Alterations in CIITA constitute a common mechanism accounting for downregulation of MHC class II expression in diffuse large B-cell lymphoma (DLBCL).
OBJECTIVE
Significant decreases in patient survival are associated with downregulation of major histocompatibility complex class II (MHC-II) antigen expression in diffuse large B-cell lymphoma (DLBCL). However, the molecular mechanisms responsible for decreased MHC-II expression in DLBCL are poorly defined. We therefore examined these mechanisms in established DLBCL cell lines.


METHODS
Human leukocyte antigen (HLA)-DR surface expression was examined by flow cytometry. Expression of the MHC-II genes and the MHC-II transcriptional activators class II transactivator (CIITA) and RFX was investigated by reverse transcriptase polymerase chain reaction. The integrity of the MHC-II genes was examined by polymerase chain reaction. Stable transfection assays were utilized to reconstitute CIITA expression.


RESULTS
Dramatic variations in the levels of cell surface HLA-DR expression were observed on the DLBCL cell lines. OCI-Ly10 cells lack HLA-DR and HLA-DQ expression due to homozygous deletions within the MHC-II locus on chromosome 6. Dyscoordinate downregulation of MHC-II beta-chain expression in OCI-Ly3 cells mediates dramatic reductions of MHC-II surface expression. In SUDHL-4 and SUDHL-6 cells, expression of the MHC-II genes is coordinately reduced and quantitatively correlated with expression of the CIITA, the master regulator of MHC-II transcription. DB cells lack expression of CIITA and all of the MHC-II genes. Stable transfection of DB cells with CIITA expression vectors resulted in coordinate upregulation of MHC-II gene expression, which demonstrates the causal relationship between the lack of CIITA and MHC-II loss.


CONCLUSIONS
These data demonstrate that downregulation of MHC-II expression occurs by multiple distinct mechanisms in DLBCL. However, decreases in CIITA expression appear to be the most prevalent mechanism.
Cycon
Kelly A
KA
Department of Immunology, Roswell Park Cancer Institute, Buffalo, NY, USA.
Rimsza
Lisa M
LM
Murphy
Shawn P
SP
D045744:Cell Line, Tumor; D002896:Chromosomes, Human, Pair 6 / Q000235:genetics / Q000378:metabolism; D015973:Gene Expression Regulation, Leukemic / Q000235:genetics; D006683:HLA-DQ Antigens / Q000096:biosynthesis* / Q000235:genetics; D006684:HLA-DR Antigens / Q000096:biosynthesis* / Q000235:genetics; D006801:Humans; D016403:Lymphoma, Large B-Cell, Diffuse / Q000235:genetics / Q000378:metabolism*; D009363:Neoplasm Proteins / Q000096:biosynthesis* / Q000235:genetics; D009687:Nuclear Proteins / Q000096:biosynthesis* / Q000235:genetics; D040641:Quantitative Trait Loci / Q000235:genetics; D015534:Trans-Activators / Q000096:biosynthesis* / Q000235:genetics
D006683:HLA-DQ Antigens; D006684:HLA-DR Antigens; C083728:MHC class II transactivator protein; D009363:Neoplasm Proteins; D009687:Nuclear Proteins; D015534:Trans-Activators