Inhibition of Peripheral Blood Mononuclear Cell Proliferation by Cardiac Glycosides

Prior studies have shown that ouabain, a cardiac glycoside that inhibits the sodium, potassium adenosine triphosphatase (Na+, K+, ATPase) enzyme, downregulates phytohemagglutinin (PHA)-induced peripheral blood mononuclear cell (PBMNC) proliferation. This study examined and compared the effects of both ouabain and digoxin, a cardiac glycoside used therapeutically in humans, on PBMNC proliferation. Peripheral blood mononuclear cells were isolated from healthy human subjects, incubated for 72 hours with and without PHA (2%) in the presence and absence of ouabain (10 -12M to 10 -4M) or digoxin (10 -9 to 10 -6M), and pulsed with 3H thymidine. For PHA-stimulated PBMNCs in the ouabain-treated group (n = 10 subjects), the mean (±STD) % uptake (% 3H thymidine uptake in absence of ouabain) was 80.5 +/− 6.0 at 10 -12M ouabain, 73.1 +/− 8.4 at 10 -10M, 47.89 +/− 13.1 at 10 -8M, 6.9 +/− 3.2 at 10 -6M, and 3.4 +/− 1.6 at 10 -4M. For PHA-stimulated cells in the digoxin-treated group (n = 9 subjects), the mean (+/−STD) % uptake (% 3H thymidine uptake in absence of digoxin) was 89.8 +/− 9.8 at 10 -9M digoxin, 92.6 +/− 8.2 at 10 -8M, 54.3 +/− 19.8 at 10 -7M and 1.0 +/− 2.4 at 10 -6M. Repeated measures ANOVA demonstrated a signifiacnt effect of concentration of both glycosides on PBMNC proliferation ( P < .01). The inhibitory effect was reversible, but was largely abbrogated if ouabain was added after 48 hours of incubation with PHA. Further, the inhibitory effect extended to PBMNCs stimulated with recall antigen (tetanus) and to fractionated PBMNCs (CD4 +, CD8 +, and CD19 +) stimulated with mitogens. Additionally, dose-response inhibitory effects of glycosides on PBMNC Na +, K +, ATPase enzyme activity and interleukin-2 (IL-2) secretion by PHA-stimulated PBMNC were also noted. Neither glycoside had an effect on spontaneous PBMNC proliferation (no PHA) or trypan blue exclusion. These studies demonstrate that both cardiac glycosides inhibited PHA-induced PBMNC proliferation, possible via Na +, K +, ATPase inhibition, but not via cell toxicity. The concentration range over which inhibition was observed was similar for both glycosides. The results raise the possibility that therapeutic or toxic doses of digoxin could have an effect on cell-mediated immunity in vivo.