IL-12 p40 monomer is different from other IL-12 family members to selectively inhibit IL-12Rβ1 internalization and suppress EAE

Multiple sclerosis (MS) is the most common human demyelinating disease of the central nervous system. The IL-12 family of cytokines has four members, which are IL-12 (p40:p35), IL-23 (p40:p19), the p40 monomer (p40), and the p40 homodimer (p40(2)). Since all four members contain p40 in different forms, it is important to use a specific monoclonal antibody (mAb) to characterize these molecules. Here, by using such mAbs, we describe selective loss of p40 in serum of MS patients as compared to healthy controls. Similarly, we also observed decrease in p40 and increase in IL-12, IL-23, and p40(2) in serum of mice with experimental autoimmune encephalomyelitis (EAE), an animal model of MS, as compared to control mice. Interestingly, weekly supplementation of mouse and human recombinant p40 ameliorated clinical symptoms and disease progression of EAE. On the other hand, IL-12, IL-23, and p40 2 did not exhibit such inhibitory effect. In addition to EAE, p40 also suppressed collagen-induced arthritis in mice. Using IL-12R beta 1(-/-), IL-12R beta 2(-/-), and IL-12R beta 1(+/-)/IL-12R beta 2(-/-) mice, we observed that p40 required IL-12R beta 1, but not IL-12R beta 2, to suppress EAE. Interestingly, p40 arrested IL-12-, IL-23-, or p40(2)-mediated internalization of IL-12R beta 1, but neither IL-12R beta 2 nor IL-23R, protected regulatory T cells, and suppressed Th1 and Th17 biasness. These studies identify p40 as an anti-autoimmune cytokine with a biological role different from IL-12, IL-23, and p40(2) in which it attenuates autoimmune signaling via suppression of IL-12R beta 1 internalization, which may be beneficial in patients with MS and other autoimmune disorders.