Impaired chromatin remodelling at STAT1-regulated promoters leads to global unresponsiveness of Toxoplasma gondii-infected macrophages to IFN-γ

Toxoplasma gondii Inhibits gamma interferon (IFN-γ)- and IFN-β-induced host cell STAT1 transcriptional activity by increasing the association of STAT1 with DNA

The gamma interferon (IFN-γ) response, mediated by the STAT1 transcription factor, is crucial for host defense against the intracellular pathogen Toxoplasma gondii, but prior infection with Toxoplasma can inhibit this response. Recently, it was reported that the Toxoplasma type II NTE strain prevents the recruitment of chromatin remodeling complexes containing Brahma-related gene 1 (BRG-1) to promoters of IFN-γ-induced secondary response genes such as Ciita and major histocompatibility complex class II genes in murine macrophages, thereby inhibiting their expression. We report here that a type I strain of Toxoplasma inhibits the expression of primary IFN-γ response genes such as IRF1 through a distinct mechanism not dependent on the activity of histone deacetylases. Instead, infection with a type I, II, or III strain of Toxoplasma inhibits the dissociation of STAT1 from DNA, preventing its recycling and further rounds of STAT1-mediated transcriptional activation. This leads to increased IFN-γ-induced binding of STAT1 at the IRF1 promoter in host cells and increased global IFN-γ-induced association of STAT1 with chromatin. Toxoplasma type I infection also inhibits IFN-β-induced interferon-stimulated gene factor 3-mediated gene expression, and this inhibition is also linked to increased association of STAT1 with chromatin. The secretion of proteins into the host cell by a type I strain of Toxoplasma without complete parasite invasion is not sufficient to block STAT1-mediated expression, suggesting that the effector protein responsible for this inhibition is not derived from the rhoptries.

Genetic and epigenetic susceptibility to early life infection

PURPOSE OF REVIEW

This review aims to interpret the current literature on the role of genetic and epigenetic factors in susceptibility to neonatal infection, a leading cause of early life mortality and morbidity.

RECENT FINDINGS

Epidemiological data indicate that the differential susceptibility to infection is partly heritable. To date there have been relatively few studies on genetic determinants of susceptibility to neonatal infection and many of these have methodological shortcomings. Most studies predominantly focus on the innate immune system. There is growing interest in the potential role of epigenetic mechanisms in disease susceptibility and data are emerging on the role of epigenetics in the maturation of the immune system in early life.

SUMMARY

Infection is a leading cause of morbidity and mortality, especially in preterm infants, but it remains unclear why neonates are so susceptible or what mediates differential risk. Genetic and epigenetic epidemiologic studies may assist in the identification of critical protective and pathogenic pathways. Despite the current relative lack of robust data, such studies may facilitate the development of interventions that ultimately decrease the significant morbidity and mortality of this highly vulnerable population.

Toxoplasma gondii triggers phosphorylation and nuclear translocation of dendritic cell STAT1 while simultaneously blocking IFNγ-induced STAT1 transcriptional activity

The protozoan Toxoplasma gondii actively modulates cytokine-induced JAK/STAT signaling pathways to facilitate survival within the host, including blocking IFNγ-mediated STAT1-dependent proinflammatory gene expression. We sought to further characterize inhibition of STAT1 signaling in infected murine dendritic cells (DC) because this cell type has not previously been examined, yet is known to serve as an early target of in vivo infection. Unexpectedly, we discovered that T. gondii infection alone induced sustained STAT1 phosphorylation and nuclear translocation in DC in a parasite strain-independent manner. Maintenance of STAT1 phosphorylation required active invasion but intracellular parasite replication was dispensable. The parasite rhoptry protein ROP16, recently shown to mediate STAT3 and STAT6 phosphorylation, was not required for STAT1 phosphorylation. In combination with IFNγ, T. gondii induced synergistic STAT1 phosphorylation and binding of aberrant STAT1-containing complexes to IFNγ consensus sequence oligonucleotides. Despite these findings, parasite infection blocked STAT1 binding to the native promoters of the IFNγ-inducible genes Irf-1 and Lrg47, along with subsequent gene expression. These results reinforce the importance of parasite-mediated blockade of IFNγ responses in dendritic cells, while simultaneously showing that T. gondii alone induces STAT1 phosphorylation.

Modulation of innate immunity by Toxoplasma gondii virulence effectors

Toxoplasma gondii is a common parasite of animals and humans and can cause serious opportunistic infections. However, the majority of infections are asymptomatic, possibly because the organism has co-evolved with its many vertebrate hosts and has developed multiple strategies to persist asymptomatically for the lifetime of the host. Over the past two decades, infection studies in the mouse, combined with forward-genetics approaches aimed at unravelling the molecular basis of infection, have revealed that T. gondii virulence is mediated, in part, by secretion of effector proteins into the host cell during invasion. Here, we review recent advances that illustrate how these virulence factors disarm innate immunity and promote survival of the parasite.

Toxoplasma gondii clonal strains all inhibit STAT1 transcriptional activity but polymorphic effectors differentially modulate IFNγ induced gene expression and STAT1 phosphorylation

Host defense against the parasite Toxoplasma gondii requires the cytokine interferon-gamma (IFNγ). However, Toxoplasma inhibits the host cell transcriptional response to IFNγ, which is thought to allow the parasite to establish a chronic infection. It is not known whether all strains of Toxoplasma block IFNγ-responsive transcription equally and whether this inhibition occurs solely through the modulation of STAT1 activity or whether other transcription factors are involved. We find that strains from three North American/European clonal lineages of Toxoplasma, types I, II, and III, can differentially modulate specific aspects of IFNγ signaling through the polymorphic effector proteins ROP16 and GRA15. STAT1 tyrosine phosphorylation is activated in the absence of IFNγ by the Toxoplasma kinase ROP16, but this ROP16-activated STAT1 is not transcriptionally active. Many genes induced by STAT1 can also be controlled by other transcription factors and therefore using these genes as specific readouts to determine Toxoplasma inhibition of STAT1 activity might be inappropriate. Indeed, GRA15 and ROP16 modulate the expression of subsets of IFNγ responsive genes through activation of the NF-κB/IRF1 and STAT3/6 transcription factors, respectively. However, using a stable STAT1-specific reporter cell line we show that strains from the type I, II, and III clonal lineages equally inhibit STAT1 transcriptional activity. Furthermore, all three of the clonal lineages significantly inhibit global IFNγ induced gene expression.

Epigenetics of host-pathogen interactions: the road ahead and the road behind

A growing body of evidence points towards epigenetic mechanisms being responsible for a wide range of biological phenomena, from the plasticity of plant growth and development to the nutritional control of caste determination in honeybees and the etiology of human disease (e.g., cancer). With the (partial) elucidation of the molecular basis of epigenetic variation and the heritability of certain of these changes, the field of evolutionary epigenetics is flourishing. Despite this, the role of epigenetics in shaping host-pathogen interactions has received comparatively little attention. Yet there is plenty of evidence supporting the implication of epigenetic mechanisms in the modulation of the biological interaction between hosts and pathogens. The phenotypic plasticity of many key parasite life-history traits appears to be under epigenetic control. Moreover, pathogen-induced effects in host phenotype may have transgenerational consequences, and the bases of these changes and their heritability probably have an epigenetic component. The significance of epigenetic modifications may, however, go beyond providing a mechanistic basis for host and pathogen plasticity. Epigenetic epidemiology has recently emerged as a promising area for future research on infectious diseases. In addition, the incorporation of epigenetic inheritance and epigenetic plasticity mechanisms to evolutionary models and empirical studies of host-pathogen interactions will provide new insights into the evolution and coevolution of these associations. Here, we review the evidence available for the role epigenetics on host-pathogen interactions, and the utility and versatility of the epigenetic technologies available that can be cross-applied to host-pathogen studies. We conclude with recommendations and directions for future research on the burgeoning field of epigenetics as applied to host-pathogen interactions.

The HAT/HDAC interplay: multilevel control of STAT signaling

Besides the transcription-promoting role of histone acetyltransferases (HATs) and the transcription-delimiting function of histone deacetylases (HDACs) through histone acetylation and deacetylation respectively, HATs and HDACs also regulate the activity of several non-histone proteins. This includes signal transducers and activators of transcription (STATs), key proteins in cytokine signaling. Unlike Tyr phosphorylation/dephosphorylation, which mainly acts as an on/off switch of STAT activity, the control exerted by HATs and HDACs appears multifaceted and far more complex than initially imagined. Our review focuses on the latest trends and novel hypotheses to explain differential context-dependent STAT regulation by complex posttranslational modification patterns. We chart the knowledge on how STATs interact with HATs and HDACs, and additionally bring a transcriptional regulatory and gene-set specific role for HDACs in the picture. Indeed, a growing amount of evidence demonstrates, paradoxically, that not only HAT but also HDAC activity can be required for STAT-dependent transcription, in a STAT subtype- and cell type-dependent manner. Referring to recent reports, we review and discuss the various molecular mechanisms that have recently been proposed to account for this peculiar regulation, in an attempt to shed more light on the difficult yet important question on how STAT specificity is being generated.