Altered structure of autoantigens during apoptosis

Tumor-associated antigen arrays for the serological diagnosis of cancer

The recognition that human tumors stimulate the production of autoantibodies against autologous cellular proteins called tumor-associated antigens (TAAs) has opened the door to the possibility that autoantibodies could be exploited as serological tools for the early diagnosis and management of cancer. Cancer-associated autoantibodies are often driven by intracellular proteins that are mutated, modified, or aberrantly expressed in tumor cells and hence are regarded as immunological reporters that could help uncover molecular events underlying tumorigenesis. Emerging evidence suggests that each type of cancer might trigger unique autoantibody signatures that reflect the nature of the malignant process in the affected organ. The advent of novel genomic, proteomic, and high throughput approaches has accelerated interest in the serum autoantibody repertoire in human cancers for the discovery of candidate TAAs. The use of individual anti-TAA autoantibodies as diagnostic or prognostic tools has been tempered by their low frequency and heterogeneity in most human cancers. However, TAA arrays comprising several antigens significantly increase this frequency and hold great promise for the early detection of cancer, monitoring cancer progression, guiding individualized therapeutic interventions, and identification of novel therapeutic targets. Our recent studies suggest that the implementation of TAA arrays in screening programs for the diagnosis of prostate cancer and other cancers should be preceded by the optimization of their sensitivity and specificity through the careful selection of the most favorable combinations of TAAs.

Nucleosome autoantibodies

The nucleosome is a large protein-nucleic acid complex involved in DNA packing and in controlling genetic information. Under circumstances described below, this component, normally sequestered in the cell nucleus, is released into the extracellular milieu and then is easily accessible to cells of the immune system. For still not completely understood reasons, nucleosomes become immunogenic under particular conditions. Thus, anti-nucleosome autoantibodies (autoAb) have been described in connective tissue diseases and especially in systemic lupus erythematosus (SLE). This review describes the mechanisms leading to nucleosome production and anti-nucleosome autoimmunity, as well as the pathogenesis associated with nucleosomes.

Nuclear protein kinase C

Protein kinase C (PKC) isozymes constitute a family of ubiquitous phosphotransferases which act as key transducers in many agonist-induced signaling cascades. To date, at least 11 different PKC isotypes have been identified and are believed to play distinct regulatory roles. PKC isoforms are physiologically activated by a number of lipid cofactors. PKC is thought to reside in the cytoplasm in an inactive conformation and to translocate to the plasma membrane or cytoplasmic organelles upon cell activation by different stimuli. However, a sizable body of evidence collected over the last 20 years has shown PKC to be capable of translocating to the nucleus. Furthermore, PKC isoforms are resident within the nucleus. Studies from independent laboratories have to led to the identification of quite a few nuclear proteins which are PKC substrates and to the characterization of nuclear PKC-binding proteins which may be critical for finely tuning PKC function in this cell microenvironment. Several lines of evidence suggest that nuclear PKC isozymes are involved in the regulation of biological processes as important as cell proliferation and differentiation, gene expression, neoplastic transformation, and apoptosis. In this review, we shall highlight the most intriguing and updated findings about the functions of nuclear PKC isozymes.

Heterogeneous nuclear ribonucleoprotein P2 is an autoantibody target in mice deficient for Mer, Axl, and Tyro3 receptor tyrosine kinases

Deficiencies in clearance of apoptotic cells predispose to the development of autoimmune disease. This is evident in mice lacking the receptor tyrosine kinases Tyro3, Axl, and Mer. Deficient mice exhibit an increased abundance of apoptotic cells in tissues and manifest diverse autoimmune conditions. To test these mice for the presence of autoantibodies to apoptotic cells, we generated spontaneous splenic B cell hybridomas and used a novel microscopy screen to detect Ab binding to apoptotic Jurkat cells. From hybridomas secreting IgG Abs reactive with apoptotic cells, we selected one that recreated the major serum specificity for apoptotic cells. The Ab LHC7.15 bound to an Ag that is differentially distributed between the nucleus and the cytoplasm in live and apoptotic cells. In late apoptotic cells, the Ag coalesces into aggregates that bleb from the cell surface. Immunopurification of the Ag, followed by mass spectrometry, identified a protein of 69 kDa whose partial sequence matched heterogeneous nuclear ribonucleoprotein P2. This multifunctional protein binds DNA, RNA, and several known ribonucleoprotein autoantigens. Our observations indicate that a ribonucleoprotein complex, formed and translocated to the cell surface in apoptosis, represents a potent stimulus for breaking tolerance and inducing systemic autoimmunity in mice with defective clearance of cell remnants.

Immune stimulation mediated by autoantigen binding sites within small nuclear RNAs involves Toll-like receptors 7 and 8

Systemic lupus erythematosus (SLE) is an autoimmune disease characterized by the production of autoantibodies to certain cellular macromolecules, such as the small nuclear ribonucleoprotein particles (snRNPs), which had been considered to be passive targets of the autoimmune response. SLE is also characterized by the increased expression of type I interferon (IFN), which appears to be associated with the development and severity of disease. Here, we show that specific, highly conserved RNA sequences within snRNPs can stimulate Toll-like receptors (TLRs) 7 and 8 as well as activate innate immune cells, such as plasmacytoid dendritic cells (pDCs), which respond by secreting high levels of type I IFN. SLE patient sera containing autoantibodies to snRNPs form immune complexes that are taken up through the Fc receptor γRII and efficiently stimulate pDCs to secrete type I IFNs. These results demonstrate that a prototype autoantigen, the snRNP, can directly stimulate innate immunity and suggest that autoantibodies against snRNP may initiate SLE by stimulating TLR7/8.

Endometriosis associated with defective handling of apoptotic cells in the female genital tract is a major cause of autoimmune disease in women

Several autoimmune diseases are far more common in women than men. The reasons are unknown. Recent studies have shown that many autoimmune diseases which predominantly affect females are characterized by the production of autoantibodies against components of apoptotic cells. Furthermore, in experimental animals, defective clearance of apoptotic cells in a pro-oxidant inflammatory environment can trigger the production of these autoantibodies and related autoimmune disease. Endometriosis is characterized by defective clearance of apoptotic endometrial cells in a pro-oxidant inflammatory environment. It is proposed that this combination of abnormalities triggers autoantibody production in women affected by endometriosis. A proportion of these women will be genetically predisposed to develop overt autoimmune disease. As endometriosis affects at least 4% of the female population of reproductive age, this phenomenon will have a major effect on the gender prevalence of several related autoimmune syndromes. The hypothesis is supported by epidemiological studies which show a strong association between endometriosis and female-predominant autoimmune disorders.

Cytotoxic T lymphocytes and autoimmunity

PURPOSE OF REVIEW

The possibility of the recognition by cytotoxic T lymphocytes of tissue autoantigens has been largely ignored in explaining organ-specific autoimmune diseases. Recent advances in the understanding of human leukocyte antigen class I-binding peptides motifs have led to the detection and the characterization of those autoreactive CD8(+) cytotoxic T lymphocytes involved in organ-specific autoimmune diseases. The purpose of this review is to discuss recent studies that shed light on the implication of cytotoxic T lymphocytes in several autoimmune disorders as well as the mechanisms underlying their stimulation.

RECENT FINDINGS

Significant progress has been made in the characterization of autoantigens targeted by cytotoxic T lymphocytes in several class I-restricted autoimmune diseases, including Behçet's disease and ankylosing spondylitis, and their implication in systemic autoimmune disease such as systemic lupus erythematosus. Moreover, the signals involved in the activation of autoreactive cytotoxic T lymphocytes have been better characterized, particularly the molecular requirements of the antigen presentation at the surface of the dendritic cell system, mainly because of a better understanding of the Toll-like receptor-induced signals or the discovery of a defect in regulatory T cells.

SUMMARY

New findings in the pathophysiology of cytotoxic T lymphocytes in autoimmunity and especially a better comprehension of their activation may give a new impetus for the development of targeted immunologic therapies in various autoimmune disorders.

Generation of novel covalent RNA-protein complexes in cells by ultraviolet B irradiation: implications for autoimmunity

OBJECTIVE

To determine whether ultraviolet B (UVB) irradiation induces novel modifications in autoantigens targeted during experimental photoinduced epidermal damage.

METHODS

To search for novel UVB-induced autoantigen modifications, lysates made from UVB-irradiated human keratinocytes or HeLa cells were immunoblotted using human autoantibodies that recognize ribonucleoprotein autoantigens. Novel autoantigen structures identified were further characterized using nucleases and RNA hybridization.

RESULTS

Human sera that recognize U1-70 kd (U1-70K) and La by immunoblotting also recognized multiple novel species when they were used to immunoblot lysates of UVB-irradiated keratinocytes or HeLa cells. These species were not present in control cells and were not observed when apoptosis was induced by Fas ligation or cytotoxic lymphocyte granule contents. Biochemical analysis using multiple assays revealed that these novel UVB-induced molecular species result from the covalent crosslinking between the U1 RNA and the hYRNA molecules with their associated proteins, including U1-70K, La, and likely components of the Sm particle.

CONCLUSION

These data demonstrate that UVB irradiation of live cells can directly induce covalent RNA-protein complexes, which are recognized by human autoantibodies. As previously described for other autoantigens, these covalent complexes of RNA and proteins may have important consequences in terms of antigen capture and processing.