Mice Lacking H2-M Complexes, Enigmatic Elements of the MHC Class II Peptide-Loading Pathway

Recent data suggest that the diversity of self peptides presented in the thymus during development contributes to positive selection of a diverse T cell repertoire. We sought to determine whether a previously defined “hole in the immunological repertoire” could be explained by the absence of an appropriate selecting self peptide. The repertoire defect in question is the inability of bm8 mice to make an H-2K-restricted response to OVA. Like other OVA-specific, H-2K-restricted receptors, OT-I-transgenic T cells are not positively selected in bm8 mice. Using criteria we had previously established for identifying positive selection ligands, we found peptides that could restore positive selection of OT-I thymocytes in bm8 mice. Thus, the T cell repertoire can be limited by a requirement for specific self peptides during development. Data with MHC-specific Abs suggested that peptides might be able to force MHC residues to adopt different conformations in Kb vs Kbm8. This shows that peptides can potentially contribute to ligand diversity both directly (via variability in the solvent-exposed side chains) and indirectly (through their effect on the MHC conformation). Our data support a model where self peptide diversity allows selection of T cells specific for a broad range of MHC conformations.

Section: Positive Selection Is Limited By Available Peptide-dependentmentioning

confidence: 99%

Positive Selection Is Limited by Available Peptide-Dependent MHC Conformations

Stefanski

Jameson

Hogquist

2000

“…Although crystallographic studies have not found any evidence for this possibility (58,60), studies with conformation-dependent Abs have suggested that peptide binding can affect the conformation of MHC class I (61, 62) and class II (43,44,59,(63)(64)(65) molecules and that such subtly altered conformations can affect recognition by TCRs (57,(65)(66)(67). Our observation that none of the hybrids reactive with A b EpIi Ϫ cells recognized H2-DM-deficient cells, and vice versa (Table I), is consistent with this idea.…”

Section: Peptide-dependence Of Alloreactive T Cell Responsesmentioning

confidence: 99%

The Alloreactive and Self-Restricted CD4+ T Cell Response Directed Against a Single MHC Class II/Peptide Combination

Kovalik

Singh

Mendiratta

et al. 2000

The cellular basis for allograft rejection derives from the strong T cell response to cells bearing foreign MHC. While it was originally assumed that alloreactive T cells focus their recognition on the polymorphic residues that differ between syngeneic and allogeneic MHC molecules, studies with MHC class I-restricted CTL have shown that MHC-bound peptides play a critical role in allorecognition. It has been suggested that alloreactive T cells depend more strongly on interactions with the MHC molecule than with the associated peptide, but there is little evidence to support this idea. Here we have studied the alloreactive and self-restricted response directed against the class II H2-Ab molecule bound with a single peptide, Ep, derived from the H2-Eα chain. This MHC class II-peptide combination was a poor target and stimulator of alloreactive CD4+ T cell responses, indicating that MHC-bound peptides are as important for alloreactive CD4+ T cells as they are for alloreactive CTL. We also generated alloreactive T cells with exquisite specificity for the Ab/Ep complex, and compared their reactivity with self-restricted T cells specific for the same Ab/Ep complex. Our results showed that peptide-specific alloreactive T cells, as compared with self-restricted T cells, were more sensitive to peptide stimulation, but equally sensitive to amino acid substitutions in the peptide. These findings indicate that alloreactive and self-restricted T cells interact similarly with their MHC/peptide ligand.

“…It also seems likely that DM actions demonstrated via these complementation assays could be influenced by many additional experimental parameters, such as abnormal expression levels and functional contributions by recipient cell lines, making it difficult to assess the significance of these observations. Current thinking about the class II pathway has been strongly influenced by studies of DM mutant mice (22)(23)(24). The loss of DM function disrupts conventional Ag presentation and causes complex defects with respect to selection of mature CD4 ϩ T cells.…”

mentioning

confidence: 99%

“…The striking accumulation of A b /CLIP complexes alongside severely reduced peptide-loading capabilities strongly argues DM activity is required to mediate CLIP release and promote occupancy by diverse peptides under physiological conditions in normal APCs. As judged by the gain of reactivity toward wild-type class II molecules, mature CD4 ϩ T cells exclusively selected on A b /CLIP complexes exhibit somewhat incomplete tolerance toward natural self-peptides (22)(23)(24). Considerable evidence suggests these animals express a semidiverse TCR repertoire (25-28).…”

mentioning

confidence: 99%

Relaxed DM Requirements During Class II Peptide Loading and CD4+ T Cell Maturation in BALB/c Mice

Bikoff

Wutz

Kenty

et al. 2001

Current ideas about DM actions have been strongly influenced by studies of mutant strains expressing the H-2b haplotype. To evaluate DM contributions to class II activities in BALB/c mice, we generated a novel mutation at the DMa locus via embryonic stem cell technology. Unlike long-lived Ab/class II-associated invariant chain-derived peptide (CLIP) complexes, mature Ad and Ed molecules are loosely occupied by class II-associated invariant chain-derived peptide and are SDS unstable. BALB/c DM mutants weakly express BP107 conformational epitopes and toxic shock syndrome toxin-1 superantigen-binding capabilities, consistent with partial occupancy by wild-type ligands. Near normal numbers of mature CD4+ T cells fail to undergo superantigen-mediated negative selection, as judged by TCR Vβ usage. Ag presentation assays reveal consistent differences for Ad- and Ed-restricted T cells. Indeed, the mutation leads to decreased peptide capture by Ad molecules, and in striking contrast causes enhanced peptide loading by Ed molecules. Thus, DM requirements differ for class II structural variants coexpressed under physiological conditions in the intact animal.