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

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Abstract
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 (MΦϕ), B lymphoma cells (B) or dendritic cells (DC). Major histocompatibility complex class II‐restricted antigen presentation by MΦϕ 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 MΦ, 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 MΦ but absent and/or unnecessary in both B lymphoma cells and DC.

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