Antigen presentation and assembly by mouse I-Ak class II molecules in human APC containing deleted or mutated HLA DM genes.

The behavior of mouse I-Ak molecules was studied in the human Ag presentation mutants T2 and 9.5.3, which contain deleted or mutated HLA DM genes. HLA class II molecules expressed by these APC are defective in presentation of native Ag and are mostly complexed with class II-associated invariant chain peptides (CLIP). In contrast to human class II molecules, a significant proportion of mouse I-Ak molecules expressed in T2 and 9.5.3 were associated with antigenic peptides, indicating that I-Ak/peptide assembly is possible in the absence of the Dm proteins. Thus, the presentation of determinants derived from hen egg lysozyme (HEL), keyhole limpet hemocyanin, and conalbumin was normal in 9.5.3Ak and a conalbumin determinant was presented normally by T2.Ak. However, the keyhole limpet hemocyanin determinant was not presented by T2.Ak, and HEL46-61 was only presented at a low level by these APC. SDS-stable, dimeric I-Ak molecules were expressed by both T2.Ak and 9.5.3Ak and formed late in their intracellular transport. Presentation of HEL46-61 was partially inhibited by disrupting vacuolar acidification in 9.5.3Ak, consistent with I-Ak/peptide assembly in a post-Golgi endosomal compartment. Accordingly, Dm is not an obligatory requirement for MHC class II/peptide assembly. We propose that Dm influences the displacement of CLIP from recently synthesized class II molecules, a process that is likely to be less critical for I-Ak because of its low affinity for CLIP.