Effects of Monoclonal Antibodies to Specific Epitopes of Rat Interleukin-1 Beta (IL-Iβ) on IL-1β-Induced ACTH, Corticosterone and IL-6 Responses in Rats

Recently, we developed a panel of monoclonal antibodies (MoAbs) to rat IL-1β and found that MoAbs binding to the aminoacid sequences 66–85 and 123–143 of mature rIL-1β inhibited the binding of rIL-1β to murine EL4 cells. Here we study whether MoAbs to these and other domains of IL-1 interfere with the biological effects of rIL-1β in adult male rats in vivo. Administration of rIL-1β (1 or 5 μg/kg i.v.) enhanced the plasma concentrations of ACTH, corticosterone (CORT) and of IL-6 in a time- (0.5–4 h) and dose-dependent manner. Because 2 h after 5μg/kg i.v., all three parameters were consistently elevated, this dose and time interval was used for further studies. Prior to injection, rIL-1β was incubated alone or in the presence of a MoAb (10 mg/kg) for 30 min at 37°C or at 4°C. Plasma ACTH, CORT and IL-6 responses to these mixtures are compared to those obtained after preincubation of rIL-1β with a non-IL-1 binding MoAb (PEN7). SILK 3, a MoAb that binds to the 66–85 domain of rIL-1β, reduced the ACTH and IL-6 responses by 48 and 45% respectively. In contrast, a MoAb to the 123–143 domain (SILK 5) and SILK 16, which binds to the 106–124 domain did not affect any of the IL-1 induced responses, whereas a MoAb directed to domain 78–97 of rIL-1β (SILK 20) enhanced the ACTH and CORT responses by 51 and 41% respectively, but not the IL-6 response. These observations lead us to conclude that the domain 66–85, harbours sequences that are important for receptor binding and for the biological actions of rIL-1β in the rat and that this biologically active domain is located at the ‘closed side’ rather than at the ‘open side’ of the IL-1β molecule where domains involved in receptor binding and biological activity are considered to be located.