Internalization of IL-2 by an IL-2-dependent murine T cell lymphoma expressing no detectable cell surface receptors for IL-2.

A murine Radiation leukemia virus-induced T cell lymphoma, 5C2, which is dependent on interleukin-2 (IL-2) for proliferation has been analyzed for interleukin-2 receptor (IL-2R) expression. Using fluorescence-activated cell sorter and electron microscopic analysis together with antibody specific for the known p55 chain of the murine IL-2R, no evidence has been obtained to suggest that these cells express detectable numbers of receptors with high affinity for IL-2. However, two different antibodies with specificity for the p55 chain of the IL-2R have been shown to inhibit 5C2 proliferation. An analysis of 125I-IL-2 binding has precluded a cell surface receptor density of greater than 80 molecules per cell. A temperature-dependent, nonspecific uptake of 125I-IL-2 has been described for 5C2. Uptake is saturated at 8.5 nM 125I-IL-2 with equilibrium being established within 60 min. When incubated at 37 degrees C in the presence of 400 pM 125I-IL-2, a maximum of approximately 2,000 molecules are internalized within 40 min. Uptake of other iodinated proteins by 5C2 was not observed. This property is unique to 5C2 and not to the control C6VL/1 cell line. Intracellular vesicles have also been found in 5C2 cells by electron microscopy which stain positively with gold-conjugated antibody specific for the p55 chain of the IL-2R. 5C2 appears to exhibit unique IL-2 regulatory characteristics.