Regulation of interleukin (IL)-18 receptor alpha chain expression on CD4(+) T cells during T helper (Th)1/Th2 differentiation. Critical downregulatory role of IL-4.

Interleukin (IL)-18 has been well characterized as a costimulatory factor for the induction of IL-12-mediated interferon (IFN)-gamma production by T helper (Th)1 cells, but also can induce IL-4 production and thus facilitate the differentiation of Th2 cells. To determine the mechanisms by which IL-18 might regulate these diametrically distinct immune responses, we have analyzed the role of cytokines in the regulation of IL-18 receptor alpha chain (IL-18Ralpha) expression. The majority of peripheral CD4(+) T cells constitutively expressed the IL-18Ralpha. Upon antigen stimulation in the presence of IL-12, marked enhancement of IL-18Ralpha expression was observed. IL-12-mediated upregulation of IL-18Ralpha required IFN-gamma. Activated CD4(+) T cells that expressed low levels of IL-18Ralpha could produce IFN-gamma when stimulated with the combination of IL-12 and IL-18, while CD4(+) cells which expressed high levels of IL-18Ralpha could respond to IL-18 alone. In contrast, T cell stimulation in the presence of IL-4 resulted in a downregulation of IL-18Ralpha expression. Both IL-4(-/)- and signal transducer and activator of transcription (Stat)6(-/)- T cells expressed higher levels of IL-18Ralpha after TCR stimulation. Furthermore, activated T cells from Stat6(-/)- mice produced more IFN-gamma in response to IL-18 than wild-type controls. Thus, positive/negative regulation of the IL-18Ralpha by the major inductive cytokines (IL-12 and IL-4) determines the capacity of IL-18 to polarize an immune response.

A heritable defect in IL-12 signaling in B10.Q/J mice. I. In vitro analysis.

B10.Q mice are normally susceptible to the induction of collagen-induced arthritis. We noted that one subline of B10.Q mice, B10.Q/J, was completely resistant to disease induction when immunized with collagen in CFA. B10.Q/J mice have a global defect in the generation of Th1 responses, and Ag-specific T cells derived from this strain failed to produce IFN-gamma. Because T cells from these mice could produce normal amounts of IFN-gamma when activated by IL-12/IL-18-independent stimuli, the defect appeared to be a failure to respond to IL-12. This defect extended to NK cells, which also failed to produce IFN-gamma when stimulated by IL-12. The capacity of NK cells, but not activated T cells, to produce IFN-gamma in response to IL-12 could be partially restored by IL-18. The expression of the IL-12R beta1- and beta2-chains on T cells and NK cells from B10.Q/J mice was normal. However, activated T cells from B10.Q/J mice did not signal normally through the IL-12R and manifested a defect in their capacity to phosphorylate Stat4. This defect was partial in that it could be overcome by increasing both the concentration of IL-12 and the incubation times in the Stat4 phosphorylation assays. Because Stat4 function is apparently intact in B10.Q/J mice, the defect in IL-12 signaling can be localized between the IL-12R complex and Stat4. This subtle abnormality in IL-12 responsiveness results in a profound defect in the generation of Th1 cells and the development of autoimmune disease.

Inhibition of autoimmune T cell responses in the DA rat by bone marrow-derived NK cells in vitro: implications for autoimmunity.

Regulation of the immune response is critical to homeostasis. While innate immunity can influence the development of adaptive immune responses, its role in regulation is less well understood. Recently, NK cells have been implicated in the control of experimental autoimmune encephalomyelitis, an animal model for multiple sclerosis. In this report, we show that rat bone marrow-derived NK cells exhibited potent inhibitory effects on T cell proliferation to both Con A as well as the central nervous system Ag myelin basic protein. There was also a significant decrease in both IFN-gamma and IL-10 production in vitro, whereas levels of the beta-chemokine monocyte chemoattractant protein-1 were significantly elevated. Flow cytometry studies suggest that the NK cells may play an important role in regulating both normal and autoimmune T cell responses by exerting a direct effect on activated, autoantigen-specific T cells.

Critical requirement for aspartic acid at position 82 of myelin basic protein 73-86 for recruitment of V beta 8.2+ T cells and encephalitogenicity in the Lewis rat.

We synthesized single amino acid-substituted peptide analogues of guinea pig myelin basic protein (MBP) 73-86 to study the importance of aspartic acid at residue 82 (QKSQRSQDENPV), which previous reports have suggested is a critical TCR contact residue. Whereas the wild-type 73-86 peptide elicited severe experimental autoimmune encephalomyelitis (EAE) in the Lewis rat, none of the peptide analogues with substitutions at position 82 were capable of inducing EAE. The inability to cause EAE was not due to a failure to bind MHC or to elicit T cell proliferation and cytokine secretion. T cells specific for MBP73-86 did not cross-react with any of the analogues tested, further indicating the importance of this residue in T cell responses to 73-86. Analysis by flow cytometry showed that only the wild-type 73-86 peptide was capable of recruiting V beta 8.2+ T cells, which have been shown previously to be important for disease induction. Reduced expression of the V beta 8.2 TCR was also seen in Lewis rats protected from EAE by coimmunization of MBP73-86 with 73-86(82D-->A), despite an increase in cytokine production when both peptides were present during in vitro culture. The data indicate that aspartic acid 82 is a critical TCR contact residue and is required for the recruitment of V beta 8.2+ T cells and the encephalitogenic activity of MBP73-86.

Delineation of two encephalitogenic myelin basic protein epitopes for DA rats.

We studied synthetic peptides that correspond to two regions of the guinea pig myelin basic protein (MBP) molecule which elicit experimental autoimmune encephalomyelitis (EAE) in DA rats. Using truncated peptides, we determined that two encephalitogenic epitopes reside within MBP63-81, a major determinant defined by MBP residues, 63-76, and a minor encephalitogenic epitope defined by residues, 66-81. Experiments with alanine-substituted analogs of MBP63-76 revealed that the HYGSLP sequence is critical for encephalitogenicity. The core epitope within a second encephalitogenic region, MBP101-120, was defined by residues, 106-119. Studies with analogs of this sequence indicated that residues, Leu 111, Phe 114 and Trp 116 are important for T-cell responsiveness.

Concordance and contradiction concerning cytokines and chemokines in experimental demyelinating disease.

Experimental autoimmune encephalomyelitis (EAE) has served as a model for human demyelinating diseases, including multiple sclerosis. EAE is mediated by CD4+ T lymphocytes of the TH1 subset. These T cells produce inflammatory cytokines and chemokines that are associated with pathogenicity. The disease is downregulated by other T cells, presumably of the TH2 subset that secrete a different pattern of cytokines which modulate the activity of the pathogenic TH1 cells. Ongoing studies should provide insight into how the interactions of T-cell subsets impact on the pathogenesis of autoimmune demyelinating diseases.

Identification of nucleotide sequences that regulate transcription of the MCF13 murine leukemia virus long terminal repeat in activated T cells.

The region downstream of the enhancer (DEN) of the long terminal repeat of the mink cell focus-forming murine leukemia virus is important for viral pathogenicity. Another important activity of DEN is its control of transcription in activated T cells, and we have determined that an NF-kappaB site is critical for this activity.