Residual Type 1 Immunity in Patients Genetically Deficient for Interleukin 12 Receptor β1 (IL-12Rβ1): Evidence for an IL-12Rβ1–Independent Pathway of IL-12 Responsiveness in Human T Cells

Genetic lack of interleukin 12 receptor β1 (IL-12Rβ1) surface expression predisposes to severe infections by poorly pathogenic mycobacteria or Salmonella and causes strongly decreased, but not completely abrogated, interferon (IFN)-γ production. To study IL-12Rβ1–independent residual IFN-γ production, we have generated mycobacterium–specific T cell clones (TCCs) from IL-12Rβ1–deficient individuals. All TCCs displayed a T helper type 1 phenotype and the majority responded to IL-12 by increased IFN-γ production and proliferative responses upon activation. This response to IL-12 could be further augmented by exogenous IL-18. IL-12Rβ2 was found to be normally expressed in the absence of IL-12Rβ1, and could be upregulated by IFN-α. Expression of IL-12Rβ2 alone, however, was insufficient to induce signal transducer and activator of transcription (Stat)4 activation in response to IL-12, whereas IFN-α/IFN-αR ligation resulted in Stat4 activation in both control and IL-12Rβ1–deficient cells. IL-12 failed to upregulate cell surface expression of IL-18R, integrin α6, and IL-12Rβ2 on IL-12Rβ1–deficient cells, whereas this was normal on control cells. IL-12–induced IFN-γ production in IL-12Rβ1–deficient T cells could be inhibited by the p38 mitogen-activated protein kinase (MAP) kinase inhibitor SB203580 and the MAP kinase kinase (MEK) 1/2 inhibitor U0126, suggesting involvement of MAP kinases in this alternative, Stat4-independent, IL-12 signaling pathway. Collectively, these results indicate that IL-12 acts as a partial agonist in the absence of IL-12Rβ1. Moreover, the results reveal the presence of a novel IL-12Rβ1/Stat4–independent pathway of IL-12 responsiveness in activated human T cells involving MAP kinases. This pathway is likely to play a role in the residual type 1 immunity in IL-12Rβ1 deficiency.