Cellular and genetic restrictions in the immunoregulatory activity of alpha-fetoprotein. III. Role of the MLC-stimulating cell population in alpha-fetoprotein-induced suppression of T cell-mediated cytotoxicity.

Alpha-fetoprotein (AFP), a normal component of fetal and newborn sera, exerts selective suppressive effects on various functions of thymus-derived (T) lymphocytes, including the T cell-mediated cytotoxic reaction. In the present study, AFP-induced suppression of the T cell-mediated allogeneic response was examined to define more precisely the mechanism by which AFP acts. Data presented indicate that AFP elicits its effect within the first 25 to 36 hr of culture. However, AFP apparently has no direct effect on T lymphocytes. T cells incubated in the presence of AFP retain full capacity to respond in MLC and CML. In contrast, AFP induces major changes in the functional status of monocyte-enriched, MLC-stimulating cell population. This monocyte-enriched population, after pretreatment with AFP, possesses strong stimulating capacity for the T suppressor cell limb of the immune response, while at the same time, suppresses the activity leading to the normal generation of cytotoxic T lymphocytes. These data correlate with the immune activity present in the newborn mouse.