Structure of HLA-B27-specific T cell epitopes. Antigen presentation in B*2703 is limited mostly to a subset of the antigenic determinants on B*2705.

The structure of HLA-B27-specific epitopes recognized by anti-B*2705 and anti-B*2703 cytotoxic T lymphocytes (CTL) from three unrelated donors was examined with site-specific mutants at various side-chain pockets in the antigen-binding site. The effect of any given mutation on allorecognition correlated strongly with its predictable effect on peptide binding. Acidic charges in the C/F pocket of HLA-B27, which binds C-terminal peptide residues, strongly modulated allorecognition. Anti-B*2705 CTL from different donors were differently affected by some mutations, indicating individual differences in the structure of epitopes recognized by alloreactive CTL from each donor. Most anti-B*2703 CTL recognized a subset of epitopes that were also present on B*2705, but differed from the bulk of allospecific epitopes on this subtype in their smaller dependence on pocket A structure, where the difference between these subtypes is located, and in their greater dependence on Glu45, in the B pocket. The structure of the very few epitopes on B*2703 not shared by B*2705 was quite different from that of the much more predominant cross-reactive epitopes. The results strongly suggest that B*2703 is antigenically defective as compared with B*2705 and that this is due to the fact that the repertoire of peptides presented by B*2703 consists mainly of a subset of the B*2705-bound peptides which do not critically require the canonic binding of the peptidic N-terminus to a B*2705-like A pocket, because they are sufficiently stabilized by other contacts through the peptide binding site.

Clonal analysis of alloreactive T cell responses against the closely related B*2705 and B*2703 subtypes. Implications for HLA-B27 association to spondyloarthropathy.

Alloreactive responses against closely related HLA-B27 subtypes were compared at the clonal level by fine specificity analysis of anti-B*2705 and anti-B*2703 CTL clones from unrelated HLA-B27- individuals with target cells expressing B*2701 to B*2706, and other HLA Ags. T cell epitope sharing between B*2705 and other subtypes was B*2705 > B*2703 > B*2702 > B*2701 > B*2704 > B*2706, and correlated with their amino acid differences. This suggests that identical or similar peptides, or peptide motifs, can be presented by multiple HLA-B27 subtypes to T cells, a feature that may be critical for the similar linkage of several subtypes to spondyloarthropathies. Other cross-reactions were predominantly with HLA-B61 and HLA-B60. Marked differences were observed in the nature and frequency of clonal reaction patterns among individuals. They correlated with structural features of the HLA-B Ags from each donor, suggesting that anti-HLA-B27 T cell responses are partially determined by the HLA-B phenotype of the responder. Ability to respond to particular HLA-B27-associated epitopes may determine differences in disease susceptibility among HLA-B27+ individuals. The anti-B*2703 and anti-B*2705 responses in the same individual were different. A major feature of anti-B*2703 CTL was that a large majority cross-reacted with B*2705. This can be explained by the effect of the single amino acid change in B*2703 on peptide binding and suggests that the B*2703-bound peptide repertoire is mainly a subset of that bound to B*2705, with few peptides being specifically presented by B*2703 to T cells.

T cell allorecognition and endogenous HLA-B27-bound peptides in a cell line with defective HLA-B27-restricted antigen presentation.

The B*2702+ lymphoblastoid cell line NW is unable to present at least some HLA-B27-restricted viral antigens to T cells. This defect was genetically inherited, and was suggested to be related to the nature of the HLA-B27 binding peptides reaching the endoplasmic reticulum in these cells (Pazmany et al., J. Exp. Med. 1992. 175: 361). In the present study 17 of 19 HLA-B27-specific alloreactive cytotoxic T lymphocyte clones recognizing the B*2702 subtype on other cells also lysed NW cells. Only two cytotoxic T lymphocyte clones failed to lyse NW while efficiently killing other B*2702+ cell lines. The high-performance liquid chromatography profiles of the B*2702+ bound peptides extracted from NW cells was similar, but not identical, to those from two other cell lines. These results indicate that the HLA-B27-bound peptide repertoire in NW cells is not fundamentally different from those in other B*2702+ cells. Our data argue against gross differences in peptide processing or transport as being responsible for the defective presentation of particular HLA-B27-restricted viral antigens to T cells, but do not rule out distinct presentation of some endogenous peptides. Differences in the capacity to present certain peptides could cause differential susceptibility among HLA-B27+ individuals to ankylosing spondylitis.

Involvement of platelet-activating factor and tumour necrosis factor in the pathogenesis of joint inflammation in rabbits.

We have studied the participation of platelet-activating factor (PAF) in antigen-induced arthritis in rabbits, as well as the possible co-operation between PAF and tumour necrosis factor (TNF) in their ability to induce joint inflammation when injected into the knees of healthy rabbits. The administration of two structurally different PAF receptor antagonists, BN52021 and Alprazolam, from 4 h before the intra-articular injection of ovalbumin in preimmunized rabbits, induced an important reduction in the synovial fluid volume, in the amount of cells infiltrating the articular cavity and the synovial membrane, as well as in the prostaglandin E2 (PGE2) concentration. Furthermore, proteoglycans of the articular cartilage, which were found diminished in animals with non-treated arthritis, were well preserved in rabbits treated with PAF antagonists. All the synovial fluids from joints with arthritis had detectable amounts of PAF. The injection of either TNF or PAF into the joints of normal rabbits induced a mild inflammation. When TNF was administered 1 h before PAF, a synergistic response was noted in the synovial fluid volume, in the accumulation of leucocytes, and in the amount of PGE2. The administration of BN50726, a hetrazepine with a potent PAF-receptor antagonist effect, induced a diminution in those parameters. Our results suggest that PAF may be an early and important mediator of joint damage, and that TNF can amplify the inflammatory response induced by PAF. PAF receptor antagonists could play some role in the treatment of inflammatory joint diseases.

[New immunopathogenic aspects of IgA nephropathy]. / Nouveaux aspects immunopathogéniques de la néphropathie à IgA.

IgA nephropathy (Berger's disease) has become recognized worldwide as one of the most common of the primary glomerulonephritis. The mesangial granular deposits suggested an immune complex disease. The available data evidence that the IgA circulating immune complexes in these patients are heterogeneous. Recent analysis, performed after dissociating the complexes, found both IgA1 and IgG. In fact, high serum levels of IgA rheumatoid factor and shared antibody idiotypes were found in a large proportion of those patients. A close relationship was noted between the presence of cross-reactive idiotypes on mesangial immunoglobulins and the existence of increase levels of serum idiotypes and many patients have increased rates of IgA synthesis either spontaneously or after stimulation of the peripheral blood mononuclear cells by various mitogens. A lot of abnormalities on B and T lymphocytes, related with the IgA immune regulation, have been described. Most of deposited IgA in the mesangium is polymeric and belongs to the IgA1 class. Patients with IgA nephropathy have very often antibodies against exogenous and endogenous antigens. Among the most frequently found are the antibodies against dietary, viral and bacterial antigens as well as against the Fc and Fab portions of immunoglobulins, nuclear and glomerular antigens. The mechanisms of mesangial damage in IgA nephropathy are not well known. Mesangial cells are capable of producing and releasing various lipidic and proteic mediators. The stimulation of mesangial cells, cultured in vitro by IgA or IgG immune complexes induced the release of PAF, PGE2 and superoxide anion. A better knowledge of the mechanism implicated in the abnormality of IgA immune regulation, as well as of the glomerular inflammation response could afford a new therapeutic approach to this nephropathy.

IgA nephropathy: association of a history of macroscopic hematuria episodes with increased production of polymeric IgA.

From the clinical point of view patients with IgA nephropathy present two main symptoms. Some patients have episodes of macroscopic hematuria, either isolated or recurrent, while others present only asymptomatic urinary abnormalities. Although these differences could be due to age-related, geographical or other unknown reasons, in this paper we studied whether patients with these two clinical manifestations differ in some IgA immunological aspects, described occasionally as abnormal in this disease. The following results were obtained: patients having a history of macroscopic hematuria episodes (n = 29) had a significant increase in the percentage of blood polymeric-IgA-producing cells (68 +/- 12%) and in the T cells with IgA Fc-receptors (T alpha cells; 14.5 +/- 3%) in relation to those having only asymptomatic urinary abnormalities (n = 13; 48 +/- 11 and 11.8 +/- 3%, respectively). However, there was no difference in the two groups of patients in relation to the serum IgA levels, percentage of IgA-bearing cells, in vitro synthesis of IgA, T-cell subpopulation and generation of Con A-IgA suppressor cells. Since these patients did not differ in the age of the apparent onset of the disease, in age at the moment of the study or in the activity of the disease determined by hematuria, our results afford an immunological support suggesting the existence of two subgroups of patients with IgA nephropathy.

Immunological abnormalities in the tonsils of patients with IgA nephropathy: inversion in the ratio of IgA: IgG bearing lymphocytes and increased polymeric IgA synthesis.

In the last few years the mucosal origin of the IgA deposited in the kidneys of patients with IgA nephropathy has been examined by several investigators. We have previously presented evidence that polymeric IgA may have a predominant role in the pathogenesis of IgA nephropathy. Taking into account that these patients often present with macroscopic haematuria following respiratory tract infections we have studied the possible existence of immunological abnormalities in the tonsils of patients with IgA nephropathy. Six patients and 13 controls suffering from chronic tonsillitis were submitted to tonsillectomy. Patients with IgA nephropathy showed a significant increase (P less than 0.00025) in IgA bearing lymphocytes (14.4 +/- 2.3) and a significant decrease (P less than 0.025) in IgG bearing lymphocytes (20.5 +/- 4.6) compared to the control group (2.9 +/- 1.4 and 31.6 +/- 3.6, respectively). After 7 days of culture with pokeweed mitogen the percentage of tonsillar cells producing polymeric IgA was significantly higher in the patients than in the controls (66.5 +/- 12.6 vs 33.4 +/- 10.3; P less than 0.005). These results also suggest a mucosal origin for the IgA deposited in the kidneys of these patients. Our data are consistent with the existence of an immunoregulatory dysfunction in the secretory immune system of patients with IgA nephropathy.