Astrocytes as antigen-presenting cells: expression of IL-12/IL-23.

Interleukin-12 (IL-12, p70) a heterodimeric cytokine of p40 and p35 subunits, important for Th1-type immune responses, has been attributed a prominent role in multiple sclerosis (MS) and its animal model, experimental autoimmune encephalomyelitis (EAE). Recently, the related heterodimeric cytokine, IL-23, composed of the same p40 subunit as IL-12 and a unique p19 subunit, was shown to be involved in Th1 responses and EAE. We investigated whether astrocytes and microglia, CNS cells with antigen-presenting cell (APC) function can present antigen to myelin basic protein (MBP)-reactive T cells, and whether this presentation is blocked with antibodies against IL-12/IL-23p40. Interferon (IFN)-gamma-treated APC induced proliferation of MBP-reactive T cells. Anti-IL-12/IL-23p40 antibodies blocked this proliferation. These results support and extend our previous observation that astrocytes and microglia produce IL-12/IL-23p40. Moreover, we show that stimulated astrocytes and microglia produce biologically active IL-12p70. Because IL-12 and IL-23 share p40, we wanted to determine whether astrocytes also express IL-12p35 and IL-23p19, as microglia were already shown to express them. Astrocytes expressed IL-12p35 mRNA constitutively, and IL-23 p19 after stimulation. Thus, astrocytes, under inflammatory conditions, express all subunits of IL-12/IL-23. Their ability to present antigen to encephalitogenic T cells can be blocked by neutralizing anti-IL-12/IL-23p40 antibodies.