Cellular microenvironment in Burkitt’s lymphoma : gene expression profiling of tumour-associated macrophages in situ

Abstract

Tumour-associated macrophages (TAM) are a major component of the inflammatory infiltrate that typifies most malignancies. Among them, Burkitt’s lymphoma (BL), a high-grade non Hodgkin lymphoma (NHL) of B-cell origin, represents a characteristic example. Studies from our group have shown that TAM in BL exert pivotal roles that are mainly supportive of tumourigenesis such as maintaining an immunosuppressive microenvironment. In order to unravel the molecular mechanisms underlying TAM functions in BL, solid tumours from a mouse xenograft model of BL have been used to obtain TAM and assess their activation status in vivo. Laser-capture microdissection has been successfully used to procure intact macrophage sections from the tumour site, allowing the production of a pure, in situ gene expression signature of TAM in BL. Tingible-body Mφ from lymph node germinal-centres and resident tissue Mφ from resting lymph nodes of non-tumour bearing mice were chosen for direct comparison with TAM. Whole-genome microarray technology has revealed a distinct TAM gene expression profile, with 454 genes being significantly up-regulated (fc ≥ 2, p<0.05) and 1293 genes being significantly down-regulated (fc ≤ -2, p<0.05) between TAM and either of the two normal Mφ populations. Further bioinformatics analysis of gene functions has highlighted matrix remodeling, phagocytosis, and immune response among the processes most highly enriched in TAM. Importantly, mRNA and tissue expression of selected differentially expressed genes relevant to these processes was validated by real-time qPCR and immunofluorescence labeling respectively. Following the generation of the TAM profile in situ, in vitro experimental approaches were undertaken in order to investigate how specific elements of the BL microenvironment drive the observed TAM signatures. Specifically, the direct role of apoptotic tumour cells, a key component of the BL microenvironment, versus that of viable tumour cells in driving TAM matrix remodelling gene expression was assessed in short-term mouse and human Mφ-NHL cell co-cultures. From the aforementioned cluster, emphasis was given to MMP12 and MMP2 transcripts: mRNA and protein expression of these MMPs was found to be up-regulated in Mφ following viable tumour cell co-cultures and this effect was further enhanced following apoptotic tumour cell co-cultures, implying that apoptotic NHL cells could directly shape TAM matrix remodeling phenotype in BL in vivo. Whereas the mRNA of both MMPs was solely Mφ-derived in this system, MMP12 and MMP2 protein was surprisingly found also to be increased in NHL cells in the apparent absence of increased mRNA. Detailed examination of MMP12 production by NHL cells revealed that it is most likely an apoptosis-dependent process, since apoptotic NHL cells generated through different apoptosis stimuli, as well as apoptotic cell-derived microparticles, showed markedly increased MMP12 protein levels. In conclusion, the data presented in this thesis, provide the first insight into the in vivo activation status of TAM in high-grade NHL, through generation of the TAM gene signature in situ. Upon further in vitro studies, apoptotic NHL cells were shown to directly modulate the matrix remodelling component of the TAM signature as well as to actively produce matrix remodelling mediators themselves, suggesting distinct roles for tumour cell apoptosis within the NHL microenvironment that can profoundly influence the disease outcome