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Protein targets in autoimmune disease vary in location, originating within cells as in SLE, or found on cell surfaces or in extracellular spaces. The term “autoantigenesis” is first defined here as the changes that arise in self-proteins as they break self tolerance and trigger autoimmune B and/or T cell responses. As illustrated in many studies, between 50 and 90% of the proteins in the human body acquire posttranslational modification. In some cases, it may be that these modifications are necessary for the biological functions of proteins of the cells in which they reside or as extracellular mediators. Summarized herein, it is clear that some posttranslational modifications can create new self-antigens by altering immunologic processing and presentation. While many protein modifications exist, we will focus on those created, amplified, or altered in the context of inflammation or other immune system responses. Finally, we will address how posttranslational modifications in self-antigens may affect the analyses of B and T cell specificity, current diagnostic techniques, and/or the development of immunotherapies for autoimmune diseases.

Protein L-isoaspartyl methyltransferase (PIMT) catalyzes repair of L-isoaspartyl peptide bonds, a major source of protein damage under physiological conditions. PIMT knock-out (KO) mice exhibit brain enlargement and fatal epileptic seizures. All organs accumulate isoaspartyl proteins, but only the brain manifests an overt pathology. To further explore the role of PIMT in brain function, we undertook a global analysis of endogenous substrates for PIMT in mouse brain. Extracts from PIMT-KO mice were subjected to two-dimensional gel electrophoresis and blotted onto membranes. Isoaspartyl proteins were radiolabeled on-blot using [methyl-3 H]S-adenosyl-L-methionine and recombinant PIMT. Fluorography of the blot revealed 30 -35 3 H-labeled proteins, 22 of which were identified by peptide mass fingerprinting. These isoaspartate-prone proteins represent a wide range of cellular functions, including neuronal development, synaptic transmission, cytoskeletal structure and dynamics, energy metabolism, nitrogen metabolism, pH homeostasis, and protein folding. The following five proteins, all of which are rich in neurons, accumulated exceptional levels of isoaspartate: collapsin response mediator protein 2 (CRMP2/ULIP2/DRP-2), dynamin 1, synapsin I, synapsin II, and tubulin. Several of the proteins identified here are prone to age-dependent oxidation in vivo, and many have been identified as autoimmune antigens, of particular interest because isoaspartate can greatly enhance the antigenicity of self-peptides. We propose that the PIMT-KO phenotype results from the cumulative effect of isoaspartaterelated damage to a number of the neuron-rich proteins detected in this study. Further study of the isoaspartate-prone proteins identified here may help elucidate the molecular basis of one or more developmental and/or age-related neurological diseases. Formation of isoaspartate (isoAsp)3 is a major source of spontaneous protein damage under physiological conditions, arising in conjunction with deamidation of asparaginyl residues and isomerization of aspartyl residues (1-6). This non-enzymatic process occurs via the formation of a succinimide intermediate (cyclic imide) following nucleophilic attack of the Asx side-chain carbonyl group by the amide nitrogen at the C-flanking amino acid (see Fig. 1). Hydrolysis of the succinimide leads to formation of isoaspartyl (isoAsp) and aspartyl products in a typical ratio of 3 to 1. The ␤-linkage characteristic of the predominant isoaspartyl form introduces a kink into the polypeptide backbone that can disrupt normal protein folding and activity. Isoaspartyl formation is strongly influenced by the amino acid that immediately follows (is C-flanking to) an Asn or Asp residue, by the degree of local polypeptide flexibility, and by environmental stressors such as high pH, heat shock, radiation, and oxidation.The enzyme activity we now call protein L-isoaspartyl methyltransferase (PIMT, EC 2.1.1.77) was first encountered in 1965 by Axelrod and Daly (7) as an S-adenosyl-L-methionine (AdoMet)-dependent...

Graft-versus-host disease (GVHD) remains a major cause of morbidity and mortality in allogeneic stem cell transplantation (alloSCT). Donor T cells that accompany stem cell grafts cause GVHD by attacking recipient tissues; therefore, all patients receive GVHD prophylaxis by depletion of T cells from the allograft or through immunosuppressant drugs. In addition to providing a graft-versus-leukemia effect, donor T cells are critical for reconstituting T cell–mediated immunity. Ideally, immunity to infectious agents would be transferred from donor to host without GVHD. Most donors have been exposed to common pathogens and have an increased precursor frequency of memory T cells against pathogenic antigens. We therefore asked whether memory CD62L–CD44+ CD4+ T cells would induce less GVHD than unfractionated or naive CD4+ T cells. Strikingly, we found that memory CD4 cells induced neither clinical nor histologic GVHD. This effect was not due to the increased number of CD4+CD25+ regulatory T cells found in the CD62L–CD44+ fraction because memory T cells depletion of these cells did not cause GVHD. Memory CD4 cells engrafted and responded to antigen both in vivo and in vitro. If these murine results are applicable to human alloSCT, selective administration of memory T cells could greatly improve post-transplant immune reconstitution