Regulation of MHC class I synthesis and expression by human neutrophils.
Human peripheral blood neutrophils (PMN) treated with granulocyte-macrophage CSF (GM-CSF) increase the amount of class I 42-kDa H chain and 12-kDa L chain, beta 2-microglobulin (beta 2m), that they synthesize by 2.1- and 2.6-fold, respectively. To determine whether the increase in translation was associated with an increase in levels of class I H chain transcript, RNA blot analysis was performed on PMN that had been cultured in the presence of GM-CSF. Under no conditions were there increased levels of class I H chain transcript when class I heterodimer protein synthesis was increased. In addition, there was neither an increase in the synthesis of H chain mRNA, as measured by transcription assay, nor an alteration in the degradation rates of class I H chain transcript in PMN cultured with GM-CSF. In situ hybridization demonstrated that both the percentage of PMN that expressed class I transcript and the relative amounts of transcript per cell in GM-CSF-cultured PMN were the same as those in control PMN. Although there is increased translation of class I heterodimer in PMN treated with GM-CSF, there is no increase in its expression on the plasma membrane. The maintenance of constant levels of class I on the plasma membrane is dependent on continued protein synthesis and is maintained by release of class I heterodimer and free beta 2m into the medium. Heterodimer is released in the context of plasma membrane-derived vesicles, whereas beta 2m is released as a soluble protein. Maintenance of constant levels of class I heterodimer on the plasma membrane is also regulated by constitutive internalization. Up to 30% of class I molecules bearing 125I-Fab-labeled mAb to class I are internalized over 2 h at 37 degrees C. Therefore, inducible synthesis of class I by PMN is likely a consequence of post-transcriptional regulation, whereas the continued synthesis of class I heterodimer is required for maintenance of its expression. Furthermore, there is no increase in class I expression, in spite of increased synthesis, due to the release of class I heterodimer and beta 2m and the internalization of class I heterodimer from the plasma membrane. Thus, PMN are capable of post-transcriptional regulation of protein synthesis and are able to modulate the expression of plasma membrane proteins by regulated expression, release, and internalization.
Neuman
E
E
Graduate Program in Cell and Developmental Biology, Harvard Medical School, Boston, Massachusetts.
Huleatt
J W
JW
Vargas
H
H
Rupp
E E
EE
Jack
R M
RM
D000944:Antigens, Differentiation, B-Lymphocyte / Q000235:genetics; D003513:Cycloheximide / Q000494:pharmacology; D004705:Endocytosis; D015870:Gene Expression / Q000187:drug effects; D016178:Granulocyte-Macrophage Colony-Stimulating Factor / Q000494:pharmacology; D015395:Histocompatibility Antigens Class I / Q000378:metabolism*; D006801:Humans; D066298:In Vitro Techniques; D008562:Membrane Glycoproteins / Q000378:metabolism; D009504:Neutrophils / Q000378:metabolism*; D009693:Nucleic Acid Hybridization; D012333:RNA, Messenger / Q000235:genetics; D011951:Receptors, Complement / Q000235:genetics; D017463:Receptors, Complement 3b; D017464:Receptors, Complement 3d; D012995:Solubility
D000944:Antigens, Differentiation, B-Lymphocyte; D015395:Histocompatibility Antigens Class I; D008562:Membrane Glycoproteins; D012333:RNA, Messenger; D011951:Receptors, Complement; D017463:Receptors, Complement 3b; D017464:Receptors, Complement 3d; D016178:Granulocyte-Macrophage Colony-Stimulating Factor; D003513:Cycloheximide