Interactions of stem cells and T lymphocytes contribute to the physiological control of cell proliferation in rapidly renewing tissues

Control of cell proliferation, in tissues which replicate rapidly, may be exercised, at least in part, by common populations of circulating cells. Thymus-derived (T) lymphocytes possess properties which would fit them for this purpose. Among these are the functional manifestations of 'help' and 'suppression' with respect to defined physiological processes, such as immunoglobulin production; and unique traffic patterns in blood and extra-vascular tissues, including those in non-lymphoid organs such as bone marrow, skin and gut epithelium. This tropism may involve specific chemotactic agents and result in a predominance of 'suppressor' cells in target tissues. A 'steady-state' of cell proliferation could be maintained by this mechanism which is subject to humoral modulation, for instance by corticosteroids. Influx of 'helper' T lymphocytes would stimulate cell production while an excess of 'suppressors' would diminish cell renewal, as has been observed in some forms of bone marrow aplasia. Fulfillment of these roles by T cells may depend on the expression of antigens in the HLA-DR complex and it has implications for further insight into the pathogenesis of auto-immunity and neoplasia.