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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.

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., Summary: Lymphoid tissue fitness can determine disease severity and affect outcome by altering the course of immune responses. We discuss how lymphoid tissue function can impact disease progression during infections, chronic inflammatory conditions and cancer.

Podoplanin is an inflammatory marker upregulated in many pathologies and correlated with invasive cell behaviour. Podoplanin is reported to facilitate both actomyosin contractility and formation of cell protrusions. However, how podoplanin can elicit these opposing phenotypes is unknown. We examined podoplanin functions in lymph node fibroblastic reticular cells (FRCs), with high endogenous podoplanin expression. We report that podoplanin expression, localisation and function are dependent on partner proteins. CLEC-2 binding up-regulates podoplanin transcription, and tetraspanin CD82 is essential for trafficking of podoplanin to the plasma membrane. At the cell surface, podoplanin regulates cytoskeletal dynamics, balanced by its membrane binding partners hyaluronan receptor CD44 and tetraspanin CD9. Both CD44 and CD9 dampen podoplanin-dependent actomyosin contractility, and in vitro , CD9/podoplanin promotes filopodia-like protrusions whereas CD44/podoplanin promotes lamellipodia formation. Both CD44 and CD9 are required to coordinate protrusion formation and spreading of FRCs in response to CLEC-2 + dendritic cells, a requirement for acute lymph node expansion. In vivo , surface expression levels of podoplanin, CD44 and CD9 are specifically upregulated on T-cell zone FRCs in the early phase of lymph node expansion. Our data support a model whereby podoplanin resides in distinct plasma membrane domains, and that CLEC-2 binding serves as a molecular switch to change podoplanin function.

We compared live zebrafish, mouse and human nasal challenge responses to the TLR7/8 agonist resiquimod (R848). We found remarkably similar induction of mediators in the three species, offering novel mucosal models of innate anti-viral immunity.

Alveolar macrophages (AMs) are one of the first cells to respond to inhaled pathogens. The RIG-I-like receptors (RLRs) are critical for recognition of respiratory syncytial virus (RSV), a common cause of lower respiratory tract infections. Upon detection of the virus, RLRs signal via MAVS to induce the synthesis of pro-inflammatory mediators, including type-I-interferons (IFNs), which trigger and shape antiviral responses and protect cells from infection. AMs are among the first immune cells to encounter RSV and the main producers of type I IFNs. The ability of AMs to restrict viral replication places AMs as ideal sensors of RSV infections and important initiators of immune responses in the lung. Whether IFNs act to prevent AMs from serving as vehicles for viral replication remains unclear. To answer this question, AMs from MAVS (Mavs-/-) or type I IFN receptor (Ifnar1-/-) deficient mice were exposed to RSV ex vivo and in vivo. Wildtype (wt) AMs but not Mavs-/- or Ifnar1-/- AMs produced inflammatory mediators in response to RSV. Furthermore, Mavs-/- and Ifnar1-/- AMs accumulated more RSV proteins compared to wt, however the infection was abortive, demonstrating that induction of pro-inflammatory mediators from AMs upon RSV infection, but not the viral restriction, depends on RLR-MAVS and IFNAR signalling. Infants are vulnerable to severe RSV infection, which may require hospitalisation, and which is associated with asthma and wheezing in adolescence. Upon RSV infection, neonatal mice lack an enhanced innate response compared to adults. In order to understand this characteristic and examine potential adjuvants that can boost an immune response, neonates were exposed to various pattern recognition receptor ligands agonists (R848, imiquimod, CpG, poly(I:C) and LPS). When compared to adults, the neonates responded strongly to R848 and not to the other stimuli, highlighting the importance of TLR7 signalling in neonates. Improving the understanding of this response and the cell types responsible can open avenues to using ligands as potential vaccine adjuvants to RSV. Open Access

Rationale: Despite relative antigenic stability, respiratory syncytial virus (RSV) reinfects throughout life. After more than 40 years of research, no effective human vaccine exists and correlates of protection remain poorly defined. Most current vaccine candidates seek to induce high levels of RSV-specific serum neutralizing antibodies, which are associated with reduced RSV-related hospitalization rates in observational studies but may not actually prevent infection. Objectives: To characterize correlates of protection from infection and the generation of RSV-specific humoral memory to promote effective vaccine development. Methods: We inoculated 61 healthy adults with live RSV and studied protection from infection by serum and mucosal antibody. We analyzed RSV-specific peripheral blood plasmablast and memory B-cell frequencies and antibody longevity. Measurements and Main Results: Despite moderately high levels of preexisting serum antibody, 34 (56%) became infected, of whom 23 (68%) developed symptomatic colds. Prior RSV-specific nasal IgA correlated significantly more strongly with protection from polymerase chain reaction–confirmed infection than serum neutralizing antibody. Increases in virus-specific antibody titers were variable and transient in infected subjects but correlated with plasmablasts that peaked around Day 10. During convalescence, only IgG (and no IgA) RSV-specific memory B cells were detectable in peripheral blood. This contrasted with natural influenza infection, in which virus-specific IgA memory B cells were readily recovered. Conclusions: This observed specific defect in IgA memory may partly explain the ability of RSV to cause recurrent symptomatic infections. If so, vaccines able to induce durable RSV-specific IgA responses may be more protective than those generating systemic antibody alone.

During viral infection, inflammation and recovery are tightly controlled by competing proinflammatory and regulatory immune pathways. Respiratory syncytial virus (RSV) is the leading global cause of infantile bronchiolitis, which is associated with recurrent wheeze and asthma diagnosis in later life. Th2-driven disease has been well described under some conditions for RSV-infected mice. In the present studies, we used the Foxp3(DTR) mice (which allow specific conditional depletion of Foxp3(+) T cells) to investigate the functional effects of regulatory T cells (Tregs) during A2-strain RSV infection. Infected Treg-depleted mice lost significantly more weight than wild-type mice, indicating enhanced disease. This enhancement was characterized by increased cellularity in the bronchoalveolar lavage (BAL) fluid and notable lung eosinophilia not seen in control mice. This was accompanied by abundant CD4(+) and CD8(+) T cells exhibiting an activated phenotype and induction of interleukin 13 (IL-13)- and GATA3-expressing Th2-type CD4(+) T cells that remained present in the airways even 14 days after infection. Therefore, Treg cells perform vital anti-inflammatory functions during RSV infection, suppressing pathogenic T cell responses and inhibiting lung eosinophilia. These findings provide additional evidence that dysregulation of normal immune responses to viral infection may contribute to severe RSV disease.