H2-M Mutant Mice Are Defective in the Peptide Loading of Class II Molecules, Antigen Presentation, and T Cell Repertoire Selection

Abstract

H2-M is a nonconventional major histocompatibility complex (MHC) class II molecule that has been implicated in the loading of peptides onto conventional class II molecules. We generated mice with a targeted mutation in the H2-Ma gene, which encodes a subunit for H2-M. Although the mutant mice express normal class II cell surface levels, these are structurally distinct from the compact SDS-resistant complexes expressed by wild-type cells and are predominantly bound by class II-associated invariant chain peptides (CLIPs). Cells from these animals are unable to present intact protein antigens to class II-restricted T cells and show reduced capacity to present exogenous peptides. Numbers of mature CD4+ T lymphocytes in mutant mice are reduced 3- to 4-fold and exhibit altered reactivities. Overall, this phenotype establishes an important role for H2-M in regulating MHC class II function in vivo and supports the notion that self-peptides contribute to the specificity of T cell positive selection.

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

“…The DMa-deficient mice (22), on a (129 ϫ C57BL/6)F 2 background, that express the H-2 b haplotype, have been maintained by brother-sister matings. The production of Ii chain mutants (33), and sublines established by backcrossing the targeted allele onto BALB/cAn (H-2 d ), or B.C-9, a strain congenic with C57BL/6, but expressing the Igh a allotype of BALB/c, has been described (10,34).…”

“…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.…”

“…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).…”

Positive Selection Is Limited by Available Peptide-Dependent MHC Conformations

“…Indeed, other data suggested that positive selection is not greatly impacted by the specificity of the self peptides presented. Here, the experimental approaches were designed to limit the available peptide/ MHC repertoire to one face and to examine the impact on repertoire size (15)(16)(17)(18). It was shown that in such "single" peptide/MHC mice, a large and diverse repertoire was selected.…”

C-terminal extension of the MHC class II-associated invariant chain by an antigenic sequence triggers activation of naive T cells

“…These co-players of antigen processing are Ii, MHC encoded DM and DO molecules and certain proteases. [1][2][3][4][5][6][7] An important role of Ii is to escort class II molecules to the endocytic route where Ii is degraded by cathepsin S or cathepsin L in MIIC vesicles. 7,8 Along the transition from multivesicular to multilaminar MIICs, DM releases the remaining Ii fragment CLIP (class II-associated Ii peptides) from class II dimers and catalyzes the uptake of antigenic peptides.…”