In secondary lymphoid organs, development and homeostasis of stromal cells such as podoplanin (Pdpn)-positive fibroblastic reticular cells (FRCs) are regulated by hematopoietic cells, but the cellular and molecular mechanisms of such regulation have remained unclear. Here we show that ablation of either signal regulatory protein α (SIRPα), an Ig superfamily protein, or its ligand CD47 in conventional dendritic cells (cDCs) markedly reduced the number of CD4 cDCs as well as that of Pdpn FRCs and T cells in the adult mouse spleen. Such ablation also impaired the survival of FRCs as well as the production by CD4 cDCs of tumor necrosis factor receptor (TNFR) ligands, including TNF-α, which was shown to promote the proliferation and survival of Pdpn FRCs. CD4 cDCs thus regulate the steady-state homeostasis of FRCs in the adult spleen via the production of TNFR ligands, with the CD47-SIRPα interaction in cDCs likely being indispensable for such regulation.

Conventional dendritic cells (cDCs) orchestrate immune responses to cancer and comprise two major subsets: type-1 cDCs (cDC1s) and type-2 cDCs (cDC2s). Compared with cDC1s, which are dedicated to the activation of CD8+ T cells, cDC2s are ontogenically and functionally heterogeneous, with their main function being the presentation of exogenous antigens to CD4+ T cells for the initiation of T helper cell differentiation. cDC1s play an important role in tumor-specific immune responses through cross-presentation of tumor-derived antigens for the priming of CD8+ T cells, whereas little is known of the role of cDC2s in tumor immunity. Recent studies have indicated that human cDC2s can be divided into at least two subsets and have implicated these cells in both anti- and pro-tumoral immune responses. Furthermore, the efficacy of cDC2-based vaccines as well as cDC2-targeted therapeutics has been demonstrated in both mouse models and human patients. Here we summarize current knowledge about the role of cDC2s in tumor immunity and address whether these cells are beneficial in the context of antitumor immune responses.

Signal regulatory protein α (SIRPα) is expressed predominantly on type 2 conventional dendritic cells (cDC2s) and macrophages. We previously showed that mice systemically lacking SIRPα were resistant to experimental autoimmune encephalomyelitis (EAE). Here, we showed that deletion of SIRPα in CD11c + cells of mice (Sirpa DC mice) also markedly ameliorated the development of EAE. The frequency of cDCs and migratory DCs (mDCs), as well as that of Th17 cells, were significantly reduced in draining lymph nodes of Sirpa DC mice at the onset of EAE. In addition, we found the marked reduction in the number of Th17 cells and DCs in the CNS of Sirpa DC mice at the peak of EAE. Whereas inducible systemic ablation of SIRPα before the induction of EAE prevented disease development, that after EAE onset did not ameliorate the clinical signs of disease. We also found that EAE development was partially attenuated in mice with CD11c + cell-specific ablation of CD47, a ligand of SIRPα. Collectively, our results suggest that SIRPα expressed on CD11c + cells, such as cDC2s and mDCs, is indispensable for the development of EAE, being required for the priming of self-reactive Th17 cells in the periphery as well as for the inflammation in the CNS.

Nonhematopoietic stromal cells contribute to the organization and homeostasis of secondary lymphoid organs by producing cytokines and chemokines. The development and maintenance of these stromal cells are thought to be regulated by innate immune cells. Indeed, we recently showed that signal regulatory protein α (SIRPα)-positive dendritic cells (DCs) are essential for the proliferation and survival of podoplanin (Pdpn)-positive fibroblastic reticular cells (FRCs) in mouse spleen. We have now established an in vitro culture system for lymph node stromal cells (LNSCs) isolated from mouse peripheral LNs. Activated DCs and TNF-α each promoted the proliferation of cultured LNSCs, most of which were found to be Pdpn + FRCs. Furthermore, ablation of SIRPα in CD11c + cells attenuated this effect of DCs on LNSC proliferation. Transplantation of activated DCs together with cultured LNSCs into the renal subcapsular space markedly increased the number of ER-TR7 + stromal cells as well as induced the accumulation of T cells and increased the expression of Ccl19 in the transplants. Ablation of SIRPα in CD11c + cells greatly impaired the development of LN-like structure in the transplants. Our findings thus suggest that SIRPα + DCs are important for the proliferation and differentiation of Pdpn + FRCs in peripheral LNs.Keywords: Dendritic cell r Fibroblastic reticular cell r Lymph node r SIRPα r Stromal cell Additional supporting information may be found online in the Supporting Information section at the end of the article.

Conventional dendritic cells (cDCs) are required for peripheral T cell homeostasis in lymphoid organs, but the molecular mechanism underlying this requirement has remained unclear. We here show that T cell–specific CD47-deficient ( Cd47  ΔT ) mice have a markedly reduced number of T cells in peripheral tissues. Direct interaction of CD47-deficient T cells with cDCs resulted in activation of the latter cells, which in turn induced necroptosis of the former cells. The deficiency and cell death of T cells in Cd47   ΔT mice required expression of its receptor signal regulatory protein α on cDCs. The development of CD4 + T helper cell–dependent contact hypersensitivity and inhibition of tumor growth by cytotoxic CD8 + T cells were both markedly impaired in Cd47  ΔT mice. CD47 on T cells thus likely prevents their necroptotic cell death initiated by cDCs and thereby promotes T cell survival and function.