Teorías cognitivas y su relación con el aprendizaje del sujeto

Integrins play a dual role in the multistep adhesion cascade, arresting rolling lymphocytes as well as mediating transendothelial migration (indeed as mentioned above there is also evidence

of their activities in the initial rolling step). The integrins are a large and abundant class of adhesion receptors. Integrins are heterodimers made up of non-covalently associated pairs of a and p integrin subunits. Integrin adhesion is divalent cation dependent (either or Mg^"^,

according to the integrin). They are widely expressed throughout the body and are important in a number of physiological processes such as embryogenesis, growth cone guidance, wound

healing, cell survival as well as of course, in leukocyte trafficking and inflammation (Hynes, 1992; Meredith and Schwartz, 1997; Springer, 1994). They are also important in pathology, most notably in tumour metastasis, where expression of (in)appropriate integrins permits cancer cells to bind and migrate into different body tissues (Stetler-Stevenson et al., 1993). Within an immunological context, an inherited defect in the integrin subunit (LAD -

leukocyte adhesion deficiency) leaves afflicted individuals susceptible to recurrent life- threatening bacterial infections (Larson and Springer, 1990). This disease is a good pointer to the importance of (P^) integrin mediated adhesion in the proper functioning of the immune

system.

So far, over 20 different integrin heterodimers have been identified. These result from a variety of pairings between the 16 different a subunits with 8 different p subunit partners (Newham

and Humphries, 1996). The principal integrins found on lymphocytes are a^p2 (LFA-1) and

a^p, (very late antigen (VLA)-4). The most abundant of these on naive lymphocytes is LFA-1. In humans, there are also low levels of integrin a^p^ on naive cells. On activated or memory

lymphocytes this integrin is expressed at high levels (Girard and Springer, 1995).

Integrins mediate two types of adhesion, cell to extracellular matrix (ECM) and cell to cell. In parallel with this, ligands tend to fall into two categories, with large ECM macromolecules such as fibronectin, collagen or laminin on the one hand and cell surface molecules such as

intercellular adhesion molecule (ICAM)-l or vascular cell adhesion molecule (VCAM)-l on the other. One study has shown that the integrin subunit itself, can act as a ligand for integrins

and VLA-4 (Altevogt et al., 1995).

Integrins are thought to exist in at least two, perhaps three activation states (inactive, partially active and fully active), determined by their conformation and determining their ability to bind ligand (Mackay and Imhof, 1993). A number of integrin activation stimuli have been described, from the artificial, such as phorbol esters, Mn^^, or even activating antibodies, to more physiological stimuli, such as cellular activation through the T cell receptor (Hughes and Pfaff, 1998; Stuiver and O'Toole, 1995). One group have isolated a monoclonal antibody that appears to act as an activation marker for the integrin LFA-1 (Dransfield and Hogg, 1989). Of particular interest to the field of lymphocyte trafficking is the emerging role of chemokines in integrin activation (Campbell et al., 1996; Laudanna et al., 1996). Recent advances in the chemokine field will impact greatly on our understanding of the integrated role of chemokine stimulation and integrin activation in the transition from rolling to firm adhesion in HEVs (Gunn et al.,

1998c; Kim and Broxmeyer, 1999).

Integrins must be able to “integrate” the interaction between the extracellular environment at one end, and the actin cytoskeleton on the other (N.B. is linked to intermediate filaments

instead). This is done through the mediation of a number of cytoskeletal linker proteins such as tahn or vinculin (Hemler, 1998). In addition to changes in receptor affinity, significant contributions are made by cytoskeletal rearrangements subsequent to ligand binding, such as integrin clustering and cell spreading (Faull et al., 1994). The priming of integrins for

subsequent activation is often referred to as inside-out or outside-in signalling, depending on whether the resultant effects act outside or within the ceU respectively (Ginsberg et al., 1992; Hynes, 1992). Under this form of classification, chemokine receptor engagement leading to integrin activation and concomitant increased integrin mediated binding, is an example of inside-out signalling. Alternatively, binding to integrins by ligand, Mn^^ or activating antibody

which led to integrin clustering or increased integrin affinity for ligands would be referred to as examples of outside-in signalling.

1.3.4.1 Integrins and lymphocyte recirculation

LFA-1 is one of the principal integrins involved in the adhesive interactions of naive

lymphocytes with HEVs (Dunon et al., 1996; Springer, 1994). Three distinct but homologous gene products named intercellular adhesion molecule (ICAM) -1,-2 and -3 have been identified as ligands for this integrin (de Fougerolles and Springer, 1992; Marlin and Springer, 1987; Staunton et al., 1989). LFA-1 is expressed on many leukocytes, including all lymphocytes. In terms of lymphocyte trafficking, ICAM-1 and ICAM-2 are particularly interesting ligands, since both are expressed by endothelial cells. In particular, ICAM-2 is expressed by HEVs (de Fougerolles et al., 1991)

The interaction of integrin with HEVs is of some interest. As well as mediating firm

adhesion to endothelium, this integrin has also been shown to act in the sequestration of lymphocytes from flow, in a similar fashion to that conventionally associated with that of the selectins and their carbohydrate ligands (Berlin et al., 1995). As with LFA-1, has been

implicated in the adhesion and migration of lymphocytes across HEVs, although in contrast to LFA-1, its involvement is largely confined to migration into gut associated lymphoid tissue such as Peyer's patches. Indeed, the cognate ligand for a^p^, mucosal addressin cell adhesion

molecule (MAdCAM) -1 is found in both Peyer’s patch as well as mesenteric lymph node HEV, but not in the HEV of peripheral lymph nodes (Streeter et al., 1988a). Naive lymphocytes express low levels of a^P^, contrasting with activated or memory lymphocytes,

which express at much higher levels.

There is a question as to the role of VLA-4 in the recirculation of naive lymphocytes in humans, since its ligand VCAM-1 has not been found on human HEV (Girard and Springer, 1995). In contrast, cultured rat HECs do express VCAM-1 and binding by primary rat lymphocytes is

inhibited by blocking V LA ^ with soluble ligand peptide CSl (May et ah, 1993). In another study, anti human monoclonal antibodies raised against and p, integrin subunits blocked the

binding of human lymphocytes to rat HECs (Szekanecz et ah, 1992) in vitro. In humans at least, VLA-4 activity is associated with lymphocyte migration to sites of inflammation, where inflammatory mediators induce VCAM-1 expression on local endothelial cells (Osborn et al., 1989). VCAM-1 gene ablation is embryonic lethal with insurmountable defects in vascular development (Kwee, et al., 1995).

In vivo antibody blocking studies provide good evidence of the importance of both LFA-1 and ot^p^ integrins in lymphocyte recirculation into secondary lymphoid organs. Antibodies to

LFA-1 significantly inhibited lymphocyte migration into peripheral lymph nodes as well as Peyer's patches (80% and 50% reduction respectively) (Hamann et al., 1994). In addition, antibodies raised against either a^p^ or MAdCAM-1 both inhibit migration into Peyer’s patches

by up to 75% (Hamann et al., 1994; Streeter et al., 1988a). More recently, the use of targetted gene disruption or gene “knockout” technology has provided both confirmation of and clarity to the functions of these integrins in lymphocyte trafficking.

LFA knockout mice show deficits in homing to Peyer’s patches, MLN and PLN in increasing order of severity (Berlin-Rufenach et al., 1999). Interestingly, the removal of LFA-1 effects revealed an element of redundancy mediated by integrins, with migration to PLN being

largely blocked by antibodies raised against a^P^ as well as VCAM-1. The chain

knockout is an embryonic lethal, although studies with chimeric mice, generated by injecting embryonic stem cells into recombination activating gene (RAG)-l or RAG-2

blastocysts did allow some homing phenotype to be pursued (Arroyo et al., 1996; Yang et al., 1995). Here, because of a persistent developmental block in B cell development, only T cell homing could be examined. T cells migrate normally to MLN, and PLN, but fail to migrate to Peyer’s patches confirming a role for the a^p^ : MAdCAM-1 pairing in this tissue. However, in contrast to deficient (T) lymphocytes in chimeric mice, with the p^ knockouts, migration

of lymphocytes to Peyer’s patches is not completely abolished, hinting at an element of

redundancy between and VLA-4 (Wagner et al., 1996).

The phenotypes of integrin knockouts are rightly interpreted with caution, particularly in light

of developmental implications of knocking out such important and versatile gene products. The fact that integrins exist as heterodimers, with the constituent chains exhibiting varying degrees

of promiscuity, adds another layer of complexity to the interpretation of data. Nonetheless, knockout data confirm the premier roles of LFA-1 and the integrins in the process of

lymphocyte homing to secondary lymphoid tissue, as well as highlighting subtle overlaps in function.

1.3.5 The Selectins

Selectins are involved at the first stage of the multistep adhesion cascade, mediating leukocyte capture from (blood) flow and their concurrent rolling along endothelial cells (Springer, 1994). This rolling behaviour is permitted by the relatively low affinity interaction of selectin adhesion molecules with their carbohydrate ligands, in particular their rapid association and dissociation rate constants (Alon et al., 1995).

^ E G F domain

^^^hortconsensus repeal

NK