One of the “perks” of pregnancy is hearing from your doctor that you should not be cleaning the kitty litter tray. Wait a second! What did you just say? Can you write that one down in an authoritative doctor’s note, please? Reason being – that if you have never been infected before, a primary infection from Toxoplasma cysts in cat feces can (rarely) be transmitted through the placenta to your baby. Apparently about a 1/3 of adults have antibodies against the parasite, Toxoplasma gondi. The infection is usually fairly insignificant. However, this infection becomes life threatening for those with poor immune systems. Think about those in the womb, post-chemotherapy or AIDS patients. Let’s face it, you cannot prevent “Fluffy” from eating a mouse, or casually strolling across your kitchen counter tops.
By looking at this parasitic infection through the lens of epigenetics, Christine Lang et al. have devised a novel therapeutic strategy using HDAC inhibitors.
Their Genome-wide expression profiling via transcriptome analysis, compared whole genome mRNA microarray of infected vs uninfected murine macrophages, both stimulated with IFN-γ. The comparison highlighted the disability of the infected macrophages to responsd to IFN-γ. This analysis was the first peek into the view of how transcription was being affected. Toxoplasma gondi infected, murine macrophages became globally unresponsive to stimulation with IFN-γ
Next the group moved on to proteomic analysis. In uninfected macrophages, they had previously shown that IFN-γ induces signaling of STAT-1 into the nucleus, where it participates in activating transcription, within a complex. In this publication, they identified another complex binding to the same sequences, which is specific to Toxoplasm-infected macrophages, regardless of IFN-γ stimulation. Sounds suspicious, right? The group performed 2D gels, followed by mass spec of the infected vs uninfected samples to identify components of this novel complex. They verified equivalent localization and expression levels of the proteins and of Stat-1.
Returning to the epigenetics angle, the group assayed promoter binding of BRG-1, a core subunit of chromatin remodeling complexes, and found binding upon IFN-γ treatment, in uninfected macrophages. Using ChIP, the group assessed assembly of BRG-1 into chromatin remodeling complexes and found a 10 fold increase in infected vs uninfected samples – with no change in overall abundance of BRG-1. Next, they used ChIP followed by RT PCR, to show that 90% of IFN-γ induced histone H4 acetylation was abolished at the CIITA promotor post infection with Toxoplasma gondi.
Toxoplasma gondi somehow makes a move to block normal chromatin remodeling complexes at IFN-γ regulated promoters, following IFN-γ treatment. That move was marked by a histone acetylation defect. Very interesting!
HDAC inhibitors were their logical focus. Fluorescence activated cell sorting (FACS) showed upregulation of H2-A/E (histone 2 acetylation expression) after IFN-γ treatment. This effect was NOT seen in populations of infected cells, (even parasite-negative cells in the population…indicating involvement of unknown soluble signaling factor). However, after HDAC inhibitor treatment H2-A/E was restored in infected cells. This effect was confirmed by RT-PCR transcriptome analysis showing responsive IFN-γ promotors, post HDAC inhibitor treatment. Finally histone acetylation at the CIITA promotor increased by ChIP analysis of HDAC inhibitor treated infected (and slightly in non-infected) cells.
Thus we now have evidence of a “parasitic” type mechanism by Toxoplasma gondi, to use our own epigenetic machinery to regulate our immunity. Most importantly…. now you can still rationalize keeping your cats.
Christine Lang, Anke Hildebrandt, Franziska Brand, Lennart Opitz, Hassan Dihazi, & Carsten G.K. Luder (2012). Impaired Chromatin REmodelling at STAT1-Regulated Promoters Leads to Global Unresponsiveness of Toxoplasma gondii-Infected Macrophages to IFN gamma. PLoS Pathog, 8 (1) : 10.1371