Induction of the murine class-II antigen-associated invariant chain by TNF-alpha is controlled by an NF-kappa B-like element

Parasite Manipulation of the Invariant Chain and the Peptide Editor H2-DM Affects Major Histocompatibility Complex Class II Antigen Presentation during Toxoplasma gondii Infection

Toxoplasma gondii is an obligate intracellular protozoan parasite. This apicomplexan is the causative agent of toxoplasmosis, a leading cause of central nervous system disease in AIDS. It has long been known that T. gondii interferes with major histocompatibility complex class II (MHC-II) antigen presentation to attenuate CD4(+) T cell responses and establish persisting infections. Transcriptional downregulation of MHC-II genes by T. gondii was previously established, but the precise mechanisms inhibiting MHC-II function are currently unknown. Here, we show that, in addition to transcriptional regulation of MHC-II, the parasite modulates the expression of key components of the MHC-II antigen presentation pathway, namely, the MHC-II-associated invariant chain (Ii or CD74) and the peptide editor H2-DM, in professional antigen-presenting cells (pAPCs). Genetic deletion of CD74 restored the ability of infected dendritic cells to present a parasite antigen in the context of MHC-II in vitro. CD74 mRNA and protein levels were, surprisingly, elevated in infected cells, whereas MHC-II and H2-DM expression was inhibited. CD74 accumulated mainly in the endoplasmic reticulum (ER), and this phenotype required live parasites, but not active replication. Finally, we compared the impacts of genetic deletion of CD74 and H2-DM genes on parasite dissemination toward lymphoid organs in mice, as well as activation of CD4(+) T cells and interferon gamma (IFN-γ) levels during acute infection. Cyst burdens and survival during the chronic phase of infection were also evaluated in wild-type and knockout mice. These results highlight the fact that the infection is influenced by multiple levels of parasite manipulation of the MHC-II antigen presentation pathway.

Maximal induction of p69 2', 5'-oligoadenylate synthetase in Daudi cells requires cooperation between an ISRE and two IRF-1-like elements

The human 2', 5'-oligoadenylate (2-5A) synthetases are members of a family interferon (IFN)-inducible anti-viral proteins. Three size classes of these enzymes: small (p40, p46), intermediate (p69, p71) and large (p100), are encoded by three genes that exhibit differential constitutive and IFN-inducible expression. Since the 5'-regulatory region of the 69 kDa isoform contains multiple putative control elements, deletion analysis and site directed mutagenesis were done to identify key regulatory motifs. The region located between bp -972 and -452 from the translational start site contains elements that slightly repress constitutive and IFN-inducible transcription. The region from bp -366 to -117 contains two positive regulatory elements that differ slightly from consensus IFN-regulated factor 1 (IRF-1) binding sites. In mobility shift assays, the proximal IRF-1 site weakly binds a novel factor found in both control and IFN-treated cells. The region from bp -117 to -10 contains a functional interferon stimulated response element (ISRE) that contributes to constitutive expression and confers IFN-inducibility on a heterologous promoter. The ISRE specifically binds an IFN-inducible factor that contains signal transducer activator of transcription (STAT) 1alpha. The ISRE and the two IRF-1-like sites cooperatively interact to control transcription. These three elements are sufficient for constitutive and IFN-inducible expression, since expression from reporter constructs containing mutations in all three elements is low in both control and IFN-treated cells.

Discordant Expression of Ii and HLA-DR in Thyrocytes: A Possible Pathogenetic Factor in Hashimoto's Thyroiditis

Hashimoto's thyroiditis is the archetype of organ-specific autoimmune disease. The key pathogenetic feature is the activation of thyroid-specific T-cells by properly presented endogenous thyroid antigens. There is strong indication that thyrocytes themselves present self-antigens, based on the finding of antigen presenting human leukocyte antigen-DR (HLA-DR) molecules on the thyrocytes in Hashimoto's thyroiditis. Because class ll-associated invariant chain (Ii) is tightly bound to the antigen-binding groove of the DR molecules in the endoplasmic reticulum and is found to compete with the endogenous peptides for binding with DR. it is speculated that Ii prevents endogenous peptides from binding to, and being presented by, DR on the cell surface. We hypothesize that in Hashimoto's thyroiditis, Ii is insufficient, leaving DR molecules open for endogenous peptides, In this article, we test our hypothesis of discordance between DR and Ii in the thyroid epithelial cells in Hashimoto's thyroiditis by immunohistochemistry. Formalin-fixed, paraffin-embedded archival tissue from eight cases of Hashimoto's thyroiditis and four cases of normal thyroid specimen were randomly selected and immunostained with monoclonal antibodies (MAbs) against HLA-DR and Ii using avidin-biotin-peroxidase method. We found that in all eight cases of Hashimoto's thyroiditis. Ii is significantly reduced relative to DR. The results support our hypothesis that presentation of self-antigen may be a result of insufficient Ii in the thyrocytes in Hashimoto's thyroiditis.

Stabilization of invariant chain mRNA by 12-O-tetradecanoylphorbol-13-acetate is blocked by IFN-gamma in a murine B lymphoma cell line

Activation of protein kinase C (PKC) by 12-O-tetradecanoylphorbol-13-acetate (TPA) increased steady-state levels of mRNA encoding the major histocompatibility complex (MHC) class II antigen I-A beta and the class II antigen-associated invariant chain (Ii, CD74) in A20 B lymphoma cells and in normal mouse B cells. The increase in Ii mRNA levels appeared to be due to a slight increase in the rate of gene transcription and an increase in the stability of Ii mRNA. The half-life of Ii mRNA increased from 12 h to >24 h following treatment with TPA, as determined by Northern blot analysis following actinomycin D treatment or by the [3H]-uridine pulse-chase method. Interferon-gamma (IFN-gamma), which has been well characterized as a cytokine that induces class II antigens and the Ii, increased Ii expression slightly in A20 cells. However, cotreatment of cells with TPA and IFN-gamma resulted in a block in the TPA-induced increase in Ii expression. Transcription of the Ii gene was minimally affected following treatment with IFN-gamma alone, and cells treated with both TPA and IFN-gamma had the same transcription rate as the control cells. IFN-gamma did, however, block stabilization of Ii mRNA by TPA. Activation of PKC by TPA, which was previously shown to lead to membrane translocation and downregulation, was not inhibited by IFN-gamma. Therefore, IFN-gamma appeared to block a downstream signal transduction pathway activated by PKC that controls stability of Ii mRNA.

Invariant chain (CD74) gene regulation: enhanced expression associated with activation of protein kinase C delta in a murine B lymphoma cell line

The invariant chain (Ii, CD74) is a transmembrane glycoprotein that is transiently associated with the MHC class II antigens in the endoplasmic reticulum and in endocytic vesicles. An activator of protein kinase C (PKC), 12-O-tetradecanoyl-1,2-phorbol 13-acetate (TPA), was found to enhance expression of Ii mRNA in the murine B lymphoma cell line, A20, 6-48 hr following treatment. In contrast, TPA did not induce the Ii in NIH 3T3 fibroblasts. TPA addition to either cell line activated PKC. Pretreatment of A20 cells with the PKC inhibitors, staurosporine or chelerythrine chloride, for 5 or 20 min prior to addition of TPA, decreased Ii mRNA levels when compared to cells treated with TPA alone. A 20 min preincubation with the highly specific PKC inhibitor, calphostin C, completely blocked the TPA enhanced expression of the Ii suggesting that activation of PKC was responsible for TPA increased Ii mRNA levels. IFN-gamma also blocked the TPA increased Ii mRNA levels. Constitutive expression of Ii mRNA was decreased by treatment with staurosporine but not chelerythrine chloride or calphostin C, suggesting that non-PKC protein kinases may also be important for maintaining high levels of Ii mRNA in these cells. Western blot analysis using PKC isotype specific antibodies showed that A20 cells express PKC delta abundantly whereas NIH 3T3 cells express primarily PKC alpha. These data suggest that a PKC delta mediated signal transduction pathway plays a crucial role in up-regulation of the Ii.

The bio-logical role of invariant chain (Ii) in MHC class II antigen presentation

Foreign antigens are internalized by antigen presenting cells by endocytosis and processed to peptides. To enable presentation of antigenic peptides by MHC class II molecules, these molecules have to be sorted to endosomal compartments where they can meet and bind the peptides. Invariant chain is complexed with MHC class II molecules and contains sorting signals responsible for MHC class II accumulation in endosomes. Invariant chain also has several other features contributing to the immune system's specific combat against invaders.