Novel insights into the molecular pathogenesis of gastric MALT lymphoma

Gastric marginal zone B-cell lymphoma of mucosa-associated lymphoid tissue (MALT lymphoma) represents a distinct class of extranodal lymphoma that evolves against a background of chronic inflammation induced by persistent infection with the bacterium Helicobacter pylori. In its early stages, MALT lymphoma is an antigen-dependent disease characterised by an indolent clinical course and in most cases is treatable by antibiotic eradication therapy alone. Low grade MALT lymphomas can eventually undergo high grade transformation to a more aggressive counterpart termed gastric diffuse large B-cell lymphoma (gDLBCL). At this stage the lymphomas grow autonomously and are refractory to Helicobacter eradication therapy. Little is known about the molecular mechanisms governing the development of low grade MALT lymphoma and its ultimate progression to gDLBCL. To improve our current understanding of the molecular pathogenesis of MALT lymphoma, a multidimensional approach was taken to investigate various aspects of this disease during the course of my PhD thesis. Although a variety of circumstantial evidence has implicated an important role for antigen in MALT lymphomagenesis, the identity of such ligands recognised by the tumour cells remain unclear. To address this issue of antigenreceptor specificity, enzyme immunoassays were employed to systematically analyse the antigen binding profile of a comprehensive panel of human and murine recombinant MALT lymphoma-derived antibodies. The majority of tumour derived immunoglobulins were found to exhibit a broad reactivity profile in line with the definition of polyreactivity. In addition, using the BALB/c mouse model of Helicobacterinduced gastric MALT lymphoma, we demonstrated that explanted tumour cells proliferated in response to a variety of cognate antigens recognised by their polyreactive B-cell receptor. Our results therefore suggest that MALT lymphoma development may be facilitated by an array of local selfand foreign antigens, providing direct antigenic stimulation of the tumour cells via their B-cell receptor. A characteristic feature of gastric MALT lymphomas is that they are typically infiltrated by large numbers of T-helper cells producing interleukin-4 (IL4) and other Thelper cell type 2 cytokines. Therefore, an additional aim of this study was to elucidate the pathogenic role of the tumour-infiltrating T-cell population. Tumour cell proliferation was strongly enhanced by the presence of intratumoural CD4+ T cells in a CD40/CD40Lindependent manner. Another key finding was that a substantial fraction of tumourinfiltrating CD4+ T cells were functional CD25+FoxP3+ regulatory T (Treg) cells. These cells were found to be recruited by the tumour cells themselves through secretion of the Tregattracting chemokines CCL17 and CCL22. Interestingly, the depletion of CD25+ T cells was as efficient as CD4+ T-cell depletion in blocking tumour growth ex vivo and in vivo. Judging from our data, we propose that MALT lymphoma cells require at least two independent signals for proliferation. One signal is received by the functional surface-bound polyreactive immunoglobulin, while the other signal is delivered by tumour-infiltrating T cells, in particular by Tregs, which are likely to play a direct role in stimulating tumour growth. To date, MALT lymphoma research has largely focused on altered expression of protein coding genes. However, recent evidence suggests that alterations of non-coding RNA, particularly microRNA (miRNA), also contribute to tumourigenesis. Using a genomewide microarray-based approach, we defined the unique miRNA expression signature associated with the development and progression of this disease. A special focus was first laid upon miR-203 and its putative tumour suppressive function during the progression from reactive Helicobacter-specific gastritis to MALT lymphoma. We identified miR-203 to be significantly downregulated in MALT lymphoma tissue due to hypermethylation of the miR203 locus. The restoration of miR-203 expression in primary MALT lymphoma cells repressed the recently identified miR-203 target, ABL1, and blocked tumour cell proliferation. Finally, pharmacological inhibition of ABL1 activity by imatinib blocked MALT lymphoma cell proliferation ex vivo and effectively eradicated tumours in vivo. Collectively, our observations suggest that ABL1 plays an important role in MALT lymphoma cell biology and support a novel potential application of imatinib in the treatment of MALT lymphoma. Our genome-wide survey further revealed a characteristic set of MYC-repressed miRNAs to be specifically downregulated in human gDLBCL compared to MALT lymphoma and gastritis. Aberrant MYC expression indeed correlated with high grade transformation as evident from our tissue microarray analysis. The re-expression of a panel of selected MYC-associated miRNAs significantly reduced the proliferation of DLBCL cells. In particular, miR-34a was found to represent the most potent tumour suppressor in DLBCL cell lines. We could further attribute the tumour suppressive effects of miR-34a to dysregulation of its target FOXP1. Accordingly, FOXP1 overexpression was found to be strongly associated with gDLBCL and the transient knockdown of FOXP1 in DLBCL cell lines significantly impaired the proliferation of the tumour cells. Taken together, our findings elucidate a novel mechanism linking the aberrant expression of MYC and concomitant repression of miR-34a to FOXP1 deregulation in high grade transformation of MALT lymphoma. Posted at the Zurich Open Repository and Archive, University of Zurich ZORA URL: https://doi.org/10.5167/uzh-50428 Originally published at: Craig, V J. Novel insights into the molecular pathogenesis of gastric MALT lymphoma. 2010, University of Zurich, Faculty of Science.