Frederick Masson

The transcription factors Blimp-1 and IRF4 jointly control the differentiation and function of effector regulatory T cells

Regulatory T cells (Treg cells) are required for peripheral tolerance. Evidence indicates that Treg cells can adopt specialized differentiation programs in the periphery that are controlled by transcription factors usually associated with helper T cell differentiation. Here we demonstrate that expression of the transcription factor Blimp-1 defined a population of Treg cells that localized mainly to mucosal sites and produced IL-10. Blimp-1 was required for IL-10 production by these cells and for their tissue homeostasis. We provide evidence that the transcription factor IRF4, but not the transcription factor T-bet, was essential for Blimp-1 expression and for the differentiation of all effector Treg cells. Thus, our study defines a differentiation pathway that leads to the acquisition of Treg cell effector functions and requires both IRF4 and Blimp-1.

Id2 expression delineates differential checkpoints in the genetic program of CD8α+ and CD103+ dendritic cell lineages

Dendritic cells (DCs) have critical roles in the induction of the adaptive immune response. The transcription factors Id2, Batf3 and Irf‐8 are required for many aspects of murine DC differentiation including development of CD8α+ and CD103+ DCs. How they regulate DC subset specification is not completely understood. Using an Id2‐GFP reporter system, we show that Id2 is broadly expressed in all cDC subsets with the highest expression in CD103+ and CD8α+ lineages. Notably, CD103+ DCs were the only DC able to constitutively cross‐present cell‐associated antigens in vitro. Irf‐8 deficiency affected loss of development of virtually all conventional DCs (cDCs) while Batf3 deficiency resulted in the development of Sirp‐α− DCs that had impaired survival. Exposure to GM‐CSF during differentiation induced expression of CD103 in Id2‐GFP+ DCs. It did not restore cross‐presenting capacity to Batf3−/− or CD103−Sirp‐α−DCs in vitro. Thus, Irf‐8 and Batf3 regulate distinct stages in DC differentiation during the development of cDCs. Genetic mapping DC subset differentiation using Id2‐GFP may have broad implications in understanding the interplay of DC subsets during protective and pathological immune responses.

TCF-1 Controls ILC2 and NKp46+RORγt+ Innate Lymphocyte Differentiation and Protection in Intestinal Inflammation

Innate lymphocyte populations play a central role in conferring protective immunity at the mucosal frontier. In this study, we demonstrate that T cell factor 1 (TCF-1; encoded by Tcf7), a transcription factor also important for NK and T cell differentiation, is expressed by multiple innate lymphoid cell (ILC) subsets, including GATA3+ nuocytes (ILC2) and NKp46+ ILCs (ILC3), which confer protection against lung and intestinal inflammation. TCF-1 was intrinsically required for the differentiation of both ILC2 and NKp46+ ILC3. Loss of TCF-1 expression impaired the capacity of these ILC subsets to produce IL-5, IL-13, and IL-22 and resulted in crippled responses to intestinal infection with Citrobacter rodentium. Furthermore, a reduction in T-bet expression required for Notch-2–dependent development of NKp46+ ILC3 showed a dose-dependent reduction in TCF-1 expression. Collectively, our findings demonstrate an essential requirement for TCF-1 in ILC2 differentiation and reveal a link among Tcf7, Notch, and Tbx21 in NKp46+ ILC3 development.

Brain Microenvironment Promotes the Final Functional Maturation of Tumor-Specific Effector CD8+ T Cells

During the priming phase of an antitumor immune response, CD8+ T cells undergo a program of differentiation driven by professional APCs in secondary lymphoid organs. This leads to clonal expansion and acquisition both of effector functions and a specific adhesion molecule pattern. Whether this program can be reshaped during the effector phase to adapt to the effector site microenvironment is unknown. We investigated this in murine brain tumor models using adoptive transfer of tumor-specific CD8+ T cells, and in spontaneous immune responses of patients with malignant glioma. Our data show proliferation of Ag-experienced tumor-specific T cells within the brain parenchyma. Moreover, CD8+ T cells further differentiated in the brain, exhibiting enhanced IFN-γ and granzyme B expression and induction of αE(CD103)β7 integrin. This unexpected integrin expression identified a subpopulation of CD8+ T cells conditioned by the brain microenvironment and also had functional consequences: αE(CD103)β7-expressing CD8+ T cells had enhanced retention in the brain. These findings were further investigated for CD8+ T cells infiltrating human malignant glioma; CD8+ T cells expressed αE(CD103)β7 integrin and granzyme B as in the murine models. Overall, our data indicate that the effector site plays an active role in shaping the effector phase of tumor immunity. The potential for local expansion and functional reprogramming should be considered when optimizing future immunotherapies for regional tumor control.

Cutting Edge: Cross-Presentation as a Mechanism for Efficient Recruitment of Tumor-Specific CTL to the Brain

The number and localization of effector cells to the tumor site are crucial elements for immune rejection of solid tumors. However, for cerebral malignancies, antitumor responses need to be finely tuned to avoid neuropathologic consequences. In this study, we determine factors that regulate CTL localization and tumoricidal function after intracerebral implantation of tumors expressing model Ag. H-2bxd mice implanted with a CW3+ murine glioma lacking H-2Kd molecules necessary to present the CW3170–179 epitope demonstrate cross-priming of H-2Kd-restricted CTL, and moreover, Ag-dependent accumulation of functional H-2Kd/CW3170–179-specific CTL within the tumor bed. This implicates a role for cross-presentation not only in priming, but also in retention of fully differentiated CTL in the tumor stroma at the effector stage of the response. Modulating cross-presentation of Ag may be the key in regulating specific immune responses in the brain: either by augmenting protective responses or by down-modulating destructive autoimmune reactions.

Immune infiltration of spontaneous mouse astrocytomas is dominated by immunosuppressive cells from early stages of tumor development

Immune infiltration of advanced human gliomas has been shown, but it is doubtful whether these immune cells affect tumor progression. It could be hypothesized that this infiltrate reflects recently recruited immune cells that are immediately overwhelmed by a high tumor burden. Alternatively, if there is earlier immune detection and infiltration of the tumor, the question arises as to when antitumor competency is lost. To address these issues, we analyzed a transgenic mouse model of spontaneous astrocytoma (GFAP-V(12)HA-ras mice), which allows the study of immune interactions with developing glioma, even at early asymptomatic stages. T cells, including a significant proportion of Tregs, are already present in the brain before symptoms develop, followed later by macrophages, natural killer cells, and dendritic cells. The effector potential of CD8 T-cells is defective, with the absence of granzyme B expression and low expression of IFN-gamma, tumor necrosis factor, and interleukin 2. Overall, our results show an early defective endogenous immune response to gliomas, and local accumulation of immunosuppressive cells at the tumor site. Thus, the antiglioma response is not simply overwhelmed at advanced stages of tumor growth, but is counterbalanced by an inhibitory microenvironment from the outset. Nevertheless, we determined that effector molecule expression (granzyme B, IFN-gamma) by brain-infiltrating CD8 T-cells could be enhanced, despite this unfavorable milieu, by strong immune stimuli. This potential to modulate the strong imbalance in local antiglioma immunity is encouraging for the development and optimization of future glioma immunotherapies.

A molecular threshold for effector CD8(+) T cell differentiation controlled by transcription factors Blimp-1 and T-bet.

T cell responses are guided by cytokines that induce transcriptional regulators, which ultimately control differentiation of effector and memory T cells. However, it is unknown how the activities of these molecular regulators are coordinated and integrated during the differentiation process. Using genetic approaches and transcriptional profiling of antigen-specific CD8+ T cells, we reveal a common program of effector differentiation that is regulated by IL-2 and IL-12 signaling and the combined activities of the transcriptional regulators Blimp-1 and T-bet. The loss of both T-bet and Blimp-1 leads to abrogated cytotoxic function and ectopic IL-17 production in CD8+ T cells. Overall, our data reveal two major overlapping pathways of effector differentiation governed by the availability of Blimp-1 and T-bet and suggest a model for cytokine-induced transcriptional changes that combine, quantitatively and qualitatively, to promote robust effector CD8+ T cell differentiation.

Fas ligand-mediated immune surveillance by T cells is essential for the control of spontaneous B cell lymphomas

Loss of function of the tumor suppressor gene PRDM1 (also known as BLIMP1) or deregulated expression of the oncogene BCL6 occurs in a large proportion of diffuse large B cell lymphoma (DLBCL) cases. However, targeted mutation of either gene in mice leads to only slow and infrequent development of malignant lymphoma, and despite frequent mutation of BCL6 in activated B cells of healthy individuals, lymphoma development is rare. Here we show that T cells prevent the development of overt lymphoma in mice caused by Blimp1 deficiency or overexpression of Bcl6 in the B cell lineage. Impairment of T cell control results in rapid development of DLBCL-like disease, which can be eradicated by polyclonal CD8+ T cells in a T cell receptor–, CD28- and Fas ligand–dependent manner. Thus, malignant transformation of mature B cells requires mutations that impair intrinsic differentiation processes and permit escape from T cell–mediated tumor surveillance.

Id2-Mediated Inhibition of E2A Represses Memory CD8+ T Cell Differentiation

The transcription factor inhibitor of DNA binding (Id)2 modulates T cell fate decisions, but the molecular mechanism underpinning this regulation is unclear. In this study we show that loss of Id2 cripples effector differentiation and instead programs CD8+ T cells to adopt a memory fate with increased Eomesodermin and Tcf7 expression. We demonstrate that Id2 restrains CD8+ T cell memory differentiation by inhibiting E2A-mediated direct activation of Tcf7 and that Id2 expression level mirrors T cell memory recall capacity. As a result of the defective effector differentiation, Id2-deficient CD8+ T cells fail to induce sufficient Tbx21 expression to generate short-lived effector CD8+ T cells. Our findings reveal that the Id2/E2A axis orchestrates T cell differentiation through the induction or repression of downstream transcription factors essential for effector and memory T cell differentiation.

Dendritic cells: driving the differentiation programme of T cells in viral infections.

Protective immunity against viral pathogens depends on the generation and maintenance of a small population of memory CD8+ T cells. Successful memory cell generation begins with early interactions between naïve T cell and dendritic cells (DCs) within the inflammatory milieu of the secondary lymphoid tissues. Recent insights into the role of different populations of DCs, and kinetics of antigen presentation, during viral infections have helped to understand how DCs can shape the immune response. Here, we review the recent progress that has been made towards defining how specific DC subsets drive effector CD8+ T‐cell expansion and differentiation into memory cells. Further, we endeavour to examine how the molecular signals imparted by DCs coordinate to generate protective CD8+ T‐cell immunity.