Schematic of findings. LPS released both GM-CSF and IL-6 through MAPKs p44/42 and p38 pathways. Previous work has shown that LPS-induced tight junction disruption (paracellular permeability) is mediated through the p44/42 MAPK pathway whereas HIV-1 transcytosis is mediated through the p38 MAPK pathway. Tight junction function as measured by TEER but not tight junction protein expression as measured by western blots was influenced by IL-6 (but not GM-CSF) through a site not blocked by antibodies and so assumed to be intracellular. GM-CSF and IL-6 both promoted HIV-1 transcytosis. The transcytotic effects of GM-CSF and IL-6 were mediated through luminal but not abluminal sites that were blocked by antibodies and therefore assumed to be extracellular.