Both in mice and humans, the microenvironment where T cells encounter DC is spanned by a 3-dimensional network of T zone fibroblastic reticular cells known to produce the extracellular matrix scaffold, including microvessels called conduits. More recently it has become clear that TRC are not only cells providing a 3D microenvironment but play an active role in adaptive immunity. They Vorapaxar web physically guide lymphocytes during their several hours-long migration across the T zone by forming a `road system’. TRC also actively recruit CCR7 expressing T cells and DC into the T zone by constitutively secreting CCL19 and CCL21. These chemokines not only retain T cells in the T zone but also promote their motility. Furthermore, incoming and resident DC adhere to TRC as well as their associated matrix structures. Finally, TRC are the major constitutive source of IL-7 in LN and access to LN TRC is critical for naive T cell survival. As the processes of selection, amplification and differentiation of antigen-specific T cells all take place within the TRC environment, it raises the possibility that TRC positively influence these steps. Several lines of evidence support this hypothesis: First, the TRC network appears to increase the frequency of DC-T cell encounters leading to a faster selection of antigen-specific T cells whose frequency is estimated to be around 1 out of 100’000 T cells for a given protein antigen. Both physical and chemical guidance cues provided by TRC are thought to contribute to this effect. Second, the homeostatic chemokines CCL19 15516710” and CCL21 act as costimulatory signals for T cell activation and proliferation in vitro. These chemokines also increase DC maturation and function. Third, IL-7 enhances T cell responses to November 2011 | Volume 6 | Issue 11 | e27618 Lymph Node Fibroblasts Limit T Cell Expansion viral infections in vivo. Together, these observations have strengthened the notion that TRC help in the induction of T cell responses by accelerating T cell priming and expansion. However, recent reports have suggested that TRC may also negatively regulate T cell responses. TRC were shown to express the inhibitory programmed death ligand 1 thereby reducing CD8 T cell mediated pathology. TRC also express self-antigens in the context of MHC class I thereby promoting CD8+ T cell tolerance . In addition, stromal cells isolated from neonatal or adult spleen were shown to induce over 12 weeks the development of DC that inhibit T cell proliferation in vitro. The spleen contains many stromal cell subsets and the precise identity of the cells used as well as 
their localization relative to DC and T cells has remained unclear. Together, these observations indicate that lymphoid tissue stromal cells may also inhibit T cell responses. Currently, the exact role of LN TRC in T cell activation and differentiation is not known. This is in part due to the difficulty of isolating sufficient numbers of TRC for in vitro experiments and the lack of appropriate tools to investigate TRC in vivo. Here we used a combination of in vitro and in vivo approaches to study the effect of TRC on CD8+ T cell priming by antigen-pulsed DC. We demonstrate that 21123673” TRC diminish T cell expansion by releasing NO. They share this property with a subset of DC. We show that NO production by TRC and DC is strongly dependent on cytokines from activated T cells suggesting a negative feedback loop once T cell priming has started. Our in vivo findings using Inos2/2 mice indicate th
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