T cells specific for the cytochrome c Ag are widely used to investigate many aspects of TCR specificity and interactions with peptide-MHC, but structural information has long been elusive. In this study, we present structures for the well-studied 2B4 TCR, as well as a naturally occurring variant of the 5c.c7 TCR, 226, which is cross-reactive with more than half of possible substitutions at all three TCR-sensitive residues on the peptide Ag. These structures alone and in complex with peptide-MHC ligands allow us to reassess many prior mutagenesis results. In addition, the structure of 226 bound to one peptide variant, p5E, shows major changes in the CDR3 contacts compared with wild-type, yet the TCR V-region contacts with MHC are conserved. These and other data illustrate the ability of TCRs to accommodate large variations in CDR3 structure and peptide contacts within the constraints of highly conserved TCR–MHC interactions.

Regeneration of propagating action potentials at nodes of Ranvier allows nerve impulses to be conducted over long distances. Proper nodal function is believed to rely on intimate associations among axons, myelinating oligodendrocytes, and perinodal astrocytes. Studies in the optic nerve, corpus callosum, and spinal cord suggest that NG2+ cells are also key constituents of CNS nodes and that these glia may influence conduction efficacy and formation of axon collaterals. However, the prevalence of NG2+ cell processes at CNS nodes of Ranvier has not been rigorously quantified. Here we used a transgenic mouse expressing membrane-targeted EGFP to visualize the fine processes of NG2+ cells and to quantify the spatial relationship between NG2+ cells and nodes of Ranvier in four distinct CNS white matter tracts. NG2+ cell processes came within close spatial proximity to a small percentage of nodes of Ranvier and approximately half of these spatial interactions were estimated to occur by chance. The majority of NG2+ cell process tips were not found in close proximity to nodes and gray matter NG2+ cells in regions of low nodal density were as morphologically complex as their white matter counterparts, indicating that attraction to nodes does not critically influence the elaboration of NG2+ cell processes. Finally, there was no difference in nodal density between small regions devoid of NG2+ cell processes and those containing numerous NG2+ cells processes, demonstrating that the function of CNS nodes of Ranvier does not require ongoing interaction with NG2+ cells.Significance StatementEffective propagation of action potentials along neuronal axons is dependent upon periodic regeneration of depolarization at nodes of Ranvier. The position, structural integrity, and function of nodes of Ranvier is believed to be regulated, in part, by intimate physical interactions between nearby glial cells and nodes. Clarifying whether oligodendrocyte precursor cells are obligate members of this nodal support system is critical for defining whether these cells contribute to pathologies in which nodal structure is compromised.

Self-peptide MHCII ligands are critical for selection of CD4+ T cells in the thymus, and maintenance in the periphery. To date, no investigation as to the exact thymic and peripheral expression of a naturally occurring positive selecting self-peptide MHCII (self-pMHCII) complex has taken place. We have generated a sensitive T cell hybridoma to functionally detect the endogenous presentation of a confirmed positive selecting self-pMHCII complex for a CD4+ transgenic T cell. Using this tool to survey and quantify the expression selecting self-pMHCII, we have shown unequivocal proof that a known CD4+ selecting ligand can be presented on both positive and negative selecting thymic APCs. We also show that peripheral presentation of this same selecting ligand is affected by activation state of the APCs. Furthermore, discrepancies between the gene expression and self-pMHCII complex presentation of this bona fide selecting ligand suggest that functional detection self-ligand complexes will be required to establish a complete view of the naturally presented endogenous self-pMHC landscape.

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