Anchorage and lymphocyte function. Spreading-capacity distinguishes common thymocytes and peripheral T lymphocytes.

Contact of T-enriched human blood lymphocytes with an adhesive surface in the presence of Concanavalin A (Con A) almost immediately induced a sequence of motile changes in virtually all cells. The initial event in this spreading process was the formation of filopodia distinct from the microvilli of lymphocytes in suspension. The filopodia were accompanied by lamellipodia, ruffles and flattening of the nucleus. Contact with a nonadhesive substratum in the presence of Con A did not trigger this sequence of changes. Cytochalasin B and D or low temperature inhibited the contact-induced changes. With the exception of a small number of cells (5-15%), T-enriched lymphocytes that were allowed to settle in the absence of Con A showed a radius of action (area occupied by the cells/translational movement per hr) of 39 micrometers 2/ less than 1 micrometer. The small 'motile' population showed a radius of action of 74 micrometers 2/8 micrometers. The Con-A-mediated spreading-process yielded a radius of action of the lymphocytes of 117 micrometers 2/6 micrometers. This augmented radius of action markedly facilitated cell-cell interaction in a high frequency of the cells and appeared to be a prerequisite for such interactions at 'low' cell density. Thymocytes reactive with OKT 6 antibodies or belonging to the 'high-density' fraction of cells attached to a Con-A-coated surface to the same extent as peripheral OKT 3 positive lymphocytes, but did not exhibit the morphological changes characteristic of a spreading-process. In contrast, OKT 6 negative thymocytes or thymocytes with a relatively low density showed spreading indistinguishable from that of OKT 3 positive peripheral lymphocytes. These results characterize the spreading-process in human T lymphocytes and demonstrate its functional importance for interactions with the environment. Spreading-capacity appears to reflect the stage of maturation of T cells.