The various embodiments relate to peptide antagonists of γc-family cytokines, Interleukin-2 (IL-2), Interleukin-4 (IL-4), Interleukin-7 (IL-7), Interleukin-9 (IL-9), Interleukin-15 (IL-15), and Interleukin-21 (IL-21). The γc-cytokines are associated with important human diseases, such as leukemia, autoimmune diseases, collagen diseases, diabetes mellitus, skin diseases, degenerative neuronal diseases and graft-versus-host disease (GvHD). Thus, inhibitors of γc-cytokine activity are valuable therapeutic and cosmetic agents as well as research tools. Traditional approaches to inhibiting γc-cytokine activity involve raising neutralizing antibodies against each individual γc-cytokine family member/ receptor subunit. However, success has been limited and often multiple γc-cytokine family members co-operate to cause the disease state. Combinatorial use of neutralizing antibodies raised against each factor is impractical and poses an increased risk of adverse immune reactions. The present embodiments overcome these shortcomings by utilizing peptide antagonists based on the consensus γc-subunit binding site to inhibit γc-cytokine activity. Such approach allows for flexibility in antagonist design. In several embodiments, peptides exhibit Simul-Block activity, inhibiting the activity of multiple γc-cytokine family members.
