In the rat, citrullinated autologous fibrinogen is immunogenic but the induced autoimmune response is not arthritogenic

Conversion of arginyl to citrullyl residues (citrullination) is essential for the formation of the epitopes recognized by rheumatoid arthritis (RA)-associated autoantibodies to citrullinated proteins (ACPA). ACPA are secreted by plasma cells of the rheumatoid synovial tissue where their major target, citrullinated fibrin, is abundant. Although numerous arguments suggest that ACPA play an important role in RA, their pathological relevance remains to be established. In the present study, we assessed the immunogenicity and arthritogenicity of complete Freund's adjuvant-emulsified autologous citrullinated (C-rFBG) or non-citrullinated (NC-rFBG) fibrinogen in Lewis (LEW) and Brown-Norway rats, which exhibit drastic differences in their susceptibility to induced autoimmune diseases. NC-rFBG induced no antibody response. In contrast, a single injection of C-rFBG induced an IgG response directed mainly to citrullinated determinants of rFBG. However, all rat strains remained devoid of clinical and histological signs of arthritis up to 3 months after C-rFBG inoculation. Next, in LEW rats, we tested whether autoimmunity to C-rFBG could aggravate acute ankle arthritis triggered by intra-articular injection of incomplete Freund's adjuvant (IFA). However, such arthritis evolved identically in the presence or absence of anti-C-rFBG autoantibodies. However, IFA-injected joints were devoid of citrullinated fibrin deposits. Therefore, citrullination allows breakdown of immunological tolerance but the autoimmune response developed is not spontaneously arthritogenic. Whether or not it can aggravate arthritis with citrullinated fibrin deposits remains to be evaluated.

Epitopes of human fibrin recognized by the rheumatoid arthritis-specific autoantibodies to citrullinated proteins

Formation of the epitopes recognized by the rheumatoid arthritis (RA)-specific autoantibodies to citrullinated proteins (ACPA) on filaggrin and on the alpha- and beta-chains of fibrin, their synovial target, requires conversion of their arginyl residues into citrullyl residues, but is also affected by their amino-acyl environment. Using competition with five citrullinated filaggrin-derived peptides bearing major ACPA epitopes, we confirmed the close cross-reactivity between filaggrin and citrullinated fibrin. To identify the sequential epitopes recognized on fibrin by ACPA, 71 citrullinated 15-mer peptides derived from all the sites of the alpha- and beta-chains of fibrin harboring arginyl residues were tested by ELISA using ACPA-positive RA sera exhibiting different reactivity profiles to the five filaggrin peptides. We identified 18 fibrin-derived peptides bearing ACPA epitopes. Regarding the ability of fibrinogen arginyl residues to be citrullinated in vitro, 11 of the peptides likely correspond to in vivo targeted epitopes. Two out of them bear major epitopes and are located in the central globular domain of the protein. In the synovial tissue, fibrin citrullination and ACPA binding could impair fibrin degradation by plasmin. The immunological conflict between ACPA and fibrin could therefore sustain synovial inflammation not only via pro-inflammatory effector mechanisms but also via impairment of fibrinolysis.

Influence of HLA-DR genes on the production of rheumatoid arthritis-specific autoantibodies to citrullinated fibrinogen

OBJECTIVE

Antibodies directed against citrullinated fibrinogen are highly specific for rheumatoid arthritis (RA). This study was undertaken to test whether RA-associated HLA-DR alleles are associated with anti-citrullinated fibrinogen in RA patient sera and whether replacement of arginyl by citrullyl residues on fibrinogen peptides modifies their binding to HLA-DR molecules and their recognition by T cells.

METHODS

Antikeratin, antifilaggrin, and anti-citrullinated fibrinogen antibodies were assayed in RA patients who had undergone HLA-DR typing. Direct assays were performed to investigate binding of citrullinated or native fibrinogen peptides (encompassing the entire alpha- and beta-chains of fibrinogen) to purified HLA-DR molecules. T cell proliferative responses to citrullinated or native fibrinogen peptides were measured in RA patients and controls.

RESULTS

HLA-DRB1*0404 was associated with anti-citrullinated fibrinogen in RA sera (P = 0.002). For the RA-associated alleles HLA-DRB1*0401 and HLA-DR1, there was a nonsignificant trend toward association (P = 0.07). Multiple peptides from the alpha- and beta-chains of fibrinogen bound many HLA-DR alleles; DRB1*0404 was the best fibrinogen peptide binder. Citrullination did not influence fibrinogen peptide binding to HLA-DR or fibrinogen peptide recognition by T cells. Peripheral blood T cells that recognized native or citrullinated fibrinogen peptides were common in RA patients but not in healthy controls.

CONCLUSION

The RA-associated HLA-DRB1*0404 allele is also associated with production of antibodies to citrullinated fibrinogen. DRB1*0401 and DRB1*01 tend to be associated with anti-citrullinated fibrinogen, but this is not statistically significant. Citrullination of fibrinogen peptide does not influence peptide-DR-T cell interaction. Finally, T cell proliferation in response to citrullinated or uncitrullinated fibrinogen peptides is frequent in RA patients and very infrequent in controls.

Fibrin deimination in synovial tissue is not specific for rheumatoid arthritis but commonly occurs during synovitides

Autoantibodies to deiminated (citrullinated) proteins are the most specific serological markers of rheumatoid arthritis (RA). Deimination is critical in generating the peptidic epitopes they recognize. In the synovial tissue (ST), deiminated forms of the alpha- and beta-chains of fibrin are their major autoantigenic targets (anti-human fibrin(ogen) autoantibodies (AhFibA)). We investigated whether the presence of deiminated fibrin in the ST was specific for RA, because this could explain why AhFibA are RA specific. In 13 patients with RA and 19 patients with various other rheumatological disorders, knee ST biopsies were collected in macroscopically inflamed areas identified under arthroscopy. Synovitis was histopathologically confirmed in all of the biopsies. By immunoblotting, using antisera to fibrin, Abs to citrullyl residues, and AhFibA purified from RA sera, deiminated fibrin was evidenced in ST extracts from all of the patients. Moreover, variations in the degree of fibrin deimination were observed that were not related to the disease. Immunohistochemical analysis, using Abs to citrullyl residues and an antiserum to fibrin on adjacent serial sections of ST, confirmed the results because deiminated proteins colocalized with fibrin in RA as well as in control patients. Therefore, fibrin deimination in the ST is a general phenomenon associated to any synovitis, which does not necessarily induce a B autoimmune response with production of AhFibA.

IgG subclass distribution of the rheumatoid arthritis-specific autoantibodies to citrullinated fibrin

In the rheumatoid synovium, deiminated ('citrullinated') forms of fibrin are the major targets of IgG autoantibodies to citrullinated proteins (ACPA), the most specific serological markers of rheumatoid arthritis (RA). To further the characterization of ACPA, we determined their subclass distribution. From a previously validated highly sensitive and specific enzyme-linked immunosorbent assay (ELISA) onto in vitro deiminated human fibrinogen - antihuman fibrin(ogen) autoantibodies (AhFibA)-ELISA - we derived and calibrated four ELISAs, using monoclonal antibodies to each of the four IgG subclasses, to determine the proportions of AhFibA subclasses in the sera. A series of 186 serum samples from RA patients was analysed. All AhFibA-positive sera contained IgG1-AhFibA, which reached the highest titres and accounted for more than 80% of AhFibA in three-quarters of the sera. One or two other subclasses were associated with IgG1 in 39% of the sera, IgG4-AhFibA being observed much more frequently and at higher titres than IgG3- or IgG2-AhFibA. IgG1 alone or IgG(1 + 4)-AhFibA were the AhFibA subclass profiles found in more than 80% of patients. AhFibA are mainly IgG1 and, to a lesser extent, IgG4. Such IgG subclass profiles may influence the effector phases of the immunological conflict between ACPA and deiminated fibrin that takes place specifically in the rheumatoid synovium and therefore may play a critical role in the self-maintenance of rheumatoid inflammation.

Clinical and pathophysiological significance of the autoimmune response to citrullinated proteins in rheumatoid arthritis

Rheumatoid arthritis (RA) is the most frequent human autoimmune disease, affecting about 1% of the adult population worldwide. A better knowledge of the autoimmune mechanisms involved is essential. We identified the epithelial targets of various autoantibodies specifically associated to RA, as variants of (pro)filaggrin. We also showed that these targets correspond to deiminated ("citrullinated") proteins, of which arginyl residues have been posttranslationally transformed into citrullyl residues by a peptidylarginine deiminase (PAD). Moreover, we and others established that citrullyl residues are indispensable elements of the epitopes recognized by these autoantibodies but only in the context of specific aminoacid sequences. We also demonstrated that these autoantibodies to citrullinated proteins (ACPA) are secreted by plasma cells of the synovial tissue and that their major targets correspond to citrullinated forms of the alpha- and beta-chains of fibrin, abundant in the tissue. These results have allowed the development of new efficient immunochemical methods for the detection of ACPA. Some of them are already commercially available. These new methods have permitted the high diagnostic value of ACPA which are present very early in the course of the disease, and also their prognostic value, to be confirmed. ACPA detection should therefore prove to be also a very valuable tool to guide the choice of therapeutic strategies, from the earliest stages of the disease. The synthesis of ACPA in the rheumatoid synovial tissue and the existence therein of a specific antigenic target constitute a strong argument for the involvement of this specific immunological conflict in the pathophysiology of RA. Indeed, it could lead to activation of effector mechanisms with pro-inflammatory effects, thus to formation in the tissue of new fibrin deposits, secondarily citrullinated. We therefore, propose a new pathophysiological model accounting for the self-maintenance and chronicity of rheumatoid inflammation. Numerous questions about the pathophysiological significance of the autoimmune response to deiminated proteins in RA remain to be answered to confirm this model.

A family based study shows no association between rheumatoid arthritis and the PADI4 gene in a white French population

BACKGROUND

Autoantibodies to citrullinated proteins (ACPA) are considered a specific marker for rheumatoid arthritis. Peptidylarginine deiminase (PAD) is the enzyme that converts arginyl into citrullyl residues; different isoforms of the enzyme are expressed in mammals. It has been suggested that the PADI4 gene may contribute to genetic susceptibility to rheumatoid arthritis, but conflicting results have been obtained in different populations.

OBJECTIVE

To test the hypothesis that the PADI4 gene may confer susceptibility to rheumatoid arthritis in a white French population, using powerful and highly reliable family based association tests.

METHODS

DNA samples were analysed from 100 families where one member was affected by rheumatoid arthritis and both parents were available for sampling. Five single nucleotide polymorphisms, located within the PADI4 gene and in its close proximity, were genotyped by restriction fragment length polymorphism, and haplotypes were constructed. The analysis involved use of the transmission disequilibrium test and genotype relative risk. ACPA were detected by ELISA on cyclic citrullinated peptides and on human deiminated fibrinogen.

RESULTS

No single SNP or haplotype was associated with the disease, or was preferentially transmitted. No association was found when patients were partitioned according to ACPA positivity.

CONCLUSIONS

No PADI4 haplotype is associated with rheumatoid arthritis in a white French population. The role of genes encoding the other PAD isoforms, or modulating tissue expression or enzyme activity, remains to be elucidated.

cDNA cloning, gene organization and expression analysis of human peptidylarginine deiminase type I

Peptidylarginine deiminases (PADs) catalyse a post-translational modification of proteins through the conversion of arginine residues into citrullines. The existence of four isoforms of PAD (types I, II, III and IV) encoded by four different genes, which are distinct in their substrate specificities and tissue-specific expression, was reported in rodents. In the present study, starting from epidermis polyadenylated RNA, we cloned by reverse transcriptase-PCR a full-length cDNA encoding human PAD type I. The cDNA was 2711 bp in length and encoded a 663-amino-acid sequence. The predicted protein shares 75% identity with the rat PAD type I sequence, but displays only 50-57% identity with the three other known human isoforms. We have described the organization of the human PAD type I gene on chromosome 1p36. A recombinant PAD type I was produced in Escherichia coli and shown to be enzymically active. Human PAD type I mRNAs were detected by reverse transcriptase-PCR not only in the epidermis, but also in various organs, including prostate, testis, placenta, spleen and thymus. In human epidermis extracts analysed by Western blotting, PAD type I was detected as a 70 kDa polypeptide, in agreement with its predicted molecular mass. As shown by immunohistochemistry, the enzyme was expressed in all the living layers of human epidermis, with the labelling being increased in the granular layer. This is the first description of the human PAD type I gene and the first demonstration of its expression in epidermis.

Detection of antibodies to deiminated recombinant rat filaggrin by enzyme-linked immunosorbent assay: a highly effective test for the diagnosis of rheumatoid arthritis

OBJECTIVE

To assay antifilaggrin autoantibodies, we developed an enzyme-linked immunosorbent assay (ELISA) using a "citrullinated" recombinant rat filaggrin. Our objectives were to assess its value for diagnosing rheumatoid arthritis (RA) and to compare the results with those obtained using 4 other reference methods for detection of antifilaggrin autoantibodies, including the commercially available ELISA that uses a modified "citrullinated" synthetic peptide derived from the sequence of human filaggrin (CCP-ELISA).

METHODS

We analyzed 711 sera from patients with well-characterized rheumatic diseases, including 240 patients with RA. Antifilaggrin autoantibodies were detected by an ELISA using a recombinant rat filaggrin deiminated in vitro as immunosorbent (ArFA-ELISA). The results considered were the differences between the optical densities obtained on deiminated and nondeiminated proteins. Antibodies to rat esophagus epithelium were detected by indirect immunofluorescence, while antibodies to human filaggrin were detected by immunoblotting and by a recently described ELISA using a deiminated recombinant human filaggrin. Finally, CCP-ELISA was performed according to the manufacturer's recommendations.

RESULTS

At the titer thresholds allowing diagnostic specificities of 0.95, 0.985, and 0.99 to be reached, the diagnostic sensitivities of the ArFA-ELISA were 0.76, 0.67, and 0.65, respectively. At these 3 thresholds, the sensitivities were significantly higher than those of the 4 other tests. Despite incomplete overlapping of the 5 tests, the high diagnostic performance of the ArFA-ELISA allows us to propose this test to replace all the other methods for antifilaggrin autoantibody detection.

CONCLUSION

ArFA-ELISA appears to be the most efficient test among those available for the detection of antifilaggrin autoantibodies, in terms of diagnostic accuracy for RA. Its diagnostic performance in early RA and its prognostic value are currently under evaluation.

Specific presence of intracellular citrullinated proteins in rheumatoid arthritis synovium: relevance to antifilaggrin autoantibodies

OBJECTIVE

To investigate the presence of citrullinated proteins in the synovial membrane of patients with rheumatoid arthritis (RA) and controls, and to analyze a possible relationship with antifilaggrin autoantibody (AFA) reactivity.

METHODS

Synovial biopsy samples were obtained from 88 consecutive patients undergoing needle arthroscopy for knee synovitis associated with RA (n = 36), spondylarthropathy (n = 35), osteoarthritis (n = 9), or other diagnoses (n = 8). Tissue sections were stained with 2 different anticitrulline polyclonal antibodies and an antifilaggrin monoclonal antibody (mAb). The phenotype of citrulline-positive cells and the colocalization with affinity-purified AFA were investigated by double immunofluorescence on frozen sections.

RESULTS

Studies with the first antibody showed that citrulline is expressed intracellularly in the lining and sublining layers of RA synovial tissue. Staining with the second antibody, monospecific for proteins containing modified citrulline, and with anti-inducible nitric oxide synthetase confirmed the presence of citrullinated proteins rather than free citrulline in the synovium. Citrulline-positive cells were detected in 50% of the RA patients (18 of 36) but in none of the controls (0 of 52). The anticitrulline reactivity colocalized with affinity-purified AFA reactivity, although stainings with the antifilaggrin mAb indicated the absence of filaggrin in the synovium.

CONCLUSION

Intracellular citrullinated proteins, which are not recognized by an antifilaggrin mAb, are expressed in RA but not in control synovium. The high specificity of this finding and the colocalization with AFA reactivity boost the interest in citrullinated proteins as possible triggers of autoimmune responses in RA. Moreover, this is the first description of a specific histologic marker for RA synovium.

Performance of two ELISAs for antifilaggrin autoantibodies, using either affinity purified or deiminated recombinant human filaggrin, in the diagnosis of rheumatoid arthritis

OBJECTIVE

To develop a standardisable enzyme linked immunosorbent assay (ELISA), using human filaggrin, for detection of antifilaggrin autoantibodies in rheumatoid arthritis (RA). To compare the diagnostic performance of the ELISA with those of reference tests: "antikeratin antibodies" ("AKA"), and antibodies to human epidermis filaggrin detected by immunoblotting (AhFA-IB).

METHODS

Two ELISAs were developed using either affinity purified neutral-acidic human epidermis filaggrin (AhFA-ELISA-pur) or a recombinant human filaggrin deiminated in vitro (AhFA-ELISA-rec) as immunosorbent. Antifilaggrin autoantibodies were assayed in 714 serum samples from patients with well characterised rheumatic diseases, including 241 RA and 473 other rheumatic diseases, using the two ELISAs. "AKA" and AhFA-IB tests were carried out in the same series of patients. The diagnostic performance of the four tests was compared and their relationships analysed.

RESULTS

The titres of "AKA", AhFA-IB, and the AhFA-ELISAs correlated strongly with each other. The diagnostic sensitivity of the AhFA-ELISA-rec, which was better than that of AhFA-ELISA-pur, was 0.52 for a specificity of 0.95. This performance was similar to those of "AKA" or AhFA-IB. However, combining AhFA-ELISA-rec with AhFA-IB led to a diagnostic sensitivity of 0.55 for a specificity of 0.99.

CONCLUSION

A simple and easily standardisable ELISA for detection of antifilaggrin autoantibodies was developed and validated on a large series of patients using a citrullinated recombinant human filaggrin. The diagnostic performance of the test was similar to that of the "AKA" and AhFA-IB. Nevertheless, combining the AhFA-ELISA-rec with one of the other tests clearly enhanced the performance.

The major synovial targets of the rheumatoid arthritis-specific antifilaggrin autoantibodies are deiminated forms of the alpha- and beta-chains of fibrin

IgG antifilaggrin autoantibodies (AFA) are the most specific serological markers of rheumatoid arthritis. In epithelial tissues, they recognize citrulline-bearing epitopes present on various molecular forms of (pro)filaggrin. Histological analysis of rheumatoid synovial membranes with an Ab to citrulline showed labeling of interstitial amorphous deposits and mononuclear cells of various types. Immunochemical analysis of exhaustive sequential extracts of the same tissues showed that they contain several deiminated (citrulline containing) proteins. Among them, two proteins, p64--78 and p55--61, present in urea-DTT and guanidine extracts, were shown by immunoblotting to be specifically targeted by AFA. By amino-terminal sequencing the proteins were identified as deiminated forms of the alpha- and beta-chains of fibrin, respectively. Their identity was confirmed using several Abs specific for the A alpha- and/or to the B beta-chain of fibrin(ogen). Moreover, AFA-positive rheumatoid arthritis (RA) sera and purified AFA were highly reactive to the A alpha- and B beta-chains of human fibrinogen only after deimination of the molecules by a peptidylarginine deiminase. Autoantibodies affinity purified from a pool of RA sera onto deiminated fibrinogen were reactive toward all of the epithelial and synovial targets of AFA. This confirmed that the autoantibodies to the deiminated A alpha-and B beta-chains of fibrinogen, the autoantibodies to the synovial proteins p64--78 and p55--61, and, lastly, AFA, constitute largely overlapping autoantibody populations. These results show that deiminated forms of fibrin deposited in the rheumatoid synovial membranes are the major target of AFA. They suggest that autoimmunization against deiminated fibrin is a critical step in RA pathogenesis.

In the rheumatoid pannus, anti-filaggrin autoantibodies are produced by local plasma cells and constitute a higher proportion of IgG than in synovial fluid and serum

IgG anti-filaggrin autoantibodies (AFA) are the most specific serological markers of rheumatoid arthritis (RA). They include the so-called 'anti-keratin antibodies' (AKA) and anti-perinuclear factor (APF), and recognize human epidermal filaggrin and other (pro)filaggrin-related proteins of various epithelial tissues. In this study we demonstrate that AFA are produced in rheumatoid synovial joints. In 31 RA patients, AFA levels were assayed at equal IgG concentrations in paired synovial fluids (SF) and sera. AFA titre-like values determined by indirect immunofluorescence and immunoblotting and AFA concentrations determined by ELISA were non-significantly different in serum and SF, clearly indicating that AFA are not concentrated in SF. In contrast, we demonstrated that AFA are enriched in RA synovial membranes, since the ELISA-determined AFA in low ionic-strength extracts of synovial tissue from four RA patients represented a 7.5-fold higher proportion of total IgG than in paired sera. When small synovial tissue explants from RA patients were cultured for a period of 5 weeks, the profile of IgG and AFA released in the culture supernatants was first consistent with passive diffusion of the tissue-infiltrating IgG (including AFA) over the first day of culture, then with a de novo synthesis of IgG and AFA. Therefore, AFA-secreting plasma cells are present in the synovial tissue of RA patients and AFA can represent a significant proportion of the IgG secreted within the rheumatoid pannus.

The Epitopes Targeted by the Rheumatoid Arthritis-Associated Antifilaggrin Autoantibodies are Posttranslationally Generated on Various Sites of (Pro)Filaggrin by Deimination of Arginine Residues

Antifilaggrin autoantibodies (AFA) are a population of IgG autoantibodies associated to rheumatoid arthritis (RA), which includes the so-called “antikeratin” Abs and antiperinuclear factor. AFA are the most specific serological markers of RA. We previously showed that they recognize human epidermal filaggrin and other profilaggrin-related proteins of various epithelial tissues. Here, we report further characterization of the protein Ags and epitopes targeted by AFA. All the Ags that exhibit numerous neutral/acidic isoelectric variants were immunochemically demonstrated to be deiminated proteins. In vitro deimination of a recombinant human filaggrin by a peptidylarginine deiminase generated AFA epitopes on the protein. Moreover, two of three filaggrin-derived synthetic peptides with a citrulline in the central position were specifically and widely recognized by AFA affinity-purified from a series of RA sera. These results indicate that citrulline residues are constitutive of the AFA epitopes, but only in the context of specific amino acid sequences of filaggrin. In competition experiments, the two peptides abolished the AFA reactivity of RA sera, showing that they present major AFA epitopes. These data should help in the identification of a putative deiminated AFA-inducing or cross-reactive articular autoantigen and provide new insights into the pathogenesis of RA. They could also open the way toward specific immunosuppressive and/or preventive therapy of RA.

Anti-perinuclear factor compared with the so called "antikeratin" antibodies and antibodies to human epidermis filaggrin, in the diagnosis of arthritides

OBJECTIVE

Antiperinuclear factor (APF), "antikeratin antibodies" ("AKA"), and antibodies to human epidermis filaggrin (AFA), are highly specific serological markers of rheumatoid arthritis (RA), which recognise epitopes on various isoforms of (pro)filaggrin. It was proposed that these antibodies are globally named antifilaggrin autoantibodies. Here the diagnostic value of the detection of each one is compared and the overlap between the three tests evaluated.

METHODS

492 serum samples were tested, including 279 RA serum samples, taken from patients in France and Belgium. APF and "AKA" titres were estimated by indirect immunofluorescence, and AFA titres by immunoblotting on filaggrin enriched human epidermis extracts.

RESULTS

By a convenient choice of the positivity thresholds, the diagnostic sensitivity and specificity of the tests were shown to be similar (0.52 and 0.97, respectively). Although the antibody titres were strongly correlated, the associations APF-AFA or AFA-"AKA" permitted more than 52% or 55% of RA to be diagnosed, with a specificity of 0.99.

CONCLUSION

APF, "AKA", and AFA detection have a similar diagnostic value. However, because the three tests do not totally overlap, associating APF with "AKA" or AFA with "AKA" can improve diagnostic sensitivity. None of the three antigens used bear all the epitopes recognised by antifilaggrin autoantibodies.

The epitopes targeted by the rheumatoid arthritis-associated antifilaggrin autoantibodies are posttranslationally generated on various sites of (pro)filaggrin by deimination of arginine residues

Antifilaggrin autoantibodies (AFA) are a population of IgG autoantibodies associated to rheumatoid arthritis (RA), which includes the so-called "antikeratin" Abs and antiperinuclear factor. AFA are the most specific serological markers of RA. We previously showed that they recognize human epidermal filaggrin and other profilaggrin-related proteins of various epithelial tissues. Here, we report further characterization of the protein Ags and epitopes targeted by AFA. All the Ags that exhibit numerous neutral/ acidic isoelectric variants were immunochemically demonstrated to be deiminated proteins. In vitro deimination of a recombinant human filaggrin by a peptidylarginine deiminase generated AFA epitopes on the protein. Moreover, two of three filaggrin-derived synthetic peptides with a citrulline in the central position were specifically and widely recognized by AFA affinity-purified from a series of RA sera. These results indicate that citrulline residues are constitutive of the AFA epitopes, but only in the context of specific amino acid sequences of filaggrin. In competition experiments, the two peptides abolished the AFA reactivity of RA sera, showing that they present major AFA epitopes. These data should help in the identification of a putative deiminated AFA-inducing or cross-reactive articular autoantigen and provide new insights into the pathogenesis of RA. They could also open the way toward specific immunosuppressive and/or preventive therapy of RA.

Immunoblotting detection of autoantibodies to human epidermis filaggrin: a new diagnostic test for rheumatoid arthritis

OBJECTIVE

We previously reported that so-called antikeratin antibodies (AKA) and antiperinuclear factor (APF) recognize epitope(s) present on human epidermal filaggrin. In the present study, we developed a new diagnostic test for rheumatoid arthritis (RA) based on detection of antifilaggrin autoantibodies (AFA) by immunoblotting.

METHODS

We tested 670 serum samples, including 190 RA. AFA titers were estimated by immunoblotting on filaggrin enriched human epidermis extracts, and AKA titers by indirect immunofluorescence (IIF) on rat esophagus epithelium. Diagnostic values of the tests were compared.

RESULTS

Each test resulted in diagnosis of more than 40% of RA samples, with a specificity of 0.99. Although the tests were strongly correlated, their association allowed the diagnosis of more than 60% of RA samples, with the same specificity.

CONCLUSION

Immunoblot detection of AFA, a simple and standardizable test, may be an alternative or complement to conventional IIF detection of AKA.

Contact with T cells modulates monocyte IL-10 production: role of T cell membrane TNF-alpha

IL-10, originally described as a cytokine synthesis inhibitory factor, is secreted by a number of cells of the immune system, including monocytes and T cells. IL-10 is a potent inhibitor of monocyte/macrophage activation, and we have shown previously this cytokine to be a major endogenous down-regulator of TNF-alpha in the rheumatoid joint. The mechanisms involved in regulating IL-10 production by cells of the monocyte/macrophage lineage are not yet clear, and most studies to date have used an exogenous triggering signal such as LPS. In this study, we have investigated the effects of cell-cell contact between human peripheral blood-derived activated T cells and monocytes in regulating monocyte IL-10 production. T cells, prestimulated with anti-CD3 mAb or with phorbol 12,13 di-butyrate and ionomycin, were fixed with glutaraldehyde and then incubated with monocytes. Fixed prestimulated T cells induced monocytes to secrete both IL-10 and TNF-alpha, and in addition, enhanced LPS-stimulated monocyte production of IL-10 and TNF-alpha in a dose-dependent manner. Stimulation of monocyte IL-10 production was abrogated when T cells were separated physically from monocytes within the tissue culture well. Using neutralizing Abs, we show that T cell contact-mediated induction of monocyte IL-10 is partially dependent on endogenous TNF-alpha and IL-1. Furthermore, T cell membrane TNF-alpha was shown to be one of the contact-mediated signals regulating monocyte IL-10 production. Endogenous IL-10 was shown to down-regulate T cell contact-mediated monocyte TNF-alpha production. Collectively, our results demonstrate that an autoregulatory loop exists involving both secreted and membrane-associated forms of IL-10 and TNF-alpha, and suggest that T cell-monocyte cognate interaction may play an important role in the regulation of monocyte cytokine production.

Contact with T cells modulates monocyte IL-10 production: role of T cell membrane TNF-alpha

IL-10, originally described as a cytokine synthesis inhibitory factor, is secreted by a number of cells of the immune system, including monocytes and T cells. IL-10 is a potent inhibitor of monocyte/macrophage activation, and we have shown previously this cytokine to be a major endogenous down-regulator of TNF-alpha in the rheumatoid joint. The mechanisms involved in regulating IL-10 production by cells of the monocyte/macrophage lineage are not yet clear, and most studies to date have used an exogenous triggering signal such as LPS. In this study, we have investigated the effects of cell-cell contact between human peripheral blood-derived activated T cells and monocytes in regulating monocyte IL-10 production. T cells, prestimulated with anti-CD3 mAb or with phorbol 12,13 di-butyrate and ionomycin, were fixed with glutaraldehyde and then incubated with monocytes. Fixed prestimulated T cells induced monocytes to secrete both IL-10 and TNF-alpha, and in addition, enhanced LPS-stimulated monocyte production of IL-10 and TNF-alpha in a dose-dependent manner. Stimulation of monocyte IL-10 production was abrogated when T cells were separated physically from monocytes within the tissue culture well. Using neutralizing Abs, we show that T cell contact-mediated induction of monocyte IL-10 is partially dependent on endogenous TNF-alpha and IL-1. Furthermore, T cell membrane TNF-alpha was shown to be one of the contact-mediated signals regulating monocyte IL-10 production. Endogenous IL-10 was shown to down-regulate T cell contact-mediated monocyte TNF-alpha production. Collectively, our results demonstrate that an autoregulatory loop exists involving both secreted and membrane-associated forms of IL-10 and TNF-alpha, and suggest that T cell-monocyte cognate interaction may play an important role in the regulation of monocyte cytokine production.

Cytokine stimulation of T lymphocytes regulates their capacity to induce monocyte production of tumor necrosis factor-alpha, but not interleukin-10: possible relevance to pathophysiology of rheumatoid arthritis

Previous studies in the laboratory have shown that the pro-inflammatory cytokine tumor necrosis factor (TNF)-alpha plays a pivotal role in the pathogenesis of rheumatoid arthritis (RA). The mechanisms involved in regulating monocyte/macrophage cytokine production are not yet fully understood, but are thought to involve both soluble factors and cell/cell contact with other cell types. We and others have previously demonstrated that T cells activated through the T cell receptor/CD3 complex induce monocyte TNF-alpha production by contact-mediated signals. In this report, we investigated further whether T cells activated by cytokines in the absence of T cell receptor stimulation also regulate monocyte cytokine production. T cells were activated in an antigen-independent manner using the cytokines interleukin (IL)-15 or IL-2 alone, or in combination with IL-6 and TNF-alpha. Subsequently, T cells were fixed and incubated with monocytes. Fixed, cytokine-stimulated T cells induced monocytes to secrete TNF-alpha in a dose-dependent manner, but did not induce secretion of IL-10, a potent endogenous down-regulator of TNF-alpha and other pro-inflammatory cytokines. Stimulation of monocyte TNF-alpha was markedly inhibited when T cells were physically separated from monocytes within the tissue culture well, confirming that T cell contact is necessary. T cell acquisition of monocyte-activating capacity was shown to be dependent on the period of cytokine stimulation, with T cells activated for 8 days more effective than T cells activated for shorter periods. Addition of interferon-gamma or granulocyte/macrophage colony-stimulating factor to the T cell/monocyte cultures enhanced T cell induction of monocyte TNF-alpha by threefold and ninefold, respectively. The results from this model of cognate interaction suggest that cytokine-stimulated T cells, interacting with macrophages in the rheumatoid synovial membrane, may contribute to the continuous excessive production of TNF-alpha observed in the RA joint, and to the imbalance of pro-inflammatory cytokines over anti-inflammatory cytokines.

Normal human epidermal keratinocytes express in vitro specific molecular forms of (pro)filaggrin recognized by rheumatoid arthritis-associated antifilaggrin autoantibodies

BACKGROUND

The so-called antikeratin antibodies and the antiperinuclear factor are the most specific serological markers of rheumatoid arthritis (RA). They were recently shown to be largely the same autoantibodies and to recognize human epidermal filaggrins and profilaggrin-related proteins of buccal epithelial cells (collectively referred to as (pro)filaggrin).

MATERIALS AND METHODS

To further characterize the target antigens, we investigated their expression by normal human epidermal keratinocytes cultured in differentiating conditions, using immunofluorescence and immunoblotting with RA sera and three different monoclonal antibodies to (pro)filaggrin.

RESULTS

On the cornified, stratified epithelial sheets obtained in vitro, RA sera with anti(pro)filaggrin autoantibodies (AFA) produced granular staining of the stratum granulosum and diffuse staining of the stratum corneum. The antigens recognized by RA sera strictly colocalized with (pro)filaggrin in keratohyalin granules. Following sequential extraction of the proteins from the epithelial sheets, the RA sera and the three monoclonal antibodies to (pro)filaggrin, recognized a series of low-salt-soluble molecules, including a neutral/acidic isoform of filaggrin and several proteins with sizes and pI intermediates between this isoform and profilaggrin. They also recognized urea-soluble high-molecular-weight profilaggrin-related molecules.

CONCLUSIONS

These results show that in vitro epidermal keratinocytes express various molecular forms of (pro) filaggrin that bear epitopes targeted by AFA of RA sera, and that some of these are absent from epidermis. Moreover, these epitopes, which are present on the keratohyalin granules of buccal epithelial cells but not on those of epidermal cells, are present on the granules of the cultured keratinocytes. This work completes the molecular characterization of the proteins targeted by AFA.

Evidence that filaggrin is a component of cornified cell envelopes in human plantar epidermis

Cornified cell envelope (CE) is generated during the late stages of epidermal differentiation and is made up of proteins covalently linked together by transglutaminases. To determine whether filaggrin is a component of this structure in humans, we analysed highly purified CE from plantar stratum corneum. An immunoelectron microscopy analysis showed specific binding of four different anti-(pro)filaggrin monoclonal antibodies to the surface of the CE, proved previously to be free of non-covalently linked proteins. Moreover, the anti-filaggrin activity of one of the antibodies was absorbed by preincubation with the plantar CE, as determined by ELISA. Convincingly, fragments of CE produced by proteolytic digestion of the structures were stained by this antibody on immunoblots. These data provide direct evidence that filaggrin is a component of CE purified from human plantar stratum corneum. Cross-linking between CE and the filaggrin-containing fibrous matrix may enhance the structural cohesion of the corneocytes and thus the resistance of the stratum corneum.