The transcription factor SpiB regulates the fibroblastic reticular cell network and CD8+ T-cell responses in lymph nodes

Fibroblastic reticular cells (FRCs) construct microanatomical niches that support lymph node (LN) homeostasis and coordination of immune responses. Transcription factors regulating the functionality of FRCs remain poorly understood. Here, we investigated the role of the transcription factor SpiB that is expressed in LN FRCs. Conditional ablation of SpiB in FRCs impaired the FRC network in the T-cell zone of LNs, leading to reduced numbers of FRCs and altered homeostatic functions including reduced CCL21 and interleukin-7 expression. The size and cellularity of LNs remained intact in the absence of SpiB but the space between the reticular network increased, indicating that although FRCs were reduced in number they stretched to maintain network integrity. Following virus infection, antiviral CD8+ T-cell responses were impaired, suggesting a role for SpiB expression in FRCs in orchestrating immune responses. Together, our findings reveal a new role for SpiB as an important regulator of FRC functions and immunity in LNs.

Lymph node homeostasis and adaptation to immune challenge resolved by fibroblast network mechanics

Emergent physical properties of tissues are not readily understood by reductionist studies of their constituent cells. Here, we show molecular signals controlling cellular, physical, and structural properties and collectively determine tissue mechanics of lymph nodes, an immunologically relevant adult tissue. Lymph nodes paradoxically maintain robust tissue architecture in homeostasis yet are continually poised for extensive expansion upon immune challenge. We find that in murine models of immune challenge, cytoskeletal mechanics of a cellular meshwork of fibroblasts determine tissue tension independently of extracellular matrix scaffolds. We determine that C-type lectin-like receptor 2 (CLEC-2)-podoplanin signaling regulates the cell surface mechanics of fibroblasts, providing a mechanically sensitive pathway to regulate lymph node remodeling. Perturbation of fibroblast mechanics through genetic deletion of podoplanin attenuates T cell activation. We find that increased tissue tension through the fibroblastic stromal meshwork is required to trigger the initiation of fibroblast proliferation and restore homeostatic cellular ratios and tissue structure through lymph node expansion.

Immune function and dysfunction are determined by lymphoid tissue efficacy

Lymphoid tissue returns to a steady state once each immune response is resolved, and although this occurs multiple times throughout life, its structural integrity and functionality remain unaffected. Stromal cells orchestrate cellular interactions within lymphoid tissue, and any changes to the microenvironment can have detrimental outcomes and drive disease. A breakdown in lymphoid tissue homeostasis can lead to a loss of tissue structure and function that can cause aberrant immune responses. This Review highlights recent advances in our understanding of lymphoid tissue function and remodelling in adaptive immunity and in disease states. We discuss the functional role of lymphoid tissue in disease progression and explore the changes to lymphoid tissue structure and function driven by infection, chronic inflammatory conditions and cancer. Understanding the role of lymphoid tissues in immune responses to a wide range of pathologies allows us to take a fuller systemic view of disease progression.

Fibroblastic Reticular Cells Control Conduit Matrix Deposition during Lymph Node Expansion

Lymph nodes (LNs) act as filters, constantly sampling peripheral cues. This is facilitated by the conduit network, a tubular structure of aligned extracellular matrix (ECM) fibrils ensheathed by fibroblastic reticular cells (FRCs). LNs undergo rapid 3- to 5-fold expansion during adaptive immune responses, but these ECM-rich structures are not permanently damaged. Whether conduit flow or filtering function is affected during LN expansion is unknown. Here, we show that conduits are partially disrupted during acute LN expansion, but FRC-FRC contacts remain connected. We reveal that polarized FRCs deposit ECM basolaterally using LL5-β and that ECM production is regulated at transcriptional and secretory levels by the C-type lectin CLEC-2, expressed by dendritic cells. Inflamed LNs maintain conduit size exclusion, and flow is disrupted but persists, indicating the robustness of this structure despite rapid tissue expansion. We show how dynamic communication between peripheral tissues and LNs provides a mechanism to prevent inflammation-induced fibrosis in lymphoid tissue.