CHANGES IN THE CYTOPLASMIC STRUCTURE OF CTLS DURING TARGET-CELL RECOGNITION AND KILLING

CTL play a critical role in immune defense by recognizing and killing virally infected or tumor cells. In this report, the structure of cyto plasm within living CTL was monitored during CTL killing of target cel ls. Living CTL were simultaneously loaded with fluorescent 70,000- and 10,000-kDa dextran particles. The relative distribution of the large and small dextrans within CTL revealed subcellular heterogeneities in the submicroscopic structure of cytoplasm. Localized alterations in cy toplasmic structure correlated with specific events during CTL killing . Recognition of target cells was accompanied by a transient increase in large dextran accessibility over a broad front near the interface b etween CTL and target cells. This region narrowed to a smaller area fr om which pseudopodia were extended toward the target. During extension , there was a large difference between regions of high dextran accessi bility within the pseudopod and more structured cytoplasm within the c ell body. Areas undergoing structural changes showed localized foci of high dextran accessibility, During retraction, cytoplasmic structure became gradually more uniform throughout the protrusion and cell body, These observations revealed subcellular regions undergoing major chan ges during early stages of the killing response, and addressed the rol e of cytoplasmic solation in controlling CTL morphology. They support mechanisms of pseudopod extension driven by hydrostatic pressure and d emonstrate a precise regulation of cortical structure to control the d irection of pseudopod extension.