Although the ontogeny of hematopoietic stem cells (HSCs) in vertebrates has been studied intensely, a lineage relationship between the HSCs found in the developmentally successive hematopoietic organs remains to be shown. By using an in situ photoactivatable cell tracer in the transparent zebrafish embryo, we demonstrated that definitive blood precursors appeared between the dorsal aorta and axial vein, validating the homology of this tissue with the AGM (aorta-gonad-mesonephros) of amniotes. These cells first migrated through the blood to a previously undescribed caudal hematopoietic tissue (CHT), where they differentiated, expanded, and further migrated to seed the definitive hematopoietic organs, the thymus and kidney. Immigrants on the way to the thymus expressed c-myb and ikaros but not rag1; they were probably no longer HSCs, however, because they lacked scl and runx1 expression, unlike immigrants to the kidney. The CHT thus has a hematopoietic function similar to that of the mammalian fetal liver.

The first leukocytes that arise in the development of vertebrate embryos are the primitive macrophages, which differentiate in the yolk sac and then quickly invade embryonic tissues. These macrophages have been considered to constitute a separate lineage, giving rise to no other cell type. Using an in vivo photoactivatable cell tracer in the transparent zebrafish (Danio rerio) embryo, we demonstrated that this lineage also gave rise to an equal or higher number of neutrophilic granulocytes. We were surprised to find that the differentiation of these primitive neutrophils occurs only after primitive myeloid progenitors have dispersed in the tissues. By 2 days after fertilization, these neutrophils have become the major leukocyte type found wandering in the epidermis and mesenchyme. Like the primitive macrophages, all primitive and larval neutrophils express PU.1 and L-plastin and they are highly attracted to local infections, yet only a small fraction of them phagocytose microbes, and to a much lesser extent per cell than the macrophages. They are also attracted to variously stressed or malformed tissues, suggesting a wider role than antimicrobial defense.

Cryptococcus neoformans is a yeast responsible for disseminated meningoencephalitis in patients with cellular immune defects. The major virulence factor is the polysaccharide capsule. We took advantage of a relevant murine model of disseminated meningoencephalitis to study the early events associated with blood-brain barrier (BBB) crossing. Mice were sacrificed at 1, 6, 24, and 48 hours post-intravenous inoculation, and classical histology, electron microscopy, and double immunofluorescence were used to study tissues and yeasts. Crossing of the BBB occurred early after inoculation, did not involve the choroid plexus but instead occurred at the level of the cortical capillaries, and caused early and severe damage to the structure of the microvessels. Seeding of the leptomeninges was not the primary event but occurred secondary to leakage of cortical pseudocysts. Organ invasion was associated with changes in cryptococcal capsule structure and cell size, which differed in terms of magnitude and kinetics, depending on both the organs involved, and potentially, on the bed structure of the local capillary. The rapid changes in capsule structure could contribute to inability of the host immune response to control cryptococcal infection in extrapulmonary spaces.

Malaria sporozoites cross the liver sinusoidal barrier, target Kupffer cells and endothelial cells with cell traversal inhibiting sporozoite clearance.

SUMMARYThe selective accumulation of different leucocyte populations during inflammation is regulated by adhesion molecules and chemokines expressed by vascular endothelium. This study examined how chemokine production and the expression of adhesion molecules and chemokine receptors vary between endothelia from different vascular beds. Human saphenous vein endothelium was compared with lung and dermal microvascular endothelia and with umbilical vein endothelium and a bone-marrow endothelial cell line. All endothelia produced CCL2 and CXCL8 constitutively, whereas CXCL10 and CCL5 were only secreted after tumour necrosis factor (TNF)-a or interferon (IFN)-g stimulation. In combination with TNF-a , IFN-g suppressed CXCL8 but enhanced CCL5 and CXCL10, whereas transforming growth factor (TGF)-b reduced secretion of all chemokines. Basal chemokine secretion was higher from umbilical vein than other endothelial cells. Chemokine receptors, CXCR1, CXCR3 and CCR3, were present on all endothelia but highest on saphenous vein. CCR4, CCR5, CCR6, CXCR2, CXCR4 and CXCR5 were also detected at variable levels on different endothelia. The variation between endothelia in chemokine secretion was much greater than the variations in adhesion molecules, both on resting cells and following cytokine stimulation. These results indicate that it is the tissue-specific variations in endothelial chemokine secretion rather than variations in adhesion molecules that can explain the different patterns of inflammation and leucocyte traffic seen in non-lymphoid tissues.

IntroductionHuman T-cell lymphotropic virus type I (HTLV-I) is the etiologic agent of an aggressive and fatal T-cell malignancy of activated CD4 ϩ CD45RO ϩ T lymphocytes termed adult T-cell leukemia/ lymphoma (ATL). 1,2 The mechanisms of leukemogenesis are not yet fully understood. Infection during infancy and a long clinical latency period of 20 to 30 years appear to be critical factors associated with the development of ATL. During this period, clonal expansion of HTLV-I-bearing T cells occurs, and, following a model of multistep oncogenesis, the accumulation of critical somatic mutations may contribute to the development of ATL. Viral protein expression from early infection to ATL may play a major role during all stages of the disease development. 3 The HTLV-I Tax protein is a 40-kDa transcriptional transactivator of the HTLV-I gene via its interaction with activation transcription factor (ATF)/CCAAT-enhancer binding protein (CREB) proteins and the transcriptional coactivators CREB binding protein (CBP) and p300. 4,5 Tax is also capable of increasing expression of other cellular genes by positively regulating nuclear factorB (NF-B) activity. 3 There is strong evidence that Tax may also play a critical role in the cellular transformation of various in vitro models, including T cells, and is capable of inducing tumors in transgenic mice. [6][7][8] In these models, Tax induction of transformation is also associated with cellular gene expression modulation via the NF-B and/or ATF/CREB pathways. 9 In ATL cells, activated protein-1 (AP-1) activity is constitutively activated 10,11 and may play a critical role in cell proliferation and transformation. AP-1 is a transcription factor complex composed of members of Fos (c-fos, FosB, Fra-1, and Fra-2) and Jun (c-Jun, JunB, and JunD) families that play a major role in the positive regulation of proliferation and activation of T-cell and cytokine production. 12,13 In nonstimulated normal T cells, the basal level of AP-1 proteins is low, but T-cell activation results in rapid induction of jun and fos genes. 14 AP-1 activity is also regulated at the posttranscriptional level by the activation of c-Jun N-terminal kinase (JNK). 15 JNK phosphorylates c-Jun, thereby increasing its DNA binding activity. 16 this pathway by inducing the expression of various members of the AP-1 family, including c-Jun, and by constitutively activating JNK. 10,13,17,18 Several reports have demonstrated that fresh ATL cells as well as ATL cell lines produce high levels of transforming growth factor ␤1 (TGF-␤1) as a consequence of the activation of AP-1 sites located in the 5Ј regulatory region of the TGF-␤1 gene. 19,20 However, the role of TGF-␤1 production by ATL cells in HTLV-I leukemogenesis remains to be elucidated.TGF-␤1 controls various aspects of cell growth and differentiation by signaling through a heteromeric complex of type I (TGF-␤1-RI) and II (TGF-␤1-RII) serine/threonine kinase transmembrane receptors. TGF-␤1 binds TGF-␤1-RII, resulting in the recruitment and the activation of TGF-␤1...

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Complex mechanisms of human immunodeficiency virus type-1 (HIV-1) brain pathogenesis suggest the contribution of individual HIV-1 gene products. Among them, the Nef protein has been reported to harbor a major determinant of pathogenicity in AIDS-like disease. The goal of the present study was to determine whether Nef protein expressed in vivo by primary macrophages could induce a brain toxicity also affecting the behavior of the rat. To achieve this goal we grafted Nef-transduced macrophages into the rat hippocampus. Two months post-transplantation, we observed that Nef induces monocyte/macrophage recruitment, expression of TNF-alpha, and astrogliosis. No apoptotic event was detected. We further demonstrated that Nef neurotoxicity is associated with cognitive deficits.