Specific Inactivation of Antigen-reactive Cells with ¹²⁵I-Labelled Antigen

Abstract

A QUESTION of current interest is whether the initial stages of an immune response—either the induction of antibody formation or of specific tolerance—may involve the reaction of an antigen with a specific lymphocyte. Naor and Sulitzeanu¹ have demonstrated, both in vivo and in vitro, a reaction of ¹²⁵I-labelled bovine serum albumin with a small proportion (about 1/5,000) of mouse spleen lymphocytes. We have since shown² that both flagellin and polymerized flagellin (Salmonella adelaide) and haemocyanin (Jasus lalandii) labelled with ¹²⁵I or ¹³¹I react in vitro with certain cells from spleens of rats and mice. Of the strongly reactive cells, almost all are mononuclear with a high nuclear/cytoplasmic ratio and 7–12 microns in diameter and, for any one antigen, comprise about 1/5,000 of the total cell population. These reactive cells adsorb between 4,000–40,000 molecules of labelled protein when allowed to react at 0° C with labelled protein (about 3 × 10¹² molecules/ml.) in 10 per cent foetal calf serum. A similar proportion of such reactive cells was observed in cell suspensions from lymph nodes and thoracic duct lymph, while peritoneal exudate contained a higher proportion and thymus a lower proportion of these cells². The reaction was not inhibited by concentrations of sodium azide which restricted the uptake of labelled antigen by macrophages but was inhibited, using mouse cells, by rabbit anti-mouse globulin serum. Spleen cells from germ-free and conventional mice reacted equally well with ¹³¹I-labelled flagellin². We wished to know whether the ability of these cells to react with antigen was immunologically significant. Experiments were devised to distinguish between the following possibilities: that the reactive cells were (1) cells present because of a prior experience of the animal with a related antigen, (2) antigen-reactive cells^3,4 which had not had prior antigenic experience but which were capable of contributing to a specific immune response such as formation and secretion of antibody, and (3) cells coated non-specifically with cytophilic antibody.