Inhibition of store-operated calcium entry in microglia by helminth factors: implications for immune suppression in neurocysticercosis

BACKGROUND: Neurocysticercosis (NCC) is a disease of the central nervous system (CNS) caused by the cestode Taenia solium. The infection exhibits a long asymptomatic phase, typically lasting 3 to 5 years, before the onset of the symptomatic phase. The severity of the symptoms is thought to be associated with the intensity of the inflammatory response elicited by the degenerating parasite. In contrast, the asymptomatic phase shows an absence of brain inflammation, which is presumably due to immunosuppressive effects of the live parasites. However, the host factors and/or pathways involved in inhibiting inflammation remain largely unknown. Recently, using an animal model of NCC in which mice were intracranially inoculated with a related helminth parasite, Mesocestoides corti, we reported that Toll-like receptor (TLR)-associated signaling contributes to the development of the inflammatory response. As microglia shape the initial innate immune response in the CNS, we hypothesized that the negative regulation of a TLR-induced inflammatory pathway in microglia may be a novel helminth-associated immunosuppressive mechanism in NCC. METHODS AND RESULTS: Here we report that helminth soluble factors (HSFs) from Mesocestoides corti inhibited TLR ligation-induced production of inflammatory cytokines in primary microglia. This was correlated with an inhibition of TLR-initiated upregulation of both phosphorylation and acetylation of the nuclear factor κB (NF-κB) p65 subunit, as well as phosphorylation of JNK and ERK1/2. As Ca(2+) influx due to store-operated Ca(2+) entry (SOCE) has been implicated in induction of downstream signaling, we tested the inhibitory effect of HSFs on agonist-induced Ca(2+) influx and specific Ca(2+) channel activation. We discovered that HSFs abolished the lipopolysaccharide (LPS)- or thapsigargin (Tg)-induced increase in intracellular Ca(2+) accumulation by blocking the ER store release and SOCE. Moreover, electrophysiological recordings demonstrated HSF-mediated inhibition of LPS- or Tg-induced SOCE currents through both TRPC1 and ORAI1 Ca(2+) channels on plasma membrane. This was correlated with a decrease in the TRPC1-STIM1 and ORAI1-STIM1 clustering at the plasma membrane that is essential for sustained Ca(2+) entry through these channels. CONCLUSION: Inhibition of TRPC1 and ORAI1 Ca(2+) channel-mediated activation of NF-κB and MAPK pathways in microglia is likely a novel helminth-induced immunosuppressive mechanism that controls initiation of inflammatory response in the CNS.