The adhesion and locomotion of mouse peripheral lymph node lymphocytes on 2-D protein-coated substrata and in 3-D matrices were compared . Lymphocytes did not adhere to, or migrate on, 2-D substrata such as serum-or fibronectin-coated glass. They did attach to and migrate in hydrated 3-D collagen lattices . When the Collagen was dehydrated to form a 2-D surface, lymphocyte attachment to it was reduced . We propose that lymphocytes, which are poorly adhesive, are able to attach to and migrate in 3-D matrices by a nonadhesive mechanism such as the extension and expansion of pseudopodia through gaps in the matrix, which could provide purchase for movement in the absence of discrete intermolecular adhesions. This was supported by studies using serum-coated micropore filters, since lymphocytes attached to and migrated into filters with pore sizes large enough (3 or 8 ,um) to allow pseudopod penetration but did not attach to filters made of an identical material (cellulose esters) but of narrow pore size (0 .22 or 0.45 Am) . Cinematographic studies of lymphocyte locomotion in collagen gels were also consistent with the above hypothesis, since lymphocytes showed a more variable morphology than is typically seen on plane surfaces, with formation of many small pseudopodia expanded to give a marked constriction between the cell and the pseudopod . These extensions often remained fixed with respect to the environment as the lymphocyte moved away from or past them . This suggests that the pseudopodia were inserted into gaps in the gel matrix and acted as anchorage points for locomotion .Though lymphocytes are motile, they adhere and move poorly on protein-coated glass or plastic under conditions in which fibroblasts, macrophages, and neutrophils adhere and move well . In consequence, generalizations about cell locomotion obtained from studies of the latter cells do not fit the more puzzling locomotor behavior of lymphocytes, a knowledge of which would help us to understand how lymphocytes recirculate, cooperate with each other and with other cell types, and infiltrate and migrate through extravascular sites .Cells moving on 2-D surfaces must make close enough contact with the substratum to provide traction for locomotion . In fibroblasts, it has been postulated that locomotion takes place because the cells form areas of contact named focal adhesions (1) where there is cross-bridging of the space between cell and substratum with an intermediate protein, fibronectin (27), which allows traction to be generated . Focal adhesions are not seen in faster-moving cells such as neutrophil leukocytes, but these cells do form rapidly shifting areas of close contact while moving on protein-coated substrata (3) .
In the present investigation we found that the presence of lymph within a 3-D collagen gel potentiates the invasion of mature recirculating lymphocytes into the gel. Preferential accumulation of lymphocytes in lymph-containing gels follows the same rules that apply for both in vitro lymphocyte adhesion to high endothelial venules of frozen sections of the lymph nodes and in vivo lymph node entry of lymphocytes. Furthermore, the results obtained suggest that lymph is chemotactic for lymphocytes. This chemoattractant activity of lymph could be assigned mainly to a protein fraction precipitating in the range of 40-60% concentration of ammonium sulphate. The biological significance of these findings for the selective process of lymphocyte emigration from blood to lymph, across the parenchyma of the nodes, is discussed.
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