We have previously demonstrated that in vivo expression of the polyomavirus DNA-binding T-antigen initiated production of IgG antibodies to T-antigen and to DNA, but not to a panel of autoantigens not related to nucleosomes, indicating an antigen-selective T cell-dependent B cell response. In this study, we demonstrate that CD4-positive T cells from both normal and systemic lupus erythematosus (SLE) patients readily proliferate in response to pure T-antigen, and also to T-antigen in complex with nucleosomes. T-antigen-specific T cell lines from both normal individuals and SLE patients proliferate in response to nucleosome-T-antigen complexes, but not to nucleosomes or histones. B cells co-cultured with T-antigen-specific T cells and stimulated with nucleosome-T-antigen complexes produce anti-T-antigen and anti-DNA antibodies, indicating that such CD4-positive T cells have the potential to interact with B cells specific for individual components of nucleosome-T-antigen complexes. Thus, a non-self DNA-binding protein like polyomavirus T-antigen may initiate and maintain an antibody response to DNA when T-antigen is actively expressed.

Objective. To investigate whether polyomavirus T antigen linked to histones through nucleosome-T antigen complexes has the potential to terminate histonespecific T cell anergy.Methods. Blood mononuclear cells from healthy individuals were used as the source to establish T cell lines initiated and maintained by T antigen, histones, nucleosome-T antigen complexes, or nucleosomes. Proliferative responses of these lines to T antigen, histones, and nucleosomes were determined.Results. Whereas T cell lines could be established using T antigen or T antigen-nucleosome complexes, histones or nucleosomes did not have this potential. However, T cell lines selected by T antigen-nucleosome complexes responded subsequently to histones and nucleosomes. Identical results were obtained with murine and human nucleosomes, provided that they were complexed with T antigen.Conclusion. T antigen-specific T cells possess the potential to proliferate when interacting with an antigen-presenting cell that presents T antigen. In the presence of T antigens complexed with nucleosomes, T antigen-specific T cells offer bystander help that may terminate histone-specific T cell anergy. These T cells may progress into functional, autoimmune T cells if histones are properly presented.

We have previously demonstrated that in vivo expression of the polyomavirus DNA‐binding T‐antigen initiated production of IgG antibodies to T‐antigen and to DNA, but not to a panel of autoantigens not related to nucleosomes, indicating an antigen‐selective T cell‐dependent B cell response. In this study, we demonstrate that CD4‐positive T cells from both normal and systemic lupus erythematosus (SLE) patients readily proliferate in response to pure T‐antigen, and also to T‐antigen in complex with nucleosomes. T‐antigen‐specific T cell lines from both normal individuals and SLE patients proliferate in response to nucleosome‐T‐antigen complexes, but not to nucleosomes or histones. B cells co‐cultured with T‐antigen‐specific T cells and stimulated with nucleosome‐T‐antigen complexes produce anti‐T‐antigen and anti‐DNA antibodies, indicating that such CD4‐positive T cells have the potential to interact with B cells specific for individual components of nucleosome‐T‐antigen complexes. Thus, a non‐self DNA‐binding protein like polyomavirus T‐antigen may initiate and maintain an antibody response to DNA when T‐antigen is actively expressed.

Objective. Investigators in this study undertook to determine whether in vitro antigen-responsive immune (polyomavirus T antigen [T-ag]-specific) and autoimmune (histone-specific) T cells from normal individuals share structural and genetic characteristics with those from patients with systemic lupus erythematosus (SLE).Methods. Histone-specific T cells were generated by stimulation of peripheral blood mononuclear cells (PBMCs) with nucleosome-T-ag complexes and were subsequently maintained by pure histones. T-agspecific T cell clones were initiated and maintained by T-ag. The frequencies of circulating histone-and T-agspecific T cells were determined in healthy individuals and in SLE patients by limiting dilution of PBMCs. T cell receptor (TCR) gene usage and variable-region structures were determined by complementary DNA sequencing. These sequences were compared between T-ag-and histone-specific T cells and between normal individuals and SLE patients for each specificity.Results. Individual in vitro-expanded histoneand T-ag-specific T cells from normal individuals displayed identical TCR V ␣ and/or V ␤ chain third complementarity-determining region (CDR3) sequences, indicating that they were clonally expanded in vivo. The frequencies of in vitro antigen-responsive T-ag-or histone-specific T cells from normal individuals were similar to those from SLE patients. Although heterogeneous for variable-region structure and gene usage, histone-specific T cells from healthy individuals and SLE patients selected aspartic and/or glutamic acids at positions 99 and/or 100 of the V ␤ CDR3 sequence. Conclusion. Autoimmune T cells from healthy individuals can be activated by nucleosome-T-ag complexes and maintained by histones in vitro. Such T cells possessed TCR structures similar to those from SLE patients, demonstrating that T cell autoimmunity to nucleosomes may be a latent property of the normal immune system.Experimental results consistently demonstrate that pathogenic autoimmune anti-DNA antibodies are secondary, antigen-driven, CD4ϩ T cell-dependent immune responses initiated by DNA itself (1-5). Since DNA most likely is not presented by antigen-presenting cells (APCs) in the context of HLA class II molecules, the contemporary paradigm favors DNA binding proteins as a sine qua non for generating T helper cell stimuli in the autoimmune anti-DNA antibody response (3,5,6). Such a hapten-carrier model, implying DNAspecific B cells and protein-specific T cells, enables the understanding of the molecular and the cellular basis for an immunoglobulin class switch and affinity maturation of anti-single-stranded DNA and anti-double-stranded DNA (anti-dsDNA) antibodies. Experimental and clinical results demonstrating that T cells specific for self or non-self DNA binding proteins may provide help for DNA-specific B cells support a model consistent with cognate B cell-T cell interaction (2,3,(6)(7)(8). So far, however, it is unclear how self-specific T cells may be initially activated. A priori, one would expect, for example, nuc...

Previous results have revealed a strong correlation between polyomavirus BK reactivation and disease activity and antinuclear auto-antibody production in the human autoimmune disease systemic lupus erythematosus. BK virus establishes a latent infection in most humans, and reactivation requires the production of the DNA-binding large T antigen. Experimentally induced expression of the polyomavirus SV40 large T antigen in mice induces both an immune response to large T antigen and autoimmune response to nuclear antigens and antinuclear antibody production. Previous results have indicated that human Tantigen-specific CD4 1 T-cell lines are stimulated equally by free, soluble and nucleosome-bound T antigen. This study was designed to determine how antigen processing of nucleosomes containing bound SV40 large T antigen may affect the specificity and response characteristics of experimentally induced T-antigenspecific CD4 1 T cells. The results indicated that CD4 1 T-cell lines generated from mice immunized with soluble, free T antigen responded very poorly in response to stimulation with T antigen bound to nucleosomes. CD4 1 T-cell lines generated from mice immunized with nucleosomes that had bound T antigen in situ responded to both free and nucleosome-bound T antigen. The T-antigen-specific, CD4 1 memory T cells induced by latent polyomavirus infections in humans may be uniquely suited to initiate autoimmunity to nuclear antigens upon virus reactivation.