Noncovalently bonded subunits of 22 and 28 kd are rapidly internalized by T cells reacted with anti-Leu-4 antibody.

Using immunofluorescence analysis in a FACS or cytofluorograph, we monitored the density of four different human T cell surface molecules (Leu-1/OKT1, Leu-2/OKT5,8, Leu-3/OKT4, and Leu-4/OKT3) at different intervals of incubation at 37 degrees C with monoclonal antibodies that react with these structures. Leu-1 and Leu-2 are probable human homologues of the murine Lyt-1 and Lyt-2,3 antigens, respectively. Leu-3 is a 55 kd antigen that is restricted to Leu-2- (helper) T cells. Leu-4 is expressed by all peripheral T cells and has at least two components of 22 and 28 kd that have an undefined structural relationship. The ultimate degree of modulation of each antigen varied considerably. Modulation of Leu-4 was particularly striking, because this antigen disappeared completely from the cell membrane within 7 hr of culture with saturating concentrations of alpha Leu-4. After modulation of surface 125I-labeled T cells, Leu-4 was found in the detergent-solubilized cell lysate, but not in the cellfree supernatant, indicating that it was internalized. Our results also indicated that the internalized antigen-antibody complex existed in at least two forms: a) as an antigen-antibody complex having only the lower m.w. (22 kd) component bound to antibody, or b) as free antigen having the 28 kd chain in noncovalent association with the 22 kd moiety. These studies indicate that the Leu-4 structure is composed of at least two distinct chains that are noncovalently associated. The rapid modulation and structural properties of the Leu-4 complex are discussed in the context of its possible functions in T cell immunity.