Assembly-dependent Structure Formation Shapes Human Interleukin-23 versus Interleukin-12 Secretion

Interleukin 12 (IL-12) family cytokines connect the innate and adaptive branches of the immune system and regulate immune responses. A unique characteristic of this family is that each member is an α:β heterodimer. For human α subunits it has been shown that they depend on their β subunit for structure formation and secretion from cells. Since subunits are shared within the family and IL-12 as well as IL-23 use the same β subunit, subunit competition may influence cytokine secretion and thus downstream immunological functions. Here, we rationally design a folding-competent human IL–23α subunit that does not depend on its β subunit for structure formation. This engineered variant still forms a functional heterodimeric cytokine but shows less chaperone dependency and stronger affinity in assembly with its β subunit. It forms IL-23 more efficiently than its natural counterpart, skewing the balance of IL-12 and IL-23 towards more IL-23 formation. Together, our study shows that folding-competent human IL-12 family α subunits are obtainable by only few mutations and compatible with assembly and function of the cytokine. These findings might suggest that human α subunits have evolved for assembly-dependent folding to maintain and regulate correct IL–12 family member ratios in the light of subunit competition.