The IL-20 Subfamily of Cytokines and Their Receptors

The interleukin (IL)-20 subfamily of cytokines is comprised of IL-19, IL-20, IL-22, IL-24, and IL-26. These cytokines are part of the larger IL-10 cytokine family, but share a common biology based on structural homology and common receptor usage. Their heterodimeric class II receptors are preferentially expressed on epithelial tissues, which uniquely positions IL-20 subfamily cytokines as a means of communication between leukocytes and epithelial cells. IL-20 subfamily cytokines generally function to enhance innate defense mechanisms, wound healing, and tissue repair at epithelial surfaces. This biology has been extensively studied in the skin where IL-20 subfamily cytokines have critical roles during the wound healing response but are also key drivers of psoriasis. These functions are mediated through their ability to stimulate epithelial cell proliferation and differentiation, to induce antimicrobial responses in epithelial cells, and to promote inflammation through the recruitment and activation of neutrophils and other leukocytes. IL-22, the best-studied member of the group, has important host-protective roles during bacterial infections of the intestine and the lung, consistent with the high expression of the IL-22 receptor on mucosal epithelial cells.