Asthma is an inflammatory disease of the airways affecting 16 million people in the United States alone and more than 300 million worldwide . In asthma, the lungs are invaded by a variety of inflammatory cells, including eosinophils and lymphocytes. These cells, in addition to mast cells that are resident in the lungs, secrete cytokines and chemokines that elicit constriction of the bronchi and secretion of mucus.
The current model for asthma suggests an abnormal shift in the Th1/Th2 balance in favor of Th2 cells and the production of IL-4, IL-5, and IL-13. Through these mediators, Th2 lymphocytes are thought to recruit additional effector cells to the lungs, and the cells recruited promote allergic inflammatory events. MSCs (mesenchymal stem cells) have been shown to have useful effects in a number of diseases and disease models. In the majority of these disorders, however, the T cell balance is shifted toward Th1 dominance. MSCs seem to modulate the immune responses and reestablish the physiological balance in a variety of autoimmune and infectious diseases and also have been able to tip the balance back to normal in an allergic environment exemplified by Th2 dominance.
Previous studies using luciferase-expressing or Q-dot–labeled MSCs demonstrated that lungs are the primary site of MSC accumulation following i.v. injection, confirming the known phenomenon of cell trapping in the pulmonary microvasculature that is partially related to cell size. Further, using Q-dot–labeled MSCs studies found significantly more MSCs retained in the lungs when rag weed-induced allergic inflammation was present compared with the unchallenged state The primary site of pathologic processes is the lung itself, and the enhanced presence of MSCs could deliver a concentrated effort to modulate pathological immune responses.
IL-10 and TGF-β, two well characterized antiinflammatory cytokines, and IFN-γ, a key Th1 cytokine capable of down-regulating Th2-mediated pathological responses, are all thought to be capable of suppressing asthma. As IL-10 has been shown to play a significant role in the beneficial effect of MSCs in sepsis, we first measured cytokines in serum and BAL (Bronchoalveolar lavage) in the treated versus untreated RW-challenged mice. Studies find no change in IL-10 or IFN-γ levels, but there was a significant increase in the level of TGF-β, suggesting a different mechanism of action than what was found in a septic environment. We then demonstrated the importance of TGF-β in vivo by treating the mice with TGF-β–specific neutralizing antibodies before MSC injection. Blocking TGF-β—but not IL-10—eliminated the beneficial effect of BMSC treatment. A possible source of TGF-β has been shown to be the MSCs themselves, which has previously been implicated as one of the possible mediators of the MSCs’ immunosuppressive effect.
Data further suggest that IL-4 and/or IL-13 bind to IL-4R receptors on MSCs activating the STAT6 pathway. This, in turn, drives the cells to produce increased amounts of TGF-β. When this TGF-β is released from the MSCs in the allergic (i.e., Th2-dominant) environment, TGF-β receptor activation on immune cells could result in a decrease in IL-4 production and ultimately leads to a shift back toward immunological equilibrium. Surprisingly, neither recombinant IL-4 nor IL-13 alone or in combination was able to elicit the elevation of TGF-β by MSCs seen with RW-conditioned BAL fluid or blood serum in vitro. This indicates that activation of the IL-4R/STAT6 pathway is necessary but not sufficient to cause TGF-β up-regulation in MSCs. In addition to IL-4 or IL-13, there must be other important factors in vivo (present in BAL fluid or serum) that contribute to the production and subsequent release of TGF-β by MSCs.