Cartilage proteoglycans as potential autoantigens in humans and in experimental animals

Proteoglycan aggrecan conducting T cell activation and apoptosis in a murine model of rheumatoid arthritis

Rheumatoid arthritis (RA) is a systemic autoimmune disease and its targeting of the joints indicates the presence of a candidate autoantigen(s) in synovial joints. Patients with RA show immune responses in their peripheral blood to proteoglycan (PG) aggrecan. One of the most relevant animal models of RA appears to be proteoglycan-induced arthritis (PGIA), and CD4⁺ T cells seem to play a crucial role in the initiation of the disease. In this review, the role of various T cell epitopes of aggrecan in the induction of autoreactive T cell activation and arthritis is discussed. We pay special attention to two critically important arthritogenic epitopes, 5/4E8 and P135H, found in the G1 and G3 domains of PG aggrecan, respectively, in the induction of autoimmune arthritis. Finally, results obtained with the recently developed PG-specific TCR transgenic mice system showed that altered T cell apoptosis, the balance of activation, and apoptosis of autoreactive T cells are critical factors in the development of autoimmunity.

Aggrecan: A Target Molecule of Autoimmune Reactions

Aggrecan in cartilage forms aggregates with hyaluronan and link protein, embedded in a collagen network. It accounts for the compressive stiffness and resilience of the hyaline cartilage. Many forms of inflammatory arthritis were shown to be accompanied with aggrecan degradation and loss from the cartilage. The loss of this major component of cartilage renders the tissue more vulnerable when exposed to abrasive forces. Therefore, aggrecan degradation may significantly contribute to cartilage destruction in arthritis. Furthermore, fragments of degraded aggrecan are released during joint inflammation. Thus, molecules of an avascular, immune-privileged tissue (hyaline cartilage) may become accessible to the cells of the immune system. Similarly, there is a "leakage" of aggrecan fragments from cartilage during aging and after joint injury, which may also lead to autosensibilisation. Autoimmune reactivity to aggrecan can be detected in human joint diseases, as well as in animal models of arthritis. The epitopes involved in these processes are currently being identified. Recent data from work with mice suggest a strong immune response focused to the N-terminal G1 domain of aggrecan that leads to arthritis and spondylitis.

Antigen-specific B cells present cartilage proteoglycan (aggrecan) to an autoreactive T cell hybridoma derived from a mouse with proteoglycan-induced arthritis

Cartilage proteoglycan (aggrecan)-induced polyarthritis in BALB/c mice is characterized by chronic inflammation and destruction of joint tissues similar to that observed in human rheumatoid arthritis. The immunization of mice with fetal human proteoglycan (PG) elicits specific antibodies to the immunizing antigen of which a population cross-reacts with native mouse PG. This (auto)antibody production is immediately followed by an explosive proliferation of autoreactive T cells, suggesting that PG-specific B cells may participate in antigen presentation of PG to autoreactive T cells. We therefore isolated B cells from the spleens and lymph nodes of PG-immunized mice and examined their ability to present PG to a PG-specific T cell hybridoma. The antigen-specific T cell responses elicited by B cells from PG-immunized mice (both arthritic and clinically asymptomatic) were markedly higher than those of non-immune mice and keyhole limpet haemocyanin (KLH)-immunized mice, and these B cells could present low PG concentrations. Levels of B cell presentation corresponded with the serum levels of PG-specific antibodies, implying that these B cells were presenting the PG specifically via their surface immunoglobulin. This B cell-T cell interaction was strongly dependent on MHC class II/T cell receptor (TCR), LFA-1/intercellular adhesion molecule-1 (ICAM-1) and CD28/B7 interactions, as antibodies to Ia, ICAM-1 and B7-2 (but not to B7-1) markedly reduced presentation. These data indicate that PG-specific B cells may play an essential role in governing the development of PG-induced arthritis.

Anti-collagen antibodies and immune complexes in equine joint diseases

An investigation was made into the possible contribution of autoimmune mechanisms to equine arthropathies. Serum and synovial fluid (SF) immune complexes and anti-collagen Type II antibodies were measured, by ELISA, in groups of horses with naturally occurring osteoarthritis (OA), osteochondritis dissecans (OCD), bone fracture, traumatised joints, synovitis, infected joints and non-diseased (control) joints. Significantly raised anti-collagen Type II antibodies were found in osteoarthritic (P < 0.02) and traumatised joint synovial fluids (P < 0.01) compared with the control, where ten of 38 (26%) OA and five of eight (63%) traumatised joint synovial fluid samples had raised anti-collagen Type II antibodies (above mean and 2 SD of control group; P < 0.05). Significantly raised levels of C1q-binding immune complexes were found in the synovial fluids of horses with OA (P < 0.001), OCD (P < 0.02), fractured articular bone (P < 0.001), infected (P < 0.01) and traumatised joints (P < 0.001) as compared with the control horses. Raised C1q-binding activity (above mean and 2 SD of control group; P < 0.05) was shown in synovial fluids in 18 of 38 (47%) OA, four of 16 (25%) OCD, four of nine (44%) fractured bone, one of eight (14%) synovitis, two of four (50%) infected joints and seven of eight (88%) traumatised joints. No raised anti-collagen Type II antibodies or C1q-binding activity were detected in sera from any clinical group. Correlations were found, in synovial fluids, between anti-collagen Type II antibodies and C1q-binding activity in OA (P < 0.02), synovitis (P < 0.01) and infected joint (P < 0.05) groups. Within individual horses, there were no correlations between the SF and sera measurements of either anti-collagen II or C1q-binding activity. The relationship between cause and effect of these immunological findings cannot be determined; as they are common to many types of equine joint disorder it is probable that they are not an initiating factor in pathology.

Mapping of arthritogenic/autoimmune epitopes of cartilage aggrecans in proteoglycan-induced arthritis

Immunization of BALB/c mice with chondroitin sulfate-depleted proteoglycan (aggrecan) of fetal human cartilage produces progressive polyarthritis and ankylosing spondylitis. The development of the disease in genetically susceptible BALB/c mice is dependent upon the expression of both cell-mediated and humoral immune responses against the host mouse cartilage proteoglycan (PG). Although cartilage PGs from various species have many biochemical and immunological similarities, only a select group of PGs from fetal and newborn human, fetal pig and canine articular cartilages, human osteophytes and human chondrosarcomas are able to induce arthritis in BALB/c mice. Arthritis develops only in mice that also develop autoantibodies to self-cartilage PGs, although autoantibodies occasionally are present in non-arthritic animals as well. The protease-sensitive auto/arthritogenic epitope(s) is located in, or close to, the chondroitin sulfate (CS) attachment region of the PG molecule. The primary structure of the core protein is responsible for the autoimmune/arthritogenic effect of this select group of PGs, whereas the core protein epitopes are masked by glycosaminoglycan (GAG)-side chains. The CS side chains seem to inhibit antigen recognition in all aggrecans with arthritogenic potential, whereas a similar effect with keratan sulfate (KS) appears only in PGs of aging cartilages.

Effect of pregnancy on proteoglycan-induced progressive polyarthritis in BALB/c mice: remission of disease activity

Proteoglycan-induced arthritis is a murine autoimmune model displaying many similarities to human rheumatoid arthritis and ankylosing spondylitis, as has been documented by clinical, immunological and histopathological studies. Since the onset of arthritis correlates with the serum antibody level to mouse cartilage proteoglycan (PG), it is believed that these autoreactive antibodies may play crucial roles in the pathological mechanisms of PG-induced arthritis. We have found that fertility in these PG-induced arthritic mice had been reduced but, unlike collagen-induced arthritis, had not been completely lost. Moreover, pregnancy had a beneficial effect upon the clinical symptoms with very little or no influence on serum antibody levels. Although fertility was retained and arthritic mothers delivered healthy offspring, the birth frequency was significantly less than in non-arthritic age-matched controls. Furthermore, the presence of anti-PG autoantibodies (predominantly IgG1 subclass) transmitted from arthritic mothers to infants transplacentally and by milk during the lactation period did not render these offspring either resistant or more sensitive to subsequent induction of arthritis. Subsequent immunization of infants with 'arthritogenic' PG revealed an unaltered susceptibility to arthritis induction.

Immunomodulation of proteoglycan-induced progressive polyarthritis by leflunomide

Proteoglycan-induced arthritis is a mouse model displaying many similarities to human rheumatoid arthritis and ankylosing spondylitis which has been documented by clinical and histopathological studies. The development of the disease in genetically susceptible BALB/c mice is dependent upon the expression of both cell-mediated and humoral immunity to host mouse cartilage proteoglycan. Since both development and regression of acute inflammatory processes in joints correlate directly with the serum antibody level to mouse cartilage proteoglycan, it is believed that these autoreactive antibodies may play a key role in the pathological mechanism of proteoglycan-induced arthritis. The treatment of arthritic animals with an immunomodulating agent (leflunomide) suppressed acute inflammatory events, protected animals from new inflammatory episodes or acute exacerbations in chronically inflamed joints and blocked pathological processes in arthritic joints, which otherwise led to progressive deformities, ankylosis and the loss of articular cartilage. We conclude that the suppressive effect of leflunomide (HWA 486) in proteoglycan-induced arthritis primarily is due to the suppression of autoantibody formation and that the drug may be a potential agent in human therapy as well. Further, we feel that this novel model of murine polyarthritis will extend further the pharmacological repertoire necessary to discover innovative antirheumatic drugs.

Synovial cells responding to a 65-kDa mycobacterial heat shock protein have a high proportion of a TcR gamma delta subtype uncommon in peripheral blood

We have analysed the ability of T cells from synovial fluid mononuclear cells (SFMC) and from peripheral blood mononuclear cells (PBMC) of inflammatory arthritic diseases to proliferate in response to mycobacterial antigens (65-kDa heat shock protein [hsp] of BCG, whole BCG) and to rat collagen type II. The SFMC demonstrated a significantly greater ability to respond to 65-kDa hsp of BCG, and to whole BCG, compared with PBMC from the same patients. With collagen type II, only a small proportion of the patients showed a proliferative response, although with this antigen also SFMC responded better than PBMC. There was no difference between SFMC and PBMC in the response to control antigen (tetanus toxoid), phytohaemagglutinin (PHA), or interleukin 2 (IL-2). A high proportion of cells in SFMC-derived short-term T-cell lines were of TcR gamma delta type, often exceeding the number of TcR gamma beta type. There was a significantly higher proportion of TcR gamma delta cells in the SFMC lines compared with the PBMC lines, and a large part of the TcR gamma delta cells in the SFMC cultures was CD8+. The SFMC lines had a high proportion of delta-TCS-1+ cells (V delta 1) among their TcR gamma delta cells, always exceeding the percentages of Ti gamma A+(V gamma 9) and BB3+ (V delta 2). In the PBMC lines, the distribution of TcR gamma delta subtypes was markedly different, with a Ti gamma A+/BB3+ population in the majority. These data argue for a different subpopulation distribution of TcR gamma delta cells in synovial fluid compared with peripheral blood of patients with inflammatory arthritic diseases.

Proteoglycan-induced polyarthritis and spondylitis adoptively transferred to naive (nonimmunized) BALB/c mice

Mononuclear cells from BALB/c mice with progressive polyarthritis and spondylitis induced by injection of fetal human articular cartilage proteoglycan (PG) were used to transfer arthritis by intravenous injection into irradiated, nonimmunized syngeneic mice. Successful transfer of arthritis to BALB/c mice required the injection of lymphocytes from mice with arthritis, along with 50 micrograms of human fetal PG, or lymphocytes stimulated in vitro with either fetal human PG or with mouse cartilage PG. In addition, interleukin-2 or immune sera from animals with arthritis significantly reduced the time to onset of transferred disease. The onset of adoptively transferred arthritis, using cells and antigen, from the time of the first injection (38.2 +/- 18.2 days, mean +/- SD) was shortened if lymphocytes from mice with transferred arthritis were reinjected (retransferred) into other, irradiated syngeneic mice (6.1 +/- 2.6 days). The appearance of autoreactive antibodies to mouse cartilage PG in the sera of mice with adoptively transferred arthritis (secondary or tertiary) preceded the appearance of the first clinical symptoms by a few days. The transfer of arthritis was blocked by pretreatment of donor (arthritic) lymphocytes with either anti-T cell or anti-B cell antibodies and complement. Exposure of mononuclear cells from mice with arthritis to PG, and its removal prior to transfer, also resulted in transfer of the arthritis. PG-induced arthritis was not transferred to nonirradiated mice, nor to irradiated mice injected with lymphocytes from animals with primary arthritis without chondroitinase ABC-digested fetal human PG. Arthritis never developed after injection of immune sera from mice with arthritis (without cells), nor when cells of nonarthritic animals were used with chondroitinase ABC-digested fetal human PG, with or without interleukin-2.