Role of the transcriptional regulators Msc and Id2 in peripheral CD8 T cell differentiation during chronic infection – TRANSMIT

CD8 T cells are essential to eliminate pathogen-infected cells or tumors. During an acute pathogen infection, CD8 T cells undergo a phase of contraction following pathogen clearance leaving a small number of memory CD8 T cells persisting over time. By contrast antigen persistence and chronic inflammation drive CD8 T cell exhaustion characterized by loss of effector functions and decreased survival. Importantly, most of the findings about CD8 T cell exhaustion during pathogen infection were observed in animal models of chronic infection (such as LCMV clone 13) mirroring overwhelming infection akin to HIV infection. By contrast much less is known about the differentiation of CD8 T cells responding to the more common type of chronic pathogen infections which establish chronic latency in targeted organs. CD8 T cell activation and differentiation is controlled by extrinsic cues that include cytokines, costimulation and the strength of TCR signaling. These signals are integrated at the transcriptional level by key “master” transcriptional regulators. We and others have demonstrated that peripheral CD8 T cell differentiation is regulated by the interplay between the class I basic helix-loop-helix protein (bHLH) family of transcriptional regulators called E proteins (E2A, HEB, E2-2) and their inhibitors the Id proteins. Indeed, loss of Id2 crippled effector differentiation and programmed CD8 T cells to adopt a memory fate in an E2A-dependent manner. Meanwhile, Id2 was recently shown to be involved in the differentiation of a CD8 T cell subset essential to control chronic LCMV infection. Interestingly, we discovered that the bHLH protein Musculin (Msc), a transcription factor (TF) originally associated with muscle development, was downregulated in virus-specific CD8 T cells upon T-cell specific deletion of Id2 in virus infected mice. Importantly, similar to Id2, Msc was previously identified as an E2A repressor in muscles and B cell lymphoma.
Our preliminary data have shown that Msc is selectively induced in effector CD8 T cells suggesting a role in effector CD8 T cell development. Accordingly, Msc-deficiency impaired the differentiation of short-lived KLRG1+ effector CD8 T cells after acute virus infection as well as their accumulation into non lymphoid tissues, a phenotype reminiscent of Id2-deficiency. Taken together these data suggest that Msc could be a critical molecular “switch” for effector CD8 T cell differentiation, however whether (and how) Msc might collaborate with Id2 to regulate CD8 T cell fate remains to be determined as well as the role of Msc and Id2 during overwhelming vs. latent chronic pathogen infections potentially to curtail memory CD8 T cell response.
In this project, using a combination of next-generation sequencing approaches, coupled with complementary genetic engineering tools and new mouse models of infection by the Toxoplasma gondii (T. gondii) parasite, we will aim to: 1. Define the molecular mechanisms by which Msc regulates CD8 T cell differentiation and more precisely, whether it acts through direct inhibition of E2A transcriptional activity, or by independent mechanisms; 2. Decipher the role of Msc and Id2 in regulating CD8 T cell responses to different levels of pathogen persistence (i.e. cleared vs. chronic latent vs. chronic overwhelming) using our new mouse models of infection by T. gondii ; 3. Identify E2A and Msc direct target genes required for CD8 T cell accumulation and differentiation during chronic infection through an shRNA-based screening. Overall, this work will shed light on the role of a new transcriptional regulator in CD8 T cell differentiation, will deepen our understanding of the molecular mechanisms controlling CD8 T cell responses against chronic infections, and finally will potentially uncover new therapeutic targets that could regulate CD8 T cell function in the context of antigen persistence as found in chronic infection but also in cancer and autoimmunity.