Interleukin 2, a lymphokine that acts as a second signal of cellular immune response by way of its action as a T-cell growth factor, was morphologically identified by immunoperoxidase staining. With the use of a monoclonal antibody to interleukin 2 and several complex-forming antisera, the lymphokine was readily distinguished in cytocentrifuge preparations of peripheral blood leukocytes stimulated with a T-cell mitogen. When preparations of cloned interleukin 2 producer and responder cells were stained by the same procedures, discrete patterns of both responder and producer cell phenotypes were revealed. Interleukin 2 producer T cells exhibited a characteristic intense, ringlike cytoplasmic staining, whereas the responder cells (as exemplified by interleukin 2-dependent cell lines) exhibited a less intensive, spotlike membrane staining. In addition, intense membrane localization of interleukin 2, reminiscent of potential capping phenomena, could be observed in stained preparations of cloned responder cells.
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