Normal and pathological lymphoid microenvironment

What is FL ?

Follicular lymphoma (FL) is the second most diagnosed lymphoma in the Caucasian population and the most common indolent lymphoma (Kridel et al., 2012). Despite an excellent response to current treatments (combination of immunotherapy and chemotherapy), patients relapse in most cases and 30% evolve into aggressive lymphoma, making FL a real public health problem. FL is characterized by the t (14; 18) translocation present in more than 90% of cases and leading to the constitutive activation of BCL2, a protein involved in the inhibition of apoptosis. However, some recurrent genetic alterations of tumor cells, identified in this pathology, are not oncogenic per se, but impact the dialogue between the FL and its microenvironment. We thus find alterations of TNFRSF14 / HVEMin 30-40% of FL, or the introduction of N-glycosylation sites in the variable part of the BCR in 90% of cases, all of which may be involved in the interaction with a microenvironment favorable to FL (Amin et al., 2015). Our team has recently demonstrated in a murine model of lymphoma that the loss of HVEM in B cells leads, in a context where BCL2 is deregulated, to an acceleration of lymphomagenesis via i) direct activation of B proliferation; ii) the establishment of a supportive microenvironment (Boice et al., 2016). The importance of the microenvironment has also been demonstrated by numerous transcriptomic and immunohistochemical studies, revealing the presence of a tumor niche close to the normal B niche of Germinal Centers (GC), but with specific characteristics associated with FL survival and proliferation and its resistance to therapies (Amé-Thomas and Tarte, 2014).

 

The tumor Niche of FL, a new therapeutic target :

The tumor niche of FL,  is particularly characterized by the presence of pro-tumor lymphoid stromal cells corresponding to cancer-associated fibroblasts (CAF) described in solid tumors.

The physiological lymphoid stroma :

In the physiological state, lymphoid stromal cells are required in secondary lymphoid organs for normal B cells activation and they include two major cellular subtypes: i) fibroblastic reticular cells (FRCs) involved in B cells, naive T cells as well as mature dendritic cells recruitment via the production of CCL19 and CCL21; ii) Follicular dendritic cells (FDCs) that regulate entry and retention in GC and are involved in the selection of B cells of high affinity for the antigen. These cells derive from local mesenchymal precursors that are still poorly characterized in humans. However, differentiation and activation of these cells with different localizations and functions depend on two common non-redundant factors : Tumor Necrosis Factor alpha (TNF) and lymphotoxin alpha1beta2 (LT) produced by lymphocyte cells (Peduto et al. 2009, Shields et al., 2010).

Results of the team :

Our team has demonstrated that lymphoid stromal cells with a FRC phenotype are able to support tumor B cell survival in vitro (Amé-Thomas et al., 2007). Mesenchymal stem cells (MSCs) from bone marrow (BM-MSCs) and adipose tissue (ASCs), described as precursors of mouse lymphoid stromal cells, but also tonsillar stromal cells (TSCs) can be induced in vitro by combination of TNF / LT to acquire this FRC type phenotype (FRC-like cells). In addition, this FRC-like phenotype is also observed during co-culture of MSC, ASC and TSC with FL B-lines or FL-B cells from patients. We have also demonstrated that bone marrow-derived MSCs derived from FL patients (FL-MSCs) have a particular gene expression profile, including a transcriptomic signature characteristic of lymphoid stromal cells. Moreover, these FL-MSCs spontaneously possess the same tumor B cell support capacities as committed cells differentiated in vitro in FRC-like cells (Guilloton et al., 2012). These results suggest that these LF-CSMs are ectopically engaged to a lymphoid stroma differentiation pathway. We have also shown that TNF production by LF B cells is associated with hyperexpression of CCL2 and IL-8 chemokines responsible for the recruitment of monocytes and neutrophils, contributing to the establishment of a supportive microenvironment in FL (Grégoire et al., 2015, Guilloton et al., 2012).
 
Thus, the stromal supporting microenvironment plays a key role in lymphomagenesis both by directly delivering growth signals to tumor cells and by promoting the organization of a pro-tumoral cell niche. However, these stromal cells are still poorly characterized and their study is mainly carried out in 2D culture models or in very imperfect mouse models, thus limiting both the relevance of the work carried out and their potential for the design of new therapeutic strategies. It therefore seems important to better characterize these cells and understand their emergence, but also to develop new approaches of in vitro, in vivo and in situ studies of these lymphoid stromal cells.
 
The areas of research currently being developed within the “microenvironment” group are :

- the study of the spatial kinetic aspects of the interaction between tumor B cells and stromal cells
- the characterization of human lymphoid stromal cell populations in a healthy context and in the context of FL
- the study of the epigenetic mechanisms involved in lymphoid stromal differentiation and their regulation / deregulation in the pro-tumor lymphoid stroma
- the development of a murine FL model to study interactions between tumor cells and stromal cells in vivo