Age-associated changes to lymph node fibroblastic reticular cells

The decreased proportion of antigen-inexperienced, naïve T cells is a hallmark of aging in both humans and mice, and contributes to reduced immune responses, particularly against novel and re-emerging pathogens. Naïve T cells depend on survival signals received during their circulation among the lymph nodes by direct contacts with stroma, in particular fibroblastic reticular cells. Gross changes to the architecture of the lymph nodes have been described, but it is unclear how lymph node stroma are altered by aging, and whether these changes contribute to reduced naïve T cell maintenance. Here, we visualized naïve T cell migration in the aged lymph node using 2-photon microscopy and determined that T cell motility is impaired when in proximity to fibrotic deposition. Adoptive transfer of young naïve T cells into young or aged hosts revealed reduced homeostatic turnover in the aged host, supporting the role of T cell-extrinsic mechanisms that regulate their survival. Further, we determined that development of early fibroblastic reticular cells of the lymph node stroma is impaired, which can contribute to declining levels of naïve T cell survival factors. Thus, our study addresses the controversy as to whether aging impacts the composition lymph node stroma and supports a model in which impaired differentiation of lymph node fibroblasts inhibits the interactions necessary for naïve T cell homeostasis.

Leukotriene B4 modulation of murine dendritic cells affects adaptive immunity

Dendritic cells (DCs) link innate and adaptive immunity. The microenvironment generated during the innate immunity affects DCs and the type of adaptive immunity generated. Lipid mediators are released early in inflammation and could modify the functional state of DCs. Leukotriene B₄ (LTB₄) has a wide range of effects on macrophages and in the present study we investigated if it also affects DCs. Murine bone marrow-derived DCs were employed and it was found that stimulation of DCs with LTB4 (10 nM) increased the gene expression of the high affinity receptor BLT-1 but not of BLT-2. It also increased the co-stimulatory molecule CD86 expression but did not affect CD80 and CD40. LTB₄-stimulated DCs acquired the capacity to present antigen to T lymphocytes, evidenced by antigen-specific proliferation of CD4⁺ lymphocytes in co-cultures of ovalbumin-loaded DCs with DO11.10 splenocytes. LTB₄-stimulated DCs induced Treg proliferation and increased Th2 cytokine IL-13 in the co-cultures. Expression of transcription factor genes, Gata3 and Foxp3 (Th2 and Treg, respectively) were also found increased. However, the expression of Th1 transcription factor (Tbet) and Th17 (RorγT) were not affected. These results indicate that LTB₄ affects DCs and modulates the type of adaptive immune response.