The transcriptional activator Sp1, a novel autoantigen

Apoptosis and immune responses to self

The mystery that surrounds autoimmunity revolves around how the immune system of patients who have systemic autoimmune diseases becomes primed to recognize intracellular antigens, how the autoantibodies thus produced contribute to the pathogenesis of the disease, and how those autoantibodies access their target proteins. By examining the mechanisms that are involved in the normal cellular process of apoptosis, we are beginning to unravel this mystery. The intracellular autoantigen targets of many systemic autoimmune diseases become altered during apoptosis in ways that may change how they are perceived by the immune system. High concentrations of self-antigens, or in the case of viral infection, complexes of foreign and self-antigens, are packaged during generation of apoptotic cells. The packages also may contain altered fragments of self-antigens that have not been encountered previously by the immune system. Under normal circumstances, apoptotic cells are cleared rapidly by macrophages and DCs. The normal consequence of that clearance is that the apoptosis-altered self-antigens are either ignored by the immune system or tolerance to those antigens is maintained. Clearance is achieved through complex mechanisms that enable macrophages and DCs to recognize apoptotic cells as nonthreatening "self" particles. Defects in this process that cause a delay in clearance could change the appearance of apoptotic cells and cause them to be recognized as "foreign invaders," thereby stimulating an inflammatory response that, in turn, activates an immune response to self-antigens. By studying the mechanisms that are involved in recognition and clearance of apoptotic cells, we are uncovering clues to the defects that may underlie the development of systemic autoimmunity.

Titin as a chromosomal protein

We identified titin as a chromosomal protein using a human autoimmune scleroderma serum. We cloned the corresponding gene in the fruitfly, Drosophila melanogaster. We have demonstrated that titin is not only expressed and localized in striated muscle but is also distributed uniformly on condensed mitotic chromosomes using multiple antibodies directed against different domains of both Drosophila and vertebrate titin. Titin is a giant sarcomeric protein responsible for the elasticity of striated muscle. Titin may also function as a molecular scaffold during myofibril assembly. We hypothesize that titin is a component of chromosomes that may function to determine chromosome structure and provide elasticity, playing a role similar to that proposed for titin in muscle. We have identified mutations in Drosophila Titin (D-Titin) and are characterizing phenotypes in muscle and chromosomes.

Identification of human autoantibodies to the transcriptional repressor ZF5

ZF5 was originally cloned as a transcriptional repressor on the mouse c-myc promoter. It contains the Kruppel-type zinc fingers and a conserved POZ domain, which is found in a growing number of zinc finger proteins and mediate protein-protein interactions. Autoantibodies against transcription factors are sometimes found in sera from patients with high levels of anti-nuclear antibody (ANA). Using Western blotting with ZF5 and sera of autoimmune disease, we detected one serum, named M6 serum, which contains the antibody against a transcriptional repressor ZF5. The confirmed epitope was specific to ZF5 and was not reactive to the other analogous factors: BCL-6, ZID, and Sp1. This epitope also has a molecular mimicry of the viral proteins. From these results, we predict that viral protein which mimics host ZF5 antigen triggers self-reactive T cell clones and induces the autoantibody in M6 serum after the destruction of host tissues.

The locus of Epstein-Barr virus terminal repeat processing is bound with enhanced affinity by Sp1 and Sp3

EBV DNA contains G-rich, repeat regions that are involved in rearrangement and recombination events including terminal repeat (TR) processing and the EBNA-2 deletion in the EBV strain P3HR-1. Cellular proteins, called terminal or tandem repeat binding proteins (TRBPs), recognize sequences at the junctions of these recombination events. In this study, using antibody supershift assays and expression of recombinant proteins, we show that Sp1 and Sp3 are the sequence-specific components of TRBP and that Ku is the nonspecific binding component. Sp1 binds other recombinogenic regions of EBV DNA, but Sp3 does not bind to the large internal repeat. The sequence GGGGTGGGG, a low affinity site for Sp1 and Sp3, is the minimal binding site within terminal repeat binding site 1 (TRBS1). However, 3' flanking sequences in the sequence GGGGTGGGGCATGGGG augment binding of Sp1 and Sp3 so that their affinity of binding is increased approximately twofold relative to a classical high-affinity Sp1 site. EBV lytic cycle induction does not alter the abundance or binding activity of any of the three identified components of TRBP. Sp1 and Sp3 may act in trans to promote EBV terminal repeat processing and possibly other viral and cellular recombination events.