Modulation of invariant NKT cell activity by cytokines and receptors in human disease

Abstract

Invariant natural killer T (NKT) cells are innate-like lymphocytes with both immunoregulatory and cytotoxic functions that play a role as activators and regulators of the immune response in many diseases. NKT cells are believed to bridge the innate and adaptive immune systems by rapidly producing large quantities of cytokines after recognition of CD1d-presented lipid antigens. NKT cells can be divided into two phenotypically and functionally distinct subsets based on the expression of CD4. How the NKT cells and their subsets are regulated, and how they integrate signals from their environment to modulate immune responses is still not fully understood.

In this thesis I have investigated how the activity of NKT cells can be modulated by factors other than the TCR, such as cell surface receptors and cytokines. These investigations have been based on blood samples from healthy controls, and from patients suffering from HIV-1 infection or atopic eczema (AE). We have found that CD4- NKT cells are able to degranulate and kill target cells in an NKG2D-dependent but TCR-independent manner in response to NKG2D stimulus. Moreover, we have shown that NKG2D+ NKT cells frequently express perforin that polarizes toward NKG2D-ligand expressing tumor cells. These data demonstrate that the CD4-subset of human NKT cells can mediate direct lysis of CD1d-negative target cells upon NKG2D engagement. We have further characterized the phenotype and function of NKT cells in patients with chronic diseases. In patients with chronic HIV-1 infection, the CD4-NKT cell subset showed increased expression of the inhibitory programmed death-1 (PD-1) receptor, and displayed severe functional defects. However, the functional impairment was not caused by PD-1 expression per se because the defect could not be reversed by PD-1 blockade.

In addition, we have studied the effect of interleukin-2 (IL-2) on NKT cells and natural killer (NK) cells in patients with chronic HIV-1 infection. Material for this study was obtained from a longitudinally study, where administration of IL-2 was added to the antiretroviral treatment (ART) for one year. We found that NKT cells and NK cells responded with different kinetics and in different ways to the IL-2 administration. The NKT cells responded with a gradual numerical increase, but with no significant functional changes. NK cells responded rapidly with an expansion of the cytotoxic CD56dim NK cell subset and increased IFN-γ production. However, the effects of IL-2 on these cells were generally not sustained post treatment. NKT cells were also studied in the chronic inflammatory skin disease AE where the patients have elevated levels of plasma IL-18. Our data provide evidence that IL-18 is a potent activator of human NKT cells promoting an acute pro-inflammatory CD1d-dependent response, even in the absence of exogenous lipid antigens. Interestingly, chronic exposure of NKT cells to IL-18 is inhibitory and skews the NKT cell pool by selectively suppressing the proliferation of CD4+ NKT cells. Importantly, our in vitro data are reflected in AE patients where reduced numbers of CD4+ NKT cells are associated with elevated levels of IL-18 and disease severity.

In conclusion, the work presented here contributes to our understanding of the function and role of NKT cells in human diseases including infections and allergies.