Role of interleukin-12 and interleukin-18 in murine immune cell regulation

Abstract

Interleukin 12 plays a central role in NK cell activation and CD4 +T cell differentiation. IL-12 acts via a receptor composed of IL-12Rbeta1 and IL-12Rbeta2 subunits. IL-12Rbeta1 plays a primary role in ligand binding whereas IL-12Rbeta2 is responsible for signaling. IL-18 shares some functional activities of IL-12. However, there is a considerable amount of contradictory data in the literature regarding the requirements for IL-12 and IL-18 responsiveness. This work examined the expression of IL-12Rbeta2 on murine NK and T cells and the requirements for acquisition of IL-12/IL-18 responsiveness. NK cells stimulated with IL-2+IL-12 or IL-2+IL-18 exhibited rapid upregulation of IFN-gamma expression followed by upregulation of IL-10 or IL-13 mRNA, respectively. NK cells from IL-12Rbeta2-deficient mice responded to IL-2+IL-18 independently of IL-12. Furthermore, flow cytometric analyses revealed that NK cells activated in vitro with IL-2 differentiate into two distinct cell subsets expressing different levels of IL-12Rbeta2. Like NK cells, NK-T cells also responded to IL-2+IL-12 or IL-2+IL-18 without stimulation of the antigen-specific T cell receptor (TCR). Previously, it was thought that all T cells require activation via the TCR in order to respond to IL-12 or IL-18. Thus, responsiveness of conventional T cells to IL-2+IL-12 or IL-2+IL-18 in the absence of TCR ligation was assessed. Naive CD4+T cells from wild type or DO11.10/Rag2-/- OVA-specific TCR transgenic mice responded to IL-2+IL-12 or IL-2+IL-18 by expressing IFN-gamma and cells grown in IL-2+IL-12 exhibited signs of polarization towards a TH1 phenotype. Transgenic BALB/c mice constitutively expressing the IL-12Rbeta2 chain were generated and the role of the IL-12Rbeta2 in TH2 phenotype development was analyzed using the TH1-dependent murine leishmaniasis model. Despite constitutive expression of the IL-12Rbeta2 chain, transgenic TH2 cells did not revert to TH1 when restimulated with IL-12 and mice remained susceptible to Leishmania. The role of IL-12Rbeta2 in TH1 cell development was also analyzed using the same model. Resistant C57BL/6 mice defective in IL-12Rbeta2 gene exhibited susceptibility to L. major infection and expressed a T H2 phenotype, consistent with a critical role for IL-12 in this model. Thus understanding the expression of IL-12Rbeta2 in NK and CD4 +T cells and its regulation by cytokines may constitute an important element in studies in cancer and autoimmune disease therapy.