Cellular, genetic, and evolutionary aspects of lymphocyte interactions with high-endothelia venules

Circulation and chemotaxis of fetal hematopoietic stem cells

The major site of hematopoiesis transitions from the fetal liver to the spleen and bone marrow late in fetal development. To date, experiments have not been performed to evaluate functionally the migration and seeding of hematopoietic stem cells (HSCs) during this period in ontogeny. It has been proposed that developmentally timed waves of HSCs enter the bloodstream only during distinct windows to seed the newly forming hematopoietic organs. Using competitive reconstitution assays to measure HSC activity, we determined the localization of HSCs in the mid-to-late gestation fetus. We found that multilineage reconstituting HSCs are present at low numbers in the blood at all timepoints measured. Seeding of fetal bone marrow and spleen occurred over several days, possibly while stem cell niches formed. In addition, using dual-chamber migration assays, we determined that like bone marrow HSCs, fetal liver HSCs migrate in response to stromal cell-derived factor-1α (SDF-1α); however, unlike bone marrow HSCs, the migratory response of fetal liver HSCs to SDF-1α is greatly increased in the presence of Steel factor (SLF), suggesting an important role for SLF in HSC homing to and seeding of the fetal hematopoietic tissues. Together, these data demonstrate that seeding of fetal organs by fetal liver HSCs does not require large fluxes of HSCs entering the fetal bloodstream, and that HSCs constitutively circulate at low levels during the gestational period from 12 to 17 days postconception. Newly forming hematopoietic tissues are seeded gradually by HSCs, suggesting initial seeding is occurring as hematopoietic niches in the spleen and bone marrow form and become capable of supporting HSC self-renewal. We demonstrate that fetal and adult HSCs exhibit specific differences in chemotactic behavior. While both migrate in response to SDF-1α, fetal HSCs also respond significantly to the cytokine SLF. In addition, the combination of SDF-1α and SLF results in substantially enhanced migration of fetal HSCs, leading to migration of nearly all fetal HSCs in this assay. This finding indicates the importance of the combined effects of SLF and SDF-1α in the migration of fetal HSCs, and is, to our knowledge, the first demonstration of a synergistic effect of two chemoattractive agents on HSCs.

New results on the migratory behavior of blood cell precursors in the early embryo might be relevant to bone marrow transplants and other clinical therapies

Cloning and expression of cDNAs for the alpha subunit of the murine lymphocyte-Peyer's patch adhesion molecule

cDNA clones encoding the alpha chain of the murine lymphocyte-Peyer's patch adhesion molecule (LPAM), which is associated with lymphocyte homing, have been isolated by screening with the human VLA-4 (alpha 4h) probe. Several alpha 4 antigenic determinants were identified on COS-7 cells after transfection. From overlapping clones, approximately 5 kb of contiguous nucleotide sequence have been determined, encoding a protein sequence of 1039 amino acids for the LPAM alpha chain (alpha 4m). LPAM is a member of the integrin family of cell-surface heterodimers, and alpha 4m is the murine homologue of the human alpha 4 h chain. The two proteins have a total sequence similarity of 84%, with an almost perfect conservation (31/32 amino acids) in the cytoplasmic domain. Like alpha 4h, alpha 4m is distinct from other integrin alpha chains because it has neither an I-domain nor a COOH-terminal cleavage site. The positions of the characteristic Cysteine residues are conserved, and a putative protease cleavage site is located near the middle of the protein sequence. The NH2-terminal part of the protein contains seven homologous repeats, and three of them include putative divalent cation-binding sites. These sites are among the most conserved between the alpha 4m sequence and other alpha chains, and may therefore be involved in the binding of integrin alpha and beta chains. An additional cDNA clone was isolated which shares a sequence of perfect homology with the alpha 4m encoding cDNAs, but has a unique 3' poly-A end. This observation correlates with the fact that three discrete murine RNA bands are observed in Northern blot experiments using alpha 4m as a probe, whereas only two human RNA species are described for alpha 4h, indicating a higher complexity for murine than for human sequences.

Human homologue of mouse lymph node homing receptor: evolutionary conservation at tandem cell interaction domains

A cDNA clone homologous to the mouse lymph node homing receptor core protein (mLHRc) was isolated from a cDNA library derived from stimulated human peripheral blood lymphocytes. Human RNA blot analysis shows a tissue and cell-line distribution of transcript expression generally parallel to that seen in the mouse, with expression confined to lymphoid tissues and some cell lines. Genomic DNA analysis suggests a low-copy gene under high-stringency conditions. The complete nucleotide sequence predicts a mature protein of 334 amino acids, identical in length to mLHRc. The protein shows striking conservation globally between human and mouse sequences. In particular, all three genre of protein interaction domains identified in the mouse--an animal lectin domain, an epidermal growth factor (EGF)-like domain, and two homologous repeat units preserving the motif of complement regulatory proteins (CRP)--are present in the human protein (hLHRc), and maintain the same tandem arrangement. The lectin and EGF-like regions are the most homologous, while the CRP domains are less conserved between species. The two CRP units in hLHRc are distinct from those in mLHRc in that they are homologous to one another rather than identical, suggesting strong pressure for maintenance of two repeats in this molecule. hLHRc is distinct from other kinds of lymphocyte adhesion molecules represented by VLA-4 (integrin) or CD44/gp90Hermes and, together with mLHRc and two other recently described molecules having a similar domain motif, defines a novel class of adhesion molecules exhibiting distinct evolutionary features. We propose that hLHRc likely represents the protein core of the human homologue of mLHRc functionally as well as structurally.

Organ specificity of tumor metastasis: role of preferential adhesion, invasion and growth of malignant cells at specific secondary sites

The locations of distant secondary tumors in many clinical cancers and animal tumors are nonrandom, and their distributions cannot be explained by simple anatomical or mechanical hypotheses based on the simple lodgment or trapping of tumor cell emboli in the first capillary bed encountered. Evidence from certain experimental tumor systems supports Paget's 'seed and soil' hypothesis on the nonrandom distributions of metastases, in which the unique properties of particular tumor cells ('seeds') and the different characteristics of each organ microenvironment ('soil') collectively determine the organ preference of metastasis. Experimentally, differential tumor cell adhesion to organ-derived microvessel endothelial cells and organ parenchymal cells, differential invasion of basement membranes and organ tissues, and differential responses to organ-derived growth-stimulatory and -inhibitory factors all appear to be important determinants in explaining the organ preference of metastasis. Each tumor system may achieve organ specificity because of its own unique set of multiple metastasis-associated properties and responses to host microenvironments. As neoplasms progress to more highly malignant states multisite metastases are more likely and organ-specific metastases may be masked or circumvented owing to stochastic events, tumor cell diversification, host selection processes, and increased production of tumor autocrine molecules that may modulate adhesion, invasion, growth, and other properties important in metastasis. The importance of each of these properties, however, appears to vary considerably among different metastatic tumor systems. These and other tumor cell and host properties may eventually be used to predict and explain the unique metastatic distributions of certain human malignancies.

Lymphocyte migration through the lymphatic sinuses of the chicken's lymph node

We have described the lymphocyte migration through the sinus wall of the lymph node. The sinuses showing lymphocyte migration are located in a given area of the node that might be comparable with the T-dependent deep cortex of the mammalian lymph nodes. The lymphocyte migration takes place through the cytoplasm of the endothelial cell presumably by formation of the transendothelial channels, which are produced by vesicles of the endothelial cell. The intensive vesiculation preceding the formation of transendothelial channels might indicate the direction of the migration. The absence of vesicles in the endothelial cell and the dilated intercellular space between the lymphocyte and endothelial cell suggest that the lymphocyte in the sinus wall may be "resting". The presence of the multivesicular body in the pseudopod or in the vicinity of the pseudopod of the lymphocyte may contribute to the migration.

In vitro alteration of receptors for vasoactive intestinal peptide changes the in vivo localization of mouse T cells

The capacity of T lymphocytes exposed in vitro to the neuropeptide vasoactive intestinal peptide (VIP) to bind VIP in vitro and to migrate to different tissues in vivo has been studied. VIP treatment of T cells resulted in a time- and dose-dependent loss of the ability of T cells to specifically bind radioiodinated VIP. Altered binding was due to a decrease in the expression of cellular receptors for VIP on the treated cells rather than an alteration in the affinity of the cells for the neuropeptide. Alteration of VIP receptor expression was not associated with a change in the expression of Thy-1, Lyt-1, or Lyt-2 surface markers by the treated cells. VIP treatment of T cells in vitro resulted, however, in a dose-dependent decrease in the ability of the treated cells to localize in mesenteric lymph nodes (MLN) and Peyer's patches of recipient animals at early times after cell transfer, and this was due to a selective decrease in the rate of accumulation of the treated cells in these tissues. There was no alteration in the distribution of VIP-treated cells in the blood, spleen, liver, or other major organs of the recipient animals. It is concluded that the presence of VIP receptors on T cells facilitates the entry of T cells into MLN and Peyer's patches in vivo, and it is proposed that this effect is mediated by T cell-VIP interactions in the vicinity of the specialized endothelium of those tissues.