Objective To investigate the effects of exosomal microRNA-1234-3p(miR-1234-3p)of lung cancer cells on the malignant behaviors of lung cancer cells,the polarization of tumor-associat-ed macrophages(TAMs),and the functions of T cells,as well as its potential molecular mecha-nisms.Methods Exosomes were extracted from lung cancer A549 cells,and their morphology was observed using transmission electron microscopy.The expression of exosomal marker proteins[tumor susceptibility gene 101(TSG101),CD63 and CD9]was detected by western blot.The relative ex-pression level of miR-1234-3p was measured using real-time fluorescence quantitative polymerase chain reaction(qRT-PCR).The expression of exosomal miR-1234-3p was inhibited by transfecting,the proliferation,migration,and invasion abilities of A549 cells were detected using the CCK-8 method,wound healing assay,and Transwell invasion assay,respectively.The exosomes were co-cultured with macrophages and T lymphocytes separately.The expression of TAM polarization markers CD206 and CD163 was detected by western blot.The secretion levels of cytokines[interleukin(IL)-6,IL-10,and transforming growth factor-β(TGF-β)]were measured using an enzyme-linked immu-nosorbent assay(ELISA).The proliferation and apoptosis abilities of T lymphocytes,as well as the expression of functional molecules[γ-interferon(IFN-γ)and programmed death receptor 1(PD-1)]were detected through cell experiments.The expression levels of proteins related to the phosphatidylinositol 3-kinase(PI3 K)/protein kinase B(AKT)signaling pathway were detected by western blot.Results The isolated exosomes were round double-layer vesicles with a diameter of 50 to 200 nm and expressed TSG101,CD63 and CD9 proteins.The expression level of miR-1234-3p in A549 cells was higher than that in human normal lung epithelial cells BEAS-2B,and the expression level of miR-1234-3p in A549 cell exosomes was higher than that in parental A549 cells,with statis-tically significant differences(P＜0.001).After inhibiting exosomal miR-1234-3p,the prolifera-tion,migration,and invasion abilities of A549 cells decreased,with statistically significantdifferenc-es(P＜0.001).After inhibiting exosomal miR-1234-3p,the levels of M2-type polarization marker proteins CD206 and CD163 in macrophages decreased,the levels of cytokines IL-10 and TGF-β de-creased,and the level of IL-6 increased,with statistically significant differences(P＜0.001),indi-cating that the polarization of macrophages toward the M2 type was inhibited.In T lymphocytes,af-ter inhibiting exosomal miR-1234-3p,cell viability increased,the apoptosis rate decreased,the ex-pression level of the functional molecule IFN-γ increased,and the expression level of PD-1 de-creased,with statistically significant differences(P＜0.01 orP＜0.001).After inhibiting exosom-al miR-1234-3p,the protein levels of phosphorylated(p)-PI3K and p-AKT in A549 cells de-creased,with statistically significant differences(P＜0.001).Conclusion Exosomal miR-1234-3p of lung cancer cells may promote the malignant behaviors of lung cancer cells,the polarization of TAMs toward the M2 type,and the dysfunction of T lymphocytes by activating the PI3K/AKT signa-ling pathway,thereby regulating the immunosuppressive microenvironment and promoting the pro-gression of lung cancer.