The Dendritic Cells’ Immunological Behaviors Modulated by the Spatial Confinements of Deposited Fibrin Matrix

Implant materials,as foreign objects to host,can cause various degrees of inflammation in most cases.The inflammation is triggered by a series of immune responses and directly impacts the tissue regeneration process,which determines the outcome of tissue repair.The immune responses are complex process involving numerous immune cells and can be divide into innate immune and adaptive immune responses.Once materials are implanted,innate immune responses are activated under the mediation of several immune cells(e.g.neutrophils and macrophages),meanwhile immature dendritic cells(imDCs)are recruited to the implant sites to recognize,internalize and process antigens.Upon antigen uptake,imDCs gradually differentiate into mature dendritic cells(mDCs)and migrate to secondary lymph nodes.In the lymph nodes,mDCs present processed antigen peptides to naive T lymphocytes and activate their antigen specific proliferation,resulting in initiation of adaptive immune responses.Due to their key position in the immune system,serving to bridge innate and adaptive immunity,DCs are crucial to guiding and modulating the immune responses caused by implanted materials.Therefore,figuring out the response of DCs to implanted materials and the exact role of DCs in tissue healing processes will provide deeper insight for the rational design of biomaterials.Previous studies on the effects of implants on immune functions of DCs are mainly focused on physical and chemical properties of the materials(e.g.released chemical composition,surface chemistry,substrate stiffness and surface topography).All these factors will change the microenvironment of the tissue around implant materials,which affect the immune functions of DCs.However,the change of microenvironment not only directly derives from the physical and chemical properties of the material(intrinsic),but also indirectly results from the remodeled extracellular matrix(ECM)caused by implanted materials.When blood or tissue fluid contact with materials after implantation,proteins(e.g.fibrin and collagen)will absorb and deposit on the surface of implants,leading to a provisionally stable matrix with microporous fibrous-liked network structure.It means that the remodeled ECM can provide adhesion sites for recruited DCs and form spatial confinement.DCs,as a kind of cells that are extremely sensitive to mechanical stimuli,theoretically,can response to the mechanical stimuli coming from spatial confinement of remodeled ECM,which may lead to a series of modulations in their cell morphologies and immune functions.Then,the remodeled ECM is a non-negligible mechanical cue.However,to the best of our knowledge,there is a lack of a simple and effective model to establish the relationship between the immune functions of DCs and remodeled ECM.Most studies on the responses of DCs to implanted materials are still based on suspension culture model,which is the normal status of DCs in vitro culture systems.In addition,the processes by which DC exerts immune functions(both endocytosis and antigen presentation)are dynamically physical interaction.It means that the changes of DCs’immune functions are highly correlated with the changes of their biomechanical characteristics caused by remodeled ECM.In this work,we have found that the ECM was remodeled by a large amount of fibrin matrix deposited on the surface of implants in the early stage of the inflammations following implantation.Thus,we used non-toxic salmon fibrin hydrogels with microporous fibrous-liked network structure to mimic the deposited fibrin matrix.Then,human monocyte-derived DCs were cultured on the surface and inside of the fibrin hydrogels to mimic the different spatial confinement states of fibrin matrix.Our results indicated that cell morphologies and cytoskeleton structures of DCs were regulated by the spatial confinement of fibrin hydrogels,resulting in generating mechanical stimuli for DCs.Furthermore,we have found that the biomechanical characteristics and the immune functions of both imDCs and mDC were also modulated.Considering the changes in surface markers,secreted cytokines and biomechanical characteristics of DCs,it indicates that the tendency and magnitude of modulations were highly associated with the spatial confinement of fibrin hydrogels.This model demonstrated that mechanical stimuli deriving from spatial confinement of deposited fibrin matrix is an important factor for regulating the biomechanical characteristics and immune functions of DCs.