Rôle de la voie de signalisation Notch dans la réponse lymphocytaire T CD8 suite à une infection aiguë ou chronique

Following an infection, naïve CD8 T cells expand and differentiate into effectors able to eliminate the pathogen by direct cytolysis or by cytokine production. At the peak of the response, two populations of effectors are distinguishable: short-lived effector cells (SLECs) (CD127lowKLRG1highT-bethighEOMESlow) meant to die by apoptosis and memory precursor effector cells (MPECs) (CD127highKLRGlowT-betlowEOMEShigh) destined to survive as memory cells that will confer long-term protection. Thus, following activation, the CD8 T cell faces a binary cell fate decision. Furthermore, following cancer or chronic infection, persistence of antigen and sustained inflammation direct the effector to differentiate into an exhausted state. As the chronic infection progresses, CD8 T cells gain expression of inhibitory receptors leading to a progressive loss of their functionality and fail to differentiate into memory CD8 T cells. In-vivo blockade of the inhibitory receptors PD-1 during chronic infection or during cancer lead to gain of function of CD8 T cells and decreased viral load or tumor burden revealing a crucial role for this receptor in CD8 T cell exhaustion. Two subpopulations of exhausted CD8 T cells exist: the progenitor subset, which expresses low level of PD1, those cells maintain residual proliferative capacity and are still able to produce some cytokines such as IFN-gamma and TNF-alpha. The other subset corresponds to the terminally differentiated subset that coexpresses PD-1 along with another IR like CD160 and Lag-3. However, PD-1/PDL1 blockade will only reinvigorate cells expressing intermediate level of PD-1 (PD-1int). Thereby, a better understanding of the molecular mechanism by which CD8 T cell differentiates following acute or chronic infection will allow to 1) improve actual and future vaccination strategies to generate large numbers of highly functional memory CD8 T cells and 2) to counteract the exhausted phenotype of CD8 T cells following chronic infection or cancer. In CD8 T cell, it was known that Notch directly controls the expression of cytokines as interferon-gamma (IFN-g) and effector molecules as granzyme B (GrzmB) but also controls the expression of the transcription factors T-bet and EOMES. However, those transcription factors are known to be important for SLEC/MPEC differentiation. Indeed, T-bet is essential for SLEC generations while EOMES is highly expressed in MPEC. Moreover, T-bet expression support  the differentiation of progenitors while EOMES defines terminally differentiated CD8 T cell following chronic infection. We hypothesized that the Notch signaling pathway controls CD8 T cell differentiation following acute or chronic infection. Thereby, we used mice in which only mature CD8 T cells are deficient for Notch1 and Notch2. As, Notch3 and Notch4 are not expressed in CD8 T cells, this allows us to study the role of this pathway. Those mice were either vaccinated with OVApulsed dendritic cells or were acutely infected with Listeria monocytogenes expressing OVA (Lm-OVA) or were chronically infected with lymphocytic choriomeningitis virus (LCMVcl13). Following infection with Listeria monocytogenes (Lm) or vaccination with antigenpulsed dendritic cells, Notch deficiency induced a decrease in cytokines production but also a decreased SLEC generation along with a weaker expression of CD25, the high affinity chain of the IL-2 receptor without impairing memory generations. This defective SLEC generation is not due to a decreased expression of the transcription factors T-bet, EOMES or BLIMP-1. However, restoration of CD25 in Notch deficient T cells did not correct SLEC differentiation. These results suggest either that Notch regulates the expression of new players involved in SLEC differentiation or Notch signalling collaborates with already known actors of SLEC differentiation such as T-bet or Blimp-1. Following a chronic infection with LCMVcl13, Notch-deficient virus-specific CD8 T cells express higher levels of IRs and produce less cytokines and granzyme B when compared to Notch-sufficient CD8 T cells at days 15 and 30 post-infection. Moreover, Notch-deficient effectors preferentially differentiate into the terminal-progeny subset at the expense of the progenitor subset. As of consequence, Notch-deficient exhausted CD8 T cells cannot be reinvigorated following PD-1/PDL-1 blockade and cannot properly control viral load. In conclusion, we uncover a new role for the Notch signaling pathway controlling CD8 T cell differentiation following acute or chronic infection. This pathway controls SLEC generation following vaccination or acute infection while protecting CD8 T cell from exhaustion following a chronic infection.

Keyphrases: Lymphocyte T CD8, CD8 T cell, Notch signaling pathway, Acute infection, Infection chronic, Chronic Infection, T cell exhaustion