Enhancement of human B lymphocyte differentiation in vitro by thyroid hormone.

A study was made of the regulatory influence of a thyroid hormone, triiodothyronine (T3), on human blood T- and B-lymphocytes responses in vitro. Blood lymphocytes were stimulated with the T-cell mitogens concanavalin A (Con A) and phytohaemagglutinin (PHA), with the T-dependent B-cell activator pokeweed mitogen (PWM), and with the T-independent B-cell activator Staphylococcus aureus Cowan I bacteria in the presence of various hormone concentrations. T3 did not stimulate T-cell proliferation. The number of immunoglobulin-containing and -secreting cells (plasmablasts) was increased in PWm and Staph. aureus cultures treated with T3. The maximal enhancement was reached at a concentration of 10(-9)-10(-7) M T3. Cell fractionation techniques revealed that T3 apparently had a direct stimulatory effect on B-cell differentiation.