Marco Vergelli

TCR ligand discrimination is enforced by competing ERK positive and SHP-1 negative feedback pathways

Functional discrimination between structurally similar self and foreign antigens is a main attribute of adaptive immunity. Here we describe two feedback mechanisms in T lymphocytes that together sharpen and amplify initial signaling differences related to the quality of T cell receptor (TCR) engagement. Weakly binding ligands predominantly trigger a negative feedback loop leading to rapid recruitment of the tyrosine phosphatase SHP-1, followed by receptor desensitization through inactivation of Lck kinase. In contrast, strongly binding ligands efficiently activate a positive feedback circuit involving Lck modification by ERK, preventing SHP-1 recruitment and allowing the long-lasting signaling necessary for gene activation. The characteristics of these pathways suggest that they constitute an important part of the mechanism allowing T cells to discriminate between self and foreign ligands.

Probing degeneracy in T-cell recognition using peptide combinatorial libraries

Recent studies have demonstrated flexibility of the T-cell receptor (TCR) with respect to recognition of peptide bound to self major histocompatibility complex (MHC). With the introduction of peptide combinatorial libraries, it has become possible to dissect the extent of degeneracy in T-cell recognition. On the basis of these novel findings, Bernhard Hemmer and colleagues propose a conceptual framework for lymphocyte selection and survival and the occurrence of autoimmunity.

Immunodominance of a low-affinity major histocompatibility complex-binding myelin basic protein epitope (residues 111-129) in HLA-DR4 (B1*0401) subjects is associated with a restricted T cell receptor repertoire.

The pathogenesis of multiple sclerosis (MS) is currently ascribed in part to a T cell-mediated process targeting myelin components. The T cell response to one candidate autoantigen, myelin basic protein (MBP), in the context of HLA-DR15Dw2, has been previously studied in detail. However, the characteristics of cellular immunity in the context of other MS-associated HLA-DR haplotypes are scarcely known. MBP-specific T cell lines (TCL) were generated from HLA-DR4 (B1*0401)-positive MS subjects. Out of 275 MBP-specific TCL, 178 (64. 7%) specifically recognized region MBP(111-129), predominantly in the context of DRB1*0401. The major T cell epitope for MBP recognition corresponded to residues MBP(116-123). These TCL expressed disparate profiles of cytokine secretion and cytotoxicity. T cell receptor analysis, on the other hand, revealed a strikingly limited heterogeneity of rearrangements. In contrast to MBP(81-99), which binds with high affinity to HLA-DR15 and is recognized by a diverse T cell repertoire, MBP(111-129) binds weakly to DRB1*0401, suggesting that only high affinity T cell receptors might be able to efficiently engage such unstable MHC/peptide complexes, thus accounting for the T cell receptor restriction we observed. This study provides new insight about MBP recognition and proposes an alternative mechanism for immunodominance of self-antigen T cell epitopes in humans.

Molecular Mimicry and Multiple Sclerosis: Degenerate T-Cell Recognition and the Induction of Autoimmunity

Various mechanisms have been proposed for the initiation of autoimmune responses by autoreactive T‐cell clones. One of these, the molecular mimicry hypothesis, postulates that myelin‐reactive T‐cell clones are activated by foreign antigens. Until recently, sequence homology between self‐ and foreign antigens was considered necessary for cross‐recognition to occur in multiple sclerosis. This article reviews current progress in T‐cell receptor immunology that led to modify this view and proposes a role for degenerate T‐cell antigen recognition in the induction of autoimmunity. Ann Neurol 1999;45:559–567

Contribution of Individual Amino Acids Within MHC Molecule or Antigenic Peptide to TCR Ligand Potency

The TCR recognition of peptides bound to MHC class II molecules is highly flexible in some T cells. Although progress has been made in understanding the interactions within the trimolecular complex, to what extent the individual components and their amino acid composition contribute to ligand recognition by individual T cells is not completely understood. We investigated how single amino acid residues influence Ag recognition of T cells by combining several experimental approaches. We defined TCR motifs for CD4+ T cells using peptide synthetic combinatorial libraries in the positional scanning format (PS-SCL) and single amino acid-modified peptide analogues. The similarity of the TCR motifs defined by both methods and the identification of stimulatory antigenic peptides by the PS-SCL approach argue for a contribution of each amino acid residue to the overall potency of the antigenic peptide ligand. In some instances, however, motifs are formed by adjacent amino acids, and their combined influence is superimposed on the overall contribution of each amino acid within the peptide epitope. In contrast to the flexibility of the TCR to interact with different peptides, recognition was very sensitive toward modifications of the MHC-restriction element. Exchanges of just one amino acid of the MHC molecule drastically reduced the number of peptides recognized. The results indicate that a specific MHC molecule not only selects certain peptides, but also is crucial for setting an affinity threshold for TCR recognition, which determines the flexibility in peptide recognition for a given TCR.

Immunosuppressive activity of 13-cis-retinoic acid and prevention of experimental autoimmune encephalomyelitis in rats.

Some activities of retinoids on cellular and humoral immunity have been described, but the available data are conflicting or obtained at concentrations that are toxic in vivo. In this study, we demonstrate that 13-cis-retinoic acid (13-cRA), a retinoid well tolerated in human therapy, can suppress T cell-mediated immunity in rats. Treatment with pharmacological concentrations of 13-cRA prevented active as well as passive transfer experimental autoimmune encephalomyelitis (EAE) and suppressed lymphocyte responsiveness to T cell mitogens, suggesting that the drug activity included suppression of an effector T cell response. In addition, mitogen- and antigen-induced lymphocyte proliferation was inhibited in vitro in the presence of concentrations of 13-cRA equivalent to or less than those achieved in vivo, further suggesting that the prevention of EAE was due to a suppressive activity on T cell-mediated immunity. The immunosuppressive activity of 13-cRA included suppression of interleukin 2, whose production was inhibited in splenocytes. These data indicate that, in an in vivo mammalian system, 13-cRA exerts a suppressive activity on T cell-mediated immunity intensive enough to suppress an ongoing immune response, and that this effect can be achieved at nontoxic concentrations that may also be attained in human therapy.

CYTOKINE PHENOTYPE OF HUMAN AUTOREACTIVE T CELL CLONES SPECIFIC FOR THE IMMUNODOMINANT MYELIN BASIC PROTEIN PEPTIDE (83-99)

Experimental allergic encephalomyelitis (EAE), an animal model resembling multiple sclerosis (MS), is mediated by myelin antigen‐specific CD4+ T cells secreting cytokines such as interferon‐γ (IFN‐γ), tumor necrosis factor‐β (TNF‐β), and the proinflammatory cytokine TNF‐α—all associated with the T‐helper‐1 (Th1) T cell subset. Based on numerous similarities between MS and EAE, it has been postulated that Th1‐like T cells are involved in the pathogenesis of MS. Production of proinflammatory cytokines such as IFN‐γ and, in particular, TNF‐α/β by autoreactive T cells is considered crucial for the initiation and amplification of inflammatory brain lesions and possibly also for direct myelin damage. In contrast, regulatory cytokines such as interleukin‐4 (IL‐4), IL‐10, and IL‐13, which are associated with the Th2‐like phenotype, may play a role in the resolution of relapses. Although the human T cell response to myelin basic protein (MBP) is well characterized in terms of antigen specificity, HLA restriction, and T cell‐receptor (TCR) usage, little is known about the cytokine pattern of these autoreactive T cells. To gain such information, conditions for studying cytokine secretion by human autoreactive T cell clones (TCC) were established. The cytokine secretion profile of human autoreactive CD4+ TCC, specific for myelin basic protein peptide (83–89) [MBP(83–99)], a candidate autoantigen in MS, was investigated. Our results show that TCC cytokine production in long‐term culture was stable. In addition, the correlation of various cytokines within specific TCC revealed differences compared to murine T cells. The comparison of 30 human MBP(83–99)‐specific TCC demonstrated heterogeneity in cytokine secretion, with a continuum between Th1‐ and Th2‐like cells rather than distinct Th1 or Th2 subsets. These data are important for further investigation of the potential role of cytokines in the inflammatory process of MS, and provide a powerful tool to investigate therapeutic interventions with respect to their influence on cytokine secretion of autoreactive T cells.

Protein tyrosine phosphatase receptor-type C exon 4 gene mutation distribution in an Italian multiple sclerosis population

In this study, we investigate the role of the C-->G mutation in position 77 of exon 4 of the protein tyrosine phosphatase receptor-type C (PTPRC) gene, coding for the CD45 molecule, for the development of multiple sclerosis (MS) in an Italian continental population. The PTPRC mutated genotype has been recently described as associated with MS in three different case-control studies carried out in German MS patients, whereas similar studies performed in the US and Swedish populations failed to demonstrate such an association. The C-->G transition in position 77 was found in a small number of Italian MS patients and in none of the matched group of healthy controls (Fisher exact test, P value=0.02). This finding suggests a role, in at least a group of patients, for the PTPRC mutation in genetic susceptibility to MS.

A synthetic glycopeptide of human myelin oligodendrocyte glycoprotein to detect antibody responses in multiple sclerosis and other neurological diseases

Glycopeptides of hMOG(30-50) containing a glucosyl moiety on the side-chains of Asn, Ser or Hyp at position 31 were synthesised. Antibody titres to hMOG(30-50) and to its glucoderivatives were measured by ELISA in sera of patients affected by different neurological diseases. Anti-hMOG(30-50) antibodies were detected only using the glycopeptide [Asn31(N-Glc)]hMOG(30-50).

IL-7-enhanced T-cell response to myelin proteins in multiple sclerosis

In this study, we investigated the in vitro proliferative response of peripheral blood T lymphocytes from MS patients and controls to MBP and MOG either in the absence or in the presence of the conditioning factor IL-7. In the absence of IL-7, T-cell reactivity to MOG and MBP was similar in MS patients and controls even if an increased MBP response was found in a subgroup of patients with active disease. In the presence of IL-7, increased T-cell reactivity to MBP was observed in MS patients suggesting that their MBP-specific T cells are in a different functional state.