Ruggero Pardi

Regulatory mechanisms in leukocyte adhesion: flexible receptors for sophisticated travelers

Unstimulated leukocytes spend extended periods circulating in the blood, punctuated by migration through lymphoid areas and peripheral tissues. During transit, strong cell-cell interactions control immune surveillance and specialized effector functions. The structures and mechanisms that allow this flexible adhesion and migration behavior are the subject of this review.

A tyrosine‐based sorting signal in the β2 integrin cytoplasmic domain mediates its recycling to the plasma membrane and is required for ligand‐supported migration

Integrins play pivotal roles in supporting shear‐ and mechanical‐stress‐resistant cell adhesion and migration. These functions require the integrity of the short β subunit cytoplasmic domains, which contain multiple, highly conserved tyrosine‐based endocytic signals, typically found in receptors undergoing regulated, clathrin‐dependent endocytosis. We hypothesized that these sequences may control surface integrin dynamics in statically adherent and/or locomoting cells via regulated internalization and polarized recycling of the receptors. By using site‐directed mutagenesis and ectopic expression of the αL/β2 integrin in Chinese hamster ovary cells, we found that Y735 in the membrane‐proximal YRRF sequence is selectively required for recycling of spontaneously internalized receptors to the cell surface and to growth factor‐induced membrane ruffles. Disruption of this motif by non‐conservative substitutions has no effect on the receptors adhesive function, but diverts internalized integrins from a recycling compartment into a degradative pathway. Conversely, the non‐conservative F754A substitution in the membrane‐proximal NPLF sequence abrogates ligand‐dependent adhesion and spreading without affecting receptor recycling. Both of these mutants display a severe impairment in ligand‐supported migration, suggesting the existence in integrin cytoplasmic domains of independent signals regulating apparently unrelated functions that are required to sustain cell migration over specific ligands.

Early Events in Cell-Mediated Recognition of Vascularized Xenografts: Cooperative Interactions between Selected Lymphocyte Subsets and Natural Antibodies

Cell-mediated early immune recognition of xenogeneic vascularized discordant grafts is poorly characterized. It has been the purpose of our studies to elucidate the role of lymphocytes in the recognition and rejection phenomena. To this end, we have utilized both ex vivo and in vitro model systems. We demonstrate that selected human lymphocyte subpopulations (mainly NK cells) rapidly and specifically adhere to xenogeneic endothelia. Efficient lysis of endothelial cells is subsequently mediated by such a population. Adhesion and cytotoxicity occur via at least two pathways, one dependent on and the other independent of the presence of human natural antibodies of the G class. Both IgG-dependent and IgG-independent adhesion and cytotoxicity can be partly inhibited by the use of anti-leukocyte integrin monoclonal antibodies. IgG-dependent adhesion and cytotoxicity can be also partly inhibited by carbohydrate structures that contain the alpha-galactosyl epitope. A possible role of these events in the eventual outcome of discordant vascularized xenogeneic transplants can be postulated.

The platelet endothelial cell adhesion molecule-1 (PECAM1) contributes to endothelial barrier function

In this study we have analyzed the role of the platelet‐endothelial cell adhesion molecule‐1 (PECAM1) in vascular barrier function. PECAM1 is an immunoglobulin gene superfamily member expressed by endothelial cells at the cell boundaries. Macromolecule permeability assays performed on cell monolayers that express native or transfected PECAM1, indicated that the molecule participates in the establishment and maintenance of vascular barrier function in vitro. This hypothesis was confirmed by the finding that in vivo injection of the specific monoclonal antibody directed against the murine vascular PECAM1 led to a detectable leakage of hepatic and renal blood vessels.

Expression of intercellular adhesion molecule-1 (ICAM-1) on cultured human endometrial stromal cells and its role in the interaction with natural killers

PROBLEM: Recent evidence emphasizes the role of natural killer cells (NKs) as potential effectors of peritoneal immune surveillance directed against the outgrowth of endometrial cells, refluxed with menstrual debris, in ectopic sites. This NK‐mediated cytotoxicity toward autologous endometrial antigens seems to be significantly decreased in endometriosis patients.

Effects of short-term lithium treatment on peripheral blood lymphocytes and granulocytes in healthy voluteers

SummaryThe effects of a short-term treatment with lithium carbonate on lymphocytes and granulocytes were studied in six normal subjects. The lithium effects are related to a direct, immediate and quickly reversible action of the drug on the bone marrow, as demonstrated by the granulocyte number and variations in Arneths index. Although lymphocyte and granulocyte functions do not seem to be affected by lithium when investigated by the usual methods, the ability of the drug to counteract the theophylline-induced inhibition of cellular functions would suggest that lithium acts mainly through the modulation of intracellular levels of cyclic nucleotides. This is confirmed by evaluation of cellular cyclic 3′, 5′-adenosine monophosphate (cAMP). The above hypothesis could explain the quick onset and loss of lithium action on granulopoiesis.

Adhesion Receptors Involved in Leukocyte Functions

Leukocytes are the only nucleated somatic cells that spend a significant portion of their life span displaying a nonadherent, circulating phenotype, being transported by the bloodstream and interstitial fluids towards their sites of action. However, most effector functions of leukocytes depend on and are induced by firm adhesion to other cells or to the extracellular matrix. A logical explanation for this apparent paradox is that while most functional responses of immunocompetent cells are triggered by intimate contacts with other cells, the effective search for potentially harmful environmental agents in a multicellular organism (i.e. immune surveillance) requires continuous patrolling of the body across anatomical barriers. Leukocytes have evolved a highly dynamic and finely regulated adhesive behaviour in order to comply with these contrasting requirements.