Regulation of Major Histocompatibility Complex and TAP Gene Products in Preimplantation Mouse Stage Embryos

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‐2D^b) 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.