Background: Adalimumab is a fully human monoclonal antibody to tumor necrosis factor (TNF), which was recently introduced as a therapy for Crohn's disease and rheumatoid arthritis. Besides neutralization, induction of apoptosis of monocytes/macrophages and T cells is thought to be an important mechanism of action of the anti-tumor necrosis factor monoclonal antibody infliximab, at least in Crohn's disease therapy.
Aim: To study caspase activation and the induction of apoptosis by adalimumab and the effect of a caspase inhibitor in vivo.
Methods: For in vitro studies, THP-1 cells (human monocytic cell line) were incubated with adalimumab, infliximab, or human immunoglobulin G, and annexin V + propidium iodide, Apo2.7, and 7-amino actinomycin-D were used to study apoptosis on the cell membrane, mitochodrial, and DNA level, respectively. Active caspase-3 was detected by intracellular staining. For in vivo studies, a chimeric human-mouse model was used, in which THP-1 cells were injected intraperitoneally in SCID-Beige mice followed by treatment with adalimumab, infliximab, or human immunoglobulin G. Effects of a pan-caspase inhibitor N-benzyloxycarbonyl-Val-Ala-Asp-fluoromethyketone on apoptosis induction were evaluated.
Results: In vitro analysis revealed that apoptosis could be induced in THP-1 cells by both adalimumab and infliximab. Activation of caspase-3 after incubation with adalimumab was demonstrated by intracellular staining. In addition, in the chimeric mouse model, a higher percentage of residual THP-1 cells were apoptotic, and lower cell numbers were recovered in the adalimumab- or infliximab-treated mouse. Apoptosis induction by adalimumab could be abrogated through in vivo pretreatment of mice with the pan-caspase inhibitor.
Conclusions: Adalimumab, besides neutralizing tumor necrosis factor, also induces apoptosis of transmembrane tumor necrosis factor-positive THP-1 cells by activating intracellular caspases. This activity is likely to be important for the clinical effect of this biodrug.