Specific and non-specific autoreactive immunity

Journal of A utoimmunity (1992) 5 (Supplement A), 37-44

Specific and Non-specific Autoreactive Immunity

Erna M611er, M a n u c h e h r Abedi Valugerdi and A n n a Ridderstad Department of Immunology, Arrhenius Laboratories for Natural Sciences, Stockholm University and Department of Clinical Immunology, Karolinska Institute Medical School, Huddinge Hospital, Stockholm, Sweden

Most autoimmune diseases are HLA-associated which supports the n o t i o n t h a t t h e y a r e d e p e n d e n t u p o n specific i m m u n e a c t i v a t i o n o f a l i m i t e d set o f T cell c l o n e s . F i n d i n g s w h i c h i m p l y t h a t i n d u c t i o n o f a u t o i m m u n e r e a c t i v i t y p r o b a b l y does n o t differ f r o m n o r m a l i m m u n e responses are discussed. The possibility of transfering autoimmune d i s e a s e u s i n g T cell c l o n e s i n d i c a t e s t h a t t a r g e t s t r u c t u r e s f o r a u t o i m m u n e a t t a c k a r e also p r e s e n t i n h e a l t h y i n d i v i d u a l s . I n t h e p r e s e n t a r t i c l e , i t is a r g u e d t h a t a u t o i m m u n e r e a c t i o n s a n d i m m u n i t y a g a i n s t n o m i n a l c o n v e n t i o n a l a n t i g e n s i n p r i n c i p l e a r e effected a n d r e g u l a t e d b y s i m i l a r m e c h a n i s m s . It is a s s u m e d t h a t p e r s i s t e n t t i s s u e d a m a g e o c c u r s i f i m m u n e a t t a c k is d i r e c t e d a g a i n s t t i s s u e s t h a t c a n n o t b e r e g e n e r a t e d , s u c h as i n d i a b e t e s , o r a r e o n l y s l o w l y r e c o n s t i t u t e d , s u c h as i n r h e u m a t o i d a r t h r i t i s . N o r m a l i m m u n e r e s p o n s e s a r e r e g u l a t e d b y various inflammatory mediators and cytoklnes]interleukins. The joint o f p a t i e n t s w i t h r h e u m a t o i d a r t h r i t i s is d i s c u s s e d as a m o d e l f o r p r o p a gation of i m m u n e r e a c t i o n s a n d tissue d e s t r u c t i o n in a u t o i m m u n e disease. O f t h e d i f f e r e n t c y t o k i n e s w h i c h a r e p r e s e n t i n t h e s y n o v i a l f l u i d o r p r o d u c e d b y cells i n t h e s y n o v i a l t i s s u e , m o s t a r e p r e s u m e d to h a v e o r i g i n a t e d i n m a c r o p h a g e s / m o n o c y t e s s u c h as IL-1, IL-6, IL-8, T N F - a a n d T G F - [ L E v e n so, T cells a r e b e l i e v e d to h a v e a n i m p o r t a n t r o l e f o r the c o n t i n u e d r e a c t i v i t y associated with a u t o i m m u n e disease. This disc r e p a n c y c a n b e e x p l a i n e d i n d i f f e r e n t ways. T cell p r o d u c t s m i g h t escape detection because they are short-lived, they are immediately consumed or they are produced only during short time intervals. However, the m o s t likely e x p l a n a t i o n m i g h t be that they have not yet b e e n i d e n t i f i e d , a n d it s h o u l d b e p o i n t e d o u t t h a t c e r t a i n c r u c i a l i m m u n o regulatory mechanisms have not yet found their lymphokine/cytokine. T h e r o l e o f c y t o k i n e s i n t h e p e r p e t u a t i o n o f t h e a u t o i m m u n e r e s p o n s e is d i s c u s s e d as well as p o s s i b i l i t i e s to i d e n t i f y c e n t r a l l y i m p o r t a n t c y t o k i n e s , w h i c h , i f c o n t r o l l e d , c o u l d b e u s e d as h a n d l e s to a r r e s t a n d c o n t r o l autoimmunity.

37 0896-8411/92/0A0037 + 08 $03.00/0

© 1992 Academic Press Limited

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E. M611er e t aL HLA association of autoimmune diseases

H L A antigens regulate T cell dependent immune responses at two distinct levels. First, H L A antigens expressed in the thymus during the ontogenetic development of immunocompetent T cells serve as self markers for the selection of T cell clonal repertoires [1]. Recent studies have clearly demonstrated that positive and negative selective processes, most probably both dependent upon the expression of self H L A molecules in association with endogenous peptides [2], have a central influence on the selection of T cell repertoires. T cells, which have successfully rearranged genes of the germ line receptor-forming genetic elements, are postulated to be positively selected if they bind specifically to self H L A - p e p t i d e complexes [3], which are expressed in the thymus, more specifically the thymic epithelium [4], and will constitute the peripheral repertoire of immunocompetent cells if they escape negative selection, presumably taking place in the thymic medulla. T h e second level of H L A antigen regulation is peripheral. H L A class I and class I I antigens in complex with exogenous and/or endogenous peptides are recognized by immunocompetent T lymphocytes. T h e allelic variations in H L A molecules primarily affect the peptide-binding portions of the molecules [5], which implies selectivity in peptide binding properties. Studies on T cell clones specifically reactive against alloantigens indicate that each individual H L A molecule can interact with many different peptides, such that an H L A - A 2 molecule has affinities for a certain set of peptides whereas the A3 molecule interacts with a different set of peptides. T h e extent of overlap in peptide binding is not yet ascertained but is under intense investigation. Foreign antigens, split into peptides, are believed to interact with different H L A molecules. An immune response might take place if any of the H L A molecules expressed in the antigen-presenting cells have affinity for one or more of the peptides formed as a result of antigen processing or are produced by genes which are integrated in the cellular genome. We have recently argued that it is likely that H L A and disease associations operate at the central level of selection of T cell repertoires [6]. However, the opposite view, i.e. peripheral control, has been advocated by others [7]. With either level of operative control, the fact that autoimmune diseases are HHLA associated and the finding that this association depends primarily on the function of H L A genes as such, would support immunological specificity. Therefore, a specific immune reactivity would be involved in the initiation of an autoreactive immunity.

Target structures for autoimmune attack

T h e target determinants for T cell-mediated autoimmune attack are as yet unknown. T h e fact that autoantibodies with certain specificities can be identified in patients with different autoimmune disorders tells nothing about the T cell specificity. T h e pathogenic role of antibodies remains to be elucidated for most diseases. It is not impossible that specific autoantibody formation is the result of activation of T cell reactivity against exogenous antigens, as a result of microbial infection. Since antibodies can react with an extremely wide spectrum of different determinants, the finding of an antibody reacting against a microsomal antigen, as one example, does not have to be the result of specific activation of B cells reactive with a microsomal

Specific and non-specific autoreactivity

39

antigen. It is not even known if autoantibody production results from a specific or a non-specific activation process. Polyclonal activation of the production of high-connectivity antibodies internally regulated by network interactions can indeed also explain a substantial fraction of autoantibody specificities found in patients with autoimmune diseases [8, 9]. Also, endogenous substances could induce autoreactivity--one possibility is via circulating antibodies. Antigen-presenting cells could phagocytose antibodies as such, or rather, antibody complexes, via the Fc receptor. Fragmentation of endogenous antibodies could yield antibody fragments which would bind to s e l f H L A and activate an H L A - s e l f restricted immune response. Such T cells would interact with B cells carrying receptors for any determinants expressed on the immune complex, since such B cells would preferentially internalize the complexes after binding by the specific Ig receptors. T h e high-connectivity (multispecific) antibodies which are parts of the internal network, have probably had the capacity to influence T cell repertoires already during thymic education. However, antibodies which are produced by somatically mutated V-region genes might also be able to activate peripheral T cells. Furthermore, it is not yet known whether the target molecules for T cell-mediated autoimmune attack are self H L A class I or II molecules in complex with peptides which are of internal or external origin. Because of the homogeneity of T cell responses with little or no cross-reactivity between different clones, it is likely that the existence of a particular T cell clone with a set combination of a[3 or 5'6 T cell receptor chains, will determine whether immune reactivity will occur or not. It is also unlikely that breaking of a state of natural tolerance is responsible for the induction of T cell autoreactivity as we have summarized elsewhere [6]. We have earlier argued that only a few of all possible 'autoantigens' serve as 'self' peptides for the selection of T cell repertoires in the thymus. Therefore, a large n u m b e r of possible T cell 'autoantigens' can occur in the normal organism without precipitating an autoimmune response. T cell clones with specificity for such antigens would not have undergone positive selection and therefore be scarce. In addition, and perhaps more importantly, the expression of a complex of self plus autopeptide on a cell in the tissues would not induce immune reactivity unless the autopeptide expressing cell can function as an antigen-presenting cell. I f the conditions for antigen presentation are fulfilled, the series of events that may follow would, however, be harmful to other cells which also express the same autopeptides in a self HLA-restricted manner.

A u t o i m m u n e reactivity in the absence o f specific T cell activation? Above, we have argued that H L A association implies involvement of specific T cell clones in autoimmune reactions. However, it is possible to achieve autoreactivity even in the absence of specific T cell activation. Byrd et al. [10] first published results showing that patients with a rare tumour, atrial myxoma, could present with autoimmune manifestations such as systemic vasculitis. Removal of the turnout led to disappearance of symptoms. When it was later shown that cardiac myxoma cells often produce interleukin-6 (IL-6), it was thought that autoantibody formation was dependent on the activity of the B cell active differentiation factor, I L - 6 [11]. Other

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E. Miiller e t aL

experiments indicating a direct role of interleukins in autoimmune manifestations have suggested a role for IL-1. Pettipher et al. [12] showed that IL-1 injected into the knee joints of rabbits led to cellular infiltration in the synovial lining cells and synovial fluid and the loss ofproteoglycan from the articular cartilage. In yet another experimental model, the MRL/Ipr mouse, which spontaneously develops a disease resembling SLE, the injection of recombinant IL-2 in a live vaccinia vector led to an arrest of the developing symptoms in these mice with prolonged survival, lack of clinical symptoms and decreased production of certain autoantibodies typical of these sick mice [13]. I m p o r t a n c e o f l y m p h o k i n e s J c y t o k i n e s for the p r o p a g a t i o n o f h u m a n a u t o i m m u n e disease

The fact that autoimmune disease in patients with rheumatoid arthritis (RA) involves the major joints, has enabled the study of the presence of various inflammatory mediators and cytokines in the synovial membrane and fluid of these patients. The massive infiltration of T cells and the high expression of H L A class II antigens on some T cells and on other cells in the synovium was instrumental in formulating an important role for T cells in autoimmune destruction (see ref. 14). However, typical T cell produced lymphokines such as IL-2, IL-4, IL-5 and interferon-gamma (IFNY) were not found in the synovial fluid, neither were T cells producing such lymphokines identified using in situ hybridization techniques and adequate lymphokine probes [ 15-17]. However, many other cytokines are present in the synovial fluid and produced by non-T cells in the synovium in RA patients. To these belong IL-1, IL6, IL-8, turnout necrosis factor alpha (TNF-~), transforming growth factor-beta (TGF-[3) and granulocyte-macrophage colony stimulating factor (GM-CSF) [1829]. The IgG2b inducing factor, which serves as a T helper factor and a B cell differentiation factor is produced by as yet unidentified cells [18, 30]. G M - C S F rather than IFN-7 could be responsible for induction of class II antigen expression in the synovial tissues [19]. Since it is well known that T cell lymphokines might be short-lived and that activated T cells only produce lymphokines during a short time period after activation, the lack of T cell produced factors does not necessarily imply absence of T cell activation and concomitant lymphokine synthesis. In addition, certain T cell factors have probably not yet been identified. To these belong the physiological factor with 'LPS-like properties' on B cells, a factor which might be the main T cell product influencing B cell growth and differentiation. Furthermore, the IgG2b inducing factor, produced by as yet unidentified cells, has B cell stimulating properties mimicking dextran sulphate (DxS) in vitro (Ridderstad, unpublished observation) and can partly correct the sex-linked B cell deficiency of the CBN/N mice [31]. If injected in vivo, the IgG2b inducing factor causes a very marked production of both IgG2b and IgG1 antibodies in mice [32]. The recently described lymphokine IL-10 [33, 34], which can be produced by T cells, B cells and by certain other cell types has not been studied in the synovial fluid. Since IL-10 is a factor, earlier referred to as CSIF (cytokine synthesis inhibitory factor), it is possible that the absence of certain T cell lymphokines in the inflamed joint can be explained by the over-production of an as yet unidentified substance which can control the expression of the others. Other

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biologically active substances include platelet-derived growth factor ( P D G F ) and new findings show that specific receptors for P D G F develop during chronic inflammations in the joint [28]. O f other cytokines which have been defined, I L - 8 is present in synovial fluid. In as yet unpublished experiments we have tested whether the I g G 2 b - i n d u c i n g factor is identical to I L - 8 . We did not find that I L - 8 has any I g G 2 b inducing activity nor has anyone else described an I L - 8 mediated activity on B cells. I L - 1 0 has not yet been studied in the synovial fluid but our own data do not support that I L - 1 0 is identical to the I g G 2 b inducing activity. Synovial fluid of RA (RA-SF) patients thus contains at least one factor with B cell stimulating properties. R A - S F when tested on h u m a n B lymphocytes acts as a T cell replacing factor ( T R F ) and B cell differentiation factor ( B C D F ) [18]. T h e selective I g G subclass induction observed on mouse splenic B lymphocytes and in vivo mice does not have a clear counterpart in the h u m a n situation. T h e r e is no selective I g G class induction by R A - S F on h u m a n blood B lymphocytes, rather an increase of both I g M , I g G and IgA. T h e same problem indeed exists for other B cell stimulating factors with a defined effect and selective help for production of specific subclasses, such as I L - 4 for IgG1 and I g E in mice, I F N - y for IgG2a, TGF-[~ and I L - 5 for IgA, I g G 2 b inducing factor etc. Using h u m a n blood B lymphocytes as responder cells, the selective sensitivity for B cells to undergo a switch process or develop into plasma cells producing one Ig class only is not yet clear. T h u s , Ig-subclass specific induction is not b r o u g h t about by the above mentioned lymphokines on h u m a n blood l y m p h o cytes, possibly with the exception of TGF-I3. Whether this is dependent on the type of target B cells used, mouse spleen versus h u m a n blood, or on the species is not yet known. N o n e of the above mentioned cytokines/lymphokines that exist in the synovial fluid of RA patients are exclusive to the inflammatory lesion in RA and it is as yet unknown whether such factors will be identified. It is not unexpected that a site for chronic inflammation such as the joint of R A patients with different kinds of inflammatory cells contains a mixture of active factors. T h e s e factors are produced by various kinds of cells and can have an autocrine effect. F u r t h e r m o r e , T N F - ~ can increase the production of IL-1 that regulate each other and in concert stimulate the production of I L - 6 . T h e c o m m o n denominator, if any, in these events has not been described.

Therapeutic possibilities for drugs or agents that interfere with cytokine activity? In our thinking, a u t o i m m u n e diseases are T cell dependent i m m u n e reactions not necessarily different from conventional i m m u n e responses against foreign antigens. I m m u n e attack on tissues that can not be regenerated would lead to p e r m a n e n t tissue damage, whereas a continuous attack against determinants expressed on cells that can regenerate slowly m i g h t lead to chronic disease. T h e continued presence of antibodies that can form i m m u n e complexes and in turn activate complement, as well as the presence of inflammatory cytokines which directly or indirectly, through activation of B cells, induce the production of antibodies, and of mononuclear cells to produce collagenase and prostaglandin E2,

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propagate tissue destructive i m m u n e events. Even cytokines, as exemplified by T N F - u , can cause destruction by activation of osteoclast activity. T h e Central cell in this cascade-like reaction might be the T cell. However, the initiating stimulus could be an autoantibody, which indirectly in a self H L A restricted m a n n e r stimulates T cells. Therefore, an approach to block autoimmunity might better be directed at both T and B target cells. In our own experiments, we have found that the synovial fluid of RA patients contains a factor that selectively augments the differentiation of cytotoxic T cells formed in a normal allogeneic M L R . Both the n u m b e r of C T L precursors and the cytotoxic efficiency on a per cell basis is increased (Ridderstad et al., submitted). It is likely that the T cell active factor is different from the I g G 2 b - i n d u c i n g factor. Whatever the connection, a T cell active factor with lymphokine/cytokine characteristics can in part be responsible for the continued participation of T lymphocytes in the local inflammatory reaction typical of the rheumatoid synovitis. All continued i m m u n e responses are dependent on a cytokine cascade. U n f o r t u nately, a possible central cytokine in the chronic RA inflammation has not been identified. I f it were to be found, it would offer possibilities of interfering locally with the production of this cytokine, or interfering with its activity by blocking the expression of cytokine receptors, using soluble receptors, receptor antagonists or alternatively by monoclonal antibodies. One p r o b l e m with the latter therapy is that monoclonal antibodies might lead to anti-idiotypic immunization which in turn could produce a series of events that inhibit the physiological activity of the cytokine and hence cause lasting immunosuppression. F o r this reason, our personal view is that adequate drug design is important for the regulation of cytokines, since medical treatment can be regulated so as to minimize the risks of harmful side effects. Acknowledgements

T h e present study was supported by grants from the Swedish Medical Research Council (No. 2334), the Swedish Association against Rheumatism, the King Gustav 80-year Foundation and the N a n n a Schwartz Foundation. References

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