[Present views of lymphocyte growth and differentiation]

In recent years, a large body of information has been accumulated on the origin and differentiation of lymphocytes. According to the present state of knowledge, their ontogeny can be outlined as follows: the pluripotent stem cell originates the lymphatic progenitors, which undergo a series of differentiation events within the microenvironment of the primary lymphatic organs (bone marrow and thymus). Thus lymphatic precursors appear, which finally give rise to mature T- and B-lymphocytes. These events are accompanied by distinct changes in cell markers, mostly surface markers, which have been clearly documented. Some controversy does exist as to whether T and B lineages stem from a common lymphatic progenitor, since recent evidence suggests that B lymphocytes, but not the T, might share a common progenitor with myeloid lineages (hemocytoblast). T-cell maturation starts in the thymic cortex, and under the influence of the soluble thymic factor(s) it progresses in the several post-thymic locations, until final maturation. This process goes from immature (To) to semi-mature (T1) and mature (T2) lymphocytes, these latter mostly characterized by the absence of TdT and by the property to form E-rosettes with SRBC and respond to mitogens. Circulating mature T-lymphocytes display a considerable degree of heterogeneity in terms of functional properties; in this way, various T-subpopulations have been identified on the ground of recognized different properties. The sequence of maturational events of B-lineage takes place within the bursa-equivalent sites. This includes an antigen-independent stage with early B precursors (B0 - B1 - B2), and an antigen-driven maturational phase which actually produces the plasma cells. Among the early B-precursors (B-virgin population), memory cells arise following the antigenic stimulation. Also at that stage, B-cells are highly susceptible to tolerance induction. B-circulating lymphocytes are mainly characterized by S-Ig, Fc receptors, and complement receptors.