Increased permeability in acute inflammation is due to
Formation of endothelial gaps in venules, i.e. immediate transient response is the most common mechanism causing increased vascular permeability in acute inflammation,
Mediators involved in this mechanism are

i. Immediate (more important): Histamine, bradykinin, leukotrienes, neuropeptide substance P.
ii. Somewhat delayed: IL-1, TNF, IFNγ
Histamine
IL-2
TGF-p
FGF