Interaction of exposed membrane-lipid regions after mixing of immune peritoneal exudate T lymphocytes (PEL) and target cells (TC) was studied by monitoring the degree of fluorescence polarization (P) of 1,6-diphenyl 1,3,5-hexatriene (DPH), a lipophilic probe, which can be transferred upon contact of lipid phases. Since the P of DPH-labeled PEL and labeled TC differed significantly, detection of DPH translocation between labeled and nonlabeled cells was possible. A decrease in P was observed after interaction of DPH-labeled PEL and nonlabeled TC. Conversely, an increase in P was detected after nonlabeled PEL were allowed to react with DPH-labeled TC. These changes in P were evident when TC were reacted with specific PEL. However, substantial changes were also monitored with PEL immunized against third-party target cells. No significant changes in P were obtained when DPH-labeled TC were reacted with normal or immune lymph node cells or thymocytes, or with anti-TC antibody. These findings suggest that nonspecific collision of exposed membrane lipid regions occurs during immune PEL-TC interaction. We propose that binding of cytotoxic lymphocytes and TC is preceded by an unstable and nonspecific hydrophobic interaction of cells, which facilitates specific and stable binding. In support of this proposal is the finding that changes in the composition of membrane lipid constituents prevent binding of PEL and TC.

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