The Mechanism Of Bystander CD4+ T Cells Activation By C-Reactive Protein

C-Reactive Protein(CRP)is a human acute phase protein consisting of five identical subunits,whose blood concentrations can rise 1000-fold in response to infection and inflammatory stimuli.When infection occurs,the body initiates innate immunity to clear pathogens,in which high-expression CRP in the acute phase provides host defense by activating the classical complement pathway,and pentameric CRP(p CRP)in the lesion can depolymerize into monomeric CRP(m CRP)by recognizing PC exposed to pathogens or damaged cell membranes.After innate immunity is initiated,Antigen-Presenting Cells(APCs)such as phagocytes and dendritic cells take up and process pathogen antigens,and then present the antigen peptides to specific Na?ve T cells through major histocompatibility complex(MHC).Once Na?ve CD4+ T cells are activated by T cell receptor(TCR)-specific recognition of homologous antigens(TCR engagement),they proliferate and differentiate into effector T cells,exerting their adaptive immune function by releasing specific cytokines or killing infected cells.Previous studies have shown that p CRP selectively binds to the surface of Na?ve CD4+ T cells and affects the differentiation of Na?ve CD4+ T cells into Th1/Th2 effector cells during adaptive immune initiation.However,the interaction between m CRP(enriched at the site of inflammation)and T cells,especially effector T cells present in lesions,remains unclear.Therefore,the interaction between m CRP and CD4+ T cells in the absence TCR engagement has become the focus of this study.First,in the absence of TCR engagement,p CRP/m CRP stimulation can induce the expression IFN-γ and related activation markers by CD4+ T cells in a short time,while m CRP-induced effect of CD4+ T cells was stronger than p CRP.Subsequently,by comparing the binding differences between p CRP or m CRP and CD4+ T cells and the activation differences induced by homologous p CRP with different depolymerization tendencies,concluding that p CRP activates CD4+ T cells by recognizing phosphorylcholine(PC)then dissociation into m CRP.The whole process of CRPinduced T cell activation does not depend on cognate antigenst,therefore,this m CRPdependent T cell activation model of CRP is classified here as TCR engagementindependent bystander activation.Next,according to the fact that mCRP can interact with membrane lipid raft cholesterol,after inhibiting the interaction between m CRP and cholesterol by extracting cell membrane cholesterol and deleting m CRP CBS,the binding and induction effects of m CRP on CD4+ T cells were both impaired.Cholesterol constitutively associates with TCR,the dissociation of cholesterol allosterically switches TCR to a primed conformation,which can initiate spontaneous signaling of TCR to activate T cells,and the mechanism of m CRP activation of T cells is related to this process.Corresponding results showed that the early signaling molecules of TCR pathway,such as CD3ζ,Lck and Zap 70,were activated to varying degrees in m CRP-stimulated CD4+ T cells,and distal events related to TCR activation,such as calcium influx,were also triggered.Finally,this study found that after weakening the interaction between m CRP and cholesterol,the induction effect of m CRP on TCR phosphorylation was also suppressed,so cholesterol is extremely important for m CRP to trigger TCR signaling.In subsequent experiments,by comparing the percentage of m CRP-bound Jurkat T cells before and after TCR knockout and the further results of cholesterol beads pull down assay showed that m CRP induced allosteric TCR signaling to promote bystander activation of T cells by competing TCR-cholesterol.This TCR activation mechanism of m CRP has certain universality and potentially induces local immune responses in mice.In summary,the all results show that CRP directly induces CD4+ T cell bystander activation by transforming conformation during innate immune recognition,thereby generating a rapid adaptive immune response.These results provides a reference for further understanding that CRP synergizes innate and adaptive immunity processes through conformational changes.On the other hand,this non-antigen-dependent T cell activation pattern through allosteric TCR-signaling of CRP provides a new paradigm for bystander T cell activation.