Immunomodulatory Properties of Mesenchymal Stromal Cells: Still Unresolved "Yin and Yang".

Mesenchymal stromal cells (MSC) are mesodermal elements characterized by the ability to differentiate into several types of cells present mainly in connective tissues. They play a key function in tissue homeostasis and repair. Furthermore, they exert a strong effect on both innate and adaptive immune response. The main current of thought considers MSC as strong inhibitors of the immune system. Indeed, the first description of MSC immunomodulation pointed out their inability to induce alloimmune responses and their veto effects on mixed lymphocyte reactions. This inhibition appears to be mediated both by direct MSC interaction with immune cells and by soluble factors. Unfortunately, evidence to support this notion comes almost exclusively from in vitro experiments. In complex experimental systems, it has been shown that MSC can exert immunosuppressive effects also in vivo, either in murine models or in transplanted patients to avoid the graft versus host disease. However, it is still debated how the small number of administered MSC can regulate efficiently a large number of host effector lymphocytes. In addition, some reports in the literature indicate that MSC can trigger rather than inhibit lymphocyte activation when a very low number of MSC are co-cultured with lymphocytes. This would imply that the ratio between the number of MSC and immune cells is a key point to forecast whether MSC will inhibit or activate the immune system. Herein, we discuss the conflicting results reported on the immunomodulatory effects of MSC to define which features are relevant to understand their behavior and cross-talk with immune cells.

Down regulation of human natural killer cell-mediated cytolysis induced by blood transfusion: role of transforming growth factor-ß(1), soluble Fas ligand, and soluble Class I human leukocyte antigen.

BACKGROUND: Human natural killer (NK) cells are thought to play a role in antiviral response and tumor immune surveillance. The molecular mechanisms of down regulation of NK-cell activity observed after red blood cell (RBC) transfusion is still undefined. STUDY DESIGN AND METHODS: Both effects of blood transfusion (ex vivo) and supernatants (SNs) derived from RBC units unstored (RBC-0) or stored for 5 or 30 days (RBC-5 or -30, respectively) in vitro were analyzed on NK cell-mediated cytolytic activity. RESULTS: We have found that NK cells isolated from transfused patients on Day 3 lysed the NK-sensitive target cells K562 to a lesser extent than before transfusion. This down regulation of NK-cell activation was evident also for NK-cell killing mediated through the engagement of NK cell-activating receptors as NKG2D, NKp30, NKp46, and CD16. Transfused patients reacquired NK cell-mediated cytolytic activity from Day 5 to Day 7 after transfusion. SN from RBC-30, but not from RBC-0 or RBC-5, strongly inhibited the generation of lymphokine-activated killer (LAK) cells and lysis of the NK-resistant target cell Jurkat in a dose-dependent manner. Transforming growth factor-ß1 (TGF-ß1) blocking antibodies partially restored the generation of LAK activity. In addition, the depletion of both soluble Class I human leukocyte antigens (sHLA-I) and soluble Fas ligand (sFasL) from SN of RBC-30 completely restored the generation of LAK activity. CONCLUSIONS: Altogether, these findings would support the idea that blood transfusion-mediated down regulation of NK-cell activity is mediated by sHLA-I, sFasL, and TGF-ß1.

Engagement of CD31 delivers an activating signal that contributes to the survival of chronic lymphocytic leukaemia cells.

The present study showed that engagement of CD31 delivers a survival signal in chronic lymphocytic leukaemia (CLL) cells. We describe two groups of CLL, showing different kinetics of apoptosis in vitro and distinct ratios between anti-apoptotic and pro-apoptotic proteins: CLL-I displayed low Bcl-x(L) /Bax and Bcl-2/Bax ratio and underwent rapid apoptosis in vitro; CLL-II had high Bcl-x(L) /Bax and Bcl-2/Bax ratio and were resistant to apoptosis for several days. Nurse-like cells, expressing vimentin, CD68 and CD31 were detected mainly in CLL-II cultures. Of note, CD31 cross-linking, obtained with a specific monoclonal antibody (mAb), induced phosphatidylinositol-3-kinase-dependent Akt phosphorylation and nuclear translocation of the nuclear factor-kBp65 and p52 subunits in both CLL groups, leading to upregulation of Bcl-2 and Bcl-x(L) transcription and increased cell survival. Binding to CD31(+) stable transfectants, could also deliver an anti-apoptotic signal in B cells of both CLL-I and CLL-II, increasing the Bcl-2 and Bcl-x(L) protein content, regardless the expression of CD38. On the other hand, the addition of the F(ab')2 (that is unable to oligomerize the target molecule) of the anti-CD31 mAb prevented these effects. These data suggest that the CD31 adhesion system may play a role also in vivo in maintaining CLL survival.

PECAM-1, apoptosis and CD34+ precursors.

Apoptosis is a physiological process that controls tissue homeostasis, in combination with survival signals delivered by distinct receptors that bind hormones, growth factors or extracellular matrix components. The extrinsic pathway of apoptosis is due to the triggering of death receptors and the activation of the caspase cascade; the intrinsic pathway is due to withdrawal of growth factors and mainly related to mitochondrial metabolism. The choice between survival or apoptosis, which is the result of such different integrated environmental signals, is crucial for the maintainance of bone marrow reservoir of hematopoietic precursors (HPC). CD34+ HPC can receive multiple survival signals during homing and maturation, due to different interactions with adhesion molecules expressed on endothelial and bone marrow stromal cells, proteins of the extracellular matrix and chemokines or growth factors. Among them, the signal delivered via platelet endothelial cell adhesion molecule-1 (PECAM-1) seems to contribute to the resistance of this cell population to starvation, and it is related to the maintainance of mitochondrial metabolism. Indeed, this molecule, originally described as an adhesion receptor belonging to the immunoglobulin superfamily, capable of homophilic and heterophilic interactions, turned out to be a signalling molecule, containing an immunoreceptor tyrosine-based inhibitory motifs (ITIM) within its cytoplasmic domain. In particular, it has been shown that PECAM-1 binds to different kinases and phosphatases, including the phosphatidylinositide-3-kinase that phosphorylates Akt, which, in turn can upregulate transcription and function of antiapoptotic proteins, such as Bcl-2 and Bcl-x or A1, responsible for the rescue from mitochondrial apoptosis. The possible role of PECAM-1 engagement in the prevention of starvation-induced apoptosis of HPC precursors and in the maintainance of their survival is discussed.