• MHC ontogeny;
  • mouse reproduction;
  • Qa2;
  • TAP1;
  • TAP2

PROBLEM: To determine the ontogeny of major histocompatibility complex (MHC) expression and TAP products in mouse embryos.

METHOD OF STUDY: mRNAs encoding MHC and associated molecules were identified by reverse transcriptase-polymerase chain reaction, and the protein products were localized by confocal microscopy.

RESULTS: mRNAs encoding class Ia (H-2Db) and class Ib (Q7/9) were present in one-cell embryos, whereas β2-microglobulin (β2-m) transcripts were not detected until the two-cell stage. Transporter TAP1, but not TAP2, transcripts were detected only in blastocysts. H-2 class Ia (classical) protein was detected on the surface of two-cell embryos, H-2 class Ib (nonclassical) protein was detected on one-cell embryos, and β2-m transcripts were detected on eight-cell embryos; TAP1 protein was present at low levels in the cytoplasm from the one-cell stage onward, increasing in expression in blastocysts.

CONCLUSIONS: In mice, MHC class I mRNAs encoding the heavy chain of H-2- and Q7/9-encoding Qa2 molecules are synthesized soon after conception prior to implantation. Similarly, the nonpolymorphic MHC class I-associated molecule β2-m also is expressed before implantation. TAP1, but not TAP2, is first detected at the blastocyst stage, thus preceding the onset of TAP2 in embryonic development.