Study of the physical meaning of the binding parameters involved in effector–target conjugation using monoclonal antibodies against adhesion molecules and cholera toxin

In earlier work, we established a mathematical model to characterize the binding properties of cytotoxic cells to target cells. These properties can be described by the values of the maximum effector and target conjugate frequencies, αmax and βmax, respectively, and the dissociation constant of the conjugates formed, KD (Garcia-Peñarrubia, P., Cabrera, L., Alvarez, R., and Galvez, J., J. Immunol. Methods 155 (1992) 133). Here, we address the problem of exploring the physical meaning of these parameters and their relationships with cytotoxicity. With this purpose, conjugation between a human leukemic NK cell line (NKL) and K562 tumor cells has been studied from binding isotherms obtained from data of effector (α) and target (β) conjugate frequencies measured by flow cytometry analysis at different effector-to-target ratios (R). The results have been compared to those obtained after target cells treatment with monoclonal antibodies recognizing adhesion molecules ICAM-1 (CD54) and LFA-3 (CD58) (which are able to block some of the receptors implicated in conjugation), as well as with cholera toxin (CTX) that can modify the state of affinity of some adhesion molecules such as LFA-1 (CD11a/CD18). The results show that: (1) blocking adhesion receptors CD54 and CD58 on the surface of target cells leads to a significant decrease of αmax and βmax, indicating that these parameters are related to the density of expression of receptors implicated in effector–target adhesion; (2) treatment of effector cells with CTX induced an increase of KD, demonstrating that this parameter is associated with the effector–target affinity of the system; and (3) parallel experiments of conjugation and cytotoxicity showed that effector–target affinity and saturability influence the cytotoxic activity of the effector population.