Wortmannin, an inhibitor of phospholipase D activation, selectively blocks major histocompatibility complex class II-restricted antigen presentation.

Wortmannin, a fungal metabolite, is a specific inhibitor of phospholipase D (PLD) activation. Presentation of defined exogenous soluble proteins to specific T cell hybridomas was studied by using different antigen-presenting cells (APC): IA-positive peritoneal macrophages (M phi), B lymphoma cells (B) or dendritic cells (DC). Major histocompatibility complex class II-restricted antigen presentation by M phi was blocked when cells were pretreated with wortmannin. However, when cells constitutively expressing IA molecules (B, DC) were used as APC, no inhibition was observed. Additionally, MHC class I antigen presentation was not impaired by wortmannin. Moreover, wortmannin does not block either peptide presentation or presentation to autoreactive T cells. This effect was time and dose dependent and occurred at the level of intracellular handling of the antigen. Mainly because it was not a toxic inhibition, it was reversible with time and neither antigen uptake and catabolism, nor IA synthesis were affected. Because M phi, but not B or DC, express PLD activity and only the former were blocked by wortmannin in antigen presentation, our results strongly suggest that a differential antigen-processing pathway exists in these disparate APC, which could be based essentially on a wortmannin-sensitive, PLD-dependent step present in M phi but absent and/or unnecessary in both B lymphoma cells and DC.