Oxpentifylline inhibits tumor necrosis factor‐α mRNA transcription and protects against arthritis in mercuric chloride‐treated Brown Norway rats

Abstract

The phosphodiesterase inhibitor oxpentifylline (OXP) has a number of potentially important immunomodulatory actions which include a selective inhibition of the Th1 subset of CD4⁺ cells in vitro and inhibition of tumor necrosis factor (TNF)-α mRNA transcription. In vivo, it has a dramatic protective effect against experimental allergic encephalomyelitis. In this animal model, tissue injury is associated with both a Th1 response and with TNF-α production, either of which could be targets for the protective action of OXP. In an attempt to clarify the relative importance of the Th cell subsets and TNF-α in pathogenesis, we investigated the effect of OXP on a Th2 model of T cell-dependent disease, mercuric chloride (HgCl₂)-induced autoimmunity in the Brown Norway rat. The effects of OXP on the Th1: Th2 response, TNF-α mRNA transcription in spleen and ankle joints, and on the incidence and severity of arthritis and cecal vasculitis have been examined and the effects in vivo have been compared with those of a soluble TNF receptor-IgG1 fusion protein (sTNFR) that neutralizes rat TNF-α. In two separate experiments, OXP significantly enhanced unstimulated levels of splenic interleukin-4 (IL-4) mRNA (median 62 %, of an artificial IL-4 mRNA construct, vs. 36.5 % in controls) and in one experiment, exaggerated the total IgE response to HgCl₂. OXP inhibited HgCl₂-induced TNF-α mRNA transcription in spleen and ankle joints. In three separate experiments, OXP had a significant protective effect against arthritis, with the mean incidence reduced from 100 % to 30 % and mean peak score reduced from 7.2 to 2.59 (experiments 1 and 2). The protection against arthritis was indistinguishable from that produced by sTNFR. There was no such protection against cecal vasculitis with either OXP or sTNFR. These results demonstrate that OXP induces a shift towards a Th2 response, inhibits TNF-α mRNA transcription locally in joint and systemically in spleen, and has a protective effect against arthritis similar to that produced by sTNFR in the HgCl₂-treated BN rat. We conclude that TNF-α is a critical cytokine in the pathogenesis of arthritis but not cecal vasculitis in this model, and that inhibition of TNF-α transcription is the most important mode of action of OXP in this situation. OXP may be a potential therapeutic agent in the treatment of other arthritides, such as human rheumatoid arthritis, in which TNF-α has been implicated in pathogenesis.