The fail-safe paradigm of immunological self-tolerance

Beneficial autoimmunity improves cancer prognosis

Many tumour antigens that do not arise from cancer cell-specific mutations are targets of humoral and cellular immunity despite their expression on non-malignant cells. Thus, in addition to the expected ability to detect mutations and stress-associated shifts in the immunoproteome and immunopeptidome (the sum of MHC class I-bound peptides) unique to malignant cells, the immune system also recognizes antigens expressed in non-malignant cells, which can result in autoimmune reactions against non-malignant cells from the tissue of origin. These autoimmune manifestations include, among others, vitiligo, thyroiditis and paraneoplastic syndromes, concurrent with melanoma, thyroid cancer and non-small-cell lung cancer, respectively. Importantly, despite the undesirable effects of these symptoms, such events can have prognostic value and correlate with favourable disease outcomes, suggesting 'beneficial autoimmunity'. Similarly, the occurrence of dermal and endocrine autoimmune adverse events in patients receiving immune-checkpoint inhibitors can have a positive predictive value for therapeutic outcomes. Neoplasias derived from stem cells deemed 'not essential' for survival (such as melanocytes, thyroid cells and most cells in sex-specific organs) have a particularly good prognosis, perhaps because the host can tolerate autoimmune reactions that destroy tumour cells at some cost to non-malignant tissues. In this Perspective, we discuss examples of spontaneous as well as therapy-induced autoimmunity that correlate with favourable disease outcomes and make a strong case in favour of this 'beneficial autoimmunity' being important not only in patients with advanced-stage disease but also in cancer immunosurveillance.

Failure of immunosurveillance accelerates aging

Immunosurveillance is generally conceived as a mechanism through which the immune system detects and eliminates (pre-)malignant cells, thus reducing the risk of developing cancer. A recent paper by Ovadya et al. demonstrates that knockout of the gene coding for perforin-1 causes accelerated accumulation of senescent cells in multiple mouse organs, thereby speeding up the aging process. These results suggest that immunosurveillance plays a much broader role in maintaining organismal health than it had been suspected.

Overcoming the Histocompatibility Barrier: Transferrins as Carriers and Modulators of Immunogenic Identity

Proteic molecules were found in the bone marrow that were later identified as transferrins. When applied to transplantation of genetically incompatible bone marrow in supralethally irradiated recipient mice, the transferrins obtained from plasma of bone marrow donors promoted engraftment, permanent hemopoietic chimerism, and donor-type immune character. A combination of donor-matched transferrins and antigens was needed for induction of xenogeneic (interspecies) “tolerance” or unresponsiveness to donor antigens in chemically immunosuppressed mice treated with human transferrins and donor leucocytes. This novel and unique property of transferrins may explain the genesis and maintenance of immunogenic identity and allow a reshaping of the immune system. © 1998 Elsevier Science Inc.

Cell surface CD28 levels define four CD4+ T cell subsets: abnormal expression in rheumatoid arthritis

CD28 is a costimulatory receptor expressed in most CD4(+) T cells. Despite the long-standing evidence for up- and downregulation of surface CD28 expression in vitro, and the key regulatory role assigned to the upregulation of CD28 counterreceptor [the CD152 (CTLA-4) molecule], in vivo CD28 induction has attracted little attention. We studied CD28 and CD152 expression and function in 33 rheumatoid arthritis (RA) patients, 20 clinically active and 13 inactive, and in 24 healthy donors. Four subsets of CD28(-), CD28(low), CD28(int), and CD28(high) peripheral blood human CD4(+) T cells were defined using three-color flow cytometry. The three CD28(+) subsets displayed a one-, two-, or threefold quantitative difference in their relative number of CD28 antibody binding sites, respectively (P < 0.01). RA patients, whether active or inactive, showed a distinct phenotype when compared to healthy donors: (i) the percentage of CD4(+)CD28(high) cells was increased twofold and the CD4(+)CD28(low) subset was reduced twofold (P < 0.01) and (ii) the CD4(+)CD28(high) cells from RA patients showed an in vivo activated phenotype, CD45RO(+)CD5(high)IL-2Ralpha(+) (P < 0.01). Active RA patients were different from inactive patients. They showed a twofold increase in mean CD28 expression (P < 0.05), whereas each of the CD28(+) subsets in the inactive RA patients showed reduced expression when compared to healthy donors. Notably, both active and inactive RA patients showed abnormal CD28 upregulation when T cells were activated in vitro with CD3 antibodies, but only inactive RA patients showed a hypoproliferative response to TCR/CD3 triggering when compared to healthy donors (P < 0.01). This defective proliferation was normalized by concurrent crosslinking with CD28 antibody. No differences were noted in the expression of CD152 or CD80, a CD28 and CD152 shared ligand. The disregulated in vivo expression of CD28 was related to the RA patients' disease activity and suggests that modulation of CD28 surface levels may be an additional mechanism to finely tune the delicate responsiveness/tolerance balance.

The clinical outcome of autoimmune thrombocytopenic purpura patients is related to their T cell immunodeficiency

In this work we have furthered the understanding of the alterations of T lymphocytes from 29 patients with active autoimmune thrombocytopenic purpura (ATP) and the clinical significance of their lymphocytes. An increased percentage of in vivo activated (CD25+ and DR+) T lymphocytes was found in ATP patients with respect to that found in 22 healthy controls. The function of these T cells measured as the proliferative response to polyclonal mitogenic signals is heterogeneously impaired in ATP patients. T lymphocytes from 65.5% (19/29) of the ATP patients showed a decreased proliferative response to these mitogenic signals. This functional alteration is associated with a redistribution of the T cell compartment in these patients' peripheral blood since a significant decrease of CD4+ T lymphocytes was found. We have also found that the impairment of the T cell function is different in the diverse clinical situations of the disease. Those with stable, untreated disease showed a marked decrease in the T cell proliferative response to mitogens. Furthermore, those patients who did not respond either to steroids or to splenectomy showed significantly reduced T lymphocyte blastogenesis after phytohaemagglutinin (PHA) stimulation in comparison to that found in responding patients.

Immune ablation with stem-cell rescue: a possible cure for systemic lupus erythematosus?

The impressive prolongation of survival has been the most important progress made in clinical systemic lupus erythematosus (SLE). Quality of life has also greatly improved, including pregnancy. However, persisting disease and therapy-related morbidity outcomes justify new approaches, different from the usual long-term palliative immunosuppression. Haematopoietic stem cells (HSCs) from healthy histocompatible mice are capable of curing murine SLE after eradication of the original HSCs with total body irradiation. Syngeneic and even autologous HSCs are also capable of curing induced experimental autoimmune diseases such as adjuvant arthritis and experimental allergic encephalomyelitis. In man allogeneic bone-marrow transplantation (BMT) is becoming progressively safer, but cannot yet be offered to SLE patients. However, syngeneic transplants from twins non-concordant for the disease would be justified. Conditioning with high-dose cyclophosphamide followed by autologous HSC rescue, from the marrow and/or from the peripheral blood, may already be regarded as a powerful immunosuppressive procedure for selected cases of SLE and other severe autoimmune diseases. Autologous transplant procedures are not saddled with the immunologic problems of allo-BMT. Although eradication of SLE may not be achieved by auto-BMT, minimal residual immunologic disease can be suppressed or controlled, and long-term self-maintained remissions may be expected.

Compartmentalization of the peripheral immune system

The periphery of the immune system--as opposed to the central lymphoid organs--contains inhomogeneously distributed B and T cells whose phenotype, repertoire, developmental origin, and function are highly divergent. Nonconventional lymphocytes bearing a phenotype that is rare in the blood, spleen, or lymph nodes of undiseased individuals are encountered at high frequency in different localizations, e.g., alpha/beta TCR+CD4-CD8- cells in the bone marrow and gut epithelium, particular invariant gamma/delta TCR+CD4-CD8 alpha+CD8 beta- and gamma/delta TCR+CD4-CD8 alpha-CD8 beta- T cells in various epithelia, or CD5+ B cells in the peritoneum. The antigen receptor repertoire is different in each localization. Thus, different gamma/delta TCR gene products dominant in each site, and the proportion of cells expressing transgenic and endogenous alpha/beta TCR and immunoglobulin gene products follows a gradient, with a maximum of endogenous gene expression in the peritoneum, intermediate values in other peripheral lymphoid organs (spleen, lymph nodes), and minimum values in thymus and bone marrow. Forbidden T cells that bear self-superantigen-reactive V beta gene products are physiologically detected among alpha/beta TCR+CD4-CD8- lymphocytes of the bone marrow, as well as in the gut. Violating previous ideas on self-tolerance preservation, self-peptide-specific gamma/delta T cells are present among intestinal intraepithelial lymphocytes, and CD5+ B cells produce low-affinity crossreactive autoantibodies in a physiological fashion. It appears that, in contrast to the bulk of T and B lymphocytes, certain gamma/delta and alpha/beta T cells found in the periphery, as well as most CD5+ B cells, do not depend on the thymus or bone marrow for their development, respectively, but arise from different, nonconventional lineages. In addition to divergent lineages that are targeted to different organs guided by a spatiotemporal sequence of tissue-specific homing receptors, local induction or selection processes may be important in the diversification of peripheral lymphocyte compartments. Selection may be exerted by local antigens, antigen-presenting cells whose function varies in each anatomical localization, cytokines, and cell-matrix interactions, thus leading to the expansion and maintenance of some clones, whereas others are diluted out or deleted. The spatial compartmentalization of lymphocytes in different microenvironments has major functional consequences and leads to a partial fragmentation of immunoregulatory circuits at the local level. Lymphocytes residing in certain antigen-exposed compartments are likely to combat tissue-specific pathogens or self-proteins.(ABSTRACT TRUNCATED AT 400 WORDS)

Dissociation of autoaggression and self-superantigen reactivity

Self-superantigens have been described as products of endogenous retroviruses of the mouse ('minor lymphocyte stimulating loci') that are capable of interacting without prior processing with conserved domains of TCR V beta chains, causing the activation and deletion of most T cells expressing products of determined V beta gene families [1-4]. The fact that superantigens activate a far higher percentage of T cells (1-20%) than conventional, peptidic antigens (< 0.1%) provides the methodological advantage that the degree of clonal deletion may be measured by the analysis of the TCR repertoire using appropriate anti-V beta antibodies. Although much information on the spatio-temporal organization of repertoire-purging has been gathered by virtue of self-superantigens, serious doubts exist as to the possibility that such structures serve as pathogenetically relevant autoantigens. Thus, certain inbred mice spontaneously develop autoimmune diseases, although they bear T-cell repertoires that appear to be purged from self-superantigen-reactive V beta products. In addition, therapeutic interventions targeted to V beta gene products that are not specific for self-superantigens are successful in preventing disease development. The lack of correlation between superantigen-related V beta deletions and autoimmune disease development is substantiated in further models of murine autoimmunity. Based on these observations, we formulate the hypothesis that self-superantigen-reactive T cells are not involved in the development of autoimmune diseases.

Functional double-negative T cells in the periphery express T cell receptor V beta gene products that cause deletion of single-positive T cells

A proportion of peripheral T cells lack surface expression of the CD4 or CD8 coreceptor molecules and hence are designated as " double negative" (DN). Most DN T lymphocytes express the gamma/delta T cell receptor (TcR), but a minor fraction of them, in both humans and mice, express the alpha/beta TcR. Whereas alpha/beta+ DN T lymphocytes are infrequent (< 1%) in conventional lymphoid organs (spleen, blood, lymph node), they account for two-thirds of the T cells residing in adult bone marrow. Analysis of the TcR V beta repertoire expressed by peripheral DN T cells revealed a high frequency of cells bearing autoreactive TcR that cause deletion of "single-positive" (SP) (CD4+CD8-or CD4-CD8+) T cells. Peripheral DN cells thus represent a cell type that is relatively resistant to clonal deletion. Furthermore, such cells have not been inactivated (anergized) in vivo since they proliferate and secrete interleukins in response to cross-linking by monoclonal antibodies specific for these V beta gene products that are deleted in SPT cells. These results might help to understand the association of peripheral expansion of DN cells and development of autoimmune diseases.

Possible role of histamine in pathogenesis of autoimmune diseases: Implications for immunotherapy with histamine-2 receptor antagonists

The immunosuppressive chemical drugs cyclosporine A (CsA) and methotrexate (Mx) have recently been shown to be of benefit in several different diseases of autoimmune origin. Cellular immune responses may play a major role in autoimmunity as autoreactive T lymphocytes appear to recognize autoantigens and major histocompatibility complex (MHC) class II restriction molecules presented by non-immune, aberrant cells, subsequently leading to damage on healthy tissues. Psoriasis is suggested to be an autoimmune disease and in severe, uncontrollable psoriasis CsA and Mx are of value in reducing disease activity. Histamine is suggested to be involved in the pathogenesis of psoriasis and the histamine-2 receptor antagonist ranitidine has been shown to be of value to reduce severe psoriatic disease. The finding that CsA and Mx efficiently reduce histamine formation and release raises the possibility, that histamine is one of the molecules involved in pathogenesis of autoimmune diseases. T cell mediated regulation and suppression of autoreactive T cells seem to be ineffective in controlling the enhanced immune reaction in patients where the discrimination between self and non-self is changed. A consequence of this may be induction of interferon-gamma (IFN-g) production and release by cytotoxic T cells, subsequently leading to expression of MHC II molecules on non-immune tissues. As immunotherapy may be of value in some autoimmune diseases the use of histamine-2 receptor antagonists should be evaluated in patients where conventional therapy is ineffective to reduce disease activity.

Mechanisms of self tolerance

The redundancy of biological systems minimizes the probability that isolated molecular and cellular defects entail deleterious consequences. This notion also applies to the establishment and maintenance of tolerance to self antigens. Thus, immune homeostasis is attributed to multiple distinct safety valves that are connected in series and intervene at defined control points of the life cycle of the developing lymphocyte to guarantee the physical elimination, functional inactivation, or regulated inhibition of self-reactive, potentially autoaggressive, B and T cells.

Management of early inflammatory arthritis. Intervention with immunomodulatory agents: T cell vaccination

Current theories of the aetiology of RA point to a central role for the trimolecular complex comprising the MHC class II molecule on the surface of the APC, the antigenic peptide and the TCR on the disease-inducing T cell. Thus the arthritogenic T cell is an important target for new therapy. However, it cannot be directly identified because the causative antigen is unknown, so indirect techniques such as TCV and TCR peptide vaccination are required. In TCV, T cells thought to mediate the disease, in an activated and attenuated form, are injected into the patient, who then develops a specific immune response against these pathogenic T cells. TCV has been shown to be effective in protecting against and treating a variety of animal models of autoimmune disease, including AA, EAE and IDDM in NOD mice. The vaccines initially comprised clones and lines of T cells shown to be capable of transferring the disease, but later unseparated LN cells were also shown to be effective, paralleling more closely the human situation. Interestingly, it has become clear that TCV does not create its own regulatory network but amplifies a natural immunoregulatory network which forms as the disease develops. The major stimulating moiety on the vaccinating T cell is its receptor (anti-idiotypic response), although there is also an anti-ergotypic (anti-activated T cell) response. For this reason the technique of TCR peptide vaccination was developed, which utilizes only a short peptide from the TCR of the disease-causing cells to stimulate an immune response against them. This is effective in the prevention and treatment of EAE, where there is a preferential usage of TCR-V beta 8 by encephalitogenic T cells. The application of both these techniques to human autoimmune disease is in its infancy. Studies of TCV in MS and RA have not shown clear-cut clinical benefit, although immunological changes have been observed; comparison of methodology with the animal work and assessment of results are complex and further studies are in progress. Studies of TCR peptide vaccination in MS and RA are handicapped by the lack of a consensus on TCR usage in these conditions, but a limited study is underway in MS.

Inability of IL-2 and IL-10 to counteract B cell clonal deletion

The B cell antigen receptor (BCR) delivers inhibitory signals in nascent B cells leading to the establishment of tolerance via clonal deletion or clonal anergy depending upon the type of antigen to which the B cells are exposed. In previous work, it has been demonstrated that activated Th2 cells, as well as some recombinant lymphokines, prevent the inhibition of growth and subsequent cell death induced through the BCR in model B cell lymphomas. Herein, we extend this work to another Th2 lymphokine, IL-10, that in contrast to IL-4 does not interfere with the deletion promoted by IgM crosslinking. The effect of individual lymphokines has also begun to be analyzed in a transgenic model of B cell clonal deletion. To this end, we have administered a recombinant vaccinia virus producing human IL-2 to mice expressing an autoreactive H-2Kk,b-specific transgenic IgMk and found that IL-2 does not abrogate B cell deletion in vivo.

Disease modification in rheumatoid arthritis with special reference to cyclosporin A

Disease modifying antirheumatic drugs improve clinical markers of synovial inflammation as well as acute phase reactants and functional status in patients with rheumatoid arthritis. Some of the disease modifying antirheumatic drugs probably also retard radiographic progression. Data from prospective observational studies and epidemiological studies indicate a modest and temporary effect. New treatment modalities are needed, including both new strategies and new drugs. Cyclosporin A improves clinical markers of synovial inflammation and is useful for individual patients with rheumatoid arthritis. However, further studies are required to investigate the long term efficacy and tolerance of cyclosporin in rheumatoid arthritis.