Invariant Natural Killer T Cells in Chronic Lymphocytic Leukaemia

Abstract

Invariant natural killer T (iNKT) cells recognise glycolipid antigens, such as the synthetic ligand alpha galactosylceramide (a-GalCer), in the context of CD1d. Upon recognition of a-GalCer on CD1d, iNKT cells become activated and rapidly produce cytokines. Recruitment of iNKT cells with a-GalCer results in potent antitumour activity in some pre-clinical cancer models. Effector mechanisms include direct cytotoxicity of iNKT cells against CD1d-expressing tumours in the presence of a-GalCer, transactivation of natural killer cells by iNKT cell-derived interferon gamma, and iNKT cell-induced maturation and activation of dendritic cells (DCs), leading to enhanced T cell responses to DC-presented peptides.

Chronic lymphocytic leukaemia (CLL) is a clonal malignancy of B lymphocytes. Chemotherapy induces remission in most patients, but most eventually relapse. Allogeneic stem cell transplantation can be curative, but is not available to most patients due to advanced age or co-morbidities. Thus, CLL is an attractive candidate for cancer immunotherapy. This thesis aims to assess the iNKT cell/CD1d axis of patients, and to explore the possibility of exploiting iNKT cells for the immunotherapy of CLL.

Peripheral blood mononuclear cells (PBMCs) were isolated from patients with CLL, and from healthy age-matched controls, and analysed by flow cytometry. Absolute number and phenotype of circulating iNKT cells was similar in patients and controls, although patients exhibited a relative reduction in iNKT cells due to expansion of other T cell populations. iNKT cell frequency did not correlate with disease stage or with subsequent progression-free survival in patients with untreated CLL. Expression of CD1d on dendritic cells and monocytes from patients with CLL was similar to controls.

The cytokine profile of patient iNKT cells was similar to that of controls. In vitro proliferation of invariant natural killer T (iNKT) cells from patients with CLL was preserved, and iNKT cell lines generated from patients exhibited cytokine and cytotoxicity profiles similar to those from healthy controls, producing both Th1- and Th2-type cytokines, and lysing a target cell in a CD1d- and a-GalCer-dependent manner. Lysis of autologous CLL cells by iNKT cell lines was inefficient.

In vitro vaccine recall responses were enhanced by a-GalCer in PBMCs containing high frequencies of iNKT cells. The treatment of leukaemic cells with a-GalCer enhanced their ability to stimulate proliferation of allogeneic PBMCs from healthy donors in vitro, largely due to iNKT cell proliferation. Leukaemic cells treated with a-GalCer induced proliferation of autologous iNKT cells, and a-GalCer treatment also led to enhanced proliferation of ‘conventional’ T cells.

These results indicate that the iNKT cell/CD1d axis is largely intact in patients with CLL and suggest that if low iNKT cell frequencies can be overcome, the adjuvant activity of iNKT cells might be exploited in cellular immunotherapy of CLL, for example by employing a-GalCer-pulsed leukaemic cells as a whole tumour vaccine.