Uridine transport in concanavalin A- and lipopolysaccharide-activated mouse lymphocytes

Leflunomide-mediated suppression of antiviral antibody and Tcell responses: differential restoration by uridine

BACKGROUND

Leflunomide is an isoxazol derivative with immunosuppressive capacities in various experimental allo- and xenotransplantation models. Two main mechanisms of action have been described: Inhibition of pyrimidine de novo synthesis and impairment of tyrosine phosphorylation of different tyrosine kinases involved in receptor signaling via B cell and cytokine receptors.

MATERIALS AND METHODS

Interference of Leflunomide with the IgM antibody responses to vesicular stomatitis virus (VSV, T-independent type 1), IgM to recombinant VSV glycoprotein (T-independent type 2), and IgG to lymphocytic choriomeningitis virus (LCMV, T-dependent) were analyzed whereas the cytotoxic T cell (CTL) response was examined after LCMV infection. Interference with the CD8+ T cell-mediated autoimmune diabetes in a transgenic mouse expressing the LCMV-glycoprotein in the pancreatic islets was studied as a model for T cell-mediated autoimmune diseases. Uridine substitution experiments were performed to differentiate between the above mentioned two mechanisms of action on different functions of the immune system in vivo.

RESULTS

Leflunomide at 35 mg/kg/day suppressed the humoral immune response against all antigens tested. Similar effects on T-independent compared to T-dependent antibody responses required two to four times higher drug doses. CTL responses to LCMV were considerably impaired. Uridine substitution prevented lethal VSV encephalitis under Leflunomide treatment by restoring the neutralizing IgM and IgG responses. However, the inhibition of LCMV specific CTLs and suppression of CD8+ T cell-mediated autoimmune diabetes remained unaffected by additional uridine treatment.

CONCLUSIONS

Leflunomide-mediated suppression of B cell and T helper cell activity but not of CTLs largely depends on inhibition of pyrimidine de novo synthesis.

Leflunomide: an immunomodulatory drug for the treatment of rheumatoid arthritis and other autoimmune diseases

Leflunomide (Arava) has recently been approved by the Food and Drug Administration for the treatment of rheumatoid arthritis (RA). The drug, due to its protective effects on structural joint damage, has been classified as a disease modifying anti-rheumatic drug (DMARD). Leflunomide is structurally dissimilar from other drugs currently used to treat RA and exhibits a different mechanism of action. It has shown to be protective in a variety of animal models of arthritis and autoimmunity based on its immunomodulatory activity. Leflunomide is rapidly converted in vivo to its pharmacologically active metabolite A77 1726. This metabolite is a potent non-cytotoxic inhibitor of the enzyme dihydroorotate dehydrogenase (DHODH), a key enzyme in the de novo synthesis of uridine monophosphate (UMP). Activated lymphocytes depend on the pyrimidine de novo syntheses to fulfill their metabolic needs for clonal expansion and terminal differentiation into effector cells. De novo synthesis of pyrimidines is not only essential to provide precursors for new RNA and DNA synthesis, but also for phospholipid synthesis and the pyrimidine sugars necessary for protein glycosylation, which support the massive expansion in membrane biosynthesis to form daughter cells. This mechanism likely contributes to leflunomide's action as a DMARD in RA and other autoimmune diseases. This review is a summary of current in vivo and in vitro data, focussing primarily on the mechanism of action of leflunomide in RA.

Species differences in the binding of [3H]nitrobenzylthioinosine to the nucleoside transport system in mammalian central nervous system membranes: evidence for interconvertible conformations of the binding site/transporter complex

The binding of [3H]nitrobenzylthioinosine (NBMPR) to specific sites in CNS membranes was investigated using cortical tissue from a variety of mammalian species. Mass law analysis of the site-specific binding of NBMPR data revealed that rat, mouse, guinea pig, and dog cortical membranes each contained an apparent single class of high-affinity (KD 0.11-4.9 nM) binding sites for NBMPR; rabbit cortical membranes, however, exhibited two distinct classes of NBMPR binding sites with KD values of 0.4 nM and 13.8 nM. Dipyridamole, a potent inhibitor of nucleoside transport, produced a biphasic profile of inhibition of the binding of NBMPR to guinea pig, rabbit, and dog membranes (IC50 less than 20 nM and IC50 greater than 6 microM for NBMPR binding sites displaying high and low affinity for dipyridamole, respectively). These results are indicative of heterogeneity of NBMPR binding sites in mammalian cortical membranes. Rat and mouse cortical membranes appear to possess only one type of NBMPR binding site, which has low affinity for dipyridamole. Detailed analysis of inhibitor-induced dissociation of NBMPR from its sites in each species led to the conclusion that these multiple forms of NBMPR binding sites are different conformations of a single site associated with the CNS nucleoside transport system, rather than two distinct sites. It is also suggested that the affinity of dipyridamole for each conformation of NBMPR site indicates the susceptibility of that conformation of the nucleoside transport system to inhibition by dipyridamole.