Background CD4+ and CD8+ T-lymphocytes are suggested to differentially affect airway inflammation in asthma. Agents which increase intracellular cAMP levels, such as PDE inhibitors, have been shown to diminish lymphocyte growth and differentiation, and to affect cytokine expression. Differences in the PDE isoenzyme profile between CD4+ and CD8+ cells might form a basis to differentially modify their functions by PDE inhibitors.
Objective The study investigates and compares the PDE isoenzyme activity profiles of human peripheral blood CD4+ and CD8+ T-lymphocytes.
Methods CD4+ and CD8+ T-lymphocytes were purified (>98%) from peripheral blood mononuclear cells by negative selection. PDE isoenzyme activity profiles were investigated using PDE isoenzyme selective inhibitors and activators.
Results In CD4+ and CD8+ T-lymphocyte homogenates, PDE IV and PDE III activities were the predominant PDE isoenzyme activities at 0.5μM cyclic nucleotide substrate concentrations. PDE IV was localized in the soluble fraction whereas PDE III was membrane bound. Low PDE I, II and V activities were detected. About 20% of total eAMP hydrolysing capacity at 0.5 μM cAMP was insensitive to PDE isoenzyme selective inhibitors and activators and therefore could not be assigned to PDE I-IV. The PDE isoenzyme pattern was not different between CD4+ and CDS+ T-lymphocytes. Moreover, representative inhibitors of PDE HI and IV activity inhibited cAMP hydrolysis in soluble fractions of both T-lymphocyte subsets with similar potency. Enzyme kinetic analysis similarly did not reveal differences between CD4h and CD8+ T-lymphocytes.
Conclusion Normal CD4+ and CD8+ T-lymphocytes are likely to be equally sensitive targets for the effects of PDE inhibitors.