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Evidence for the Binding of a Biologically Active Interleukin-2 to Human α2-Macroglobulin (*)

Human α2-macroglobulin (α2M), which irreversibly entraps proteinases through a drastic conformational change, has also been reported to bind various cytokines. The meaning of cytokine binding to native and/or transformed α2M molecules is, however, not understood. In an attempt to elucidate this question, we have studied the interaction of radioiodinated recombinant human interleukin-2 (125I-rhIL-2) with native and chymotrypsin (α2M-C)- or methylamine-transformed (α2M-MA) α2M. Our results show that native and α2M-MA are able to bind 125I-rhIL-2, with binding occurring only with the latter in a covalent manner, whereas the labeled cytokine is proteolyzed when incubated with α2M-entrapped chymotrypsin. The degradation of uncomplexed 125I-rhIL-2 has also been observed in the presence of trypsin, whereas 125I-rhIL-2 bound to α2M-MA is protected. Moreover, the proliferative activity of this cytokine on responsive cells is still maintained either with native α2M- or α2M-MA-complexed rhIL-2 in comparison with that observed with the cytokine alone. Our results, which lead us to consider α2M molecules as IL-2-binding proteins, emphasize the possible role of these molecules as immune response regulators.