Reports from the 15th European Histocompatibility Conference

and Poster Viewing Sessions Thorsten Zacher, Erlangen, Germany The first set of reported abstracts gave an overview of the current progress in understanding the regulation of the transcription factor CIITA, focusing on DNA methylation as a way of silencing HLA class II but also class I gene expression. In the second set of abstracts I concentrate on all presented abstracts dealing with HLA antibody mediated apoptosis. Constitutive and induced HLA class II gene expression is tightly regulated within antigen presenting cells in regard to their state-of-activation and differentiation. One of the key elements in this process of transcription seems to be the protein class II transactivator (CIITA). The CIITA gene itself is regulated by four distinct promoters, whereas promoter III (CIITA-PIII) directs constitutive expression in B cells. Nienke van der Stoep of the group of Peter J. van den Elsen examined the possible transcription factors responsible for CIITA transcription by protein-DNA interaction assays and transfection studies. Their findings indicate that, due to a putative CRE site in the CIITA-PIII, transcription factors CREB-1 and ATF-1 play an essential role in CIITA gene regulation in B cells lines. This effect seems to be enhanced by the cofactor CBP. Interestingly, these factors have impact on constitutive CIITA expression in B cells, but not HeLa cells or dendritic cells. The group of Peter J. van den Elsen presented a study on promoter IV of the CIITA gene, which regulates IFN−γ inducible expression of this transcription factor. Recent studies showed that methylation of CIITA-PIV at CpG sites prevented promoter activity and therefore HLA class II expression in foetal trophoblast cells. Being thought to be one mechanism of tolerance to the foetus, down-regulation of MHC class II molecules on tumour cells may also be considered as a tumour escape mechanism. Comparing developing and differentiated tumour cell lines for the induction of CIITA via IFN−γ, van der Stoep and colleagues found no CIITA expression in all developing tumours tested. This was in contrast to the more differentiated cells. Surprisingly, the developing tumours contained all transcription factors needed for promoter occupation as shown by transient transfection of constructs containing CIITA-PIV. Analysis of genomic DNA and demethylation experiments using the agent 5-aza-2’-deoxycytidine revealed inhibition of the CIITA-PIV by methylation. As the developing tumours originate from different kind of tissue, the authors consider a general, natural mechanism of silencing CIITA expression during development rather than an escape mechanism. That methylation of DNA has indeed negative influence of an anti-tumour response was the issue of the poster presented by Serrano and co-workers. They showed a loss of HLA-A and -B transcription after hypermethylation of these genes in a tumour cell line (MSR3-mel) generated from a melanoma patient after MAGE-3-specific immunotherapy. Studies on demethylation using 5-aza-2’deoxycytidine allowed HLA-A,-B transcription followed by tumour recognition by MAGE-specific CTLs. Reasoning on these findings the authors suggested a modulation of DNA methylation in cancer with impairment to immunotherapy. Due to the unwanted toxicity of current available agents new strategies for demethylation are needed. Although the mayor role of HLA class I and II molecules is antigen presentation recent studies indicate a signal transduction cascade inside the cell after HLA engagement. One of the events, which might occur is apoptosis, featured in three poster presentations at the congress. In the first poster, by Dinara Kaioumova and colleagues, programmed cell death was triggered by the monoclonal antibody W6/32 recognising a monomorphic epitope in the α2 and α3 domain of HLA class I. Their experimental setup included the T-cell line Jurkat and peripheral blood lymphocytes. In both systems, 50% of cells conducted apoptosis after a 24 hr treatment as detected by annexin V binding due to phosphatidylserine exposure. Interestingly, the authors were not able to detect any DNA fragmentation and caspase inhibiton of caspases-3, -8, -9 or with Z-VAD-FMK showed no influence on apoptosis induction. The involvement of membrane depolarization in mitochondria was detected in less than 50% of apoptotic cells. Rescue experiments, by transfecting the anti-apoptotic protein Bcl-2 into Jurkat did not prevent cell death in 2/3 of the cells. The presented results as a whole seem to lead to a pathway of apoptosis induction without implicating caspases. Professional antigen presenting cells like monocytes, macrophages and dendritic cells are crucial in immune surveillance and inflammation. Regulating the inflammatory process by programmed cell death after HLA-DR mediated T-cell activation might be a mechanism to overcome potential harmful effects by secreted mediators of these cells. J.G. Castaigne and co-workers introduced their in vitro model to study HLA-DR mediated apoptosis using the IFN−γ treated monocytic cell line THP-1 and the antibody L243.