Genetic Alterations of the MHC Class II Transactivator CIITA Are Frequent in Primary Mediastinal Large B-Cell Lymphoma and Associated with Diminished MHC Class II Expression

Introduction: Constitutive MHC class II expression is a hallmark of antigen-presenting cells, including B cells, and is indispensable for the initiation of antigen specific immune responses. It has been shown that certain B cell lymphoma entities are able to evade immune recognition by downregulation of MHC molecules on the tumor cell surface. We have previously identified recurrent chromosomal rearrangements of CIITA, the master regulator of MHC class II transcription, as one possible mechanism to reduce MHC class II expression in primary mediastinal large B-cell lymphoma (PMBCL) and classical Hodgkin lymphoma (cHL) (Steidl et al., Nature 2011). Furthermore, we have recently described a 1.6kb breakpoint cluster region within intron 1 of CIITA and have shown in a small sample set of PMBCL cases that deletions, insertions and single nucleotide variants (SNV) are commonly found within this genomic region (Steidl, ASH abstract # 437, 2011). Therefore, we aimed to explore the frequency of these alterations and the correlation with CIITA and MHC class II protein expression in a larger cohort of PMBCL cases and to further characterize their functional significance.

Methods: We have comprehensively analyzed 45 diagnostic PMBCL samples for the presence of coding sequence mutations as well as alterations within the promoter III region and the first 3kb of intron 1 using deep amplicon sequencing (Illumina TruSeq) and/or Sanger sequencing. In addition, we characterized the PMBCL-derived cell lines U2940 and Med-B1 by whole transcriptome paired-end sequencing (RNA-seq). To elucidate the functional consequences of the coding sequence mutations identified in these two cell lines we performed retroviral transductions of wild type CIITA and CIITA mutants in a CIITA and HLA-DR expression-negative cell line (DEV, nodular lymphocyte predominant Hodgkin lymphoma-derived). We subsequently analyzed CIITA mRNA expression using qRT-PCR and HLA-DR surface expression using flow cytometry. Furthermore, we applied immunohistochemistry (IHC) to determine expression levels of CIITA and HLA-DR in a large cohort of PMBCL samples represented on two tissue microarrays (TMA, n=149). The TMAs were also used for fluorescence in-situ hybridization (FISH) to evaluate the presence of copy number alterations or translocations of the CIITA locus.

Results: FISH was interpretable in 115 samples with a CIITA break-apart (CIITA-ba) frequency of 33.9% (39/115). Correlative analyses revealed that decreased CIITA protein expression by IHC was significantly correlated with the presence of CIITA-ba (P=0.019), whereas HLA-DR expression was not correlated with CIITA-ba status alone (P=0.219). However, we could demonstrate a positive correlation between protein expression of CIITA and HLA-DR (Pearson r=0.45, P<0.0001). Within the subset of 45 PMBCL cases that were analyzed for the presence of genomic alterations, 39% were CIITA-ba positive (16/41), and in 31.8% (14/44) we observed coding sequence mutations and/or alterations affecting the promoter III region. 45.5% (20/44) of the cases presented indels and/or SNVs in intron 1.

Using RNA-seq, we have detected two missense mutations in the Med-B1 cell line affecting both alleles in functionally relevant protein domains. Furthermore, we identified a novel NUBP1-CIITA fusion transcript in U2940 also harboring an SNV on the other allele resulting in the transcription of an elongated protein due to the loss of the original stop codon. Ectopic expression of these CIITA mutants in DEV, which has been shown to have undetectable levels of CIITA and HLA-DR due to a biallelic CIITA inactivation, revealed that these individual SNVs showed a diminished capability to restore HLA-DR surface expression in comparison to wild type CIITA as measured by flow cytometry.

Conclusions: Here we show that the presence of CIITA rearrangements is significantly associated with low CIITA protein levels, and we could demonstrate that protein expression of CIITA and HLA-DR are positively correlated in PMBCL. Furthermore, CIITA is frequently targeted by coding sequence mutations and intronic deletions in PMBCL cell lines and clinical samples. Functional studies demonstrate that genomic alterations in CIITA contribute to downregulation of MHC class II expression in malignant lymphomas and therefore represent a potent mechanism of acquired immune privilege and escape from immune surveillance.