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Inhibition of polyamine synthesis suppresses human lymphocyte proliferation without decreasing cytokine production or
M A McCarthy1, J P Michalski, E S Sears
1Louisiana State University Medical Center, New Orleans.
Abstract:
Difluoromethylornithine (DFMO) irreversibly inhibits ornithine decarboxylase (ODC), a crucial enzyme in polyamine synthesis, and impairs mitogen-induced lymphocyte proliferation. To examine the mechanism of action of DFMO, we studied the effect of this ODC inhibitor on lymphokine production and interleukin 2 (IL 2) receptor expression. DFMO decreased thymidine uptake of peripheral blood mononuclear cells stimulated by the mitogens phytohemagglutinin, concanavalin A, phorbol myristate acetate and ionomycin 60-70% compared with untreated cells, and the inhibition could be completely reversed by 10 mM spermidine. DFMO had no effect on IL 1 production by monocytes exposed to silica particles. Concentrations of IL 2 increased 7-fold in DFMO-treated, PHA-stimulated PBMC cultures, compared with untreated cells; whereas IL 2 receptor expression as measured by the anti-Tac monoclonal antibody was not affected by the inhibition of ODC. Mixing experiments using cells cultured with or without DFMO indicated that the inhibition by DFMO was not mediated by suppressor cells. Our results strongly support the concept that polyamines are required for a relatively late event in lymphocyte activation occurring after the interaction of IL 2 and its receptor.
Insights
Difluoromethylornithine (DFMO) inhibits ornithine decarboxylase (ODC), impacting lymphocyte proliferation. Polyamines are essential for lymphocyte activation after interleukin-2 (IL-2) receptor interaction, not for IL-2 receptor expression itself.
Area of Science:
- Immunology
- Biochemistry
- Molecular Biology
Background:
- Polyamines are vital for cell growth and proliferation.
- Ornithine decarboxylase (ODC) is the rate-limiting enzyme in polyamine synthesis.
- Difluoromethylornithine (DFMO) is a known irreversible inhibitor of ODC.
Purpose of the Study:
- To investigate the mechanism of action of DFMO on lymphocyte activation.
- To determine the effect of ODC inhibition on lymphokine production and IL-2 receptor expression.
- To elucidate the role of polyamines in lymphocyte activation.
Main Methods:
- Peripheral blood mononuclear cells (PBMCs) were stimulated with mitogens (PHA, ConA, PMA, ionomycin).
- DFMO's effect on thymidine uptake and IL-2 production was measured.
- IL-2 receptor expression was assessed using the anti-Tac monoclonal antibody.
- Mixing experiments were conducted to rule out suppressor cell involvement.
Main Results:
- DFMO significantly decreased mitogen-induced thymidine uptake in PBMCs (60-70% inhibition), reversible by spermidine.
- DFMO did not affect IL-1 production by monocytes.
- IL-2 concentrations increased 7-fold in DFMO-treated PHA-stimulated PBMCs.
- IL-2 receptor expression remained unaffected by ODC inhibition.
Conclusions:
- Polyamines are required for a late stage of lymphocyte activation, following IL-2 and IL-2 receptor interaction.
- DFMO's inhibitory effect on lymphocyte proliferation is not mediated by suppressor cells.
- ODC inhibition impacts lymphocyte activation downstream of IL-2 receptor signaling.