HIV-1 tat protein suppresses cholangiocyte toll- like receptor 4 expression and defense against cryptosporidium parvum

Biliary cryptosporidiosis is associated with acquired immunodeficiency syndrome (AIDS) cholangiopathy and occurs almost exclusively in adult patients with AIDS. Infection of biliary epithelial cells (cholangiocytes) with Cryptosporidium parvum induces Toll-like receptor (TLR) 4 expression and stimulates a TLR-dependent response against infection. Here, we tested whether human immunodeficiency virus type 1 (HIV-1) Tat affects TLR expression and, hence, anti-C. parvum defense responses. Using an in vitro model of human biliary cryptosporidiosis, we found that recombinant Tat protein increased TLR4 mRNA expression in both uninfected and C. parvum-infected cholangiocytes. Conversely, Tat decreased TLR4 protein levels and suppressed C. parvum-induced TLR4 protein expression. Using actinomycin to inhibit transcription,wefound that Tat increased the half-life ofTLR4mRNAfrom̃25 to 60 min, and RNA gel-shift assays demonstrated direct binding of Tat to TLR4 mRNA. In vitro transcription/translation studies suggested that Tat does not affect transcription but does decrease TLR4 translation. Importantly, more parasites were foundin Tat-treated cellsthanin control cells48hafter infection. Thesefindingssuggest thatTatinhibits cholangiocyte TLR4proteinexpressionthroughtranslational inhibition. Theseeventsappeartodiminishtheabilityofcholangiocytes to initiate an innate immune response to C. parvum. We suggest that these findings may contribute to the unusual susceptibility of HIV-infected individuals to biliary cryptosporidiosis.