26. – Interferon-Gamma
Publisher Summary 
This chapter focuses on interferon-gamma (IFN-γ). The single-copy hIFN-γ gene, situated on chromosome 12 in the p12.05-qter region, contains three introns. The production of IFN-γ is limited to T cells and natural killer (NK) cells. T cells are stimulated to produce IFN-γ either in a polyclonal manner, via mitogens or antibodies, or in a clonally restricted, antigen-specific manner. Human CD8 and murine (m) Ly-2, isolated from individuals after viral infection or vaccination, release IFN-γ upon exposure to the corresponding viral antigens in vitro. IFN-γ induces indoleamine 2,3-dioxygenase (IDO), an enzyme of tryptophan catabolism that is responsible for the conversion of tryptophan to kynurenine. The IDO activity has been implicated in the killing of intracellular parasites such as Toxoplasma gondii or Chlamydia trachomatis and Chlamydia psittaci. In patients with advanced HIV-1 infections, the enhanced production of IFN-γ is probably responsible for the significant reduction of tryptophan levels in the serum, and a proposal has been made that this could contribute to the neurological symptoms frequently associated with HIV infection. IFN-γ acts as a regulatory agent in the determination of Ig isotype responses because it stimulates the expression of immunoglobulin of the IgG2a isotype while inhibiting the production of IgG3, IgG1, IgG2b, and IgE.