Receptors for hematopoietic regulatory cytokines: overview of structure and function

The production of blood cells is regulated by the action of external factors, cytokines, that can be released by many cell types. In the first place, a population of multipotent stem cells, mostly in the resting Go phase of the cell cycle, but with self-renewal capacity, gives rise to progenitor cells that are predetermined for differentiation into all kinds of blood cells. Expression of genes for cytokine receptors leads to external regulation of hematopoiesis by cytokines which bind to the receptors, resulting in modifications of proliferation and differentiation, as cytokines are not only growth factors, but are also maturation factors capable of directing hematopoiesis towards functionally competent cells. What is more, they are survival factors capable of suppressing programmed cell death (apoptosis). This is of particular importance for the viability of stem cells which must be preserved. Thus cytokines can act at all positions of the hematopoietic family tree, and the response can differ from proliferation and differentiation of progenitor cells to functional activation of mature cells. Under physiological conditions, during constitutive hematopoiesis, the regulatory cytokines are produced locally, for instance by stromal ceils of the microenvironment, and act locally in a paracrine manner [2].