a recall T cell response. However, activated CD2high pDCs induced the proliferation of naive T cells much more efficiently than did CD2low pDCs, and these cells also up-regulated CD80 and secreted higher levels of IL-12 p40. This discovery that human pDCs can be divided into functionally distinct subsets will be important to the understanding of the wide variety of immune responses in which these cells participate.
Crawling Out of Blood Vessels
Leukocyte crawling via αMβ2 integrin (Mac-1) on the endothelium is a newly described step in the recruitment of leukocytes to sites of inflammation. Cells adhere to the endothelium via αLβ2 integrin (LFA-1) and then crawl to junctions through which they transmigrate. To clarify the mechanism behind the transition from adhesion to crawling, Phillipson et al. (p. 6870) analyzed the role of Vav1, which regulates cellular activation downstream of LFA-1, in neutrophil trafficking. In wild-type mice, neutrophils were found to crawl via mechanotaxis in a direction perpendicular to blood flow until they reached an endothelial junction, at which point they changed direction to follow the junction and emigrated out of the vessel. In contrast, Vav1−/− neutrophils appeared to be dragged in the direction of flow in a Mac-1-independent manner and did not change direction at junctions. Vav1−/− neutrophils also demonstrated impaired adhesion to the endothelium and consequently emigrated less frequently than did wild-type cells. Decreased neutrophil recruitment to the peritoneal cavity was observed in Vav1−/− mice in a model of infectious peritonitis, demonstrating the biological relevance of the impaired neutrophil crawling in these mice. Thus, Vav1 is essential for optimal Mac-1-mediated intravascular mechanotactic crawling during innate immunity.
T Cells Begotten from Stem Cells
Research using human embryonic stem cells (hESC) has not yet reached its full potential but is predicted to lead to great advances in the understanding of development and potential treatment of disease. It has not been clear whether hESC-derived hematopoietic progenitor cells (HPC) can develop into T cells in vitro. In this issue, Timmermans et al. (p. 6879) describe the in vitro generation of functionally mature T cells from hESC, indicating that hESC could be used to study T cell development and could serve as clinically relevant sources of T cells. To induce HPC development, hESC were cultured on OP9 cells where they formed structures resembling the “blood islands” that appear during embryonic development. Upon transfer of these structures, dubbed hematopoietic zones, onto OP9-DL1 cells, CD34highCD43low HPC differentiated into CD34highCD45+CD7+c-kit+cyCD3ε+ T/NK progenitors and then into mature T cells. Interestingly, analysis of the phenotype of these cells during development suggested that CD7 expression alone did not signify lymphoid commitment. The in vitro-generated T cells consisted of both αβ and γδ T cells and bore a polyclonal TCR repertoire. Functional maturity was demonstrated by the ability of these T cells to proliferate and to secrete IFN-γ upon restimulation. Thus, the authors have created a useful tool for future research on T cell differentiation and for potential T cell-based immunotherapy.
A Degrading Death
Galectin-1 is an endogenous lectin that triggers T cell apoptosis through a mechanism distinct from the traditional extrinsic and intrinsic apoptotic pathways. Fodrin, a spectrin family member that links CD45 to the actin cytoskeleton, is degraded during many apoptotic pathways, but the mechanism by which it is cleaved is not known. As galectin-1 binds to CD45, Pang et al. (p. 7001) investigated the potential role of fodrin in galectin-1-mediated T cell death. Galectin-1 treatment of T cells led to fodrin cleavage by m-calpain through a process that required CD45 and was likely regulated by CD45 tyrosine phosphatase activity. In other cell death pathways fodrin degradation is coupled to many of the hallmarks of apoptosis, but in galectin-1-mediated apoptosis this cleavage occurred independently of phosphatidylserine externalization and DNA degradation. However, inhibition of fodrin degradation decreased apoptotic membrane blebbing and inhibited phagocytic clearance of dying T cells, indicating a specific role for fodrin cleavage in these processes. These data advance our understanding of galectin-1-mediated T cell apoptosis and of the role of the cytoskeletal tether fodrin in T cell signaling and fate decisions.
An Unconventional Exit for MIF
Macrophage migration inhibitory factor (MIF) is a proinflammatory cytokine that regulates a wide variety of immune processes. This molecule lacks a signal sequence and is secreted through an unconventional pathway that is not well understood. In a search for new molecules involved in MIF secretion, Merk et al. (p. 6896) identified the Golgi-associated protein p115 as a specific MIF binding partner in the cytoplasm of macrophages. Although p115 has been previously suggested to have a general intracellular trafficking function, this study indicated that p115 was necessary for the secretion of MIF but not of other cytokines from LPS-stimulated monocytes. MIF and p115 were cosecreted from macrophages, and depletion of p115 in these cells inhibited MIF secretion in response both to LPS and to infection with an intracellular pathogen. The mechanism of the known MIF inhibitor 4-iodo-6-phenylpyrimidine (4-IPP) was found to involve the blocking of MIF secretion through alteration of the interaction between MIF and p115. Taken together, these data begin to clarify the unconventional mechanism by which MIF is secreted from monocytes and macrophages and suggest that inhibition of the MIF:p115 interaction could have therapeutic utility in dampening inflammatory responses.
The All-Powerful HuR
The RNA-binding protein HuR is a pleiotropic regulator of mRNA stability and translation. In thymocyte subsets, Papadaki et al. (p. 6779) observed that HuR expression was regulated by TCR signaling and apoptotic stimuli. Because deletion of HuR causes embryonic lethality, the authors developed a conditional knockout mouse lacking HuR in thymocytes (LckCre+Elavl1fl/fl) to address this molecule’s potential role in thymocyte development. The LckCre+Elavl1fl/fl mice demonstrated hypercellular thymi and reduced peripheral T cell counts, which were associated with defects at multiple stages of development. At the double-negative stage, HuR was implicated in cell cycle control via modulation of the expression of the p53 tumor suppressor. HuR was also found to promote positive selection through its effects on TCR signaling. The increased thymic cellularity in LckCre+Elavl1fl/fl mice could be linked to a decrease in the deletion of thymocytes during negative selection coupled to inhibition of the chemokine signals that promote thymic egress of single-positive thymocytes. Finally, microarray analysis delineated some of the pathways through which HuR could control thymocyte development. Taken together, these data paint a picture of HuR involvement in thymocyte differentiation through its control of the cell cycle, TCR signaling, chemokine responses, and apoptosis. This multitude of activities may also be applicable to the modulation of peripheral T cell responses.
A View to Killing Cancer Stem Cells
Colorectal tumors contain a small population of cancer stem cells (CSC) that are refractory to chemotherapy, causing colon cancer to be one of the most deadly forms of cancer. Human Vγ9Vδ2 T cells can kill tumor cells that have down-regulated MHC expression, and it has previously been shown that amino-bisphosphonates such as zoledronate can sensitize tumor cells to γδ T cell-mediated lysis. Todaro et al. (p. 7287) have found that although differentiated colon cancer cells could be effectively killed by γδ T cells, CSC were not susceptible unless they were pretreated with zoledronate. Zoledronate-sensitized CSC induced expansion of Vγ9Vδ2 T cells and stimulated these cells to produce the proinflammatory cytokines IFN-γ and TNF-α and the cytotoxic molecules TRAIL and BLT-esterase. These Vγ9Vδ2 T cells then killed the sensitized CSC via a perforin-mediated mechanism that involved the TCR, NKG2D, and mevalonate metabolites. These data suggest that zoledronate could be applied to the development of immunotherapeutic strategies aimed at attacking treatment-resistant CSC in colon cancer.
Infections during pregnancy can threaten fetal health. TLRs are expressed in the placenta and may regulate the inflammatory response to gestational infection. Dulay et al. (p. 7244) hypothesized that the recently described soluble form of TLR2 (sTLR2) would be found in human amniotic fluid and would have a role in down-modulating the inflammatory response to infection. Amniotic fluid samples were analyzed from healthy pregnant women and from women undergoing preterm labor either with or without bacterial infection. sTLR2 was found to be constitutively expressed in preterm amniotic fluid irrespective of infection, but levels of sTLR2 decreased at term. Analysis of membrane-bound TLR2 expression suggested that sTLR2 was secreted from the trophoblast and amnion, and monensin treatment of placental villous explants indicated that sTLR2 was produced by intracellular posttranslational processing. In the placental villous explant system, sTLR2 was able to inhibit inflammatory cytokine production induced by the TLR2 ligand Pam3Cys. Thus, sTLR2 in amniotic fluid was proposed to serve mainly as a decoy receptor that down-regulates the inflammatory response to Gram-positive bacteria. This molecule may be part of a complex system of inflammatory regulation in pregnancy and may be relevant to studies of the role of infection in preterm labor.
Summaries written by Jennifer Hartt Meyers, Ph.D.