ANTIGEN PRESENTATION AND ASSEMBLY BY MOUSE I-A(K) 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 prese ntation 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-assoc iated invariant chain peptides (CLIP). In contrast to human class II m olecules, a significant proportion of mouse I-A(k) molecules expressed in T2 and 9.5.3 were associated with antigenic peptides, indicating t hat I-A(k)/peptide assembly is possible in the absence of the Dm prote ins. Thus, the presentation of determinants derived from hen egg lysoz yme (HEL), keyhole limpet hemocyanin, and conalbumin was normal in 9.5 .3A(k) and a conalbumin determinant was presented normally by T2.A(k). However, the keyhole limpet hemocyanin determinant was not presented by T2.A(k), and HEL(46-61) was only presented at a low level by these APC. SDS-stable, dimeric I-Ak molecules were expressed by both T2.A(k) and 9.5.3A(k) and formed late in their intracellular transport. Prese ntation of HEL(46-61) was partially inhibited by disrupting vacuolar a cidification in 9.5.3A(k), consistent with I-A(k)/peptide assembly in a post-Golgi endosomal compartment. Accordingly, Dm is not an obligato ry requirement for MHC class II/peptide assembly. We propose that Dm i nfluences the displacement of CLIP from recently synthesized class II molecules, a process that is likely to be less critical for I-A(k) bec ause of its low affinity for CLIP.