Unexpected Role for Granzyme K in CD56bright NK Cell-Mediated Immunoregulation of Multiple Sclerosis

Abstract

Functional NK cell deficiencies are associated with autoimmune diseases, including multiple sclerosis (MS). NK cells can promote or inhibit adaptive immunity via either cytokine production or cytotoxicity towards immature dendritic cells and activated T cells. In humans, this immunoregulatory role resides in the CD56bright NK cell subset, which is selectively expanded by daclizumab, a CD25-blocking antibody that suppresses MS-associated inflammation. The objective of this study was to investigate the molecular mechanisms underlying the cytotoxicity of NK cells toward activated T cells. We demonstrated that NK cells induce caspase-independent apoptosis that requires NK cell degranulation and causes mitochondrial dysfunction in activated T cells. While both granzymes A (GrA) and K (GrK) can mediate this form of apoptosis, quantitatively we observed preferential transfer of GrK to target cells. Consequently, gene silencing of GrK in the NK-92 cell line, which retains functional characteristics of CD56bright NK cells, profoundly inhibited the ability of NK-92 to kill activated syngeneic T cells. Finally we demonstrated that daclizumab treatment significantly enhanced this newly defined mechanism of cytotoxicity by CD56bright NK cells. Our study represents the first example of the important physiological role GrK plays in immunoregulation of adaptive immunity in humans and indicates that therapeutic exploitation of this pathway is beneficial in controlling autoimmunity.

In Vivo Maintenance of Human Regulatory T Cells during CD25 Blockade

“…In addition, daclizumab has profound effects on cells of the innate immune system, including an expansion of CD56 bright NK cells (20,21,49,50). It has been proposed that CD56 bright NK cells play an immunoregulatory role through a granzyme K-mediated targeting of activated effector T cells (49). Thus, expansion of CD56 bright NK cells might contribute to the efficacy of daclizumab and the maintenance of immune tolerance by keeping autoreactive T cells in check.…”

“…Reduction in Tregs must be considered in the context of the effects of CD25 blockade on effector T cells. In addition, daclizumab has profound effects on cells of the innate immune system, including an expansion of CD56 bright NK cells (20,21,49,50). It has been proposed that CD56 bright NK cells play an immunoregulatory role through a granzyme K-mediated targeting of activated effector T cells (49).…”

Innate‐like CD8+ T‐cells and NK cells: converging functions and phenotypes

“…48, 100 However, both CD56 bright NK cells51 and CD161++ CD8+ T‐cells (including the MAIT cells)90 can upregulate GrB and perforin, and become efficient killer cells, after activation. For example, the CD56 bright NK cell population can kill activated autologous CD4+ T‐cells in MS 101, 102. Furthermore, immature DCs are killed by CD56 bright NK cells in lymph nodes in a TRAIL‐dependent manner,103, 104 and IL‐2‐activated peripheral blood CD56 bright NK cells can become efficient killers 50, 51, 52.…”

“…For instance, the expansion of CD56 bright NK cells in patients with MS, receiving daclizumab treatment, is associated with better prognosis, due to their ability to kill pathogenic, highly activated CD4+ T‐cells 102. Cytotoxicity is mainly mediated through the expression of GrK101 and NKG2D 135. MAIT cells are also enriched in MS lesions and indeed early papers suggested that they play a regulatory role 136, 137.…”

Human CD56bright NK Cells: An Update

“…Indeed, the expanded CD56 bright NK cells after treatment with CD25-blocking Ab kill autologous activated T cells in vitro (105,106). The mechanisms behind this include the degranulation of granzymes A and K, followed by activation of a caspaseindependent apoptosis that induces a mitochondrial dysfunction in activated T cells (107). Furthermore, IL-2/IL-2Ab therapy (mimicking daclizumab treatment) enhances the production of IFN-g and MIP-1a by CD56…”