Elucidating the Role of Different Human nTreg Subsets in Glioblastoma Multiforme

Clare Baecher-Allan, Ph.D.

Brigham and Women's Hospital

Neurology Department
Funded in December, 2008: $400000 for 3 years
LAY SUMMARY . ABSTRACT . BIOGRAPHY .

LAY SUMMARY

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How Deadly Brain Tumor Cells Interact With the Subsets of Immune Cells

Dr. Baecher-Allan’s research will explore how deadly brain tumor cells interact with the subsets of immune cells that regulate immune responses and suppress tumor-attacking immune T cells.   

Glioblastoma Multiforme (GBM) tumors are deadly malignant brain tumors that are resistant to all current forms of treatment.  Few patients live for more than a year following diagnosis.  GBM tumor cells proliferate quickly and relentlessly, and take over normal brain cells because the patients’ immune systems are highly suppressed.  In fact, there is growing evidence that this suppression is directed by immune “regulatory” cells that reside in GBM tumors.  Ordinarily, immune regulatory cells curtail attacks that are mounted by faulty immune T cells against the body’s own tissues.  In GBM patients, however, these immune regulatory cells, called “natural T regulatory cells” or “nTregs” are found in GBM tumors in large numbers.  Evidence suggests that these regulatory cells suppress the ability of immune T cells to attack tumors.  A substance called IL-10, which is strongly produced in the GBM tumors, appears to enhance or inhibit the growth and activity of different subsets of regulatory T cells.  Potentially a double edged sword, IL-10 also suppresses immune T cells from attacking the tumor.

Three subsets of regulatory immune cells have been identified by Dr. Baecher-Allan and colleagues in GBM patients.  Her initial evidence indicates that one of the three subsets (called IL-7R-) is particularly adept at suppressing the tumor-infiltrating immune T cells that are poised to attack the tumor, limiting the T cells’ abilities to secrete a toxic substance (called Granzyme B) that they use to kill tumor cells.  In contrast, a second regulatory subset (called IL-7R+) produces a substance (called IL-17) that may actually enhance tumor growth.  She hypothesizes that either one of these subsets of regulatory immune cells may be highly responsible for the extremely poor prognosis in GBM patients and the tumor’s resistance to drug therapy.

She will undertake a series of experiments to test this hypothesis and determine (1) which type of regulatory immune subset is most prevalent in GBM tumors as compared to other types of tumors that can occur in the brain, but have better prognosis than GBM tumors; (2) how the regulatory subset inhibits anti-tumor T cells from producing Granzyme B to kill tumor cells; and (3) whether GBM tumors disproportionately alter this specific regulatory subset’s suppressive activities.

ABSTRACT

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Elucidating the Role of Different Human nTreg Subsets in Glioblastoma Multiforme

Glioblastoma Multiforme (GBM) tumors are the most aggressive malignant glioma and are highly heterogeneous and refractory to current modes of treatment.  The immune system in cancer patients has been shown to be in a highly suppressed state, making it difficult to elicit anti-tumor responses.  In addition to tumor production of anti-inflammatory TGF, PGE2 and IL-10 molecules, numerous reports have documented that the circulation and tumors in cancer patients are highly enriched for the immunosuppressive population of T cells referred to as natural regulatory T cells or nTregs.  These intra-tumor nTregs can inhibit the induction of anti-tumor responsive cells as well as the activity of matured anti-tumor effector cells, thereby decreasing the efficacy of both arms of immune-targeted therapies.

We have recently demonstrated that there are three functionally distinct subsets of CD4+CD25highFoxP3high human nTregs that differ in their ex vivo expression of HLA Class II and CD127 (the IL-7 receptor, IL-7R).  These subsets exhibit marked differences in their ability to suppress, proliferate, produce IL-10 and IL-17, and express granzyme B (GzmB), a protease typically found in the cytotoxic granules of NK and CD8 T cells and implicated in the killing of tumor cells.  Our initial studies indicate that GBM tumors are significantly enriched for the presence of CD25high nTregs that express HLA Class II proteins (“DR+nTregs”) as compared to meningiomas, or healthy donor and patient peripheral blood.  Our hypothesis is that the preferential enrichment of DR+nTregs in GBMs may contribute to their extremely poor prognosis and resistance to therapy, as peripheral blood derived DR+nTregs are the most potent nTreg subset.  However, the identity of the tumor-enriched nTregs needs to be supported by functional criteria as strong activation can also induce the expression of HLA Class II by “IL-7R+” or “IL-7Rneg” nTregs, which differ in their capacity to secrete the inhibitory cytokine IL-10, and to inhibit the expression of GzmB in effector T cells.  Additional preliminary data indicate that IL-10 enhances the proliferation of all three types of nTregs.  Thus, as IL-10 is strongly produced in GBM tumors, these data may suggest a mechanism for the observed tumor-associated increase in nTregs.

The studies outlined in this proposal will determine both the ex vivo frequency and gene expression signatures of the functionally distinct types of nTregs that are FACS-sorted from different grade or types of CNS tumors that include GBM, meningiomas, pilocytic astrocytomas, and CNS metastases, and from PBLs from these same patients as well as healthy donors.  Subsequently, we will examine the functional capacity of the distinct nTreg subsets isolated from these ex vivo tumor samples to determine how they differ in frequency or activity from those isolated from healthy donor or from patient peripheral blood.  Due to the importance of Granzyme B in the effector arm of anti-tumor immunity, we will identify molecules and pathways that are utilized by nTregs to inhibit its expression in effector T cells.  Lastly, using in vitro multi-component co-culture models, we will examine how communication between the nTreg subsets, and effector T cells is modulated by the presence of GBM tumor cells.  The ultimate goal of these studies is to identify therapeutic targets to selectively inactivate the specific subsets of nTregs that may be differentially associated with particular types of tumors, and provide an alternative to the current nTreg-targeted treatments that kill all IL-2R expressing cells which can also deplete beneficial anti-tumor effector T cells.

INVESTIGATOR BIOGRAPHIES

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Clare Baecher-Allan, Ph.D.

Clare M Baecher-Allan, Ph.D., is an assistant professor in the neurology department at Brigham and Women’s Hospital and Harvard Medical School in Boston, MA.  After having received her doctorate for her immunology research at the University of Rochester in New York, she expanded her training by performing three distinct postdoctoral fellowships centered on genetics, B cell biology and lastly, human immunology.  Having always been interested in studying human disease-based immunology, her current research focuses on understanding how the cells of the immune system regulate their own activities.

The goal of her research is to understand the mechanisms by which a recently identified population of T cells, known as regulatory T cells, inhibits the activation of other T cells and other types of hematopoietic cells.  Having studied mouse models of cancer and transplantation during her training, her focus for the past eight years has been on the study of human cells.  She has been working to understand how potentially aberrant processes and loss of control by immune-regulating cells can result in the immune system actually potentiating diseases rather than inhibiting them. 

Although her research has primarily focused on identifying regulatory T cells and mechanisms by which deficiencies in the activity of these cells may lead to autoimmunity in patients with multiple sclerosis, she is now applying her efforts to understanding how cancers suppress immune responses.  Specifically, she is studying how cancers may actually stimulate regulatory T cell activity to hinder tumor-specific immune responses and allow tumors to escape immune surveillance.

Her studies on the basic biology of these rare regulatory T cells, isolated from the circulation of patients and healthy individuals, have helped to advance the relatively new field of “Treg” biology.  Her continued efforts in trying to understand how we can manipulate the activity of these regulating T cells will likely be applicable to the design of therapeutics for the treating of various types of diseases and cancers.