Role of gamma-secretase in human umbilical-cord derived mesenchymal stem cell mediated suppression of NK cell cytotoxicity.

BACKGROUND: Mesenchymal stem cells (MSCs) are increasingly considered to be used as biological immunosuppressants in hematopoietic stem cell transplantation (HSCT). In the early reconstitution phase following HSCT, natural killer (NK) cells represent the major lymphocyte population in peripheral blood and display graft-vs-leukemia (GvL) effects. The functional interactions between NK cells and MSCs have the potential to influence the leukemia relapse rate after HSCT. Until date, MSC-NK cell interaction studies are largely focussed on bone marrow derived (BM)-MSCs. Umbilical cord derived (UC)-MSCs might be an alternative source of therapeutic MSCs. Thus, we studied the interaction of UC-MSCs with unstimulated allogeneic NK cells. RESULTS: UC-MSCs could potently suppress NK cell cytotoxicity in overnight cultures via soluble factors. The main soluble immunosuppressant was identified as prostaglandin (PG)-E2. Maximal PGE2 release involved IL-1ß priming of MSCs after close contact between the NK cells and UC-MSCs. Interestingly, blocking gamma-secretase activation alleviated the immunosuppression by controlling PGE2 production. IL-1 receptor activation and subsequent downstream signalling events were found to require gamma-secretase activity. CONCLUSION: Although the role of PGE2 in NK cell-MSC has been reported, the requirement of cell-cell contact for PGE2 induced immunosuppression remained unexplained. Our findings shed light on this puzzling observation and identify new players in the NK cell-MSC crosstalk.

IL-1ß limits the extent of human 6-sulfo LacNAc dendritic cell (slanDC)-mediated NK cell activation and regulates CD95-induced apoptosis.

To function optimally, human blood natural killer (NK) cells need to communicate with other immune cells. Previously, it has been shown that NK cells communicate with 6-sulfo LacNAc dendritic cells (slanDCs), which are able to stimulate NK cells in vitro. In this study, we investigated how slanDCs regulate the level of NK cell activation. The secretion of interleukin (IL)-1ß by slanDCs during coculture with NK cells increased as a result of signaling via intercellular adhesion molecule-1 on slanDCs following its interaction with lymphocyte function-associated antigen-1 on NK cells. IL-1ß induced the expression of Fas receptor (CD95) on NK cells. The binding of Fas ligand (CD178) to CD95 induced the apoptosis of activated NK cells. Moreover, IL-1ß also induced increased cyclooxygenase-2 expression in slanDCs, which in turn enabled the cells to secrete prostaglandin (PG)-E2. Consequently, PGE2 acted as a suppressing agent, tuning down the activation level of NK cells. In summary, IL-1ß limits the level of NK cell activation by inducing apoptosis and suppression as a homeostatic regulatory function.

Terminal Differentiation of CD56(dim)CD16(+) Natural Killer Cells Is Associated with Increase in Natural Killer Cell Frequencies After Antiretroviral Treatment in HIV-1 Infection.

HIV-1 infection results in immunological abnormalities of natural killer (NK) cells such as disturbed distribution of NK cell subsets and downmodulation of activating and upregulation of inhibitory receptors thereby diminishing NK cell killing capacity and cytokine secretion. Antiretroviral treatment (ART) is known to restore phenotype and functions of NK cells. However, the effects of ART on NK cell terminal differentiation, activation, and disturbed distribution have not been studied yet longitudinally. Here, we analyzed the effects of ART on these parameters of peripheral blood NK cells in a longitudinal as well as in a cross-sectional study. We observed that expanded CD56(-)CD16(+) NK cell frequency is inversely correlated with the frequency of CD56(dim)CD16(+) NK cells in treatment-naive HIV-1 patients. Loss of CD56(dim)CD16(+) and expansion of CD56(-)CD16(+) NK cells again restore to the levels of healthy controls after ART. Enhanced immune activation of different NK cell subsets is partially restored after ART. Terminal differentiation of CD56(dim)CD16(+) NK cells is enhanced after ART as measured by CD57 expression. Frequencies of CD57(+)CD56(dim)CD16(+) NK cells are directly correlated with the frequencies of total NK cells suggesting that an increase in the frequencies of CD57(+)CD56(dim)CD16(+) NK cells is reflected by increased frequencies of total NK cells after ART. Taken together these data demonstrate that ART has an effect on the immune restoration of NK cells and is enhanced in the terminal differentiation of CD56(dim)CD16(+) NK cells, which is associated with increased frequencies of total NK cells after ART.

Human Umbilical Cord-Derived Mesenchymal Stem Cells Utilize Activin-A to Suppress Interferon-γ Production by Natural Killer Cells.

Following allogeneic hematopoietic stem cell transplantation (HSCT), interferon (IFN)-γ levels in the recipient's body can strongly influence the clinical outcome. Human umbilical cord-derived mesenchymal stem cells (UC-MSCs) are lucrative as biological tolerance-inducers in HSCT settings. Hence, we studied the molecular mechanism of how UC-MSCs influence natural killer (NK) cell-mediated IFN-γ production. Allogeneic NK cells were cultured in direct contact with UC-MSCs or cell-free supernatants from mesenchymal stem cell (MSC) cultures (MSC-conditioned media). We found that soluble factors secreted by UC-MSCs strongly suppressed interleukin (IL)-12/IL-18-induced IFN-γ production by NK cells by reducing phosphorylation of STAT4, NF-κB, as well as T-bet activity. UC-MSCs secreted considerable amounts of activin-A, which could suppress IFN-γ production by NK cells. Neutralization of activin-A in MSC-conditioned media significantly abrogated their suppressive abilities. Till date, multiple groups have reported that prostaglandin (PG)-E2 produced by MSCs can suppress NK cell functions. Indeed, we found that inhibition of PGE2 production by MSCs could also significantly restore IFN-γ production. However, the effects of activin-A and PGE2 were not cumulative. To the best of our knowledge, we are first to report the role of activin-A in MSC-mediated suppression of IFN-γ production by NK cells.