[ADAPTHERA-Statewide cross-sectoral care network for patients with early rheumatoid arthritis shows sustained remission in standard care]. / ADAPTHERA ­ Landesweit transsektorales Versorgungsnetzwerk für Patienten mit früher rheumatoider Arthritis zeigt anhaltende Remissionen in der Regelversorgung.

BACKGROUND/OBJECTIVE: The majority of patients in Germany miss out on the necessity of early diagnosis and initiation of therapy for rheumatoid arthritis (RA) caused by considerable structural deficits in the health care system. The challenge is to reconcile the individual demand for the best possible therapy result with a sustainable expenditure of resources. METHODS: The cross-sectoral regional care network ADAPTHERA aims to improve early RA diagnosis and treatment in Rhineland-Palatinate. The retrospective triage analyses of suspected early onset RA patients was performed by tracing the selection process of all available enquiries (n = 1045). For analysis of the clinical course of the disease, a subset comprising 143 patients with a minimum observation time of 12 months (5 consecutive visits) was available. Clinical and laboratory parameters were collected quarter yearly, self-administered questionnaires were filled out and the treatment was adapted if necessary. RESULTS: A total of 454 patients were included. The mean waiting time was 23.9 (SD = 18) days. The mean observation period in the subcohort was 29.2 (SD = 12.7) months, with about 50% of the patients presenting within 3 months. Almost 75% of the patients were in remission after 2 years. A sustained remission could be described for 74.8% (6 months) and 53.5% (12 months), respectively. Especially patients with rapid remission induction benefited in terms of longer remissions (p = 0.03). A very early stage of the disease (VERA) was associated with a rarely necessary biologic therapy (p = 0.022). DISCUSSION: The approach of a supply network is not a panacea, but it might improve healthcare for patients with early onset RA. In order to minimize resource utilization, a pinpoint referral and accurate triage of potential cases are crucial.

[The regional network ADAPTHERA : Rheumatology care through coordinated cooperation: comprehensive, trans-sectoral, covering all health insurance. Initial results]. / Das landesweite Netzwerk ADAPTHERA : Erste Ergebnisse einer flächendeckenden, krankenkassenübergreifenden und transsektoralen rheumatologischen Versorgung.

The aim of the rheumatology network ADAPTHERA ("risk-adapted rheumatology therapy") is to achieve a comprehensive improvement in rheumatology care by coordinating treatment in a regional, trans-sectoral network. Accompanying biomedical research projects, training concepts, and the construction of a rheumatology register (gathering data and biomaterials) should furthermore ensure the stable and sustainable optimisation of care. In the pilot phase (2012-2015) the focus of the ADAPTHERA network, required as a "regional key project" within the framework of the Initiative on Health Economy of Rheinland-Palatinate (RL-P), Germany, was placed on the optimisation of the early diagnosis of rheumatoid arthritis, where it is well-known that there is a significant care deficit.Through the intensive, stable, and coordinated cooperation of all health care partners in the field of rheumatology (registered general practitioners and orthopaedic specialists, registered core rheumatologists as well as the Association of Rheumatology of RL-P) a unique regional, comprehensive offer with verifiable care optimisation has been established in RL-P. The network is supported by outstanding collaboration with the Association of Statutory Health Insurance Physicians and the self-help organisation Rheumatology League.The aims that were established at the start of the project will be achieved by the end of the pilot phase:- significant improvement in the early diagnosis of rheumatoid arthritis (an average of 23.7 days until diagnosis by rheumatologists)- access covering all health insurance (regardless of the particular scheme the patients belong to)- comprehensive (verifiable participation of general practitioners from all over RL-P)- data and biomaterials collection, established as a basis for biomarker research, and a rheumatology register for RL-P.

Characteristics of joint involvement and relationships with systemic inflammation in systemic sclerosis: results from the EULAR Scleroderma Trial and Research Group (EUSTAR) database.

OBJECTIVE: To determine the prevalence of and independent factors associated with joint involvement in a large population of patients with systemic sclerosis (SSc). METHODS: This study was cross-sectional, based on data collected on patients included in the European League Against Rheumatism (EULAR) Scleroderma Trials and Research (EUSTAR) registry. We queried this database to extract data regarding global evaluation of patients with SSc and the presence of any clinical articular involvement: synovitis (tender and swollen joints), tendon friction rubs (rubbing sensation detected as the tendon was moved), and joint contracture (stiffness of the joints that decreased their range of motion). Overall joint involvement was defined by the occurrence of synovitis and/or joint contracture and/or tendon friction rubs. RESULTS: We recruited 7286 patients with SSc; their mean age was 56 +/- 14 years, disease duration 10 +/- 9 years, and 4210 (58%) had a limited cutaneous disease subset. Frequencies of synovitis, tendon friction rubs, and joint contractures were 16%, 11%, and 31%, respectively. Synovitis, tendon friction rubs, and joint contracture were more prevalent in patients with the diffuse cutaneous subset and were associated together and with severe vascular, muscular, renal, and interstitial lung involvement. Moreover, synovitis had the highest strength of association with elevated acute-phase reactants taken as the dependent variable. CONCLUSION: Our results highlight the striking level of articular involvement in SSc, as evaluated by systematic examination in a large cohort of patients with SSc. Our data also show that synovitis, joint contracture, and tendon friction rubs are associated with a more severe disease and with systemic inflammation.

Increased titres of anti-nuclear antibodies do not predict the development of associated disease in the absence of initial suggestive signs and symptoms.

OBJECTIVE: To determine whether patients with elevated anti-nuclear antibodies (ANA), absent extractable nuclear antigen (ENA) reactivity, and no definite associated disease develop an ANA-associated disease (AAD). METHODS: Patients with ANA titres of at least 1:320 and no ENA reactivity were identified by searching the database of our laboratory serving a tertiary care university hospital between 1998 and 2002. Medical records of this index time point were reviewed to exclude patients with active AAD at screening. Case patients were contacted by questionnaire between 2004 and 2005 and invited for a clinical visit to ascertain the individual disease status. RESULTS: Seventy-six patients were evaluated after a median follow-up of 32 months. An AAD was diagnosed in eight patients: connective tissue disease (CTD) in three, autoimmune hepatitis in two, rheumatoid arthritis in one, encephalomyelitis disseminate in one, and lymphoma in one. The only predictive factor associated with the development of AAD was the suspicion of an autoimmune disease by the treating physician at the initial evaluation. In the absence of initial suspicion for an autoimmune disease, only two out of 54 patients developed AAD, whereas six out of 22 patients with initial disease suspicion developed a defined AAD. CONCLUSION: In the absence of a clinical suspicion, elevated ANA titres have a low positive predictive value of 4% for developing AAD for the upcoming 3 years.

Antibiotic treatment of Lyme borreliosis: what is the evidence?

Antibiotic treatment of all disease manifestations after infection with Borrelia sensu lato spp aims at resolving the presenting disease manifestation and preventing late stage disease. The goals are resolution of the preventing manifestation and prevention of the spread of bacteria to prevent late disease like arthritis. Different borrelial species prevail in Europe. The natural disease course of European borreliosis is not well defined and the effect of antibiotic treatment is unclear.

Standardised work-up programme for fever of unknown origin and contribution of magnetic resonance imaging for the diagnosis of hidden systemic vasculitis.

BACKGROUND: Fever of unknown origin (FUO) is a diagnostic challenge. Rheumatologists are often in charge of patients with FUO because the vasculitides, especially, are potential and common causes of FUO. OBJECTIVE: To evaluate the value of a standardised investigation to identify the cause of FUO. METHODS: A standardised work-up programme for patients with FUO was started at the beginning of September 1999. The rate of identified causes of FUO was compared between all patients with FUO admitted to a tertiary care centre of rheumatology between January 1996 and August 1999 (control group) and September 1999 and January 2003 (work-up group). In January 2002 magnetic resonance imaging (MRI) was added to the investigation. RESULTS: 67 patients with FUO were identified--32 before and 35 after institution of the work-up programme. Before implementation 25% of all patients with FUO remained undiagnosed, after implementation 37%. After institution of the investigation the percentage of patients with vasculitides increased significantly from 6% (n = 2) to 26% (n = 9, p = 0.047, Fisher's exact test). This increase could be attributed to the addition of MRI in 2002. When all patients with FUO before 2002 (n = 55) and thereafter (n = 12) were compared the prevalence of systemic vasculitis increased from 11% (n = 6) to 42% (n = 5, p = 0.021). CONCLUSION: Implementation of a standardised work-up programme for FUO did not improve the overall rate of diagnosis. Addition of MRI significantly increased the diagnosis of systemic vasculitis as the underlying cause of FUO. MRI should be included in the investigation of patients with FUO when vasculitis is suspected.

The genetically-engineered secretory B27/Q10 chimeric molecule inhibits HLA-B27 restricted alloreactive T-lymphocytes.

OBJECTIVES: Intracellularly persisting bacterial infections and high association with HLA-B27 are the hallmarks of reactive arthritis. Soluble HLA-B27 molecules are induced by bacterial infection; however their biological role in arthritis is unknown. It was the aim of this study to generate soluble HLA-B27 molecule and to analyze its effect on cytotoxic HLA-B27 alloreactive CD8+ T-lymphocytes in order to better understand potential functional links between persistent infection and HLA-B27 association. METHODS: Using PCR Exons 1 through 4 of HLA-B*2705 were fused to Exon 5 of the soluble murine MHC class I variant Q10 and stably transfected into Hela-cells. Transfectants were analyzed using specific PCR, RT-PCR and intracellular and extracellular staining with anti-HLA-B27 monoclonal antibody ME1. Secretion of B27Q10 in the supernatant was examined by isoelectric focusing (IEF). The effect of B27Q10 on T-cells was analyzed using either HLA-B27- or HLA-A2-restricted alloreactive T-cells in a standard 51Cr-release assay. RESULTS: PCR and RT-PCR demonstrated the DNA and mRNA of B27Q10 in the transfectants. By intracellular and extracellular staining with ME1 B27Q10-molecule was detected intracellularly but was not expressed in the cell membrane. Using IEF soluble B27Q10-molecules were found in supernatants of transfectants in a concentration of up to 1.342 microg/ml. Soluble B27QJO-molecule inhibited specifically the cytotoxicity of HLA-B27-restricted alloreactive T-cells by about 30%. CONCLUSION: The secretory non-membrane-expressed molecule B27Q10 inhibits HLA-B27 specific T-cells. The inhibition of cytotoxic T-cells by bacteria induced soluble HLA-B27 may thus enable bacterial persistence.

Chlamydia and Borrelia DNA in synovial fluid of patients with early undifferentiated oligoarthritis: results of a prospective study.

OBJECTIVE: More than 50% of patients with synovitis involving 1-4 joints remain classified as having undifferentiated oligoarthritis (UOA) after 1 year of disease. The clinical presentation is often similar to that of reactive arthritis (ReA) and other spondylarthropathies or to Lyme arthritis. We therefore determined how often Chlamydia trachomatis (Ct) and Borrelia burgdorferi (Bb) can be identified in patients with UOA, by using an extensive laboratory approach. METHODS: We prospectively studied 52 patients with UOA who presented at an early synovitis clinic in a region highly endemic for Lyme disease. Patients were examined by standardized clinical and immunoserologic procedures. Synovial fluid was screened for the presence of Ct and Bb DNA by polymerase chain reaction (PCR). Urine was tested for Ct DNA by ligase chain reaction, and serum was tested for Ct antibodies by enzyme-linked immunosorbent assay and Bb antibodies by hemagglutination test and Western blotting. PCR results in the UOA patients were compared with the results in cohorts of patients with definite rheumatoid arthritis (RA), Lyme arthritis, and Chlamydia-induced arthritis (CIA). RESULTS: In the synovial fluid of 9 of 52 patients with UOA (17%), we found Ct DNA, and in 6 of the 52 patients (12%), Bb DNA was found. The frequency of bacteria-specific DNA was 50% (7 of 14) in CIA patients and 69% (11 of 16) in patients with Lyme arthritis. No Bb or Ct DNA was found in the synovial fluid of the 31 RA patients. CONCLUSION: With optimized PCR protocols, it is possible to detect considerable levels of Bb and Ct DNA in the synovial fluid of patients with UOA. Although the presence of bacterial DNA does not unequivocally prove its etiologic significance, we suggest that at least one-third of patients with UOA may have a form of ReA that involves asymptomatic primary infection.

The modulation of chlamydial replication by HLA-B27 depends on the cytoplasmic domain of HLA-B27.

OBJECTIVE: The intracellular persistence of viable Chlamydia trachomatis (CT) within the joint is thought to initiate and maintain the inflammatory process in CT-induced arthritis. CT-induced arthritis is associated with HLA-B27. Recently it was shown that HLA-B27, besides being a T-cell restriction element, can directly influence the invasion and/or replication of enterobacteriae and alters salmonella-induced signal transduction. It was the aim of this study to analyze the effect of HLA-B27 on CT-invasion and replication in human host cells. METHODS: Human Hela cells and Hela cells transfected with either HLA-B27 cDNA or controls (HLA-A1 cDNA; HLA-B27 mutant = HLA-B27 without cytoplasmic tail; B27Q10 = HLA-B27 Exon 1-4 linked to Exon 5 of murine Q10) were infected with CT. By direct immunofluorescence chlamydial invasion was determined 4 hours post infection (p.i.), chlamydial replication 2 days and 4 days p.i. The number of infective CT in the different cell lines was determined by titration of the cell lysates on Hep-2 cells with subsequent immunoperoxidase staining. RESULTS: Invasion was not affected by HLA-B27. However, formation of chlamydial inclusion bodies and replication was suppressed by HLA-B27. Genetically engineered mutants of HLA-B27 (HLA-B27 mutant, B27Q10) lacking the cytoplasmic tail of HLA-B27 did not affect replication. CONCLUSION: The reduction of chlamydial replication by HLA-B27 depends on the cytoplasmic domain of HLA-B27, thus providing a new hypothesis for chlamydial persistence in HLA-B27 positive reactive arthritis.

Infection of human monocyte-derived macrophages with Chlamydia trachomatis induces apoptosis of T cells: a potential mechanism for persistent infection.

Viruses can escape T-cell surveillance by infecting macrophages and thereby induce apoptosis of noninfected T cells. This ability had not been demonstrated for bacteria. We investigated whether infection of macrophages with the important human pathogen Chlamydia trachomatis can induce T-cell apoptosis. Because Chlamydia-Mycoplasma coinfection is a frequent event, the ability of Mycoplasma fermentans-infected macrophages to induce T-cell apoptosis was also studied. Infected macrophages were cocultivated with autologous T cells in different activation states. Propidium iodide-based fluorescence-activated cell sorter analysis demonstrated that macrophages infected with viable chlamydiae induced T-cell death. Apoptosis was identified as the mode of death induction by using a terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling assay. Induction of T-cell death was macrophage dependent. Incubation of T cells with infectious chlamydiae in the absence of macrophages did not lead to T-cell apoptosis. UV irradiation of chlamydiae diminished the ability to induce death. T-cell death was induced by a cell-free supernatant of infected macrophages. Not only phytohemagglutinin-preactivated T cells but also non-mitogen-preactivated T cells were susceptible to C. trachomatis-induced apoptosis. In contrast, M. fermentans infection of macrophages did not induce T-cell death. Coinfection had no additional effect. In summary, intracellular chlamydial infection of macrophages can induce T-cell apoptosis. Apoptosis induction by chlamydiae possibly explains how persistently infected macrophages escape T-cell surveillance and why the Chlamydia-specific T-cell response is diminished during persistent chlamydial infection.

[Reactive arthritis after cat bit: a rare manifestation of cat scratch disease--case report and overview]. / Reaktive Arthritis nach Katzenbiss: eine seltene Manifestation der Katzenkratzkrankheit--Kasuistik und Ubersicht.

Cat scratch disease (CSD) is a rarely recognized infectious disease in Germany. Only a few years ago the causative agent, Bartonella henselae, could be isolated. The typical clinical manifestations of CSD consist of skin changes at the inoculation site and a benign lymphadenopathy; other manifestations are rare. We report the case of a 47 year old woman, who developed a reactive spondylarthropathy with synovitis of finger joints, polyarthralgias of large- and medium-sized joints, and inflammatory spinal pain after a cat bite. The rheumatic manifestations resolved after 10 months by treatment with non-steroidal antirheumatic drugs. Only a few cases of rheumatic manifestations associated with CSD have been described in the literature. Because the prevalence of Bartonella henselae infection of cats is high in Europe, rheumatic manifestations might be more frequent. Diagnosis of CSD is now improved by the development of serological tests. We provide an overview of the clinical manifestations and the diagnostic criteria.

Multiple mechanisms support oligoclonal T cell expansion in rheumatoid synovitis.

BACKGROUND: The synovial T cell infiltrate in rheumatoid arthritis (RA) is diverse but contains clonally expanded CD4+ populations. Recent reports have emphasized that RA patients have a tendency to develop CD4+ T cell oligoclonality which also manifests in the peripheral blood. Clonal dominance in the tissue may thus result from antigen specific stimulation in the synovial membrane or may reflect the infiltration of expanded clonotypes present throughout the lymphoid system. We have explored to what extent clonal populations amongst tissue CD4+ T cells display joint specificity as defined by their restriction to the joint, their persistence over time, and their expression of markers indicative for local activation. MATERIALS AND METHODS: Matched samples of peripheral blood and synovial fluid or synovial tissue were collected from 14 patients with active RA and CD4+ IL-2R+ and CD4+ IL-2R- T cells from both compartments were purified. Clonal populations of CD4+ T cells were detected by RT-PCR amplification of T cell receptor (TCR) transcripts with BV and BJ specific primers followed by size fractionation and direct sequencing of dominant size classes of TCR transcripts. RESULTS: Clonal CD4+ T cells were detected in the synovial fluid and synovial tissue of all patients. All patients carried synovial clonotypes that were undetectable in the blood but were present in independent joints or at several non-adjacent areas of the same joint. These joint restricted CD4+ clonotypes were generally small in size, were preferentially found in the IL-2R+ subpopulation, and persisted over time. A second type of clonogenic T cells in the synovial infiltrate had an unrestricted tissue distribution and was present at similar frequencies amongst activated and nonactivated T cells in the blood and affected joints. Ubiquitous clonotypes isolated from two different patients expressed sequence homologies of the TCR beta chain. CONCLUSIONS: Two types of expanded CD4+ clonotypes contribute to the T cell infiltrate in rheumatoid synovitis. Differences in the distribution pattern and in molecular features suggest that distinct mechanisms are supporting the clonal outgrowth of these two groups of clonotypes. Clonally expanded T cells restricted to the joint but present in several independent joints appear to respond to locally residing antigens. Clonogenic cells with an unrestricted distribution pattern and widespread activation in the blood and tissue may react to a different class of antigens which appear to be shared by multiple patients. T cell recognition in RA may be involved at several different levels and may be related to more than one pathomechanism.

[Hepatitis C virus in rheumatic diseases]. / Das Hepatitis C-Virus und rheumatische Erkrankungen.

HCV-infection is an important infectious disease in rheumatology. It is the cause of mixed cryoglobulinemia and other rheumatic manifestations develop frequently during HCV-infection. These comprise: Sicca-syndrome, thromboembolic events associated with anti-cardiolipin antibodies and fibromyalgia. Also associated with HCV-infection is a non-erosive polyarthritis. This synovitis often fulfills the ACR-criteria for rheumatoid arthritis, but the disease course is different with frequent remissions and non-erosive joint involvement. The following autoantibodies are associated with HCV-infection: Cryoglobulins, rheumatoid factor, antinuclear antibodies (ANA), antismooth muscle antibodies (SMA), anti-phospholipid-antibodies and anti-thyroid-antibodies. In HCV-associated sicca-syndrom, antibodies against Ro (SSA) and La (SSB) are not detected. The course of HCV-infection is often occult, without elevation of liver enzymes. We summarize the clinical and serological signs and symptoms when HCV-infection should be suspected and when HCV-testing should be performed in a rheumatological setting. The identification of HCV-infection in rheumatic patients is important to minimize the risk of aggravating hepatitis by prescription of hepatotoxic drugs and because of the availability of alpha-interferon as a potential virus eradicating agent.

Emergence of oligoclonal T cell populations following therapeutic T cell depletion in rheumatoid arthritis.

OBJECTIVE: To examine the compartment of CD4+ T cells in patients with rheumatoid arthritis (RA) who have developed persistent lymphopenia following antibody-mediated T cell depletion and to investigate why T cell depletion is of limited therapeutic efficacy. METHODS: Circulating T lymphocytes from 10 patients with seropositive RA treated with the monoclonal antibody (MAb) CAMPATH-1H were longitudinally monitored by fluorescence-activated cell sorter analysis with MAb. To assess the molecular diversity of repopulating T cells, random samples of T cell clones from the peripheral blood of 3 patients were analyzed by sequencing the T cell receptor (TCR) beta chains. At the time of recurring disease, the synovial tissue was examined by immunohistochemistry, and the repertoires of peripheral and synovial tissue T cells were compared by TCR beta-chain sequencing and by semiquantitative hybridization with oligonucleotides specific for the V-D-J beta junctional region of selected clones. RESULTS: The reconstitution of the peripheral T cell compartment was very slow. A mean CD4+ T cell count of 105/microliters was reached 34 weeks following MAb treatment. After treatment, the percentage of CD4+ T cells with the CD45RO+ phenotype was significantly increased (P = 0.001), indicating the expansion of antigen-primed memory T cells. TCR beta-chain sequences revealed a marked restriction in the diversity of repopulating T cells with the emergence of dominant clonotypes. Despite the low counts of peripheral CD4+ T cells, the synovial tissue was infiltrated by CD4+ T cells to a similar extent as that in RA patients not treated with MAb. Selected clonotypes that had emerged in the peripheral blood compartment dominated the repertoire of tissue-infiltrating T cells in the synovium. CONCLUSION: In patients with RA, T cell depletion induces a long-term imbalance in T cell homeostasis. Clonal proliferation of CD4+ T cells severely restricts the diversity of available T cell specificities and results in the emergence of dominant clonotypes, which accumulate in the synovial tissue despite peripheral lymphopenia.

Dominant clonotypes in the repertoire of peripheral CD4+ T cells in rheumatoid arthritis.

Clonal expansion of T cell specificities in the synovial fluid of patients has been taken as evidence for a local stimulation of T cells. By studying the T cell receptor (TCR) repertoire of CD4+ T cells in the synovial and peripheral blood compartments of patients with early rheumatoid arthritis (RA), we have identified clonally expanded CD4+ populations. Expanded clonotypes were present in the peripheral blood and the synovial fluid but were not preferentially accumulated in the joint. Dominant single clonotypes could not be isolated from CD4+ cells of HLA-DRB1*04+ normal individuals. Clonal expansion involved several distinct clonotypes with a preference for V beta 3+, V beta 14+, and V beta 17+CD4+ T cells. A fraction of clonally related T cells expressed IL-2 receptors, indicating recent activation. The frequencies of clonally expanded V beta 17+CD4+ T cells fluctuated widely over a period of one year. Independent variations in the frequencies of two distinct clonotypes in the same patient indicated that different mechanisms, and not stimulation by a single arthritogenic antigen, were involved in clonal proliferation. These data support the concept that RA patients have a grossly imbalanced TCR repertoire. Clonal expansion may result from intrinsic defects in T cell generation and regulation. The dominance of expanded clonotypes in the periphery emphasizes the systemic nature of RA and suggests that T cell proliferation occurs outside of the joint.

Soluble HLA-DR molecules in patients with HLA class II versus class I associated disorders.

HLA genes have been identified as key genetic factors contributing to many chronic diseases characterized by autoimmune features. The role of HLA encoded molecules in the pathogenesis of these diseases is unresolved. We have now analysed soluble HLA-DR molecules circulating in the serum of patients with different autoimmune diseases and have defined parameters controlling serum levels. Patients with HLA-DR associated diseases were characterized by elevated serum concentrations of HLA-DR molecules and were clearly distinct from patients with HLA-B27 associated disorders. We did not find evidence for a correlation between disease activity, laboratory abnormalities and elevated serum concentrations of soluble HLA-DR molecules. Studies in normal individuals indicated that soluble HLA-DR molecules are at least partially regulated by the HLA haplotype. Highest serum concentrations were found in individuals carrying the HLA-DR3 or HLA-DR4 haplotype raising the possibility that the phenomenon of HLA-disease association reflects differences in the genetic control of soluble HLA-DR molecules. Interferon-gamma treatment caused an increase in serum concentrations of soluble HLA-DR molecules, whereas a decrease of circulating HLA-DR molecules was associated with an immunosuppressive with cyclosporine A. These data suggest that the patient's immunoresponsiveness represents a second important mechanism controlling circulating HLA-DR molecules.

Structural and functional characterization of HLA-DR molecules circulating in the serum.

HLA molecules are highly polymorphic glycoproteins with a single binding site for immunogenic peptides. The complex formed by HLA-DR molecules and peptides is the entity specifically recognized by the antigen receptor of CD4+ helper T lymphocytes. This biological function has been linked to the constitutive cell surface expression of HLA molecules on antigen-presenting cells which provide immunogenic peptides through denaturation or fragmentation of antigen. Here, we report that HLA-DR molecules can be identified in a soluble form circulating in the serum. Biochemical characterization and recognition of such soluble HLA-DR molecules by a monoclonal antibody reactive to conformational determinants on the alpha-beta HLA complex indicates that they are circulating as intact heterodimers. Upon activation, human T cells and B cells release soluble HLA-DR molecules. The expression and secretion of HLA-DR by T lymphocytes is greatly enhanced in the presence of macrophages indicating that the production of HLA-DR molecules might be regulated and controlled through factors derived from antigen-presenting cells. An alloreactive human T cell clone can be stimulated to proliferate in response to soluble HLA-DR molecules purified and enriched from serum suggesting that the soluble form of the natural ligand of the T cell receptor might have immunoregulatory functions.