Changes In The Target Tissue

In an inflamed tissue, the expression of many surface molecules increases, usually because cytokines increase the transcription of the genes. The example of IFN-y triggering the transcription of the MHC class I and II genes in vivo is the best known, but many adhesion molecules can also be induced on endothelium, inflammatory cells, and parenchymal cells. We shall outline the features of the induction of class I and II MHC molecules by IFN-y.

The Transcriptional Regulation of MHC Genes184-185

The level of MHC expression determines the immunogenicity of tissues and their sensitivity to immune injury, and some increased MHC expression is invariably seen in acute T cell-mediated rejection. The expression of MHC proteins in tissues is primarily regulated by transcriptional control.

MHC genes behave as "housekeeping genes" (as opposed to tissue specific genes), which are either expressed or expressible in most tissues, but to varying degrees. This implies that the chromatin structure of their regulatory regions is available for transcription factors in many tissues.

In the normal mammal, constitutive class I expression is widespread but highly variable between cell types. A component of IFN-y-induced expression is common even in normal hosts.186 Constitutive class II expression is confined to B cells. The class II expression in dendritic cells and some macrophages in normal individuals may reflect low levels of cytokines such as IFN-y, IL-4, and GM-CSF. Class II expression found in some normal epithelia probably also reflects cytokine induction.

MHC Promoters

The level of MHC expression is closely related to the steady state mRNA levels and probably reflects the activity of the promoter in regulating transcription. The class I and II promoters are highly conserved.

The characteristic DNA sequence in the class I promoter is a class I regulatory element (CRE) at about -160 to -200 bp from the start site of transcription, overlapping an interferon consensus sequence (ICS) at about -140 to -160 bp. The CRE is a series of overlapping palindromic sequences which are sites of binding of transcriptional regulatory proteins and are necessary for the tissue specific basal and induced expression of class I. Protein binding to the CRE seems to correlate with constitutive class I expression.184,185,187 The ICS probably binds proteins which

MCR-1, MCAF

Target cells

Neutrophils

Monocytes, T cells are regulated by IFN-a/p, and IFN-y and acts by increasing transcription in concert with the CRE. TNF-a also acts on the CRE, probably through NF-kB proteins.

The class II promoter contains a conserved region at about -60 to -100 bp which contains sequences termed the X box, Y box, and a spacer between them. The X and Y boxes are occupied by proteins in the basal state and probably are the elements giving class II genes their characteristic patterns of regulation in the basal and cytokine induced state, but other elements participate.188 The key regulation of class II genes is the class II transactivator, or CIITA.

Normal and Induced IFN-y Production and Regulation

IFN-y is produced by T cells (CD4, CD8) and NK cells. IFN-y production is an important event in rejection, with both adverse and favorable effects. IFN-y-medi-ated MHC induction is probably necessary but not sufficient for rejection, and IFN-y can induce accelerated rejection.189 IFN-y is produced by the specifically triggered T cells and is also capable of triggering its own release, probably from NK cells with the appearance of large granular lymphocytes (LGL).190 Thus the LGLs may serve as an amplifier to increase the release of IFN-y.

IFN-y Receptor Triggering

Two IFN-y receptors bind the IFN-y homodimer, each engaging the N terminal of one unit and the C terminal of the other.129 Receptor crosslinking leads to membrane-to-cytoplasm signal transduction via mechanisms involving the large intracytoplasmic domain of the receptor. The mechanism involves a protein kinase: the receptor becomes phosphorylated,191-193 and one tyrosine in the intracytoplasmic portion of the receptor has been shown to be essential to the biological activity of the receptor.194 The receptor has additional subunits, encoded on chromosome 21 and chromosome 16 in the human. Tyrosine kinases (JAK1 and JAK2) then phosphorylate the cytoplasmic form of a transcription factor, interferon stimulated gene factor 3, in particular, the p91 component, now called STAT-1. This then moves into the nucleus to activate transcription of genes with IFN-y activated sites. Some of these induced mRNAs encode products which are transcription factors.

The details of the pathway from the IFN-y receptor to MHC promoters remain to be elucidated; it is unclear why MHC expression tends to be induced later than some other genes, e.g., 24-48 hours after IFN-y administration. It is likely that MHC induction requires the synthesis of IFN-induced transcription factors such as IRF-1. In the case of class I induction, the signal transduction pathway used by IFN-y seems to require some of the same steps as are used by IFN-a/p.195 These proteins probably affect the ICS. In the case of class II, the new protein induced by IFN-y is CIITA.195a

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