Interferon gamma (IFN-γ) is a multifunctional protein first observed as an antiviral activity in cultures of Sindbis virus-infected human leukocytes stimulated by PHA (1). Produced by Tlymphocytes and natural killer (NK) cells, IFN-γ ?is now known to be both an inhibitor of viral replication and a regulator of numerous immunological functions. Human IFN-γ? is reported to be active only on human and non-human primate cells (5). The biochemistry and biological activities of the interferons have been extensively reviewed (2-9).

Human IFN-γ ?is a 143 amino acid residue, 20 or 25 kDa glycoprotein that demonstrates little sequence homology to IFN-α? or -β?(10-13). Naturally occurring IFN-γ ?is found as either of two molecular weight species, differing in degree of glycosylation. Human IFN-γ? apparently exists as a head-to-tail dimer in solution with the C-terminus of one monomer aligned with the N-terminus of the other monomer (14,15) A receptor for IFN-γ has been identified and its gene localized to chromosome 6 (16,17) Apparently the product of a single gene, the receptor is a single chain 90 kDa glycoprotein that shows a high degree of species-specific binding of IFN-γ?(18-21).

Functionally, IFN-γ ?produces a variety of effects. Produced by CD8+, NK, gd, and TH1 T helper cells, IFN-γ ?has documented antiviral, antiprotozoal and immunomodulatory effects on cell proliferation and apoptosis, as well as the stimulation and repression of a variety of genes (9, 22-25) he antiprotozoal activity of IFN-γ ?against Toxoplasma and Chlamydia is believed to result from indoleamine 2,3-dioxygenase activity, an enzyme induced by IFN-γ ?(26).The immunomodulatory effects of IFN-γ? are extensive and diverse. In monocyte/macrophages, the activities of IFN-γ ?include: increasing the expression of class I and II MHC antigens; increasing the production of IL-1, platelet-activating factor, H2O2, and pterin; protection of monocytes against LAK cell-mediated lysis; downregulation of IL-8 mRNA expression that is upregulated by IL-2; and, with lipopolysaccharide, induction of NO production.Finally, IFN-γ?has been shown to upregulate ICAM-1, but not E-Selectin or VCAM-1, expression on endothelial cells.