STRUCTURAL ANALOGS OF INTERLEUKIN-2 - A POINT MUTATION THAT FACILITATES BIOLOGICAL RESPONSE

Interleukin-2 (IL-2) is an immunoregulatory cytokine whose biological effects are mediated through interaction with specific receptors on th e surface of target cells. Due to its presumed role in generating a no rmal immune response, IL-2 is being evaluated for the treatment of a v ariety of tumors, in addition to infectious diseases. During the study of the structure-activity relationships for IL-2 and its receptors, o ne analog in which threonines at positions 41 and 51 were replaced by prolines (T41/51P) was found to possess apparent signaling abnormaliti es. Bioassays and receptor binding assays with human peripheral blood lymphocytes revealed the EC(50) and K-d values of this analog to be 20 0 pM and 5.9 nM, respectively. Although the EC(50) is greater and the receptor affinity of T41/51P is much weaker than that of wild-type IL- 2, receptor occupancy versus biological response comparisons indicated that a much lower receptor occupancy was required to generate an equi valent biological response. Competitive receptor binding analyses with both intermediate affinity (beta/gamma subunit complex) and low affin ity (alpha subunit) receptors were carried out to assess the origin of this phenomenon. Similar analyses of the singly substituted T41P and T51P analogs were carried out. From these studies, it was apparent tha t facilitated signaling was mainly attributable to position 51, wherea s mutations at position 41 primarly influenced low affinity binding. T he observation that the T51P analog facilitates response, compared wit h wild-type IL-2, may indicate a signaling-dependent conformational ch ange in IL-2 upon receptor binding.