Effects of infliximab on apoptosis and reverse signaling of monocytes from healthy individuals and patients with Crohn's disease

Objectives: Infliximab, an anti-tumor necrosis factor (TNF) monoclonal antibody, might exert some of its long-term therapeutic effects in Crohn's disease (CD) by interacting directly with cells of the immune system such as monocytes and T lymphocytes via membrane TNF and by inducing apoptosis. Accordingly, the effects of infliximab on monocyte apoptosis and down-regulation of proinflammatory cytokines (reverse signaling) were assessed. Methods: To assess apoptosis, monocytes from healthy individuals (controls) and CD patients were incubated in the presence or absence of infliximab or the apoptotic agent gliotoxin for 24 hours. Annexin V staining and the terminal deoxynucleotidyl transferase (TdT)-mediated dUTP-FITC nick end labeling assay were used to measure early and late apoptosis. To measure the effects of infliximab on reverse signaling, monocytes from healthy individuals pretreated in vitro with infliximab were stimulated with lipopolysaccharide or staphylococcal enterotoxin A, and the induction of the proinflammatory cytokines, TNF-alpha, interleukin (IL)-1beta, IL-6, and IL-8 was measured by reverse transcription polymerase chain reaction. The effect of in vivo infliximab treatment of monocytes was similarly determined by comparing the responses of monocytes from CD patients before and immediately after infliximab infusion. Results: Infliximab did not induce apoptosis of monocytes from either healthy individuals or CD patients but rather stabilized them. However, monocytes from healthy individuals treated with infliximab in vitro, or from CD patients infused with infliximab, produced significantly less TNF and other proinflammatory cytokines when stimulated with the bacterial products lipopolysaccharide and staphylococcal enterotoxin A. Conclusions: Apoptosis of monocytes is not responsible for the therapeutic effects of infliximab. However, some of the therapeutic effects of infliximab may be caused by its ability to down-regulate proinflammatory cytokines production by monocytes exposed to bacterial antigens.