Blocking of interleukin 2 (IL 2) binding to the IL 2 receptor is not required for the in vivo action of anti‐IL 2 receptor monoclonal antibody (mAb). I. The production, characterization and in vivo properties of a new mouse anti‐rat IL 2 receptor mAb that reacts with an epitope different to the one that binds to IL 2 and the mAb ART‐18

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Abstract
A mouse anti‐rat interleukin 2 (IL 2) receptor (IL 2R) monoclonal antibody (mAb), ART‐65, has been developed. As shown by fluorescence‐activated cell sorter analysis and immunoprecipitation studies, ART‐65 recognizes in a species‐specific manner the same molecule as does ART‐18, a mAb which has been shown previously to recognize the rat receptor for IL 2. ART‐65 and ART‐18 do not competitively inhibit the binding of each other to activated T cells. ART‐65, in contrast to ART‐18, does not inhibit the binding of IL 2 to cells nor does it have any inhibitory effect in vitro on IL 2‐driven proliferation of rat T lymphoblasts. Therefore, ART‐65 is another mAb recognizing the rat IL 2 receptor, but binding to an epitope distinct from that recognized by either IL 2 or ART‐18. We compared the in vivo activity of the mAb ART‐65 and ART‐18 with that of the W3/25 mAb in a local graft‐vs‐host reaction (GVHR). Similar to the anti‐W3/25 treatment, ART‐65 and ART‐18 inhibited GVHR. The results demonstrate that GVHR depends on a small subpopulation of IL 2R+ cells present in the W3/25+ T cell population because IL 2R‐targeted therapy was as effective as the treatment with W3/25 mAb which reacts with the entire T helper cell population. Moreover, the results argue against the possibility that anti‐IL 2R mAb act via blockade of the IL 2 binding to IL 2R+ cells and/or by inhibiting the IL 2‐driven expansion of the antigen‐activated clones. The results support the view that IL 2R‐targeted therapy results in the elimination of the IL 2R+ cells.