Smad2/4 Signaling Pathway Is Critical for Epidermal Langerhans Cell Repopulation Under Inflammatory Condition but Not Required for Their Homeostasis at Steady State

Abstract

Epidermal Langerhans cells (LCs) are skin-resident dendritic cells that are essential for the induction of skin immunity and tolerance. Transforming growth factor-β 1 (TGFβ1) is a crucial factor for LC maintenance and function. However, the underlying TGFβ1 signaling pathways remain unclear. Our previous research has shown that the TGFβ1/Smad3 signaling pathway does not impact LC homeostasis and maturation. In this study, we generated mice with conditional deletions of either individual Smad2, Smad4, or both Smad2 and Smad4 in the LC lineage or myeloid lineage, to further explore the impact of TGFβ1/Smad signaling pathways on LCs. We found that interruption of Smad2 or Smad4 individually or simultaneously in the LC lineage did not significantly impact the maintenance, maturation, antigen uptake, and migration of LCs in vivo or in vitro during steady state. However, the interruption of both Smad2 and Smad4 pathways in the myeloid lineage led to a dramatic inhibition of bone marrow-derived LCs in the inflammatory state. Overall, our data suggest that canonical TGFβ1/Smad2/4 signaling pathways are dispensable for epidermal LC homeostasis and maturation at steady state, but are critical for the long-term LC repopulation directly originating from the bone marrow in the inflammatory state.

ALK3 Is Not Required for the Embryonic Development, Homeostasis, and Repopulation of Epidermal Langerhans Cells in Steady and Inflammatory States

“…In the canonical TGFb signaling, TGFb1 binds TGFbR1 and TGFbR2 complex, mediating the phosphorylation of SMAD2 and SMAD3 complex, which forms a heterotrimeric complex with SMAD4 and translocates into the nucleus and regulates TGFb1-targeting genes (Derynck and Zhang, 2003). However, our genetic mouse models with deletion of Smad3 (Xu et al, 2012), Smad2, and Smad4 (Huang et al, 2020) showed no effect of SMAD pathway loss on LCs development at steady state but did show an impact on LC repopulation under inflammatory state. Nevertheless, recent studies revealed that both TGFb family members, TGFb1 and BMP7, can potentially signal through BMPR1A or ALK3 to induce LC differentiation in vitro (Borek et al, 2020;Yasmin et al, 2013), and BMP7-ALK3 signaling leads to the differentiation and proliferation of inflammation-associated LCs from bone marrowederived precursors in psoriatic lesions, which promotes the psoriatic epidermal changes in patients with psoriasis as well as in a psoriatic mouse model (Borek et al, 2020).…”

“…LCs could be derived from bone marrow under inflammatory conditions, and we found that SMAD2 and/or SMAD4 is required for LC repopulation from bone marrow in UVC-treated skin (Huang et al, 2020). To explore the role of ALK3 in LC repopulation, the epidermal LCs (CD45 þ MHCIIhi CD207 hi ) in Csf-1R Cre ALK3 fl/fl and WT littermates were analyzed at 2 weeks after UVC treatment and were found comparable (Figure 1g), indicating that ALK3 is not required for LCs repopulation.…”

USP2 promotes experimental colitis and bacterial infections by inhibiting the proliferation of myeloid cells and remodeling the extracellular matrix network

“…In contrast, we found that USP2 inhibits the TGF-β signaling and the proliferation of myeloid cells. In support of this notion, a number of studies have shown that TGFβ signaling promotes the proliferation of myeloid cells ( Bataller et al., 2019 ; Huang et al., 2020 ; Meng et al., 2016 ). Moreover, a recent study has reported that USP2 inhibits TGF-β pathway by promoting deubiquitylation of SMAD7 in glioblastoma cells that has been assumed to originate from glial-type cells and resembles some features of glial macrophages ( Tu et al., 2022 ).…”

The Roles of Skin Langerhans Cells in Immune Tolerance and Cancer Immunity

“…Phagocytosis of apoptotic cells by macrophages and DC leads to inflammatory immune response suppression [12,21]. Transforming growth factor-β1 (TGF-β1) is a fundamental regulator of immune cell development and function and this molecule is a crucial regulatory factor for LC development, maintenance, and function with context-dependent down-stream signaling pathways [22][23][24]. Axl is a TAM (Tyro3, Axl, and Mer) receptor tyrosine kinase family member, which functions as an inhibitor of innate inflammatory responses in dendritic cells and is essential for the prevention of lupus-like autoimmunity [25].…”

“…The pivotal role of TGF-β in LC development and epidermal residence has been established, in which TGF-β is absolutely required for LC development [113,114], and the activity of TGF-β is dependent on integrins αvβ6 and αvβ8 that are expressed on KC [115,116]. In addition, TGF-β is required for inflammation-induced LC repopulation, in which alternative TGF-β downstream signaling pathways are involved, which is different from steady-state LC maintenance [22,23]. Given the critical role of TGF-β and specific downstream signaling pathways in the epidermal retention and maintenance of LC, as well as the critical involvement of TGF-β in the induction of tolerogenic function in both DC and TRM in multiple tissues, it is tempting to speculate a potential role for TGF-β in programming the LC tolerogenic state.…”