Early age-related atrophy of cutaneous lymph nodes precipitates an early functional decline in skin immunity in mice with aging

Secondary lymphoid organs (SLO; including the spleen and lymph nodes) are critical both for the maintenance of naive T (TN) lymphocytes and for the initiation and coordination of immune responses. How they age, including the exact timing, extent, physiological relevance, and the nature of age-related changes, remains incompletely understood. We used time-stamping to indelibly mark cohorts of newly generated naive T cells (a.k.a. recent thymic emigrants - RTE) in mice, and followed their presence, phenotype and retention in SLO. We found that SLO involute asynchronously. Skin-draining lymph nodes (LN) atrophied early (6-9 months) in life and deeper tissue-draining LN and the spleen late (18-20 months), as measured by the loss of both TN numbers and the fibroblastic reticular cell (FRC) network. Time-stamped RTE cohorts of all ages entered SLO and successfully completed post-thymic differentiation. However, in older mice, these cells were poorly retained, and those found in SLO exhibited an emigration phenotype (CCR7loS1P1hi). Transfers of adult RTE into recipients of different ages formally demonstrated that the defect segregates with the age of the SLO microenvironment and not with the age of T cells. Finally, upon intradermal immunization, RTE generated in mice as early as 6-7 months of age barely participated in de novo immune responses and failed to produce well-armed effector cells. These results highlight changes in structure and function of superficial secondary lymphoid organs in laboratory mice that are earlier than expected and are consistent with the long-appreciated and pronounced reduction of cutaneous immunity with aging.

Development of follicular dendritic cells in lymph nodes depends on retinoic acid mediated signaling

Specialized stromal cells occupy and help define B- and T cell domains, which is crucial for proper functioning of our immune system. Signaling through lymphotoxin and TNF-receptors is crucial for development of different stromal subsets, which are thought to arise from a common precursor. However, mechanisms that control the selective generation of the different stromal phenotypes are not known. Using in vitro cultures of embryonic mouse stromal cells, we show that retinoic acid mediated signaling is important for the differentiation of precursors towards the Cxcl13pos follicular dendritic cell (FDC) lineage, while blocking lymphotoxin mediated Ccl19pos fibroblastic reticular cell (FRC) lineage differentiation. Accordingly, at day of birth we observe the presence of Cxcl13posCcl19neg/low and Cxcl13neg/lowCcl19pos cells within neonatal lymph nodes. Furthermore, ablation of retinoic acid receptor signaling in stromal precursors early after birth reduces Cxcl13 expression, while in addition, complete blockade of retinoic acid signaling prevents formation of FDC networks in lymph nodes.

Fibroblastic reticular cell response to dendritic cells requires coordinated activity of podoplanin, CD44 and CD9

In adaptive immunity, CLEC-2+ dendritic cells (DCs) contact fibroblastic reticular cells (FRCs) inhibiting podoplanin-dependent actomyosin contractility, permitting FRC spreading and lymph node (LN) expansion. The molecular mechanisms controlling LN remodelling are incompletely understood. We asked how podoplanin is regulated on FRCs in the early phase of LN expansion, and which other proteins are required for the FRC response to DCs. We find that podoplanin and its partner proteins CD44 and CD9 are differentially expressed by specific LN stromal populations in vivo, and their expression in FRCs is coregulated by CLEC-2. Both CD44 and CD9 suppress podoplanin-dependent contractility. We find that beyond contractility, podoplanin is required for FRC polarity and alignment. Independently of podoplanin, CD44 and CD9 affect FRC-FRC interactions. Further, our data show that remodelling of the FRC cytoskeleton in response to DCs is a two-step process requiring podoplanin partner proteins CD44 and CD9. Firstly, CLEC-2/podoplanin-binding inhibits FRC contractility, and secondly FRCs form protrusions and spread which requires both CD44 and CD9. Together, we show a multi-faceted FRC response to DCs, which requires CD44 and CD9 in addition to podoplanin.

Goose Nephritic Astrovirus infection of Goslings Induces Lymphocyte Apoptosis, Reticular Fiber Destruction, and CD8 T-Cell Depletion in Spleen Tissue

The emergence of a novel goose nephritic astrovirus (GNAstV) has caused economic losses to the Chinese goose industry. High viral load is found in the spleen of goslings infected with GNAstV, but pathological injuries to the spleen due to GNAstV are largely unknown. In this study, 50 two-day-old goslings were infected orally with GNAstV, and 50 goslings were treated with PBS as control. Spleens were collected at different times following infection to assess damage. GNAstV infection caused visceral gout and urate deposition in joints, and resulted in 16% mortality. GNAstV was found in the lymphocytes and macrophages within the spleen. Lymphocyte loss, especially around the white pulp, and destruction and decline in the number of reticular fibers was observed in GNAstV-infected goslings. Moreover, in GNAstV-infected goslings, ultrahistopathological examination found that splenic lymphocytes exhibited condensed chromatin and apoptotic bodies, and reticular cells displayed damage to plasma membrane integrity and swollen mitochondria. Furthermore, TUNEL staining confirmed apoptosis of lymphocytes, and the mRNA levels of Fas and FasL were significantly increased in the GNAstV-infected goslings. In addition, GNAstV infection reduced the number and protein expression of CD8. In conclusion, GNAstV infection causes lymphocyte depletion, reticular cell necrosis, reticular fiber destruction, lymphocyte apoptosis, and reduction in CD8 levels, which contribute to spleen injury.

Environment-imposed constraints can make Brownian walkers efficient searchers

Pathogen-specific CD8 T cells face the problem of finding rare cells that present their cognate antigen either in the lymph node or infected tissue. To optimize the search for rare targets it has been proposed that T cells might perform a random walk with long displacements called Levy walks enabling superdiffusive behavior and shorter search times. Many agents ranging from cells to large animals have been found to perform Levy walks suggesting that Levy walk-based search strategies may be evolutionary selected. However, whether random walk patterns are driven by agent-intrinsic programs or shaped by environmental factors remains largely unknown. To address this problem we examined the behavior of activated CD8 T cells in the liver where both the movement of the cells and the underlying structural constraints can be clearly defined. We show that Plasmodium-specific liver-localized CD8 T cells perform short displacement, Brownian-like walks and yet display superdiffusive overall displacement, the cardinal feature of efficient Levy walks. Because liver-localized CD8 T cells are mainly associated with liver sinusoids, simulations of Brownian or Levy walkers in structures derived from the liver sinusoids illustrate that structure alone can enforce superdiffusive movement. We show that linear structure of the sinusoids is sufficient to cause T cells to superdiffuse even when cell-intrinsic movement lengths are Brownian given preference for forward movement. Moreover, Brownian walkers require less time to find a rare target when T cells search for the infection in physiologically-derived liver structures. Importantly, analysis of fibroblastic reticular cell networks on which CD8 T cells move in lymph nodes also allows for superdiffusion in simulations, though this is not observed experimentally, suggesting that structure is not the only factor determining movement patterns of T cells. Our results strongly suggest that observed patterns of movement of CD8 T cells are likely to result from a combination of cell-intrinsic movement programs, physical constraints imposed by the environmental structures, and other environmental cues. Future work needs to focus on quantifying the relative contributions of these factors to the overall observed movement patterns of biological agents.