Follicular dendritic cells, conduits, lymphatic vessels, and high endothelial venules in tertiary lymphoid organs: Parallels with lymph node stroma

Role of the Lymphotoxin/LIGHT System in the Development and Maintenance of Reticular Networks and Vasculature in Lymphoid Tissues

Lymphoid organs are meeting zones where lymphocytes come together and encounter antigens present in the blood and lymph or as delivered by cells migrating from the draining tissue bed. The exquisite efficiency of this process relies heavily on highly specialized anatomy to direct and position the various players. Gated entry and exit control access to these theaters and reticular networks and associated chemokines guide cells into the proper sections. Lymphoid tissues are remarkably plastic, being able to expand dramatically and then involute upon resolution of the danger. All of the reticular scaffolds and vascular and lymphatic components adapt accordingly. As such, the lymph node (LN) is a wonderful example of a physiologic remodeling process and is potentially a guide to study such elements in pathological settings such as fibrosis, chronic infection, and tumor metastasis. The lymphotoxin/LIGHT axis delivers critical differentiation signals that direct and hone differentiation of both reticular networks and the vasculature. Considerable progress has been made recently in understanding the mesenchymal differentiation pathways leading to these specialized networks and in the remodeling that occurs in reactive LNs. In this article, we will review some new advances in the area in terms of developmental, differentiation, and maintenance events mediated by this axis.

Positive and negative influence of the matrix architecture on antitumor immune surveillance

The migration of T cells and access to tumor antigens is of utmost importance for the induction of protective anti-tumor immunity. Once having entered a malignant site, T cells encounter a complex environment composed of non-tumor cells along with the extracellular matrix (ECM). It is now well accepted that a deregulated ECM favors tumor progression and metastasis. Recent progress in imaging technologies has also highlighted the impact of the matrix architecture found in solid tumor on immune cells and especially T cells. In this review, we argue that the ability of T cells to mount an antitumor response is dependent on the matrix structure, more precisely on the balance between pro-migratory reticular fiber networks and unfavorable migration zones composed of dense and aligned ECM structures. Thus, the matrix architecture, that has long been considered to merely provide the structural framework of connective tissues, can play a key role in facilitating or suppressing the antitumor immune surveillance. A new challenge in cancer therapy will be to develop approaches aimed at altering the architecture of the tumor stroma, rendering it more permissive to antitumor T cells.

Therapeutic use of dendritic cells to promote the extranodal priming of anti-tumor immunity

Ectopic lymphoid tissue, also known as tertiary lymphoid organs (TLO) develop adaptively within sites of chronic tissue inflammation, thereby allowing the host to efficiently crossprime specific immune effector cells within sites of disease. Recent evidence suggests that the presence of TLO in the tumor microenvironment (TME) predicts better overall survival. We will discuss the relevance of extranodal T cell priming within the TME as a means to effectively promote anti-tumor immunity and the strategic use of dendritic cell (DC)-based therapies to reinforce this clinically preferred process in the cancer-bearing host.

Stromal cell regulation of homeostatic and inflammatory lymphoid organogenesis

Secondary lymphoid organs function to increase the efficiency of interactions between rare, antigen-specific lymphocytes and antigen presenting cells, concentrating antigen and lymphocytes in a supportive environment that facilitates the initiation of an adaptive immune response. Homeostatic lymphoid tissue organogenesis proceeds via exquisitely controlled spatiotemporal interactions between haematopoietic lymphoid tissue inducer populations and multiple subsets of non-haematopoietic stromal cells. However, it is becoming clear that in a range of inflammatory contexts, ectopic or tertiary lymphoid tissues can develop inappropriately under pathological stress. Here we summarize the role of stromal cells in the development of homeostatic lymphoid tissue, and assess emerging evidence that suggests a critical role for stromal involvement in the tertiary lymphoid tissue development associated with chronic infections and inflammation.

Lymphatic vessel function in head and neck inflammation

BACKGROUND

Serious infections of the head and neck cause lymphedema that can lead to airway compromise and oropharyngeal obstruction. Lymphangiogenesis occurs in the head and neck during infection and after immunization. The goal of this project was to develop tools to image lymphatic vessels in living animals and to be able to isolate individual lymphatic endothelial cells in order to quantify changes in single cells caused by inflammation.

METHODS

The ProxTom transgenic red-fluorescent reporter mouse was developed specifically for the purpose of imaging lymphatic vessels in vivo. Prox1 is a transcription factor that is necessary for lymphangiogenesis in development and for the maintenance of lymphatics in adulthood. Mice were immunized and their lymphatic vessels in lymph nodes were imaged in vivo. Individual lymphatic endothelial cells were isolated by means of their fluorescence.

RESULTS

The ProxTom transgene has the red-fluorescent reporter td-Tomato under the control of Prox1 regulatory elements. tdTomato was faithfully expressed in lymphatic vessels coincident with endogenous Prox1 expression. We show lymphangiogenesis in vivo after immunization and demonstrate a method for the isolation of lymphatic endothelial cells by their tdTomato red-fluorescence.

CONCLUSIONS

The faithful expression of the red-fluorescent reporter in the lymphatic vessels of ProxTom means that these mice have proven utility for in vivo study of lymphatic vessels in the immune response. ProxTom has been made available for distribution from the Jackson Laboratory: http://jaxmice.jax.org/strain/018128.html .